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UBC Theses and Dissertations

Addition polymerization of phosphorus-carbon double bonds Yam, Man Sheung Mandy 2007

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ADDITION POLYMERIZATION OF PHOSPHORUS-CARBON DOUBLE BONDS by MAN SHEUNG MANDY YAM B.Sc., The University of British Columbia, 2000 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (Chemistry) THE UNIVERSITY OF BRITISH COLUMBIA August 2007 © Man Sheung Mandy Yam, 2007 Abstract The distillation of phosphaalkene MesP=CPh2(I, Mes = 2,4,6-trimethyiphenyl) has afforded poly(methylenephosphine) (II) in the residue with a molecular weight of ca. 30,000 gmoF’. Polymer II can also be obtained using radical and anionic initiators. The backbone of polymer II consists of phosphine moieties which can be chemically functionalized using hydrogen peroxide and elemental sulfur to afford air-stable polymers [-MesP(=O)CPh2jand [-MesP(=S)CPh2],respectively. The thermogravimetric analyses (TGA) of these polymers showed a high thermal stability to weight loss (Tonset = ca. 300 °C). Using the base-catalyzed phospha-Peterson reaction, we have synthesized several P-Mes phosphaalkenes including MesPC(4-FC6H4)2(III), Me5PCPh(4-FC6H4(IV), MesPC(4- MeOC6H4)2(V), MesP==CPh(4MeOC6H4(VI), and MesPCPh(2-pyridyl) (VII). X-ray quality crystals were obtained for phosphaalkenes III, VI, and VII. Attempts to prepare P-adamantyl (P Ad) phosphaalkenes with this method were unsuccessful. In an attempt to prepare AdPCPh2, the product isolated was found to be a 1 ,2-diphosphetane (AdPCPh2)by X-ray crystallography. The effects of reaction temperature, residence time, and initiator loading on the radical polymerization of! were studied. The polymerization conditions of 1% VAZO 88 initiator [1,1’- azobis(cyclohexanecarbonitrile)] at 100 °C for 24 hours are shown to give a higher molecular weight (M = ca. 16,000 gmoi’) and yield for II for radical polymerizations. Employing these conditions, phosphaalkenes Ill—WI were polymerized to give C-functionalized poly(methylenephosphine)s with molecular weight in the range of 6,000 to 20,000 gmof’. The radical copolymerization of phosphaalkenes with styrene offers a convenient route to incorporate phosphine moieties into organic polymers. Preliminary estimation of the monomer reactivity ratios for phosphaalkene I and styrene was conducted using Fineman—Ross and Kelen— 11 TUdOs methods (rpA 2 and rsT 1). This study suggests that this copolymer may consist of poly(methylenephosphine) sequences, randomly, distributed between styrene moieties. To expand the addition polymerization to other PC systems, several new P-fluoroaryl phosphaalkenes were prepared using base-induced 1,3-hydrogen rearrangement. Employing 2,6- bis- or 2,4,6-tris-(trifluoromethyl)phenyl (ArF’ and ArF) substituents, the rearrangement reaction becomes a viable route to isolable phosphaalkenes with a C-methyl substituent (i.e., ArF’PCMe2,ArF’P=CMePh, and ArpP=CMe2). 111 Table of contents Abstract ii Table of contents iv List of tables x List of figures xi List of abbreviations xiii Acknowledgements xvii Statement of co-authorship xviii Chapter One Introduction: Polymers: A New Challenge in Inorganic Chemistry 1 1.1 Addition polymerization 1 1.1.1 Inorganic polymers 6 1.1.2 Research objectives 7 1.2 Possible inorganic multiple-bond systems for addition polymerization 8 1.2.1 Challenges to prepare stable unsaturated inorganic compounds 9 1.2.2 Examples of potential monomers 11 1.3 The selection of P=C bonds for polymerization studies 15 1.3.1 Similarities between P=C and CC bonds 16 1.3.2 The P=C/C=C analogy in molecular chemistry 16 1.4 Feasibility of the addition polymerization of P=C bonds 18 1.4.1 Estimation of the enthalpy of the polymerization of P=C bonds 19 iv 1.4.2 Consequences of unstable phosphaalkenes 20 1.4.3 Stabilization of P=C bonds for polymerization studies 25 1.5 Outline of thesis 27 1.6 Contributions by other researchers to this thesis work 28 1.7 References 29 Chapter Two The Addition Polymerization of a P=C Bond: A Route to New Phosphine Polymers 38 2.1 Introduction 38 2.2 Results and discussion 40 2.2.1 Synthesis and characterization of poly(methylenephosphine) 40 2.2.2 Properties and chemical modification of poly(methylenephosphine) 41 2.2.3 Polymerization of phosphaalkene using a radical initiator 43 2.2.4 Polymerization of phosphaalkene using anionic initiators 44 2.3 Summary 45 2.4 Experimental section 46 2.4.1 Preparation of MesPCPh2(2.1) 47 2.4.2 Isolation of poly(methylenephosphine) (2.2) from the distillation residue 48 2.4.3 Preparation of poly(methylenephosphine oxide) (2.3) 49 2.4.4 Preparation of poly(methylenephosphine sulfide) (2.4) 49 2.4.5 Radical polymerization of 2.1 using VAZO 88 50 V 2.4.6 Anionic polymerization of 2.1 using MeLi 50 2.4.7 Anionic polymerization of 2.1 using BuLi 51 2.4.8 Reaction of 2.1 with MeLi (1 equiv) 52 2.4.9 Anionic polymerization of 2.1 using 2.5 52 2.5 References 53 Chapter Three Scope and Limitations of the Base-Catalyzed Phospha-Peterson P=C Bond Forming Reaction 57 3.1 Introduction 57 3.2 Results and discussion 59 3.2.1 Synthesis of P-mesityl phosphaalkenes 59 3.2.2 Attempted synthesis of P-adamantyl phosphaalkenes 66 3.3 Summary 70 3.4 Experimental section 71 3.4.1 Preparation of MesP=CPh2(3.la) 72 3.4.2 Preparation ofMesP=C(4-FC6H4)(3.lb) 72 3.4.3 Preparation of MesP=C(Ph)(4-FC (3.lc) 73 3.4.4 Preparation of MesPC(4-MeOC6H4)2(3.ld) 74 3.4.5 Preparation of MesPC(Ph)(4-MeOC (3.le) 75 3.4.6 Preparation of Me5PC(Ph)(2-py) (3.11) 76 3.4.7 Preparation of AdP(SiMe3)2 76 3.4.8 Preparation of AdP(H)SiMe3 77 3.4.9 Preparation of AdP(Li)SiMe3 78 3.4.10 Preparation of (AdPCPh)2(3.3a) 78 3.4.11 Attempted preparation of AdPCR’R” (3.2b—d) 79 vi 3.5 References . 80 Chapter Four Radical Initiated Polymerization of Phosphaalkenes: The Effects of Temperature, Initiator, Time, and Substituent 86 4.1 Introduction 86 4.2 Results and discussion 87 4.2.1 Effect of temperature 88 4.2.2 Effect of initiator concentration 89 4.2.3 Effect of residence time 92 4.2.4 Mechanistic implications of termination 94 4.2.5 Substituent effects in the radical polymerization of P=C bonds 95 4.2.6 Polymer properties 99 4.3 Summary 102 4.4 Experimental section 103 4.4.1 General procedures for radical polymerization of 4.la-f 104 4.4.1.1 Characterization of polymer 4.2a 104 4.4.1.2 Characterization of polymer 4.2b 105 4.4.1.3 Characterization of polymer 4.2c 105 4.4.1.4 Characterization of polymer 4.2d 105 4.4.1.5 Characterization of polymer 4.2e 106 4.4.1.6 Characterization of polymer 4.2f 106 4.4.2 Preparation of poly(methylenephosphine oxide) 4.3a ... 106 4.4.3 Preparation of poly(methylenephosphine sulfide) 4.4a .. 107 vii 4.4.4 Solution polymerization of 4.la 107 4.4.5 Polymerization of 4.la using benzoyl peroxide 107 4.4.6 Polymerization of 4.la using VAZO 88 (0.1 mol%) 108 4.5 References 108 Chapter Five Radical Copolymerization of Phosphaalkenes with Styrene 110 5.1 Introduction 110 5.2 Results and discussion 111 5.2.1 Copolymerization of phosphaalkene 5.1 and styrene 111 5.2.2 Reactivity ratios of P-mesityl phosphaalkene and styrene 117 5.2.3 Copolymerization of 4-methoxy-substituted phosphaalkene with styrene 122 5.3 Summary 123 5.4 Experimental section 124 5.4.1 General procedures for synthesis of copolymers (5.3) 125 5.5 References 126 Chapter Six A Convenient Synthesis of New Isolable Phosphaalkenes Using the Base-Induced Hydrogen Rearrangement of Secondary Vinyiphosphines 129 6.1 Introduction 129 6.2 Results and discussion 132 6.3 Summary 139 6.4 Experimental section 140 6.4.1 Preparation of2,6-(CF3)CHPC1 (6.1: R1 = H) 140 viii 6.4.2 Preparation of2,4,6-(CF3)C6HPC1(6.1: R1 CF3) 141 6.4.3 Preparation of2,6-(CFP(H)C(CH)= H(6.2a) 142 6.4.4 Preparation of2,6-(CF3CP(H)C(Ph)=CH (6.2b) 143 6.4.5 Preparation of2,4,6-(CFP(H)C H)= H(6.2c) 143 6.4.6 Preparation of2,6-(CF3CP=C H (6.3a) 144 6.4.7 Preparation of(E,Z)-2,6-(CFP=C(Ph)(CH (6.3b) 145 6.4.8 Preparation of2,4,6-(CF3)6HP=C(CH (6.3c) 145 6.4.9 Preparation of MesP(H)C(CH)CH(6.4) 146 6.4.10 Preparation of MesPC(CH3)2(6.5) 147 6.5 References 147 Chapter Seven Overall Conclusions and Future Work 150 7.1 Summary of thesis work 150 7.2 Future work 154 7.2.1 Studies of P-fluoroaryl phosphaalkenes 154 7.2.2 Free-radical (co)polymerization of phosphaalkenes 154 7.2.3 Attempts to polymerize other inorganic multiple-bond systems using addition polymerization 154 7.3 Closing remarks 155 7.4 References 155 ix List of tables Table 1.1. Bond energy of several multiple-bond systems 10 Table 1.2. Stable multiple bonds reported for low-coordinate heavier group 15 compounds 11 Table 1.3. Standard enthalpies, entropies, and free energies for the polymerization of various vinyl monomers at 25 °C 20 Table 1.4. Possible enthalpy estimation (Afl,°) for polymerizations of H2C=CH and HP=CH2 20 Table 3.1. X-ray crystallographic data for 3.lb, E-3.le, E-3.lf, and 3.3a+THF 61 Table 3.2. Important metrical parameters for phosphaalkenes bearing P-Mes and C-Ar substituents 62 Table 3.3. Synthesis and the 31P NMR chemical shifts of phosphaalkenes 3.1 and 3.2, RP=CR’R” 69 Table 4.1. Data for the radical polymerization of 4.la 90 Table 4.2. Data for the radical polymerization of substituted phosphaalkenes 4.la—f 97 Table 5.1. Initial data for copolymerization of 5.1 with styrene at 140 °C for 14 h 112 Table 5.2. Data for copolymerization of 5.1 and styrene with VAZO 88 (1 mol%) atlOO°Cfor24h 113 Table 5.3. Fineman—Ross parameters to access the reactivity ratios calculated 118 Table 5.4. Kelen—TUdOs parameters to access the reactivity ratios calculated 120 Table 5.5. Monomer reactivity ratios of phosphaalkene 5.1 (rpA) and styrene (rsT) by Fineman—Ross and Kelen—TUdOs methods for copolymer 5.3 121 Table 5.6. Copolymerization data for the 4-methoxy-phosphaalkene/styrene system 123 x List of figures Figure 1.1. Figure 1.2. Figure 1.3. Figure 1.4. Figure 2.1. Figure 2.2. Figure 3.1. Figure 3.2. Figure 3.3. Figure 3.4. Figure 3.5. Figure 4.1. Figure 4.2. Figure 4.3. North American production of some major plastics in 2005 1 Canadian exports of plastics from 1996 — 2005 2 HOMO and HOMO- 1 energy of HP=CH2 16 Energy profile of the polymerization of P=C bonds 26 31P NMR spectra (CDCI3)of: (a) crude polymerization mixture from the thermolysis of crude 2.1; (b) poly(methylenephosphine) 2.2 after purification 41 ‘3C{’H} NMR spectra (CDC13)of: (a) Mes(Me)PCPh2H; (b) poly(methylenephosphine) 2.2 42 Molecular structure of MesP=C(4-FC6H42(3.lb) 63 Molecular structure of MesPC(Ph)(4-MeOC)(E-3. 1 e) 64 Molecular structure of MesP=C(Ph)(2-py) (E-3.lf) 65 The reaction of AdP(Li)SiMe3and O=CPh2in THF 67 Molecular structure of (AdPCPh2)(3.3a) 68 Plots showing the effect of temperature (x-axis) on M and isolated yield of polymer 4.2a in the polymerization of 4.la (1%VAZO 88, 1 d) 88 Plots showing the effect of initiator loading (x-axis) on M and isolated yield of polymer 4.2a in the polymerization of 4.la (100 °C, 1 d) 91 Plots showing the effect of residence time (x-axis) versus M11 and isolated yield for polymer 4.2a in the polymerization of 4.la (1%VAZO 88, 100 °C) 93 xi Figure 4.4. TGA traces of poly(methylenephosphine) 4.2a, poly(methylenephosphine oxide) 4.3a, and poly(methylenephosphine sulfide) 4.4a 100 Figure 4.5. TGA traces of polymer oxides 4.3a, difluoro 4.3b, and methoxy 4.3e .. 101 Figure 4.6. TGA traces of poly(methylenephosphine) 4.2a at a heating rate of 10 °C/min under a nitrogen and an oxygen atmosphere 101 Figure 4.7. Wide angle X-ray scattering (WAXS) pattern for polymer oxide 4.3a .. 102 Figure 5.1. ‘H NMR spectrum (CD2C1)of a purified copolymer 5.3b’ 114 Figure 5.2. 31P NMR spectra (THF) of copolymers 5.3a—h [-(MesP—CPh2)-(CH- CHPh)-] with different incorporation of phosphaalkene 5.1 116 Figure 5.3. Determination of the reactivity ratios of phosphaalkene 5.1 (rpA) and styrene (rsT) by the Fineman—Ross method (Equation 5.2) for copolymer 5.3 ... 118 Figure 5.4. Determination of the reactivity ratios of phosphaalkene 5.1 (rpA) and styrene (rsT) by the Fineman—Ross method (Equation 5.3) for copolymer 5.3 ... 119 Figure 5.5. Determination of the reactivity ratios of phosphaalkene 5.1 (rpA) and styrene (rsT) by the Kelen—TüdOs method (Equation 5.6) for copolymer 5.3 .... 120 Figure 6.1. ‘H NMR spectrum (CDC13)of distilled 6.2a 133 Figure 6.2. 13C{1H} NMR spectrum (CDC13)of distilled 6.2a 133 Figure 6.3. 1H NMR spectrum (C6D)of distilled phosphaalkene 6.3a 134 Figure 6.4. ‘3C{’H} NMR spectrum (CDC13)of distilled phosphaalkene 6.3a 135 Figure 6.5. 31P NMR spectrum (CDC13)of distilled phosphaalkene 6.3a 135 Figure 6.6. 31P NMR spectrum (CDC13)of distilled phosphaalkene 6.3b 137 Figure 6.7. “P NMR spectrum (CDC13)of distilled phosphaalkene 6.5 139 xii List of abbreviations abs. = absolute Ad = Adamantyl Anal. = Analysis Ar = Aryl ArF = 2,4,6-tris(trifluoromethyl)phenyl;C6H2-2,4,6-(CF3) ArF’ = 2,6-bis(trifluoromethyl)phenyl;C6H3-2,6-(CF) A Wp = Atomic mass of phosphorus Bbt 2,6-bis[bis(trimethylsilyl)methyl]-4- [tris(trimethylsilyl)methyl]phenyl =C6H2-2,6- {CH(SiMe3)]-4-C bp = boiling point Bu = n-Butyl ‘Bu = tert-Butyl °C = degree Celsius Calcd = Calculated cat. catalytic cony, conversion d = day = Free energy change for polymerization = Enthalpy change for polymerization = Entropy change for polymerization DABCO = 1 ,4-diazabicyclo[2.2.2]octane DBU = 1, 8-diazabicyclo[5 .4. Ojundec-7-ene xl” DMAP = 4-dimethylaminopyridine DP = Degree of Polymerization dn/dc = refractive index increment equiv = equivalent(s) E or E’ = Inorganic atom E = Energy El = Electron Impact Et = Ethyl f= Initiator efficiency FWpA = Formula weight (or molar mass) of phosphaalkene FWST = Formula weight (or molar mass) of styrene GPC = Gel Permeation Chromatography h = hour HOMO = Highest Occupied Molecular Orbital HOTf = Trifluoromethanesulfonic acid [I] = Initiator Concentration k = Rate of initiation = Rate of propagation or polymerization LUMO = Lowest Unoccupied Molecular Orbital [M] = Monomer Concentration Me = Methyl MeO = Methoxy Mes = Mesityl = 2,4,6-trimethylphenyl;C6H2-2,4,6-Me3 xiv Mes* = Supermesityl = 2,4,6-tri-tert-butyiphenyl;C6H2-2,4,6-tBu3 mm = minute = Number average molecular weight Weight average molecular weight mol mole MpA = Mole percentage of phosphaalkene in the feed composition MST = Mole percentage of styrene in the feed composition mpA = Mole percentage of phosphaalkene in the copolymer composition msT Mole percentage of styrene in the copolymer composition MS = Mass Spectrometry v = Kinetic chain length NMR = Nuclear Magnetic Resonance PDI = Polydispersity Index PE = Polyethylene %PEA = Weight percentage of phosphorus obtained by elemental analysis Ph = Phenyl PP Polypropylene 1Pr = isopropyl PS = Polystyrene PVC = Poly(vinyl chloride) 2-py 2-pyridyl ref. = reference rel. = relative xv = Rate of initiation = Rate of propagation or polymerization rpA = Monomer reactivity ratio of phosphaalkene rsT = Monomer reactivity ratio of styrene rt = Room temperature R, R’, or R” = Side group Rgw = Radius of gyration obtained from the weight average molecular weight Rhw = Hydrodynamic radius obtained from the weight average molecular weight s = second t112 = Half life T or Temp. = Temperature Tonset = Onset temperature of weight loss Tbt = 2,4,6-tris[bis(trimethylsilyl)methyllphenyl —C6H2-2,4,6-[CH(SiMe3)] TMEDA = N N N’-tetramethylethylenediamine TGA = Thermogravimetric Analysis THF = Tetrahydrofuran Trip = 2,4,6-isopropylphenyl;C6H2-2,4,6-1Pr3 VAZO 88 = 1,1 ‘-Azobis(cyclohexanecarbonitrile) WAXS Wide-Angle X-ray Scattering wt% = Weight percentage xvi Acknowledgements Professor Derek P. Gates (Supervisor) For his advice, encouragement, patience, and support Dr. Chi.-Wing Tsang (Postdoctoral Fellow) For the collaboration, advice, friendship, and fun Dr. Vince Wright, Bronwyn Gillon, Kevin Noonan, Josh Bates, Cindy Chun, and Paul Siu (Graduate Colleagues) For the co times proof-reading and friendship Baharnaz Baharloo, Jonathan Chong, and David Riendi (Undergraduates) For the collaboration and the preliminary work on different projects Staff in Department of Chemistry (UBC) For their assistance NSERC, UBC, and Department of Chemistry (UBC) For financial support Parents For their love, patience, cares, and support Eugenia, Lorraine, Venus, Carey, Grace, Stephen, Miranda, Karen, Luke Fellowship, and KEC For their prayer support, rides, laughter and tears, and karaoke Wayne Kwok For his love and being there for me Jesus Christ For everything xvii Statement of co-authorship The material reported in Chapter Two was published in 2003. Chi-Wing Tsang, Mandy Yam, and Derek P. Gates, “The Addition Polymerization of a PC bond: A Route to New Phosphine Polymers,” J Am. Chem. Soc. 2003, 125, 1480—148 1. A version of Chapter Three was published in 2006. Mandy Yam, Jonathan H. Chong, Chi-Wing Tsang, Brian 0. Patrick, Anita E. Lam, and Derek P. Gates, “Scope and Limitations of the Base-Catalyzed Phospha-Peterson P=C Bond-Forming Reaction,” Inorg. Chem. 2006, 45, 5225—5234. A portion of material reported in Chapter Five was published in 2004. Chi-Wing Tsang, Baharnaz Baharloo, David Riendi, Mandy Yam, and Derek P. Gates, “Radical Copolymerization of a Phosphaalkene with Styrene: New Phosphine-Containing Macromolecules and Their Use in Polymer-Supported Catalysis,” Angew. Chem. mt. Ed. 2004, 43, 5 682—5685. A version of Chapter Six was published. Mandy Yam, Chi-Wing Tsang, and Derek P. Gates, “A Convenient Synthesis of New Isolable Phosphaalkenes Using the Base-Induced Rearrangement of Secondary Vinylphosphines,” Inorg. Chem. 2004, 43, 37 19—3723. The rest of the results (i.e. Chapter Four and Chapter Five) presented herein will be submitted for publication in due course. A detailed contribution by other coauthors in each publication is included in Section 1.6 (Page 28). xviii 1Chapter One Introduction: Polymers: A New Challenge in Inorganic Chemistry 1.1 Addition polymerization Beginning in the 1950s, society entered the age of plastics.’ Marking this era was the enormous growth in consumption and the widespread adoption of plastics in our daily lives. As an indication of their importance, the worldwide trade value of plastics was measured at six hundred billion US dollars in 2005,2 and the North American production of plastics exceeded thirty million metric tons (Figure 1 .1). In particular, Canadian exports of plastics have increased 160% to eleven billion dollars over the last ten years (Figure 1 .2).2 These huge figures indicate the tremendous importance of plastics to the world today. All commonly used plastics are synthetic materials which are derived from a variety of organic polymers. 20.0 r ‘;‘i r CK31 15.0 +C-C-I- +C-C II I II I[HHJ [HH 10.0 PE PP EH Phi EH cii0 II II III 5.0 -l-C-C—+ -I-C-C Hii C n n 0.0 — — PS PVC Figure 1.1. North American production of some major plastics in 2005. PE: Polyethylene; PP: Polypropylene; PS: Polystyrene; and PVC: Poly(vinyl chloride) and its copolymers. References start on page 29 16.3 8.1 PE PP PS vc Chapter One 2 14.0 12.0 i 4.0 2.0 0.0 1996 1998 2000 2002 2004 Figure 1.2. Canadian exports of plastics from 1996 — 2005.2 The first description of the chain structure of a polymer, or a macromolecule, appeared in Staudinger’s famous, but controversial, publication in 192O. He proposed that polymers are long-chain macromolecules composed of repeating units of a small molecule. At that time, it was difficult to believe that molecules could attain such high molecular weights.5’6The postulate of a giant molecule was indeed startling to the scientific community. A few years later, the existence of natural macromolecules was confirmed by Meyer and Mark who analyzed the X-ray crystal structures of cellulose and natural rubber, which are naturally occurring macromolecules.7’8In addition, Carothers and Hill isolated the first man-made macromolecules, polyesters as well as nylons, both of which are industrially important polymeric materials that we use today.91’The success in the preparation of these artificial fibers also lent support to Staudinger’ s hypothesis of macromolecules. In 1953, Staudinger was awarded the Nobel prize for his significant “discoveries in the field of macromolecular chemistry”.’2Currently, polymers are involved in virtually every aspect of our daily lives, including clothing, cosmetics, glasses, pharmaceuticals, References start on page 29 Chapter One 3 food and drink containers, medical devices, computers, and automobile parts. The vast majority of these products are comprised of polymers synthesized by addition polymerization. Addition polymerization involves the enchainment of unsaturated monomers via a chain- growth process, where monomeric molecules, such as vinyl compounds (H2C=CHR), are added to the reactive end of the propagating chain (Scheme 1.1). This chain-growth polymerization affords polymers with high molecular weight at an early stage of the reaction process.’3 Furthermore, lengthening the reaction time and increasing monomer conversion increases only the number of polymer molecules. This is a typical observation in addition polymerization and it allows for the ready and direct access of high-molecular weight polymers in good yields. High molecular weight is a crucial factor for a macromolecule to show some useful material properties. In addition, various interesting properties can be introduced to a polymer by attaching different side groups (R) to the C=C bond in the monomer. Subtle changes to the side group (R) of the vinyl monomers lead to the most commercially important commodity polymers such as polyethylene (PE, R = H), polypropylene (PP, R = Me), polystyrene (PS, R = Ph), and poly(vinyl chloride) (PVC, R = Cl). CH2=C R CH2=C R CH2=C R HH HHHH 1 terminationX±C_CH_C_C* - - -- - +-C-C II II I I II I [HHJ HH [HH m n Scheme 1.1. Remarkably, the discovery of addition polymers was serendipitous.5’14, For example, Simon obtained an oil-like material from the distillation of storax resin in 1839.16 Interestingly, this material, which he believed was monomeric styrene oxide, was in fact polystyrene (PS). In References start on page 29 Chapter One 4 an analogous case, poly(vinyl chloride) (PVC) was obtained from the unexpected photochemical polymerization of vinyl chloride. In 1872, Baumann observed the formation of PVC upon exposure of a vinyl chloride liquid to sunlight.’7Polyethylene (PE) was accidentally obtained by Gibson and Fawcett from Imperial Chemical Industry Co. in 1933.’ 18, 19 Their attempts to prepare ketones from ethylene and benzaldehyde eventually yielded a waxy solid, which was later determined to be PE. This unintentional polymerization of ethylene represents the first industrial synthesis of PE, a polymer that has revolutionalized our society. The advancement of addition polymerization chemistry towards practical applications stemmed from the development of well-defined methods to initiate polymerization. In the early industrial syntheses of PE the polymerization reactions required exceedingly high pressures (1000 —2000 atm) and temperatures (ca. 200 °C). The major breakthrough was the discovery of organometallic catalysts to polymerize ethylene by the Nobel Prize winners Ziegler and Natta.2° Ziegler—Natta catalysts permit a substantially lower ethylene pressure (even atmospheric pressure) for polymerization. In addition to improving reaction conditions, a very significant property of Ziegler—Natta catalysts is their ability to induce high stereoregularity in polyolefins (i.e. tacticity in polypropylene PP). Since the microstructure of a polymer determines its bulk properties and is highly dependant on the initiator, much effort was made to explore new polymerization catalysts. During Swarc’s investigation of anionic initiators for styrene polymerization, he discovered that the chain-end carbanion can propagate with fresh monomers without termination.21This so-called “living” anionic polymerization led to the first block copolymers and offered a route to unprecedented custom designed materials for specific purposes. The developments of initiator technology have led to the enormous industrial activity in polymer synthesis that we see today. The growth in developing useful polymerization References start on page 29 Chapter One 5 initiators continues and the extension of the “living” reaction to radical polymerization was one of the major successes in the last decade. The three most important “living” radical polymerization methods are atom transfer radical polymerization (ATRP),22’3nitroxide mediated polymerization (NMP),24’5 and reversible addition-fragmentation chain-transfer polymerization (RAFT).26’7 To date, addition polymerization has been almost exclusively limited to C=C bonds in organic chemistry. Apart from CC bonds, only a few other organic multiple-bond systems have been polymerized using addition polymerization. These organic polymers include 1.1-1.6 which are obtained, respectively, from the addition polymerization of: (i) CO bonds in aldehydes (1.1),2830 (ii) CN bonds in isocyanates (1.2)’ and azastyrene (1.3),’ (iii) CS bonds in thiocarbonyl compounds (1.4),3638 (iv) CC bonds in acetylenes (1.5),’ 40 and in rare cases, (v) CN bonds in nitriles (1.6).’ Many of the aforementioned systems would be classified as organic polymers.45 Inorganic polymers are macromolecules in which the main chain includes atoms other than carbon, oxygen, and nitrogen. These macromolecules are of considerable interest because of the fascinating properties that inorganic moieties may impart to the polymer. Addition polymerization has often been dismissed as a possible synthetic route to inorganic polymers due to the lack of suitable precursors. In the next section, examples of inorganic polymers are given to illustrate the unusual properties and useful applications of these materials. References start on page 29 Chapter One 6 1.1.1 Inorganic polymers The synthesis of inorganic polymers is an area of rapid growth because these attractive materials offer wide-ranging physical properties, chemical reactivities, and in many instances higher thermo-oxidative stability that are not attainable using organic polymers.46For example, polysiloxanes (1.7) offer high flexibility and substantial thermal stability due to the special properties of the Si—O backbone. As a consequence of the long Si—O bond (1.64 A vs 1.54 A for a C—C bond), the large bond angle at oxygen (LSi—O—Si, ca. 143° vs LC—C—C, Ca. 109°), and the two-coordinate oxygen atoms, there is a large increase in the free volume in 1.7 as compared to polyolefins. Therefore, 1.7 has found numerous commercial applications in biomedical materials, sealants, and silicone oils. Another example of common inorganic polymers is polyphosphazenes (1.8), whose backbone consists of alternating P—N and P=N bonds. Due to the very stable phosphorus—nitrogen bonds in the backbone, polymers such as 1.8 possess extremely high temperature resistance and are excellent fire retardants. Starting with poly(dichlorophosphazene) (1.8, R = R’ = Cl), chemical modification of the polymer can be achieved simply by replacing the chloro groups with other organic functionalities via nucleophilic substitution. Consequently, a variety of macromolecules with desirable properties are accessible for different applications. In one case, the attachment of glucosyl and glyceryl side groups imparts water solubility as well as hydrolytic instability to the polymer for potential biomedical applications in drug delivery.47’8 r 1 fSi-O-j- +FN L’ in LRT 1.7 1.8 The development of viable synthetic routes to inorganic polymers remains the major challenge in this field. To date, polycondensation and ring-opening methods are the two major References start on page 29 Chapter One 7 processes used, probably due to the ready accessibility and the straightforward synthesis of polymer precursors. However, the addition method is limited to organic monomers and is virtually unexplored for inorganic multiple-bond systems. We, therefore, hypothesized that addition polymerization could be an attractive approach to inorganic macromolecules. 1.1.2 Research objectives Owing to the unexplored potential of addition polymerization as a synthetic route to inorganic polymers and the general interest in these fascinating materials, the principle objective of my Ph.D. project was to extend this industrially important methodology from C=C systems to inorganic E=E’ bonds (Scheme 1.2). This research endeavour involved the synthesis and characterization of suitable inorganic monomers containing multiple bonds and the evaluation of possible methods for polymer initiation. In particular, we were interested in the addition polymerization of PC bonds to prepare new phosphorus-containing polymers. In the following sections, I will outline the background material which is necessary to understand how we arrived at PC systems as the starting point to developing addition polymerization for inorganic multiple bonds. This includes: • Possible inorganic multiple-bond systems for addition polymerization (Section 1.2) • Reasons for P=C bonds to be selected for polymerization studies (Section 1.3) • Feasibility of the addition polymerization of P=C bonds (Section 1.4) J-C-C-1 EE) fl / “ LI I],., / \ [I I],, Scheme 1.2. References start on page 29 Chapter One 8 1.2 Possible inorganic multiple-bond systems for addition polymerization In order to prepare inorganic polymers by addition polymerization, we sought stable unsaturated compounds with a multiple bond involving at least one inorganic element. Possible candidates for addition polymerization studies comprised all isolable compounds containing an inorganic multiple bond. Over the past forty years, there has been a great deal of research into the synthesis of unsaturated inorganic compounds.4952 Recent exciting advances include the development of the first stable triple bond between two silicon centers, SiSi system (1.9), by Sekiguchi and coworkers.53The synthesis of disilyne 1.9 represents a milestone in main-group chemistry because it completes the series of homonuclear triple bonds of group 14 elements. Another remarkable achievement was the preparation of the dichromium compound 1.10 by Power and coworkers in 2OO5. This dichromium compound is the first example of a stable quintuple-bond system. These fascinating unsaturated molecules represent the most recent developments in the field of inorganic multiple bonds; however, they are unlikely candidates for polymerization studies at this time. The purpose of this section is not to provide a comprehensive overview for every multiple-bond system containing heavier elements, but rather, it is to survey the field to identify potential monomers for polymerization studies. Me3S1 Me3Si(M 3 7SIMe3 Pr Me3SiS.M Me3Si SiMe3 1.9 1.10 References start on page 29 Chapter One 9 1.2.1 Challenges to prepare stable unsaturated inorganic compounds The synthesis of an isolable monomer containing a multiple bond between heavier elements (n>2) is very challenging and early attempts to isolate these compounds were rarely successful. For example, in the base-catalyzed condensation of PhPH2and PhPC12,Köhler and Michaelis believed that they had prepared “phosphobenzene”,55the heavier analogue of the well- known azobenzene, PhN=NPh. However, the products were later determined to contain cyclic oligomers of (PhP)5 and (PhP)6molecules (Scheme 1 •3)•56 Cyclic oligomers were also obtained with compounds initially proposed to possess homonuclear double bonds of arsenic (As=As)58 and silicon (Si=Si),59 and heteronuclear silicon-carbon double bond (Si=C)6065 Oligomerization is a very important observation in the context of this thesis because it suggests that (i) unstable/transient E=E’ compounds readily oligomerize to low-molecular weight molecules; and (ii) isolable E=E’ monomers are necessary for possible high polymer formation. Ph Ph PhPH2 Ph P Phbase “PhP=PPh” _____ Ph.p_P\ Ph P P+ “phosphobenzene’ PhPCI2 Ph’ Ph P Ph Ph Ph Scheme 1.3. What causes these inorganic unsaturated compounds to be unstable? The classical “double-bond” rule was formulated to answer this question. This often cited rule stated that molecules containing (p-p)3t bonds between two elements with a principal quantum number greater than two will not be stable.66 A common example used to illustrate this idea is Si02 for which the polymeric, rather than the molecular, structure is formed. The absence of (p-p)t bonds between heavier elements (n>2) results from the thermodynamic preference for two a bonds References start on page 29 Chapter One 10 over a (a+3t) bond. For heavier elements, the more diffuse p-orbitals contribute to less efficient overlap between two parallel p-orbitals on adjacent atoms. Therefore, the 3t bond is weaker between heavier elements (n>2) as compared to lighter elements (n=2). This is illustrated in Table 1.1. Table 1.1 shows the a- and the at-bond energy of several multiple-bond systems of solely second row elements (left side) versus those involving the third row (right side) of the periodic table. Table 1.1. Bond energy of several multiple-bond systems.a System Bond energy System Bond energy (n=2) (n3) E0+’ E Ef 0=0 493 142 351 S=S 425 226 199 NN 942 167 388 PP 481 201 140 C=0 799 358 441 Si=0 626e 452 209 a Units are kJmoF’ and data are from Reference 67 unless otherwise indicated. b Bond dissociation energy of the multiple-bond systems. Bond dissociation energy of the single-bond systems. d Calculations for E= (E0+ — E) for double bonds and E= (Ec,+,, — Ec,) ÷ 2 for triple bonds. e Calculation for E÷,(Si0) = E,(Si0) +E0(Si—0). 1Data from Reference 68. As shown in Table 1.1, the multiple bonds involving heavier third-period elements possess a r-bond energy lower than the a-bond energy, in contrast to those between two second- row atoms. Consequently, elements in the third row or below in the periodic table have a thermodynamic preference for single bonds over multiple bonds. For example, in the most stable and simplest form, oxygen exists as diatomic species with one a and one 3t bond. However, the thermodynamically stable allotrope of the heavier congener sulfur is the homocyclic ring S8 (1.11), which consists solely of S—S single bonds. Similarly, the simplest form of nitrogen is N2 with one a and two t bonds, whereas the analogous elemental phosphorus forms P4 (1.12), a tetrahedron where each P atom forms three a bonds. The next example showing the favored formation of single bonds in molecules containing heavier elements is the case of CO2 versus References start on page 29 Chapter One 11 Si02. The gaseous molecule CO2consists of two CO double bonds, whereas the analogous silicon molecule does not contain any Si0 bonds. Rather, Si02 is linked by a series of Si—O single bonds to form a polymeric structure (1.13). These examples provide experimental support for the double-bond rule (i.e. o bonds will be favored in molecules containing heavier elements where n>2). Hence, the preparation of stable unsaturated monomeric molecules involving heavier elements poses a real challenge to my thesis objectives. S R R R/\ wI-Q-j-O-j-OSS P___-.P i F R 1.11 1.12 1.13 1.2.2 Examples of potential monomers Our goal was to mimic the addition polymerization of CC bonds with main-group elements (e.g. EE’ or EE’). In particular, we were interested in the multiple bond containing low-coordinate heavier group 15 elements because a vast number of isolable unsaturated molecules of pnictogens have already been synthesized and are readily available (Table 1.2). To introduce the diverse range of possible monomers for polymerization studies, this section outlines the fascinating developments of compounds containing multiple bonds involving heavier group 15 elements. Table 1.2. Stable multiple bonds reported for low-coordinate heavier group 15 compounds. C Si Ge Sn P As Sb Bi P -P=C< -PSi< -PGe< -PSn< -PP- -P=As- -PSb- -PBi Pc As -As=C< -As=Si< -As=As AsC Sb -Sb=C< -Sb=Sb- -Sb=Bi Bi -Bi=Bi References start on page 29 Chapter One 12 The synthesis of low-coordinate phosphorus multiple-bond systems is an active field of research. The first observation of a molecule with a genuine phosphorus-carbon multiple bond can be traced back to 1961, when Gier detected phosphaacetylene (1.14) in the gas phase.69 It is relevant to this thesis that, reportedly, this reactive species slowly polymerized at —130 °C to a black pyrophoric solid that analyzed as (HCP). To avoid such uncontrolled polymerization, unsaturated phosphorus compounds were first stabilized by thermodynamic methods. For example, thermodynamic stability was conferred to phosphamethine cyanine cations 1.15 through the conjugation of the P=C bond with the N=C bond in a positively charged system.7° Importantly, this compound, reported by Dimroth and Hoffmann in 1964, represents the first room-temperature stable low-coordinate phosphorus-carbon doubly bonded system. The first neutral P=C molecule was 2,4,6-triphenylphosphinine (1.16) isolated by Märkl in 1966.’ Five years later, Ashe reported the unsubstituted phosphinine (1.17), which represents the heavier analogue of pyridine.72The P=C bond in both phosphinine 1.16 and 1.17 is thermodynamically stabilized by 3r-delocalization in an aromatic ring. These unsaturated compounds are unlikely to undergo polymerization because of the thermodynamic stability of the PC bonds in 1.15—1.17 through it-delocalization. Moreover, due to t-delocalization of PC moieties, the PC bond may not selectively react with polymerization initiators. Therefore, it is important to adjust the thermodynamic stabilization of P=C bonds so that they are isolable but controllably polymerizable. Thermodynamic as well as kinetic stabilization will be discussed in more detail in Section 1.4. PC-H PhXPh Q BE4 1.14 1.15 1.16 1.17 References start on page 29 Chapter One 13 Conceivably, using steric protection instead of electronic stabilization would lead to more appropriate monomers. Steric hindrance is a means of kinetic stabilization and can be employed to stabilize inorganic multiple bonds and as such has led to the rapid growth of multiple-bond chemistry. A landmark in the area of unsaturated phosphorus chemistry was the preparation of phosphaalkene 1.18 (E P) by Becker in 1 976, which represents the first acyclic molecule with a PC bond stabilized using bulky siloxy and tert-butyl(tBu) substituents. Using identical substituents, Becker synthesized the first arsaalkene 1.18 (E = As), which is the arsenic analogue of phosphaalkene 1.18 (E = P).74 Similarly, the first examples of stable phosphorus-carbon triply bonded (PC) systems were isolated using sterically encumbering substituents by Becker (1.19) and Appel (1.20) separately in 1981.75.76 BulkyC6H2-2,4,6-’Pr3(Trip) ligands were also used to stabilize the first As=Si bond in In addition, the 2,4,6-tri-tert-butylphenyl group (Mes* = C6H2-2,4,6-’Bu3)is an extensively employed substituent of large steric bulk. This bulky Mes* substituent has been utilized in stabilizing compounds containing the first P=P (1.22),78 P=Si (1.23, E = Si),79 P=Ge (1.23, E = Ge),8°PSn (1.24),81 P=As (1.25),82 As=As (1.26),83 and AsC (1.27)84 bonds. Taking advantage of kinetic stabilization, unsaturated phosphorus and arsenic molecules 1.18—1.27 were synthesized with polymerizable multiple bonds. ‘Pr — tBu — ,SiMe3 iPr P=C-C—SiMe3 As—Si ‘Pr Pt( OSiMe3 PCtBu ‘iMe3 Pr3Si’ — /E=P,As ‘Pr —, 1.18 1.19 1.20 1.21 ‘Pr Me3Si Mes )—SiMe3 Me3Si Me3Si ,Mes P=E’ PSn )—S1Me3 )—SIMe3 AsEC_Mes* Mes* Mes* Mes Mes*” )—SiMe3 PAs AsAs E Si, Ge Me3S1 Mes* Mes* 1.22 1.23 1.24 1.25 1.26 1.27 References start on page 29 Chapter One 14 A substantial increase in steric bulk is necessary to stabilize the very reactive multiple bonds involving the heaviest elements, antimony and bismuth. Tokitoh’s group developed the extremely bulkyC6H2-2,4,6-[CH(SiMe3)](Tbt) andC6H2-2,6-[CH(SiMe3)]-4-C (Bbt) substituents for isolation of compounds containing homonuclear Sb=Sb (1.28, E = Sb)85 and Bi=Bi (1.28, E = Bi)86 bonds, and heteronuclear P=Bi (1.29)87 and Sb=Bi (1.30)88 bonds.89 Significantly, the preparation of the stable distibene and dibismuthene (1.28) completes the series of homonuclear double bonds of group 15 elements. An analogous series of dipnictenes (1.31) were reported bearing the exceedingly congested terphenyl ligands,C6H3-2,6-Trip (Trip = C6H2-2,4,6-’Pr3)andC6H3-2,6-Mes groups.90’Last but not least, theC6H3-2,6-Trip ligand allowed the isolation of the first solution-stable phosphastibene 1.32.92 These kinetically stabilized unsaturated molecules also meet the criteria for polymerization studies. Me3Si SiMe3 Me3S1NifSiMe Me3Si’M 3 Tbt =C6H2-2,4,6-[CH(SiMe3)](R = H) Bbt = -2,6-[CH(SiMe4-CH(Si (R = SiMe3) /Tbt E= E Tbt” E = Sb, Bi 1.28 /Mes* / P=Bi 1.29 Bbt Bbt / Sb=Bi / Bbt 1.30 C6H3-2,6-Trip (R = ‘Pr) -2,6-Mes (R = Me) Ar Ar =C6H3-2,6-Mes E=E E=P,As,Sb,Bi A/ Ar =C6H3-2,6-Trip E = As, Sb, Bi 1.31 Ar / P =Sb /Mes* Ar =C6H3-2,6-Trip 1.32 References start on page 29 Chapter One 15 The use of gigantic protecting groups can certainly confer sufficient stability to the extremely reactive bonds involving antimony and bismuth. However, these unsaturated pnictogen molecules may be too bulky for subsequent molecular reactions, let alone polymerization. Another potential problem associated with the large dangling side groups is their possible reactivity with polymerization initiators. Typical initiators may react with the bulky substituents rather than the target multiple bond, thus preventing polymer formation. The optimal monomer should mimic the property of ethylene in that it should be stable enough to be isolated and purified, but not too stable that the E=E’ bond is inert towards initiators. To choose such an appropriate monomer, we hypothesized that the ability of an unsaturated inorganic molecule to polymerize through the E=E’ bond should relate to its chemical reactivity. In other words, this postulate suggested that any inorganic multiple-bond systems showing comparable reactivity to olefins should also be polymerizable. 1.3 The selection of P=C bonds for polymerization studies As mentioned in Section 1.1, the addition polymerization of C=C bonds is arguably one of the most important reactions in chemistry. In Section 1.2, the breadth of unsaturated pnictogen molecules as monomer choices was discussed. Of particular interest was the P=C bond because it is cited as a C=C parallel (Scheme 1 •4)•93 For this reason, we chose the P=C system as the starting point of our polymerization studies. In this section, the similarities between olefins and phosphaalkenes will be outlined to show how CC and P=C systems are analogous to each other in their bonding and chemical reactivity. \ / __ / C=C 4p PC / \ / \ Scheme 1.4. References start on page 29 Chapter One 16 1.3.1 Similarities between P=C and C=C bonds The (p-p)t bond in P=C and C=C systems strongly resemble each other in terms of their polarities and ionization potentials. The (2p-2p)2t component of ethylene is apolar. In methylenephosphine (HP=CH2),the (3p-2p)t bond is only slightly polarized and can be considered as apolar.95Ab initio calculations by Lacombe et al. revealed that the highest occupied molecular orbital (HOMO) of methylenephosphine is the 3tp_c orbital which has an ionization potential higher than the lone pair orbital (np_c) (Figure 1 •3)96 Moreover, the ionization potential of the 3tp_c orbital (—10.30 eV) is very close to that of the 3tc_c orbital in ethylene (—10.50 eV). This comparable polarity and ionization potential between the (P=C)3t and (C=C)rt bonds suggested that phosphaalkenes should show chemical reactivity analogous to olefins. In the next section, some of these chemical reactivities of the PC bond are described. HOMO Energy HY ‘ H P-C P=C. HOMO -10.30eV tC.C HccH H0 0H HOMO-1 -10.51 eV H.. Hflp “H -10.70 eV Figure 1.3. HOMO and HOMO-1 energy of HP=CH2.96 1.3.2 The P=CIC=C analogy in molecular chemistry Since the synthesis of the first phosphaalkene (1.18, E P) with a localized P=C bond in 1976, the reactivity of the P=C bond has been intensely studied.93’478The reactions of the P=C bond that mimic those of the more well-known C=C bond are shown in Scheme 1.5. Path A References start on page 29 Chapter One 17 and Path B represent the phosphorus analogues of hydrogenation and epoxidation reactions, respectively.99’100 It is noteworthy that in these two cases, complexation of the phosphaalkene to a metal is necessary to protect the reactive phosphorus lone pair. Analogous to alkenes, phosphaalkenes can undergo a 1,2-addition reaction (Path C).’°’’°3Addition reactions with protic reagents usually afford products with a C—H bond due to the slightly polarized P—C bond (Po+C&) Interestingly, the polarity of the P=C bond can be reversed (PCo+) by attaching electron-rich substituents to carbon, thus leading to the rare P—H product. These so-called inversely polarized phosphaalkenes are less common and have been reviewed by Weber.’°4 H H % I P-c / \ A H2[Rhi cat (= g— / [0] P-C-.... P=C - P-C / \ E / \ B / [M] H X X H P—C or P—C P=C / \ / \/ \ Scheme 1.5. ri2-Metal coordination is an important reaction of CC bonds and has been extended to P=C bonds (Path D))°5’1°The [2+4] cycloaddition of a phosphaalkene and a butadiene to a six membered phosphorus heterocycle represents the phosphorus variant of the Diels—Alder reaction (Path E).’°3’111117 In addition, phosphaalkenes are known to react with various (unsaturated) molecules to yield [2+11,118 [2+2],” [2+31,120 and [2+81 121 cycloaddition products. Last but not least, the phosphorus analogue of the Cope-rearrangement reaction has been extensively studied References start on page 29 Chapter One 18 (Scheme 1.6).122131 31P NMR spectroscopy revealed that the two-coordinate phosphorus atoms in Y were in equilibrium with the three-coordinate phosphorus in X at room temperature.’24’7 ____ PhPh PhPh x Y Scheme 1.6. Evidently, phosphaalkenes are able to undergo a diverse range of organic reactions. For this reason, phosphorus has even been called a “carbon-copy” and the field of phospha-organic chemistry has emerged.94 Given that the chemistry of the PC bond is similar to that of the CC system, we hypothesized that phosphaalkenes would be ideal monomers for polymerization studies (Scheme 1.7). \ / __ liii / __ F I n C=C ±C-C+ IEE) fl P=C ±P-C/ “ LI I], / \ LI I Scheme 1.7. 1.4 Feasibility of the addition polymerization of PC bonds In the last section, I described how the analogy between the P=C and C=C bonds in molecular chemistry led to the selection of phosphaalkenes as potential candidates for addition polymerization studies. This section will provide an estimation of the energies involved in the addition polymerization of PC bonds in order to demonstrate the thermodynamic feasibility of this reaction. The requirements for a suitable phosphaalkene monomer will also be outlined after discussing previous examples of several suspected polymerizations of phosphaalkenes. References start on page 29 Chapter One 19 1.4.1 Estimation of the enthalpy of the polymerization of PC bonds From a thermodynamic point of view, the feasibility of a polymerization relies on a negative free energy (AG°<O), which depends on the enthalpy (AI0°), entropy (AS°), and temperature (I) of a reaction (Equation 1.1). Addition polymerization reactions are usually = — mS,,° (1.1) highly exothermic (H°<O) because energy is released from the formation of a series of a bonds in the polymer for every multiple bond (a+3t) dissociated from the monomer. In general, the entropy is negative (iS°<O) because fewer molecules are present in the product compared to the reactant. This trend can be supported by the negative AS° values obtained for the polymerization of several vinyl compounds that are listed in Table 1.3.132 All of these vinyl monomers show a negative change in enthalpy, entropy, and free energy for polymerization at 25 °C. Despite this unfavorable entropy change for polymerization, the AI]° term often outweighs the TAS° term, especially at lower temperatures. Since the free energy (AG°) is dominated by the enthalpy change (AH°) in the polymerization, we can predict the feasibility of the polymerization of P=C bonds via enthalpy calculations (Table 1.4). To simplify this calculation using bond energy data, the enthalpy of the polymerization is estimated for the simplest P=C molecule, methylenephosphine (HP=CH2).For comparison, a similar estimation for ethylene is also included in Table 1.4. The slight difference between the calculated AR,° value from bond energy for ethylene (—79 kfmo[’ in Table 1.4) and the more accurate experimental value (—88.8 kJmor’ in Table 1 3)132 is caused by the inaccuracy of mean bond energy data. Importantly, the approximation of enthalpy change for methylenephosphine polymerization using mean bond energy gives a negative AI1° value (—71 kfmolj. This suggests that the addition polymerization of P=C bonds will be thermodynamically feasible. References start on page 29 Chapter One 20 Table 1.3. Standard enthalies, entropies, and free energies for the polymerization of various vinyl monomers at 25 c.13 —zfl° (kfmol’) —iXS° (Jdeg’mor’) —AG° (kJmo[’) Ethylene 88.8 100 58.6 Formaldehyde 31 80 7.1 Methyl methacrylate 55.3 120 21 a-Methylstyrene 35 104 4.2 Styrene 69.9 105 39 Tetrafluoroethylene 160 112 121 Table 1.4. Possible enthalpy estimation (AN1,°) for polymerizations ofH2CCH and HP=CH2. Polymerization ofH2C=CH Polymerization of HPCH2 H H FHH1 H [ H ‘ / II II I I c=c +c-c-I- n p=C —I-P-C / ‘ II I , ‘ II IH H [HHin H H LHHn Energy required to n x (a+3t) bonds = 661n kJ mol’ n x (a+3r) bonds = 483n kJ mol’break the bonds Energy released 2n x a bonds 2n x ci bonds from new bond = 2n x 370 = 740 n kJ moF’ = 2n x 277 = 554 n kJ mor’formation zH° (661n—740n)+n (483n—554n)+np = — 79 kfmol = — 71 kJmol a The bond energy used here is based on those estimated by Schleyer and Kost.’33 b The number of a bonds formed after polymerization is calculated for a cyclic structure. 1.4.2 Consequences of unstable phosphaalkenes The thermodynamic preference for PC bonds to polymerize can be further supported by some literature reports of unstable phosphaalkenes. Interestingly, the literature on the preparation of low-coordinate P=C molecules is full of the mention of “polymers”. The term “polymer” was often employed to describe the undesirable product yielded in attempts to isolate phosphaalkenes with substituents of insufficient steric bulk. None of these so-called polymers were fully characterized and it is likely that only low-molecular weight oligomers were formed. Several of these sterically under-protected phosphaalkenes will be outlined below. References start on page 29 Chapter One 21 After the first phosphaalkene (1.18, E = P) was isolated using bulky substituents,73the preparation of phosphaalkenes with decreased steric protection was attempted. Unfortunately, in many cases, these phosphaalkenes were unstable and could not be isolated. For example, although C-diphenyl phosphaalkenes with a mesityl(C6H2-2,4,6-Me3)or a 2,6-dimethylphenyl (C6H3-2,6-Me)substituent at phosphorus were isolable, attempts to decrease the steric bulk of the P-substituent to a 2-methyiphenyl or a phenyl group were unsuccessful (Scheme 1.8).b02 Bickelhaupt and his coworkers speculated that the preparation of these unstable phosphaalkenes yielded a “polymeric material” instead of the monomer. Since no other characterization was provided for the product, it is uncertain whether or not this material is in fact a polymer. R, ,Ph DBU Ph PC\ CH “Polymer”? CI H - HCI R Ph R = Ph, j=ç Scheme 1.8. In other cases, some phosphaalkenes are merely stable at low temperatures in bulk and become reactive at higher temperatures. One of these examples is the fluorinated phosphaalkene, CF3P=CF2.This phosphaalkene was prepared by pyrolysis of Me3SnP(CF3)2and, subsequently, trapped at —196 °C (Scheme 1.9).134 After warming the crude product to room temperature, the reactive phosphaalkene formed a self-addition product [i.e. (F3CPCF2)j,which the authors referred to as “polymer”. The analysis of the product mixture showed that this material consists mainly of low-molecular weight cycloadducts (dimers:trimers = 85:15). As a result, it is debatable whether this species should be a “polymer”. Nevertheless, it shows the propensity for the addition of phosphaalkenes. References start on page 29 Chapter One 22 300 - 340 °c, io torr -196 °C to ii (F3CPCF2) Me3SnP(CF)2 PC - Me3SnF F3C F “Polymer”? Scheme 1.9. Another example of a phosphaalkene stable only at low temperatures is the simplest phosphaalkene derivative HP=CH2prepared via dehydrochiorination ofH2P—CC1H (Scheme 1.l0).’ Although the phosphaalkene was condensed at —196 °C, it gradually decomposed to a low-solubility species. Again, the authors referred to this insoluble material as a “polymer”. This decomposition became instantaneous in low-melting point NMR solvents (CFC13 or Me20) at —120 °C. It is unclear whether this low-solubility species is really a high-molecular weight polymer because no characterization was reported. H Flash Vacuum Vacuum Gas1’ Thermolysis or Solid Reaction — T’ —196 °C; solid state “a polymer of P—C--H P—C H’ Cl io- torr H’ H —120 “C; cFcI3 or Me20 low solubility Scheme 1.10. A similar “polymerization” at low temperatures was mentioned for the transient phosphaalkene, HP=CHC1. Denis et al. speculated that the treatment ofC12H—PH with DABCO at —80 °C results in some “polymeric products” (Scheme 1.1 1).136 These products could result from intermolecular reactions between either of the unsaturated molecules, HPCHC1 and H-CP. Due to the low-temperature stability of H-CP (stable below —20 °C), the intermediate species was postulated to be the unstable phosphaalkene HPCHCl. Again, no convincing spectroscopic or molecular weight data were provided to confirm whether this was indeed a polymer. References start on page 29 Chapter One 23 DABCO, THE 8O °C CI2H—PH ‘Polymer”? H Cl Scheme 1.11. Quin and coworkers discovered that the thermolysis of some masked phosphaalkenes afforded unstable species which in the absence of trapping agents, rapidly transformed to phosphine-containing species (Scheme 1.12). 137 Broad resonances were observed in the 31 NMR spectra of each reaction mixture in attempts to prepare transient phosphaalkenes MeP=CH2 and PhP=CH2.In the case of MeP=CH2,31P NMR resonances in the range of—40 to —50 ppm suggested the presence of tertiary phosphines. The authors speculated that these products resulted from intermolecular head-to-head or head-to-tail reactions of the reactive phosphaalkenes. It is noteworthy that the molecular weights of these possible oligomerized products were not studied. Me4..CQQMe 40°C, 5 h — Products with — )l 30 °C Ph—c/i 70 °C in few mm MH broad 31P NMR PH COOMe resonances Scheme 1.12. The final example of so-called polymers obtained from unstable phosphaalkenes comes from the 31P NMR analysis of the product derived from in situ generated PhP=C(H)(’Bu). This phosphaalkene was prepared via hydroboration of PhP=C(’Bu)(OSiMe3)and subsequent elimination of siloxyborane (Scheme 1.13). 138 Due to its kinetic instability, PhP=C(H)(’Bu) was hypothesized to “polymerize” to a product mixture with 31P NMR signals at —10, —15, and —17 ppm. No assignments of these NMR signals nor molecular weight analyses were reported for the product. Hence, the formation of “polymer” is arguable, but is indeed interesting. References start on page 29 Chapter One 24 R2BH tBU -R2BOSiMe3 BU p=c Ph—l—9-OSiMe3 P=C “Polymer”? Ph” bSiMe3 R2B H Ph H Scheme 1.13. As a result of insufficient kinetic stability, the isolation of phosphaalkenes with substituents of low to moderate steric bulk, such as those mentioned in this section, and summarized in Scheme 1.14 was unsuccessful. In many cases, these reactions afforded an undesired product which was speculated to be a “polymer”. The hypothesis of “polymer” formation is debatable because no definitive evidence of the polymeric structure was given in any case. In particular, no molecular weight analysis was reported. Moreover, these so-called polymers are more likely to be self-oligomerized products and are likely of low molecular weight due to monomer impurity and the lack of a specific initiation step. To avoid these undesirable reactions which inhibit high polymer formation, we hypothesized that the ideal phosphaalkene monomer must be stabilized, yet maintain its polymerizability, similar to ethylene. Ph ,!BU p=cH3C / / ‘ — PhPh H H Ph p=c’ \ / P=c Ph” ‘H P1( Ph uncharacterized “Polymers”??? / \ Me H / F3C F P=c P=c I-I ‘Cl H H Scheme 1.14. References start on page 29 Chapter One 25 1.4.3 Stabilization of PC bonds for polymerization studies Stability can be conferred to the reactive P=C bond via thermodynamic and/or kinetic methods (Figure 1.4). In the first route (Method A), the ground-state energy of the P=C monomer is lowered with respect to the -(P—C)- polymer. This method represents a thermodynamic stabilization and may be accomplished through ir-electron delocalization of the P=C bond with neighboring functionalities. Examples of thermodynamically-stabilized P=C bonds are illustrated by phosphamethine cyanine cations 1.15, phosphinines 1.16 and 1.17. However, the functional moieties may alter the PC bond character and prevent chain propagation of the PC bond using polymerization initiators. From the perspective of polymerization, the preferred method for stabilizing monomers is kinetic stabilization (Method B) in which the transition-state energy (Ea) is increased with minimal effect on enthalpy (AH). This can be achieved by attaching bulky substituents to the PC bond. These bulky substituents must be carefully selected for the PC monomer, otherwise the reaction of the reactive bulky substituents with the initiator will become more favorable than polymerization. In addition, the steric bulk of the substituent must be tuned so the PC bond is not over-protected and inhibited from polymerization (i.e. Ea too large). In other words, the kinetic stability of these compounds should be tuned to allow the P=C monomer to be isolable and thermodynamically favor for polymerization. References start on page 29 Chapter One 26 Energy / TB ‘ Reaction Coordinate Figure 1.4. Energy profile of the polymerization of P=C bonds. Stabilization of PC bond via thermodynamic (A) and kinetic (B) methods. In summary, the requirements for the ideal phosphaalkene monomer for polymerization studies will possess the following characteristics: • Potential monomers must be isolable, preferably stabilized kinetically using sterically encumbering substituents, yet maintain enough thermodynamic instability with respect to polymerization reactions. In other words, suitable monomers must remain stable, i.e. without any decomposition or self-oligomerization, until a polymer initiator is introduced. • Polymer initiators should only activate the P=C moiety in the monomer to form polymers. No other undesirable products inhibiting polymerization, such as those resulting from intramolecular reactions with side groups, should be afforded. / fl P=C / \ H ‘i References start on page 29 Chapter One 27 Interestingly, these are some properties that polymerizable olefins possess. This suggests that a careful combination of a polymerizable P=C bond with unreactive protecting groups is important for monomer selection. In conclusion, the design of a suitable phosphaalkene monomer requires a delicate balance of kinetic and thermodynamic stability. 1.5 Outline of thesis As mentioned in Section 1 .1, addition polymerization is an industrially important reaction for organic polymer synthesis; however, it has often been dismissed as a route to inorganic macromolecules. To explore addition polymerization as a viable route to new inorganic polymers, we sought suitable inorganic multiple-bond systems for this study. As illustrated in Sections 1.2 and 1.4, the synthesis of these stable compounds is challenging because insufficient kinetic protection of the multiple bond can result in undesirable oligomerization. Suitable monomers thus require a careful choice of substituents to provide adequate kinetic stabilization for their isolation as well as a thermodynamic preference for polymer initiation. As a starting point, we selected the PC bond for polymerization studies because it exhibits many parallels to the C=C bond (Section 1.3), including some observations of ill-characterized materials that may be polymeric. Therefore, the purpose of this thesis is to extend the analogy of P=C/C=C bonds to addition polymerization, and to prepare and characterize well-defined new polymers that contain phosphorus in the backbone. Chapter Two highlights the first addition polymerization of a P=C bond. This achievement has opened up a new synthetic route to phosphorus-containing polymers. The next goal was to generalize this polymerization method to other P=C systems. Chapter Three describes the preparation of several P-mesityl C-aryl phosphaalkenes and the attempted synthesis References start on page 29 Chapter One 28 of some P-adamantyl phosphaalkenes using the base-catalyzed phospha-Peterson reaction. Chapter Four deals with the addition polymerization of stable P-mesityl phosphaalkenes and different effects on the radical polymerization of the P=C bond. In light of the success of the homopolymerization, we speculated that the radical copolymerization of P=C and G=C bonds should afford a hybrid inorganic-organic macromolecule. Chapter Five introduces this fascinating synthesis as another facile method for the incorporation of phosphorus atoms into the backbone of organic polymers. To broaden the range of phosphaalkene monomers, a novel route to P-fluoroaryl phosphaalkenes bearing methyl substituents at carbon was developed and is reported in Chapter Six. This significant advance represents the first quantitative formation of phosphaalkenes from the base-catalyzed 1,3-hydrogen rearrangement of secondary vinyiphosphines. The last chapter, Chapter Seven, concludes the findings of this thesis and proposes some possible future directions for this research. 1.6 Contributions by other researchers to this thesis work Some projects mentioned in this thesis were done in collaboration with other researchers and the coauthors involved in the related publications are listed in the Statement of co-authorship (Page xviii). In Chapter Two, the distillation of phosphaalkene to the first poly(methylenephosphine) was jointly investigated with Dr. Chi-Wing Tsang. The early studies conducted with Dr. Tsang included the radical and anionic polymerization and the functionalization of the phosphine polymer. I performed the thermogravimetric analysis of the resulting polymers and also the experiments involving the proof of the anionic polymerization mechanism. All radical polymerizations and the preparation of various P-mesityl phosphaalkenes described in this thesis were performed independently by me. The synthesis of P-adamantyl References start on page 29 Chapter One 29 phosphaalkenes reported in Chapter Three was initially carried out by the undergraduate student Jonathan Chong under my supervision. I obtained X-ray quality crystals of the dimer. In Chapter Five, the radical copolymerization of phosphaalkene with styrene was started by Dr. Tsang and two undergraduate students, David Riendl and Baharnaz Baharloo. The undergrads worked under joint supervision by myself and Dr. Tsang. All the copolymers for monomer reactivity ratio studies were prepared solely by me. In Chapter Six, Dr. Tsang conducted the initial preparation of ArF’P=CMe2[ArF’ =C6H3-2,6-(CF)]and the preliminary studies of ArFP=CMe2 [ArF =C6H2-2,4,6-(CF3)].I optimized the methods for the preparation of AIFP=CMe2and extended the synthetic method to prepare ArFP=CMePh. 1.7 References 1. Mark, H., Coming to an Age of Polymers in Science and Technology. In Histoiy of Polymer Science and Technology; Seymour, R. B., Ed.; Marcel Dekker: New York, 1982; pp 1—9. 2. United Nations Commodity Trade Statistics Database. Department of Economic and Social Affairs, Statistics Division. http://comtrade.un.org/db/default.aspx (accessed Jan 2007). 3. Chem. Eng. News 2006, 84 (Jul 10), 35—72. 4. Staudinger, H. Ber. 1920, 53, 1073. 5. Morawetz, H., Polymers: The Origins and Growth ofa Science; Wiley-Interscience: New York; 1985. 6. Furukawa, Y., Inventing Polymer Science: Staudinger, Carothers & the Emergence of Macromolecular Chemistry; University of Pennsylvania Press: Philadelphia, PA, 1998. 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References start on page 29 38 Chapter Two The Addition Polymerization of a P=C Bond: A Route to New Phosphine Polymers* 2.1 Introduction As mentioned in Chapter One, the synthesis of inorganic polymers has attracted considerable attention because of the interesting properties and potential applications these materials can offer.”2 In the last few years, several new inorganic polymers have been synthesized with their backbone consisting of various main-group elements.3’However, the development of new inorganic macromolecular materials has been hindered by the difficulty in finding suitable synthetic methods for their preparation. There has been widespread interest in the synthesis and study of stable compounds containing multiple bonds involving heavier p block elements (n>2). 14-23 Surprisingly, although the addition polymerization of C=C bonds is an industrially important route to organic polymers, the polymerization of inorganic multiple bonds seems to have been overlooked. This is likely attributed to the belief that inorganic unsaturated precursors were not suitable for polymerization. For example, it has been postulated that most Si=Si precursors would be too unstable to be isolated and would polymerize (or oligomerize) immediately upon their formation.24 On the other hand, the kinetically stabilized disilenes are thought to be too sterically hindered to be capable of polymerization.24The few attempts to form high polymers from unsaturated compounds have met with little success.25 To investigate the addition polymerization of a heavier element multiple bond, we hypothesized that phosphaalkenes would be excellent candidates as monomers for polymerization studies because * A version of this chapter has been published. Chi-Wing Tsang, Mandy Yam, and Derek P. Gates, “The Addition Polymerization of a P=C bond: A Route to New Phosphine Polymers,” I Am. Chem. Soc. 2003, 125, 1480—148 1. Copyright 2003 American Chemical Society. Chapter Two 39 P=C bonds exhibit many parallels to C=C bonds.2628 Sterically unhindered phosphaalkenes are unstable and the difficulty in isolating phosphaalkenes bearing small substituents has often been attributed to undesired polymerization reactions.2934 These so-called polymerizations of impure monomers were likely uncontrolled, leading to short chain oligomers. Moreover, molecular weight analysis was not reported for the so-called “polymers”. For a phosphaalkene monomer to be suitable for polymerization studies, we hypothesized that it must be isolable and sterically protected to a minimal degree, analogous to ethylene (H2CCH).Ethylene is kinetically stable, yet thermodynamically unstable with respect to polymerization. Initial attempts to initiate the polymerization of isolable Mes*P=CH2 (Mes* = 2,4,6-tri-tert-butylphenyl) with GaC13,A1C13, and HOTf afforded products resulting from the intramolecular C—H activation of the tert-butyl group in the PMes* substituent which effectively prevented chain propagation.35’6 To avoid any undesired reactions with the P substituent, phosphaalkene (2.1) containing a mesityl (Mes = 2,4,6-trimethyiphenyl) group at phosphorus attracted our attention.29’37 Interestingly, although 2.1 is isolable, attempts to prepare an analogue of 2.1 using the sterically less hindered 2-methylphenyl group in place of Mes reportedly afforded a “polymeric material” rather than a stable phosphaalkene.29This subtle difference in stability suggests that the slightly bulkier P-mesityl substituent offers minimal kinetic stabilization yet it should be thermodynamically unstable with respect to polymerization. This is analogous to the case of ethyleneH2CCH stated above. Therefore, phosphaalkene 2.1 is a good candidate as a monomer for addition polymerization. In this chapter, the first addition polymerization of a phosphaalkene (2.1) is described (Scheme 2.1). A new functional polymer, poly(methylenephosphine) (2.2), which contains a backbone of alternating carbon and phosphorus(III) atoms, is prepared and fully characterized. References start on page 53 Chapter Two 40 Ph initiator F Ph P=C Mes’ Ph 150 C L Mes Ph] 2.1 2.2 Scheme 2.1. 2.2 Results and discussion 2.2.1 Synthesis and characterization of poly(methylenephosphine) The preparation of phosphaalkene 2.1 was based on the procedures reported by Becker’s group.38 After distilling the crude product mixture under vacuum (bp = 150 °C, 0.1 mmHg), we observed a viscous, pale-brown residual material. The 31P NMR spectrum of this residual material revealed a strong signal at 233 ppm, several sharp signals, and a broad resonance at —10 ppm [Figure 2.1(a)]. This suggested the presence of phosphaalkene 2.1 (6np = 233) along with some phosphine moieties [631p = —10 (br)]. Repeated precipitations of concentrated THF solutions of the residue with hexanes afforded polymer 2.2 as a white solid (yield = 7%). The 31P NMR spectrum of the isolated polymer displayed a single broad resonance at —10 ppm [Figure 2.1(b)]; moreover, ‘H and 13C{1H} NMR spectra also exhibited signals which were consistent with the polymeric structure of poly(methylenephosphine) 2.2. In addition, the NMR spectra of 2.2 exhibited signal broadening, which is typical of polymeric species. Since no other polymers with a similar backbone have been synthesized, Mes(Me)PCPh2Hwas prepared as a model compound for polymer 2.2. This model compound [Mes(Me)PCPhH]represents polymer 2.2 with a repeating unit of one (n = 1). The‘3C{’H} NMR spectrum of 2.2 [Figure 2.2(b)] strongly resembles that of the model compound Mes(Me)PCPh2H[Figure 2.2(a)]. In particular, the presence of the quaternary carbon in the polymer backbone [P—C(Ph2) P] is identified to be the References start on page 53 Chapter Two 41 broad signal (6 51.3) in thel3C{1H} NMR spectrum of 2.2. The molecular weight determination of polymer 2.2 by gel permeation chromatography (GPC, in THF, vs polystyrene standards) revealed an estimated number average molecular weight (rel. M11) of 11,500 gmor1 with a polydispersity index (PDI; M/M) of 1.25. ppm Figure 2.1. 31P NMR spectra (CDC13)of: (a) crude polymerization mixture from the thermolysis of crude 2.1; (b) poly(methylenephosphine) 2.2 after purification. Reproduced with permission from J. Am. Chem. Soc. 2003, 125, 1480—148 1. Copyright 2003 American Chemical Society. 2.2.2 Properties and chemical modification of poly(methylenephosphine) Poly(methylenephosphine) 2.2 possesses stability towards air and moisture in the solid state for several months under sunlight. However, upon exposure of a CH21 solution of 2.2 to air for four days, the polymer gradually oxidizes to phosphine oxide 2.3. To improve the air stability of the phosphorus polymer, chemical modification of 2.2 to air-stable polymers 2.3 or 2.4 was conducted (Scheme 2.2). The oxidation of 2.2 with excess H20 (30% in H20) afforded (a) (b) 250 200 150 100 50 0 -50 -100 -150 References start on page 53 Chapter Two 42 2.3 immediately (isolated yield = 59%). The air- and moisture-stable polymer 2.3 was characterized by 31P NMR spectroscopy (5 47) and GPC (rel. M = 8,800 gmoP’; PDI = 1.27). Analogous to the reaction with oxygen, the phosphine polymer 2.2 reacts with elemental sulfur to give polymer 2.4 (yield = 84%; 6ip= 52) with similar molecular weight (rel. M = 11,900 gmol’; PDI = 1.24) to the unsulfurized 2.2 (rel. M = 11,500 gmo[’; PDI = 1.25). To obtain a more accurate molecular weight for the phosphorus polymer, an absolute molecular weight was determined for the sulfurized polymer 2.4 using triple detection GPC with a light-scattering detector and a viscometer. Remarkably, triple detection GPC showed that the absolute molecular weight of 2.4 (abs. M = 32,000 gmo[’) was substantially higher than the molecular weight estimated relative to polystyrene standards (rel. M = 11,900 gmotj. The thermal stability of (a) IiLi H Me-—qPh Mes Ph L lii (b) * * F Phi LMsP’hJn I I — I — I — I — U — I I I 160 140 120 100 80 60 40 20 ppm Figure 2.2.‘3C{’H} NMR spectra (CDC13)of: (a) Mes(Me)PCPh2H;(b) poly(methylenephosphine) 2.2. (*) indicates CDCI3.Reproduced with permission from J. Am. Chem. Soc. 2003, 125, 1480—148 1. Copyright 2003 American Chemical Society. References start on page 53 Chapter Two 43 these phosphorus-containing polymers was studied with thermogravimetric analysis (TGA) which showed onset decomposition temperatures at 265 °C (2.2), 320 °C (2.3), and 220 °C (2.4), and 85 — 95% of the polymer weight was lost above these temperatures. More details of the thermal stability of these and other related poly(methylenephosphine)s will be given in Chapter Four. F S Phi S8 F Phi H202 F C Phi H—P—C-I— H—P—C-I— —I-—P—C I I ii Ii ii Ii I L Mes Ph] L Mes Ph] [Mes Ph 2.4 2.2 2.3 Scheme 2.2. 2.2.3 Polymerization of phosphaalkene using a radical initiator Although the NMR spectroscopic measurements and molecular weight characterization of poly(methylenephosphine) 2.2 are in agreement with the assigned polymeric structure, the mechanism of the phosphaalkene polymerization is not clear. At high temperatures (150 °C), any radical impurities in the monomer mixture might be potential initiators for polymerization. To evaluate whether such a radical mechanism is plausible, we conducted a radical polymerization of phosphaalkene 2.1 with VAZO 88 as the initiator at 200 °C. After 48 hours, the initially free- flowing liquid became very viscous. The viscous polymer mixture was dissolved in THF and was precipitated with hexanes (3x) to afford poly(methylenephosphine) 2.2 (rel. M = 5,700 gmol1;PDI = 1.10). Two broad overlapping resonances at —10 and —40 ppm were observed in the 31P NMR spectrum of the isolated polymer which suggested that there might be head-to-tail and head-to-head linkages in the polymer backbone. The 31P NMR spectra of the radical polymers obtained from other attempted radical polymerization reactions of 2.1 differed References start on page 53 Chapter Two 44 significantly; however, polymer 2.2 was still isolated and verified by GPC and 31P NMR spectroscopy in each polymerization. 2.2.4 Polymerization of phosphaalkene using anionic initiators In view of Bickelhaupt’ s lithiation of phosphaalkene 2.1 with MeLi,39 we speculated that 2.1 should undergo anionic polymerization. Hence, we treated phosphaalkene 2.1 with MeLi (0.05 equiv) in a minimal amount of THF (Ca. 0.2 mL) in a sealed tube at 150 °C. After 24 hours, a viscous polymeric solution was obtained and subsequently quenched with MeOH and precipitated with hexanes. Polymer 2.2 was isolated (yield = 30%; rel. M = 6,600 gmot1;PDI = 1.55) and the 31P, ‘H, and‘3C{’H} NMR spectra of this polymer were consistent with those of 2.2 obtained from distillation. The polymerization of 2.1 with MeLi is reproducible and afforded phosphine polymers with relative molecular weights between 5,000 and 10,000 gmof’. The anionic polymerization of phosphaalkene 2.1 can also be initiated with “BuLi (0.05 equiv) and polymer 2.2 was isolated with a similar yield (25%) and molecular weight (rel. M = 5,400 gmol’; PDI 1.15). We were interested in verifying the mechanism of the anionic polymerization. The addition reactions of P=C and PC bonds with anionic reagents (i.e. MeLi) have been described in the literature.39’40 For instance, Bickelhaupt et al. reported that the treatment of phosphaalkene 2.1 with MeLi followed by MeOH afforded Mes(Me)P(O)CHPh239Furthermore, in the presence of MeLi (1 equiv), Arifet al. proposed that phosphaalkyne Mes*CEP (2 equiv) dimerized to give the linear MeP=C(Mes*)P=C(Mes*)Li.4°The carbanion 2.5 was believed to be the intermediate species in these two addition reactions. However, when Yoshifuj i’s group reacted Mes* CP (2 equiv) with ‘BuLi (1 equiv) and, subsequently, quenched the reaction mixture with MeOH, they References start on page 53 Chapter Two 45 obtained a cyclic 1 ,3-diphosphacyclobutene, instead of a linear molecule.4’This interesting cyclic compound suggested that addition reactions with different anionic reagents could, sometimes, result in products with different configuration. This is not surprising because the addition reactions of P=P bonds gave various products for each RLi reagent used.42 To verify the mechanism of the anionic polymerization of phosphaalkene 2.1 using MeLi, we obtained the carbanion 2.5 and employed it as a polymerization initiator. We have repeated the addition reaction of 2.1 with MeLi (1 equiv) (Scheme 2.3). Quantitative conversion to carbanion 2.5 (631p —45) was observed and subsequent treatment with water gave Mes(Me)PCPh2H(5ip — —24). Presumably, the carbanion 2.5 is the initial propagating species in the anionic polymerization of 2.1 with MeLi. Therefore, we performed an anionic initiation of 2.1 with the carbanion 2.5 (0.05 equiv; 150 °C; 24 h) and obtained polymer 2.2 (yield 41%; rel. M = 6,200 gmo[’; PDI = 1.29). The involvement of carbanion 2.5 in the anionic polymerization of 2.1 using RLi reagents is also supported by the fact that no polymer was isolated from heating purified phosphaalkene 2.1 without any initiators at 150 °C. These results are consistent with an anionic polymerization mechanism. 0 ,Ph 1 equiv MeLI e Li P=c . / ‘. ILA Dh Mes Ph 2.1 2.5 Scheme 2.3. 2.3 Summary I have demonstrated the first addition polymerization of a P=C bond to yield a new class of phosphorus macromolecules. This method has opened a new synthetic route to inorganic polymers containing phosphorus(III) atoms in the backbone. Poly(methylenephosphine) 2.2 can References start on page 53 Chapter Two 46 be obtained through distillation of phosphaalkene 2.1, or using radical and anionic initiators. Furthermore, the reaction of phosphine polymer 2.2 with chemical reagents has afforded the air- and moisture-stable polymers poly(methylenephosphine oxide) 2.3 and poly(methylenephosphine sulfide) 2.4. 2.4 Experimental section General procedures. All manipulations of air and/or water sensitive compounds were performed under pre-purified nitrogen (Praxair, 99.998%) using standard high vacuum or Schlenk techniques or in an Innovative Technology Inc. glovebox. Hexanes and CH21 were dried by passing the solvent through activated alumina columns.43 Tetrahydrofuran (THF) was distilled from sodium/benzophenone immediately prior to use. Methanol and distilled water were degassed prior to use. CDC13 (CIL) and cyclohexane were distilled from P205 and degassed. Benzophenone was purchased from Aldrich and sublimed prior to use. KOH was purified by recrystallization from ethanol and subsequent heating in vacuo.44 VAZO 88 [1,1’- azobis(cyclohexanecarbonitrile)], sulfur, and H20 (30% in H20) were purchased from Aldrich and used as received. MeLi (in diethyl ether), and‘1BuLi (in hexane) were purchased from Aldrich and were titrated prior to use.45 MesP(SiMe3)2was prepared following a literature procedure.46 Equipment. ‘H, 31P, and‘3C{’H} NMR spectra were recorded at room temperature on a Bruker Avance 300 MHz spectrometer. Chemical shifts are reported relative to: residual CHC13 (5 = 7.24 for 1H); 85% H3P04as an external standard (5 = 0.0 for 31P); CDC13 ( = 77.0 for‘3C{’H}). Mass Spectra were acquired using Kratos MS 50 instrument. Relative molecular weights were References start on page 53 Chapter Two 47 estimated by gel permeation chromatography (GPC) using a Waters liquid chromatograph equipped with a Waters 515 HPLC pump, Waters 717 plus autosampler, Waters Styragel® columns (4.6x300mm) HR2, HR4, and HR5E and a Waters 2410 differential refractometer (refractive index detector). A flow rate of 0.3 mL/min was used and samples were dissolved in THF (ca. 1 mg/mL) and filtered before injection. Narrow molecular weight polystyrene standards were used for calibration purposes. Thermogravimetric analyses (TGA) were carried out on a TA Instruments 2000 instrument with a TGA5 1 module, heating under dry nitrogen at 10 °C/min from 20 °C to 950 °C. Elemental analyses were performed on several representative polymer samples by Mr. Minaz Lakha in the Departmental Microanalysis Facility. Triple detection GPC analysis of 2.4 was performed by Dr. Scott Clendenning (University of Toronto) using a Waters Associates 2690 separation module equipped with a Waters 410 differential refractometer and a Viscotek T6OA dual detector connected in parallel. The refractive index increment (dn/dc) of the polymer solutions was obtained with a Chromatix KMX- 16 differential refractometer operating at a wavelength of 632.8 nm. The instrument was calibrated with NaC1 solutions. 2.4.1 Preparation of MesPCPh2(2.1). To a stirred solution of MesP(SiMe3)2(20.0 g, 0.067 mol) and benzophenone (12.3 g, 0.067 mol) in THF (300 mE) was added a suspension of finely ground anhydrous KOH (0.38 g, 6.8 mmol) in THF (40 mL). The pale yellow reaction mixture was stirred for 1 h and an aliquot was removed and analyzed by 31P NMR spectroscopy. The observation of a single signal (6 = 233) indicated the quantitative formation of 2.1. The solvent References start on page 53 Chapter Two 48 was removed in vacuo and subsequently, the product 2.1 was dissolved in hexanes, the solution was filtered and the solvent was removed in vacuo. Crude yield (yellow oil) = 18 g (85%). Purification of 2.1 by vacuum distillation: crude 2.1 (18 g) was transferred into a short-path distillation apparatus and was heated under vacuum in an oil bath (200 °C, 0.01 mmHg). The refluxing yellow liquid distilled at 150—160 °C over a 3 h period and was collected in a Schlenk flask and was recrystallized from a minimum amount of cyclohexane (3x) at room temperature. Yield of 2.1 = 12.0 g (67%). 31P NMR (CDC13): 233; ‘H NMR (CDC13): 6 = 7.52-6. 87 (m, 1OH; aryl H), 6.70 (s, 2H, Mes H), 2.27 (s, 6H; o-CH3), 2.20 (s, 3H;p-CH).Anal. Calcd forC22H1P: C, 83.52; H, 6.69. Found: C, 83.42; H, 6.74. Characterization of 2.1 agreed with those reported previously in the literatures.29’38 i Phi 2.4.2 Isolation of poly(methylenephosphme) (2.2) from the distillation L Mes Ph] residue. The gummy pale brown residue from the distillation was dissolved in CH21 and 31P NMR spectroscopic analysis of the solution showed the presence of 2.1 and several sharp resonances and a broad resonance at —10 ppm. Addition of a concentrated solution of the residue to stirred hexanes (ca. 100 mL) resulted in the precipitation of a colorless fibrous material. The polymer was precipitated four times from a concentrated THF solution with hexanes. The solid was dried in vacuo overnight. Yield = 1.20 g (7%). 31P NMR (CDC13): 6-10 (br); ‘H NMR (CDC13): 6 7.8—6.6 (br, 12H, aryl H), 2.5-1.9 (br, 911, p CR3, o-CH3);13C{H} NMR (CDC13): 6 146.7 (br, Mes-C), 143.2 (br, Mes-C), 138.0 (br, Mes C), 132-125 (br mult, Ph-C), 52.2(br, P-C-P), 23.1 (br, o-CH3), 21.0 (br, p-CR3);GPC (f11F, vs. References start on page 53 Chapter Two 49 polystyrene): rel. M = 11,500 gmo[’, rel. M 14,400 gmol’, PDI = 1.25; Anal. Calcd. for (C22H1P): C, 83.52; H, 6.69. Found: C, 82.00; H, 6.53. Phi 2.4.3 Preparation of poly(methylenephosphine oxide) (2.3). To a solution of -+---- [Ms Ph] polymer 2.2 (0.16 g, 0.51 mmol, rel. M = 11,500 gmol1)in CH21 (5 mL) was added 30% H20 in water (3 mL). The reaction mixture was stirred and shaken for 3 hours. Analysis of the reaction mixture by 31P NMR showed quantitative formation of a new product with a broad signal at 47 ppm. The aqueous layer was removed and the CH21 layer was extracted with H20 three times in air. The organic layer was dried with MgSO4and the solvent removed in vacuo leaving a colorless solid. The polymer 2.3 was purified by precipitation of a concentrated THF solution with hexanes and dried in vacuo. Yield = 0.10 g (59%). 31P NMR (CDCI3): 6 47 (br); ‘H NMR (CDC13): 67.8—6.6 (br, 12H, aryl H), 2.5—1.9 (br, 9H, p CH3, o-CH3);GPC (THF, vs. polystyrene): rel. M = 8,800 gmoi’, rel. M = 11,200 gmo[1,PDI = 1.27. s Phi 2.4.4 Preparation of poly(methylenephosphine sulfide) (2.4). A solution of H—P—c-I— L Mes Ph] polymer 2.2 (0.30 g, 0.95 mmoL, rel. M = 11,500 gmol1)in CH21 (20 mL) was added to a suspension of S8 (0.15 g, 0.58 mmol) in CH21 (10 mL). The stirred suspension was monitored by 3’P NMR spectroscopy and after 48 h the quantitative formation of a new product (6 = 52 ppm) was observed. The excess sulfur was removed by filtration and subsequent sublimation of the solid (80°C, 0.01 mm Hg). The polymer 2.4 was precipitated from a concentrated CH21 solution with hexanes. The polymer was obtained as a colorless solid. Yield = 0.27 g (82%). References start on page 53 Chapter Two 50 31P NMR (CDC13): 652 (br); ‘H NMR (CDC13): 6 7.8—6.6 (br, 12H, aryl H), 2.5-1.9 (br, 9H, p CH3, o-CH3);‘3C{’H} NMR (CDC13): 6 146.6 (br, Mes-C), 142.4 (br, Mes-C), 137.7 (br, Mes C), 132-124 (br mult, Ph-C), 52.2 (br, P-C-P), 23.1 (br, o-C!-13), 20.8 (br, p-Cl-I3),GPC (THF, vs. polystyrene): rel. M = 11,900 gmo[1,rel. M = 14,700 gmol’, PD! = 1.24; GPC (THF, triple detection): abs. M = 32,000 gmo[’, abs. M = 35,000 gmo11,PD! = 1.08, dn/dc 0.152 mL/g, Rgw 4.27 nm, Rhw 3.27 nm; Anal. CaLcd. for (C22H1PS): C, 75.80; H, 6.10. Found: C, 72.20; H, 6.19. F Phi 2.4.5 Radical polymerization of 2.1 using VAZO 88. In a typical experiment a —I— P—c-I— L Mes Phj Pyrex tube was charged with 2.1 (1.00 g, 3.16 mmol) and VAZO 88 (0.08 g, 0.32 mmol) and the tube was flame-sealed in vacuo. The mixture was heated (200 °C, 48 h) in an oven equipped with a rocking tray over which time the reaction mixture became increasingly viscous. 31P NMR analysis of the product in THF showed that approximately half the monomer 2.1 was consumed and several new broad resonances were observed. The molecular impurities were removed by repeated precipitation of concentrated THF solutions with hexanes (3x). Yield = 0.16 g (16%) 31P NMR (CDC13): 6—10 (br), —40 (br); GPC (THF, vs. polystyrene): rel. M = 5,700 gmo[1, rel. M = 6,300 gmo11,PD! = 1.10. Ph 2.4.6 Anionic polymerization of 2.1 using MeLi. To a Pyrex tube containing 2.1 (1.00 g, 3.16 mmol) and a small amount of THF (Ca. 0.2 mL) was added LMeS Phj ,-, MeLi (1.12 M, 0.14 mL, 0.16 mmol). The contents of the tube were immediately frozen (liquid nitrogen) and the tube was flame sealed in vacuo. In an oven equipped with a rocking tray was References start on page 53 Chapter Two 51 heated the tube (150 °C, 24 h) in which the reaction mixture became increasingly viscous over the 24 h period. The product was dissolved in THF (Ca. 30 mL) and quenched with degassed MeOH (Ca. 0.1 mL). The sample was concentrated in vacuo (ca. 1 mL) and degassed water (30 mE) was added to precipitate the polymer. Precipitated of the polymer from THF with MeOH (lx) and hexanes (lx) afforded a pale yellow solid, which was further washed with hexanes and isolated by filtration and dried in vacuo. Yield 0.30 g (30%). This experiment was repeated several times and identical spectroscopic data were obtained and the polymers that had M’ s in the range of 6,000 to 11,000 gmor’ were obtained. GPC (THF, vs. polystyrene): rel. M = 6,600 gmol’, rel. M 10,300 gmol1,PDI = 1.55. The 31P, ‘H, and 13C{’H} NMR spectra were identical to those of polymer 2.2 obtained by distillation as in Section 2.4.2 (Page 48). Ph 2.4.7 Anionic polymerization of 2.1 using BuLi. To a Pyrex tube containing LMesPhj 2.1 (1.00 g, 3.16 mmol) was added ‘BuLi (1.44 M, 0.11 mL, 0.16 mmol). The contents of the tube were immediately frozen (liquid nitrogen) and the tube was flame sealed in vacuo. In an oven equipped with a rocking tray was heated the tube (150 °C, 24 h) in which the reaction mixture became increasingly viscous over the 24 h period. The product was dissolved in minimal amount of THF (ca. 2 mL), precipitated with degassed MeOH (30 mL) and isolated by filtration. After repeated precipitations from a minimal amount of CH21 (ca. 1 mL) with hexanes (3x30 mL), the pale yellow polymer was isolated by filtration and dried in vacuo. Yield = 0.25 g (25%). GPC (THF, vs polystyrene): rel. M = 5,400 gmo[’, rel. M = 6,200 gmol’, PDI = 1.15. The 31P References start on page 53 Chapter Two 52 NMR spectrum was identical to those of polymer 2.2 obtained by distillation as in Section 2.4.2 (Page 48). H 2.4.8 Reaction of 2.1 with MeLi (1 equiv). To a cooled (—80 °C) stirred solution Me-r— Ph Mes Ph of 2.1 (1.07g. 3.38 mmol) in diethyl ether (20 mL) was added dropwise MeLi (1.05 M, 3.9 mL, 4.1 mmol). The reaction mixture changed color from brown to red and finally to orange over 30 mm and the cooling bath was then removed. Upon warming to room temperature, degassed water (10 mL) was added to the reaction mixture and the solution became a pale yellow color. The ether portion was separated from the aqueous layer and dried with MgSO4.The solvent was removed in vacuo leaving a yellow oil which solidified over several days. The product was recrystallized from a minimum amount of hexanes to give colorless crystals. Yield = 0.92 g (82%) 31P NMR (CDC13): 5 —24; ‘H NMR (CDC13): ö 7.60—7.58 (d, 2H, aryl H), 7.35—7.00 (m, 8H, aryl H), 6.75 (s, 2H, Mes H), 4.88 (d, 2PH = 5.3 Hz, 1H, -C(H)Ph2),2.51 (s, 6H, o-CH3), 2.16 (s, 3H, p-CH3);‘3C{’H} NMR (CDC13): 144.4 (m, Mes-C), 144.2 (m, Mes-C), 138.6 (s, Mes-C), 129.5 — 125.8 (m, Ph-C), 125.8 (d, Mes-C, 1J = 1.3 Hz), 51.3 (d, ‘J = 16.0 Hz, MesP(CH3)- C(H)Ph2),23.3 (s, o-CH3), 23.1 (s, o-CH3), 20.7 (s,p-CH3),9.84 (d, 1J = 18.7 Hz, MesP(C1-13)- C(H)Ph2);MS (El, 70 eV): m/z (%): 333 (4), 332 (15) [M1, 168 (15), 167(100) [C(H)Ph21. Anal. Calcd. for (C23H5P)11:C, 83.10; H, 7.58. Found: C, 82.78; H, 7.46. F Phi 2.4.9 Anionic polymerization of 2.1 using 2.5. Compound 2.5 was prepared 1Mes from 2.1 (0.513 g, 1.6 mmol) and MeLi (1.38 M, 1.4 mL, 1.9 mmol) in 2.5 mL THF. The quantitative formation of 2.5 was determined using 31P NMR spectroscopy. To a References start on page 53 Chapter Two 53 Pyrex tube containing 2.1 (0.95 g, 3.00 mmol) was added a solution of 2.5 (0.415 M, 0.38 mL, 0.16 mmol). The contents of the tube were immediately frozen (liquid nitrogen) and the tube was flame sealed in vacuo. The mixture was heated (150 °C, 24 h) in an oven equipped with a rocking tray. The reaction mixture became increasingly viscous over the 24 h period. The product was dissolved in THF (ca. 5 mL) and quenched with degassed MeOH (ca. 0.1 mL). The sample was concentrated to approximately 2 mL, and 30 mL hexanes was added to precipitate the polymer. The polymer was precipitated from THF with degassed water (ca. 30 mL), washed with MeOH (2x), dried, and reprecipitated from THF with 50 mL hexanes (2x). The pale yellow polymer was isolated by filtration and dried in vacuo. Yield = 0.39 g (4 1%). GPC (THF, vs. polystyrene): rel. M = 6,200 gmo[’, rel. M = 8,000 gmol’, PDI = 1.29. The 31P and NMR spectra were identical to those of polymer 2.2 obtained using distillation as in Section 2.4.2 (Page 48). 2.5 References 1. Mark, J. E., Allcock, H. R., West, R., Inorganic Polymers, 2nd ed.; Oxford University Press: New York, 2005. 2. Archer, R. D., Inorganic and Organometallic Polymers. Wiley-VCH: New York, 2001. 3. Vanderark, L. A.; Clark, T. J.; Rivard, E.; Manners, I.; Slootweg, J. C.; Lammertsma, K. Chem. Commun. 2006, 31, 3332. 4. Wright, V. A.; Patrick, B. 0.; Schneider, C.; Gates, D. P. 1 Am. Chem. Soc. 2006, 128, 8836. 5. Wright, V. A.; Gates, D. P. Angew. Chem. mt. Ed 2002, 41, 2389. 6. Smith, R. C.; Protasiewicz, J. D. I Am. Chem. Soc. 2004, 126, 2268. References start on page 53 Chapter Two 54 7. Jin, Z.; Lucht, B. L. I Am. Chem. Soc. 2005, 127, 5586. 8. Jin, Z.; Lucht, B. L. I Organomet. Chem. 2002, 653, 167. 9. Lucht, B. L.; St. Onge, N. 0. Chem. Commun. 2000, 2097. 10. Morisaki, Y.; Aiki, Y.; Chujo, Y. Macromolecules 2003, 36, 2594. 11. Dorn, H.; Rodezno, J. M.; Brumihöfer, B.; Rivard, E.; Massey, J. A.; Manners, I. Macromolecules 2003, 36, 291. 12. Dorn, H.; Singh, R. A.; Massey, J. A.; Nelson, J. M.; Jaska, C. A.; Lough, A. J.; Manners, I. I Am. Chem. Soc. 2000, 122, 6669. 13. Naka, K.; Gelover-Santiago, A.; Chujo, Y. I Polym. Sci. Part A: Polym. Chem. 2004, 42, 5872. 14. Power, P. P. Chem. Rev. 1999, 99, 3463. 15. Jutzi, P. Angew. Chem. mt. Ed 2000, 39, 3797. 16. Power, P. P. 1 Chem. Soc., Dalton Trans. 1998, 2939. 17. Yoshifuji, M. I Chem. Soc. Dalton. Trans. 1998, 3343. 18. Driess, M.; Grützmacher, H. Angew. Chem. mt. Ed 1996, 35, 828. 19. Norman, N. C. Polyhedron 1993, 12, 2431. 20. Niecke, E.; Gudat, D. Angew. Chem. mt. Ed Engl. 1991, 30, 217. 21. Regitz, M. Chem. Rev. 1990, 90, 191. 22. West, R. Angew. Chem. mt. Ed Engi. 1987, 26, 1201. 23. Cowley, A. H. Polyhedron 1984, 3, 389. 24. Mark, J. E., Alicock, H. R., West, R., Inorganic Polymers; 2nd ed.; Oxford University Press: New York, 2005; pp 208-209. References start on page 53 Chapter Two 55 25. The spontaneous polymerization of PhCP has been reported, however the oligomers isolated were of low molecular weight and contained few P=C bonds. See, Loy, D. A.; Jamison, G. M.; McClain, M. D.; Alam, T. M. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 129. 26. Mathey, F. Angew. Chem. mt. Ed 2003, 42, 1578. 27. Dillon, K. B., Mathey, F., Nixon, J. F., Phosphorus: The Carbon Copy. Wiley: New York, 1998. 28. Appel, R., In Multiple Bonds and Low Coordination in Phosphorus Chemistry; Regitz, M., Scherer, 0. J., Eds.; Georg Thieme Verlag: Stuttgart, 1990; pp 157—219. 29. Kiebach, T. C.; Lourens, R.; Bickeihaupt, F. J Am. Chem. Soc 1978, 100, 4886. 30. Grobe, J.; Le Van, D.; Schuize, J.; Szameitat, J. Phosphorus Suifur 1986, 28, 239. 31. Pellerin, B.; Guenot, P.; Denis, J.-M. Tetrahedron Lett. 1987, 28, 5811. 32. Quin, L. D.; Hughes, A. N.; Pete, B. Tetrahedron Lett. 1987, 28, 5783. 33. Guillemin, J. C.; Le Guennec, M.; Denis, J. M. J Chem. Soc., Chem. Commun. 1989, 988. 34. lonkin, A. S.; Ignatév, S. N.; Nekhoroshkov, V. M.; Efremov, J. J.; Arbuzov, B. A. Phosphorus, Sulfur, and Silicon 1990, 53, 1. 35. Tsang, C.-W.; Rohrick, C. A.; Saini, T. S.; Patrick, B. 0.; Gates, D. P. Organometallics 2002, 21, 1008. 36. Tsang, C.-W.; Rohrick, C. A.; Saini, T. S.; Patrick, B. 0.; Gates, D. P. Organometallics 2004, 23, 5913. 37. Van der Knaap, T. A.; Klebach, T. C.; Visser, F.; Bickeihaupt, F.; Ros, P.; Baerends, E. J.; Stam, C. H.; Konijn, M. Tetrahedron 1984, 40, 765. References start on page 53 Chapter Two 56 38. Mundt, 0.; Becker, G.; Uhi, W.; Massa, W.; Birkhahn, M. Z. Anorg. Aug. Chem. 1986, 540/541, 319. 39. Van der Knaap, T. A.; Bickeihaupt, F. Tetrahedron Lett. 1982, 23, 2037. 40. Arif, A. M.; Barron, A. R.; Cowley, A. H.; Hall, S. W. J Chem. Soc. Chem. Commun. 1988, 171. 41. Yoshifuji, M.; Sugiyama, H.; Ito, S. J Organomet. Chem. 2005, 690, 2515. 42. Weber, L. Chem. Rev. 1992, 92, 1839. 43. Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J. Organometallics 1996, 15, 1518. 44. Armarego, W. L. F.; Perrin, D. D. Purification ofLaboratory Chemicals, 4th ed.; Butterworth Heinemann Press: Oxford, 1998, P. 429 on purification of NaOH. A similar procedure was used for KOH. 45. Burchat, A. F.; Chong, J. M.; Nielsen, N. I Organomet. Chem. 1997, 542, 281. 46. Becker, G.; Uhl, W.; Wessely, H.-J. Z Anorg. Aug. Chem. 1981, 479, 41. References start on page 53 57 Chapter Three Scope and Limitations of the Base-Catalyzed Phospha—Peterson P=C Bond Forming Reaction* 3.1 Introduction Having been able to successfully conduct the first addition polymerization of P=C bonds as shown in Chapter Two,’ I investigated the generality of this method to prepare a new class of phosphorus-containing polymers. To facilitate the generalization of this new polymerization reaction, a convenient preparative route to isolable phosphaalkenes possessing a variety of substituents is desirable. Several important methods for the synthesis of acyclic phosphaalkenes are shown in Scheme 3.1. One of the most general preparative routes to acyclic phosphaalkenes is 1 ,3-silatropic migration (Path A) which was employed by Becker to synthesize the first stable acyclic phosphaalkene (R = Ph, Me; R’ = tBu; R” = OSiMe3)in 1976.217 This reaction involves a condensation followed by rearrangement to the stable Si—O tautomer. The resulting bulky siloxy substituent effectively protects the PC bond through kinetic and thermodynamic stabilization. The second synthetic route to phosphaalkenes is 1,2-elimination (Path B) which is driven by the thermal or base-induced removal of a small molecule from the phosphine precursor. 18-29 A similar phosphaalkene synthesis which also relies on the ready removal of small molecules is the condensation of a primary phosphine with a methylene hailde (Path C)3033 or with an aldehyde (Path D).3437 Another important access to phosphaalkenes is phospha-Wittig reaction (Path E), which usually involves a phospha-Wittig ylid and a carbonyl.3845 To prepare a phosphaalkene * A version of this chapter has been published. Mandy Yam, Jonathan H. Chong, Chi-Wing Tsang, Brian 0. Patrick, Anita E. Lam, and Derek P. Gates, “Scope and Limitations of the Base-Catalyzed Phospha-Peterson P=C Bond Forming Reaction,” Inorg. Chem. 2006, 45, 5225—5234. Copyright 2006 American Chemical Society. Chapter Three 58 ,Li A SiMe3 A A F R’ R_P\ + O=C R_R\ + 0C SiMe3 A” SiMe3 CI X H -LiOSiMe” A Base (cat) BaseR A R”=CH3 -2HX Hp—c A—P + CR’A”X2 / \‘ G cH OH2 H — +Me - OPM \<20 ,H R’ P=ML + o=c + o=c R—P + O=C / n / \ \ A A’ A Me A” H A” Scheme 3.1. with less steric bulk at phosphorus, metal-complexed Wittig reagents are required to provide more stabilization to the P=C bond.42’3 One limitation of the phospha-Wittig reaction is that the carbonyl precursors are limited to aldehydes only. However, with anionic phosphoryiphosphine metal complexes, ketones will react to form C-disubstituted phosphaalkenes.44’5In addition, transition metals are involved in phosphaalkene synthesis through the formation of terminal metal-phosphinidenes (Path F).4648 The R—P groups can be transferred to phosphaalkenes from a tantalum-,46 zirconium-,47 or iridium-complexed phosphinidene in a metathesis reaction.48 Path G represents a 1,3-hydrogen rearrangement of secondary vinyiphosphines to phosphaalkenes.49’5° This reaction resembles a keto-enol tautomerism and is usually induced by a base catalyst or thermally. We are particularly interested in the reaction of cc-silyl phosphides with ketones or aldehydes as a route to phosphaalkene monomers for polymerization studies (Path H). This so- called the phospha-Peterson reaction is an analogue of the Peterson olefination reaction,51 and has been used extensively to prepare isolable phosphaalkenes bearing a variety of bulky References start on page 80 Chapter Three 59 substituents. 52-61 An alternate base-catalyzed phospha-Peterson reaction (Scheme 3.2) has been reported for the preparation of phosphaalkene 3.la.53’62 This route represents a potentially more convenient method of P=C bond formation since it is not necessary to generate RP(Li)SiMe3in situ and the sole byproduct (hexamethyldisiloxane) is volatile and easy to remove. Other than 3.la, only a few phosphaalkenes have been prepared by this method. ,SiMe3 ,Ph KOH ,Ph Mes-P + O=C • P=C + O(S1Me3)2 “SiMe3 Ph (cat) Me Ph 3.1 a Scheme 3.2. In this chapter, the scope of the base-catalyzed phospha-Peterson reaction as a route to P mesityl and P-adamantyl phosphaalkenes is explored. 3.2 Results and discussion 3.2.1 Synthesis of P-mesityl phosphaalkenes The base-catalyzed phospha-Peterson reaction can be used to prepare P-mesityl phosphaalkenes bearing several C-aryl substituents (Scheme 3.3). By slightly modifying Becker’s original procedure for 3.la,53 several isolable P-mesityl phosphaalkenes (3.lb—f) have been prepared from MesP(SiMe3)2and the appropriate ketone in the presence of a catalytic quantity of anhydrous KOH or NaOH.63 After vacuum distillation and/or recrystallization the isolated yields of analytically pure phosphaalkene were between 43 and 72% and each could be prepared on multi-gram scale. Interestingly, we have found that compounds 3.la—e are amenable to aqueous work-up provided they are kept free from oxygen. Aqueous work-up allows for easy removal of KOH (or KOSiMe3)and obviates the need for distillation. Unlike the other phosphaalkenes, 2- References start on page 80 Chapter Three 60 pyridyl-substituted phosphaalkene (3.11) is very sensitive to moisture and must be recrystallized from a mixture of acetonitrile and hexanes. Compound 3.lf is a rare example of a 2-pyridyl- substituted phosphaalkene and is of interest as a bidentate chelating ligand for transition metals. Previously reported pyridyl-substituted phosphaalkenes have employed the more bulky supermesityl substituent at phosphorus [e.g. Mes*P=CH(2py),5SMes*P=CH(2,6py)HC= PMes*,55 Mes*P=C(R)(2py) (R = H, SiMe3 ‘Bu; Mes* = 2,4,6-tri-tert-butylphenyl)j.59’4 SiMe3 ,R1 KOH Mes—R + O=C P=C + O(SIMe3)2 SiMe3 R2 (cat) Me R2 3.laR1=R2Ph 3.lb =4-FC6H 3.lc R1=Ph;R2=4-FC6H 3.ld2=4-(MeO)C 3.le R1=Ph; =4-(MeO)C 3.lf R1=Ph;R2=2-Pyridyl Scheme 3.3. The presence of two signals of equal intensity in the 31P NMR spectra of the phosphaalkene reaction mixtures suggests that the compounds 3.lc, 3.le, and 3.lf form as mixtures of E- and Z-isomers in approximately 1:1 ratios. X-ray structure determinations (discussed below) for single crystals of 3.le and 3.lf reveal that each mixture crystallizes as the E-isomer. Interestingly, the 31P NMR spectra of these crystals dissolved in CDC13,THF, or C6H show signals for both E- and Z- isomers, and therefore, it can be concluded that a facile cis/trans isomerization is taking place in solution. The cis/trans isomerization of phosphaalkenes has been observed previously; however, it is usually photochemical requiring UV-irradiation.54’6165-67 In the case of E-3.le and E-3.lf, 31P NMR spectroscopic studies in C6H reveal that isomerization occurs in the absence of light (Ca. 48 h to equilibrium), but is faster when solutions are exposed to sunlight (< 24 h to equilibrium). Interestingly, equilibrium is reached in just 30 mm when a C6H solution of E-3.le is irradiated with IJV light (> 290 nm, 25 °C). References start on page 80 Chapter Three 61 Table 3.1. X-ray crystallographic data for 3.lb, E-3.le, E-3.lf, and 3.3a+THFf1 compound 3.lb E-3.le E-3.lf 3.3a+THF formula C22H19PF C23H0P C21H0NP C50H8P20 fw 352.36 346.38 317.35 736.96 cryst syst triclinic monoclinic monoclinic monoclinic space group P 21/n P 21/c C2/c color yellow yellow yellow clear a(A) 9.3394(2) 12.5689(8) 11.016(1) 22.447(2) b (A) 14.1908(3) 10.4921(6) 17.254(2) 15.2803(9) c (A) 15.6207(3) 15.6160(10) 9.475(1) 13.533(1) a(deg) 69.626(8) 90.0 90.0 90.0 fl(deg) 71.699(8) 109.345(3) 107.89(1) 122.037(3) y(deg) 86.12(1) 90.0 90.0 90.0 V(A3) 1840.6(1) 1943.1(2) 1713.8(3) 3934.8(5) T(K) 173.0 173.0 173.0 173.0 Z 4 4 4 4 4u(Mo Ka) (cmj 1.69 1.48 1.60 1.49 cryst size (mm) 0.50 x 0.25 x 0.15 0.40 x 0.40 x 0.20 0.20 x 0.20 x 0.05 0.35 x 0.30 x 0.20 calcd density (Mg m3) 1.271 1.184 1.230 1.244 2max) (deg) 55.7 55.7 55.6 55.7 no.ofrflns 16453 37662 42105 18463 no.ofuniquedata 7452 4594 4044 4621 R(int) 0.036 0.0309 0.061 0.053 rfln/param ratio 16.52 20.33 19.08 15.32 Rib 0.041;I> 3cj(J) 0.0389;I> 2o(I) 0.043;I> 2u(I) 0.042;I> 2a(I) wR2(alldata)C 0.123 0.1204 0.124 0.112 GOP 1.07 1.091 1.05 1.00 a Reproduced with permission from Inorg. Chem. 2006, 45, 5225—5234. Copyright 2006 American Chemical Society. b Ri = IF0 — IFIIIIF0. wR2(F2 [all data]) = {[w(F02—)]/[w(F0}1/2 A search of the Cambridge Crystallographic Database found twenty P=C compounds with P-mesityl substituents of which several were metal complexes or (3, 5) systems. In order to further investigate the structural features and bonding in uncomplexed P-mesityl phosphaalkenes we have analyzed three representative compounds by X-ray crystallography. A summary of cell constants and data collection parameters for 3.lb, E-3.le, and E-3.lf are included in Table 3.1. The molecular structures of phosphaalkenes 3.1 b, E-3. le and E-3. if are shown in Figures 3.1, 3.2, and 3.3, respectively. Important metrical parameters for 3.lb, E-3.ie and E-3.lf are tabulated in Table 3.2 and, for comparison, the metrical parameters are also provided for the closely related phosphaalkenes 3.la,26’68 E-MesPCPh(2-’PrCH4),69MesPC(4-BrC6H4)(4-MeO,7°and 3.ld.7° References start on page 80 Chapter Three 62 Table 3.2. Important metrical parameters for phosphaalkenes bearing P-Mes and C-Ar substituents.’ Compound 3.la 3.la 3.lbc 3.ldc E-3.le E-3.lf Z-MesP=C E-MesP=C (4-BrC6H) (2-’PrC6H4)Ph (4-MeOC Bond lengths P=C 1.692(3) 1.693(2) 1.688(2) 1.698(3) 1.7082(13) 1.7043(16) 1.692(5) 1.682(2) 1.691(2) 1.696(3) PCMes 1.828(3) 1.830(2) 1.831(2) 1.821(3) 1.8418(13) 1.8378(16) 1.827(5) 1.835(2) 1.830(2) 1.820(3) CCirans 1.491(5) 1.493(5) 1.486(2) 1.481(3) 1.4884(17) 1.496(2) 1.482(7) 1.500(2) 1.49 1(2) 1.484(4) C—C1 1.487(4) 1.489(2) 1.484(2) 1.479(4) 1.4908(17) 1.498(2) 1.491(7) 1.482(2) 1.482(2) 1.486(4) Bond angles LCMesP=C 107.5(2) 107.6(2) 107.14(9) 108.8(13) 106.51(6) 107.80(7) 105.8(2) 106.20(8) 108.17(8) 108.8(13) LP=CCirans 116.2(2) 118.0(2) 117.8(1) 115.11(19) 115.66(9) 116.87(11) 115.9(3) 116.4(1) 114.9(1) 114.9(2) LP=C—C01 127.2(2) 124.8(2) 126.5(1) 128.01(19) 126.51(9) 125.41(11) 127.0(4) 128.5(1) 128.3(1) 127.6(2) LC,—C— 116.6(2) 117.1(3) 115.7(2) 116.8(2) 117.72(10) 117.48(13) 117.1(4) 115.1(1) Ctrans 116.8(1) 117.5(2) Angles between planes’ Mes 71 72.2 71.8 64.5 71.9 70.4 70.4 69.5 70.1 68.9 Ar,rans 36.6 21.4 45.1 45.8 33.7 22.4 37.4 66.6 32.9 41.4 Ar1. 42.9 59.2 54.8 37.1 49.0 57.6 46.5 47.9 56.0 37.1 Reference 26 68 This work 70 This work This work 70 69 a Reproduced with permission from Inorg. Chem. 2006, 45, 5225—5234. Copyright 2006 American Chemical Society. b The angle between the mean plane of the specified aryl ring-atoms to the mean plane Cjpso—P=C—(C,rans)(Ccis) atoms. Two independent molecules are present in the asymmetric unit. Data on the top line is for Molecule 1 the bottom line is for Molecule 2. The methoxy-substituted E-3.le and 2-pyridyl-substituted E-3.lf possess slightly longer P=C bond lengths [3.le, 1.7082(13) A; 3.lf, 1.7043(16) A] than in 3.la [1.692(3), 1.693(2) A] and 3.lb [1.688(2), 1.691(2) A]. Overall, the P=C bonds in P-mesityl phosphaalkenes are at the long end of the range typically found for C-substituted phosphaalkenes (1.61 — 1.71 A)7’ but are shorter than the PC bonds in inversely polarized phosphaalkenes (1.70 — 1.76 A).72 Interestingly, the P—CMeS bonds for the P-mesityl phosphaalkenes (ca. 1.83 A) are short compared with a typical P-C single bond (range: 1.85 — 1.90 A).73 The slight elongation of the PC bond References start on page 80 Chapter Three 63 and shortening of the PCMes bond suggests some it-conjugation between the Mes group and the P=C bonds. Figure 3.1. Molecular structure of MesP=C(4-FC6I-14)2(3.lb). Two virtually identical molecules appear in the asymmetric unit. Metrical parameters are given for one of the two molecules. Ellipsoids are drawn at the 50% probability level, hydrogen atoms are omitted for clarity. Selected bond lengths (A) and angles (°): P(1)-C(1) 1.688(2), P(1)-C(14) 1.83 1(2), C(1)-C(2) 1.486(2), C(1 )-C(8) 1.484(2), F( 1 )-C(5) 1.358(2), F(2)-C( 11) 1.363(2); C( 1 )-P(1)-C( 14) 107.14(9), P(1)-C(1)-C(2) 117.8(1), P(1)-C(1)-C(8) 126.5(1), C(2)-C(1)-C(8) 115.7(2). Reproduced with permission from Inorg. Chem. 2006, 45, 5225—5234. Copyright 2006 American Chemical Society. However, the large angles between the Mes and P=C planes in each phosphaalkene (ca. 710) are consistent with less it-conjugation between P=C and Mes than between the PC and Arfr03 (angle between planes: 2 1.4°— 45.1°). For comparison, the angles between the P=C bond and Ph planes in the C-H functional phosphaalkenes EMes*PCHPh74and E,E-PhHCPArPCHPh38are 14.2° and 22°, respectively. Of course, in addition to intramolecular electronic and steric effects, Fl C4 C3 C6 P1 Cl C20 C18 6 F2 References start on page 80 Chapter Three 64 the metrical parameters are also influenced by intermolecular crystal packing effects. Nevertheless, the data do support the notion that there is some ir-interaction between the aryl substituents and the P=C bond in P-Mes phosphaalkenes. C18 C22 Figure 3.2. Molecular structure of MesP=C(Ph)(4-MeOC6H4)(E-3.le). Ellipsoids are drawn at the 50% probability level, hydrogen atoms are omitted for clarity. Selected bond lengths (A) and angles (°): P(1)-C(1) 1.7082(13), P(1)-C(14) 1.8418(13), C(1)-C(2) 1.4884(17), C(1)-C(8) 1.4908(17); C(2)-C(l)-P(1) 115.66(9), C(8)-C(1)-P(l) 126.51(9), C(1)-P(l)-C(14) 106.51(6), C(2)-C(1)-C(8) 117.72(10), C(5)-O(1)-C(23) 117.86(13). Reproduced with permission from Inorg. Chem. 2006, 45, 5225—5234. Copyright 2006 American Chemical Society. C23 C4 C3 C6 P1 c1 Cl 6 C17 C12 Cli References start on page 80 Chapter Three 65 Figure 3.3. Molecular structure of MesP=C(Ph)(2-py) (E-3.lf). Ellipsoids are drawn at the 50% probability level, hydrogen atoms are omitted for clarity. Selected bond lengths (A) and angles (°): P(l)-C(1) 1.7043(16), P(1)-C(13) 1.8378(16), C(1)-C(2) 1.496(2), C(1)-C(7) 1.498(2); C(2)- C(1)-P(1) 116.87(11), C(7)-C(1)-P(1) 125.41(11), C(1)—P(1)—C(13) 107.80(7), C(2)—C(1)-C(7) 117.48(13). Reproduced with permission from Inorg. Chem. 2006, 45, 5225—5234. Copyright 2006 American Chemical Society. The bond angles at phosphorus, LC—PC in all the P-Mes phosphaalkenes are ca. 108°. This is consistent with a high degree of s-character in the lone pair on phosphorus with the a- bonds being higher in p-character. The geometry at Cl is essentially planar in each compound [sum of angles 360(1)° in each case]. It is interesting to note that Ar (LP=C—C: Ca. 127°) bends further away from the PC bond than Artrans (LPCCtrans ca. 116°). This fact reflects the greater steric congestion between the Mes and Ph groups in the former. C3 I Cl 7 C8 Cl 4 C9 C20 References start on page 80 Chapter Three 66 3.2.2 Attempted synthesis of P-adamantyl phosphaalkenes Thus far, we have shown that the base-catalyzed phospha-Peterson reaction is a clean and simple method to phosphaalkenes bearing P-mesityl substituents and C-aryl substituents. To extend the range of phosphaalkenes from P-aryl to P-alkyl substituents, we set out to prepare P adamantyl (P-Ad) phosphaalkenes using the base-catalyzed phospha-Peterson reaction. Therefore, under analogous conditions to those used to prepare 3.la—f, we treated AdP(SiMe3)2with benzophenone in the presence of a trace of KOH. The reaction was monitored by 31P NMR spectroscopy and signals were observed that could be attributed to unreacted AdP(SiMe3)2 (6 = —106), the desired phosphaalkene (3.2a) (6 286), and an unknown product later determined to be 3.3a (6= 28) (Scheme 3.4; AdP(SiMe)2:3.2a:3.3a = 35:37:28). Despite several attempts to improve the reaction conditions or the isolation procedures, 3.2a could not be generated quantitatively nor could it be separated from the other species. Ad Ad ,SiMe3 ,Ph cat. KOH ,PhAd—P + O=C P=c + I I S1Me3 Ph - (Me3Si)20 Ad Ph Ph, — Ph 3.2a 3.3a Scheme 3.4. Given these difficulties, we attempted to use the standard phospha-Peterson route (Scheme 3.1, Path H) to access 3.2a. The complete lithiation of AdP(SiMe3)2requires reflux conditions in THF (Scheme 3.5). It is more convenient to prepare AdP(Li)SiMe3(önp —96.9) [Figure 3.4(a)] from the room temperature reaction of AdP(H)SiMe3and MeLi (1 equiv). In contrast to the base catalyzed reaction described above, treating the lithium phosphide with benzophenone results in the clean, near quantitative, formation of 3.2a (&ip = 286) [see Figure 3.4(b)]. Unfortunately, References start on page 80 Chapter Three 67 3.2a is not isolable and slowly (ca. 48 h) dimerizes to 3.3a (6 28) according to 31P NMR spectroscopy [Figure 3 .4(c)J. SiMe3 Ad—Fç MeLi SiMe3 THF (refIuI 1) O=CPh2 Ad Ad - Me4Si ,Li 2) Me3SiCI ,Ph ca 48 hAd—F ,P=q SiMe3 - Lid Ad Ph I MeLi - (Me3Si)20 H 3.2a 3.3a Ad—F( THF (-78°C -> rt) SiMe3 -CH4 Scheme 3.5. (C) ._1L .-lt-. JJ-Ir 1111t.. TL-4 (b) .- (a). -... 300 250 200 150 100 50 0 -50 -100 ppm Figure 3.4. The reaction of AdP(Li)SiMe3and O=CPh2 in THF. (a) 31P NMR spectrum of AdP(Li)SiMe3in THF (prepared from AdP(H)SiMe3+ MeLi); (b) 31P NMR spectrum of 3.2a (6 = 286) recorded immediately after O=CPh2was added to AdP(Li)SiMe3(c) 31P NMR spectrum of 3.3a ( = 28) formed from 3.2a after 2 days in THF. Reproduced with permission from Inorg. Chem. 2006, 45, 5225—5234. Copyright 2006 American Chemical Society. The molecular structure of the 1 ,2-diphosphetane (3.3a), formed from the dimerization of 3.2a, was confirmed using X-ray crystallography (Figure 3.5). The [2+21-cycloaddition of phosphaalkenes is often observed when insufficient thermodynamic and/or kinetic stability is conferred to the P=C bond although formally the dimerization is symmetry forbidden.53’62571578 References start on page 80 Chapter Three 68 Phosphaalkenes have been observed to dimerize with both head-to-tail (to 1,3-diphosphetanes) and head-to-head (to 1 ,2-diphosphetanes) regiochemistry with head-to-tail cycloaddition being most common. However, it has been proposed that phosphaalkenes bearing larger P- than C substituents will favor head-to-head dimerization since the long P—P bonds and short C—C bonds in the 1 ,2-diphosphetane will reduce intramolecular steric repulsions better than in a 1,3- diphosphetane containing four intermediate length P—C bonds.75 The observed head-to-head dimerization of 3.2a, which possesses bulkier P- than C-substituents (Ad vs. Ph), is consistent with these arguments. C20* Figure 3.5. Molecular structure of (AdPCPh2)(3.3a). The structure contains THF of crystallization. The THF and all hydrogen atoms are omitted for clarity. Ellipsoids are drawn at the 50% probability level. Selected bond lengths (A) and angles (°): P(l)P(l)* 2.1888(8), P(l) C(l) 1.9452(15), C(1)_C(l)* 1.622(3), P(1)-C(14) 1.8929(16); C(l)_P(l)_P(1)* 79.04(4), P(1)- C(l)_C(l)* 94.93(5), C(14)-P(1)-C(l) 118.79(7), C(14)_P(l)_P(l)* 107.52(5), C(2)-C(l)-P(l) 113.41(10), C(8)—C(1)-P(1) 109.41(10), C(2)_C(l)_C(1)* 114.27(13), C(8)_C(1)_C(l)* 114.65(13), C(8)-C(1)-C(2) 109.49(12). Reproduced with permission from Inorg. Chem. 2006, 45, 5225—5234. Copyright 2006 American Chemical Society. C5* References start on page 80 Chapter Three 69 1 ,2-Diphosphetane 3.3a crystallizes with a molecule of THF in the unit cell; however, there is no close contact between the THE and 3.3a. The two P-C units of the non-planar P2C ring in 3.3a are related by a two-fold rotation axis with the P-Ad substituents in an anti- configuration to minimize steric repulsion. Interestingly, the ring bond lengths in 3.3a [P(l)-C(1), 1.9452(15) A, C( 1 )-C( 1) *, 1.622(3) A] are significantly longer than typical P—C and C—C single bonds (1.85, 1.55 A, respectively) and are longer than those found in the seven related compounds with l,2-PCrings [range(A), average(A): P—C, 1.85 — 1.93, avg. 1.90; C—C, 1.47— 1.62, avg. 1.55].737985 In contrast, the P—P bond [P(1)P(1)*, 2.1888(8) A] is slightly shorter than a typical P—P bond (2.22 A) and shorter than those found in other 1,2-diphosphetanes [range(A), avg.(A): P—P, 2.20—2.25, avg. 2.23].737985 The exocyclic P—CAd bond [P(1)-c(14) = 1.8929(16) A] is considerably shorter than the aforementioned endocyclic P—C bonds [P(1)-c(l), 1.9452(15) A]. Presumably, these bond lengths reflect the strain of the four-membered ring, consistent with this notion of ring strain in 3.3a are the very small ring bond angles at phosphorus [C(1)P(l)P1* = 79.04(4)°] and carbon {P(1)c(1)C(1)* = 94.93(5)°]. Table 3.3. Synthesis and the 31P NMR chemical shifts of phosphaalkenes 3.1 and 3.2, RP=cRIRh?.a compound R R’ R” 31P NMR (ppm) 3.la Mes Ph Ph 233 3db Mes 4-Fc6I 1 4-FC6H 234 3.lc Mes Ph 4-FC6H 234 (Z), 233 (E) 3.ld Mes 4-MeOc6H 4-MeOC 217 3.le Mes Ph 4-MeOc6H 226 (Z), 224(E) 3.lf Mes Ph 2-py 260 (E), 242 (Z) 3.2a Ad Ph Ph 286 3.2b Ad Me Me 247 3.2c Ad Mes H 299, 293 3.2d Ad ‘Bu tBu not formed a Reproduced with permission from Inorg. Chem. 2006, 45, 5225-5234. Copyright 2006 American Chemical Society. References start on page 80 Chapter Three 70 Given that 3.2a dimerizes readily, we attempted to prepare P-adamantyl phosphaalkenes bearing varying degrees of steric bulk at carbon with the objective of finding an isolable species. The success of each reaction was measured by monitoring each reaction using 31P NMR spectroscopy and the spectroscopic data are shown in Table 3.3. In two instances, 3.2b and 3.2c phosphaalkenes were detected; however, these species rapidly decomposed or seif-oligomerized during attempted purification. Attempts to prepare the phosphaalkene 3.2d bearing bulky C-tBu substituents were unsuccessful as AdP(Li)SiMe3did not react with this very hindered ketone. We conclude that P-adamantyl phosphaalkenes bearing solely C-aryl or C-alkyl substituents will be difficult to isolate at ambient temperature. To our knowledge, the only isolable P-adamantyl phosphaalkene is the C-heteroatom-substituted [AdP=C(OSiMe3)’Bu]86 which was prepared using the Becker reaction (Scheme 3.1, Path A). Based on the fact that 3.la (P-Mes) is isolable and 3.2a (P-Ad) readily dimerizes to 3.3a, it is tempting to conclude that the it-conjugation between the Mes group and the PC bond is a key factor in the higher stability of 3.la compared to 3.2a. In addition, Mes may provide better steric protection to the P=C bond than Ad. In contrast, the opposite trend in stability is observed for phosphaalkynes; namely, AdCP can be stored indefinitely at ambient temperature,87 whilst MesCP decomposes slowly at 25 °C.3 These observations reinforce the delicate balance of steric and electronic factors which affect the stability of low-coordinate phosphorus compounds. 3.3 Summary We have shown that the base-catalyzed phospha-Peterson reaction is a general and convenient synthetic route to P-mesityl phosphaalkenes bearing C-aryl substituents. These compounds have been thoroughly characterized and 31P NMR spectroscopic analysis of E-3.le References start on page 80 Chapter Three 71 suggests that a facile thermal or photochemical E-/Z-isomerization occurs in solution. Interestingly, the X-ray crystal structures of 3.lb, E-3.le and E-3.lf are consistent with some it- conjugation between the P=C bond and the aryl substituents. Attempts to extend the base- catalyzed phospha-Peterson reaction to the preparation of P-adamantyl phosphaalkenes were unsuccessful as the intermediate phosphaalkenes were observed to seif-oligomerize on workup. In one instance, a 1 ,2-diphosphetane dimer (3.3a) was isolated and structurally characterized. The P-mesityl phosphaalkenes reported herein are attractive monomers for addition polymerization studies. This investigation will be reported in the next chapter. 3.4 Experimental section General procedures. All manipulations of air and/or water sensitive compounds were performed under a nitrogen atmosphere using standard Schienk or glovebox techniques. Hexanes and dichloromethane were deoxygenated with nitrogen and dried by passing through a column containing activated alumina. THF was freshly distilled from sodium!benzophenone ketyl. Distilled water was degassed prior to use. CDCI3(CIL), C6H and acetonitrile were distilled from P205and degassed. Benzophenone (Aldrich) was sublimed prior to use. KOH and NaOH were made anhydrous by following a literature procedure (recrystallization from EtOH and subsequent heating in vacuo) •88 4,4’-Difluorobenzophenone, 4-fluorobenzophenone, 4,4’- dimethoxybenzophenone, 4-methoxybenzophenone, 2-benzoylpyridine, and mesitaldehyde were purchased from Aldrich and used as received. C6D was used as received. MesP(SiMe3)2,53 AdPH2,89 and OC(EBu)290 were prepared following literature procedures. Equipment. ‘H, 31P, ‘9F, and‘3C{’H} NMR spectra were recorded at room temperature on Bruker Avance 300 MHz or 400 MHz spectrometers. Chemical shifts are reported relative to: References start on page 80 Chapter Three 72 residual CHC13(6 = 7.24 for 1H); C6D5H (6 = 7.15 for ‘H); 85% H3P04as an external standard (6 0.0 for 31P); CFC13 in CDC13 as an external standard (6= 0.0 for ‘9F); CDC13(6= 77.0 for ‘3C{’H}) and C6D (6= 128.1 for‘3C{’H}). Assignments of NMR spectra were made with the aid of ‘H-’H COSY, ‘H-’3C HMQC, and ‘H-13C HMBC experiments. The E- and Z-isomers of 3.lc, 3.le, and 3.lf were assigned with the aid of previously published spectral data and X-ray crystal structural data obtained in this work.54’66 Elemental analyses were performed in the UBC Chemistry Microanalysis Facility. Mass Spectra were recorded on a Kratos MS 50 instrument in El mode (70 eV). Samples of E-3.le were irradiated using a 450 W medium pressure mercury lamp equipped with a Pyrex filter (Corning #7740, transmits> 290 nm). 3.4.1 Preparation of MesP=CPh2(3.la). The experimental procedure for 3.la was reported in the last chapter (Section 2.4.1, p.47). To avoid the time consuming distillation, the KOH can be removed from 3.la by extracting a CH21 solution of the crude product with degassed water (3x), drying the organic layer with Mg504,followed by recrystallization from hexanes (3x). Yield: 72%. The spectroscopic characterization of 3.la was the same as those reported previously in Section 2.4.1. 3.4.2 Preparation of MesP=C(4-FC6H4)2(3.lb). MesP(SiMe32 (13.1 g, 0.044 mol), 4,4’-difluorobenzophenone (8.5 g, 0.039 mol) and H3C finely ground anhydrous KOH (4 mg, 0.07 mmol) were mixed and then dissolved in THF (150 mL). The mixture was stirred and the reaction progress was monitored by 31P NMR spectroscopy. After 1 d F References start on page 80 Chapter Three 73 compound 3.lb was formed quantitatively (6 234) and the solvent was removed in vacuo. The solid residue was dissolved in hexanes (100 mL), filtered, and the solvent was removed in vacuo leaving a yellow oil. The crude product was purified by vacuum distillation (bp = 120 — 140 °C; 0.01 mmHg). Yellow crystals suitable for X-ray diffraction were obtained from slow evaporation of a hexanes solution. Yield: 6.2 g (45%). 31P NMR (CDC13): 6234 (s); ‘9F NMR (CDC13): 6—112.8, —113.3; ‘H NMR (CDC13): 67.48 (m, 2H, o-cis-Ar), 7.01 (dd, 3Jin,i 8 Hz, 3JFH = 8 Hz, 2H, rn-cis-Ar), 6.82 (m, 2H, o-trans-Ar), 6.74 (dd, 3JHH = 9 Hz, 3JFH = 9 Hz, 2H, rn-trans-Ar), 6.72 (s, 2H, rn-Mes), 2.25 (s, 6H, o-CH3), 2.21 (s, 3H,p-CH);‘3C{’H} NMR (CDC13): 6190.7 (d, ‘Jpc = 44Hz, P=C), 163.6 (dd, 5Jpc = 5 Hz, ‘JFC = 249 Hz,p-cis-Ar), 162.0 (d, 1JFC = 248 Hz, p-trans-Ar), 140.8 (d, 2Jpc = 25 Hz, i-cis-Ar), 140.2 (d, 2Jpc = 7 Hz, o-Mes), 139.1 (d, 2Jpc = 15 Hz, i-vans-Ar), 138.7 (s,p-Mes), 135.9 (d, 1Jpc = 43 Hz, i-Mes), 130.4 (dd, 3Jpc = 7 Hz, 3JFC = 7 Hz, o-irans-Ar), 129.3 (dd, 3Jpc = 19 Hz, JFC = 8 Hz, o-cis-Ar), 128.4 (s, rn-Mes), 115.2 (d, 2JFC = 22 Hz, rn-cis-Ar), 114.5 (d, .JFC = 22 Hz, m-trans Ar), 22.1 (d, 3Jpc = 9 Hz, o-CH3), 21.0 (s,p-CH3);MS (El, 70eV): 353, 352 [24, 100; Mj; 351 [43; M— H]; 258, 257 [4, 20; M— 4-F-C6H];256 [26, M— 4-F-C6H+H]; 203 [46; M—MesP + H]. Anal. Calcd forC22H19FP:C, 74.99; H, 5.44. Found: C, 74.77; H, 5.36. F 3.4.3 Preparation of MesPC(Ph)(4-FC6H4)(3.lc). MesP(SiMe32 CH3 0 (12.7 g, 0.043 mol), 4-fluorobenzophenone (8.6 g, 0.043 mol) and H3C / PC anhydrous KOH (6 mg, 0.1 mmol) were mixed as solids and then — dissolved in THF (80 mL). The mixture was stirred and the reaction progress was monitored by 31P NMR spectroscopy. After 4 d compound 3.lc was formed quantitatively (6 = 234, 233) and the solvent was removed in vacuo. The solid residue was References start on page 80 Chapter Three 74 dissolved in hexanes (50 mL), filtered and the solvent was removed in vacuo leaving a dark red oil. The crude product was purified by vacuum distillation (170— 180 °C, 0.01 mmHg). The product is a viscous yellow oil which does not solidify on standing. Yield: 8.3 g (58%). 31P NMR (CDC13): 6234 (Z), 233 (E); ‘9F NMR (CDC13)(E/Z-mixture): 6—113.8, —114.6; ‘H NMR (CDC13)(E/Z-mixture): 6 7.5 — 6.7 (m, 1 1H, Ar), 2.23 (s, 6H o-CH3), 2.20, 2.18 (s, 3H, p CH3); MS (El, 70eV): 336, 335, 334 [3, 22, 100; M]; 333 [51; M — H]; 186, 185 [9, 45; M— MesP + H]. Anal. Calcd forC22H0FP: C, 79.03; H, 6.03. Found: C, 78.79; H, 5.87. OMe 3.4.4 Preparation of MesP=C(4-MeOC6H4)2(3.ld). CH3 0 MesP(SiMe3)2(13.0 g, 0.044 mol), 4,4’-dimethoxybenzophenone H3C / PC (10.6 g, 0.050 mol) and anhydrous KOH (0.10 g, 1.8 mmol) were CH / \ — mixed as solids and then dissolved in THF (150 mL). The reaction OMe mixture was stirred and the reaction progress was monitored by 31P NMR spectroscopy. After 2 d compound 3.ld was formed quantitatively (6 = 217) and the solvent was removed in vacuo. The solid residue was extracted with hexanes (3x50 mL), filtered and solvent was removed in vacuo leaving a yellow solid. The crude product was purified by recrystallization from a mixture of hexanes and THF at —70°C. Yield: 8.0 g (49%) 31P NMR(CDC1):6217; 111 NMR (CDC13): 67.61 (d,3JHH = 7 Hz, 2H, o-cis-Ar), 6.97 (d, HH = 7 Hz, 2H, m-cis-Ar), 6.94 (d, HH = 8 Hz, 2H, o-trans-Ar), 6.83 (s, 211, m-Mes), 6.68 (d, 3J = 9 Hz, 2H, rn-trans-Ar), 3.87 (s, 3H, Z-OCH3),3.73 (s, 3H, E-OCH3),2.40 (s, 611, o-CH3),2.30 (s, 3H, p-CH3); ‘C{’H} NMR (CDC13): 6 192.1 (d, 1J = 44Hz; P=C), 160.4 (d, 5J = 4 Hz,p-cis-Ar), 158.8 (s,p-trans-Ar), 140.2 (d, 2J = 7 Hz, o-Mes), 137.8 (s,p-Mes), 137.7 (d,2J= 25 Hz, i-cis Ar), 136.7 (d, ‘J = 43 Hz, i-Mes), 136.0 (d,2J= 15 Hz, i-trans-Ar), 130.1 (d, 3J = 6 Hz, o trans-Ar), 129.0 (d, 3J, = 19 Hz, o-cis-Ar), 128.1 (s, m-Mes), 113.4 (s, rn-cis-Ar), 112.6 (s, rn References start on page 80 Chapter Three 75 trans-Ar), 55.0 (s, Z-OCH3),54.7 (s, E-OCH3),22.1 (d, 3J = 9 Hz, o-CH3),20.9 (s, p-Cl-I3);MS (El, 70eV): 378, 377, 376 [4, 27, 100; Mj; 375 [30; M — H]; 362, 361 [4, 17; M— CH3]; 270, 269, 268 [4, 28, 93; M—4-MeOC6H4— H]; 228, 227 [6, 28; M— MesP + H]. Anal. Calcd for C24H50P: C, 76.58; H, 6.69. Found: C, 76.30; H, 6.71. OMe 3.4.5 Preparation of MesP=C(Ph)(4-MeOC6H4)(3. le). CH3 0 MesP(SiMe3)2(4.05 g, 0.016 mol), 4-methoxybenzophenone H3C / PC (2.90 g, 0.014 mol) and anhydrous KOH (4 mg, 0.07 mmol) were CH3 — mixed as solids. The solids were dissolved in THF (50 mL) and stirred for 1 d. Analysis of an aliquot removed from the reaction mixture by 31P NMR spectroscopy showed the quantitative formation of phosphaalkenes (6= 226, 224). After solvent removal, the solid residue was extracted in hexanes (3x20 mL), filtered and solvent removed in vacuo revealing a yellow liquid. The crude product was purified by distillation at 189 °C (0.01 mmHg). Yellow crystals suitable for X-ray diffraction were obtained from slow evaporation of a hexanes solution. Yield: 2.05 g (43%). 31p NMR (CDC13): 6226 (Z), 224 (K); ‘H NMR (CDC13)(E-isomer): 67.48 (dd, JHH =9 Hz, 4Jp,-i = 3 Hz, 2H; o-trans-Ph), 7.11-7.01 (m, 3H; m,p-cis-Ph), 6.87 (d, 3JHH = 7 Hz, 2H, o-cis-Ph), 6.84 (d, 3JHH = 9 Hz, 2H; rn-trans-Ph), 6.68 (s, 2H; m-Mes), 3.82 (s, 3H; -OCH3), 2.26 (s, 6H; o-CH3), 2.18 (s, 3H;p-CH);‘3C{’H} NMR (CDC13)(E-isomer): 6 192.9 (d, ‘Jpc = 44 Hz; P=C), 160.6 (d, 5Jpc = 4 Hz; p-Ar), 143.3 (d, 2Jpc = 14 Hz; i-Ar), 140.4 (d, 2Jpc = 7 Hz; o-Mes), 138.2 (s,p-Mes), 137.7 (d, 2Jpc = 25 Hz; i-Ph), 136.4 (d, ‘Jpc = 43 Hz; i-Mes), 128.9 (d, 3Jpc = 20 Hz; a-Ph), 128.6 (d, 3Jpc = 7 Hz; 0-Ar), 128.1 (s, rn-Mes), 127.3 (s, rn,p-Ph), 113.6 (s, rn-Ar), 55.4 (s, -OCH3),22.3 (d, 3Jpc = 9Hz, o-Cl-13), 21.0 (s,p-CH3);MS (El, 70eV): 348, 347, 346 [3, 24, 100; M]; 345 [27; References start on page 80 Chapter Three 76 M — H]; 331 [7; M— CH3]; 269 [6; M— Ph]; 240, 239, 238 [4, 16, 72; M— 4-MeOC6HH]; 198, 197 [6, 30; M— MesP + H]. Anal. Calcd (%) forC23H0P: C, 79.75; H, 6.69. Found: C, 80.15; H, 6.35. 3.4.6 Preparation of MesP=C(Ph)(2-py) (3.11). MesP(SiMe32(16.2 CH3 N ‘) H — g, 0.055 mol), 2-benzoylpyridine (10.0 g, 0.055 mol) and anhydrous CH3 NaOH (10 mg, 0.25 mmol) were mixed. The solids were dissolved in THF (100 mL) and stirred for 3 d. Analysis of an aliquot removed from the reaction mixture by 31P NIvfR spectroscopy showed the quantitative formation of phosphaalkenes ((5= 260, 242). After solvent removal, the solid residue was extracted into hexanes (50 mL), filtered and solvent removed in vacuo revealing a yellow green solid. The crude product was purified by slow evaporation of an acetonitrile:hexanes (3:1) mixture to afford yellow crystals. Yield: 8.3 g (48%) 31P NMR (CDC13): (5260 (E), 242 (Z); ‘H NMR (CDC13)(E/Z-mixture): (58.67 (d, 3HH =4 Hz, 1H; E-o-py), 8.46 (d, 3HH =4 Hz, 1H; Z-o-py), 7.6— 6.7 (br m, 16H; E/Z-ArIPh), 6.68,6.67 (s, 4H; E,Z-m-Mes), 2.28, 2.26 (s, 12H; Z,E-o-CH3),2.17,2.16 (s, 611; E,Z-p-CH3);MS (El, 70eV): 319, 318, 317 [3, 24, 100; Mj; 316 [9; M — H]; 303, 302 [3, 11; M— CH3]; 241, 240 [7, 40; M— Ph]; 239 [14; M—C6H]; 226 [34; M — py — H]; 169, 168, 167 [32, 97; M— MesP + H]; 167 [32; M — MesP]; 149 [33; MesP + H]. Anal. Calcd forC21H0NP: C, 79.47; H, 6.35; N, 4.41. Found: C, 79.38; H, 6.35; N, 4.36. 3.4.7 Preparation of AdP(SiMe3)2.To a solution of AdPH2 (3.45 g, - Si Me3 20.5 mmol) in THF (40 mL) was added MeLi (1.4 M, 58 mL, 81 mmol) at SiMe3 —78 °C. The reaction mixture was slowly warmed to room temperature whereupon it was stirred References start on page 80 Chapter Three 77 for a further 30 mm. The resultant dark orange solution was cooled to —78 °C, Me3SiC1 (11 mL) was added, and the reaction mixture was again warmed to room temperature. An aliquot was taken for analysis by 31P NMR spectroscopy [AdP(SiMe3)2:6 = —106 (s)]. Often after the first silylation step, we observed a mixture of AdP(SiMe3)2,AdP(H)SiMe3and AdPH2.In this instance, the solvent/Me3SiC1 was removed in vacuo, and the resultant mixture was redissolved in freshly distilled THF, relithiated and silylated following the above procedure. Typically, after one relithiation AdP(SiMe3)2was formed quantitatively (according to 31P NMR). The solvent was then removed in vacuo and the yellow solid residue was extracted with hexanes (3x50 mL), filtered and the solvent removed. The crude product (a colorless liquid) was purified by distillation (bp = 110 °C, 5 mmHg). Yield: 4.68 g (73%) 31P NMR (C6D): 6 —106 (s); ‘H NMR (C6D): 62.04 (m, 6H; 2-Ad), 1.83 (s, 3H; 3-Ad), 1.62 (s, 6H; 4-Ad), 0.35 (d, 3JpH = 4Hz, 18H; CH3);‘3C{’H} NMR (C6D): 646.4 (d, 2Jpc = 8 Hz; 2-Ad), 36.8 (s; 4-Ad), 35.2 (d, 1Jpc 14 Hz; 1-Ad), 29.9 (d, 3Jpc = 8 Hz; 3-Ad); 3.89 (d, 2Jpc = 12 Hz, CH3). 3.4.8 Preparation of AdP(H)SiMe3.To a solution of AdPH2 (1.16 g, 6.9 ..Si Me3 mmol) in Et2O (20 mL) was added MeLi (1.5 M, 7 mL, 10.5 mmol) at H —78 °C. The reaction mixture was slowly warmed to room temperature whereupon it was stirred for a further 45 mm. The resultant yellow suspension was cooled to —78 °C, Me3SiCl (1.5 mL, 12 mmol) was added, and the reaction mixture was again warmed to room temperature for 30 mm. An aliquot was taken for analysis by 31P NMR spectroscopy [AdP(H)SiMe3:6 —84.7 (d)j. The solvent was then removed in vacuo and the yellow solid residue was extracted with hexanes References start on page 80 Chapter Three 78 (3x20 mE), filtered and the solvent removed in vacuo. The crude product (a colorless liquid) was purified by distillation (bp = 80 °C, 5 mmHg). Yield: 1.19 g (72%) 31P NMR (C6D): 6 —84.7 (d, 1PH = 194 Hz) ; ‘H NMR (C6D): 62.32 (d, 1pH = 194 Hz, lH; PR), 1.88 (m, 6H; 2-Ad), 1.80 (s, 3H; 3-Ad), 1.59 (s, 611; 4-Ad), 0.25 (d, 3JpH = 4 Hz, 9H; CH3). 3.4.9 Preparation of AdP(Li)SiMe3.Method A: To a solution of .SiMe3 AdP(SiMe3)2(1.66 g, 5.3 mmol) in THF (25 mE) was added MeLi (1.4 M, Li 7.1 mL, 9.9 mmol). The reaction mixture was heated to reflux overnight. The quantitative formation of AdP(Li)SiMe3was confirmed by 31P NMR [6 = —96.9 (s)]. Method B: To a solution of AdP(H)SiMe3(1.2 g, 5.0 mmol) in THF (20 mL) was added MeLi (1.5 M, 4.0 mL, 6.0 mmol) at —78°C. The reaction mixture was warmed to room temperature and the quantitative formation of AdP(Li)SiMe3was established by 31P NMR [6 = —96.9 (s) 1. c;\ 3.4.10 Preparation of (AdPCPh)2(3.3a). To a solution ofpp AdP(Li)SiMe3(4.9 mmol) was added dropwise a solution of PhPh Ph Ph O=CPh2 (1.08 g, 5.9 mmol) in THF (10 mL). An aliquot of the dark orange reaction mixture was removed and analyzed by 31P NMR spectroscopy. A singlet resonance was observed at 286 ppm suggesting quantitative formation of a phosphaalkene; however, over a few hours a second signal at 28 ppm was detected in addition to the signal assigned to phosphaalkene 3.2a. The reaction mixture was quenched with Me3SiC1 (0.76 mL, 6.0 mmol). Over a period of ca. 2 d the 31P NMR spectrum of the reaction mixture showed only one signal at 28 ppm. After solvent removal, the solid residue was extracted with hexanes (3x20 mL), filtered and the solvent was removed in vacuo affording a yellow oil. Yellow crystals suitable for References start on page 80 Chapter Three 79 X-ray diffraction were obtained by slow evaporation of a THF solution. Yield: 0.71 g (43%) 31P NMR (CDC13): 628 (s); ‘H NMR (CDC13): 6 8.17 (d, 3JHH = 8Hz, 4H; o-Ph), 7.48 (t, 3JHJ-, = 7 Hz, 4H; rn-Ph), 7.11 (t, .JHH 7Hz, 4H; rn-Ph), 6.82 (m, 4H;p-Ph), 6.54 (d, 4H; o-Ph), 1.8-1.4 (m, 30H; Ad); MS (70eV, El): 665, 664 [0.10, 0.17; M]; 333, 332 [3, 12; AdPCPh], 168, 167 [10, 50; (AdPH) or (HCPh2)j, 136, 135 [14, 100; Adj. RT 3.4.11 Attempted preparation of AdP=CR’ R” (3.2b-d). To a cooled P=c R” (—78 °C) solution of AdP(Li)SiMe3(ca. 1 mmol) in THF (5 mL) was added dropwise a solution of ketone/aldehyde (1 equiv) in THF. An aliquot was immediately removed from the reaction mixture and was analyzed by using 31P NMR spectroscopy. The NMR spectroscopic data are summarized in Table 3.3. None of these phosphaalkenes were isolable. X-ray crystallography. All single crystals were immersed in oil and were mounted on a glass fiber. Data were collected on a RigakuJADSC CCD diffractometer (3.lb and 3.3a) or a Bruker X8 APEX diffractometer (E-3.le and E-3.lf) with graphite monochromated Mo Ka radiation. All structures were solved by direct methods and subsequent Fourier difference techniques and refined anisotropocally for all non-hydrogen atoms. All data sets were corrected for Lorentz and polarization effects. All calculations on crystal 3.lb and 3.3a were performed using the teXsan91 crystallographic software package of Molecular Structure Corporation, while all refinements ofE 3.le and E-3.lf were performed using the SHELXTL92 crystallographic software package of Bruker-AXS. Compound E-3.lf is disordered by a 180° rotation about the C1—C2 bond. The disorder was modeled by placing partial nitrogen and carbon in both positions ortho to C2 (using Shelxl References start on page 80 Chapter Three 80 EXYZ and EADP functions) and refining their respective populations [occupancy 0.84(1) and 0.16(1)]. Compound 3.3a crystallizes with a half-molecule of THF (on a C2 axis) and the adamantyl group was disordered. The disorder was modeled in two orientations with relative populations of 0.86 and 0.14 for the major and minor fragments, respectively. Additional crystal data and details of the data collection and structure refinement are given in Table 3.1. 3.5 References 1. Tsang, C.-W.; Yam, M.; Gates, D. P. 1 Am. Chem. Soc. 2003, 125, 1480. 2. Brym, M.; Jones, C. Dalton Trans. 2003, 3665. 3. Mack, A.; Pierron, E.; Allspach, T.; Bergstrafier, U.; Regitz, M. Synthesis 1998, 1305. 4. Pietscbnig, R.; Niecke, E.; Nieger, M.; Airola, K. I Organomet. Chem. 1997, 529, 127. 5. GrUnhagen, A.; Pieper, U.; Kottke, T.; Roesky, H. W. Z Anorg. Aug. Chem. 1994, 620, 716. 6. Martens, R.; du Mont, W.-W. Chem. Ber. 1992, 125, 657. 7. Haber, S.; Boese, R.; Regitz, M. Angew. Chem., mt. Ed. Engi. 1990, 29, 1436. 8. Issleib, K.; Schmidt, H.; Leissring, E. I Organomet. Chem. 1990, 382, 53. 9. Appel, R.; Hünerbein, J.; Knoch, F.; Nieger, M. I Organomet. Chem. 1988, 346, 379. 10. Appel, R.; Foiling, P.; Schubn, W.; Knoch, F. Tetrahedron Lett. 1986, 27, 1661. 11. Weber, L.; Reizig, K.; Boese, R.; Polk, M. Organometallics 1986, 5, 1098. 12. Weber, L.; Reizig, K.; Frebel, M.; Boese, R.; Polk, M. I Organomet. Chem. 1986, 306, 105. 13. Weber, L.; Reizig, K.; Frebel, M. Chem. Ber. 1986, 119, 1857. 14. Becker, G.; Becker, W.; Uhl, G. Z Anorg. Aug. Chem. 1984, 518, 21. References start on page 80 Chapter Three 81 15. Becker, G.; Mundt, 0. Z Anorg. Aug. Chem. 1978, 443, 53. 16. Becker, G. Z. Anorg. Aug. Chem. 1977, 430, 66. 17. Becker, 0. Z Anorg. Aug Chem. 1976, 423, 242. 18. Klebach, T. C.; Lourens, R.; Bickeihaupt, F. J Am. Chem. Soc 1978, 100, 4886. 19. Cornet, S. M.; Dillon, K. B.; Goeta, A. E.; Howard, J. A. K.; Roden, M. D.; Thompson, A. L. .J Organomet. Chem. 2005, 690, 3630. 20. Toyota, K.; Kawasaki, S.; Yoshifuji, M. Tetrahedron Lett. 2002, 43, 7953. 21. Decken, A.; Carmalt, C. J.; Clyburne, J. A. C.; Cowley, A. H. Inorg. Chem. 1997, 36, 3741. 22. Gaumont, A.-C.; Pellerin, B.; Cabioch, J.-L.; Morise, X.; Lesvier, M.; Savignac, P.; Guenot, P.; Denis, J.-M. Inorg. Chem. 1996, 35, 6667. 23. Karsch, H. H.; Rupprich, T.; Heckel, M. Chem. Ber. 1995, 128, 959. 24. Cowley, A. H.; Nunn, C. M.; Pakulski, M. Polyhedron 1989, 8, 2087. 25. Boyd, B. A.; Thoma, R. J.; Watson, W. H.; Neilson, R. H. Organometallics 1988, 7, 572. 26. Van der Knaap, T. A.; Klebach, T. C.; Visser, F.; Bickelhaupt, F.; Ros, P.; Baerends, E. J.; Stam, C. H.; Konijn, M. Tetrahedron 1984, 40, 765. 27. Cowley, A. H.; Jones, R. A.; Lasch, J. 0.; Norman, N. C.; Stewart, C. A.; Stuart, A. L.; Atwood, J. L. I Am. Chem. Soc. 1984, 106, 7015. 28. Van Der Knaap, T. A.; Bickelhaupt, F. Chem. Ber. 1984, 117, 915. 29. Appel, R.; Peters, J.; Westerhaus, A. Tetrahedron Lett. 1981, 22, 4957. 30. Appel, R.; Casser, C.; Immenkeppel, M.; Knoch, F. Angew. Chem. mt. Ed Engl. 1984, 23, 895. References start on page 80 Chapter Three 82 31. Tsang, C.-W.; Robrick, C. A.; Saini, T. S.; Patrick, B. 0.; Gates, D. P. Organometallics 2004, 23, 5913. 32. Grobe, J.; Le Van, D.; Lüth, B.; Hegemami, M. Chem. Ber. 1990, 123, 2317. 33. Appel, R.; Immenkeppel, M. Z Anorg. Aug. Chem. 1987, 553, 7. 34. lonkin, A. S.; Arbuzov, B. A. Bull. Acad Sci. USSR, Div. Chem. Sd. 1990, 39, 1489. 35. Romanenko, V. D.; Ruban, A. V.; Chernega, A. N.; Povolotskii, M. I.; Antipin, M. Y.; Struchkov, Y. T.; Markovskii, L. N. I Gen. C’hem. USSR. 1989, 59, 1528. 36. Romanenko, V. D.; Ruban, A. V.; Povolotskii, M. I.; Polyachenko, L. K.; Markovskii, L. N. I Gen. Chem. US.R. 1986, 56, 1044. 37. Oehme, H.; Leissring, E.; Meyer, H. Tetrahedron Lett. 1980, 21, 1141. 38. Shah, S.; Concolino, T.; Rheingold, A. L.; Protasiewicz, J. D. Inorg. Chem. 2000, 39, 3860. 39. Shah, S.; Protasiewicz, J. D. Chem. Commun. 1998, 1585. 40. Shah, S.; Protasiewicz, J. D. Coord Chem. Rev. 2000, 210, 181. 41. Shah, S.; Simpson, M. C.; Smith, R. C.; Protasiewicz, J. D. I Am. Chem. Soc. 2001, 123, 6925. 42. Le Floch, P.; Marinetti, A.; Ricard, L.; Mathey, F. I Am. Chem. Soc. 1990, 112, 2407. 43. Le Floch, P.; Mathey, F. Synlett 1990, 171. 44. Marinetti, A.; Bauer, S.; Ricard, L.; Mathey, F. Organometallics 1990, 9, 793. 45. Marinetti, A.; Mathey, F. Angew. Chem. mt. Ed. Engi. 1988, 27, 1382. 46. Cummins, C. C.; Scbrock, R. R.; Davis, W. M. Angew. Chem. mt. Ed Engi. 1993, 32, 756. 47. Breen, T. L.; Stephan, D. W. I Am. Chem. Soc. 1995, 117, 11914. References start on page 80 Chapter Three 83 48. Termaten, A. T.; Nijbacker, T.; Schakel, M.; Lutz, M.; Spek, A. L.; Lammertsma, K. Organometallics 2002, 21, 3196. 49. Mercier, F.; Hugel-Le Goff, C.; Mathey, F. Tetrahedron Lett. 1989, 30, 2397. 50. Veits, Y. A.; Karlstedt, N. B.; Beletskaya, I. P. Tetrahedron Lett. 1995, 36, 4121. 51. Van Staden, L. F.; Gravestock, D.; Ager, D. J. Chem. Soc. Rev. 2002, 31, 195. 52. Daugulis, M.; Brookhart, M.; White, P. S. Organometallics 2002, 21, 5935. 53. Becker, G.; Uhi, W.; Wessely, H.-J. Z Anorg. Aug Chem. 1981, 479, 41. 54. Van der Does, T.; Bickeihaupt, F. Phosphorus and Suifur 1987, 30, 515. 55. Jouaiti, A.; Geoffroy, M.; Bernardinelli, G. Tetrahedron Lett. 1992, 33, 5071. 56. Jouaiti, A.; Geoffroy, M.; Bernardinelli, G. Tetrahedron Lett. 1993, 34, 3413. 57. Jouaiti, A.; Geoffroy, M.; Bernardinelli, G. Chem. Commun. 1996, 437. 58. Kawanami, H.; Toyota, K.; Yoshifuji, M. I Organomet. Chem. 1997, 535, 1. 59. lonkin, A. S.; Marshall, W. J. Heteroatom Chem. 2002, 13, 662. 60. Termaten, A.; van der Sluis, M.; Bickelhaupt, F. Eur. I Org. Chem. 2003, 2049. 61. Yoshifuji, M.; Toyota, K.; Inamoto, N. Tetrahedron Lett. 1985, 26, 1727. 62. Becker, G.; Becker, W.; Mundt, 0. Phosphorus and Sutfur 1983, 14, 267. 63. A 31P NMR study has mentioned phosphaalkenes 3.lc—e and their chemical shifts were reported. These compounds were prepared in situ using the phospha-Peterson route (Scheme 3.1, Path H) however no experimental details were provided and they were not isolated. See ref. 54. 64. Van der Sluis, M.; Beverwijk, V.; Termaten, A.; Gavrilova, E.; Bickelhaupt, F.; Kooijman, H.; Veidman, N.; Spek, A. L. Organometallics 1997, 16, 1144. 65. Appel, R.; Knoll, F.; Ruppert, I. Angew. Chem. mt. Ed Eng?. 1981, 20, 731. References start on page 80 Chapter Three 84 66. Yoshifuji, M.; Toyota, K.; Inamoto, N.; Hirotsu, K.; Higuchi, T. Tetrahedron Lett. 1985, 26, 6443. 67. Gudimetla, V. B.; Rheingold, A. L.; Payton, J. L.; Peng, H.-L.; Simpson, M. C.; Protasiewicz, J. D. Inorg. Chem. 2006, 45, 4895. 68. Mundt, 0.; Becker, G.; Uhi, W.; Massa, W.; Birkhahn, M. Z Anorg. Aug. Chem. 1986, 540/541, 319. 69. Smeets, W. J. J.; Spek, A. L.; van der Does, T.; Bickeihaupt, F. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1987, 43, 1838. 70. These structures were reported as private communications to the CCD. Details of the synthesis were not provided. Spek, A. L.; Smeets, W. J. J.; Lutz, M.; van der Does, T.; Bickelhaupt, F.; 2004, submission codes: IQOKUU and IQOKOO. 71. Appel, R., In Multiple Bonds and Low Coordination in Phosphorus Chemistry; Regitz, M., Scherer, 0. J., Eds.; Georg Thieme Verlag: Stuttgart, 1990; pp 157—219. 72. Weber, L. Eur. I Inorg. Chem. 2000, 2425. 73. Characteristic Bond Lengths in Free Molecules. CRC Handbook ofChemistry and Physics, 84th ed.; CRC Press: Boca Raton, FL, 2003. 74. Yoshifuji, M.; Toyota, K.; Matsuda, I.; Niitsu, T.; Inamoto, N.; Hirotsu, K.; Higuchi, T. Tetrahedron 1988, 44, 1363. 75. Becker, G.; Becker, W.; Uhi, G.; Uhl, W.; Wessely, H.-J. Phosphorus and SuiXur 1983, 18, 7. 76. Zhou, X.-G.; Zhang, L.-B.; Cai, R.-F.; Wu, Q.-J.; Weng, L.-H.; Huang, Z.-E. I Organomet. Chem. 2000, 604, 260. 77. Grobe, J.; Le Van, D. Angew. Chem. mt. Ed Engi. 1984, 23, 710. References start on page 80 Chapter Three 85 78. Fild, M.; Jones, P. G.; Ruhnau, K.; Thöne, C. Z. Naturforsch. 1994, 49b, 1361. 79. Brieden, W.; Kellersohn, T. Chem. Ber. 1993, 126, 845. 80. Weber, L.; Frebel, M.; MUller, A.; Bogge, H. Organometallics 1991, 10, 1130. 81. Huy, N. H. T.; Ricard, L.; Mathey, F. Organometallics 1991, 10, 3958. 82. Weber, L.; Frebel, M.; Boese, R. Chem. Ber. 1990, 123, 733. 83. Appel, R.; Knoch, F.; Kunze, H. Chem. Ber. 1984, 117, 3151. 84. Chernega, A. N.; Antipin, M. Y.; Struchkov, Y. T.; Kukhar, V. P.; Shevchenko, I. V.; Kolodyazhnyi, 0. I.; Boldeskul, I. E. Z Strukt. Khim. 1984, 25, 122. 85. Appel, R.; Barth, V.; Halstenberg, M.; Huttner, G.; von Seyerl, J. Angew. Chem. mt. Ed Engi. 1979, 18, 872. 86. Goerlich, J. R.; Schmutzler, R. Phosphorus, Su(fur, and Silicon 1995, 101, 245. 87. Alispach, T.; Regitz, M.; Becker, G.; Becker, W. Synthesis 1986, 31. 88. Armarego, W. L. F.; Perrin, D. D. PurUlcation ofLaboratory Chemicals, 4th ed.; Butterworth Heinemann Press: Oxford, 1998, P. 429 on purification of NaOH. A similar procedure was used for KOH. 89. Stetter, H.; Last, W.-D. Chem. Ber. 1969, 102, 3364. 90. Collins, D. J.; Jacobs, H. A. Aust. I Chem. 1987, 40, 1989. 91. teXsan: Crystal Structure Analysis Package; Molecular Structure Corporation: The Woodlands, Texas (USA), 1985 and 1992. 92. SHELXTL Version 5.1.; Bruker AXS Inc.: Madision, Wisconsin, USA, 1997. References start on page 80 86 Chapter Four Radical Initiated Polymerization of Phosphaalkenes: The Effects of Temperature, Initiator, Time, and Substituent* 4.1 Introduction The addition polymerization of phosphaalkene (4.la) has opened a new route to novel phosphorus-containing polymers as reported in Chapter Two.’ In order to examine the generality of this addition polymerization reaction, we have recently prepared a series of P-Mes phosphaalkenes bearing various C-aryl substituents (4.lb-t) and their syntheses were described in the last chapter.2 The incorporation of C-functionalities in the polymers is expected to lead to the ability to tune the properties of poly(methylenephosphine) through post-polymerization modification with functional side-chains. Our initial attempts to polymerize 4.la with radical initiators used high temperatures (200 °C), high initiator loading (10 mol%), and long reaction times (>48 h) to afford polymer 4.2a.1 These polymers were found to have low molecular weights (M = 5,700 gmof’; PDI = 1.10, vs. polystyrene standards) and were isolated in low yields (16%). In order to improve these extreme conditions, we set out to study the conditions for the radical polymerization of 4.la before attempting the polymerization of various phosphaalkene monomers. In particular, our objective was to maximize the molecular weight and the isolated yield of the polymer. Three parameters were varied systematically and the results will be discussed in this chapter: (i) the polymerization temperature, (ii) the initiator concentration, and (iii) the residence time (i.e. length of reaction). Furthermore, the effect of the C-aryl substituents will be discussed and the properties of poly(methylenephosphine) derivatives will be given. * This chapter will be submitted for publication in due course. References start on page 108 Chapter Four 87 4.2 Results and discussion Our initial report of the radical polymerization of 4.la was performed neat in molten monomer (Chapter Two).’ However, to lower viscosity and dissipate heat most radical polymerizations of vinyl monomers are conducted in solution. Our attempts to polymerize 4.la (THF, toluene, and benzene) in solution using VAZO 88 as initiator (1 mol%, 140 °C, 16 h) afforded only very low monomer conversions and isolated yields of polymer. For example, the attempted radical polymerization of 4. la in THF (1 mL, 140 °C, 16 h) afforded only a trace amount of 4.2a (cony. 5%, yield < 1%). The formation of polymer depends on the rate of chain propagation, which also depends on monomer concentration (r = k[Mj).3 Since adding solvents may reduce the monomer concentration as well as the rate of propagation, the polymer yield may be decreased. Furthermore, solvent molecules may promote chain transfer reactions and may thus inhibit polymerization in some cases. Hence, all subsequent polymerization experiments were performed in neat molten monomer at temperatures above its melting point (83 — 85 °C).4 Although monomer 4.la can be polymerized with benzoyl peroxide (1 mol%, 100 °C, 24 h) to afford polymer 4.2a (yield = 6%; M = 18,700 gmol’; PDI 1.17), VAZO 88 (NCC6H,0N=N10C )was chosen for the present polymerization studies (Scheme 4.1) because of the higher yield it offered at the same polymerization conditions (yield = 19%). Ph VAZO88 E Ph] P=c —f—P—C± Me Ph [Msh] 4.la 4.2a Scheme 4.1. References start on page 108 Chapter Four 88 4.2.1 Effect of temperature In order to study the effect of temperature on the radical polymerization of 4.la, experiments were performed to systematically vary the temperature whilst keeping the initiator concentration and residence time constant (1 mol% VAZO 88, time = 24 h). Bulk polymerizations must be performed above the activation temperature of VAZO 88 (88 °C) and above the melting point of 4.la (83 — 85 oc).4 The results of polymerization experiments performed between 90 and 180 °C are summarized in Table 4.1 and each entry is representative of two or more trials performed at each temperature. The molecular weight (Ma) and yield are plotted against temperature (Figure 4. 1). The data reveal that molecular weight decreases as polymerization temperature increases and that the maximum isolated yield is obtained at 120 °C. This effect of temperature on the molecular weight of polymers is not unexpected. In general, for thermally initiated radical polymerization, it is known that the degree of polymerization (DP), which is related to molecular weight, decreases with increasing temperature.3This is 20,000 100 :: a10,000 5,000 20 0 0 80 100 120 140 160 180 200 Temperature (°C) Figure 4.1. Plots showing the effect of temperature (x-axis) on M (shown as .) and isolated yield (shown as .) of polymer 4.2a in the polymerization of 4.la (1%VAZO 88, 1 d). The trendlines are included to guide the eye and do not represent fits to the data. References start on page 108 4 Chapter Four 89 consistent with the trend observed in the radical polymerization of 4.la. The most dramatic effect of temperature on the isolated yield of polymer is observed in the polymerization at 120 °C, above which temperature a significant drop in the isolated yield occurred. This decrease may be attributed to the depolymerization reaction of 4.2a. A complicating factor in the polymerization of 4.la is that the monomer—polymer equilibrium becomes an important factor at high temperatures (Scheme 4.2). Although the rate of depolymerization is zero at low temperatures, it increases with the reaction temperature and eventually equals the rate of polymerization at the ceiling temperature. Therefore, at high temperatures, polymer chains may depolymerize into shorter chains and monomers. In addition, the low-molecular weight poly(methylenephosphine)s are only slightly soluble in hexanes, and will be fractionated out during precipitation. Since most of the polymer chains of 4.2a obtained at high temperatures are short and were removed by precipitation, only a small amount of macromolecule remained. In particular, the lowest isolated yield was observed for polymer 4.2a prepared at 180 °C (yield = 2%; Table 4.1, entry 6). To obtain the optimal molecular weight while maintaining sufficient yields of polymer for analysis, we chose 100 °C as the reaction temperature for other studies. Mes Mes . Mes Mes Mes I ,Ph po’ymerization I I I + p =c P...1.P..1.., M / Ph depolymerization -‘. ..‘- .‘-PhPhPhPh es PhPh PhPhPhPh Scheme 4.2. 4.2.2 Effect of initiator concentration The effect of free-radical initiator loading was studied at constant temperature and residence time (100 °C; 24 h). Concentrations between 0.1 — 20 mol% of VAZO 88 initiator References start on page 108 Chapter Four 90 Table 4.1. Data for the radical polymerization of 4.la. The residence time, initiator concentration and temperature of polymerization were systematically varied to find the conditions for the maximum molecular weight and yield of poly(methylenephosphine) 4.2a. Polymerization Conditions Polymer Characterization Time Initiatora Temp b PDI1, Conversionc YieldEntr’ (d) (mol%) (°C) (gmol’) (%) (%) Variation of Polymerization Temperature 1 1 1.0 90 18,600 1.23 11 13 2 1 1.0 100 16,200 1.20 19 19 3 1 1.0 120 14,200 1.14 34 27 4 1 1.0 140 9,150 1.09 38 18 5 1 1.0 160 8,430 1.08 41 5 6 1 1.0 180 6,650 1.28 39 2 Variation of Initiator Concentration 7 1 0.1 100 18,300 1.16 <1 8 8 1 1.0 100 16,200 1.20 19 19 9 1 2.0 100 16,200 1.16 23 27 10 1 5.0 100 14,800 1.14 30 26 11 1 10.0 100 11,000 1.20 39 37 12 1 20.0 100 10,600 1.15 53 45 Variation of Residence Time 13 0.25 1.0 100 12,800 1.20 5 6 14 0.5 1.0 100 15,100 1.16 9 10 15 1 1.0 100 16,200 1.20 19 19 16 2 1.0 100 14,800 1.19 36 15 17 3 1.0 100 13,500 1.18 51 33 18 4 1.0 100 12,900 1.23 64 52 19 5 1.0 100 14,000 1.21 70 51 20 6 1.0 100 13,000 1.25 73 61 21 7 1.0 100 13,100 1.24 75 56 a VAZO 88 was used as initiator in all cases. b Molecular weight (Ms) and polydispersity (PDI) were determined using GPC equipped with a light scattering detector, a visocometer, and a refractive index detector. Monomer conversion was determined from the relative integration of signals corresponding to 4.la vs 4.2a in the 31P NMR spectrum of the crude polymerization mixture. A suitably long delay time (2 s) was used to ensure the accuracy of integration. were chosen for these investigations and the results are listed in Table 4.1 and shown graphically in Figure 4.2. Not surprisingly, these data show that an increase in VAZO 88 loading results in a lowering of molecular weight. Although the maximum molecular weight was obtained at the References start on page 108 Chapter Four 91 lowest initiator loading, the lowest isolated yield was also obtained under these conditions (Table 4.1, entry 7). Such a low isolated yield is expected because fewer polymer chains are initiated when the initiator concentration is low. At high initiator concentrations, the radical polymerization afforded high yields of polymers; however, a decrease in molecular weight is observed. The kinetic chain length of a propagating radical (v) is inversely proportional to the rate of initiation (v = r/r1), which depends on the initiator concentration (r1 = 2fic[I] ; fis the initiator efficiency, 1c = rate constant of initiation, [I] = initiator concentration).3Therefore, in a typical radical polymerization, the molecular weight of a polymer decreases with increasing initiator concentration. This corresponds to the observed effect of initiator concentration on the molecular weight of 4.2a in the radical polymerization of phosphaalkene 4.la. As a result, the best concentration of VAZO 88 to give a reasonably high molecular weight with sufficient yield of polymer is 1 mol%. 5 10 15 Initiator Loading (mol%) 80 — 0 0 60 40 a 20 Figure 4.2. Plots showing the effect of initiator loading (x-axis) on M (shown as .) and isolated yield (shown as .) of polymer 4.2a in the polymerization of 4.la (100 °C, 1 d). The trendlines are included to guide the eye and do not represent fits to the data. 10020000 __ 15,000 0 E , 10,000 5,000 0 0 0 20 References start on page 108 Chapter Four 92 In addition to molecular weight and isolated yield, an interesting effect of initiator concentration was observed in the 31P chemical shifts of polymers 4.2a. As mentioned in Chapter Two, apart from the broad 31P NMR signal at —10 ppm, another broad resonance at —40 ppm with relatively low intensity was also observed in the spectrum of polymer 4.2a obtained using 5% VAZO 88 at 200 °C for 48 h. Initially, we speculated that the signal at —40 ppm was due to head- to-head or head-to-tail linkages in the polymer backbone and might be typical for radical polymerization of 4.la. Surprisingly, this signal was not often observed in the 31P NMR spectra of polymers 4.2a obtained for this polymerization study. The —40 ppm signal was only observed in the 31P NMR spectra of polymers prepared using high initiator loading (i.e. ?5% VAZO 88) or long reaction time (e.g., at 100 °C, time ? 48 h). This is consistent with the observation reported in Chapter Two. The presence of the signal at —40 ppm observed may be due to the large intensity of these abnormal linkages when more initiator or long reaction time was employed. More experiments are required to confirm the 31P NMR signal assignments for the polymer structure of 4.2a. 4.2.3 Effect of residence time The effect of residence time on the radical polymerization of 4.1 a was examined by performing the experiments at 100 °C with 1 mol% VAZO 88 for a range of time intervals (6 h to 7 d) and the results are shown in Table 4.1 and Figure 4.3. In a typical addition polymerization, the isolated yield and the monomer conversion are initially low. As the residence time is extended, they both increase but should not have any effect on molecular weight.6 Similarly, in the addition polymerization of 4.la with a radical initiator, the molecular weight of the isolated polymer approached a constant value (M = ca. 13,000 gmol’; time 3 d) after reaching a maximum in 24 h (M = 16,200 gmol’). As predicted, the most dramatic effect of References start on page 108 Chapter Four 93 residence time is on the monomer conversion and isolated yield, which reached a maximum after approximately five days. This dependence of the isolated yield on residence time is consistent with a typical addition polymerization. 20,000 100 15,000 80 60 à,10,000 400 5,000 20 0 0 0 4 6 8 Residence Time (d) Figure 4.3. Plots showing the effect of residence time (x-axis) versus M11 (shown as .) and isolated yield (shown as .) for polymer 4.2a in the polymerization of 4.la (1%VAZO 88, 100 °C). The trendlines are included to guide the eye and do not represent fits to the data. According to the half life of VAZO 88 at 100 °C (half life = 8.5 mm), all initiator molecules are consumed after 24 h.7 Interestingly, although no new radicals were formed from the initiator, the conversion of monomer to polymer continued for several days (Table 4.1; entry 13—21). This has a number of implications. Firstly, these data suggest that the growth of new monomer chains may result from chain transfer reaction. One possibility is that the radical of an active propagating chain is transferred to a molecule YZ, where YZ is a monomer or some impurity (Scheme 4.3). The original polymer chain is thus terminated but the new radical species Z may initiate the polymerization of the remaining monomers. As monomer concentration decreases over time, the newly formed polymer chain will have a slightly shorter length (v cc 4 I. 2 References start on page 108 Chapter Four 94 [Mj; v = r/r and r k[M]). This process may also account for the formation of slightly lower molecular weight polymers at long residence times (M = ca. 13,000 gmo[’; time 3 d). As the number of shorter polymer chains increases over time, the size distribution (i.e. PDI) of the polymer becomes broader. This broader molecular weight distribution is often observed in high- conversion radical polymerization.3Similarly, a slightly more polydisperse distribution was observed in phosphaalkene polymerization for extended heating time [PDI: 1.24 (7 d) vs 1.20 (1 d)]. Although the change in polydispersities is admittedly very small, the greater difference in monomer conversion and isolated yield of 4.2a [cony. = 75%; yield = 56% (7 d) vs cony. = 19%; yield = 19% (1 d)J is consistent with the formation of oligomers. These oligomers are likely removed by hexanes precipitation, resulting in a narrowing of the molecular weight distribution of 4.2a. Mes Mes Mes Mes wWUFC Fc. + YZ y? + z PhPhPhPh PhPhPhPh Scheme 4.3. In conclusion, to obtain poly(metheylenephosphine)s with reasonably high molecular weights and acceptable isolated yields, the radical polymerization of phosphaalkene should be conducted for 24 h or longer. 4.2.4 Mechanistic implications of termination In radical polymerization, the two principle termination reactions are coupling and disproportionation. In reality, a combination of these two termination mechanisms is often observed. The coupling mechanism involves the recombination of two propagating radical References start on page 108 Chapter Four 95 chains, leading to high molecular weight (DP = 2v). In a disproportionation reaction, a polymer chain is terminated by transferring the chain-end radical to another molecule, thus the degree of polymerization is identical to the kinetic chain length (DP = v). Since one molecule of VAZO 88 generates two active radical species, we would expect 1 mol% initiator to give a DP of 50 for disproportionation mechanism, or a DP of 100 for coupling reaction. As seen in the radical polymerization of 4.la using 1 mol% VAZO 88 at 100 °C for 24 h (Table 4.1, entry 2), the molecular weight of 4.2a (M = 16,200 gmoP’) corresponds to a DP of ca. 50 (molecular weight of 4.la = 316 gmo11).This DP value suggests that a disproportionation mechanism is likely occurring. In the previous section, it was speculated that chain transfer reactions contribute to the continuous consumption of monomer as polymerization time increases (Scheme 4.3). This chain transfer reaction shows how a polymer chain is terminated by disproportionation. As a preliminary speculation, the radical polymerization of phosphaalkene 4.la is likely to be terminated through a disproportionation mechanism. 4.2.5 Substituent effects in the radical polymerization of P=C bonds To explore the scope of the addition polymerization of P=C bonds, several phosphaalkenes with various C-aryl substituents (4.lb-f) were synthesized as new monomers for polymerization studies.2 The radical polymerization of each phosphaalkene (4.la—f) was conducted using conditions optimized in the previous section. Namely, polymerizations were performed at 100 °C using VAZO 88 (1 mol%) as a radical initiator for different time intervals (time = 1, 3, and 6 d) (Scheme 4.4). The polymerization data for 4.la—f are summarized in Table 4.2. Monomer conversion of 4.la—f was determined from the crude reaction mixture by 31P NMR spectroscopy (7 — 83%). Repeated precipitation of the polymeric mixture from THE or CH21 into hexanes afforded polymers 4.2a—f isolated in low to moderate yields (4 — 56%). References start on page 108 Chapter Four 96 Using triple detection GPC, we estimated that various samples of polymers 4.2a—f had absolute molecular weights (Me) between 5,100 and 18,800 gmol’ with narrow polydispersities (PDI = 1.08 — 1.24). The polydispersities of these polymers are relatively low compared to the conventional radical polymerization of alkenes (usually 2—5). As discussed in previous sections, the low PDI’ s are likely a consequence of the removal of low-molecular weight polymers by hexanes precipitation so that only high molecular weight polymers remained. ji 1 mol% VAZO 88 [ 1 P=C -H—P—C / .‘ or’ I I IMes R2 ivy i Mes R2 1,3or6d L 4.1 4.2a—f 4.laR1=R2Ph 4.1 b =4-FC6H 4.lc R1=Ph;R2=4-FC6H 4.ld2=4-(MeO)C 4.le R1=Ph; 4-(MeO)C 4.lf R1=Ph;R2=2-Pyridyl Scheme 4.4. The molecular weights of the fluorinated polymers 4.2b and 4.2c (M= 14,200— 18,800 gmof’) are slightly higher than those of the unsubstituted polymer 4.2a (M = 13,000 — 16,400 gmol1),which is not surprising when taking into account the mass of the fluorine atoms in 4.2b and 4.2c. Despite the additional weight from the methoxy and nitrogen substituents, polymers 4.2d—f showed a lower molecular weight than 4.2a. Furthermore, the monomer conversions and isolated yields of 4.2d—f are also lower than those for 4.2a (Table 4.2), although a similar effect of residence time on these parameters is observed (i.e., isolated yield increases with time). In particular, the 2-pyridyl polymer 4.2f and the dimethoxy 4.2d showed the lowest M of all para substituted polymers. These differences suggest that the presence of methoxy and 2-pyridyl substituents have a stronger influence on the radical polymerization than does the fluoro group. References start on page 108 Chapter Four 97 Table 4.2. Data for the radical polymerization of substituted phosphaalkenes 4.la—f. a Time M11 b b Conversionc YieldEntry Polymer (d) (g mo11) PDI (%) (%) 1 1 16,400 1.16 22 17 2 4.2a 3 15,100 1.16 59 41 3 6 13,000 1.24 75 56 4 1 17,300 1.21 10 11 5 4.2b 3 18,800 1.16 24 17 6 6 14,900 1.17 48 42 7 1 15,600 1.18 10 10 8 4.2c 3 18,700 1.10 24 22 9 6 14,200 1.13 57 41 10 1 10,000 1.22 <1 4 11 4.2d 3 10,500 1.20 7 8 12 6 10,300 1.18 14 18 13 1 15,400 1.18 10 10 14 4.2e 3 13,500 1.18 25 27 15 6 11,500 1.19 45 41 16 1 - - - - 17 4.2f 3 6,900 1.10 11 5 18 6 5,100 1.08 83 35 a Polymerization conditions: 1 mol% VAZO 88 at 100 °C. b Molecular weight (Ma) and polydispersity (PDI) were determined using GPC equipped with a light- scattering detector, a visocometer, and a refractive index detector. Monomer conversion was determined from the relative integration of signals corresponding to phosphaalkenes 4.la—f vs 4.2a—f in the 31P NMR spectrum of the crude polymerization mixture. A suitably long delay time (2 s) was used to ensure the accuracy of integration. A similar effect has been observed for substituted styrenes bearing p-methoxy and p-fluoro substituents. For example, in the polymerization ofp-methoxystyrene investigated by Imoto8 and Coote9 (Imoto: 71 Lmo[’s’; Coote: 60 Lmo1’s’), the propagation rate constant (kr) is smaller than that ofp-fluorostyrene (Imoto: 112 Lmol’s’; Coote: 104 Lmo1’s’) and even styrene (Imoto: 106 Lmor’ s’; Coote: 110 L mor1. 1) 8,9 The effects these para substituents have on the rate constant for styrene are analogous to the observed trend in molecular weight and monomer conversion of the analogous phosphaalkenes. Although the propagation rate constants of these phosphaalkenes were not determined, the relationship between the rate constant and the References start on page 108 Chapter Four 98 molecular weight was mentioned earlier (v k[M]). Unfortunately, the similar behavior of PC and C=C bonds does not follow for 2-pyridyl-substituted systems. In one case, the rate constant of the free-radical polymerization of 2-vinylpyridine (k = 186 Lmor1Os’) is considerably larger than that of styrene.’° This would lead one to predict a high molecular weight for 2-pyridyl poly(methylenephosphine) 4.2f; however, the lowest molecular weight was observed in the polymerization of 2-pyridyl phosphaalkene 4.lf. To account for this low molecular weight, we speculate that the C-aryl substituents may affect the reactivity of the phosphaalkene monomer and, consequently, the molecular weight of poly(methylenephosphine). In an analogous case, the para substituents have a strong effect on the reactivity of the styrene monomers in the atom transfer radical polymerization of some para-substituted styrenes.” Insight into the monomer reactivities of phosphaalkenes can be obtained by looking at the chemical shifts by 31P NMR spectroscopy. The 31P chemical shifts of phosphaalkenes 4.la—c are identical (4.la: 6 = 233; 4.lb: 6 = 234; 4.lc: 6 234, 233); whereas those for methoxy phosphaalkenes are shifted upfield (4.ld: 6 = 217; 4.le: 6 = 226, 224).2 12 These shielded resonances suggest that the methoxy substituents have a greater electronic influence on the P=C bond than the fluoro groups, leading to more stable monomers. Likewise, the 31P NMR signals of 4.lf (6 = 260, 242) shift to a lower field than that of 4.la suggesting substantial monomer stabilization through resonance delocalization between the P=C bond and the 2-pyridyl ring.2 Furthermore, this is also seen in the X-ray crystallography analysis of 4.lf, where the P=C bond in 4.lf [1.7043(16) A] is slightly longer than that in 4.la [1 .692(3) and 1.693(2) A].2’413 This increased conjugation may account for the fact that low molecular weights and monomer conversions are obtained for the 2-pyridyl polymers 4.2f despite the large propagation rate constant expected based on the polymerization of analogous 2-vinylpyridine. At this point, this is merely speculation. Detailed kinetic studies on the radical polymerization of these References start on page 108 Chapter Four 99 phosphaalkenes with different C-aryl substituents are required to confirm these hypotheses. This is beyond the scope of this thesis. 4.2.6 Polymer properties The thermal stability of poly(methylenephosphine)s was investigated by thermogravimetric analysis (TGA) under a nitrogen atmosphere with a heating rate of 10 °C!min. In Chapter Two, I have reported the onset temperatures of weight loss for 4.2a (T0= 265 °C), the oxidized 4.3a (T0 320 °C), and the sulfurized 4.4a (Tonset = 220 oc).l The thermograms of these phosphorus-containing polymers showed that complete weight loss occurred by ca. 430 °C for 4.2a, whereas a ceramic yield of Ca. 10% was observed for the oxidized and sulfurized polymers (Figure 4.4). We speculate that this residual yield may be due to the formation of thermally stable phosphine oxide or sulfide compounds. i Ph -H—P—C—-F H—P—C—H— [Ms R2 [Ms Ph j 4.3aR1=R2Ph 4.4a 4.3b =4-FC6H 4.3e R1=Ph;2=4-(MeO)C6H The thermal stability of the para-substituted polymers was studied for the oxidized polymers to avoid oxidation due to exposure to air. The thermograms of polymer oxides 4.3b and 4.3e showed a small weight loss (ca. 5 — 10%) before a major decrease at Ca. 300 °C (Figure 4.5). This may result from the presence of molecular impurities (i.e. solvent) that are trapped inside the polymers. Similar to the polymer oxide 4.3a, the ceramic yields of 4.3b and 4.3e at ca. 500 °C can be attributed to the presence of thermally stable phosphine oxide. Although the fluoro and methoxy substituents are incorporated at the para position of the C-aryl rings, no significant influence on the thermal stability of poly(methylenephosphine) was observed. References start on page 108 Chapter Four 100 120 - 100 - 80 - 60 - 40 20 0- 0 1000 Temperature (°C) Figure 4.4. TGA traces of poly(methylenephosphine) 4.2a (----), poly(methylenephosphine oxide) 4.3a (— - — -), and poly(methylenephosphine sulfide) 4.4a (—). Collected under a nitrogen atmosphere at a heating rate of 10 °C/min. Reproduced with permission from J. Am. Chem. Soc. 2003, 125, 1480—148 1. Copyright 2003 American Chemical Society. Similarly, para substituents do not show a significant impact on the thermal stability of some p substituted polystyrenes {Tonset (PS) and Tonset [poly(p-methoxystyrene)] 340 °C; T100% (PS) = 440 °C and Tlm% [poly(p-methoxystyrene)J = 425 oC}.14 The aforementioned TGA experiments were performed under an inert nitrogen atmosphere. To test the thermal stability in oxygen, polymer 4.2a was heated under a constant flow of oxygen gas. Figure 4.6 displays multiple steps of weight loss for 4.2a under an oxygen atmosphere. This interesting weight-loss pattern may be attributed to the concurrent oxidation of phosphorus at high temperatures. Nevertheless, the Tonset was not affected by oxygen, suggesting polymer 4.2a is fairly stable to an oxygen atmosphere, and even air. The polymer is fully decomposed by 800 °C, which is consistent with the complete decomposition temperature of the oxidized 4.3a. 200 400 600 800 References start on page 108 Chapter Four 101 — — ‘II 4 . % •I_ _ __ % — — —.- I I I I I I I I 0 200 400 600 Temperature (°C) Figure 4.5. TGA traces of polymer oxides 4.3a (----), difluoro 4.3b ( ), and methoxy 4.3e (— — —). Collected under a nitrogen atmosphere at a heating rate of 10 °Clmin. 0 200 400 600 800 1000 Temperature (°C) Figure 4.6. TGA traces of poly(methylenephosphine) 4.2a at a heating rate of 10 °C/min under a nitrogen (— — —) and an oxygen ( ) atmosphere. The thermal stability of poly(methylenephosphine)s to weight loss is comparable to other phosphorus polymers. For example, it is similar to the thermal stability of polyphosphazene [poly(dichlorophosphazene)1 = 350 oc}ls and poly(ferrocenylphosphine) (T0= 385 OC)lo 100 BO lE 800 1000 100 80 60 540 20 0 References start on page 108 Chapter Four 102 The wide angle X-ray scattering (WAXS) pattern of the polymer oxide 4.3a is shown in Figure 4.7. The lack of sharp signals and observation of a broad halo at the diffraction angle 20 = 7.07° in the scattering pattern suggests that the oxidized 4.3a lacks high degree of crystallinity. The amorphous nature of the polymer may arise from the atactic arrangement of the P-mesityl group along the polymer chain. 60 co4O C 20 0 60 70 20 Figure 4.7. Wide angle X-ray scattering (WAXS) pattern for polymer oxide 4.3a. (*) Scattering due to the plastic sample holder. 4.3 Summary The reaction conditions for the free-radical polymerization of phosphaalkene 4.la were investigated. The molecular weight and isolated yield of poly(methylenephosphine) 4.2a can be improved by varying reaction temperature, initiator concentration, and residence time. Using milder polymerization conditions, we have polymerized 4.lb—f with a radical initiator to give new phosphorus-containing polymers 4.2b—f. This suggests that addition polymerization can be generalized to different P=C systems with various C-aryl substituents. More new phosphine polymers may be derived from the existing 4.2b-e by chemical modification of the para substituents with other side groups. * 310 20 30 40 50 References start on page 108 Chapter Four 103 4.4 Experimental section Materials. All manipulations of air and/or water sensitive compounds were performed under pre-purified nitrogen (Praxair, 99. 998%) using standard high vacuum or Schienk techniques or in an Innovative Technology Inc. glovebox. Hexanes, toluene, and dichioromethane were dried by passing through activated alumina columns.’7 Tetrahydrofuran was distilled from sodiumlbenzophenone immediately prior to use. MeOH and distilled water were degassed prior to use. CDC13(CIL) and benzene were distilled from P205and degassed. 1,1’- Azobis(cyclohexanecarbonitrile) (VAZO 88) was purchased from Aldrich and recrystallized from petroleum ether before use. Benzoyl peroxide was purchased from Aldrich and precipitated from pet ether before use.’8 Phosphaalkene monomer 4.la—fwas synthesized according to the literature procedures.2 Measurements. ‘H, 31P, and‘3C{’H} NMR spectra were recorded at room temperature on Bruker Avance 300 MHz or 400 MHz spectrometers. Chemical shifts are reported relative to: residual CHC13( = 7.24 for ‘H); 85%H3PO4asan external standard (ó = 0.0 for 3’P); CDC13 ( = 77.0 for ‘3C). Absolute molecular weights were estimated by triple detection gel permeation chromatography (GPC — triple detection) using a Waters liquid chromatograph equipped with a Waters 515 HPLC pump, Waters 717 plus autosampler, Waters Styragel® columns (4.6x300mm) HR2 x 2 and HR4, Waters 2410 differential refractometer (refractive index detector, ). = 940 nm), Wyatt tristar miniDAWN (laser light scattering detector, ?. 690 nm) and a Wyatt ViscoStar viscometer. A flow rate of 0.3 mL/min was used and samples were dissolved in THF (Ca. 2 mg/mL). The refractive index increment (dn/dc) of 4.2a was determined using a Waters 410 differential refractometer (?. = 940 nm). Thennogravimetric analyses (TGA) were carried References start on page 108 Chapter Four 104 out on a TA Instruments 2000 instrument with a TGA5 1 module, heating under dry nitrogen or oxygen (Union Carbide) at 10 °C/min from 20 to 1000 °C. Wide-angle X-ray scattering (WAXS) analyses were performed by Dr. Brian Patrick and Dr. Anita Lam using a Bruker D8 Advance X ray Diffractometer. The powdery sample was smeared on a plastic sample holder. The scans were collected from 3.00° to 70.00° 20 at 0.0 10 per step and 0.8 sec per step at 25°C. 1 4.4.1 General procedures for radical polymerization of 4.la-f. In a typical P—c -rMS experiment a Pyrex tube was charged with 4.la (0.50 g, 1.6 mmol) and 1,1’- azobis(cyclohexanecarbonitrile) (VAZO 88) (4 mg, 0.02 mmol), and the tube was flame-sealed in vacuo. The mixture was heated at 100 °C for 24 h in an oven equipped with a rocking tray over which time the reaction mixture became increasingly viscous. The polymer was isolated by repeated precipitation of concentrated THF or CH21 solution with hexanes (3x). The polymer was then dried in vacuo at room temperature for 1 d. E Phi 4.4.1.1 Characterization of polymer 4.2a. Yield = 95 mg (19%); GPC (THF, L Mes Ph] triple detection): M = 16,200 gmol’, PDI 1.20; TGA (N2 atmosphere): plateau 25—200 °C, 5% weight loss 100—280 °C, 95% weight loss 280—430 °C; TGA (02 atmosphere): plateau 25—200 °C, 3% weight loss 100—280 °C, 40% weight loss 280—360 °C, 52% weight loss 360—800 °C, ceramic yield 5% at 800 °C; WAXS: 11.5° 20 (br). Spectroscopic data for 4.2a was previously reported in Chapter Two (Page 48). References start on page 108 Chapter Four 105 F 4.4.1.2 Characterization of polymer 4.2b. Yield = 57 mg (11%); 31P NMR (CDCl): 6 —12 (br); ‘9F NMR (CDC13): 6 —115 (br); ‘H NMR (CDC13): 6 7.7- [v1sjj ],., 6.8 (br mult, 1OH, aryl H), 2.4-1.7 (br mult, 9H,p-CH3o-CH3);‘3C{’H} NMR F (CDC13): 6 163.2 (br,p-Ar), 159.9 (br,p-Ar), 140.6 (br mult, Mes, Ar), 132- 131 (brmult, Mes, Ar), 53.7 (br, P-C-P), 24.6 (br, o-CH3), 20.7 (br,p-CH3);GPC (THF, triple detection): M 17,300 gmol’, PDI = 1.21; TGA (4.3b, N2 atmosphere): 10% weight loss 25— 330 °C, 80% weight loss 330—500 °C, plateau 500—700 °C, 10% weight loss 700—900 °C. F 4.4.1.3 Characterization of polymer 4.2c. Yield = 50 mg (10%); 31P NMR (CDCl): 6 —13 (br);‘9FNMR(CDC13:6 —117.1 (br), ‘HNMR(CDC13: [Mes Ph 6 7.0 (br, 1111, arylH), 2.5-1.4 (br, 9H,p-CH3o-CH3);‘3C{’H} NMR (CDC13): 6 163.4 (br,p-Ar), 159.9 (br,p-Ar), 142-137 (brmult, Mes, Ar), 134-125 (brmult, Mes, Ar), 115.1 (br, rn-Ar), 55.0 (br, P-C-P), 24.7(br, o-CH3), 20.9 (br,p-CH3);GPC(THF, triple detection): M 15,600 gmo[’, PDI = 1.18. MeO 4.4.1.4 Characterization of polymer 4.2d. Yield = 18 mg (4%); 31P NMR 1 (CDC13): 6 —10(br); ‘HNMR(CDC13:6 8.1-5.8(br, 1OH,arylH),3.7(br, P—c 1Ms 6H, -OCH3), 3.0-1.2 (br, 9H,p-CH3o-CH3);‘3C{’H} NMR(CDC13):6 158.3 MeO (br, Ph), 143-133 (br mult, Mes, Ar), 135-128 (br mult, Ph, Mes), 113.6 (br, m Ar), 55.0 (br, -OCH3), 54.3 (br, P-C-P), 24.5 (br, o-CH3), 21.0 (br,p-CH3);GPC (THF, triple detection): M = 10,000 gmo[’, PDI = 1.22. References start on page 108 Chapter Four 106 MeO 4.4.1.5 Characterization of polymer 4.2e. Yield = 49 mg (10%); 31P NMR 1 (CDC13): 6 —9 (br); ‘H NMR (CDC13): 6 7.2-6.7 (br mult, 1 1H, aryl H), 3.7 (br,H-P-cH [Ms Ph j 3H, -OCH3), 2.7-1.2 (br, 9H,p-CH3o-CH3); GPC (THF, triple detection): M 15,400 gmol’, PDI = 1.18; TGA (4.3e, N2 atmosphere): 5% weight loss 25—300 °C, 80% weight loss 300—560 °C, plateau 560—750 °C, 15% weight loss 750—1000 °C. 4.4.1.6 Characterization of polymer 4.2f. Similar procedure to that described ri -4-—p—c-—I— above except residence time = 3 d. Yield = 27 mg (5%); 3’P NMR (CDC13): 6 [Mes Ph ‘‘ —6.7 (br); ‘H NMR (CDC13):6 8.2 (br, 2H; E/Z-o-py), 7.3-6.9 (br mult, 20H, E/Z-Ar, E,Z-m-Mes), 2.2-1.8 (br mult, 18H,p-CH3o-CH3);‘3C{’H} NMR (CDC13): 6 161.5 (br, Pyd-C), 149-12 1 (br mult, Ph-C, Mes-C, Pyd-C), 54.5 (br, P-C-P), 23.2 (br, o-CH3), 20.8 (br, p-CH3);GPC (THF, triple detection): M = 6,900 gmo[1,PDI = 1.10. For convenience, isolated yields and GPC data for radical polymerization of 4.la—f with various reaction temperatures, initiator loading, and residence time are also listed in Tables 4.1 and 4.2. o Ph 1 4.4.2 Preparation of poly(methylenephosphine oxide) 4.3a. Experimental MsF procedures, yield, and spectroscopic data were reported in Chapter Two (Section 2.4.3, Page 49). TGA (N2 atmosphere): plateau 25—125 °C, 5% weight loss 125—280 °C, 90% weight loss 280—450 °C, plateau 450—800 °C, 5% weight loss 800—93 0 °C. WAXS: 20 = 7.07° and d-spacing = 11.5 A. References start on page 108 Chapter Four 107 S Ph 4.4.3 Preparation of poly(methylenephosphme sulfide) 4.4a. Experimental procedures, yield, and spectroscopic data were reported in Chapter Two (Section 2.4.4, Page 49). TGA (N2 atmosphere): plateau 25—220 °C, 90% weight loss 220—420 °C, ceramic yield 10% at 900 °C F Phi 4.4.4 Solution polymerization of 4.la. In a typical experiment a pyrex tube was -+-P- [Ms Ph] charged with la (1.00 g, 3.16 mmol), VAZO 88 (8 mg, 0.03 mmol), and THF (1 mL) the tube was flame sealed in vacuo. The mixture was heated (140 °C, 16 h) in an oven equipped with a rocking tray. The crude reaction mixture was analyzed by 31P NMR spectroscopy (cony. = 5%). Attempts to precipitate with hexanes (100 mL) afforded hardly any polymer 4.2a. Yield < 5 mg (<1%). Similar procedures are followed for radical polymerization of 4.la in toluene and benzene. F Phi 4.4.5 Polymerization of 4.la using benzoyl peroxide. A pyrex tube was charged —I—p—c-- [rvies Ph] with 4.la (0.50 g, 1.6 mmol) and benzoyl peroxide (4 mg, 0.02 mmol) and the tube was flame sealed in vacuo. The mixture was heated at 100 °C for 24 h in an oven equipped with a rocking tray over which time the reaction mixture became increasingly viscous. The molecular impurities were removed by repeated precipitation of concentrated THF or CH21 solutions with hexanes (3x). The polymer was then dried in vacuo at room temperature for 1 d. Yield = 31 mg (6%). GPC (THF, triple detection): M = 18,700 gmo[’, M = 21,900 gmo1’, PDI = 1.17. The 31P NMR spectrum was identical to that for polymer 4.2a obtained using VAZO 88. References start on page 108 Chapter Four 108 F Phi 4.4.6 Polymerization of 4.la using VAZO 88 (0.1 mol%). A pyrex tube was -+-P- L Mes Ph] charged with 4.la (0.50 g, 1.6 mmol) and VAZO 88 (0.032 M in benzene, 50 p.L, 1.6 tmo1) and the tube was flame sealed in vacuo. The mixture was heated at 100 °C for 24 h in an oven equipped with a rocking tray over which time the reaction mixture became increasingly viscous. The molecular impurities were removed by repeated precipitation of concentrated THE or CH21 solutions with hexanes (3x). The polymer was then dried in vacuo at room temperature for 1 d. Yield = 39 mg (8%). GPC (THF, triple detection): M = 18,300 gmol’, M = 21,300 gmo11,PDI = 1.16. The 31P NMR spectrum was identical to that for polymer 4.2a obtained using neat VAZO 88. 4.5 References 1. Tsang, C.-W.; Yam, M.; Gates, D. P. .1 Am. Chem. Soc. 2003, 125, 1480. 2. Yam, M.; Chong, J. H.; Tsang, C.-W.; Patrick, B. 0.; Lam, A. E.; Gates, D. P. Inorg. Chem. 2006, 45, 5255. 3. Odian, G., Principles ofPolymerization, 4th ed.; Wiley-Interscience: New Jersey, 2004; Chapter 3. 4. Van der Knaap, T. A.; Kiebach, T. C.; Visser, F.; Bickelhaupt, F.; Ros, P.; Baerends, E. J.; Stam, C. H.; Konijn, M. Tetrahedron 1984, 40, 765. 5. A polymerization conducted at 200 °C afforded only low molecular weight oligomers and may indicate the ceiling temperature for the radical polymerizaiton of 4.la. 6. Odian, G., Principles ofPolymerization, 4th ed.; Wiley-Interscience: New Jersey, 2004; pp 7—8. 7. The half life of VAZO 88 at 100 °C was computed from the equation: log (t112) = 7660 (1/1) - 18.39, (T= Temp. in Kelvin) provided in Grade Selection Guide. References start on page 108 Chapter Four 109 http ://www2.dupont.com!Vazo/en_US/products/grades/grade_selector.html (accessed May 2007). 8. Imoto, M.; Kinoshita, M.; Nishigaki, M. Makromol. Chem. 1965, 86, 217. 9. Coote, M. L.; Davis, T. P. Macromolecules 1999, 32, 4290. 10. Bengough, W. I.; Henderson, W. Trans. Faraday Soc. 1965, 61, 141. 11. Qiu, J.; Matyjaszewski, K. Macromolecules 1997, 30, 5643. 12. Van der Does, T.; Bickeihaupt, F. Phosphorus and Sutfur 1987, 30, 515. 13. Mundt, 0.; Becker, G.; Uhi, W.; Massa, W.; Birkhahn, M. Z Anorg. Allg. Chem. 1986, 540/541, 319. 14. Still, R. H.; Whitehead, A. J Appl. Polym. Sd. 1977, 21, 1215. 15. Ailcock, H. R.; Kugel, R. L.; Valan, K. J. Inorg. Chem. 1966, 5, 1709. 16. Peckham, T. J.; Massey, J. A.; Honeyman, C. H.; Manners, I. Macromolecules 1999, 32, 2830. 17. Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timniers, F. J. Organometallics 1996, 15, 1518. 18. Armarego, W. L. F.; Perrin, D. D. PurUication ofLaboratory Chemicals, 4th ed.; Butterworth Heinemann Press: Oxford, 1998, p.105. References start on page 108 110 Chapter Five Radical Copolymerization of Phosphaalkenes with Styrene* 5.1 Introduction The development of synthetic methodologies to incorporate phosphorus atoms into the main chain of macromolecules has attracted considerable attention because of the novel properties and chemical functionality phosphorus can impart to such One of the major achievements made during the course of this thesis work was the first addition polymerization of a PC bond (in phosphaalkene 5.1), as discussed in Chapter Two.’9 This reaction afforded poly(methylenephosphine) (5.2) containing alternating phosphorus and carbon atoms in the main chain (as shown in Scheme 5.1). I have subsequently improved the molecular weight of the radical phosphine polymer and employed the addition method to polymerize a variety of C-aryl phosphaalkenes with a radical initiator. These studies were reported in Chapter Four. This fascinating addition route represents a new synthetic methodology for inorganic macromolecules and the extension of the remarkable PC/CC analogy to polymer science. Taking advantage of the polymerizability of both P=C and C=C bonds, we hypothesized that these unsaturated systems might undergo copolymerization to form phosphine-containing polymers. The copolymerization of phosphaalkenes with olefins would offer another convenient method to incorporate inorganic phosphine moieties into the organic backbone of polyolefins. Our group recently reported the preparation of block copolymers from a phosphaalkene and styrene using living anionic polymerization.20This chapter describes the radical copolymerization of phosphaalkene 5.1 with styrene to afford inorganic-organic hybrid * A portion of this chapter has been published. Chi-Wing Tsang, Baharnaz Baharloo, David Riendi, Mandy Yam, and Derek P. Gates, “Radical Copolymerization of a Phosphaalkene with Styrene: New Phosphine-Containing Macromolecules and Their Use in Polymer-Supported Catalysis,” Angew. Chem. mt. Ed. 2004, 43, 5682—5685. The rest of the results presented herein will be submitted for publication. Chapter Five 111 copolymers 5.3. A preliminary estimation of the reactivity ratios for this monomer pair is conducted using two simple graphical methods. Ph initiator F Ph 1 P=C Me Ph [MsIhj 5.1 5.2 Scheme 5.1 Ph / Ph\ /H HVazo88 .. I I Ipc + =‘ ( p_c4/(—c-c Mes Ph Ph \Mes PhJ \H Ph 5.1 5.3 Scheme 5.2. 5.2 Results and discussion 5.2.1 Copolymerization of phosphaalkene 5.1 and styrene The homopolymerization of phosphaalkene 5.1 using radical and anionic initiators was described in Chapters Two and Four (Scheme 5.1) and has been published.19 The purity of the starting materials required for radical polymerization is usually not as strict as that for anionic initiation. Therefore, we decided to employ conventional radical initiation for copolymerization studies. Using 1,1 ‘-azobis(cyclohexanecarbonitrile) (VAZO 88) as the free-radical initiator, we copolymerized phosphaalkene 5.1 with styrene (Scheme 5.2). The first set of copolymerization experiments was performed at 140 °C for 14 h and the data for these copolymers (5.3a’—c’) are given in Table 5.1. Since these data have been published, they will not be discussed in this thesis.2’ After conducting the molecular weight optimization studies (as mentioned in Chapter Four), I decided to perform a second set of experiments at the optimized reaction conditions (100 References start on page 126 Chapter Five 112 °C, 24 h, 1% VAZO 88). This new set of results from copolymers 5.3a—h (Table 5.2) was used to give a preliminary estimation of the monomer reactivity ratios for phosphaalkene and styrene and will be discussed in Section 5.2.2. Table 5.1. Initial data for copolymerization of 5.1 with styrene at 140 °C for 14 hf VAZO 88c Rel. M e Isolated IncorPorateclgCopolymer (mol%) (mol%) (gmol’)” PDI Yield (%) (V) (%) 5.3a’ 10 0.5 9,000 1.7 37 1.28 5 5.3b’ 20 1.0 7,000 1.7 48 2.16 9 5.3c’ 40 2.0 3,600 1.4 18 6.44 39 a Reproduced with permission from Angew. Chem. mt. Ed. from Wiley-VCH, 2004, 43, 5682—5685. b %5.1 = [mol 5.1/(mol 5.1 + mol styrene)] x 100. %VAZO 88 = [mol VAZO 88/(mol 5.1 + mol styrene)1 x 100. d Molecular weights were estimated by GPC versus polystyrene standards. PDI = polydispersity index (estimated by GPC vs PS standards). ‘ %P determined by elemental analysis. Estimated from %P and calculated by Equation 5.1. In each copolymerization experiment, a sealed tube was charged with monomer 5.1 and VAZO 88 (1 mol%), partially dissolved in neat styrene. After heating at 100 °C for 24 h, the crude polymer mixture was dissolved in minimal THF and, subsequently, precipitated with hexanes (3x). Poly(methylenephosphine)-co-polystyrene 5.3 was isolated after drying in vacuo for at least 24 h (yield = 29 — 49%). Table 5.2 lists the data for each copolymerization reaction. Copolymer 5.3 is soluble in some common organic solvents, such as C112,THF, and toluene. Dry copolymer 5.3 shows reasonable stability towards hydrolysis and slowly oxidizes in air. The oxidation of the phosphine copolymer 5.3 can be achieved readily by treatment with H20to give an air- and moisture-stable oxide polymer. The triple-detection GPC analysis of each copolymer sample revealed a moderate molecular weight with a narrow polydispersity index (abs. M = 10,500 — 17,500 gmot’, PDI = 1.10 — 1.26). Importantly, the GPC trace was monomodal, which is consistent with the proposed References start on page 126 Chapter Five 113 copolymer structure for macromolecule 5.3. If the product was a blend of polystyrene and homopolymer 5.2, the GPC trace would be expected to show a bimodal molecular weight distribution. Table 5.2. Data for copolymerization of 5.1 and styrene with VAZO 88 (1 mol%) at 100 °C for 24 h. Monomer Feed Incorporated e e Isolated Copolymer Styrene’ Abs. Abs. PDI Yield (mol%) (mol%) (wt%) (mol%) (g mo ) (g mo ) (%) 5.3a 5 95 1.02 4 17,100 21,600 1.26 49 5.3b 10 90 1.61 6 13,700 16,500 1.21 31 5.3c 20 80 4.24 20 10,500 11,700 1.12 29 5.3d 40 60 7.78 56 12,100 13,300 1.10 35 5.3e 50 50 8.83 75 11,000 12,600 1.15 40 5.3f 60 40 8.80 75 13,900 15,700 1.13 42 5.3g 80 20 9.27 85 14,500 16,600 1.14 47 5.3h 90 10 9.55 93 17,500 20,300 1.16 39 a %5.1 = [mol 5.1/(mol 5.1 + mol styrene)J x 100. b %Styrene = 100% — %5.1 Determined by elemental analysis. d Estimated from %P and calculated by Equation 5.1. Molecular weights were estimated by triple-detection GPC. ‘PDI = polydispersity index (estimated by triple-detection GPC). The ‘H NMR spectra of the series of copolymers 5.3 showed broad overlapping signals in the aliphatic and aromatic regions corresponding to phosphaalkene and styrene fragments. An example spectrum of copolymer 5.3b’ is shown in Figure 5.1. Definitive assignments of the spectra are not possible and signals were only assigned to aliphatic and aromatic protons. In principle, the percent incorporation of phosphorus into 5.3 can be estimated from the ratio of the aliphatic and aromatic ‘H NMR signals. However, the ratio of (aryl H) :(alkyl H) for the two monomer units (i.e. 5.1 and styrene) in the 1H NMR spectrum is not very sensitive to large changes in the percent incorporation of 5.1 into the copolymer 5.3. For this reason, we believe that ‘H NMR spectroscopy does not give an accurate measure of the phosphaalkene content. For References start on page 126 Chapter Five 114 example, a polymer consisting entirely of the methylenephosphine unit would show a (aryl H): (alkyl H) ratio of 1.33 (i.e. 1 2H/9H), whereas the ratio for polystyrene would be 1.66 (5H/3H). As a result, we employed elemental analysis to determine the phosphorus content in the copolymers (%PEA). Subsequently, using this %PEA value, we can estimate the degree of phosphaalkene incorporation into the copolymers 5.3 from Equation 5.1. Percent incorporation of phosphaalkene into copolymer = (5.1) 100xAW -(FWPA -FW)(%PEA) where: FWST is the molar mass of styrene; FWpA is the molar mass of phosphaalkene 5.1; A Wp is the atomic mass of phosphorus. A sample calculation of the phosphaalkene incorporation is performed as follows. In Table 5.2, copolymer 5.3a prepared from 5 mol% of 5.1 and 95 mol% of styrene contains 1.02 wt% phosphorus. This corresponds to 4% phosphaalkene (i.e. x = 0.04n in 5.3a) and 96% styrene (i.e. y = 0.96n in 5.3a) incorporation in 5.3a. Given the sensitivity of elemental analysis to small amounts of impurity, these numbers are estimates only. 1 I ppm 8 6 4 2 0 Figure 5.1. ‘H NMR spectrum (CD2C1)of purified copolymer 5.3W. Reproduced with permission from Angew. Chem. mt. Ed. from Wiley-VCH, 2004, 43, 5682—5685. (*) Residual CDHC12. * References start on page 126 Chapter Five 115 Interestingly, there are two major broad resonances at —9 and 4 ppm in the 31P NMR spectra of copolymers 5.3a—h [Figure 5.2(a—h)], whereas only one broad resonance (ip= —10) is found in the spectrum of homopolymer 5.2 [Figure 5.2(i)]. We hypothesize that the resonance at —9 ppm corresponds to the fragments of (styrene)-(P—C)-(P—C) (A) and/or (P—C)-(P—C)-(P—C) (B) and the signal at 4 ppm is due to the (styrene)-(P—C)-(styrene) segments (C). The changes in the intensities of these two signals provided some insight into the complicated microstructure of 5.3. Phosphaalkene incorporation increases with the intensity of the resonance at —9 ppm, but decreases with that at 4 ppm. In general, increasing the phosphaalkene feed should lead to formation of more fragments A and B in the copolymer, thus a relatively lower occurrence of segment C. This is consistent with our 31P NMR signal assignments of the proposed phosphine moieties (A—C). In addition to the two major resonances, there are some minor signals observed in the 31P NMR spectrum of 5.3. We speculate that these signals result from partial regioirregular enchainment (i.e., P—CH2 and P—CHPh linkages) or the tacticity of the polymer. The additional 31P NMR resonances may also contribute to the phosphine moieties incorporated at or near the chain end of the polymer. Nevertheless, the relative intensities of these minor signals should not be influenced dramatically by the phosphorus content. More studies are necessary in order to fully elucidate the microstructure of copolymer 5.3 and confirm these tentative assignments. HH Ph Ph Ph Ph Ph I •. I I I b1P = -9: C-C—P-C—P-C..m I I I I I I I I I I I H Ph MesPh MesPh MesPh MesPh MesPh A B HH Ph HH 31p = 4: H Ph MesPh H Ph C References start on page 126 Chapter Five 116 Ph\\ ‘HH ‘III— —f—C-C *Mes \H Ph )O96n In i.LLLJ (b) (c) 1/ Ph\ /HH [\Mes PhJøn \H Ph 094n ilL. Ph\ /HHL \Mes Ph JO.20n \ H Ph (d) H I’’— —f—C-C *Mes Ph/056n \H Ph L]n ‘I (e) (f) Ph” /HH )n]n[\Mes Ph75n \\ H Ph (g) Ph” /HH Ln]n[\Mes Phh75n \H Ph (h) Ph\ /HH l5n]nRMes Ph,85fl \H Ph Ph\ ‘HH )o7nInL\Mes Ph)093, \H Ph (i) I Ph] L MesPhj1, 80 60 40 20 0 —20 —40 —60 —80 ppm Figure 5.2. 31P NMR spectra (THF) of copolymers 5.3a—h [-(MesP—CPh)-(CH—CHPh)-] with different incorporation of phosphaalkene 5.1: (a) x = 0.04n; y = 0.96n (5.3a); (b) x = 0.06n; y = 0.94n (5.3b); (c) x = 0.20n; y = 0.80n (5.3c); (d) x = 0.56n; y = 0.44n (5.3d); (e) x = 0.75n; y = 0.25n (5.3e); (f) x = 0.75n; y = O.25n (5.31); (g) x = 0.85n; y = 0.1 5n (5.3g); (h) x = 0.93n; y = 0.07n (5.3h); (i) x = n; y = 0 (5.2). References start on page 126 Chapter Five 117 5.2.2 Reactivity ratios of P-mesityl phosphaalkene and styrene To better understand the phosphorus incorporation versus the monomer feed for copolymerization, I set out to estimate the monomer reactivity ratios for phosphaalkene 51. (rp) and styrene (rsT). The monomer reactivity ratios were determined by the Fineman—Ross and the Kelen—TüdOs methods.22’3 These two methods offer a direct determination of the monomer reactivity ratios simply through a linear plot. In general, the Fineman—Ross and Kelen—Tüdös equations are valid for copolymerization reactions with low monomer conversion (usually less than 10%) because of the possible change in monomer concentration throughout the copolymerization reaction. However, low-conversion experiments are currently not feasible with the phosphaalkene/styrene systems due to the need for large amounts of phosphaalkene monomers. At such a low conversion, large quantities of monomers are necessary to produce sufficient polymeric material for analyses. This is particularly important for 31P NMR spectroscopic analyses of copolymers with low phosphorus incorporation. Therefore, results from bulk polymerization are used for the calculation of monomer reactivity ratios in this thesis, realizing that these are preliminary estimates of these parameters. The formulae derived for the parameters used in the linear graphical method by Fineman and Ross are given in Equations 5.2 and 5322 G=rpAF—rST (5.2) G/F = —rST/F + rpA (5.3) Gx(y—1)/y (5.4) (5.5) where: x is the feed composition ratio of 5.1 to styrene (MpA:MsT); y is the copolymer composition ratio of 5.1 to styrene (mpA:msT); MpA and MST are the mole percentage of phosphaalkene 5.1 and styrene, respectively, in the feed composition; mpA and msT are the mole percentage of phosphaalkene 5.1 and styrene, respectively, in the copolymer composition. References start on page 126 Chapter Five 118 Both Equations 5.2 and 5.3 are plotted because, as stated by the authors, a better linear fit may sometimes result from one of the two equations. The parameters involved in the two plots are listed in Table 5.3. The plot of the Fineman—Ross parameters (Equation 5.2, G vs F) is given in Figure 5.3. The reactivity ratio of phosphaalkene (rpA) and styrene (rsT) can be obtained from the slope and the negative of the y-intercept, respectively. Figure 5.4 shows the plot of Fineman— Ross parameters GIF vs 1/F, which affords rpA and rsT from the negative of the slope and they- intercept, respectively. Table 5.3. Fineman—Ross parameters to access the reactivity ratios calculated. Copolymer MpA mpA b c G e F e G/F e 1/F 5.3a 5 4 0.05 0.04 -1.32 0.072 -18.27 13.82 5.3b 10 6 0.11 0.06 -1.60 0.191 -8.42 5.25 5.3c 20 20 0.25 0.25 -0.74 0.249 -2.99 4.02 5.3d 40 56 0.67 1.27 0.14 0.349 0.41 2.87 5.3e 50 75 1.00 3.03 0.67 0.330 2.03 3.03 5.3f 60 75 1.50 2.93 0.99 0.769 1.28 1.30 5.3g 80 85 4.00 5.87 3.32 2.727 1.22 0.37 5.3h 90 93 9.00 13.09 8.31 6.187 1.34 0.16 a MpA is the mole percentage of 5.1 in the feed composition. b mpA is the mole percentage of 5.1 in the copolymer composition. x is the feed composition ratio calculated by x = MpAIMST. d is the copolymer composition ratio calculated by y = mpA/msT. Fineman—Ross parameters: F = x2/y and G = x(y — l)Iy. 10.0 8.0 6.0 4.0 G 2.0 0.0 -2.0 -4.0 -2.0 0.0 2.0 4.0 6.0 8.0 F Figure 5.3. Determination of the reactivity ratios of phosphaalkene 5.1 (rpA) and styrene (rsT) by the Fineman—Ross method (Equation 5.2) for copolymer 5.3. References start on page 126 Chapter Five 119 6.0 2.0 -2.0 -6.0 G/F -10.0 -14.0 -18.0 -22.0 -2.0 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 1/F Figure 5.4. Determination of the reactivity ratios of phosphaalkene 5.1 (rpA) and styrene (rsT) by the Fineman—Ross method (Equation 5.3) for copolymer 5.3. The monomer reactivity ratios were directly obtained from the slope and the intercept of the two Fineman—Ross plots. The two sets of reactivity ratios are significantly different from each other (rpA = 1.50 ± 0.13 and TST = 0.8 ±0.3 from Equation 5.2 versus rpA = 3.0 ± 1.1 and rsT = 1.5 ± 0.2 from Equation 5.3). The inconsistency of results is not surprising and is commonly found when analyzing data using the Fineman—Ross method.2326 The major drawback of the Fineman—Ross equations is that the copolymerization data are not equally weighted. The uneven distribution of data points is observed in the two Fineman—Ross plots as shown in Figures 5.3 and 5.4. For example, the data from the high phosphaalkene/low styrene concentration (5.3g: mpA = 85% and msT = 15%; 5.3h: mpA 93% and msT = 7%) showed a greater impact on the slope of Equation 5.2 (Figure 5.3). In contrast, the plot of Equation 5.3 was influenced significantly by the single data point of low phosphaalkene/high styrene content (Figure 5.4, 5.3a: mpA = 4% and msT = 96%). As a result, we propose that the inconsistency of the reactivity ratios from the two Fineman—Ross equations results from the imbalanced weight of data. References start on page 126 Chapter Five 120 In an effort to overcome the drawbacks of the Fineman—Ross method, the reactivity ratios were re-estimated using the linear method by Kelen and TUdôs (Equation 5.6).23 The Kelen— TüdOs equation offers a better spread of data points by introducing an arbitrary constant a. (rpA + rsT/a)—rsT/a (5.6) =G/(a+F) (5.7) F/(a+F) (5.8) a = gFm x Fmjn (5.9) where Fmax and Fmjn are the maximum and minimum values ofF. Table 5.4. Kelen—TudOs parameters to access the reactivity ratios calculated. Copolymer MPAa mpA c 5.3a 5 4 —1.78 0.10 5.3b 10 6 —1.87 0.22 5.3c 20 20 —0.81 0.27 5.3d 40 56 0.14 0.34 5.3e 50 75 0.67 0.33 5.3f 60 75 0.69 0.53 5.3g 80 85 0.98 0.80 5.3h 90 93 1.21 0.90 a MpA is the mole percentage of 5.1 in the feed composition. b mpA is the mole percentage of 5.1 in the copolymer composition. Kelen—TüdOs parameters i] = GI(a + F) and FI(a + F), where a = (Fmin X Fmax)°5 = 0.67 ± 0.36. 2.0 1.0 0.0 T] -1.0 -2.0 -3.0 0.00 0.20 0.40 0.60 0.80 1.00 Figure 5.5. Determination of the reactivity ratios of phosphaalkene 5.1 (rpA) and styrene (rsT) by the Kelen—TüdOs method (Equation 5.6) for copolymer 5.3. References start on page 126 Chapter Five 121 Table 5.5. Monomer reactivity ratios of phosphaalkene 5.1 (rpA) and styrene (rsT) by Fineman— Ross and Kelen—Tüdös methods for copolymer 5.3. Methods rpA rsT Fineman—Ross (Equation 5.2) 1 .50 ± 0.13 0.8 ± 0.3 Fineman—Ross (Equation 5.3) 3.0 ± 1.1 1.5 ± 0.2 Kelen—Tüdôs (Equation 5.6) 1.9 ± 1.5 1.1 ± 0.9 Average 2.1 ± 0.9 1.2 ± 0.5 Figure 5.5 shows the plot for the data shown in Table 5.4 with i as the ordinate and as the abscissa. The reactivity ratios were estimated from the extrapolation of = 0 and = 1 of Equation 5.6 (rpA = 1.9± 1.5 and rsT = 1.1 ± 0.9). The Kelen—TUdOs and the Fineman—Ross estimates for the monomer reactivity ratios are summarized in Table 5.5. Interestingly, the pair of reactivity ratios obtained using Kelen—TüdOs agrees with the two sets of Fineman—Ross values within the range of errors. Although it is rare to have both reactivity ratios greater than unity, several examples of monomer pairs of this type have been reported in the literature.2732 Some authors proposed that in these exceptional cases, both monomers have a tendency to form block copolymers rather than alternating copolymers.2933 The reactivity ratio of phosphaalkene 5.1 (rpA = 2.1 ± 0.9) implied that the phosphaalkyl radical reacting with a phosphaalkene monomer is about twice as likely and is faster than with a styrene comonomer. Nevertheless, the styryl radical does not show any preference to either monomers (rsT 1.2 ± 0.5). This suggests that copolymers 5.3 may consist of blocks of phosphaalkene moieties, alternating with (short sequences of) styrene. The presence of the poly(methylenephosphine) blocks in the copolymer can be further supported by the observation of the broad 31P NMR signal at —9 ppm in all 31P NMR spectra of copolymers 5.3. This particular 31P NMR resonance is assigned to -(P—C)-(P— C)-(P—C)- (B) linkages, which are present in the structure of homopolymer 5.2. The homopolymer-like structure is found in each copolymer sample of 5.3, even those of very low phosphorus incorporation. In particular, the 31P NMR spectrum of 5.3 with 4 mol% References start on page 126 Chapter Five 122 phosphaalkene content would be expected to show only the 4 ppm signal due to high styrene loading and thus the unlikelihood of many -(P—C)-(P—C)-(P—C)- linkages (6= —9 ppm). Although the major signal is at 4 ppm, the presence of a broad 31P NMR signal at —9 ppm suggests that copolymer 5.3 contains small amounts of -(P—C)-(P—C)-(P—C)- linkages [Figure 5.2(a)]. Hence, we speculate that the radical copolymerization of phosphaalkene with styrene yields a copolymer with sequences of phosphaalkene distributed amongst the styrene units, regardless of the initial loading of phosphaalkene versus styrene. It is noteworthy that this section is only intended to provide some preliminary speculation about the structure of the copolymer from radical initiation through a rough estimation of the monomer reactivity ratios. More thorough studies and repeat runs of copolymerization are necessary to confirm these conclusions. 5.2.3 Copolymerization of 4-methoxy-substituted phosphaalkene with styrene To generalize the radical copolymerization method to substituted phosphaalkenes, the methoxy-phosphaalkene 5.4 was reacted with styrene in the presence of a radical initiator to form copolymer 5.5 (Scheme 5.3). Several copolymerization reactions were performed at 140 °C for 16 h using VAZO 88 (5%) as the radical initiator to give copolymers 5.5a—c. These experimental data as well as that of the homopolymerization of 5.4 (polymer 5.5d) are summarized in Table 5.6. The triple-detection GPC analyses of polymers 5.5a—d revealed absolute Ms’s of 4,900 — 6,900 gmoP’ and monomodal molecular weight distributions (PDI = 1.07 — 1.09) in all three cases. This is consistent with the proposed copolymer structure for 5.5. The 31P NMR spectra of 4-methoxy-substituted 5.5 were analogous to those of unsubstituted copolymers 5.3. These spectra exhibited two major resonances at —9 and 4 ppm with their References start on page 126 Chapter Five 123 intensities depending on the initial phosphaalkene loading. Based on the GPC and the NMR spectroscopic data, it can be concluded that copolymer 5.5 is a single copolymer, although phosphorus elemental analysis was not performed on these samples. R / R\ /HH/ Vazo 88 ( •. I I P=C + =\ I ‘9t/t99 Mes Ph Ph \Mes PhJ \H Ph 5.4 R = 4-(MeO)C6H 5.5 Scheme 5.3 Table 5.6. Copolymerization data for the 4-methoxy-phosphaalkene/styrene system.a 5.4 Styrene Abs. M Abs. M Isolated YieldPolymer (%) (%) (g.mol’) (gmol’) PDI (%) 5.5a 10 90 6,900 7,500 1.09 31 5.5b 20 80 5,800 6,300 1.07 24 5.5c 40 60 4,900 5,300 1.09 35 5.Sd 100 0 8,900 9,600 1.09 10 a using VAZO 88 (5 mol%) at 140 °C for 16 h. 5.3 Summary In conclusion, inorganic phosphine moieties have been successfully incorporated into the backbone of organic macromolecules from the radical copolymerization of two different phosphaalkenes with styrene. This work represents the first example of the copolymerization of P=C and C=C bonds, and further extends the parallels between the chemistry of phosphaalkenes and olefins to copolymerization reactions. The preliminary estimation of monomer reactivity ratios provides some useful information on the microstructure of the radical copolymer. The access to this new inorganic-organic hybrid macromolecule also introduces a convenient, one step synthetic route to copolymers from a large variety of phosphaalkenes and olefins. In References start on page 126 Chapter Five 124 particular, generalizing the copolymerization to methoxy-substituted phosphaalkene allows the incorporation of phosphine fragments as well as C-functional side groups. 5.4 Experimental section General procedures. All manipulations of air and/or water sensitive compounds were performed under a nitrogen atmosphere using standard Schienk or glovebox techniques. Hexanes and dichioromethane were deoxygenated with nitrogen and dried by passing through a column containing activated alumina.34 THF was freshly distilled from sodium/benzophenone ketyl. CD21 (CIL, ampoule) and H20 (30% in H20) (Aldrich) were used as received. Styrene (Aldrich) was stirred over MgSO4for 24 h and distilled from fresh MgSO4under partial vacuum. 1,1 ‘-Azobis(cyclohexanecarbonitrile) (VAZO 88) (Aldrich) was recrystallized twice from ethanol (99%) and dried at least 24 h under vacuum. Monomers MesP=CPh2(5.1) and MesP=C(Ph)(4-MeOC6H4)(5.4) were prepared following the previously reported procedures.35 Equipment. ‘H and 31P NMR spectra were recorded at room temperature on Bruker Avance 300 MHz or 400 MHz spectrometers. Chemical shifts are reported relative to: residual CHDC12(6 = 5.32 for ‘H) and 85% H3P04as an external standard (6 = 0.0 for 31P). Relative molecular weights reported in Table 5.1 were estimated by gel permeation chromatography (GPC) using a Waters liquid chromatograph equipped with a Waters 515 HPLC pump, Waters 717 plus autosampler, Waters Styragel® columns (4.6x300mm) HR2, HR4, and HR5E and a Waters 2410 differential refractometer (refractive index detector). A flow rate of 0.3 mL/min was used and samples were dissolved in THF (ca. 1 mg/mL) and filtered before injection. Narrow molecular weight polystyrene standards were used for calibration purposes. Absolute molecular weights reported in Tables 5.2 and 5.6 were determined by triple detectjon GPC using a Waters liquid References start on page 126 Chapter Five 125 chromatograph equipped with a Waters 515 HPLC pump, Waters 717 plus autosampler, Waters Styragel columns (4.6x300mm), HR2 x 2 and HR4, Waters 2410 differential refractometer (refractive index detector, = 940 nm), Wyatt tristar miniDAWN (laser light scattering detector, = 690 nm) and a Wyatt ViscoStar viscometer. A flow rate of 0.3 mL/min was used and samples were dissolved in THF (ca. 2 mg/mE). The dn/dc for each sample was determined using the software. Elemental analyses on 5.3a—h were performed by the Canadian Microanalytical Service Ltd. The best-fit lines of the plots in Figures 5.3—5.5 were obtained using the least squares method and the errors were estimated using the standard error function in the software Microsoft Excel. Ph H H 5.4.1 General procedures for synthesis of copolymers (5.3).4 N Mes Ph Ph The experimental procedures for 5.3a are given below as an example: To a large pyrex tube was added styrene (1.25 g, 12.0 mmol) and phosphaalkene 5.1 (0.20 g, 0.63 mmol) and VAZO 88 (0.03 1 g, 0.14 mmol). The tube was flame sealed in vacuo. After heating at 100 °C in an oven equipped with rocking tray for 24 h, the polymerization mixture became very viscous. The tube was broken, and the contents were dissolved in THF (5 mL) and precipitated into hexanes (100 mL). Precipitation was repeated three times. A white powder was obtained and dried in vacuo for at least 24 h. Yield = 0.72 g (49%) 31P NMR (CD2C1): 54, —9 (br); ‘H NMR (CD2C1): ô = 7.5 — 6.3 (br, aryl H from phosphaalkene and styrene), 2.3 — 1.2 (br, CH3 from phosphaalkene, CH, CH2 from the styrene). Anal. Found for Copolymer 5.3a: C, 90.56; H, 7.81; P, 1.02. The same procedures applied to the preparation of copolymers 5.5, except with 5% VAZO 88, at 140°C, 16h. References start on page 126 Chapter Five 126 5.5 References 1. Mark, J. E., Alicock, H. R., West, R., Inorganic Polymers, 2nd ed.; Oxford University Press: New York, 2005. 2. Ailcock, H. R., Chemistiy and Applications ofPolyphosphazenes; Wiley & Sons: New Jersey, 2003. 3. Manners, I. Angew. Chem. mt. Ed. Engl. 1996, 35, 1602. 4. Neilson, R. H.; Wisian-Neilson, P. Chem. Rev. 1988, 88, 541. 5. McWilliams, A. R.; Dorn, H.; Manners, I. Top. Curr. Chem. 2002, 220, 141. 6. Vanderark, L. A.; Clark, T. J.; Rivard, E.; Manners, I.; Slootweg, J. C.; Lammertsma, K. Chem. Commun. 2006, 31, 3332. 7. Wright, V. A.; Patrick, B. 0.; Schneider, C.; Gates, D. P. 1 Am. Chem. Soc. 2006, 128, 8836. 8. Jin, Z.; Lucht, B. L. I Am. Chem. Soc. 2005, 127, 5586. 9. Smith, R. C.; Protasiewicz, J. D. I Am. Chem. Soc. 2004, 126, 2268. 10. Dom, H.; Rodezno, J. M.; Brunnhöfer, B.; Rivard, E.; Massey, J. A.; Manners, I. Macromolecules 2003, 36, 291. 11. Morisaki, Y.; Aiki, Y.; Chujo, Y. Macromolecules 2003, 36, 2594. 12. Majoral, J.-P.; Caminade, A. M.; Maraval, V. Chem. Commun. 2002, 2929. 13. Jin, Z.; Lucht, B. L. I Organomet. Chem. 2002, 653, 167. 14. Ailcock, H. R.; Reeves, S. D.; de Denus, C. R.; Crane, C. A. Macromolecules 2001, 34, 748. 15. Walker, C. H.; St. John, J. V.; Wisian-Neilson, P. 1 Am. Chem. Soc. 2001, 123, 3846. References start on page 126 Chapter Five 127 16. Hay, C.; Fischmeister, C.; Hissler, M.; Toupet, L.; Réau, R. Angew. Chem. mt. Ed 2000, 39, 1812. 17. Lucht, B. L.; St. Onge, N. 0. Chem. Commun. 2000, 2097. 18. Peckham, T. J.; Massey, J. A.; Honeyman, C. H.; Manners, I. Macromolecules 1999, 32, 2830. 19. Tsang, C.-W.; Yam, M.; Gates, D. P. J Am. Chem. Soc. 2003, 125, 1480. 20. Noonan, K. J. T.; Gates, D. P. Angew. Chem. mt. Ed 2006, 45, 7271. 21. Tsang, C.-W.; Baharloo, B.; Riendi, D.; Yam, M.; Gates, D. P. Angew. Chem. mt. Ed. 2004, 43, 5682. 22. Fineman, M.; Ross, S. D. I Polym. Sci. 1950, 5, 259. 23. Kelen, T.; TüdOs, F. I Macromol. Sci., Chem. 1975, A9, 1. 24. Tidwell, P. W.; Mortimer, G. A. I Polym. Sci.: Part A 1965, 3, 369. 25. Tidwell, P. W.; Mortimer, G. A. I Macromol. Sci. Rev. Macromol. Chem. 1970, C4, 281. 26. Joshi, R. M. I Macromol. Sci., Chem. 1973, A 7, 1231. 27. Tamikado, T.; Iwakura, Y. I Polym. Sci. 1959, 36, 529. 28. Majumdar, R. N.; Yang, S. L.; James, H. I Polym. Sci., Polym. Chem. Ed 1983, 21, 1717. 29. Wiley, R. H.; Mathews, W. K.; O’Driscoll, K. F. I Macromol. Sci., Chem. 1967, Al, 503. 30. Saini, G.; Leoni, A.; Franco, S. Macromol. Chem. 1971, 144, 235. 31. Saini, G.; Leoni, A.; Franco, S. Macromol. Chem. 1971, 147, 213. 32. Fernández-Monreal, M. C.; Cuervo, R.; Madruga, E. L. I Polym. Sci., Part A: Polym. Chem. 1992, 30, 2313. References start on page 126 Chapter Five 128 33. Odian, G., Principles ofPolymerization, 4th ed.; Wiley-Interscience: New Jersey, 2004; p 475. 34. Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J. Organometallics 1996, 15, 1518. 35. Yam, M.; Chong, J. H.; Tsang, C.-W.; Patrick, B. 0.; Lam, A. E.; Gates, D. P. Inorg. Chem. 2006, 45, 5255. References start on page 126 129 Chapter Six A Convenient Synthesis of New Isolable Phosphaalkenes Using the Base-Induced Hydrogen Rearrangement of Secondary Vinylphosphines* 6.1 Introduction After exploring the addition polymerization of phosphaalkenes with a variety of aromatic substituents in previous chapters, we were interested in generalizing this new polymerization reaction to P=C bond systems consisting of C-alkyl substituents. Alkyl functionalities can offer polymers with useful properties such as higher flexibility and improved solubility in hydrocarbon solvents, which are not found for the triaryl-substituted poly(methylenephosphine).’ To synthesize an isolable alkyl-substituted phosphaalkene for addition polymerization, sterically encumbering (i.e. Mes* = 2,4,6-tri-tert-butylphenyl) and/or it-delocalizing (i.e. NR2,OSiMe3) substituents are necessary to confer kinetic stability to the PC bond.25 However, the large or electron-delocalizing substituents may inhibit the polymerization of the P=C bond. Consequently, the preparation of polymerizable C-alkyl phosphaalkenes is challenging and requires a delicate balance of substituent properties. We set out to synthesize C-methyl phosphaalkenes, with the rationale that the simplest alkyl group might provide enough stability without hindering addition polymerization. P=c - / IkyI alkyl * A version of this chapter has been published. Mandy Yam, Chi-Wing Tsang, and Derek P. Gates, “A Convenient Synthesis of New Isolable Phosphaalkenes Using the Base-Induced Rearrangement of Secondary Vinyiphosphines,” Inorg. Chem. 2004, 43, 37 19—3723. Copyright 2004 American Chemical Society. Chapter Six 130 The phosphorus variant of the well-known allylic tautomerism (double-bond migration) of vinyl compounds [Scheme 6.1(a)] is a potential route to C-methyl phosphaalkenes. The double-bond migration of secondary vinyiphosphines to phosphaalkenes involves a 1,3-hydrogen rearrangement reaction of XP(H)C(Y)=CH2to XP=C(Y)(CH3)[Scheme 6.1(b)]. The thermolytic H..C.C...CH (a) x x H..p.C..CH p..C...CH (b) x x Scheme 6.1. or base-induced 1,3-hydrogen rearrangement of XP(H)C(Y)=CH2(X = Me, Ph, Mes; Y = H or Me) has been reported; however, this procedure did not afford any isolable phosphaalkenes.6’7 Transient phosphaalkenes (X = CH3 or Ph; Y = H) are thermally unstable and must be trapped with chemical reagents.7Furthermore, stable but inseparable equilibrium mixtures of phosphaalkenes and vinylphosphines have been observed in several reactions: the double-bond migration of MesP(H)C(Me)CH2,6the phospha-Cope rearrangement of diphosphine [ArPCHCH(Ph)CH(Ph)CHPAr],8and the formation of ArFP(H)CH=CHPh [ArF 2,4,6- tris(trifluoromethyl)phenyl] from the hydrophosphination of HCCPh with ArFPH2.9 Presumably, in the aforementioned cases, the mixture results from the lack of a significant thermodynamic preference for the phosphaalkene tautomer over the vinylphosphine species. We speculated that the presence of an electron-withdrawing group at phosphorus may favor the phosphaalkene tautomer by increasing the acidity of the P—H bond in the vinylphosphine. References start on page 147 Chapter Six 131 Fluorinated mesityl groups such as 2,4,6-tris(trifluoromethyl)phenyl (ArF) and 2,6- bis(trifluoromethyl)phenyl (ArF’) groups have been extensively employed in the low-coordinate group 15 chemistry because they are bulky, unreactive, and electron withdrawing. 10-26 particular, the synthesis of several low-coordinate phosphorus compounds has employed the ArF substituent. Examples include, stable diphosphenes (ArFP=PArF),’°12iminophosphines (ArFP=NArF’3and ArF’P=N’Bu’4),crystalline phosphenium salts (ArFPNR2),’5and phosphides (ArFPW).’6’17 We were interested in employing ArF or ArF’ substituents in phosphaalkene synthesis because in our attempts to polymerize Mes*P=CH2with Lewis and protic acids, an intramolecular C—H activation of the ortho-CH3group in Mes* occurs.27’8 These fluoroaryl groups possess C—F bonds which are resistant to insertion reaction and provide strong electron- withdrawing abilities as well as sufficient stabilization to low-coordinate phosphorus molecules because they possess moderate steric bulk (between Mes and Mes*). Surprisingly, ArFP=CC12 ArFP=C(SiMe3)H,and ArFP=C(H)Ph are the only reported phosphaalkenes involving the C6H2- 2,4,6-(CF3)group (Arr).24 We reported the first examples of phosphaalkenes containing the more economicalC6H3-2,6-(CF)substituent (ArF’), ArF’PCMe2(6.3a) and ArF’P=C(Ph)(Me) (6.3b), which are discussed in further detail below.29 After these results were published, Dillon’s group reported the metal coordination of ArF’P=CC12as a precursor to a phosphaalkyne.3° In this chapter, the quantitative base-induced rearrangement of secondary vinyiphosphines is reported as a facile route to stable and isolable P-fluoroaryl phosphaalkenes with methyl substituents at carbon. References start on page 147 Chapter Six 132 R1--PCI2 i) HCRMgBr Rf-Ø-PcH2 cat. Base 6.1 6.2a (R1=H;R2=CH3) 1 6.3a-c 6.2b (R1=H; =C6H5 6.2c (R=CF3;R2=CH3) Scheme 6.2. 6.2 Results and discussion In an earlier report, dichiorophosphine 6.1 (Ri = H) was prepared by the lithiation of 1,3- bis(trifluoromethyl)benzene (ArF’H) with ‘BuLi/TMEDA (IVNN’,N’ tetramethylethylenediamine), and subsequent addition of PC13.’° To ensure monosubstitution of ArF’ groups at phosphorus, we reacted the lithiated benzene [2,6-(CF)HLi]with the protected phosphine C1P(NEt2),followed with the removal of the amine groups using anhydrous HC1. Subsequently, we treated dichlorophosphine 6.1 (Ri = H) with the isopropenylmagnesium bromide (1 equiv) which was freshly prepared in a mixture of ether/hexanes (5:1) (Scheme 6.2). The 31P NMR spectrum of the reaction mixture revealed a chemical shift at 76 ppm, which suggested the formation of ArF’PC1C(Me)=CH2.Simplifying the procedure, we directly reduced the secondary chiorophosphine with LiA1H4in the same pot, affording the vinyiphosphine 6.2a (531p = —57.9, JPFI = 233 Hz) quantitatively, as determined by 31P NMR spectroscopy (Scheme 6.2). Compound 6.2a was purified by vacuum distillation (bp = 35 °C; 0.01 mmHg) as a colorless liquid (yield 77%). The ‘H and‘3C{’H} NMR spectra of 6.2a are shown in Figures 6.1 and 6.2, respectively. The NMR signal assignments of 6.2a were based on the resonances of the known ArF’ compound 6.1 (R1 = H).’° References start on page 147 Chapter Six Figure 6.1. ‘H NMR spectrum (CDC13)of distilled 6.2a. (*) Residual CHC13;(**) CH21. * Figure 6.2.‘3C{’H} NMR spectrum (CDC13)of distilled 6.2a. (*) CDC13. 133 Pure vinylphosphine 6.2a is stable at room temperature under an inert atmosphere. Upon the addition of DBU (5%) to a CH21 solution of 6.2 a, a new resonance at 196 ppm was observed in the 31P NMR spectrum of the reaction mixture. This suggested the formation of phosphaalkene 6.3a (öp = 196) from vinyiphoshine 6.2a (&ip = —57.9). The conversion of the a H CF3 CH3 PCH2 a b P d,e H CH CF3 d e C C b 9876543210ppm f h d /Ka PCH2 \ 9 e CF3 e d g a h 200 . . . . . . 6d 5o 25 ppm References start on page 147 Chapter Six 134 secondary vinyiphosphine to the phosphaalkene became quantitative in three days. Initially, the purification of 6.3a using vacuum distillation was ineffectual as DBU co-distilled with the phosphaalkene and could not be separated from the distilled product. Interestingly, DBU can be extracted from the reaction mixture with degassed water, without any decomposition of 6.3a. After drying the organic layer with MgSO4,removal of the solvent affords only the crude phosphaalkene. Analytically pure phosphaalkene 6.3a was isolated (yield = 89%) as a colorless liquid by vacuum distillation (bp = 35 °C; 0.01 mmHg). The product 6.3a was fully characterized by elemental analysis, mass spectrometry, and multinuclear NMR spectroscopy. The ‘H,‘3C{’H}, and 31P NMR spectra of phosphaalkene 6.3a are shown in Figures 6.3—6.5, respectively. a C d d FC a HJ’CHs b C b 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 ppm Figure 6.3. ‘H NMR spectrum (C6D)of distilled phosphaalkene 6.3a. (*) Residual C6D5H. References start on page 147 Chapter Six 150 * 135 Figure 6.4.‘3C{1H} NMR spectrum (CDC13)of distilled phosphaalkene 6.3a. (*)cDcl3and (**) CH21. 300 200 100 0 -100 Figure 6.5. 31P NMR spectrum (CDC13)of distilled phosphaalkene 6.3a. g f’LJ FC aF33c P=C d /(b CH3 d e a b g I 200 175 ** 125 100 I I I I I I I I I I I I I I I I I I I 75 50 25 0 ppm ppm References start on page 147 Chapter Six 136 To generalize the 1,3-hydrogen rearrangement reaction as a synthetic route to isolable P fluoroaryl phosphaalkenes containing C-methyl substituents, we set out to prepare secondary vinyiphosphines 6.2b and 6.2c (Scheme 6.2). Following analogous protocols to those described for 6.2a, we conducted the vinyl substitution and the reduction with LiA1H4 in one pot. The 31P NMR spectrum of each reaction mixture displayed a single resonance which suggested quantitative formation of the secondary vinylphosphine (6.2b, 6 —61.5, ‘JPH 235 Hz; 6.2c, 6 —54.9, 1Jp1 = 231 Hz). After the vacuum distillation of both 6.2b and 6.2c, these vinyiphosphines were purified and characterized using NMR spectroscopy. The 1,3-hydrogen rearrangement of secondary vinyiphosphines 6.2b and 6.2c with trace amounts of base (DBU or DABCO) afforded phosphaalkenes 6.3b and 6.3c quantitatively as confirmed by 31P NMR spectroscopy. Phosphaalkene 6.3b was distilled (bp = 113 °C; 0.01 mmHg) to give a pure product as a colorless liquid (isolated yield = 57%). The isolated yield for 6.3b (0.88 g, 57%) is lower than that for 6.3a (3.1 g, 89%). We speculate that a relatively high percentage loss of product results from a smaller scale of starting material for the reaction (1.54 g of 6.2b vs 3.5 g of 6.2a). Interestingly, the 31P NMR spectrum of the distilled 6.3b showed two resonances with different intensities (6 = 209, >99%; 6 = 201, <1%), which suggested the presence of E- and Z isomers in phosphaalkene 6.3b (Figure 6.6). We speculate that the major resonance is due to the sterically less congested E-isomer (631p 209) and that the minor signal should correspond to the Z-isomer(631p 201). These tentative 31P NMR assignments of phosphaalkene 6.3b are based on steric arguments. Further NMR spectroscopic analysis or X-ray diffraction studies may be required to confirm the structure of 6.3b. However, to date, we have not been able to obtain single crystals of 6.3b for X-ray analysis. References start on page 147 Chapter Six 137 Ph F3C P—ct CH3 fLCF3 ,CH3 F3C PC Ph fCF3 -— 220 210 200 ppm 250 200 150 100 50 0 -50 -100 -150 -200 -250 ppm Figure 6.6. 31P NMR spectrum (CDC13)of distilled phosphaalkene 6.3b. CH CH3 CH3 100°C7h H3C H3CCH2 H3C Scheme 6.3. Intriguingly, the 1,3-hydrogen rearrangement of vinylphosphines with mesityl substituents at phosphorus could not afford phosphaalkenes quantitatively. Mathey’s group discovered that the thermolytic rearrangement of vinyiphosphine 6.4 at 100 °C gave 8 0—90% conversion to phosphaalkene (6.5) after 7 hours (Scheme 6.3).6 By using competitive References start on page 147 Chapter Six 138 coordination of the two tautomers (6.4 and 6.5) to tungsten, they were able to separate these species as their P-complexes. A theoretical study of the 1,3-hydrogen rearrangement of H2PCH(=CH)to HP=C(H)(CH3)suggested that this reaction follows either a concerted or radical pathway.31 A preparation of phosphaalkene 6.5 using a room temperature, base-catalyzed (DBU, 5%) double-bond route from vinylphosphine 6.4 was attempted. Similarly, a tautomeric mixture of 6.4 (<5%) and phosphaalkene 6.5 (>95%) was obtained (Figure 6.7). We attempted to separate these tautomers by distillation; however, equilibrium was observed in the distilled phosphaalkene. Consequently, isolable P-mesityl phosphaalkenes cannot be prepared using the double-bond migration of secondary vinyiphosphines. In contrast, we have shown that the base- induced rearrangement is a viable route to isolable phosphaalkenes containing fluoroaryl substituents at phosphorus. These new phosphaalkenes (6.3a—c) are stable under an inert atmosphere without any observation of their vinylphosphine tautomers, even after a few years. We speculate that reasonable stability is conferred to these phosphaalkenes through electron- withdrawing substituents. The fluorinated phenyl substituents have increased the acidity of the P—H proton in vinylphosphine 6.2 and this is supported by ‘H NMR spectroscopic studies and theoretical calculations.32In the ‘H NMR spectra of P-fluoroaryl and P-mesityl vinylphosphines, the signal assigned to the P—H proton in P-fluoroaryl phosphine 6.2 (6 = 5.11 — 5.35) is shifted to a lower field as compared to that for P-mesityl phosphine 6.4 (6 4•75)6 Recently, photoelectron spectroscopic studies and theoretical calculations for phosphines ArF’PH2 [ArF’ = C6H3-2,6-(CF)]and PhPH2 showed that the fluorinated species possesses a higher P—H acidity.32 Two factors were proposed to account for the predicted trend in the acidity of the P—H bond. First, the conjugate base ArF’PH is stabilized because of the better at-overlap between the phosphorus lone pair and the aromatic system. Secondly, the destabilization of the acid ArF’PH2 References start on page 147 Chapter Six 139 results from the strong steric repulsion between the phosphorus and fluorine lone pairs. As a consequence of the increased P—H acidity in phosphines substituted with electron-withdrawing groups, the 1,3-hydrogen rearrangement of secondary vinyiphosphines becomes a viable method for the synthesis of isolable phosphaalkenes provided appropriate substituents are employed. H3C H3 CH3 )‘..}—CH3 H3C CH3 CH3 H3C—c—FcCH2 CH3 ffLL. i’1LTh. jt1 ii flj ti s,tl$fl&. !ipj4tit -% J4”.JU UJrWq4J1i Litt__ 300 250 200 150 100 50 0 -50 -100 -150 ppm Figure 6.7. 31P NMR spectrum (CDC13)of distilled phosphaalkene 6.5. 6.3 Summary In this chapter, I have demonstrated that the base-induced 1,3-hydrogen rearrangement is a convenient method for the preparation of phosphaalkenes with methyl substituents at carbon, provided with an electron-withdrawing P-substituent. This also represents the first double-bond migration of secondary vinyiphosphines to isolable phosphaalkenes. In addition, I have prepared the first example of phosphaalkenes (6.3a—b) possessing a 2,6-bis(trifluoromethyl)phenyl substituent at phosphorus. References start on page 147 Chapter Six 140 6.4 Experimental section General procedures. Unless otherwise stated, these procedures were followed for all compounds prepared in this thesis. All manipulations of air-sensitive compounds were performed under nitrogen either in an Innovative Technology glovebox, or a high-vacuum line, or using standard Schlenk techniques. ‘H, ‘3C{’H}, and ‘9F NMR spectra were recorded at room temperature on a Bruker Avance 300 MHz spectrometer. Chemical shifts are reported relative to the following: residual CHC13 (6= 7.24 for ‘H); C6D5H (6 7.15 for ‘H); CDCI3 (6= 77.0 for ‘3C{’H}); 85% H3P04as an external standard (6= 0 for 3’P); CFC13 in CDC13 as an external standard (6= 0 for ‘9F). Mass spectra were acquired using a Kratos MS 50 instrument. Elemental analyses were performed by Mr. Minaz Lakha in the Departmental Microanalysis Facility. Materials. Hexanes, Et20, and CH2I were dried by passing through an activated alumina column.33 THF (from sodiumlbenzophenone), CDC13 (CIL, fromP205), and IVN,A’,IV’ tetramethylethylenediamine (TMEDA) (from Na) were freshly distilled prior to use. 1,3- (CF3)26H4,1,3,5-(CF)C6HDABCO and DBU were purchased from Aldrich and degassed prior to use. MesPC12was prepared according to a literature procedure.34C6D (CIL) was used as received. Powdered Mg was purchased from Strem and used as received. Stock solutions of Grignard reagentsH2C=C(Me)MgBr andH2CC(Ph)MgBr were freshly prepared from H2C=C(Me)Br andH2C=C(Ph)Br, respectively, at room temperature using powdered Mg in a mixture of ether/hexanes (5:1) or THF and were used after filtration. CF3 6.4.1 Preparation of2,6-(CF3)CHPC1(6.1: R1 = H). A modification of the literature procedures was followed to prepare 2,6-(CF3)CHPC1.’°ClPt2) CF3 was used instead of Pd3.To a solution of 1,3-(CF)2C6H4(20.0 g, 93.4 mmol) References start on page 147 Chapter Six 141 and TMEDA (11.0 mL, 73 mmol) in Et20 (100 mL),‘1BuLi (80 mL, 1.6 M, 130 mmol) was added at 0 °C. This solution was warmed to room temperature and was stirred for 30 mm and subsequently was cooled to —78 °C, when C1P(NEt2)(20.0 g, 95 mmol) was added dropwise. The 31P NMR spectroscopic analysis of an aliquot removed from the reaction mixture suggested the presence of2,6-(CF3)CHP(NEt(6 115). After solvent removal, ArF’P(NEt2)was extracted into hexanes (3x100 mL), the solution was filtered, and the hexanes and remaining 1,3- (CF3)26H4were removed in vacuo. A CH21 solution (150 mL) of ArF’P(NEt2)was slowly purged with HC1(g) (ca. 30 mm). The solvent was removed in vacuo, the product was extracted into hexanes (3x50 mL) and filtered, and the solvent was removed in vacuo. The product was purified by vacuum distillation (bp = 52 °C, 0.1 mmHg). Yield = 8.68 g (30%). 31P NMR (CDC13): 6149 (sept, 4JPF 61 Hz); ‘9F NMR (CDC13): 6-53.5 (d, 4JPF =61 Hz); ‘H NMR (CDC13): 68.05 (d, 3JHH = 8 Hz, 2H, rn-Ar), 7.81 (t, JHH = 8 Hz, 1H,p-Ar). CF3 6.4.2 Preparation of2,4,6-(CF3)C6HPC1(6.1: R1 = CF3). A F3C* PCI2 modification of the literature procedures was followed to prepare 2,4,6- CF3 (CF3)6H2P 1.”C1P(NEt2)was used instead of PC13. To a solution of 1,3,5-(CF)C6H(1.00 g, 3.54 mmol) in Et20 (20 mL) was added “BuLi (2.4 mL, 1.6 M, 3.9 mmol) at 0 °C. This solution was warmed to room temperature and was stirred for 30 mm and subsequently was cooled to —78 °C, when C1P(NEt2)(1.8 g, 8.6 mmol) was added dropwise. Analysis of an aliquot removed from the reaction mixture by 31P NMR spectroscopy showed 2,4,6-(CF3)C6HP(NEt(6 = 115). After solvent removal, ArFP(NEt2)was extracted into hexanes (3 x 20 mL), the solution was filtered, and the hexanes and remaining 1,3,5-(CF)C6H2 were removed in vacuo. A CH21 solution (50 mL) of ArFP(NEt2)(50 mL) was slowly purged References start on page 147 Chapter Six 142 with HC1(g) (ca. 15 mm). The solution was filtered and the solvent was removed in vacuo. The product was purified by vacuum distillation (bp 45 °C, 0.1 mmHg). Yield = 0.78 g (57%). 31P NMR (CDC13): 6 146 (sept, 4JPF = 61 Hz); ‘9F NMR (CDC13): 6 —53.7 (d, 4JPF = 61 Hz, 6F, o-CF3),—64.5 (s, 3F,p-CF); ‘H NMR (CDC13): 68.28 (s, rn-Ar). CF3 9H3 6.4.3 Preparation of2,6-(CFP(H)C(CH)= H(6.2a). To a cooled /=:z< PCH2 / (—80 °C) solution of2,6-(CF3CHPC1(5.0 g, 16 mmol) in a mixture of CF3 Et20 (50 mL) and hexanes (10 mL) was added dropwiseH2CC(CH3)MgBr (32 mL, 0.5 M, 16 mmol). The reaction mixture was checked by 3’P NMR to confirm the quantitative formation of2,6-(CF3CHPC1(C(CH)= H(6 76). LiA1H4 (0.60 g, 16 mmol) was added as a slurry in Et20 at —80 °C, the slurry was warmed to room temperature, and subsequently H20 was added to quench excess LiA1H4.The water layer was extracted with fresh Et20 (2x). The ether layers were combined and dried with MgSO4,and solvent was removed by distillation at 1 atm. The product was distilled (bp = 35 °C, 0.01 mmHg). Yield = 3.5 g (77%). 3’P NMR (CDC13): 6—57.9 (d, ‘JPH = 233 Hz); ‘9F NMR (CDC13): 6 —57.8 (d, 4JpF = 27 Hz); ‘H NMR (CDC13): 6 7.94 (d, JHH = 8 Hz, 2H, rn-Ar), 7.58 (t, 3JHH = 8 Hz, 1H, p-Ar), 5.38 (d, 3JPH = 30 Hz, 1H, O=CHH), 5.11 (d, JPH = 231 Hz, lH, P1]), 4.93 (d, 3JPH = 13 Hz, 1H, CCHR), 1.83 (d, JpH = 8 Hz, 3H, CH3);‘3C{’H} NMR (CDC13): 6 140.4 (d, Jcp = 17 Hz, i-Ar), 136.3 (qd, 2Jcp = 8 Hz, JCF = 30 Hz, o-Ar), 134.2 (d, 1Jcp = 41 Hz, P—CC), 130.0 (d, 3Jcp = 6 Hz, rn-Ar), 129.2 (s,p-Ar), 123.7 (q, JCF = 275 Hz, CF3), 123.2 (d, 2Jcp = 25 Hz, P—CC), 23.7 (d, 2Jcp = 12 Hz, CR3). References start on page 147 Chapter Six 143 CF3 Fh 6.4.4 Preparation of2,6-(CF3)CHP(H)C(Ph)=CH (6.2b). To a solution — CCH2 of2,6-(CF3)CHPC1(3.0 g, 9.5 mmol) in Et20 (20 mL) was added CF3 dropwiseH2C=C(Ph)MgBr (30 mL, 0.317 M, 9.5 mmol). After being stirred for 1 h, the solution turned yellow and a white precipitate formed. The reaction was checked by 31p NMR to confirm the quantitative formation of2,6-(CF3)CHPC1(C(Ph)=CH (6 71). A slurry of LiA1H4 (0.18 g, 4.7 mmol) in Et20 (5 mL) was added at 0 °C. After the mixture was warmed to room temperature and excess LiA1H4was quenched with water (10 mL), the organic layer was dried with MgSO4 and filtered and the solvent was removed in vacuo. The liquid product was distilled (bp = 104 °C, 0.01 mmHg) and subsequently recrystallized from a minimal amount of hexanes at —70 °C. Yield = 1.54 g (47%). 31P NMR (CDC13): 6 —61.5 (d, 1JpH = 235 Hz); ‘9F NMR (CDC13): 6—58.2 (d, 4JpF = 27 Hz); ‘H NMR (CDC13): 67.99 (d, JHH = 8 Hz, 2H, rn-Ar), 7.64 (t, 3JHH 8 Hz, 1H,p-Ar), 7.42—7.27 (m, 511, Ph), 5.64 (dd, 3Jp,- = 17 Hz, 2JHH =2 Hz, 1H, C=CHH), 5.35 (d sept, ‘Jp- = 235 Hz, JFH 2 Hz, 1H, P11), 4.67 (dd, 3JpH 7 Hz, 2JHH 2 Hz, 1H, CCHR). F3 CH3 6.4.5 Preparation of2,4,6-(CF3)6P(H)C H= H(6.2c). To a CCH2 solution of2,4,6-(CF3)C6HPC1(0.78 g, 2.0 mmol) in a mixture of H Et20 (20 mL) and hexanes (10 mL) was added dropwise H2C=C(CH3)MgBr (4.9 mL, 0.5 M, 2.4 mmol) in Et20. The reaction mixture was checked by 31P NMR, and 2,4,6-(CF6PC1(C H) H(6 = 75) was formed quantitatively. LiAIH4 (0.04 g, 1.05 mmol) was added as a slurry in Et20 at —80 °C, the mixture was warmed to room temperature. Subsequently the ether layer was extracted with H20 to quench excess LiA1H4 and References start on page 147 Chapter Six 144 dried with MgSO4,and solvents were removed by distillation at 1 atm. The liquid product was distilled (bp = 35 °C, 0.01 mmHg). Yield = 0.50 g (70%). 31P NMR (CDC13): 6—54.9 (d, 1JPH = 231 Hz); ‘9F NMR (CDC13): 6 —58.6 (d, 4JpF = 26 Hz, 6F, o-CF3), —64.2 (s, 3F,p-CF); ‘H NMR (CDC13): 68.12 (s, 2H, rn-Ar), 5.43 (d, 3JpH = 34 Hz, 1H, C=CHH), 5.12 (d, 1JPH = 231 Hz, 1H, PH), 5.02 (d, 3JpH 14 Hz, 1H, C=C1-IH), 1.79 (d, 3Jpj- = 7 Hz, 3H, CH3);‘3C{’H} NMR (CDC13): 6 140.2 (d, 1Jcp = 17 Hz, P—CC), 139.3 (d, ‘Jcp = 17 Hz, i-Ar), 137.4 (qd, 2Jcp = 8Hz, JCF = 31 Hz, o-Ar), 131.6 (q, 2Jcp = 34 Hz,p-Ar), 126.8 (s, m Ar), 125.2 (d, 2Jcp = 29 Hz, P—C=C), 123.0 (q, 1JCF = 275 Hz, o-CF3), 122.6 (q, ‘JCF = 273 Hz,p CF3), 24.0 (d, 2Jcp = 9 Hz, CH3). ,CH3 6.4.6 Preparation of2,6-(CF3)CHP=C H (6.3a). To a solution of 2,6- F3C /L ‘CH3 (CF)2P(H)C(CH= H(3.5 g, 12.2 mmol) in CH21 (20 mL) was \j—CF3 added DBU (0.1 g, 0.6 mmol). The solution was stirred for 3 d and the reaction progress was monitored by NMR. Upon complete consumption of 2,6- (CF3)26HP(H)C(CHHthe reaction mixture was extracted with degassed H20 (3 xl 0 mL) to remove DBU, dried with MgSO4,and filtered and the solvent removed under N2 (1 atm). The product was distilled (bp = 35 °C, 0.01 nimHg). Yield = 3.1 g (89%). 31P NMR (CDC13): 6 196 (m); ‘9F NMR (CDC13): 6—59.6 (d, 4JPF 22 Hz); ‘H NMR (CDC13): 6 7.90 (d, JHH = 8 Hz, 2H, rn-Ar), 7.55 (t, JHH = 8 Hz, 1H,p-Ar), 2.25 (d, 3Jp11 = 25 Hz, 3H, =CCH3), 1.72(d, Jp-{= 13Hz, 3H, CCH3); ‘HNMR(C6D):67.39(d, JHF{ 8Hz, 2H, m Ar), 6.69 (t, JHH = 8 Hz, 1H,p-Ar), 1.96 (d, 3JpH = 25 Hz, 3H, =CCH3), 1.55 (d, 3JpH = 13 Hz, 3H, =CCH3);‘3C{’H} NMR (CDC13): 6 193.2 (d, 1Jcp = 42 Hz, PC), 141.8 (d, ‘Jcp = 74 Hz, i Ar), 134.5 (q, 2JCF = 29Hz, o-Ar), 129.0 (d, 3Jcp = 5 Hz, rn-Ar), 128.2 (s,p-Ar), 123.6 (q, 1JCF = References start on page 147 Chapter Six 145 275 Hz, CF3), 27.9 (d, 2Jcp = 47 Hz, CH3), 27.1 (d, 2Jcp = 17 Hz, CH3); MS (El, 70 eV): 287, 286 [3,24;M];272,27l [11, 1O0;M—CH];Ana1. CalcdforC,,H9F6P:C,46.17;H,3.17. Found: C, 46.49; H, 3.13. Ph 6.4.7 Preparation of (E,Z)-2,6-(CF32C6HP=C(Ph)(CH (6.3b). To a F3C p=c ‘CH3 solution of2,6-(CF)P(H)C(Ph)=CH (1.54 g, 4.42 mmol) in CH21 \ / CF3 (5 mL) was added DBU (0.1 g, 0.6 mmol). The solution turned slightly yellow, and the quantitative formation of2,6-(CF3)CHPC(Ph)(CH was observed over 3 d as determined by “P NMR. The reaction mixture was extracted with degassed H20 (3x15 mL) to remove DBU, the organic layer was dried with MgSO4,and filtered and the solvent was removed in vacuo. The product was distilled (bp 113 °C, 0.01 mmHg). Yield 0.88 g (57%). 31P NMR (CDC13): 6209 (>99%); E-isomer, tentative), 201 (<1%; Z-isomer); ‘9F NMR (CDC13): 6 —59.5 (d, 4.JPF = 20 Hz); ‘H NMR (CDC13): 67.97 (d, 3JHH = 8 Hz, 2H, rn-Ar), 7.67 (m, 2H, o Ph), 7.62 (t, 3JHH = 8 Hz, 1H,p-Ar), 7.41—7.36 (m, 3H, rn,p-Ph), 2.13 (d, 3JpH = 14 Hz, 3H, CH3); ‘3C{’H} NMR (CDC13): 6 193.3 (d, 1Jcp = 41 Hz, PC), 143.5 (d, 2Jcp 27 Hz, i-Ph), 141.7 (d, 1Jcp 74 Hz, i-Ar), 134.6 (q, 2JCF 30 Hz, o-Ar), 129.6, 129.2 (d, 3”4Jcp 5 Hz; 3”4Jcp = 4 Hz; rn-Ph and Ar), 128.9, 128.5 (s, p-Ph and Ar), 125.3 (d, 3Jcp = 21 Hz, o-Ph), 123.7 (q, ‘JCF = 275 Hz, CF3), 25.4 (d, 2Jcp = 16 Hz, CH3); MS (El, 70 eV): 349, 348 [16, 88; Mj; 334, 333 [16, 100; M — CH3]; 271 [36; M — Ph]; 103 [50; M — PAr]; Anal. Calcd forC,6H1FP:C, 55.19; H, 3.18. Found: C, 55.42; H, 3.01. CH3 6.4.8 Preparation of2,4,6-(CF3)C6HPC(CH (6.3c). This reactionF3c CH was performed on an NMR scale. In an NMR tube, DABCO (1.0 mg, 0.01 / CF3 F3C References start on page 147 Chapter Six 146 mmol) was mixed with 2,4,6-(CF3)C6HP(H)C H= H(0.10 g, 0.28 mmol) in THF (1 mL). The 6.3c was formed quantitatively over 3 d as determined by 31P NMR. The solvent was removed in vacuo, and the liquid residue was dissolved in CH2I (2 mL) and extracted with degassed H20. The CH2I solution was dried with MgSO4,and filtered and the solvent removed leaving a small amount of 6.3c as a colorless liquid. An isolated yield was not determined. 31P NMR (CDC13): 6 192 (m); 19F NMR (CDC13): 6—60.2 (d, 4JpF 22 Hz, 6F, o-CF3), —64.0 (s, 3F,p-CF);HNMR(CDC1):68.13 (s, 2H, rn-Ar), 2.28 (d, 3JPH = 25Hz, 3H, CCH3), 1.75 (d, 3JPH = 13 Hz, 3H, CCH3). CH3 CH3 6.4.9 Preparation of MesP(H)C(CH3)=CH2(6.4). To a cooled H3C*cCH2 (-80 °C) solution of MesPC12(20.0 g, 90.5 mmol) in a mixture of THF CH3 (100 mL) and hexanes (200 mL) was added dropwise H2C=C(CH3)MgBr (120 mL, 0.8 M, 96 mmol). The reaction mixture was checked by 31P NMR to confirm the quantitative formation of MesPC1(C(CH3)CH2(6= 68). The solvent was removed in vacuo leaving a brown mixture and redissolved in a mixture ofEt20 (130 mL) and hexanes (130 mL). LiA1H4(1.7 g, 45 mmol) was added as a slurry in Et20 (70 mL) at —80 °C, the slurry was warmed to room temperature, and subsequently H20 was added to quench excess LiA1H4.The water layer was extracted with fresh Et20 (2x). The ether layers were combined and dried with MgSO4,and the solvent was removed in vacuo. The product was distilled (bp = 57 — 59 °C, 0.01 mmHg). Yield = 10.5 g (60%). 31P NMR (CDC13): 6—69.8 (d, ‘JpH = 233 Hz). Characterization of 6.4 agreed with those reported previously in the literature.7 References start on page 147 Chapter Six 147 H 6.4.10 Preparation of MesP=C(CH3)2(6.5). To a solution of3 P=C bH3 MesP(H)C(CH)=C112(6.4) (2.0 g, 10.4 mmol) in CH2I (5 mL) was / CH H3C added DBU (0.75 g, 4.9 mmol). The solution was stirred for 1 week and the reaction progress was monitored by 31P NMR. When 6.4 stopped converting to the product, the reaction mixture was extracted with degassed H20 (5x10 mL) to remove DBU, dried with MgSO4,and filtered and the solvent removed in vacuo. The product was distilled (bp 80 °C, 0.01 mmHg). Yield 1.3 g (65%). 31P NMR (CDC13): 6214 (>95%); —69.8 (d, <5%). Characterization of 6.5 agreed with those reported previously in the literature.7 6.5 References 1. Allcock, H. R., Lampe, F. W., Mark, J. E. Contemporary Polymer Chemistry, 3rd ed.; Pearson Education: Upper Saddle River, NJ, 2003; Chapter 22. 2. Mathey, F. Angew. Chem. mt. Ed 2003, 42, 1578. 3. Dillon, K. B., Mathey, F., Nixon, J. F., Phosphorus: The Carbon Copy. Wiley: New York, 1998. 4. Appel, R., In Multiple Bonds and Low Coordination in Phosphorus Chemistry; Regitz, M., Scherer, 0. J., Eds.; Georg Thieme Verlag: Stuttgart, 1990; pp 157—2 19. 5. Appel, R.; Knoll, F. Adv. Inorg. Chem. 1989, 33, 259. 6. Mercier, F.; Hugel-Le Goff, C.; Mathey, F. Tetrahedron Lett. 1989, 30, 2397. 7. Gaumont, A. C.; Guillemin, J. C.; Denis, J. M. J Chem. Soc. Chem. Commun. 1994, 945. 8. Appel, R.; Winkhaus, V.; Knoch, F. Chem. Ber. 1987, 120, 125. 9. Karlstédt, N. B.; Borisenko, A. A.; Foss, V. L. Z. Obsch. Khim. 1992, 62, 1516. References start on page 147 Chapter Six 148 10. Escudie, J.; Couret, C.; Ranaivonjatovo, H.; Lazraq, M.; Satge, J. Phosphorus and Suifur 1987, 31, 27. 11. Scholtz, M.; Roesky, H. W.; Stalke, D.; Keller, K.; Edelmann, F. T. .1 Organomet. Chem. 1989, 366, 73. 12. Abe, M.; Toyota, K.; Yoshifuji, M. Chem. Lett. 1992, 2349. 13. Ahlemann, J.-T.; Roesky, H. W.; Murugavel, R.; Parisini, E.; Noltemeyer, M.; Schmidt, H.-G.; MUller, 0.; Herbst-Irmer, R.; Markovskii, L. N.; Shermolovich, Y. G. C’hem. Ber./Recueil 1997, 130, 1113. 14. Miqueu, K.; Sotiropoulos, J.-M.; Pfister-Guillouzo, G.; Rudzevich, V. L.; Gornitzka, H.; Lavallo, V.; Romanenko, V. D. Eur. I Inorg. Chem. 2004, 2289. 15. Dumitrescu, A.; Gornitzka, H.; Schoeller, W. W.; Bourissou, D.; Bertrand, G. Eur. I Inorg. Chem. 2002, 1953. 16. Rudzevich, V. L.; Gornitzka, H.; Miqueu, K.; Sotiropoulos, J.-M.; Pfister-Guillouzo, G.; Romanenko, V. D.; Bertrand, G. Angew. Chem. mt. Ed 2002, 41, 1193. 17. Davidson, M. G.; Dillon, K. B.; Howard, J. A. K.; Lamb, S.; Roden, M. D. I Organomet. Chem. 1998, 550, 481. 18. Wallace, T. C.; West, R.; Cowley, A. H. Inorg. Chem. 1974, 13, 182. 19. Batsanov, A. S.; Cornet, S. M.; Dillon, K. B.; Goeta, A. E.; Hazendonk, P.; Thompson, A. L. I Chem. Soc. Dalton Trans. 2002, 4622. 20. Burford, N.; Macdonald, C. L. B.; LeBlanc, D. J.; Cameron, T. S. Organometallics 2000, 19, 152. 21. Voelker, H.; Labahn, D.; Bohnen, F. M.; Herbst-Inner, R.; Roesky, H. W.; Stalke, D.; Edelmann, F. T. New I Chem. 1999, 23, 905. References start on page 147 Chapter Six 149 22. Ahlemann, J.-T.; Künzel, A.; Roesky, H. W.; Noltemeyer, M.; Markovskii, L.; Schmidt, H.-G. Inorg. Chem. 1996, 35, 6644. 23. Weber, L.; Schumann, H.; Boese, R. Chem. Ber. 1990, 123, 1779. 24. Dillon, K. B.; Goodwin, H. P.1 Organomet. Chem. 1994, 469, 125. 25. Dumitrescu, A.; Rudzevich, V. L.; Romanenko, V. D.; Man, A.; Schoeller, W. W.; Bourissou, D.; Bertrand, G. Inorg. Chem. 2004, 43, 6546. 26. Cornet, S. M.; Dillon, K. B.; Goeta, A. E. Inorg. Chim. Acta 2005, 358, 844. 27. Tsang, C.-W.; Rohrick, C. A.; Saini, T. S.; Patrick, B. 0.; Gates, D. P. Organometallics 2004, 23, 5913. 28. Tsang, C.-W.; Rohrick, C. A.; Saini, T. S.; Patrick, B. 0.; Gates, D. P. Organometallics 2002, 21, 1008. 29. Yam, M.; Tsang, C.-W.; Gates, D. P. Inorg. Chem. 2004, 43, 3719. 30. Cornet, S. M.; Dillon, K. B.; Goeta, A. E.; Howard, J. A. K.; Roden, M. D.; Thompson, A. L. I Organomet. Chem. 2005, 690, 3630. 31. Nguyen, M. T.; Landuyt, L.; Vanquickenborne, L. G. Chem. Phys. Lett. 1993, 212, 543. 32. Miqueu, K.; Sotiropoulos, J.-M.; Pfister-Guillouzo, G.; Rudzevich, V.; Romanenko, V.; Bertrand, G. Eur. I Inorg. Chem. 2004, 281. 33. Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J. Organometallics 1996, 15, 1518. 34. Becker, G.; Uhi, W.; Wessely, H.-J. Z Anorg. Aug. Chem. 1981, 479, 41. References start on page 147 150 Chapter Seven Overall Conclusions and Future Work 7.1 Summary of thesis work Despite the tremendous importance of addition polymerization in the industrial production of organic polymers, this method was often dismissed in inorganic polymer synthesis before the start of this thesis research. The major objective of this thesis was to establish addition polymerization as a viable route to inorganic polymers. Due to the well-known parallels between PC and C=C chemistry, we hypothesized that phosphaalkenes might mimic olefins ability to undergo addition polymerization.1’2By adjusting the kinetic and thermodynamic stability of the monomer using sterically hindered substituents, an isolable and polymerizable phosphaalkene monomer 7.la was discovered. ,Ph initiator E Fh1 P=C H—P—C-I— Me Ph 150 °C [Mes Ph] 7.la 7.2a Scheme 7.1. We have successfully polymerized 7.la via distillation, radical, and anionic initiation to afford poly(methylenephosphine) (7.2a) with a molecular weight in an excess of >10 gmo[’ (Scheme 7.1) and these studies are reported in Chapter Two.3 The air and moisture stability of 7.2a was improved via oxidation with H20 or S8 to a phosphorus(V) species (Scheme 7.2). These new phosphorus-containing polymers showed thermal stability to weight loss to Ca. 300 °C. The addition polymerization of phosphaalkenes and subsequent chemical functionalization References start on page 155 Chapter Seven 151 have established new synthetic routes to polymers with a backbone of phosphorus atoms. More importantly, the first addition polymerization of a P=C bond has laid a foundation for the addition polymerization of other inorganic multiple-bond systems. F S Ph] S8 F Ph] H202 F Ph] —f—P—C-I— -+—P—C±- —f—P—C I I II I II II I L Mes Ph]0 L Mes Ph] , L Mes Ph ,. 7.1 a Scheme 7.2. After demonstrating the polymerizability of P=C bonds, the next goal was to generalize this addition polymerization method to other P=C systems. Chapter Three outlines the synthesis of several P-mesityl phosphaalkenes with various C-aryl substituents using the base-catalyzed phospha-Peterson reaction (Scheme 73)•4 Crystals suitable for X-ray diffraction analysis were obtained for P-Mes phosphaalkenes 7.lb, 7.le, and 7.lf. Attempts to prepare several P adamantyl phosphaalkenes using the phospha-Peterson reaction were unsuccessful. In particular, the phosphaalkene 7.3 dimerized to give an interesting 1 ,2-diphosphetane (7.4), which was characterized by X-ray diffraction studies. ,SiMe3 R1 cat. KOH Ad Ad Mes—P + O=C P=C / “SiMe3 — (Me3S1)20 Mes’ R2 ,P=R Ad Ph Ph Ph 7.laR1=R2Ph 7.lb =4-FC6H 7.3 7.4 7.lc R1=Ph;R2=4-FC6H 7.1 d2=4-(MeO)C 7.le R1=Ph; =4-(MeO)C 7.lf R1=Ph;R2=2-Pyridyl Scheme 7.3. References start on page 155 Chapter Seven 152 In Chapter Four, the effects of reaction temperature, residence time, initiator loading, and substituent on the radical polymerization of phosphaalkene 7.la were studied and polymers were obtained from all phosphaalkenes (7.lb-f). The molecular weights of radical polymers 7.2 were increased (to> 16,000 gmor’) by lowering reaction temperature (90 — 100 °C) and initiator loading ( 1% VAZO 88). The monomer conversion and isolated yield could be improved by extending the residence time (e.g. at 100 °C, 1% VAZO 88, yield> 50% when time > 4 d). By applying similar conditions to other P=C systems (7.lb—f), polymers with molecular weights up to 20,000 gmol’ were prepared (Scheme 7.4). This addition polymerization provides a facile method to generate a series of phosphine polymers with different C-functionalities. 1%VAZO88 i 1 P=c —I---P—c—± Me R2 100 °C [Mes R2 j 7.1 7.2 7.laR=R2Ph 7.lb1=4-FC6H 7.lc R1=Ph;R2=4-FC6H 7.ld2=4-(MeO)C 7.leR1=Ph; =4-(MeO)C 7.lf =Ph;R2=2-Pyridyl Scheme 7.4. Since both olefins and phosphaalkenes can undergo homopolymerization, we speculated that the copolymerization of P=C and C=C bonds might be a possible method to prepare phosphine-functional organic macromolecules. Chapter Five reports the success in the radical copolymerization of phosphaalkenes with styrene to a new hybrid inorganic-organic polymer (7.5) (Scheme 7•5)5 We have thus introduced a direct path for incorporating inorganic functionalities (in this case, phosphorus) into an organic polymer backbone. To better understand the copolymer backbone, the monomer reactivity ratios were estimated for phosphaalkene 7.la References start on page 155 Chapter Seven 153 and styrene using the Fineman—Ross and Kelen—TUdOs graphical methods. Preliminary studies showed that the reactivity ratios for this monomer pair are rpA 2 and rsT 1. These values suggest that phosphaaalkene-monomer blocks [i.e. -(P—C)-(P—C)-(P—C)-j are present in the polystyrene backbone of copolymers 7.5. ,R 1% VAZO 88 / \ / + I Me Ph Ph 100°C, 24 h \,Mes Ph J Ph , 7.laR=Ph 7•5 7ie R = 4-MeOC6H Scheme 7.5. To broaden the range of polymerizable phosphaalkene monomers, several P-fluoroaryl phosphaalkenes bearing methyl substituents at carbon were prepared. Chapter Six describes the synthesis of these molecules using the base-catalyzed 1,3-hydrogen rearrangement reaction (Scheme 7.6).6 In previous reports, this reaction could only be used to prepare phosphaalkenes in equilibrium with secondary vinyiphosphine tautomers, which were inseparable.7’8However, employing the electron-withdrawing P-ArF or P-ArF’ [ArF =C6H2-2,4,6-(CF3);ArF’ =C61-13-2,6- (CF3)2]substituents changes the equilibrium constant and phosphaalkenes 7.6 were synthesized quantitatively. This advance represents the first quantitative formation of phosphaalkenes from secondary vinyiphosphines via the 1,3-hydrogen rearrangement route. CF3 R2 R2 R1—--cH2 cat. Base R1=H;R2=CH3 7.6 R1=H; =C6H5 =CF3;R2=CH3 Scheme 7.6. References start on page 155 Chapter Seven 154 7.2 Future work 7.2.1 Studies of P-fluoroaryl phosphaalkenes • Generalize the addition polymerization of P=C bonds to P-fluoroaryl phosphaalkenes; o Investigate the use of different anionic, cationic, and radical initiators for polymerization of P-fluoroaryl phosphaalkenes 7.6; o Copolymerize P-fluoroaryl phosphaalkenes with vinyl monomers; • Synthesize new P-fluoroaryl phosphaalkenes with long C-alkyl substituents using 1,3- hydrogen rearrangement reaction and investigate their polymerizability. 7.2.2 Free-radical (co)polymerization of phosphaalkenes • Copolymerize P-mesityl phosphaalkenes 7.lb-f with styrene; • Estimate the monomer reactivity ratios for each monomer pair; • Synthesize various hybrid inorganic-organic macromolecules by copolymerizing phosphaalkenes with different vinyl monomers; • Replace the methoxy or fluoro groups of the poly(methylenephosphine)s with other functionalities via coupling reactions; • Prepare new phosphaalkene monomers with different P- or C-functional substituents (R1P=CR23)by the base-catalyzed phospha-Peterson reaction ofR1P(SiMe3)2and the corresponding ketones (0CR2R3). 7.2.3 Attempts to polymerize other inorganic multiple-bond systems using addition polymerization • Search for a suitable inorganic multiple-bond system as monomer; References start on page 155 Chapter Seven 155 • Prepare sufficient amounts of monomers for polymerization; • Polymerize these monomers with cationic, anionic, or radical initiators. 7.3 Closing remarks The addition polymerization of phosphaalkenes can be conveniently performed via homopolymerization or copolymerization with anionic or radical initiators. Importantly, the objective of this thesis was achieved and the analogy between P=C and C=C bonds is extended to polymer chemistry (Scheme 7.7). The simple access to high-molecular weight macromolecules using addition polymerization is certainly an attractive, convenient methodology for inorganic polymer synthesis. The hidden potential of these new fascinating macromolecules, poly(methylenephosphine) and its copolymers, is undoubtedly enormous. \ / __ / CC 4p PC / \ / \ \ / ___ Ii ii / ___ Ii ii /C=C\ fl /P=C\ Scheme 7.7. 7.4 References 1. Dillon, K. B., Mathey, F., Nixon, J. F., Phosphorus: The Carbon Copy. Wiley: New York, 1998. 2. Mathey, F. Acc. Chem. Res. 1992, 25, 90. 3. Tsang, C.-W.; Yam, M.; Gates, D. P. J Am. Chem. Soc. 2003, 125, 1480. 4. Yam, M.; Chong, J. H.; Tsang, C.-W.; Patrick, B. 0.; Lam, A. E.; Gates, D. P. Inorg. Chem. 2006, 45, 5255. References start on page 155 Chapter Seven 156 5. Tsang, C.-W.; Baharloo, B.; Riendi, D.; Yam, M.; Gates, D. P. Angew. Chem. mt. Ed. 2004, 43, 5682. 6. Yam, M.; Tsang, C.-W.; Gates, D. P. Inorg. Chem. 2004, 43, 3719. 7. Mercier, F.; Hugel-Le Goff, C.; Mathey, F. Tetrahedron Lett. 1989, 30, 2397. 8. Gaumont, A. C.; Guillemin, J. C.; Denis, J. M. I Chem. Soc. Chem. Commun. 1994, 945. References start on page 155

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