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UBC Theses and Dissertations

Bis (ditertiaryphosphine) complexes of rhodium, and catalytic asymmetric hydrogenation Mahajan, Devinder 1979

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B I S ( D I T E R T I A R Y P H O S P H I N E ) C O M P L E X E S O F R H O D I U M , A N D C A T A L Y T I C A S Y M M E T R I C H Y D R O G E N A T I O N by D E V I N D E R [ M A H A J A N M . S c , University of British Columbia, 1976 A T H E S I S S U B M I T T E D I N P A R T I A L F U L F I L M E N T O F T H E R E Q U I R E M E N T S F O R T H E D E G R E E O F D O C T O R O F P H I L O S O P H Y i n THE FACULTY OF GRADUATE STUDIES Department of Chemistry We accept this thesis as conforming to the required standard The University of British Columbia May, 1979 © Devinder Mahajan, 1979 I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l ' f i l l f i l m e n t o f t h e r e q u i r e m e n t s f o r a n a d v a n c e d d e g r e e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l m a k e i t f r e e l y a v a i l a b l e f o r r e f e r e n c e a n d s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s m a y b e g r a n t e d b y t h e H e a d o f my D e p a r t m e n t o r b y h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t b e a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . D e p a r t m e n t n f C H l-M i S T R Y T h e U n i v e r s i t y o f B r i t i s h C o l u m b i a 2 0 7 5 W e s b r o o k P l a c e V a n c o u v e r , C a n a d a V 6 T 1W5 D a t e D E - 6 B P 75-51 1 E - i i -ABSTRACT Rhodium(I)-bis(ditertiaryphosphine) complexes of the general formula Rh(PrP)0Cl[P/^P = Ph„P(CH„) PPh„, n = 1-4, and (+)-diop (diop = 2,3-0-z / z n. i — — isopropylidene 2,3-dihydroxy-l,4 bis(diphenylphosphino)butane] have been prepared by treating [RhCcyclooctene^CJi^ with the appropriate ditertiary-phosphine. The n=l, and n=4 and diop species are five-coordinate in the solid state and in non-polar solvents, while the n=2 and 3 species contain ionic chloride. The cationic complexes RMP^^^X were prepared from the Rh(P/^P)2CJl species by adding AgX[X=SbF^,PFg,BF^] . Reaction of the chloro complexes with borohydride has yielded the hydrides, HRh(P/'"NP)2, for the 1 31 n=2 and 3 diphosphines, and for (+)-diop. H and P nmr, as well as visible spectral data, are presented: a solvent-dependent deshielding of ortho protons of the phenyl groups is observed in some of the complexes, and the ligand GH^  protons are coupled to the rhodium in the Rh(Ph PCI^PPh^^ cation; the P atom in this bis(diphenylphosphino) ligand shows an unusual highfield shift on coordination to rhodium. Preliminary kinetic data for catalytic hydrogenation of methylene succinic 0 acid or itaconic acid (IA) show that the cationic and hydrido complexes are more active than the corresponding chloro complexes, and that activity generally increases with increasing chain length of the diphosphine. The rhodium-bis(diop) complexes efficiently catalyze the asymmetric hydrogenation of a number of prochiral substrates, optical purities of >90% being obtained in the hydrogenation of N-acylaminoacrylic acids. Steric factors at the olefinic bond, and coordination of the -NHCOR group through the "^ C=0 moiety, seem important in determining the hydrogenation rates. The rates are slower in the more strongly coordinating DMA compared to n-butanol-toluene mixtures. The solvent medium has l i t t l e effect on the + - i i i -degree of asymmetric induction, when using thre Rh[(+)-diop] 2^ or HRh[(+)-d i o p ] 2 complexes, but reversal of product configuration i s observed when using the Rh[(+)-diop]^Cl complex i n DMA or i n n-butanol-toluene mixtures. An unusual increase i n op t i c a l purity of the product with increasing temperature has been observed i n the hydrogenation of IA. Detailed k i n e t i c and spectroscopic studies on the hydrogenation of IA catalyzed by HRh[(+)-diop] are explained i n terms of a mechanism involving the formation of a metal a l k y l v i a coordination of the o l e f i n i c substrate, followed by reaction with H 2 to y i e l d the saturated product (S.P.) and regenerated catalyst (equations [l ] - [ 3 ] ) . A monodentate diop(diop*) i s invoked: HRh(diop) 2 _ HRh(diop)(diop*) [1] HRh(diop) (diop*) + o l e f i n k Rh(diop) (diop*) (alkyl) [2] Rh(diop)(diop*)(alkyl) + > HRh(diop)(diop*) + S.P. [3] The i n i t i a l hydride catalyst i s slowly decomposed by protons of the acidic substrate to give Rh(diop) 2 +. To avoid t h i s complication, a mechanistic study was carried out on the HRh(diop) 2"Styrene-H 2 system, which was found to proceed v i a the same mechanism as outlined i n equations [ l ] - [ 3 ] , A mechanistic study on the Rh(diop) 2 +BF^ -catalyzed hydrogenation of IA shows that the reaction proceeds mainly v i a the 'hydride' route: Rh(diop) 2 + + H 2 ^ Rh(diop) 2H 2 + [4] Rh(diop) 2H 2 + + IA v=^Rb(diop) (diop*) ( H ) 2 ( I A ) + [5] Rh(diop) (diop*) ( H ) 2 ( I A ) + > Rh(diop) 2 + + S.P. [6] - i v -A complete inhibition of the hydrogenation by small amounts of added diop(diop:Rh>0.2) is tentatively attributed to formation of an inactive polymeric species: nRh(diop) 2H 2 + + diop > [Rh(diop)(diop*)H 2 +] n [7] The forward step of Reaction [4] was studied in detail in the absence of olefinic substrate. Spectroscopic and kinetic data are best explained in terms of dihydride formation via the consecutive reactions outlined in equation [8]: Rh(diop)„+ s . •> Rh(diop)(diop*)S + — — • Rh(diop) (diop*) (H) „S + [8] Rh(diop) 2H 2 + The dehydrogenation reaction was also studied. The reactions of [Rh(P~P)2]A complexes (A=C£,BF4) with C0,C>2,H2 and HC&(g) yield several new complexes. Thus the [Rh(P P)2XY] BF^ complexes rs rs r\ (P P = dpm,dpp;XY=CO, P P=dpm,dpe,dpp;XY=02, P P=dpp, (+)-diop: XY=H2, and rs P P=dpm,dpe,dpp;XY=HC&) were isolated and characterized. The solution 31 structures were determined using especially variable temperature P nmr spectroscopy. The formally six-coordinate rhodium(III) dioxygen and the dihydrido complexes were assigned cis geometries, whereas the HC£ complexes were more fluxional and cis geometries could only be assigned with certainty to the dpp complex; for dpm and dpe complexes, the limiting spectra could not be achieved even at -60°C. For the five-coordinate rhodium(I) CO complexes, the dpp complex has been assigned a TBP structure but the dpm complex is fluxional even at -60°C. Some stopped-flow kinetic data are presented for the addition of CO, -v-0 2, and B.^ to the Rh(P P>2 complexes. For the dpp system, the rate increased in the order CO>H2>02, although the reactions are not simple 1:1 single step additions, solvated species probably playing an important role (cf. equation [8]). - v i -TABLE OF CONTENTS Page ABSTRACT i i TABLE OF CONTENTS v i LIST OF TABLES ix LIST OF FIGURES xi ABBREVIATIONS xv ACKNOWLEDGEMENTS x v i i i CHAPTER I. INTRODUCTION 1 1.1 General 1 1.2 Scope of this thesis 3 1.3 Modes of hydrogen activation 5 1.4 Rhodium catalysts in homogeneous hydrogenation 9 1.5 Homogeneous hydroformylation 12 1.6 Solvent transfer hydrogenation of unsaturated compounds 13 1.7 Asymmetric homogeneous hydrogenation 14 CHAPTER II. GENERAL EXPERIMENTAL 23 11.1 Materials 23 11.1.1 Solvents 23 11.1.2 Gases 23 11.1.3 Diphosphines 23 11.1.4 Olefins 24 11.1.5 Inorganic silver salts 24 11.2 Instrumentation 25 11.3 Gas-uptake apparatus 25 11.4 Gas-uptake experimental procedure 27 11.5 Solubility determination 28 11.6 Stoichiometric gas-uptake measurements 29 11.7 Spectrophotometric kinetic measurements 30 11.8 Fast reaction measurements 32 11.9 Work-up of the hydrogenated products 32 11.9.1 Itaconic, citraconic, maleic, and atropic acids 32 11.9.2 N-a-acetamido-acrylic and -cinnamic acids 32 11.9.3 Liquid olefins 32 11.9.4 Acrylamide 33 11.10 Optical rotation measurements 33 CHAPTER III.HOMOGENEOUS HYDROGENATION OF ACIDIC AND 34 NON-ACIDIC SUBSTRATES USING THE HYDRIDOBIS(DIOP)-RHODIUM(I) COMPLEX 111.1 Introduction 34 111.2 Experimental 35 111.3 The summary of studies done on the Itaconic acid - HRh[(+)-diop]2 system 37 111.4 The HRh[(+)-diop]2-Styrene system 44 111.4.1 Results 44 111.4.2 Discussion 51 - v i i -Page III.5 Conclusions 56 CHAPTER IV. BIS(DITERTIARYPHOSPHINE) COMPLEXES OF RHODIUM(I). 57 SYNTHESIS, SPECTROSCOPY, AND ACTIVITY FOR CATALYTIC HYDROGENATION IV.1 Introduction 57 IV.2 Experimental 58 IV.2.1 Rh(dpm)2C£ 58 IV.2.2 Rh(dpe)2C£ and Rh(dpp)2C£ 58 IV.2.3 Rh(dpb)2C£ 59 IV.2.4 Rh[(+)-diop]2C£ 59 IV.2.5 Rh(P /l >) 2 +BF 4- (P'°P = dpm, dpe, dpp, dpb, and ( + ) - d i o p ) 59 IV. 2. 6 HRh(P^P)2(P'l> = dpe, dpp, and (+)-diop) 59 IV.3 Results and Discussion 60 IV. 4 Catalytic hydrogenation 82 CHAPTER V. ASYMMETRIC HOMOGENEOUS HYDROGENATION OF 89 PROCHIRAL OLEFINIC SUBSTRATES USING RHODIUM-DIOP COMPLEXES AS CATALYSTS V. l Introduction 89 V.2 Experimental 89 V. 2.1 Preparation of complexes 89 V. 2.2 Determination of optical purities 90 V.3 Results 91 V. 4 Discussion 98 CHAPTER VI. ASYMMETRIC HOMOGENEOUS HYDROGENATION OF ITACONIC 109 ACID USING BIS(DIOP)RHODIUM(I)TETRAFLUORO-BORATE AS CATALYST VI. 1 Introduction 109 VI.2 Experimental 110 VI.3 Results 110 VI. 3.1 Reaction of hydrogen with Rh(diop) 2 +BF 4 _ 110 VI.3.1.1 Gas-uptake studies 110 VI.3.1.2 Spectrophotometric studies 110 VI.3.1.3 3 1P nmr data 115 VI.3.2 Catalytic hydrogenation studies 121 VI.3.2.1 Kinetic data 121 VI.3.2.2 Spectroscopic data 121 VI. 4 Discussion 128 VI.4.1 The hydrogen reaction 128 VI.4.2 The catalytic hydrogenation mechanism 13 4 CHAPTER VII. REACTIONS OF BIS(DITERTIARY)PHOSPHINE-RHODIUM(I) 14 2 COMPLEXES WITH THE GAS MOLECULES CO,02,H2, and HC£ VII. l Introduction 14 2 VII.2 Experimental 14 3 - v i i i -Page VII.2.1 CO complexes 143 VII.2.1.1 Rh(P°P)2C0 BF 4 (P P = dpm,dpp) _ 143 VII.2.1.2 Reaction of CO with Rh(dpe) 2 +BF 4 150 VII.2.1.3 Reaction of CO with Rh(dpb) 2 +BF 4~ 150 VII. 2.1.4 Reaction of CO with RhCdiop^+BF^ 150 and Rh(diop)2CJi VII.2.2 Dioxygen complexes _ 151 VII.2.2.1 Rh(dpm)202+BF4_ 151 VII. 2. 2.2 Rh(lO>) 20 2 +BF 4 (P1> = dpe, dpp) 151 VII.2.2.3 Reaction of 0 2 with Rh(dpb)2 BF 4~ 151 VII.2.2.4 Reaction of 0 2 with Rh(diop) 2 +BF 4" 151 VII.2.3 Dihydride complexes _ 151 VII. 2.3.1 Reaction of H 2 with Rh(p1>)2 BF 4 151 (P'~P = dpm, dpe) VII.2.3.2 Rh(dpp)2H2 BF4"_ 152 VII.2.3.3 Rh(diop) 2H 2 BF 4" _ 152 VII.2.3.4 Reaction of H2 with Rh(dpb)2 BF 4 152 VII.2.4 HRh(Pl >) 2C£ +A~ complexes 152 VII.2.4.1 HRh(dpm)2C«+CJr 152 VII.2.4.2 HRh(P'~P)2Cr,"CJr(Pl> = dpe and dpp) 152 VII.2.4.3 Reaction of HC£(g) with 153 [Rh(P'~P)2]A (Pl> = dpb and diop; A=C£, BF4) VII. 2.4.4 HRh(P^>)2CJl+A"(P'~P = dpm, dpe, 154 and dpp; A=BF4, PFg, SbFg) VII.3 Results and Discussion 155 VII.3.1 The five-coordinate carbonyl 155 complexes of rhodium(I) VII.3.2 Six-coordinate complexes of rhodium(III) 161 VII.4 Attempted use of HRh(pHp) 2C£ +A _ complexes in catalytic 172 hydrogenation VII.5 Preliminary kinetic studies 172 CHAPTER VIII GENERAL CONCLUSIONS, AND RECOMMENDATIONS FOR FUTURE WORK 179 REFERENCES FOR CHAPTER I 184 CHAPTER II 194 CHAPTER III 194 CHAPTER IV 195 CHAPTER V 198 CHAPTER VI 200 CHAPTER VII 201 - i x -LIST OF TABLES Table number Page 111.1 Maximum hydrogenation rates for the hydro- 46 genation of styrene in n-butanol-toluene (2:1) at 45°C using HRh[(+)-diop]2 111.2 Variation of maximum rate with temperature 50 111.3 Variation of maximum rate with added 50 (+)-diop IV.1 Analytical and molar conductivity (A) data 61 for the rhodium(I) complexes IV.2 Visible spectral data for the rhodium(I) 63 complexes IV.3 Infrared and high-field H^ nmr data for 64 various rhodium(I) complexes IV.4 31p n m r data for the rhodium(I) complexes 65 at 25°C IV.5 IH nmr data for the rhodium(I) complexes 66 at 25°C IV. 6 Catalytic hydrogenation of methylenesuccinic 83 acid using the rhodium(I) complexes V. l Asymmetric homogeneous hydrogenation 93 of unsaturated substrates using the HRh[(+)-diop]2 catalyst V.2 Optical purities of products from 94 hydrogenation of olefinic substrates using the Rh[(+)-diop]2 +BF^ - complex V.3 Asymmetric homogeneous hydrogenation of 95 prochiral olefins using the Rh[(+)-diop]2C£ catalyst V.4 The Rh[(+)-diop] 2H 2 +BF4~ - catalyzed hydrogena- 96 tion of some prochiral substrates V.5 Asymmetric hydrogenation of prochiral unsaturated 97 substrates using the [RhC£(CO)(+)-diop]2 complex V. 6 Asymmetric hydrogenation of unsaturated 99 substrates using various rhodium(I)-mono(diop) complexes at 25°C and 760 mm total pressure VI. 1 Hydrogen - uptake data for Rh(diop) 2 +BF4~ 112 using ampoule technique VI.2 Spectrophotometric determination of f i r s t - 119 order rate constants for the dehydrogenation of Rh(diop)2H2+BF4_ in different solvents at 30°C VI.3 Linear rate data for the hydrogenation of 123 itaconic acid in n-butanol-toluene (2:1) catalyzed by Rh(diop) 2 + B F4~ a t 15°C. VI.4 Analysis of various stopped-flow data at 30°C 133 in terms of two first-order reactions Table number VII.1 Analytical and molar conductivity data (A) for the Rh(I) and Rh(III) complexes VII.2 Gas-uptake data for various rhodium(I)-bis(ditertiaryphosphine) complexes at 25°C VII.3 High-field 3-H nmr and infrared data for the rhodium(I) and rhodium(III) complexes VII.4 1H nmr data for various rhodium-bis(diterti-aryphosphine) complexes VII.5 31p{lfl} n m r data for various rhodium-bis(dit« tiaryphosphine) complexes VII.6 Pseudo first-order rate constant k at 30°C for the reaction Rh(P^P) 2 +BF 4~ + XY > Rh(P/^P)2XY+BF4_ VII.7 Data at 30°C for the determination of equilibrium constant for the reaction Rh(dpe) 2 + + 0 2 — i - Rh(dpe) 20 2 + - x i -LIST OF FIGURES Figure number Page 1.1 Structure of (-)-(2R,3R)-DI0P 3 1.2 Mechanism for ruthenium(II) chloride- 7 catalyzed hydrogenation of olefins 1.3 Scheme for neutral and cationic 10 rhodium(I) complexes 1.4 Pathways for olefin hydrogenation and 11 isomerization 1.5 Some effective ligands used in catalytic 17 asymmetric hydrogenation 1.6 Stereochemical model proposed for olefin 18 hydrogenation catalyzed by neutral mono(diop)-rhodium complexes 1.7 Possible conformations of saturated five- 19 membered chelate rings 1.8 Crystal structure of the [Rh(COD)(S,S- 19 chiraphos)] + complex 11.1 Constant pressure gas-uptake apparatus 26 11.2 Ampoule technique 30 11.3 Anaerobic spectral c e l l 31 111.1 Solubility of hydrogen in n-butanol- 36 toluene (2:1) mixture at various pressures at 30°C 111.2 Solubility of hydrogen in n-butanol-toluene 36 (2:1) mixture at various temperatures 111.3 Rate plots for the hydrogenation of itaconic 38 acid using HRh(diop)2 as catalyst 111.4 Maximum rate dependence on itaconic acid in 40 n-butanol-toluene(2:1) at 30°C using 1.5xlO~^M Rh; curve a, 740 mm H2; curve b, 132 mm H2 111.5 31p{ljj} nmr spectra in n-butanol-toluene- 42 acetone-d6 mixture at 25°C of: (a) HRh(diop)2(130 mg in 2 ml solvent) under Ar; (b) (a) + 130 mg IA; (c) (b) under H 2 111.6 High-field -*-H nmr spectra in n-butanol- 43 toluene-acetone-dg mixture at 25°C of: (a) HRh(diop) 2 (130 mg in 2 ml solvent) under Ar; (b)(a) + 130 mg IA under H2 111.7 Rate plot for the hydrogenation of styrene 45 using HRh(diop)2 catalyst 111.8 Maximum rate dependence on HRh(diop)2 47 concentration 111.9 Plot of maximum rate against styrene 48 concentration 111.10 Dependence of maximum rate on H2 pressure 49 - x i i -Figure number Page 111.11 Absorption spectra of HRh(diop)2(1.4xlO~3M) 52 under argon in n-butanol-toluene (2:1) in the presence of styrene (0.IM) at 45±2°C 111.12 Arrhenius plot for the hydrogenation of 53 styrene using HRh(diop)2 catalyst 111.13 Absorption spectra of HRh(diop)2(1.4xlO~4M) 55 in n-butanol-toluene (2:1) in the presence of diethyl maleate (1.0xlO~2M) at 30±1°C: (a) solution under argon; (b) solution under hydrogen IV. 1(A) Visible absorption spectra of the Rh(p""p)2C£ 68 complexes in methanol under argon at 25°C (PP = a.dpm, b.dpe, c.dpp, and d.dpb) IV.1(B) Visible spectra of the Rh(P'^P)2C£ complexes 69 in toluene under argon at 25°C (P P = e.dpm, f. dpb, and g. diop). IV.2 Visible spectra of various Rh(P'^P)2+BF4" 70 complexes in methanol under argon at 25°C (P P = a. dpm, b. dpe, c. dpp, d. dpb, and e. diop) IV.3 Absorption spectra of the HRhCEO>)2 complexes in 71 toluene under argon at 25°C (P P = a. dpe and b. dpp) . IV.4 High-field 1H nmr spectra of (1) HRh(dpe)2 73 (2) HRh(dpp)2 (3) HRh(diop)2 in CgDg at 25°C IV.5 X-ray crystal structure of the HIr(dpe) 2 complex 74 IV.6 Crystal structure of the HRh[(+)-diop]2 complex 75 IV.7 3 1P{ 1H} nmr spectrum of HRh[(+)-diop]2 under 76 argon in n-butanol-toluene-acetone-dg (a) at 25°C and (b) at -50°C. IV.8 3 1P nmr spectrum of Rh(dpm)2C& in (a) CgD5 and 77 (b) CH2C£2-acetone-d6 at 25°C. IV.9 H^ nmr spectrum of Rh(dpm)2+BF4~ in acetone-d6 80 at 25°C IV.10 % nmr (phenyl region) spectra of (1) HRh(dpe)2 81 (2) HRh(dpp)2 (3) diop (4) HRh(diop)2 in C 6D 6 at 25°C IV.11 Rate plot for the hydrogenation (100%) of 84 methylenesuccinic acid using HRh(dpp)2 as catalyst IV.12 Rate plots for the hydrogenation (50%) at one 85 atmosphere pressure of methylenesuccinic acid using: a. HRh(dpe)2, b. Rh(dpe)2+BF4~, c. Rh(dpp) 2 +BF 4~, d. Rh(dpb)2C£ IV. 13 Rate plots for the hydrogenation (50%) at one 86 atmosphere pressure of methylenesuccinic acid using: a. HRh(diop)2, b. Rh(diop)2 +BF 4~, c. Rh(dpb) 2 +BF 4" V. l Some substrates used in catalytic asymmetric 92 hydrogenation - x i i i -Figure number Page V.2 Proposed intermediates for hydrogenations 102 catalyzed by rhodium-mono(diop) complexes V.3 Crystal structure of the [Rh(dpe)(PhC(H)= 102 C(C00CH3)(NHC0CH3))]+ cation V.4 Plot of variation of optical yield with 106 temperature of: 1. (+)-2-methylsuccinic acid using a. HRh[(+)-diop]2; b. Rh[(+)-diop] 2 +BF4~ as catalysts in n-butanol-toluene (2:1); 2. N-acetyl-S(+)-phenylalanine using the Rh[(+)-diop]2C£ complex in n-butanol-toluene (2:1) V. 5 Variation of optical yield of (+)-2-vinyl- 106 norbornane with temperature using a nickel catalyst in chlorobenzene VI. 1 Solubility of hydrogen in n-butanol-toluene 111 (2:1) mixtures at various pressures at 15°C. Inset gives data at different temperatures VI.2 Visible spectrum of Rh(diop) 2 +BF4~ in n-butanol- 113 toluene (2:1) at 25°C VI.3 Visible spectrum of Rh(diop) 2 +BF4~ at 25°C in 114 n-butanol-toluene (2:1)(in absence, or presence of diop(diop:Rh=3:1)). (a) under H 2 and (b) under Ar VI. 4 Plot of log (At-A^) vs time of the stopped-flow 116 data for H2-uptake by Rh(diop )2 +BF 4 _ in DMA ' at 30°C VI.5 Spectral changes obtained for H 2 loss from 117 Rh(diop) 2H 2 +BF4 _ in methanol (under argon) at 30°C VI. 6 Plot of log(A t-A a >) vs time for spectral 118 changes in Fig.VI.5 VI.7 3 1P{ 1H} nmr spectra of Rh(diop) 2 +BF 4~ in 120 CH2C£2-acetone-d£ a. under argon: 1. at 25°C and 2. at -50°C; b. under hydrogen at 25°C VI.8 H2-uptake plot for the Rh(diop) 2 +BF4~-catalyzed 122 hydrogenation of itaconic acid VI.9 Plot of linear rate against Rh(diop) 2 +BF4~ 124 concentration VI.10 Dependence of linear rate on IA concentration 125 VI.11 Plot of linear rate against hydrogen pressure 126 VI.12 Spectral changes observed on adding IA to 127 solutions of Rh(diop) 2 + B F4 f n n-butanol-toluene^: 1) (under argon) at 30°C (in the absence, or presence of added diop) VI.13 Analysis of data from Fig. VI.4 in terms of two 13 2 first-order reactions (A and B) VI.14 Arrhenius plot for the Rh(diop) 2 +BF4~ - 13 5 catalyzed hydrogenation of IA VI.15 Plot of [ I A ] - 1 vs linear rate" 1 140 -xiv-Figure number Page VII.l 3 1P nmr spectra of Rh(dpm)2CO+BF4~ in CH 2C£ 2- 1 5 8 acetone-d6(2:l=V/V). a. Under Ar at 25°C. b. Under Ar at -50°C. c. Under CO at -50°C VII.2 3 1P nmr spectra of Rh(dpp)2CO+BF4 in CH 2C£ 2- 159 acetone-d6(2:l=V/V). a. Under Ar at 25°C. b. Under CO at 25°C. c. Under CO at -50°C. VII.3 3 1P nmr spectra in CH2C£2-acetone-d6(2:l=V/V) at 164 25°C of: a. Rh(dpm) 20 2 +BF 4 . b. Rh(dpp) 20 2 +BF 4" VII.4 High-field ^ -H nmr spectra at 25°C (under H2) of: 166 a. Rh(dpp)2H2 BF 4~ in acetone-dg. b. Rh(diop) 2H 2^BF 4" in CDCA3 VII.5 3 1P{ 1H} nmr spectra in CH2C£2-acetone-d6 167 (2:1=V/V) of: (a) Rh(dpp)2H2*BF4" at -50°C and (b) Rh(diop) 2H 2 +BF 4 at (1) 25°C, (2) -50°C, and (3) -85°C. VII.6 High-field % nmr spectra of: (a) HRh(dpm)2C£+C£", 169 (b) HRh(dpe)2C£+C£-, and (c) HRh(dpp)2C£+C£" in CDC£ 3 at 25°C. VII.7 3 1P{ 1H} nmr spectra of HRh(dpe)2C£+C£_ in CH 2C£ 2- 170 acetone-d 6 (2:1=V/V) at a. 25°C and b. -60°C. VII.8 31P{lH}nmr spectra of HRh(dpp)2C£+C£- in CH 2C£ 2- 171 acetone-d 6 (2:1=V/V) at: a. 25°C, b. -50°C, and c. -85°C. VII.9(A) Spectral changes obtained for 0 2 loss from 175 Rh(dpe) 20 2 +BF 4~ in methanol at 30°C (solution under argon) VII.10(A) Spectral changes obtained at 30°C on adding 0 2 175 (1 atm) to solutions of Rh(dpe) 2 +BF 4~ in methanol VII.9(B) Plot of log(A t-A ) vs time for spectral changes 176 in Figure VII.9(A) VII.10(B) Plot of log(A t-A ) vs time for spectral changes in 176 Figure VII.10(A)°° - X V -ABBREVIATIONS A anion, SbF, , PF, , BF. , C£ 6 6 4 acetone-d, deuterated acetone (CD„-C-CD~) 0 Ar argon atm atmosphere benzene-dg deuterated benzene (C^D^) c concentration in g/100 ml Calc. calculated Cat* chiral catalyst ^8^14 cis-cyclooctene COD 1,5-cyclooctadiene config. configuration cps cycles per second, Hz d day(s) (+)-diop [(+)-(2S,3S)-0-isopropylidene-2,3-dihydroxy-l, 4-bis(diphenylphosphino) butane] diop* 'monodentate' or 'dangling' diop DMA N,N-dimethylacetamide (H3CCON(CH3)2) DMA.HCA N,N-dimethylacetamidehydrochloride (H3CCON(CH3)2.HC£) DMSO dimethylsulfoxide (CH^SO dpm bis (diphenylphosphino) methane (<J>2P-CH2-Pt}>2) dpe 1,2-bis (diphenylphosphino) ethane (<j>2P-(CH2)-2P<f>2) dpp 1,3-bis (diphenylphosphino) propane (<|>2P-(CH2}3Pct>2) dpb 1,4-bis (diphenylphosphino) butane (<}>2P-(CH2)-4P<j>2) dppp 1,2-bis(diphenylphosphino) propane e.e. enantiomeric excess Et ethyl Fig. Figure(s) g gram(s) h hour(s) Hz cps IA itaconic acid i r infrared J coupling constant, cps or Hz - x v i -L In log M Max. Me MeOH-d. 4 min NBD nm nmr O.D. o l Ph or <f> ppm P P P.S. py R Ref. R.T. s S SA t T THF TMS UV V/V Vis vpc X T 6 ligand natural logarithm common logarithm molarity, metal atom maximum methyl deuterated methanol (CD^OD) minute(s) norbornadiene -9 nanometer (10 meter) = 1 millimicron (my) nuclear magnetic resonance o p t i c a l density, absorbance o l e f i n phenyl 31 proton-decoupled P parts per m i l l i o n a chelating diphosphine "proton sponge" (1,8-bis(dimethylamino) naphthalene) pyridine a r y l ; a l k y l reference room temperature second(s) substrate; solvent succinic acid time Temperature tetrahydrofuran tetramethylsilane u l t r a v i o l e t volume by volume v i s i b l e vapor phase chromatography halogen chemical s h i f t , ppm chemical s h i f t , ppm; T = 10-6 - x v i i -A molar conductivity A wavelength of maximum absorption max ° _^ r v frequency, cm £ molar extinction coefficient For olefinic substrates abbreviations, see p.92. - x v i i i -ACKNOWLEDGEMENTS I wish to thank Professor B. R. James for his expert advice and valuable suggestions during the course of this work. I would like to express my gratitude to Dr. K. J. Reimer for introducing me to some useful synthetic techniques, to Dr. M. Preece for proof-reading this manuscript, and to Drs. R. Ball and J. Trotter for the crystal structure determination of a rhodium-diop complex. Thanks are also due to Ms. Anna Wong for doing a superb job in typing this thesis. CHAPTER I INTRODUCTION 1.1 General The separation of two enantiomeric forms of sodium ammonium tartrate by Louis Pasteur in 1848"^  has proved to be the dawn of a new era in chemistry. With an ever increasing understanding of processes going on in the human body and together with isolation of some chemical compounds involved in these processes, the importance of the role that enantiomeric forms play in our existence has been realized. Although enantiomeric (or mirror image) forms differ only in the direction that they rotate the plane of polarized light, clockwise or counterclockwise, usually for a specific process only one form is effective while the other may be useless or may even inhibit the particular process. Nature has provided enzymes to carry out the stereoselective chemistry necessary for l i f e processes. However, most compounds in nature exist as racemic 'DL' mixtures and a ready separation of these biologically inactive compounds remains a challenging problem. Numerous compounds e.g. single 2 3 2 3 A isomers of monosodium glutamate ' , lysine ' , and menthol have been produced efficiently using biochemical processes. Generally, production of optically active compounds involves use of large quantities of optically active reagents and optical yields of the stereoisomers are often quite low. An alternative'approach for resolution is available using catalytic asymmetric synthesis. There are many natural products where a hydrogen atom is on the chiral centre, usually carbon (C*). Such compounds can thus be generated in principle by hydrogenation of some unsaturated precursors. -2-Since most industrial processes involve heterogeneous catalysis, asymmetric hydrogenation studies were f i r s t started in the heterogen-eous phase. In 1956 Akabori and coworkers^succeeded in affecting asymmetric hydrogenation of various oxime and oxazolone derivatives to optical yields of upto 35% using a catalyst consisting of metallic palladium adsorbed on s i l k . The same group subsequently achieved 50% e.e. in the hydrogenation of methylacetoacetate using Raney nickel modified with asymmetric molecules.'' Today heterogeneous asymmetric catalysis appears less promising than the use of transition metal complexes (see below), although methylacetoacetate and amino-acid precursors have been hydrogenated with 90% optical yields in some 8 9 cases. ' The mechanism of these heterogeneous hydrogenation reactions is poorly understood, however, because of d i f f i c u l t i e s in defining the active sites at the metal surfaces. Since Wilkinson's discovery"*"^'"^ in 1965 that solutions of rhodium-phosphine complexes catalyzed hydrogenation of olefins under mild conditions, studies for asymmetric synthesis have been focused on the use of soluble transition metal complexes containing chiral ligands. The overall process involves activation of both substrate and hydrogen by the complex (cat*). During the hydrogenation process the chiral ligand on the metal may interact with the substrate in such a way that hydrogen subsequently adds preferentially to one face of the olefin and produces one enantiomer in excess over the other. Thus prochiral substrates can yield optically active products when the reaction is catalyzed by catalysts containing chiral ligands (equation [1.1]): -3-R l X G = C / R 3 " 2 > R l \ g _ _ g / R 3 R 2 / N' R4 " C A T * R / I l \ [1.1] H H Since 1968 many soluble metal complexes have been used for the asymmetric hydrogenation of various prochiral substrates, and the most efficient catalysts have been found to be rhodium-chiral phosphine 12-17 complexes (see section 1.7). Recently a-N-acylaminoacrylic acids have been hydrogenated to corresponding products with almost complete ... . 18-20 stereospeciflcity. 1.2 Scope of this thesis Synthesis of a new chiral bis(ditertiaryphosphine) ligand '(-)-diop' [diop=2,3-0-isopropylidine-2,3-dihydroxy-l,4-bis(diphenylphosphino)butane] 21 (Figure 1.1) by Dang and Kagan from cheap, naturally occurring (+)-tartaric acid, and subsequent use of the ligand with rhodium has proved 21-23 very effective for asymmetric hydrogenation of olefins and asymmetric 23 hydrosilylation of ketones. Since the earlier work, numerous reports have appeared involving the use of rhodium-mono(diop) complexes for asymmetric hydrogenation of olefins. H = chiral centre X= P h (-)-(2R,3R)-Diop Figure 1.1 -4-The present work i s based on the i s o l a t i o n of the f i r s t bis(diop)-rhodium complex, HRh[(+)-diop] , prepared i n t h i s laboratory 29 by Cullen and coworkers. The complex proved to be an e f f i c i e n t and e f f e c t i v e c a t a l y s t f o r asymmetric hydrogenation of it a c o n i c a c i d and 29 a-N-acetamidoacrylic acid. This f i n d i n g contradicted a previous 30 report that rhodium-bis(diphosphine) complexes were i n a c t i v e as hydrogenation c a t a l y s t s . Detailed studies involving syntheses of various rhodium-bis(ditertiaryphosphine) complexes and t h e i r p o t e n t i a l as homogeneous hydrogenation c a t a l y s t s were thus undertaken i n the present work (chapter IV). The f i r s t k i n e t i c studies on the use of a bis(ditertiaryphosphine)-rhodium complex were presented recently and 31 these described the hydridobis(diop)rhodium(I)-itaconic acid system ; t h i s work showed that i n the hydrogenation of such acid substrates, the ca t a l y s t slowly decomposed to Rh(diop) 2 species v i a reaction [1.2]: HRh[(+)-diop] 2 + H + > Rh[(+)-diop] 2 + H 2 [1.2] To avoid such a complication i t was decided to carry out k i n e t i c and mechanistic studies on styrene, a non-acidic substrate (chapter I I I ) . Various complexes containing diop, which are described i n chapter IV, were then used for the asymmetric hydrogenation of several p r o c h i r a l substrates (chapter V). The e f f e c t of temperature, solvent, and other parameters on o p t i c a l p u r i t i e s was also studied i n d e t a i l and a general comparison made of the various mono(diop)-and bis(diop)-systems (chapter V) The decomposition reaction [1.2] inadvertently produces a new c a t a l y t i c a l l y a c t i v e c a t i o n i c species and since many of the reported -5-rhodium catalysts used in asymmetric hydrogenation are cationic, a kinetic study was also done on the Rh[(+)-diop]^-itaconic acid system 32 (chapter VI). Kinetic and mechanistic studies on a neutral and a 33 cationic mono(diop)-rhodium system appeared during the course of the present studies, and these proved useful for comparison purposes (chapter VI). 34 35 Following reports by Pruchnik ' that [HRt^dpe^CftJCJl is one of the most active homogeneous hydrogenation catalysts for olefin hydrogenation, various compounds of the type [HRh(PP)2C£]A (P P=dpm,dpe, and dpp;A=BF4 or CJL) were prepared and tested for such hydrogenation to establish the effect of increasing chain length of the diphosphines (chapter VII). Several other complexes were also isolated by adding C0,02, or H 2 to complexes of the type Rh(P P ) 2 described in chapter IV. The dihydride complexes so isolated proved useful in helping establish hydrogenation mechanisms discussed in chapters III, IV, and VI. 31 Variable temperature P nmr was primarily used to establish the structure of the various phosphine complexes in solution (chapters IV and VII), while the solid state structure of HRh[(+)-diop]2 has also been established using X-ray crystallography (chapter IV). 1.3 Mod s of hydrogen activ ti n In general, mechanism of homogeneous hydrogenation of olefins at a metal centre involves three basic processes: (a) hydrogen activation, (b) substrate activation, and (c) a subsequent hydrogen transfer to give saturated products. It appears that irrespective of the detailed -6-hydrogenation mechanism, molecular hydrogen is at some stage s p l i t by the catalyst. This splitting results in the formation of a metal-hydride complex that is an active intermediate in hydrogenation reactions. Thus catalytic activity depends to a considerable extent on the sta b i l i t y and l a b i l i t y of these hydrido complexes. Effective hydrogenation catalysts were i n i t i a l l y recognized as those containing 6 8 group VIII metals in their lower oxidation (d -d ) states. More recently 2 3 several complexes of T i , Hf, Zr and Nb with d or d configurations 36 have been reported to be effective catalysts. Three recognizable pathways have been observed in the hydrogen activation process -(a) Heterolytic splitting 37-39 This process has been observed in cases where the complex usually has a substitutional l a b i l i t y . The overall hydrogen addition to the complex results in formation of a stable hydride complex with concomitant release.of a proton. The released proton can be stabilized 40 by some base that is present. The process is thus a net substitution of a hydride ligand for another ligand with no change in the oxidation state of the metal (for example, equations [1.3] and [1.4]): ^ L , ML + H 0 • M^ — — — Z l ^ ML ,H +H .base + L [1.3] H - H + 1 1 1 3- III 3- + - 17 RuC£ & + H 2 HRuC£ 5 + H + C£ [1.4] For example, in hydrogenations catalyzed by a ruthenium(II) -7-41 chloride system , the olefin molecule f i r s t coordinates to the metal; subsequent heterolytic splitting of B.^ followed by insertion into the Ru-H bond yields a o-alkyl complex. Electrophilic attack by a proton at the carbon attached to the metal then results in release of the 42 saturated product and regeneration of the Ru(II) catalyst (Figure I.]): Mechanism for ruthenium(II) chloride-catalyzed hydrogenation of olefins. Figure 1.2 (b) Homolytic splitting In this case hydrogen addition to the complex results in an increase of the oxidation state and, generally, the coordination number 43-45 of the metal by one. The driving force for the reaction is the susceptibility of the metal to oxidation and the ab i l i t y to expand its coordination shell (equations [1.5] and [1.6]): -8-2ML + H 0 n 2 2ML -H + 2L n-l [1.5] 2[Co I i :(CN) 5] 3 + H 2 5 5 = ^ 2[HCo I I I(CN) 5] 3" [ 1 . 6 ] In this particular cobalt example, subsequent olefin insertion into the hydrido-cobalt complex gives an alkyl complex which in turn can react with another mole of hydride complex to produce saturated 46 47 product and release the catalyst via equation [1.7]. ' [(NC) 5Co-alkyl] 3 + [HCo(CN) ] 3 • > 2[Co(CN) 5] 3 + alkane [1.7] The hydrogenation of arenes and alkenes, catalyzed by metal carbonyl hydrides, may also proceed through mechanisms in which intermediate free radicals are formed by H-atom transfer from the metal 48 49 49 hydride to the olefin ' (for example, equations [I.8]-[I.10]) : HMn(C0)5 + A ^ *Mn(C0)5 + 'AH [1.8] HMn(C0)5 + "AH 3*'Mn(C0)5 + AH2 [1.9] H 2 2Mn(C0) =3> Mn 2(CO) 1 0 *.2HMn(CO)5 [1.10] (A=a-methylstyrene) (c) Dihydride formation Hydrogen activation can also be achieved i f the metal atom has the a b i l i t y to increase i t s oxidation state and coordination number by two"^'^, i.e. via oxidative addition (equations [1.11] and [1.12]): -9-MLn + H 2 „ MLnH2 [I.11] IrC£(CO)(PPh 3) 2 + H 2 .^ ^ Ir I I ] :(H 2)C£(CO) (PPh 3) 2 [1.12] The formation of products and release of the catalyst can proceed via a subsequent coordination of olefin at the metal followed by con-secutive transfer of two hydrogen atoms via a o-alkyl hydride inter-mediate. It should be noted that an overall heterolytic splitting of H 2 can also result via i n i t i a l oxidation adduct followed by reductive 53 elimination of H-base, e.g. equation [1.13] : II IV II + RuC£ 2(PPh 3) 2 + H 2 ^=>r [RuH 2C£ 2(PPh 3) 2] 5S==S HRuC£ (PPh^ 2+H +C£ [1.13] I.4 Rhodium catalysts in homogeneous hydrogenation Although hydrogenation of inorganic and organic substrates catalyzed 54 55 by rhodium(III)-amine complexes was known even back in 1935 ' i t was not unt i l about 1956, when Halpern's group"^ started investigating hydrogen activation by various metal ions in aqueous solution that such systems were shown to be homogeneous. The discovery of RhC£(PPh 3) 3 in 1965, as a versatile catalyst for homogeneous hydrogenation of o l e f i n s ^ \ established the catalytic activity of rhodium-phosphine complexes. Two basic mechanistic pathways have been established for the hydrogenation of olefinic compounds using either the neutral or the later discovered^ ^ cationic rhodium complexes (Figure 1.3). The K^k^ 'unsaturate' route involves i n i t i a l formation of a rhodium-olefin complex, followed by hydrogen addition and subsequent transfer of -10-hydrogen via a hydridoalkyl intermediate to give saturated product (H^A) and Rh(I) catalyst. The K^l^ 'hydride' route proceeds via an i n i t i a l dihydride complex which after coordination to the olefin passes through the same hydridoalkyl intermediate before regenerating Rh(I) complex and giving the saturated product. Extensive studies^ ^ on the RhCACPPh^)^ complex have shown that this complex generally hydrogenates via the 'hydride' route with i n i t i a l formation of a cis-dihydride intermediate complex RhCA(H)2(PPh.j) that has an extremely labile phosphine. The 'unsaturate' route has been found to operate in the hydrogenation of organic substrates with (A=olefin; H2A=alkane) Scheme for neutral and cationic rhodium(I) complexes. Figure 1.3 -11-RhL n - R H R h H ^ A ) , — RhH2RLn-4-H. RhHRLn PathC RhLnA R h H ^ c < ^ > RhHLn P a t h A 2 RhRL n > RhHLn(Aj (A=olefin, n=2 or 3, R=alkyl, RH=alkane, L=ligand, S and S are omitted) x y Pathways for olefin hydrogenation and isomerization. Figure 1.4 7 6 chlororhodate(III) species, and other phosphine free systems in DMA. Hydrogenations of hex-l-ene using HRhCPPh^)^ or other related monohydride 78 complexes (e.g. HRh(CO)(PPh^)^) also proceed via 'unsaturate' routes. One of the most active series of homogeneous catalysts is that provided by cationic complexes of the type Rh (diene) L^ "1" (L=tertiary 60—63 phosphine, phosphite,or arsine), f i r s t used by Osborn's group for hydrogenation of olefins, acetylenes, and ketones. These systems were subsequently studied extensively^ ^  and appear to operate via a 'hydride' route (equation [1.15]). The important equilibria governing formation of the active catalysts are given in equations [I.14]-[1.16]: Rh(diene)L + 2H0 n l RhL S + + H 0 n x 2 RhL H„S + n 2 y alkane + RhL S + n x RhL H„S n 2 + H + + RhHL S n z [1.14] [1.15] [1.16] (n=2 or 3; x, y and z unknown; S=solvent) -12-An interesting feature of the cationic system is the reversibility of the two active catalysts illustrated in equation [1.16]. This e q u i l i -brium is sensitive to the nature of L and S, and(can be shifted by adding acid or base to give solutions containing the dihydride or monohydride, respectively. The monohydride species is a powerful hydrogenation catalyst for simple olefins as well as an isomerization catalyst, whereas the d i -hydride is a moderately active olefin hydrogenation catalyst and does not give rise to concomitant isomerization. The catalytic cycles involved in both cases are summarized in Figure 1.4. Many other group VIII metal complexes especially those of Fe(0), Co(I), Ni(II), Pd(II), Ir(I), Pt(II), Pt(0), and Ru(II), have also been used for the catalytic reduction of olefins. The literature upto 1972 has been covered in a book by James"*"2 and other more recent reviews"'"'7'^'^ have appeared. The a b i l i t y to fix transition metal complexes (particularly of 81 rhodium) to various polymers, f i r s t described in patents , led Grubbs's 82 83 group ' to use such polymer-supported catalysts for hydrogenation of olefins. The resin used was commonly a copolymer of styrene and divinyl-benzene incorporated with diphenylphosphino groups as potential coordinating sites for the metal. An advantage of using these systems is the easy separation of catalyst from the reaction mixture after the hydrogenation process. Many reviews have appeared on the subject of polymer-supported catalysts or the so called heterogenized homogeneous c a t a l y s t s . ^ I.5 Homogeneous hydroformylation Cobalt and rhodium complexes have been found most effective for -13-hydroformylation of o l e f i n s to give aldehydes, and both metals have been j . j 4- • i T 100,101 , , . 102-109 used i n d u s t r i a l l y . Several papers and reviews ind i c a t e that rhodium complexes are e s p e c i a l l y promising for the production of o p t i c a l l y a c t i v e aldehydes v i a asymmetric hydroformylation (for example, f T ,-,,*107 equation [1.17]) : R' H CO,Cat* ^ / C = C H ? 9 r o r r , o n v o n > R'-CH-CH„CH0 [1.17] p^ *> 2 i-i C,benzene | 2 J Cat*=ln situ[HRh(C0)(PPh 3) 3+(-)-diop] (R)-22.7% e.e. for R'=H; R"=C,H C o o I.6 Solvent transfer hydrogenation of unsaturated compounds This method involves hydrogenation of unsaturated bonds using solvent as a hydrogen source instead of molecular hydrogen. An example i s the hydrogenation of ketones using a l c o h o l i c solvents ;usually isopropyl alcohol^with various c a t a l y s t s (equationfl.18]): R1R2C=0 + R3CH20H -» R^CHOH + R CHO [1.18] (substrate) (solvent) Widely studied compounds used i n such hydrogenation of o l e f i n s , saturated and unsaturated ketones, and aldehydes, are phosphine complexes 1 1 f)-l 1 ? 11 V of rhodium(I) and ruthenium(II), e.g. RhC£(PPh 3) 3 , HRh(PPh 3) 4 , R u C £ 2 ( P P h 3 ) 3 1 1 4 " 1 1 6 , and H 2 R u ( P P h 3 ) 4 1 1 7 " 1 1 9 . H I r C £ 2 ( D M S O ) 3 1 2 0 ' 1 2 1 and 122 RhC£(DMSO)(PPh 3) 2 are also e f f e c t i v e . A not very well understood mechanism has been proposed for these hydrogen transfer reactions''""'"^'^"'"' 113,115,118,119 , . 123,124 that have been reviewed. -14-The fact that solvent is involved in the hydrogen transfer process 116 led Descotes and Sinou to use RuCZ^(PPh^)^ with a chiral glucofuranose derivative as adonor solvent. Asymmetric hydrogenation of prochiral a,g-unsaturated ketones to the saturated ketones was achieved with upto 34% e.e. in the case of cyclohexene-3-one. lu^CJl^Cdiop)^ and RuCJ^L^tl^ PAMePhR(R=o- or p-anisyl, benzyl, propyl) or neomenthyldiphenylphosphine] have been used for the hydrogenation of activated olefins using racemic 125—128 alcohols such as 1-phenylethanol ; these systems are of interest in that they lead to slight resolution of the donor alcohol. An in situ rhodium(I) catalyst with neomenthyl phosphine has also been used 129 similarly. Optical purities upto 22% have been achieved in the transfer hydrogenation of a-methylcrotonic acid by a chiral ruthenium-130 phosphine complex in alcoholic solvents. 1.7 Asymmetric homogeneous hydrogenation 131-133 In 1968, Horner and coworkers reported the hydrogenation of a-ethylstyrene and a-methoxystyrene to S-(+)-2-phenylbutane (7-8% e.e.) and R-(+)-l-methoxy-l-phenylethane (3-4% e.e.), respectively, using an * n in situ rhodium catalyst containing the chiral phosphine S-(+)-P MePhPr . 134 Knowles and Sabacky at about the same time hydrogenated atropic acid and itaconic acid to hydratropic acid (15% e.e.) and 2-methylsuccinic * i acid (30% e.e.), respectively, using RhC£ 3L 3(L=P PhMePr ). The optical yield with atropic acid was reduced drastically from 15% to 1% e.e. with L=PPh(CH2CHMeEt)£ where the chirality was on the carbon atom of an alkyl group rather than on the phosphorus atom. Optical yields of >50% were then obtained in the hydrogenation of -15-methyl-3-phenyl-but-2-enoate to methyl-3-phenylbutanoate using McQuillin's catalyst formed from [py2SRhC£2BH4J where S is a chiral amide solvent such as PhCHMeNHCHO, PhMeNHCHO etc..." ' Morrison et al. achieved the reduction of (E)-a- and (E)-g-methylcinnamic acids and atropic acid to products with 61% e.e., 52% e.e., and 28% e.e. respectively using RhC£L ^(L =1 in Figure 1.5) complex as catalyst. 21 22 It was in 1972 when Kagan and Dang ' f i r s t reported the pre-paration of diop (Figure 1.1), and used i t effectively to hydrogenate a-acylaminoacrylic acids in >70% e.e. with in situ rhodium catalyst. 138 139 Optical yields of upto 90% were later achieved by Knowles et a l . ' in some cases. Various other rhodium complexes have been used for the asymmetric hydrogenation of prochiral olefinic substrates. Figure 1.5 l i s t s some representative ligands (I to XV) which form effective catalysts when bound to a rhodium centre. Other metals, e.g. Ru(II), Pt(II), Ni(II), Ir ( I ) , Co(II), etc., have also been used but generally these have been found to be less effective or less efficient at least for the asymmetric hydrogenation of a,3 unsaturated acids and ketones."'"'7 Since mono(diop)-rhodium systems have been found to be efficient 21—28 and effective catalysts , studies have been directed using these complexes to establish the mechanism by which hydrogenation takes place. This work was published during the course of the present studies. A 27 mechanism involving a 'hydride' route, proposed by Gelbard and Kagan for a neutral mono(diop)-rhodium system is almost certainly incorrect 31 judging by a more recent P nmr study on an analogous cationic mono(diop)-33 rhodium complex inhere i t was shown that hydrogenation proceeded via an 'unsaturate' route. This later study parallels other work done on -16-I. (+)-Neomenthyldiphenylphosphine (NMDPP). II'. (-) - (2R, 3R) -O-Isopropylidene-2,3-dihydroxy-l, 4-bis (diphenyl-phosphino)butane (Diop), X=Ph. III. (+)-0-Anisylcyclohexylmethylphosphine (ACMP). IV. (R,R)-l,2-Ethanediylbis[0-anisylphenylphosphine] (BAPPE). V. (2S,3S)-Bis(diphenylphosphino)butane (CHIRAPHOS). VI. (S)-a-[(R)-2-Diphenylphosphinoferrocenyl]ethyl dimethylamine ([S,R]-PPFA),X =H;X2=NMe2. VII. (+)-l,2,2-Trimethyl-l,3-bis(diphenylphosphinomethyl)cyclopentane (CAMPHOS). VIII. cis-Myrtanyldiphenylphosphine (MYRTPHOS). IX. (IS,2S)-trans-l,2-Bis(diphenylphosphinomethyl)Cyclobutane. X. Menthylmethylphenylphosphine (MMPP). XI. (2S,4S)-4-Diphenylphosphine-2-diphenylphosphinomethylpyrrolidine (PPM), R=H. XII. 2,2-Bis(diphenylphosphinitemethyl)-l,1-binaphthyl, X=0. XIII. trans-1,2-Bis(diphenylphosphinoxy)cyclohexane (BDPCH), n=4. XIV. 1,2-Bis(N-diphenylphosphino,N-methylphenylmethyl amino)ethane. XV. Methyl 2,3-bis-0-diphenylphosphino-4,6-0-benzylidene-a-D-glucopyranoside. -17-PPt^ 137 CH 3 CH H-^E PPh„ PP V 2 19 H hu (a) X - Ph (ti X= Ph(3-OM4 (Figure 1.1) II :F*-CKj (^rocH. 39 -OCH, r139~,18 (^T0CH3 IV CHMeX. ,PPi (a) X1 - H } X^—NMe. (b) x L = P P h 2 } X 2 = ' NMa, VI VII 16,141 VIII 16,141 :PMePh ,143 : H 2 P P h 2 ^J-X—PPhu X—PPh, (a) X = ChL (b) x - o7 .145 XI 144 - U ^ C l ^ — CH£ ° (o) n = 3 (b) n - 4 6^ XII XIII XIV 148 H OPPh-149 1 Chiral centre (*) XV Some effective ligands used in catalytic asymmetric hydrogenation (chirality is at carbon, or phosphorus, or both). Figure 1.5 -18-[Rh(NBD)dpe]+ by Halpern et al}^®, but the findings are in contrast to rhodium-bis(monodentate phosphine) systems where hydrogenation proceeds n , .1 i 64-66,75 vxa a hydride route. A detailed kinetic study using an in 32 situ [RhC£(diop)S] (S=solvent) catalyst has also appeared , and suggests that the hydrogenation proceeds via both the 'hydride' and 'unsaturate ' routes. _ .142,151-155 . , „ -Glaser et a l . , m order to rationalize the cause of asymmetric induction, have presented a stereochemical model for neutral mono(diop)-rhodium catalyzed hydrogenations using space-filling CPK-type molecular models, considering only steric effects. Thus using the model shown in Figure 1.6, they were able to predict correctly the configuration of the major products from hydrogenation of various amino acids e.g. CH2=C(S)(L) except in two cases (S=C02Me,L=Ph and S=Et, L=Ph). H Rh CL S=small L=large Stereochemical model proposed for olefin hydrogenation catalyzed by neutral mono(diop)-rhodium complexes. Figure 1.6 r 1 , , , , , 142,151-155 . ^ . Glaser s group has also done studies m terms of determining the effect on optical purity of various substituents at the double bond of the substrate. These studies have been augmented by the determination of the crystal structure of a related iridium complex [Ir(COD)(diop)C£], which is a distorted trigonal bipyramid in which diop -19-1s an apical-equatorial bidentate ligand. 19 Bosnich's group have reported the preparation of [Rh(COD)(S,S-chiraphos)]+CiO^~ (chiraphos=V in Figure 1.5), and with this catalyst a-N-acylaminoacrylic acids were hydrogenated with 91-100% optical yields. These excellent optical yields were explained on the basis that of the two normally interconvertible 157 conformations for saturated five-membered chelate rings (Figures 1.7 (a) and (b)), chiraphos (a five-membered ring substituted at the aliphatic link) adopts only one static conformation (a). (a) (b) Possible conformations of saturated five-membered chelate rings Figure 1.7 This has been confirmed by determination of the crystal structure + 158 of the [Rh(COD)(S,S-chiraphos)] complex (Figure 1.8). Rh \ Me Crystal structure of the [Rh(COD)(S,S-chiraphos)]+complex Figure 1.8 -20-From the structure i t appeared that the o r i e n t a t i o n of phenyl groups held by the puckered chelate r i n g was responsible for the diastereotopic i n t e r a c t i o n that discriminates the p r o c h i r a l faces of the substrate. A s i m i l a r explanation has also been presented by Knowles's g r o u p " ^ ' t o explain the greater than 90% e.e. obtained i n the hydrogenation of a-N-acylaminoacrylic acids using [Rh(l,5-C0D)L 2] + C£0^ where i s the ligand shown i n Figure I.5(IV). Good o p t i c a l y i e l d s i n other cases, e.g. hydrogenation of a-benzamidocinnamic acids with a [Rh(COD)(diop)] + c a t a l y s t , have been at t r i b u t e d to (1) conformational r i g i d i t y of the diphosphine, and (2) the p a r t i c i p a t i o n of the acetamido group which coordinates to rhodium 31 33 metal v i a the 0 atom; a recent v a r i a b l e temperature P nmr study confirms bonding by the amide group. Asymmetric hydrogenation of saturated ketones to alcohols has also been achieved using various c h i r a l phosphine ligand systems. ^ ® O p t i c a l p u r i t i e s upto 83% have been achieved for the hydrogenation of pyruvic to l a c t i c acid using a Rh-BPPFOH c a t a l y s t 1 ^ (Figure I.5(VI), X^=PPh2,X2=0H). Pyruvates are also converted to l a c t a t e s i n 65-75% e.e. CH 3C0C0 2R + H 2 >• CH 3CH(0H)C0 2R [1.19] 1 n l - 1 M f-using BPPM and CPPM ligand systems (Figure I.5(XI), BPPM (R=C02"Bu ) and CPPM (R=C0 2C 2-,H 4 5)). The C=N bond of the S c h i f f base PhMeC=NCH2Ph has also been reduced with 22% e.e. using the [Rh(NBD)(diop)] +ClO^~ complex 1 i • 1 . 1 6 4 xn a l c o h o l i c solvents. Ketones and imine groups can also be reduced r e a d i l y v i a a hydro-s i l y l a t i o n procedure followed by acid hydrolysis to give the o p t i c a l l y -21-active alcohols or amines (equation [1.20]): R l R l + R T 'C=X + R SiH — > NCH-X-SiR~ > CHXH [1.20] R-^ R^ ' X=0 (ketones); X=N-R (imines) This procedure gives optical purities upto 60-85% for the hydro-genation of pyruvates and glyoxalate substrates R^C0C02R2 (R^=Me,Ph; R2=alkyl or menthyl) . Secondary imines have been isolated in upto 65% e.e. while aromatic amines give products upto 39% e.e.2^'"*"^^ These and other references have been catalogued in a recent review by James.^ A clearer insight generally into the cause of asymmetric induction has been provided by Bosnich's group"^'"'"^ who discuss two types of inter-action that occur between chiral molecules: (a) a total interaction which may be quite large, and (b) part of interaction (a) which is discriminatory and leads to asymmetric induction. This asymmetric induction is further attributed to three types of interactions: (a) ligand-olefin (b) metal-olefin, and (c) ligand-metal. To understand the importance of these interactions, a study was conducted on [cis-PtCJ^(S-MPTSO)(olefin)] complexes(MPTSO=methylp-tolyl sulfoxide; olefin=ethylene, propene, butenes, styrene, etc). The following conclusions were drawn on the basis of nmr data:-(1) The olefin moiety in the two diastereomers is rotating as seen from fluxional behaviour on the nmr time scale. This fluxional exchange can -22-occur either by complete rotation about the olefin-platinum axis (e.g. as with ethylene), or by an oscillatory motion, where one end of the olefin always passes through the same cis group but is unable to pass the other under the same conditions (e.g. as with but-l-ene, propene 168 etc.). These conclusions were based on an earlier study by Lewis on [cis-PtC^(L)(olefin)] complexes, where i t was shown that a cis halogen offered more hindrance to rotation than other cis ligands. (2) The total interaction between the olefin and the sulfoxide un-doubtedly increased as the bulk of the olefinic side-chains was increased, but contrary to expectation only marginal increase in diastereotopic interaction was observed. Furthermore, the chiral sulfoxide generated diastereomers of opposite configuration with the two closely related olefins, propene and 1-butene, suggesting that the chiral sulfoxide ligand i t s e l f was rotating. With rotation of the chiral sulfoxide ligand, the distinction between the prochiral faces of coordinated olefins is essentially 'washed out', and even increasing the bulk of substituent groups on the olefin gave very small asymmetric induction. -23-CHAPTER II  GENERAL EXPERIMENTAL A l l solutions were handled under an argon atmosphere using Schlenk techniques.^ II.1 Materials 11.1.1 Solvents - A.R. grade solvents were used without purification, but were stored over molecular sieves (BDH,type 5A) for synthetic purposes and were vacuum-degassed before use. For hydrogenation experiments a l l the solvents were d i s t i l l e d before use. Toluene was refluxed with calcium hydride, alcohols with magnesium metal and iodine mixture, while DMA was stirred overnight with calcium hydride and then vacuum d i s t i l l e d . After d i s t i l l a t i o n , a l l solvents were stored in flasks under an argon atmosphere over molecular sieves (BDH, type 5A), except alcohols which were stored without molecular sieves. DMA was stored in the dark. 11.1.2 Gases - Research grade hydrogen and CP. grade carbon monoxide were obtained from the Matheson Gas Co.. Purified argon and oxygen were supplied by Canadian Liquid Air Limited. Hydrogen was passed through a "deoxo" catalytic purifier before use. Argon was purified by passing i t through a calcium chloride drying tower containing fresh Y^O^ and molecular sieves (BDH,type 5A). Lecture bottles of anhydrous hydrogen chloride were obtained from the Matheson Gas Co. and were used without further purification. 11.1.3 Diphosphines - Dpm, dpe, dpp, and (+)-diop diphosphines were obtained 2 from Strem Chemicals Inc.. A literature method with a slight modification was used to prepare dpb as follows -26.5 g (0.12 mole) of P(C AH c ;) 9C£ in 150 ml of dioxane (dried over molecular sieve) was refluxed with 12.5 g (0.5 mole) of sodium under nitrogen for 7h with strong mechanical stirring to generate P^^H^^Na. The yellow reaction mixture was cooled to room temperature, and 100 ml of anhydrous THF then added. 9.72 g (0.045 mole) of 1,4-dibromobutane in 50 ml THF was then added dropwise and the mixture stirred for 2h. The solid NaBr was filtered off, washed with benzene, and the f i l t r a t e then evaporated to dryness on the rotary evaporator. The remaining residue was dissolved in 60 ml ethanol which was then cooled to 0°C. After few hours, a yellowish white solid precipitated and this was filtered and washed with cold ethanol. The crude diphosphine was re-crystallized twice from ethanol to give pure white crystals of dpb (yield ^60%). A l l diphosphines were recrystallized from hot ethanol before use. 11.1.4 Olefins - Olefinic substrates were obtained as CP. grade. Itaconic acid, mesaconic acid, maleic acid, and a-methylstyrene were supplied by Eastman Kodak Co., acrylamide by K & K Laboratories Inc., a- and g-methylcinnamic acids by Aldrich Chemical Co. Inc., a-N-acetamido-cinnamic and -acetamidoacrylic acids, and citraconic acid by Fluka Chemical Co. Hex-l-ene was obtained from ICN Pharmaceuticals Inc., and styrene and 3 cyclohexene from MCB Co.. Atropic acid was prepared by a known procedure. Liquid olefins were d i s t i l l e d and passed through an alumina column before use. Solid olefins were recrystallized from hot ethanol. 11.1.5 Inorganic silver salts - AgSbF^, AgPF^, and AgBF^ were supplied by Alpha Inorganics and Cationics Inc., and were stored in a vacuum desiccator for protection from moisture. RhCA^.3H?0 was a loan from Johnson Matthey Limited (39%Rh). -25-II.2 Instrumentation A l l infrared (ir) spectra were recorded on a Perkin Elmer 457 grating spectrophotometer. Solid state spectra were obtained as Nujol mulls between NaC£, KBr, or Csl plates and solution spectra were recorded using 0.1 mm NaC& cell s . Visible spectra were recorded on Perkin Elmer 202 or Cary 14 spectrometers, both instruments being fit t e d with thermostated c e l l compartments. Quartz cells of path lengths 1 mm, 2 mm, and 10 mm were used. A l l "4l nmr spectra were measured using a T-60 or XL100 spectro-meter. Tetramethylsilane (TMS) was used as an external or internal standard. 31 P Nmr spectra were obtained at 40.5 MHz using a XL100 spectro-meter operating in the Fourier Transform mode and equipped with a variable temperature attachment. A P^ O^  capillary was used as an external standard for room temperature spectra, while solutions of triphenylphosphine were used as a standard for low temperature spectra. A l l chemical shifts were then converted to values relative to 85% H^ PO^ , with upfield shift taken as positive. Conductivity measurements were made at 25°C under argon using a Thomas Serfass conductivity bridge and c e l l . Melting points were recorded using a Fisher Johns Melting Point apparatus and are uncorrected. Elemental analyses were performed by Mr. P. Borda of this department. II.3 Gas-uptake apparatus The constant pressure apparatus shown in Fig. II.1 was used for Figure II.1. Constant pressure gas-uptake apparatus. -27-kinetic and for stoichiometric gas uptake experiments. A flexible glass spiral tube connected a capillary manometer D at tap C to a pyrex two-necked reaction flask A equipped with a dropping side arm bucket. The reaction flask was thermostated in an o i l bath B and shaken by means of a piston-rod and driven by an offset wheel connected to a Welch variable speed electric motor. The manometer D contained n-butyl phthalate and was connected to a gas-measuring burette consisting of a mercury reservoir E and a 10 ml pipette N of known diameter. The gas burette was connected via an Edwards high-vacuum metering valve M to the gas-handling part of the apparatus. This part consisted of a mercury manometer F, gas inlet Y, and vacuum pump G. The capillary manometer and gas burette were thermostated at 25°C in a perspex water-bath. Thermostating of the o i l bath and water bath was controlled by Jumo thermo-regulators and Merc to Merc rely control circuits, with heating accomplished by a 40 watt elongated light bulb. The baths were well stirred, and the o i l bath insulated, the temperature being held to ±0.05°C. A vertical mounted cathetometer followed the gas uptake in the burette, and time was recorded with a Labchron 1400 timer. II.4 Gas-uptake experimental procedure In a typical gas-uptake experiment, 5 ml of solvent was placed in the 25 ml reaction flask A. Weighed substrates were added to the solvent directly and weighed catalyst via the bucket after the solvent was degassed and the flask f i l l e d with reactant gas. Degassing for DMA solvent was effected by pumping on the solvent while shaking. For higher vapour pressure solvents the freeze thaw under static vacuum technique was employed. For both methods a degas-refill cycle was repeated three times. -28-I n i t i a l l y the reaction flask was f i l l e d with reactant gas via 0 at a pressure somewhat less than that required for the experiment (Fig.II.1). The taps C and P were then closed and the reaction flask complete with spiral disconnected from 0 and attached to H and the shaker rod. The whole system up to tap C was then pumped down with taps H, K, L J and M open. Reactant gas was admitted to this part of the system at a pressure greater than that in the reaction flask but less than that desired for the reaction. After thermal equilibration of the reaction flask was attained (-15 min ), tap C was opened and the pressure of the whole system adjusted to the desired reaction pressure by introduction of gas through Y. Shaking of the reaction vessel was then done to saturate the solvent with gas at the reaction pressure (-5 min.). An experimental run was then started by dropping the catalyst bucket, starting the shaker, closing taps K and L and starting the timer. Gas-uptake was indicated by a difference in o i l levels in manometer D. The manometer was balanced by admitting gas into the burette through the metering valve M. Corresponding changes in mercury levels in the pipette N were translated to moles of gas reacted. Diffusion control of the reaction was eliminated by using fast shaking rates and a large indented reaction flask. II.5 Solubility determination 5 ml of the solvent was put into the reaction flask A which was then connected by the spiral and tap C to the gas-handling part of the apparatus at 0. The solution was then degassed by alternate cooling with pumping and warming, and then taps C and P were closed. The flask and spiral arrangement were disconnected from 0 and transferred into the -29-thermostated o i l bath with the s p i r a l connected to the oil-manometer D through tap H. Needle valve M and tap H were opened and a f t e r the a i r between taps H and C was pumped out, tap Q was closed Tap C was opened and the i n i t i a l reading of the mercury l e v e l i n N was taken. The desired gas was admitted to the gas uptake apparatus at a desired pressure. Taps K and L were closed a f t e r c l o s i n g tap J and needle valve M. The timer and the shakes were started simultaneously and the uptake of the gas was measured. II.6 Stoichiometric gas-uptake measurements Several of the complexes i n t h i s work were s e n s i t i v e i n the s o l i d state to gases, and so ampoules instead of buckets were used i n the uptake procedure. A bulb was blown on a glass tube, a c o n s t r i c t i o n was placed i n the stem and a glass rod was melted onto the bottom of the bulb to weaken i t . The l a t t e r t r i c k was e s s e n t i a l to guarantee breakage of the ampoule. After being tested for leaks, the bulb was f i l l e d with sample and then sand, and then sealed o f f under argon using the hydrogenation balloon to regulate the pressure as i n Figure II.2. These precautions minimize gas uptake when the ampoule breaks. The glass was sealed off i n the shape of a hook (Figure II.2) so that the ampoule can be hooked to the bucket dropper of the uptake f l a s k . Hook formed during seal-off Figure II.2 II.7 Spectrophotometric kinetic measurements A l l optical density measurements in the ultraviolet visible region were carried out with the c e l l shown in Figure II.3. In a typical reaction involving gas-uptake, use of a relatively large volume of gas -3 -4 (^ 25 ml), with low rhodium concentrations (^ 10 - 10 M) in small volumes of solution (^ 5 ml), ensured that the partial pressure of the gas over the solution remained essentially constant throughout the reaction. In a typical experiment, a known amount of a compound was added into the quartz part of the c e l l through A (Figure II.3) and the solvent and the substrate (if any) were added through B using a pipet. The solvent was degassed three times and the required gas was then admitted. Upto this point, the c e l l was handled carefully so as not to wet the solid. The c e l l was then placed in the thermostated c e l l compartment to allow the solution temperature to equilibrate. The solvent and the solid were then quickly mixed and the spectrum was recorded at regular intervals. During spectral measurements, the c e l l was agitated to ensure thermal and physical equilibration of the solution. -31-HIGH VACUUM TEFLON STOPCOCK B 7 SOCKET QUARTZ CELL Figure II.3 Anaerobic spectral c e l l -32-11.8 Fast reaction measurements A l l fast reactions were studied by means of a Durrum 110 stopped-flow spectrophotometer equipped with a 2 cm path length cuvette and a thermostated c e l l compartment. During a typical experiment, a solution -4 of the complex (^ 10 M) in a solvent, under argon, was mixed with the same solvent saturated with the desired gas at a known pressure. 11.9 Work-up of the hydrogenated products The solution mixture resulting from hydrogenation experiments was evaporated to give a gummy residue. The hydrogenated products of the following olefins were separated as follows -11.9.1 Itaconic, citraconic, maleic, and atropic acids - The residue was dissolved in 25 ml of 5% NaOH solution, stirred for a few minutes and filtered through Celite to give a colorless f i l t r a t e . After acidi-fying with 10% HC£ the solution was extracted twice with diethyl ether (25 ml). Anhydrous MgSO^  was added to the ether extract. After ^%h the solution was fi l t e r e d and the fi n a l product obtained after evaporation of the ether. 11.9.2 N-a-acetamido-acrylic and-cinnamic acids - The residue was dissolved in C^CJ^ 0^ 10 ml) and the mixture stirred for few minutes when a yellowish white compound separated; this was washed with CE^C^ t o give a pure white product that was recrystallized by dissolution in water 0^ 5 ml) and extraction with chloroform (^ 15 ml). The aqueous layer was then freeze-dried to recover the pure product. 11.9.3 Liquid olefins - The hydrogenated solutions from styrene and a-methylstyrene were vacuum-distilled, and that from hex-l-ene was d i s t i l l e d at one atmosphere, a microscale d i s t i l l a t i o n apparatus being used. - 3 3 -II.9.4 Acrylamide - The residue was heated to 100°C under vacuum when white crystals of propionamide sublimed. The products so obtained were identified from their "4l nmr spectra. 11.10 Optical rotation measurements A l l optical rotation values were recorded on a Perkin Elmer 141 spectrometer at room temperature using a one decimeter path-length c e l l which could hold 1 ml of solution. The rotations were measured at the sodium-D line (589 nm). The specific rotation of any chiral compound can be calculated using equation [II.1]: T where [ o t ] ^ = specific rotation at temperature T measured at the sodium-D line. a = observed rotation (+) or (-). I = path length of the c e l l in decimeters. c = concentration of solution in g/ml. The enantiomeric excess (e.e.) of the hydrogenated product was then determined using equation [II.2]: T [a] (from equation [II.1]) e.e. = x 100 [II.2] [a]^ of either pure form of the product -34-CHAPTER III HOMOGENEOUS HYDROGENATION OF ACIDIC AND NON-ACIDIC SUBSTRATES USING  THE HYDRIDOBIS(DIOP)RHODIUM(I) COMPLEX II I . l Introduction In the last decade, the area of asymmetric homogeneous 1-3 hydrogenation has been studied extensively. General reviews have appeared from time to time to keep pace with the important developments in this f i e l d . One general conclusion, drawn from the data obtained so far using various transition metal complexes, is that rhodium-chiral phosphine complexes have proved to be the most effective asymmetric hydrogenation catalysts (see section 1.7, chapter I). Indeed quite 4 recently, Bosnich and Fryzuk have used rhodium-bis(diphosphine) catalysts for the reduction of N-acylaminoacrylic acids to amino-acid derivatives in almost 100% e.e.; some other rhodium complexes are almost as effective (section 1.7, chapter I). A widely used and commercially available chiral phosphine is the so-called "diop" (Figure 1.1), i n i t i a l l y synthesized by Kagan's group"*'^, and the catalysts have usually been generated in situ by adding diop to rhodium(I) precursors such as [RhC£(diene)]^ or [RhC£(olefin)^]^. The active species have been written as RhC£(diop)(solvent) ' although in the polar medium usually employed, cationic species seem like l y , and + 2 3 7 8 [Rh(diene)(diop)] complexes are equally effective. ' ' ' Recently, some mono(diop)-rhodium complexes have been isolated and formulated, for 9 example, as [RhC£(diop) (C^ -H^ ) ] ^ . These isolated complexes, along with other in situ mono(diop)-rhodium complexes, have been used effectively for the hydrogenation of olefins and ketones (for details see section 1.7, -35-chapter I). Synthesis of a bis(diop)-rhodium(I) complex, HRh[(+)-diop]2» and i t s effective use in asymmetric hydrogenation"^ was of interest because the related chelated complex Rh(dpe)2+ is relatively inactive for hydrogenation under mild conditions.^"'' A kinetic and mechanistic 12 13 study was thus undertaken on the HRh[(+)-diop]2 _itaconic acid system ' , and since this work formed the basis of many of the studies described in this thesis, a summary of this mechanistic study is given in section 31 1 III.3 together with some new P and H nmr data. Studies involving a non-acidic substrate, styrene, w i l l then be described. III.2 Experimental HRh[ (+)-diop] 2 was prepared using the literature method."*"^  In a typical preparation, 0.13 g (0.5 mmol) of RhCJ^.S^O in ethanol (5 ml) and 0.20 g (3.6 mmol) of KOH in ethanol (10 ml) were successively added to a vigorously stirred and boiling solution of 0.55 g (1.1 mmol) of (+)-diop in 15 ml ethanol. A yellow solid separated instantaneously and this was f i l t e r e d , washed with ethanol, water, ethanol, and f i n a l l y with n-hexane. The yellow solid was recrystallized from warm n-hexane to give yellow flaky needles (yield ^70% based on Rh), and was characterized by high-field 1H nmr (21.0T in benzene-dg at 20°C) and i r (-V(Rh-H)=2040 cm 1 in Nujol), as well as by elemental analysis. The solubility of hydrogen in n-butanol-toluene mixtures at different pressures and temperatures was calculated from the data in Figures I I I . l and III.2 respectively, assuming Henry's law is applicable at each temperature. It is surprising that the solubility increases with temperature (Fig.III.2) but the data were reproducible and are -36-' P H , mm of Hg Figure I I I . l Solubility of hydrogen in n-butanol-toluene (2:1) mixture at various pressures at 30°C. Figure III.2 Solubility of hydrogen in n-butanol-toluene (2:1) mixture at various temperatures. -37-considered valid. Other experimental procedure follows that given in chapter II. III.3 The summary of studies done on the Itaconic acid - HRh[(+)-diop]2  system The itaconic acid system proved convenient for a kinetic study since this substrate could be completely hydrogenated in a reasonable period of time; Figure III.3 shows the typical S-shaped l^-uptake plots obtained for the IA system in n-butanol-toluene. Measurement of the _3 maximum rates over concentration ranges of (0.4-3.0)xl0 M HRh[(+)-diop]2, -2 (0.5-12.0)xl0 M IA, and 56-740 mm H2, demonstrated a first-order depend-ence on Rh, and between zero-and first-order on both IA and H2. Addition of small amounts of (+)-diop had a remarkable inhibiting effect on the rate of catalytic hydrogenation (for example, a 100-fold decrease using added diop: rhodium = 1:3). Solutions of the HRh[(+)-diop]2 complex under argon showed an absorption maximum at 350 nm (e=12,300M ^  cm "^) in a spectrum that was invariant with time and was unaffected by addition of diop, or H2. On the basis of kinetic and visible data, the following hydro-genation mechanism was proposed (equations [III.1]-[III.3]): HRh(diop)2 • K -» HRh(diop) (diop ) [III.l] * k l * HRh (diop) (diop ) + A — R h (diop) (diop )(alkyl) [III.2] k2 Rh(diop)(diop )(alkyl) + H 2 —1—> HRh(diop)(diop ) + [III.3] _.N saturated product (A=IA) where diop refers to a monodentate diop. T r~ i i : i r 0 ) o E V r> o </> n a CN X I 00 I 200 500 800 1100 Time, Sec 1400 1700 2000 Figure III.3 Rate plots for the hydrogenation of itaconic acid using HRhCdiop)^ as catalyst. (30°C, 740 mm P , 2.0 x 1 0 - 2 M itaconic acid, n-butanol-toluene (2:1)=5 ml, (a) [Rh]=1.0 X 2 1 0 ~ 3M ; (b) [Rh]=2.0 x 1 0 " 3 M ) . Taken from ref.12. The maximum rate in terms of total rhodium, and assuming K <<1 ft (i.e.[HRh(diop)2]>>[HRh(diop)(diop ) ] ) , was given by K*k k [Rh] [IA][H ] Rate = — — - [III.4] k^+k^H^l+kjK'^IA] which accounted for the kinetic dependences on Rh, R^, and IA; a slow build-ft up of the HRh(diop)(diop ) species to a steady state concentration explained the S-shaped curves shown in Fig. III.3. Rate law [III.4] was readily analyzed using standard inverse plots (e.g. rate vs [IA] "*"). The unusual marked diop dependence was attributed to a further equilibrium [III.5], giving rise to an inactive species containing two monodentate diop ligands: ft ft ^ 9 ft HRh (diop) (diop ) + diop - HRh (diop) (diop*)2 [III.5] Rate law [III.4] suggests that the f a l l - o f f from f i r s t to zero order in IA with increasing [IA] should be more readily attained at lower [U^] • 12 Some data obtained since completion of my Master's thesis verify this prediction (Fig.III.4), and further substantiate rate law [III.4]. Decomposition of the hydride catalyst Visible spectral studies under argon at 30°C on yellow solutions of HRh(diop)2 and itaconic acid (and also maleic and succinic acids) indicated some alkyl formation (equation [III.2]) followed by a very slow formation of the orange Rh(diop)„+ species (A 442 nm), which has Z ITL3X been synthesized by another route (chapter IV). The f i n a l solution ab-12 sorbed B.^ reversibly, and these findings were attributed to reactions [III.6] and [III.7]: HRh(diop) 2 + H + *" y Rh(diop) 2H 2 + [III.6] -40-15 Figure III.4 Maximum rate dependence on itaconic acid in n-butanol-toluene (2:1) at 30°C using 1.5 x 10~3M Rh; curve a, 740 mml^; curve h, 132 mm H£ (Curve a taken from ref. 12). -41-Rh(diop) 2H 2 + k Rh(diop) 2 + + H 2 [III.7] Solutions of the dihydride were pale yellow with a low intensity continuum in the 400-450 nm region. Equilibria [III.6] and [III.7] have now been confirmed using high-1 31 31 1 fi e l d H nmr and P nmr spectroscopy. The P{ H} nmr spectrum of the HRh(diop)2 complex in an n-butanol-toluene-acetone-dgmixture at 25°C under argon showed a doublet at -21.0 ppm with J =149 Hz (Fig. 111.5(a)) . Rn—r The high-field "'"H nmr of the same solution gave a doublet of quintets (Fig. 111.6(a)). Both spectra were invariant over two days and the yellow colour of the solution remained unchanged. The same solutions of HRh(diop)2 in the presence of itaconic acid 31 1 under argon changed from yellow to deep orange in ^ %h. The P{ H} nmr spectrum of this solution was a broadened doublet at 25°C (Fig. 111.5(b)), but was a complicated pattern at -50°C; no hydride signal was detected in the high-field \ l nmr spectrum. When the experiment was repeated in the presence of H 2 instead of Ar, the color of the solution changed from 31 1 yellow to light yellow after "»2h. The P{ H} nmr spectrum of this f i n a l solution was a doublet of doublet of triplets (Figure 111.5(c)) while the "'"H nmr spectrum was a pair of multiplets centered around 20t (Fig. 111.6(b)). The nmr data are consistent with reaction [III.6] and [III.7]. 31 The broadened P nmr doublet (Fig.Ill.5(b)) and absence of the high-f i e l d hydride signal are attributed to the formation of some alkyl intermediate but there could also be some Rh(diop) 2 + present formed according to equilibria [III.6] and [III.7]. The complicated nature of 31 1 the P{ H} nmr spectrum at -50°C w i l l be discussed in chapter IV. In U 9 -42-141 103 (a) -21.0 PPM Figure III.5 (b) * = J R h - P ' H z -9A -1< 90' (c) PPM 9.9 -3.8 L PPM 3 1 Pf1!!} nmr spectra in n-butanol-toluene-acetone-dg mixture at 25°C of: (a) HRh(diop)2 (130 mg in 2 ml solvent) under Ar. (b) (a) + 130 mg IA. (c) (b) under -43-210 I. acetone-d6 mixture at 25°C of: (a) HRh(diop)2 (130 mg in 2 ml solvent) under Ar (b) (a) + 130 mg IA under H2. the presence of K^* the spectra obtained (Figs.III.5(c) and 111.6(b)) are due to the c i s - R h ( d i o p ) s p e c i e s (see chapter VII). The reversible dihydride formation is well documented in various other rhodium-tertiary-31 1 phosphine systems where similar P{ H} and high-field nmr patterns have been observed.^ 4 III.4 The HRh[(+)-diop32-Styrene system III.4.1 Results The hydrogenation of styrene was achieved at 45°C in n-butanol-toluene (2:1) solutions at a conveniently measurable rate using the HRh(diop)2 catalyst. S-shaped l^-uptake curves were obtained (Figure III.7), and the hydrogenation rates were readily determined from the maximum slope. The reproducibility of the rates (±5%) was established by repeating several experiments. The kinetic data obtained are listed in Table I I I . l . The uptake rates at constant hydrogen and olefin concentrations varied linearly with increasing catalyst concentration over the range _3 studied (2.0-6.0)xl0 M (Fig. III.8), and the reaction was first-order both in styrene (0.05-0.25M, Fig. III.9) and H 2 (110-718 mm, Fig.III.10). The rates increased with temperature (Table III.2), and decreased drastically on addition of even small amounts of excess (+)-diop (Table III.3). The solution remained yellow during and after the hydrogenation reactions (see below). On adding 0.1M styrene to catalytic concentrations of HRh(diop)2 under argon at 45°C, there were practically no visible spectral changes -46-Table I I I . l Maximum hydrogenation rates for the hydrogenation of styrene in n-butanol-toluene (2:1) at 45°C using HRh[(+)-diop], [Rh]xl03,M [StyreneJxlO^M H£ ,mm— [H 2]xl0 3,M^ Max.rate x 10 Ms"1 1 «~2 -1 k,M s 2.0 1.0 718 5.37 8.74 8.14 3.0 I I 11.12 6.90 4.0 I I 17.04 7.93 5.0 I I 19.40 7.23 6.0 I I 25.60 7.95 3.0 0.5 » 5.80 7.20 I I 1.5 17.78 7.36 11 2.0 » 22.22 6.90 I I 2.5 27.60 6.85 I I 1.0 110 0.83 1.74 7.00 I I I I 262 1.95 4.82 8.10 I I I I 414 3.09 6.76 7.30 M I I 566 4.24 9.08 7.14 -2 -1 Average k = 7.38 M s a. A solvent pressure of 42 mm Hg at 45°C was calculated for the 2:1 n-butanol-toluene mixture assuming i t obeyed Raoult's Law. b Calculated from Figures I I I . l and III.2. -47-Figure III.8 Maximum rate dependence on HRh(diop)2 concentration ([styrene]=0.1M, P H =718 mm, n-butanol-toluene (2:1), 45°C). 2 -48-Figure III.9 Plot of maximum rate against styrene concentration (45°C, P R =718 mm, [Rh]=3.0 x 10_3M, n-butanol-tolu (2:1)). 2 - 4 9 -12.5 -50-Table III.2 Variation of maximum rate with temperature -3 ([styrene]=0.1M, [HRh(diop)0]=3.0x10 M, P„ =674 mm, n-butanol-toluene(2:1)). 2 n.^ 3 T,°C Vapour pressure of [H^JxlO ,M 6 —1 Max.Rate x 10 ,Ms k,M s the solvent,mm 45 42 5.05 11.00 7.26 50 54 17.46 11.52 55 68.5 28.36 18.72 60 86 38.40 25.35 Table III.3 Variation of maximum rate with added (+)-diop (45°C, [styrene]=0.1M, [HRh(diop)2]=3.0 x : 10_3M, P„ =718 mm, n-butanol-toluene (2:1)). [(+)-diop] x 104,M Max.Rate x 10 6, Ms 1 0.0 11.12 1.0 10.28 2.0 5.44 3.0 2.64 10.0 a a_ Immeasurably slow. -51-(Fig. III.11). Similarly on sampling the solution during a catalytic hydrogenation, the visible spectrum indicated the presence of only HRh(diop)2; more concentrated solutions used for hydrogenation showed the same high-field "*"H nmr spectrum as that of starting hydride. Thus only HRh(diop)2 is detected during the catalytic hydrogenation. III.4.2 Discussion The kinetics for the hydrogenation can be written as: -d[H2] d t = k[Rh][styrene][H 2] [III.8] where k is an overall third-order rate constant of average value -2 -1 7.38 M s (Table I I I . l ) . The kinetics are consistent with the mechanism 12 postulated previously (equations [III.1]-[III.3]; A=styrene), but in which ' a l l ' the rhodium is present as HRh(diop)2 (as observed spectros-copically). The S-shaped uptake curve (Fig. III.7) is thought to result ft from a build-up of a kinetically significant amount of Rh(diop)(diop ) (alkyl); the rate would then become: K"k k [Rh] [styrene][H ] K l * R2[H2] With k2[H2]<<k_1, rate [III.9] reduces to the observed rate law [III.8] ft ft with k = K k^k^/k_^ or K where = k^/lc- ^ . Variation of k with temperature yields a reasonable Arrhenius plot (Fig. III.12) and the activation' parameters AH^=17.7±1 Kcal.mole \ E =18.3±1 Kcal.mole \ * , 3 ft and AS =1±4 eu. ; these presumably refer to the composite constant K K^2. These values are close to those measured for the itaconic acid system Figure III.11 Absorption spectra of HRh(diop)2 (1.4 x 10 M) under argon in n-butanol-toluene (2:1) in the presence of styrene (0.1M) at 45±2°C. -53-Figure III.12 Arrhenius plot for the hydrogenation of styrene using HRh(diop) 2 catalyst ([Rh]=3.0 x 1(T3M, [styrene]=0.1M, P H 0 = 6 7 4 m m> n-butanol-toluene (2:1)=5 ml). -54-(E =16.4±1 Kcal.mole"1, AH^=15.8±1 Kcal.mole"1, and AS^=-6 e.u.). 1 2 However, these parameters were measured under conditions that were zero-order in H^  ft and first-order in itaconic acid and so refer to K k^. The marked inhibition on added diop in the styrene system is of the same order of magnitude as that measured for the itaconic acid system, and is attributed to equilibrium [III.5], Species containing ft 15 monodentate diop ligands are now well established,RuCtf^ (diop)(diop ) ft 16 and [Rh(CO)2(diop)(diop )]C£ complexes having been isolated and charac-terized . 12 In the diethyl maleate system discussed previously , the visible spectral data in the absence of (under argon) at 30°C was surprising since i t essentially showed " f u l l " formation of Rh(diop)2 + with an excellent isosbestic point at 450 nm (Fig. 111.13(a)). High-field "*~H nmr of this dark orange solution did not show any hydride signal. The data show that the cation must be formed in some way from the alkyl complex. A reasonable explanation involves protonolysis via butanol: Rh (diop) (diop*) (alkyl) + BuOH -r-—> Rh(diop) 2 +0Bu~ + saturated [III.10] product Addition of to the dark orange solution resulted in a slow change of colour to yellow; high-field "'"H nmr spectra of this yellow solution showed the characteristic hydride peak (Fig. 111.6(a)) and the visi b l e spectrum resembled that of HRh(diop)2 (Fig. 111.13(b)). A distinct possibility is establishment of equilibrium [III.6]. Such 14 an equilibrium has been suggested by Schrock and Osborn for some related tertiary phosphine and arsine systems, although they were unable to isolate the monohydride species. In the case of acidic substrates -55-Time, ht ' ( 0 0 C3) 2 w 22 (io) 5(mirO (13) 1 12 151 400 1 X 1 500 600 WAVELENGTH,nm 700 Figure III.13 -4 Absorption spectra of HRh(diop) 2 (1.4 x 10 M) i n n-butanol-toluene (2:1) i n the presence of diethyl maleate (1.0 x 10~2M) at 30±1°C. (a) Solution under argon. (b) Solution under hydrogen. (Data taken from ref. 12). -56-(e.g. IA) equilibrium [III. 6] is not observed, the H*" concentration presumably maintaining dihydride formation. Strong evidence for the involvement of butanol in cation production using non-acidic substrates (reaction [III. 10]) is that in toluene alone no 440 nm peak is generated, and the slight decrease in absorbance in the vi s i b l e spectrum and absence of the high-field hydride signal are attributed to the formation of the alkyl complex. In the styrene system, the presence of a high-field hydride signal due to the i n i t i a l HRh(diop)2 complex, and insignificant spectral changes in Fig. III.11 are due to equilibrium [III.2] lying far to the l e f t consistent with rate law [III. 9]. Reaction [III.10] is not observed in this case due to insignificant amounts of alkyl present. III.5 Conclusions From sections III.3 and III.4 i t is clear that the nature of substrates.has a pronounced effect on the course of hydrogenation reactions. Although the basic mechanism involved in both IA and styrene systems is the well established 'unsaturate' route (Fig. 1.4), dependences of re-action rates on various parameters differ in both cases and yield different rate laws [III.4] and [III. 9] respectively. Spectrophotometric studies indicate that non-acidic substrates definitely involve HRh(diop)(diop ) as the active catalytic species whereas the acidic substrates may involve the Rh(diop) 2 + complex as well (depending on the rate of hydrogenation) pro-duced by slow decomposition of the hydride catalyst by protons of acidic substrates. Detailed kinetic studies using the Rh(diop)2 + complex w i l l be presented in chapter VI. CHAPTER IV BIS(DITERTIARYPHOSPHINE) COMPLEXES OF RHODIUM(I). SYNTHESIS, SPECTROSCOPY, AND ACTIVITY FOR CATALYTIC HYDROGENATION IV.1 Introduction Although the bis(ditertiaryphosphine) Ph2P(CH^^PPt^ complexes of ~ 1-4 iridium(I) and iridium(III) have been studied f a i r l y extensively , few details have been reported for the rhodium analogues. The Rh(dpe)2X complexes, X=H, Cl, ClO^, are well-known^, and in terms of activation of small gas molecules and catalytic activity, the crystal structure of the -t~ — 6 cationic dioxygen complex Rh(dpe) 20 2 PF^ has been reported. The 2 5 Rh(dpe)2C£ complex is a 1:1 electrolyte in polar solvents ' , and is ineffective as a catalyst for hydrogenation of olefinic substrates under mild conditions. 2''' The Rh(dpe) 2 + cation is unreactive toward H2.^'^a Recently, the [Rh(dpe)2]C£ and [Rh(dppp)^\Cl complexes have been used 9b for the catalytic decarbonylation of aldehydes. Our interest in these bis(diphenylphosphino) systems increased after our finding that an isolated HRh[(+)-diop]2 complex was an efficient catalyst for asymmetric hydrogenation of certain prochiral olefinic carboxylic acids under mild conditions."'"'"1'"'""'" The diop ligand (Fig.I.l) can be considered, for example, as a derivative of the dpb ligand, and this led us to look at the bis[PPh2(CH^^PPh,,] systems, including the hydrido, chloro and cationic complexes, with a view to learning more of reactivity patterns generally as a function of _n. Such data have appeared on the use of 1:1 rhodium(I)/Ph 2P(CH 2) nPPh 2 ~ 12 hydrogenation catalysts formed in situ from rhodium(I) precursors , -58-13 and depending on the media, the catalysts may be neutral or cationic. The synthesis and characterization of various bis(diphenylphosphino) complexes are reported in this chapter along with some preliminary data concerning their catalytic activity for hydrogenation of the prochiral 11 14 substrate, methylenesuccinic acid. We and Kagan's group have reported earlier on the characterization of the cationic complexes + + 15 Rh(diop) 2 and Rh(diop) 2H 2 , and Slack and Baird reported on some in situ dpp and diop systems while this work was in progress. The lumines-cence of the Rh[PPh„(CH„) PPh„]„ + cations (n=l-3) has been discussed but Z Z u Z Z — 16 17 no other details on the complexes were given. ' IV.2 Experimental A l l solutions were handled under an argon atmosphere using Schlenk techniques. Preparation of Complexes - The cyclooctene dimer [RhC£(C„H1,)„]„ o 14 Z Z 18 was prepared by a literature procedure ; yields of 90% were obtained on leaving the reaction mixture for lOd. IV.2.1 Rh(dpm)2C& To a benzene solution (5 ml) of [RhC£(C0H_.)„]„ (0.30 mmol, 0.215 g) o 14 Z Z was added dpm (1.32 mmol, 0.50 g) in benzene (5 ml), and the resulting solution was freeze-dried. The residue obtained was recrystallized from CH 2C£ 2 ~ether to give orange needles. IV. 2. 2 Rh(dpe)2C>t and Rh(dpp)2C& A benzene solution (10 ml) of the phosphine (1.1 mmol, 0.44 g dpe or 0.45 g dpp) was added to a red, benzene solution (10 ml) of [RhC£(CgH l 4) 2] 2 (0.25 mmol, 0.18 g). The precipitated yellow dpe complex or light yellow dpp complex was washed with warm benzene and -59-dried in vacuo. The complexes have been prepared previously by other u J 5,16 methods. IV.2.3 Rh(dpb)2C£ A benzene solution (15 ml) of [RhC£(CgH.^)2]2 (0.30 mmol, 0.215 g) and dpb (1.32 mmol, 0.56 g) was refluxed under Ar for 3h. Concentration to a 5 ml volume, followed by addition of n-hexane gave a yellow solid that was fi l t e r e d , recrystallized from C l ^ C ^ - ether and dried in vacuo. IV.2.4 Rh[(+)-diop]2C& A benzene solution (15 ml) of [RhC£ (C0H, ,)<->] - (0.30 mmol, o 14 z z 0.215 g) and (+)-diop (1.32 mmol, 0.66 g) was refluxed under Ar for 3h. The solution was evaporated to dryness under vacuum. The resulting residue was recrystallized from n-hexane to give an orange powder. IV.2.5 Rh(P~P)„+BF ~ (p"p = dpm,dpe,dpp,dpb, and (+)-diop) z • The complexes were prepared in each case from the corresponding chloro complex. For example, a mixture of a CH2C&2 solution (5 ml) of Rh(dpm)2CX, (0.30 mmol, 0.27 g) and a methanol solution (5 ml) of AgBF^ (0.30 mmol, 0.58 g) was stirred for 30 m. Removal of the AgC£ and evaporation to dryness l e f t a residue that was recrystallized from CH2C12 - ether to give red crystals of Rh(dpm)2+BF4 . The dpb analogue was dark red, while the dpe, dpp, and diop complexes were light orange. IV. 2.6 HRh(P^P) (pHp=dpe,dpp, and (+)-diop) The literature method^, involving treatment of the chloro complexes (0.50 mmol) with a 3 mole excess of sodium borohydride in ethanol (10 ml), was used in attempts to prepare the corresponding hydrides. The red dpe"*, orange dpp, and yellow diop complexes were recrystallized -60-from benzene-ethanol. The diop complex has been prepared earlier by 19 a different route (section III.2,chapter III). Use of the dpm ligand yielded a deep red precipitate but this decomposed even under Ar, so the compound could not be characterized. Borohydride reduction of Rh(dpb)2C£ yielded a mixture of complexes that have not been separated; borohydride species are almost certainly present as judged by i r bands in the 2340--1 20 2390 cm region. Attempts to synthesize the HRh(dpb)2 complex via the routes used to synthesize the diop analogue from RhCA^' 3H20"'~^''^'^ were similarly unsuccessful. The yields of a l l the isolated complexes described in this section were in the 75-90% range. IV.3 Results and Discussion Cleavage of the chloride bridge in [RhCA(diene)J complexes by a Lewis base L, including monodentate phosphines, can yield both RhCA (diene) L and Rh (diene) L 2 +C£ species, depending on the amount of 21 L used and the polarity of the solvent used. Reaction of the diene dimer with 2 moles of a chelating diphosphine is assumed to yield RhC£ (diphosphine) (solvent) or Rh (diene) (diphosphine)"1", depending on the i , . i . . ... . , 12-15,22-24 solvent, and the cationic complexes are readily synthesized. We have used the cyclooctene precursor [RhC£(C H .)_] to prepare the o 14 Z Z Rh(P°P)2C£ complexes (Table IV.1) via reaction [IV.1]. While these 25 studies were in progress, a report appeared describing the synthesis [RhC£(C gH 1 4) 2] 2 + 4P P 2Rh(P ?)2CSL + 4C gH 1 4 [IV.1] of a [RhCA(diop)(C^Hg)]2 complex via a similar reaction, but using the Table IV.1. Analytical and molar conductivity (A) data for the rhodium(I) complexes Complex Decomposition Analysis A* (ohm ^cm2 point(°C)£ %C %H Found Calculated Found Calculated Rh(dpm)2Cl 97 62.03^ - 66.20 4.46 4.85 71 Rh(dpe)2Cl 205-209 66.64 66.80 5.27 5.20 61 Rh(dpp)2Cl 162-165 67.43 67.33 5.30 5.40 74 (49)-Rh(dpb)2Cl 108 65.66^ 67.85 5.85 5.65 49 Rh(diop) 2Cl 79 64.97- 65.58 5.96 5.64 35 Rh(dpm)2BF4 124 61.50^ 62.65 4.55 4.59 79 Rh(dpe)2BF4 273-276 61.40^ 63.31 4.86 4.87 78 Rh(dpp)2BF~ 180 63.14- 63.92 5.05 5.13 81 Rh(dpb)2BF4 169-173 63.22- 64.51 5.40 5.38 75 Rh(diop) 2BF 4 174-177 60.91- 62.74 5.09 5.40 78 HRh(dpe)2 167 69.70 69.33 5.50 5.44 71 (2)-HRh(dpp)2 143-146 69.35 69.84 5.65 5.71 51 (12)-HRh(diop)2 128 66.70 67.64 6.28 6.00 4 (2)* moi ) a_ Uncorrected, in air. b_ 10~3M in CH3N02 at 25°C under Ar. c_ Complex hygroscopic. d_ Value in DMA. e^  Solids 02~sensitive. f_ Low carbon due to presence of C^C^, detected in nmr; the calculated values are uncorrected. -62-bis(ethylene) dimer precursor with 2 moles of the diphosphine. Reaction [IV.1] at room temperature precipitates in analytically pure forms the dpe and dpp complexes, and probably the dpm complex but this is very hygroscopic and does not give satisfactory analytical data. The preparations are simpler than ones reported earlier using rhodium(I) carbonyl precursors. ~*'"^  Synthesis of the dpb and diop complexes via reaction [IV.1] requires a refluxing procedure; the complexes are air-sensitive and this could explain the slightly low carbon analyses. The Rh(P- P ) 2 A complexes (A =BF^ , PF & , SbFg ), readily prepared from the chloro complexes by treatment with Ag+A , can be recrystallized from CH2C&2 - ether; the carbon analyses are somewhat low due to the presence of dichloromethane in the crystals (Table IV.1). Details are given only for the tetrafluoroborate complexes since these proved to have more convenient solubility properties for the catalytic studies (see section IV.4). The borohydride method of Sacco and Ugo^ was used to synthesize the HRh(P P ) 2 complexes from the chloro analogues. A presumedly dpm species decomposed spontaneously, at least at room temperature, and i t was not possible to isolate the dpb by this route or one developed by 27 Robinson's group starting from rhodium trichloride. This latter method, in fact, proved to be unsatisfactory for a l l the hydrides, 19 except HRh(diop) 2 > Tables IV.1-IV.5 report conductivity and spectroscopic data for the various complexes. The molar conductivity data (Table IV.1) show that dpm, dpe, and dpp chloro complexes are 1:1 electrolytes in nitromethane, and the -63-Table IV.2. Visible spectral data for the rhodium(I) complexes— -3 -1 -1 Complex Medium A nm (e x10 M . cm ) max — Rh(dpm)2Cl methanol 385 (3.8), 446 toluene 406^ (3.3) Rh(dpe) 2Cl methanol 406^ (4.5) Rh(dpp) 2Cl methanol 4I(£ (2.6) Rh(dpb) 2Cl methanol 435^ toluene 420 sh (1.6) Rh(diop) 2Cl methanol 440^ toluene continuum Rh(dpm)2BF~ methanol 385 (3.7), 446 Rh(dpe) 2BF 4 methanol 407- (5.0) Rh(d P P) 2BF 4 methanol 412- (2.8) Rh(dpb)2BF~ methanol 435 (3.3) Rh(diop)2BF~ methanol 442 (3.6) HRh(dpe) 2 toluene 406 (8.1) HRh(dpp)2 toluene continuum HRh(diop)2 toluene 355 (13.4) a. Under Ar at room temperature. b_ In solid state, -410 nm. c^  Intensity increases with time, d In solid state, -412 nm. -64-Table IV.3. Infrared and high-field "^H nmr data for various rhodium(I) complexes. Complex a h Nmr data— i r data— Rh-H,T J p_ R,Hz J R h_ R,Hz v(Rh-H),cm 1 v(Rh-CA),cm"1 Rh(dpb)2C£ 281 (w) Rh(diop)2C£ 284 (w) HRh(dpe)2 20.1 18.0 10.0 1900 (s) HRh(dpp)2 19.8 20.8 8.0 2100 (s) HRh(diop) 21.0 17.0 6.0 2040 (w) Free ligand— i r data— v(280-400 cm 1 bands) dpm 350(s), 387(s) dpe 345(s), 393 (s) dpp 303(a), 335(s), 393(s) a In C &D 6 at 25°C under argon. b_ In Nujol; s=sharp, w=weak. £ Dpb and diop ligands have no absorption bands in the 280-400 cm region. -65-31 Table IV.4. P nmr data for the rhodium(I) complexes at 25°C. Complex Medium o(P)- JRh-P' H z Coordination Shift (A)b 5p p m Rh(dpm)2C£ CH„C£„-acetone-d, 2 2 6 (2:1 V/V) +22.80 115.3 +0.55 C6 D6 +17.38 105.3 -4.87 Rh(dpe)2C£ CH 2C£ 2-C 6D 6(2:1 V/V) -57.18 134.2 -69.69 Rh(dpp)2C£ CH„C£„-acetone-d, 1 1 6 (2:1 V/V) -7.79 132.3 -25.04 Rh(dpb)2C£ I I See text Rh(diop)2C£ I I See text Rh(dpm)2BF4 CH 2C£ 2-C 6D 6(2:1 V/V) +23.50 117.0 +1.25 Rh(dpe)2BF~ I I -57.22 132.7 -69.73 Rh(dpp)2BF 4 I I -7.38 130.5 -24.63 Rh(dpb)2BF4 I I -21.09 132.0 -37.04 Rh(diop)2BF~ I I -9.22 140.0 -31.72 HRh(dpe) C6 D6 -56.43 142.5 -68.94 HRh(dpp ) 2 I I -18.26 141.8 -35.51 HRh(diop)2 I I -22.46 146.0 -44.96 a. Measured in ppm (upfield positive) from 85% I^PO^. Shifts (CH 2C£ 2 or CDC£ 3) for the free ligands occur at +22.25 (dpm), +12.51 (dpe), +17.25 (dpp), +15.95 (dpb), and +22.50 (diop). b A = 6 - 6 — complex ligand -66-Table IV.5. 1H nmr data for the rhodium(I) complexes at 25°C. Complex Aliphatic Phenyl—  Rh(dpm)2C£ 3.4m3- 6.5m,7.2m3-Rh(dpe)2C£ 2.15m 7.2m Rh(dpp)2C£ 1.8m,2.2m 7.1m Rh(dpb)2C£ 1.6m,2.0m 7.3m Rh(diop)2C£ 1.10s,2.3m,3.6m 6.7m,7.2m Rh(dpm)*BF~ 5.1m (4.85m)- 7.3m(7.4m,7.6m)-Rh(dpe)^BF4 2.15m 7.25m Rh(dpp) BF~ 1.85m,2.3m 7.2m Rh(dpb)2BF4 1.7m,2.15m 7.15m,7.4m Rh(diop) 2BF~ 1.10s,2.3m,3.8m 7.2m,7.4m HRh(dpe). 2.2m3- 7.0m,7.em3-HRh(dpp)2 1.6m,2.1m3- 7.1m,7.7m3-HRh(diop) 2 1.26s,2.2m,3.4m, 7.0m,7.lm,7.5m, 3. 6m3- 7. 9m3-Free diphosphine dpm 2.85t(2.75t)- 7.3m(7.0m,7.4m)-dpe 2.1t(2.2m)- 7.2m(7.0m,7.3m)-dpp 1.7m,2.2m(1.55m,2.0m)- 7.2m(7.Im,7.3m)-dpb 1.5m,1.9m(1.45m,1.8m)- 7.lm(7.Im,7.4m)-diop 1.28s,2.3m,3.8m 7.2m 1.28s,2.4m,4.0m3- 7.0m,7.3m3-a_ Measured in ppm (downfield positive) from TMS in CBCl^ unless otherwise indicated; s=singlet, t=triplet, m=multiplet. a' In C,D,. — 6 6 ID For phenyl region, lower shift when discernible is due to m- and p-protons and the higher shift due to o-protons; for HRh(diop) 2, the 2 lower shifts are due to m-,p-protons, and the higher shifts due to o-protons. c In acetone-d,.. -67-visible spectral data in methanol (Fig.IV.1(A)) correspond also to those of the BF^- salts (Fig.IV.2;Table IV.2). The solid state spectra of the dpe and dpp chloro complexes also show absorption maxima at wave-lengths similar to the solution values, indicating the same ionic structure in the solid state. These complexes have always been con-sidered as ionic in the solid state with square planar cations (or approximately s o ^ ) , and this appears to be based on electronic absorption 16 17 28 and emission spectral data. ' ' The dpm complex, unlike the other two, is soluble in toluene and the vis i b l e absorption spectrum in this solvent (Fig.IV.1(B)) and the solid state differs from that in methanol (Fig. IV.1(A)). We thus consider Rh(dpm)2C£ to be five-coordinate in the solid state and toluene. Detection of possible Rh-C£ stretching fre-quencies in the i r is prevented, due to absorption by the dpm (and dpe, dpp) ligands in the 400-280 cm ^ region (Table IV.3). The dpb and diop chloro complexes also appear to be five-coordinate in the solid state and in non-polar solvents; the conductivity data in nitromethane suggest incomplete dissociation of chloride, and their solubility in toluene (the corresponding fluoroborates being insoluble) i s also consistent with coordinated chloride. Further, weak i r bands at 281 and 284 cm ^ for the dpb and diop complexes, respectively, (not present in the BF^ analogues) are attributed to v(Rh-C£). The visible spectral data are solvent dependent and, in methanol, correspond to those of the cations, thus showing loss of ionic chloride in this medium. The absorption maximum in the cationic systems moves to higher wavelengths as ri increases from 2 to 4 (Fig.IV.2). The dpm system is different in showing two maxima 31 in the 380-450 nm region; "anomalous" P nmr shifts are also noted with -68-350 400 500 WAVELENGTH, nm Figure IV.1(A) V i s i b l e absorption spectra of the Rh(P P) 2C£ complexes i n methanol under argon at 25°C (P P = a.dpm, b.dpe, c.dpp, and d.dpb). -69-Figure IV.1(B) V i s i b l e spectra of the^Rh(P P) 2C£ complexes i n toluene under argon at 25°C (P^P = e.dpm, f.dpb, and g.diop). -70-o.o WAVELENGTH,nm Figure IV. 2 V i s i b l e spectra of various Rh(P P^^BF^ complexes i n methanol under argon at 25°C (P"T> = a.dpm, b.dpe, c.dpp, d.dpb, and e.diop). -71-0.0 WAVELENGTH,nm Figure IV.3 Absorption spectra of the HRh(j£~P)2 complexes in toluene under argon at 25°C (P^P = a.dpe and b.dpp). -72-this system (see below). Measurement of the conductivity of the five-coordinate hydrides in nitromethane gave unexpectedly high values for the dpe and dpp complexes, while the diop species exhibited normal behaviour (Table 29 IV.1). A l l the hydrides were essentially non-conducting in polar DMA. Reaction of the hydrides with the nitromethane is indicated, although the limited solubility of the complex and the extreme sensitivity of the solutions to oxygen presented d i f f i c u l t i e s for further investi-gations. Reaction of platinum metal hydride complexes with nitroalkanes is not unprecedented; several aromatic and aliphatic compounds have been reduced to corresponding amines catalyzed, for example, by HRuC£(PPh 3) 3 30 in benzene-ethanol(l:l V/V) (equation [IV.2]): R R VCH-N0„ + 3Hn HRuCA(PPh3)3 , N CH-NH. + 2Ho0 [IV. 2] R R The hydride ligand of the HRh(P T) ^  complexes is readily detected by i r , and the high f i e l d nmr, a well resolved doublet of quintets at room temperature (Table IV.3, Fig.IV.4), is consistent with four equivalent phosphorus atoms. The same nmr pattern with similar coupling constants (J _R=7 Hz, J p_ H-18 Hz) has been reported for HRh[Ph 2P(CH 3)] 4 at -60°C, Rh 31 and this was attributed to a tetragonal pyramid structure , although 32 others have considered that a fluxional C 3 v structure is more li k e l y . The solid state structure of HRh(PPh3)4 with the hydrogen atom omitted is tetrahedral, and a threefold axis of symmetry implies that the hydrogen 33 34 l i e s on this axis or must be randomly disordered in the crystal. ' A recent structure determination of Hlrtdpe^ shows i t to be approximately 20 . l t I 19.8 t 21.0 Z I ( 1 ) (3) i I Figure IV.4 High-field H nmr spectra of (1) HRh(dpe) (2) HRh(dpp)„ (3) HRh(diop)„ in C,D^  at 25°C. 6 6 -74-35 trigonal bipyramidal with the hydride presumed to be at an axial site (Figure IV.5); the high f i e l d *H nmr i s the quintet expected of a fluxional solution structure, and by analogy the HRh(P P ) 2 structures are li k e l y to be similar. Ph H p. ph / Ph Ph Ph Ph X-ray crystal structure of the HIr(dpe) 2 complex. Figure IV.5 A single crystal x-ray structure determination of HRh[(+)-diop]2, now in progress, has revealed a distorted trigonal bipyramid structure (Figure IV.6 ) , very similar to that shown in Figure IV.5 for HIr(dpe) 2. Various bond lengths and bond angles are listed with Figure IV.6. As 36 37 expected, due to the high trans influence of the hydride ligand ' , o the Rh-P^ bond length (2.340A) is found to be longer than those for the 31 1 other three Rh-P bonds. The P{ H) nmr signal of this complex is a doublet at 25°C but gives rise to a complicated spectrum at -50°C (Figure IV.7). The correlation between the solid state structure and the low temperature solution spectrum of this complex needs further elucidation. 31 The P nmr signals (proton-decoupled for the hydrides) for a l l the complexes at 25°C appear as sharp doublets due to equivalent phosphorus atoms coupling to the rhodium (Table IV.4; e.g. Figure IV.8). No general trends in <5 values are apparent, although the dpm ligand is different in undergoing upfield shifts (A positive) on coordination to the metal. Such -75-•1.6 P-C(aryl) mean = 1.85(2) P-C(alkyl) mean = 1.854(3) Angles P2-Rh-P3 P4-Rh-Pl Pl-Rh-P2 P3-Rh-P4 P2-Rh-P4 P3-Rh-Pl Pl-Rh-H P2-Rh-H P3-Rh-H P4-Rh-H (deg) 122.94 97.03 102.23 106.09 117.34 107.42 164.70 76.21 85.47 71.07 Figure IV.6 Crystal Structure of the HRh[(+)-diop] complex. -76-(a) - 2 1 . 0 L _ PPM Figure IV.7 P{ H}nmr spectrum of HRh[(+)-diop]2 under argon in n-butanol-toluene-acetone-dfi(a) at 25°C and (b) at -50°C. 17.38 I -77-22.80 I (a) (b) H 1 17.0 20.0 PPM 23.0 31. Figure IV.8 P nmr spectrum of Rh(dpm)oC£ in (a) C,T>, and D O (b) CH 2C£ 2-acetone-d 6 at 25°C. -78-upfield shifts have been reported for both dpm and dpp ligands in some 3 iridium complexes (e.g. in [Ir (CO) (dpm^jCJl and [Ir (CO) (dpp)^]c&), and 38 39 the factors determining such shifts have been enunciated ' in terms of metal-ligand interactions and rehybridization effects on coordination of ligand to the metal. The substantially large downfield shifts noted for the dpe complexes provide further examples of the large degree of 39 deshielding apparent in five-membered ring systems. The data for the chloro and tetrafluoroborate complexes are consistent with the dpe 31 and dpp complexes being ionic, and the variation of the P chemical shift for Rt^dprn^Cft with solvent is also consistent with ionic character in polar solvents and covalent character in benzene (Table 31 IV.4; Fig.IV.8). Complex P spectra were obtained at 25°C for the Rh(dpb)„C£ and Rh(diop)„C£ complexes in both C,D, and CH„C£ - acetone-d,. z z o o z z o Complications could arise by (a) partial ionization in the polar medium, (b) a more rigid five-coordinate structure [erf. the distorted trigonal bipyramidal structure of HRh[(+)-diop] (Fig.IV.6)], since increasing 2 3 chelate ring size leads to decreased fluxional behaviour ' , (c) the occurrence of multinuclear species, which are becoming increasingly 40 41 evident in rhodium-diphosphine chemistry. ' The simple doublet pattern of Rh(diop) 2 +^4 i n CD^ OD also gives way to a complex spectrum at -60°C; formation of associated species seems the most likely explan-ation. More detailed variable temperature nmr studies have been done to help c l a r i f y the complications and w i l l be discussed in chapter VI. Data for the "^H nmr in the phosphine region are given in Table IV.5. The shift of ligand protons on coordination is generally small except for the CB.^ protons of dpm which move downfield by -0.6 and -2.3 -79-ppm in the RMdpm^CJl and Rh(dpm)2+ BF^ complexes, respectively, the different shifts further confirming the non-ionic character of the chloro complex. These resonances are somewhat solvent dependent (Table IV.5), and appear as a sextet due to "virtual coupling" with the rhodium (Figure IV.9) Such a phenomenon is not unprecedented and 42 has been observed by Schrock and Osborn for CH^  resonances in [Rh(COD){P(C,Hj0CH„}] which appeared as a complex multiplet due to o _> z j rhodium to methyl coupling ( J _ , -3 Hz). The ortho protons of the Rh—LH^ phenyl groups in the dpe and dpp complexes experience an anisotropic deshielding compared to the. meta and para protons. The phenomenon is more apparent from data in benzene-d^ (Figure IV. 10) since in CDCSL^ the phenyl absorption region is broad; for example, free dpe gives multiplets (3:2) at 6^7.0 (meta,para) and 7.3 (ortho), while in the HRh(dpe)2 complex these absorptions appear at 7.0 and 7.6 ppm, respectively (Table IV.5). 43 Similar deshielding of ortho protons has been noted, for example , in some iridium alkyl complexes, IrC^(CO)(PPh^)^(alkyl), (alkyl=C nH2 n +^ (n=2-5, and 7); CH^^C^H^), although a shielding of the ortho protons + 44 occurs in the cis-octahedral complexes Ir(dpe)2X2 (X=0,S,Se) , in various cis-dpe complexes of Ir(III), Ru(III), Cr(0), Mo(0), and W(0) and in the Rh(dpm)2C£ complex reported here (Table IV.5). The low f i e l d "*"H nmr of the HRh (diop) 2 complex is of interest (Fig.IV.10) in that the aromatic protons appear as four distinct multiplets of relative intensity 3:3:2:2 at 67.0, 7.1 (meta, para) and 67.5, 7.9 (ortho); the deshielding through coordination is again well demonstrated, and there are now two sets of phenyl groups. These very likely approximate an edge-face conformation, a geometry that has been demonstrated recently in several Pt^P PPl^ c o m P l e x e s > including Figure IV.10 XH nmr (phenyl region) spectra of (1) HRh(dpe)2 (2) HRh(dpp)2 (3) diop (4) HRh(diop)2 in CgDg at 25°C. -82-45 + 46-48 IrCA(COD)(diop) and Rh(COD)(diphosphine) species , where COD= 1,5-cyclooctadiene and diphosphine=l,2-bis[(anisole)(phenylphosphino)] ethane and 2,3-bis(diphenylphosphino)butane. The orientation of the phenyl groups appears c r i t i c a l for the high efficiency of such complexes as catalysts for asymmetric hydrogenation of prochiral olefinic subs-trates . 1 3 » ^ » ^ A l l the complexes reported here are oxygen-sensitive in solution; in the solid state, only the chloride and fluoroborate complexes containing dpe and dpp are reasonably air-stable. IV.4 Catalytic hydrogenation Some preliminary kinetic data for hydrogenation of methylene-succinic acid to 2-methylsuccinic acid are summarized in Table IV.6. Figures IV.11-IV.13 show typical gas-uptake plots for the hydro-genation of methylenesuccinic acid at convenient conditions using some of the complexes listed in Table IV.6. The shape of the curves vary with the catalyst used, and only maximum rates were measured. 2 7 + -In agreement with others ' , i t has been found that Rh(dpe)2 Cl shows very low activity, and un t i l our discovery of the activity of H R h ( d i o p ) 1 1 , we and probably other workers, because of the early report^, had not considered rhodium(I)-bis(diphosphine) chelate complexes as likely catalysts; a reason was considered to be the d i f f i c u l t y in providing a 49 vacant coordination site. Such complexes can clearly lead to quite high activity (Table IV.6), and in the case of HRh(diop)2, the vacant site is thought to arise by one of the diphosphine ligands becoming monodentate 1 1'(chapter III). I n i t i a l l y surprising i s that Rh(dpe)2+BF4 shows about ten times the activity of Rh(dpe)2+C& under corresponding conditions, and a similar difference in behaviour is noted for the dpb complexes. Pre-sumably chloride must coordinate and deactivate some intermediate (e.g. -83-Table IV.6. Catalytic hydrogenation of methylenesuccinic acid using the rhodium(I) complexes— Complex [Rh] x 10 3 M [Substrate] M Max. rate x 106,M s" 1 t l ^ x 10"3 s Rh(dpm)*BF~ I 4 4. .0 0. .2 4.3 27 Rh(dpe) 2Cl 4. .0 0. .2 v.slow 72^ Rh(dpe)+BF4 4, .0 0. ,2 9.6 10(6)-4. .0 0. ,2 v.slow— 72^ 4. ,0 0. .1 9.3 5 2. .0 0. .1 8.0 6 2. .0 0. .05 6.8 4 1. .0 0. .05 4.6 7 HRh(dpe) _ 4. ,0 0. ,1 7.<£ 7 Rh(dpp) 2BF 4 2. .0 0. ,05 31.6 2 1. .0 0. ,05 17.8 3 1. .0 0. ,1 25.4 4 HRh(dpp)2 4. ,0 0. 1 41^ 3 Rh(dpb) 2Cl 2. ,0 0. 1 18. 2s-'- 3 Rh(dpb)2BF~ 2. ,0 0. 1 130s-'- 0.4 2. ,0 0. ,1 7 Rh(diop)„Cl 1. ,0 0. 1 — Rh(diop) 2BF 4 1. ,0 0. 1 135 0 . 4 ^ HRh(diop)2 1. ,0 0. 1 150 0 . 4 ^ a_ Typical experiment involved 5 ml n-butanol, at 1 atm total pressure and 60°C. jr Approx. reaction time for 50% hydrogenation of the substrate, c^  30% hydrogenation. c[ In DMA. j2 20% hydrogenation. _f In n-butanol-toluene (2:1 V/V). £ At 30°C. h In ethanol. 71me,s Figure IV.11 Rate plot for the hydrogenation (100%) of methylenesuccinic acid using HRh(dpp)2 as catalyst ([Rh]=4xlO~3M, [S]=0.1M, 60°, n-butanol-toluene (2:1)=5 ml, P =1 atm). 900 2100 3300 4500 Timers Figure IV.12 Rate plots for the hydrogenation (50%) at 1 atmosphere pressure of methylenesuccinic acid using a. HRh(dpe)2 ([Rh]=4xlO~3M, [S]=0.1M, 60°, n-butanol-toluene(2:1)=5ml). b. Rh(dpe)2BF4 ([Rh]=4xlO~3M, [S]=0.1M, 60°, n-butanol=5ml). c. Rh(dpp)2BF~ ([Rh]=4xlO~3M, [S]=0.1M, 60°, n-butanol=5ml) d. Rh(dpb)2C£ ([Rh]=2xl0~3M, [S]=0.1M, 30°, 2-methoxyethanol=5ml). 2 5 200 400 T ime,s i 00 ON I 600 800 Figure IV.13 Rate plots for the hydrogenation (50%) at 1 atmosphere pressure of methylenesuccinic acid using a. HRh(diop), ([Rh]=1.5xl0 M,[S]=0.10M, 30°, n-butanol-toluene(2 :l)=5ml) b. Rh(diop)2BF4([Rh]=2xl0 3M,[S]=0.10M,15°, n-butanol-toluene(2:l)=5ml) c. RMdpb^BF" ([Rh]=2xlO_3M,[S]=0.10M,30°, 2-methoxyethanol=5ml). -87-an alkyl) in the catalytic cycle. The activity of the Rh(diop) 2 + cation, however, is essentially the same whether the associated anion be chloride or tetrafluoroborate. The cations generally are much less active in the polar and more strongly coordinating DMA. For example, under similar conditions, Rh(dpb) 2 +BF 4 hydrogenates the substrate ten times faster in ethanol (max. rate = 13x10 ~*Ms ^) than in DMA (max.rate = 13xl0~ 6Ms _ 1)(Table IV.6). The hydrides show activity comparable to that of the corresponding fluoroborate salts. The two species are interconvertible by the equi l i -bria outlined in equation [IV.3]^' 1 1, although these are unimportant for HRh(P°P) 2 + H + 5 j=i Rh(p"p) 2H 2 + H 2 + Rh(P^P) 2 + [IV.3] the diop complex in n-butanol-toluene with the methylenesuccinic acid substrate used. 1 1 Equilibrium [IV.3] has been studied in more detail for the P P = diop system and w i l l be discussed in chapter VI; the corresponding dpe and dpp systems have not been studied. Comparison of the data for the fluoroborate salts at substrate concentrations 5>0.1 M shows that catalytic activity increases with chain 12 length, n=4 (and diop) > n=3 > n=2 > n=l (Table IV.6). Kagan's group reported a similar trend for rhodium(I)-mono(diphosphine) systems formed in situ from [RhC£(C 2H^) 2] 2 for the hydrogenation of a-acetamidocinnamic acid in benzene-ethanol at 20°C, the n=l system being inactive; however, the n=6 system had activity comparable to that for n=3. Although these types of data are of use in designing better catalysts (including chiral ones), rationalizations to explain such trends, based solely on rate data -88-measured at a set of convenient conditions, should not be attempted. These systems typically show saturation behaviour in their kinetics; that i s , the rates level off at higher substrate and/or concen-trations"'"''", and the rate vs concentration profiles w i l l vary for different systems. The limited data in Table IV.6 show that the rates for the Rh(dpe)2+BF4 system are close to zero order in olefinic substrate and are fractional in catalyst, while for the Rh(dpp)2+BF4 system the rates are close to f i r s t order in both substrate and catalyst. Detailed kinetic studies are necessary to elucidate the mechanisms, and then comparable rate constants can possibly be extracted by suitable analysis. In this particular case, i t has been found that while Rh(dpe)2+ g does not form a dihydride at ambient conditions [as noted by others ], the dpp analogue readily gives an isolable cis-Rh^dpp^l^"*" complex (see chapter VII), and the mechanisms for catalytic hydrogenation may well di f f e r . A quite different reactivity pattern has also been found, for example, for the monodiphosphine systems referred to above, when used to 12 hydrogenate styrene in benzene [n=3 > n=l,5 > n=4 > diop > n=2]. The diop systems RMdiop^X (X=C£,BF4,H) a l l gave rise to similar optical enrichment in the 2-methylsuccinic acid product (chapter V). These diop complexes have also been used effectively for the asymmetric hydro-genation of other prochiral substrates (chapter V). -89-CHAPTER V ASYMMETRIC HOMOGENEOUS HYDROGENATION OF PROCHIRAL OLEFINIC SUBSTRATES  USING RHODIUM-DIOP COMPLEXES AS CATALYSTS V.1 Introduction Since the preparation of the "diop" ligand by Dang and Kagan1, well characterized as well as many in situ mono(diop)-rhodium(I) complexes have 2 3 been used for the hydrogenation of various substrates. ' In some cases, 4 for example enamides, optical yields up to 92% have been obtained , while 2 4-8 N-acylaminoacrylic acids have been hydrogenated ' in ^ 80% e.e. proving the versa t i l i t y of such catalysts. Diop systems have been studied extensively in terms of the effects on the optical purities of the hydrogenated product by varying, for example a) concentrations of catalyst and substrate 1, b) 8 1 anions associated with the catalyst , c) temperature , d) groups attached 1 4 5 7 9-13 to the substrate ' ' ' ' , e) substitution at the phenyl groups of the 6 4 5 diop ligand , f) solvent medium. ' On the other hand, bis(diop)-rhodium complexes which overall should be "more r i g i d " than corresponding mono(diop) 14 15 systems have not been studied, except for work from this laboratory. ' In this chapter, the potential of several bis(diop)-rhodium complexes, as well as a carbonyl mono(diop) complex, for catalytic asymmetric hydro-genation w i l l be described. Optical purities of the hydrogenated substrates are compared with those obtained using mono(diop)-rhodium(I) systems. V.2 Experimental V.2.1 Preparation of complexes - The preparations of HRh[ (-t-)-diop]^, Rh[ (+)-diop] , and Rh [ (-t-)-diop] 2 + B F 4 a r e already described in chapter IV. The synthesis of Rh[ (-f-)-diop] 2 H 2 + B F 4 ~ w i l 1 b e described in chapter VII. -90-16 [RhC£(CO) 2] 2 was prepared by the l i t e r a t u r e method. [RhC&(C0)(+)-diop] 2 To a pale yellow s o l u t i o n of [RhC£(C0) 2] 2 (0.194 g, 0.50 mmol) i n degassed benzene (5 ml) was added (+)-diop (0.50 g, 1.0 mmol) i n benzene (3 ml). The s o l u t i o n , which turned lemon yellow and evolved CO, was concentrated (ca. 5 ml), and cooled to 0°C a f t e r adding d i e t h y l ether (10 ml). The r e s u l t i n g lemon yellow powder was f i l t e r e d , washed with ether, and dried i n vacuo. Calculated for [RhC£ (CO) (H-)-diop] 2 :C=57 .80%; H=4.82%. Found: C=58.10%; H=5.09%. The in f r a r e d (Nujol) showed bands due to v(CO) and v(Rh-C&) at 1970 and 307 cm res p e c t i v e l y . The compound was reported 2'"'"^ and characterized as a dimer with bridging diop ligands"'"7 2 17 while our studies were i n progress; both syntheses reported ' involved carbonylation of Rh(I)/diop solutions. V.2.2 Determination of o p t i c a l p u r i t i e s - A l l experiments were performed on the gas-uptake apparatus described previously i n chapter II (Fig.II.1). In a t y p i c a l experiment, 0.2 g of the o l e f i n i c substrate was dissolved i n 10 ml of the solvent and the so l u t i o n was degassed before adding the ca t a l y s t . The reaction was monitored t i l l completion. The f i n a l s o l u t i o n was worked-up to i s o l a t e the hydrogenated product as described i n section II.9. The product (0.1 g) was dissolved i n the appropriate solvent and the o p t i c a l r o t a t i o n of the s o l u t i o n was measured at room temperature using a Perkin Elmer 141 polarimeter. Some experiments were repeated to check the accuracy of the r o t a t i o n values obtained (±2%). The enantiomeric excesses (e.e.) were calculated based on r o t a t i o n values for pure isomers as follows: R(+)- and S(-)- 2-methylsuccinic acid [a] 2 5=±17.09° (clO.5, C ^ O H ) ; 1 8 R(+)- and S ( - ) - N-acetylalanine [a] 2 5=±66.5< (c2, H 2 0 ) ; 1 9 R(-)- and S(+)- 2-phenylpropionic acid [a] 2 5=+76.1° (c8.06, -91-20 2S 8 CHCA ) ; N-acetyl-[(S) or (R) ]-phenylalanine [a] D =±46.5° ( c l , C^OH) . V.3 Results Hydrogenation of various prochiral a,3-unsaturated carboxylic acids was achieved at one atmosphere total pressure using bis(diop)-rhodium complexes (Tables V.1-V.4) and the [RhC£(CO)(+)-diop]2 complex (Table V.5). Itaconic acid was hydrogenated several hundred times faster than one of i t s isomers, citraconic acid (Tables V.1-V.5). a-Methylcinnamic acid was hydrogenated very slowly (Tables V.2 and V.3), while no hydro-genation of ethyl methyl ketone was observed (Tables V.l and V.2). The catalyst as isolated seems pure as either a powder or crystallized 25 form (the [a]^ of both forms of the catalyst were, identical[-3.4° in benzene]) and the optical yields remained unchanged: for R(+.)-2^methylsucc:inic acid product when using either sample of HRh[(+)-diop]2 (Table V . l ) . Data in Tables V.l and V.2 for the hydrogenation of itaconic acid show an increase in optical purity with increasing temperatures when either HRh[(+)-diop]2 or Rh[(+)-diop]^BF^ was used as catalyst. However, a decrease in optical purity with increasing temperature was observed for the hydrogenation of N-acetamidocinnamic. acid using Rh[(+)-diop]2C£ as catalyst (Table V.3). Solvent medium had l i t t l e effect on optical purities of products except in the case of citraconic acid which gave the R(+) and the S(-) isomers of 2-methylsuccinic acid product in n-butanol-toluene (2:1) and in DMA, respectively, using the Rh[(+)-diop] 2 + B F4~ catalyst (Table V.2). Strong solvent effects were noted for the hydrogenation of N-acetamidocin-namic acid which afforded S(+)- or R(-)-N-acetylphenylalanine in n-butanol-toluene (2:1) or DMA, respectively, irrespective of temperature using -92-^CH2COOH H 2C = C N COOH H.C H 3 \ / c = c x HOOC' COOH Methylenesuccinic acid or Itaconic acid C i t r a c o n i c acid 'COOH Atropic acid H2C - C NHCOCH / H C = C "^COOH N-or ce-Acetamidbacrylic acid Ph -NHC0CHo \ / 3 ,C = C / \ H COOH Ph CH„ x c = c H COOH (Z)-N-Acetamidocinnamic acid (E)-a-methylcinnamic acid H 3C H 5C 2 \ C = 0 Ethyl methyl ketone Some substrates used i n c a t a l y t i c asymmetric hydrogenation. Figure V . l Table V.l Asymmetric homogeneous hydrogenation of unsaturated substrates using the HRh[(+)-diop] catalyst.— 3 1 Product , Approx.total Substrate,S [Rh]xlO ,M [S]xlO ,M Temp.,°C Solvent .config. ,%e.e.- reaction time,t Itaconic acid 2.0 2.5 15 n-butanol-toluene (2:1) R(+),26 8h II II 3.0 27 R(+),37 2.5h II 2.0 30 " R(+),40 2h II II II 30 R(+) ,4(H(20) 2.5h II II 3.0 60 " R(+),48 30 min 11 1.0 11 80 " R(+),52 15 min II II 11 80 DMA R(+),56 30 min Citraconic acid 2.0 2.5 70 DMA S(-),6 4d N-Acetamidoacrylic acid II 4.0 30 DMA S(-),7(£ 5d II II 3.0 30 n-butanol-toluene (2:1) S(-),96l 1.5d i i i t 4.0 30 DMA S(-),71 4d (Z)-N-Acetamidocinnamic acid 5.0 • 2.5 80 DMA S(+),87 9d Atropic acid 2.0 4.0 50 n-butanol-toluene (2:1) R(-),37 2.5d II i t II 60 II R(-),33 e 1.5d Ethyl methyl ketone II II 30 II — a. At 760 mm total pressure using HRh[(+)-diop]^ crystallized from n-hexane. b_ % e.e. corresponds to uncrystallized product. £ Catalyst used without recrystallization (the value in brackets taken from Ref. 15 is in error). d_ After the product was twice recrystallized. JS No hydrogenation. Table V.2 Optical purities of products from hydrogenation of olefinic substrates using the Rh[(+)-diop]^ BF complex.iL Substrate,S [Rh]xl03,M [SjxlO1,!-! Temp.°C Solvent Product config.,%e.e. Approx.to reaction t ime,t Itaconic acid 1.0 2.5 15 n-butanol-toluene^ : 1) R(+),19 8h I I I I 3.0 27 I I R(+),36 2h I I I I I I 30 I I R(+),37 2h I I I I I I 60 I I R(+),50 20 min I I i i I I 80 I I R(+),52 15 min I I I I I I 80 DMA R(+),56 45 min Citraconic acid 2.0 2.5 70 DMA S(-),6 3d I I tt It 70 n-butanol-toluene^ :1) R(+),6 1.5d N-Acetamidoacrylic acid tt 1.5 30 i t S(-),70 16h I I 1.5 2.5 60 I I S(-),73 2h (Z)-N-Acetamidocinnamic acid 2.0 1.5 80 I I S(+),94 7d I I 6.0 3.0 80 DMA S(+),86 6d Atropic acid 2.0 2.5 50 n-butanol-toluene^ :1) R(-),41 2d I I 1.0 tt 60 I I R ( - ) , 3 g 2.5d (E)-a-Methylcinnamic acid 6.0 3.0 70 n-butanol toluene(2:1) Ethyl methyl ketone 2.0 4.0 30 i i c a_ At 760 mm total pressure. _b_ ^10% reaction complete in 5 days. c^  No hydrogenation. Table V.3 Asymmetric homogeneous hydrogenation of prochiral olefins using the Rh[(+)-diop]^Ci catalyst 3-~ , Product Approx.total Substrate,S [Rh]xlO ,M [S]xlO ,M Temp.°C Solvent config.,%e.e. reaction t ime,t Itaconic acid 2.0 2.5 15 n-butanol-toluene (2:1) R(+),45 8h I I ii I I 30 I I R(+),49 2h I I ii TI 40 I I R(+),48 lh Citraconic acid ii I I 70 DMA S(-),3 3d N-Acetamidoacrylic acid I I I I 30 n-butanol-toluene (2 :1) S(-),81 8h (Z)-N-Acetamidocinnamic acid it 1.2 20 it S(+),92 20h tt I I I I . 40 I I S(+),68 6h it I I it 40 DMA R(-),13 2d ii it I I 70 n-butanol-toluene (2 :1) S(+),14 4h ii it II 70 DMA R(-),5 6h it I I 2.0 80 DMA R(-),4 4h Atropic acid ti 2.5 50 n-butanol-toluene (2:1) R(-),23 Id (E)-a-Methylcinnamic acid it 1.5 80 I I D a. At 760 mm total pressure, b Reaction too slow to measure Table V.4 The Rh[(+)-diop]„H9 BF. -catalyzed hydrogenation of some prochiral substrates."" i Product Approx. ti Substrate,S [Rh]xlO ,M [S]xl0 ,M Temp.°C Solvent config.,%e.e. reaction time, t Itaconic acid 1.0 3.0 30 n-butanol-toluene(2:l) R(+),28 2h I I II i i 60 I I R(+),42 20 min Citraconic acid 3.0 II 70 DMA S(-),5 2.5d N-Acetamidoacrylic acid 2.0 II 30 n-butanol- S(-),83 13h toluene^ : 1) (Z)-N-Acetamidocinnamic acid 5.0 i t 80 DMA S(+),88 6.5d Atropic acid 2.0 II 60 n-butanol-toluene^: 1) R(-),33 26h ON I ja At 760 mm total pressure. Table V.5 Asymmetric hydrogenation of prochiral unsaturated substrates using the [RhC£(CO)(+)-diop]2 complex.-o -i Product Approx. total Substrate,S [Rh]xlO ,M [S]xlO ,M Temp.°C Solvent config.,%e.e. reaction tlme,t Itaconic acid 2.0 3.0 40 DMA R(+),52 8d Citraconic acid 4.0 2.5 80 " S(-),22 lOd N-Acetamidoacrylic acid 4.0 " 50 " S(-),73 2.5d (Z)-N-Acetamidocinnamic acid 6.0 2.0 80 " S(+),52 8d Atropic acid 4.0 2.5 80 " R(-),10 4d ji At 760 mm total pressure. -98-R h [ ( + ) - d i o p ] a s catalyst (Table V.3). The data for some substrates suggest that hydrogenation is faster in n-butanol-toluene (2:1) than in DMA (Tables V.1-V.3). Table V.6 l i s t s selected data from the literature concerning hydrogenation of prochiral substrates using mono(diop)-rhodium(I) complexes. V.4 Discussion HRh[ (-f-)-diop] 2 effects the catalytic hydrogenation of unsaturated carboxylic acids under mild conditions (Table V.l ) . Itaconic and N-acetamidoacrylic acids, substrates containing a terminal = CR^  group, are most readily hydrogenated using HRh[(+)-diop]^• Substitution at the a-carbon also affects the hydrogenation rates as seen from the data for atropic acid which requires much longer reaction times. Hydrogenation of N-acetamidocinnamic acid, containing a trisubstituted olefinic group = CHCgH^, is very much slower again (Table V.l ) . Similar rate trends are noted for the Rh[(+)-diop] 2 + complex (Table V.2), the Rh[(+)-diop] 2H 2 + complex (Table V.4), and the Rh[(+)-diop]2C£ complex (Table V.3), although the last mentioned catalyst requires much milder conditions to effect hydrogenation. For example, N-acetamidocinnamic acid can be hydrogenated to products with 92% optical yield at 20°C using Rh[(-f-)-diop] 2C£, while the hydrido and cationic systems hydrogenate the same substrate very slowly indeed even at 80°C. The results indicate that steric factors at the olefinic group are certainly important in terms of the rates of these hydro-genation reactions. The participation of the acid function of substrates does not seem important, at least in terms of hydrogenation rates, in the HRh [ (-t-)-diop] 2 -catalyzed systems since under similar conditions styrene was —6 —1 (max. rate ^14x10 Ms at 50°C) hydrogenated somewhat faster than atropic Table V.6 Asymmetric hydrogenation of unsaturated substrates using various rhodium(I)-mono(diop) complexes at 25°C and 760 mm total pressure. Complex Substrate,S Solvent [Rh]xl03,M [S],M /o 6 • 6 • (config.) t l / 2 ' m i n Ref ([(C 2H 4) 2RhC£] 2+2(+)-diop)- H2° C^NHC0CH3 ethanol \JQ benzene 4.0 0.2 42.5(R) 5 5 11 = 1/1 I I n 30(R) 20 I I it = 1/9 tt I I 39(S) 66 it I I = 0/1 it I I 44(S) 76 I I [(C0D)Rh(+)-diop]+C£04~ ethanol 3.0 0.3 38. 5 (R) 17 4 ii benzene I I M 68 (R) 5.5 ti ([ (CgH14) 2RhCil ] 2+2 (- ) -diop)-0 NHC0CHo \=C< 3 „• ^COOH rl ethanol _ 0 ^ benzene 3.0 0.3 70(R)- 1.5 1 it it I I 1.0 0.54 72(R) c ti " +(Et3N/Rh=3/l) II it 2.0 0.06 70(R) c ti [(COD)Rh(-)-diop]+BF4" - it ethanol 3.0 0.3 81.3(R) 10.5 8 [(COD)Rh(-)-diop]+BF4" - ti it I I It 82.1(R) 10.5 I I I VO VO I a_ In situ b At 50°C. c^  Not reported. d^  Freshly prepared. j; Used after 48h under argon Table V.6 contd. Complex Substrate,S Solvent [Rh]xlO ,M [S],M %e.e. . (config.) t^ 2,min Ref. ([(COD)RhC£]2+2(-)-diop)-0 NHCOCH„ c=c' „/ vCOOH ri ethanol 3.0 0.3 81.9(R) 12 ([(C2H4)2RhC£]2+2(+)-diop)-0 NHCOCH. N c = c r 3 *C00CH„ COOH \ > c ' • N^HCO0 ethanol benzene = 3/1 81(S) 55(S) 70(S) 6.5 1.5 0 H o c = C v M U ^ n P U ethanol = 2 / ± k Q 2 N^HCOCH^  benzene v ' \ _ / 0 R c/ C\NHC0CH3 " " " 83 (R) s C - c ' H 3C \NHC0C,Hc 6 5 0.1 73 (R) Table 6 contd. Complex Substrate,S Solvent [Rh]xlO ,M [S],M %e.e. (config.) tjy 2,min Ref, [(COD)Rh(+)-diop] C£0 H /0 H> C-NHCOCH 3 benzene 3.0 0.3 92(R) 11 H0C=C 2 \ N^HCOCH^  COOH ethanol 0.15 42(S) 42 ^NHCOC^ H2 C = C-COOCH 3 benzene 60(S) ([(C 2H 4) 2RhC£] 2+2(-)-diop)- ,C=C, 0 .NHC0CHo \ / 3 •COOH ethanol benzene 2/1 3.0 0.3 81(R) o h- 1 I " + -P(2-MeC6H4)2- 27 (R) 200 " + -P(3-MeC6H4)2- 87.5(R) J [ Both 0 groups attached to -P09 moieties of the diop ligand are substituted. -102-acid (max rate ^8x10 ^ Ms 1 at 50°C). The Monsanto group 2 1 had i n i t i a l l y considered H-bonding (between the carboxylate group of unsaturated acidic substrate and 0 and H atoms of the catalyst ligand) important in their very effective systems using o-anisolephosphine derivatives, but more recent 22 studies tend to deemphasize such interactions. Coordination of the -NHCOR group ( i f present) at rhodium cannot be ruled out, however, because a vari -31 23 able temperature P nmr study o'f the hydrogenation of (Z)-N-acetamido-cinnamic acids and their methyl esters by [Rh(NBD)(+)-diop]+A~ (A=BF^,Cl) complexes has revealed such coordination (Figure V.2(A)). A similar intermediate (Figure V.2(B)) was proposed earlier by Kagan et.al? to [<+)-dioP]-Rh<r / n h H H ! ,.H (diop) ; R h : - -R.j=H, R2=Ph; Rx=Me, R2=Ph; R^H, R2=Me (A) ' NH (B) Proposed intermediates for hydrogenations catalyzed by rhodium-mono(diop) complexes. Figure V.2 24 explain good optical yields for enamide substrates. Quite recently , such an interaction has been further substantiated by the crystal structure of a related cationic complex [Rh(dpe)(PhC(H)=C(C00CH„)(NHC0CH„))] (Figure V.3 ). Crystal structure of the [Rh(dpe) (PhC(H)=C(C00CH,) (NHCOCH ))] + cation. Figure V.3 -103-In our bis(diop)-rhodium systems, such an interaction could at least explain why N-acetamidocinnamic acid hydrogenates faster than a-methyl-cinnamic acid. The data in Tables V.l and V.2 suggest that the Rh[(+)-diop] * system hydrogenates substrates slightly faster than the HRh[(+)-diop]2 system although the %e.e. of hydrogenated products are almost the same in most cases. The optical purities did not vary appreciably when pre-formed Rh[ (+)-diop] 2H-2+BF4 rather than Rh[ (+)-diop] ^  w a s used as catalyst (Table V.4). These results suggest that the geometries around the hydride-metal-substrate part of the intermediates are similar in a l l three systems (e.g. HRh(diop)(diop*)(olefin) for the HRh[(+)-diop]2~catalyzed hydro-genation (chapter III) and Rh(diop) (diop*) (Il) 2(olefin) for the Rh[(+)-dipp] 2 +-catalyzed hydrogenation (chapter VI)). 9 13 25 26 Glaser's group ' ' has presented a stereochemical model (Fig.I.6) for [RhC&(diop)S]-catalyzed hydrogenation of various substrates based on space-filling CPK-type molecular models to explain the effect of olefin substitution on optical purities; the rationalization considered only steric effects and ignored the cis-trans geometry of the olefins. Major products in terms of R and S configuration were correctly predicted except 9 in a few cases. In the bis(diop)-rhodium systems, any model w i l l necessitate incorporation of two diop ligands, one of which is 'monodentate1. Since the stereochemistry of this intermediate species (e.g. HRh(diop)(diop*)(olefin) for the HRh[(+)-diop]2~ catalyzed reactions, (chapter III)) is yet to be established, the effect of substituents of olefinic substrates on the configuration of the hydrogenated product could not be predicted. The data in Tables V.1-V.3 show that hydrogenation rates are affected by the nature of the solvent. In general, hydrogenation proceeds more slowly -104-in DMA (a basic and coordinating solvent) than in n-butanol-toluene mixture. With four and five coordinate rhodium(I) complexes, Rh[(+)-diop] 2 + and HRh[(+)-diop]2> which do not completely dissociate a diop ligand in solution (see chapters III and VI), solvent effects appear to be unimportant generally (Tables V.l and V.2), although there is a change in configuration with change in solvent in the case of citraconic acid with the Rh[(+)-diop] 2 catalyst (Table V.2). The reason for this is not clear but different mechanisms are clearly operating in the two solvents at the higher temperatures used for this substrate. The more marked solvent effects observed using Rh [(+)-diop]2C£ as catalyst (Table V.3) are in contrast to the hydride and cationic analogs. Although the hydrogenation mechanism using the chloride has not been elucidated, visible spectral studies have shown that this complex does not obey Beer's law and is in equilibrium with some other species (Table IV.2, chapter IV). Interestingly, these solvent effects are comparable to those obtained using mono(diop)-rhodium(I) species where a change of solvent medium has a dramatic effect on the configuration of the hydrogenated product."* For example, using an in situ [RhC£( (+)-diop)S] (S=solvent) catalyst, the substrate (H2C=C(0)NHCOCH3) was hydrogenated to product which was 42.5% e.e., (R), in ethanol and 44% e.e., (S), in benzene (Table V.6). These solvent effects can be rationalized qualitatively, since in the mono(diop) system solvent occupies coordination site(s) on the rhodium and thus can probably affect the configuration of the hydro-genated product. It is possible that the mechanism for the chloride system involves loss of one diop ligand but this has not been substantiated. In the hydrogenation of itaconic acid using HRh[(+)-diop]2 or Rh[(+)-diop]„+ catalysts (Tables V.l and V.2), optical yields of the product -105-at different temperatures show? an unusual trend (Figure V.4): optical purity increases generally from 20^50% with increasing temperature but levels off at about 80°C. A decrease in optical purity with increasing 21 temperature has usually been observed for hydrogenation and other 27 27 asymmetric syntheses. An example of the latter is found in the codimerization of norbornene(I) with ethylene in chlorobenzene at +10°C to -97°C to give optically active exo-(+)-2-vinylnorbornane(II), using a catalyst prepared from ir-allylnickel chloride, triethyldialuminum t r i -chloride, and optically active (-)-isopropyldimethylphosphane. The variation of optical yield with temperature (Figure V.5) shows a linear increase in optical purity with decreasing temperature. The Rh[(+)-diop]^ l system behaves differently and shows roughly a linear decrease in optical purity with increasing temperature for the hydrogenation of N-acetamido-cinnamic acid (Figure V.4), behaviour similar to that observed for reaction [V.l], but in contrast to a previous temperature study"*" on an in situ [RhC£(diop)S] system for hydrogenation of the same substrate that showed an independence of optical purity with temperature (70% e.e. at 50°C, and 72% e.e. at 30°C). At present, there are no satisfactory rationalizations for these variable temperature behaviours. More ri g i d structures at lower temperatures would be expected to lead to high optical enrichment, but Figure V.4 Plot of variation of optical yield with temperature of: 1. (+)-2-methylsuccinic acid using a. HRh[(+)-diop]2; b. Rh[(+)-diop] 2BF 4 as catalysts in n-butanol-toluene(2:1). 2. N-acetyl-S(+)-phenylalanine using the Rh[(+)-diop]2C& complex in n-butanol-toluene (2:1). 0 -25 - 50 -75 -100 T.t — Figure V.5 Variation of optical yield of (+)-2-vinylnorbornane with temperature using a nickel catalyst in chlorobenzene (data taken from ref.27). -107-different mechanisms could be operative at higher temperatures. The hydrogenation of N-acetamidocinnamic acid at 20°C gives 92% e.e. S(+)-N-acetylphenylalanine using the Rh[(+)-diop]^Cl catalyst is noteworthy in that the asymmetric induction is among the highest yet recorded for this substrate. Asymmetric hydrogenation of substrates is also effected using the [RhC£(C0)(+)-diop]2 dimer (Table V.5). The hydrogenation of itaconic acid at 40°C and citraconic acid at 80°C afforded 52% e.e. of R(+) and 22% e.e. of S(-) isomers of 2-methylsuccinic acid, respectively. These 3 values are among the highest reported so far for these substrates. A drawback of this catalyst is that i t is far less active than the in situ [RhC£(diop)S] or the bis(diop) complexes. The introduction of a ir-acid carbonyl ligand into a complex generally reduces activity for catalytic hydrogenation, and this is usually traced to a less favoured oxidative addition step somewhere in the catalytic cycle, due to reduced electron density at the metal center. Comparison of data in Tables V.1-V.6 shows that mono(diop)-rhodium complexes hydrogenate several times faster than bis(diop)-rhodium species, but the latter can give better optical yields in some cases. Optical yields as high as 96% are observed with N-acetamidoacrylic acid (Table V.l). There are only a few other catalysts reported to be equally 22,26,29,30 effective. The results obtained in the present studies (Tables V.1-V.5) together with representative literature data (Table V.6) show the effect of a number of variables on the induced asymmetry in the hydrogenated 121 8 product. These include: temperature ' , the age of the catalyst , the 1 8 31 concentrations of catalyst and olefin , the nature of the anion ' , added -108-bases , and systematic ligand effects (e.g. varying substituents on the phenyl groups of diphenylphosphino moieties^). However, i t remains impossible to match a p r i o r i substrates and catalyst system to give optimum o p t i c a l y i e l d s ; this i s s t i l l very much an empirical a r t . -109-CHAPTER VI ASYMMETRIC HOMOGENEOUS HYDROGENATION OF ITACONIC ACID USING BIS(DIOP)- RHODIUM(I)TETRAFLUOROBORATE AS CATALYST VI.1 Introduction During an investigation of itaconic acid hydrogenation using the hydridobis(diop)rhodium(I) complex, spectroscopic studies indicated that the i n i t i a l hydride complex was slowly decomposed by protons from the substrate to a corresponding cationic species and this was also thought to be an important active species during hydrogenation of other acidic substrates^ (chapter III): HRh [(+)-diop ] 2 + H + >• Rh[(+)-diop] 2 + H 2 [VI.1] The cationic bis(diop)rhodium(I) complex was thus isolated via a more convenient route starting from the cyclooctene dimer [RhC&(C0H.. ,) „] „ o 14 2 2 (chapter IV): [RhCUCgH 1 4) 2] 2 + 4(+)-diop > Rh[ (+)-diop] f l [VI.2] Rh[(+)-diop]2Cil + AgA > Rh[(+)-diop]2A" [VI.3] (A=BF4,PF6,SbF6) This cation was then found to be a very effective asymmetric hydro-genation catalyst for certain substrates, with optical yields of >90% being obtained in some cases (see chapter V). A kinetic study was therefore carried out on the Rh[(+)-diop] 2BF 4~IA system in an attempt to elucidate the hydrogenation mechanism involved. -110-VI.2 Experimental + - + -The Rh [ (+)-diop] and Rh[ ^ - d i o p ^ R ^ A complexes were prepared by procedures given in chapters IV and VII, respectively. The hydrogen solubility data are summarized in Figure VI.1. VI.3 Results VI. 3.1 Reaction of hydrogen with RhCdiop^BF^ VI.3.1.1 Gas-uptake studies - The uptake of by RhCdiop^"1" was measured on the gas-uptake apparatus using the ampoule technique to establish the stoichiometry of the reaction. A rapid uptake was observed which slowed down after ^1 min and the f i n a l stoichiometry was obtained after ^%h; the reaction being accompanied by a colour change from red to light yellow. Using this technique the complex dissolves relatively slowly, and i t is impossible to use the uptake curves for quantitative kinetics. The H^Rh ratio was found to be essentially independent of the associated anion, the solvent medium, hydrogen pressure, temperature, added Et^N, added proton sponge, and the rhodium concentration (Table VI.1). VI.3.1.2 Spectrophotometric studies - n-Butanol-toluene (2:1) or DMA I —3 l\ solutions of Rt^diop^BF^ under argon obeyed Beer's Law between ^ 10 -10 M concentration range and showed an absorption maximum at 442 nm (e=3550±100 M 1 cm "*") that was independent of added diop (Figure VI. 2 (a)). Some changes were observed on exposing the solution to air over long periods of time (Figure VI.2(b)). Addition of to a solution of the cation resulted in a rapid disappearance of the 442 nm band (orange ->- yellow) , followed by relatively slow and small spectral changes to give a f i n a l steady spectrum after ^ %h (Figure VI.3(a)); these changes were independent of added diop. On pumping - I l l -Figure VI. 1 Solubility of hydro gen in n—butanol—toluene (2:1) mixtures at various pressures at 15°C. Inset gives data at different temperatures. -112-+_ Table VI. 1 Hydrogen-uptake data for RhCdiop^BF^ using the ampoule technique . [Rh]xl03,M T,°C a Pressure,atm Solvent b H2/Rh 10 15 1.00 n-butanol-toluene 0.86 (2:1) 10 15 0.50 f l 0.80 10 15 0.25 If 0.83 10 25 1.00 n-BuOH 0.84-5 25 1.00 1! 0.70 2.5 25 1.00 I I 0.75 5+(+)-diop 25 1.00 I I 0.70 (5xl0~3M) 6 12 1.00 DMA 0.81-10 12 1.00 I I 0.82-5* 25 1.00 I I 0.76 5+Et3N(0.1M) 25 1.00 n-BuOH 0.73 5+P.S. (5xl0~3M) 25 1.00 I I 0.75 a Total pressure . b Ratio corresponds use of the ampoule to uptake after ^ h ; technique. values (±0.1) in error due to c^  Based on an average of two experiments cl Using Rh(diop)2SbF~ • -113-0.0 0.5 h-L U o < CD or o LO CD < 1.0 / / / 1 / A b/ / / \ / \ / \ / v. 'a 1.51 I I I 350 450 550 WAVELENGTH, nm Figure VI. 2 Visible spectrum of RhCdiop^BF, in n-butanol-toluene (2:1) at 25°C. a. Under argon. b. 20h after exposing solution (a) to a i r . -114-0.0 1.5 350 Figure VI.3 450 WAVELENGTH, nm 550 Visible spectrum of RtKdiop^BF^ at 25°C in n-butanol-toluene (2:1) (in absence, or presence of diop (diop:Rh=3:1)). (a) Under H„ (b) Under Ar -115-hydrogen off from these solutions, a band at 442 nm was regenerated but the optical density in the 350-400 nm region increased and was different from that of the i n i t i a l spectrum of the Rh(diop) 2 cation (Figure VI.3(b)). + -4 The hydrogen-reaction with Rh(diop) 2 (^1x10 M) in DMA at 30°C was studied using the stopped-flow spectrometer. The data obtained do not analyze for a simple first-order reaction (Fig. VI.4), even though the concentration is in about a 10:1 excess (the solution saturated with at one atmosphere total pressure becomes diluted by a factor of two in the mixing chamber of the stopped-flow apparatus). Similar data were obtained in CH 2C£ 2, DMA, and MeOH. The loss of H 2 from solutions of the synthesized RhCdiop^H^BF^ complex was measured spectrophotometrically under argon in methanol at 30°C (Figure VI.5). The spectral changes analyzed for a first-order -4 -1 reaction with slope -2.1x10 s (Figure VI.6). In CH 2C£ 2 and DMA very -4 -1 -4 -1 similar changes analyzed to give slopes of =2.1x10 s and 3.4x10 s , respectively (Table VI.2). VI.3.1.3 3 1 P nmr data 31 + -The P nmr spectrum of Rh(diop) 2BF 4 in CH2C£2-acet.one-d6 at 25°C under argon showed a doublet at -9.3 ppm, J T>=140 H Z , (Figure VI.7(a.l)), Rh—r but a more complex pattern was observed at -50°C (Figure VI.7(a.2)). This spectrum remained invariant on further cooling to -85°C and essentially consisted of three main doublets at -10.6 ppm (J„u -0=140 Hz), -15.5 ppm Rn—r (J , T>=134 Hz), and -16.5 ppm (J =134 Hz). When the solution was put Kn—r Rn—P under H 2 at 25°C, a doublet, of doublet of triplets was obtained (Figure VI. 7(b)). The "'"H nmr data are described in section VI.3.2. 2. + 0.7 + 0.3 o -0.1 - 0 . 5 0 50 Figure VI.4 o 100 Time,s 150 200 250 Plot of log (A -A- ) vs time of the stopped-flow data for H„-uptake by RhCdiop^BFj (l,lxlO _ 4M) in DMA at 30°C. -117-Figure VI.5 Spectral changes obtained for H„ loss from Rh(diop) 2H 2+BF (4.2 x 10 _ 4M) i n methanol (under argon) at 30°C. Figure VI.6 Plot of log^-Ao) vs time for s p e c t r a l changes i n F i g . VI.5. -119-Table VI.2 Spectrophotometric determination of first-order rate constants in different solvents at 30°C for the reaction Rh(diop) 2H 2 + ' A ~ 1 > Rh(diop) 2 + H 2 4 — — 4 - 1 [Rh]xlO ,M Solvent k^xlO ,s 4.21 MeOH 4.85 4.21 CH 2C£ 2 4.85 4.12 DMA 7.74 a 10 ml. b_ Determined in a 1cm' c e l l shown in Figure II.3 under one atm of argon. 140* a.1 .9.67 i 103* -170 -10.0 I l -19.9 I PPM PPM 31 1 + -Figure VI.7 P{ H} nmr spectra of Rh(diop)2BF, in CH-CJc. -acetone-d (Similar spectra were obtained xn MeOH-d^f. a. Under argon: 1. at 25°C. 2. at -50°C b. Under hydrogen at 25°C. -121-VI.3.2 Catalytic hydrogenation studies VI.3.2.1 Kinetic data - The hydrogenation of itaconic acid was achieved at 15°C in n-butanol-toluene (2:1) using the RMdiop^BF^ complex. Figure VI.8 shows a typical H^-uptake plot. The i n i t i a l faster uptake slowed down somewhat, and the hydrogenation rates were measured in the subsequent linear region (Table VI.3). The rates varied linearly with rhodium in the concentration _3 range (0.5-2.5)xl0 M (Figure VI.9). The dependence on itaconic acid varied from first-order at lower concentrations to considerably less than first-order at ^0.2M (Figure VI.10). A zero-order dependence on hydrogen pressure was observed in the 144-752 mm pressure range studied (Figure VI.11). Addition of even small amounts of added diop completely inhibited the hydrogenations (Table VI.3, footnote c). The rates increased with temperature (Table VI.3). VI.3.2.2 Spectroscopic data - Very slow spectral changes were observed on adding IA to solutions of RhCdiop)^ under argon; for the same experi-ment in the presence of added diop, no spectral changes were observed (Figure VI.12). Use of succinic acid instead of itaconic acid gave very similar spectral changes. The high-field ~*~H nmr spectrum of a light yellow solution of Rh(diop)^BF4 in methanol-benzene-dg, under IL, and at 25°C, was a pair of multiplets centered around 20x (Fig. 111.6(b)). This spectrum and the colour, immediately (5 min) after adding a saturated methanol solution O^ IM) of itaconic acid, were unchanged at 5°C, a lower temperature being used to slow down the hydrogenation reaction. After ^15 minutes the solution turned red and the hydride signal disappeared; the vi s i b l e spectrum of the red solution corresponded to that of Rh(diop ) l . -122-4 0 Time.s Figure VI.8 R^-uptake plot for the Rh(diop)2 BF^-catalyzed hydrogenation of itaconic acid (15°C,[Rh]=1.0xl0-3M, [IA]=0.2M, PR2=752 mm, n-butanol-toluene(2:1)=5 ml; uptake shown corresponds to 35% hydrogenation). -123-Table VI.3 Linear rate data for the hydrogenation of itaconic acid in n-butanol-toluene (2:1) catalyzed by Rh(diop) 2 +BF~ at 15°C, unless stated otherwise. [Rh]xl0 3, [IAjxlO 1, P [H 2]xl0 3, Linear rate x 10 5 k, -M M 2 ' ^ Ms"1 -1 -1 mm M— M s 0.5 1.0 752 2.0 1.41 0.28 l.<£ I I I I I I 3.12 0.31 1.5 I I I I I I 5.10 0.34 2.0 I I I I I I 7.24 0.36 2.5 I I I I I I 8.70 0.35 1.0 0.4 I I I I 1.57 0.39 I I 0.7 I I I I 2.42 0.35 I I 1.4 I I I I 3.64 -I I 1.8 I I tt 4.30 -M £ 2.0 I I I I 4.16 -1.0 1.0 144 0.36 3.00 0.30 I I I I 296 0.77 2.92 0.29 I I I I 448 1.18 3.22 0.32 I I I I 600 1.58 3.30 0.33 "d I I 752 2.0 5.52 0.552 ne I I I I I I 7.54 0.754 "I I I I I I I 13.56 1.356 a. Calculated from Figure VI. 1. b_ See text _^ c_ Addition of >2xl0 M diop completely inhibited the hydrogenations d At 20°C e At 25°C f At 30°C -124-10 Figure VI.9 Plot of linear rate against Rh(diop) BF concentration (15°C, 0.1M IA, 752 mm P , n-butanol-toluene(2:1)). t i 2 -125-Figure VI.10 Dependence of linear rate on IA concentration (15°C, [Rh]=1.0xl0"3M, 752 mm P , n-butanol-toluene (2:1)). H2 -126-4 i n . cu -k-> a c_ t_ CJ CD C 0 / r 300 1 600 P. , mm of Hg " 2 900 Figure VI.11 Plot of linear rate against hydrogen pressure (15°C, 0.1M IA, [Rh]=1.0xlO_3M, n-butanol-toluene (2:1)). -127-Figure VI.12 Spectral changes observed on adding itaconic acid to solutions of Rh(diop) 2 +BF 4~(3.4xlO _ 4M) in n-butanol-toluene (2:1)(under argon) at 30°C. a. No added diop _3 b. In the presence of diop (1x10 M). -128-On adding IA (0.02 or 0.2M) to solutions of Rh(diop) 2' r (^5x10 4M) at 25°C under 1 atm very small spectral changes were observed in the 350-400 nm region; no peak due to Rh(diop) 2 + was detected. The resulting f i n a l spectrum corresponds to the fi n a l one of Fig. VI.3(a).~ VI.4 Discussion VI.4.1 The hydrogen reaction Treatment of solutions of Rh(diop) 2 + with molecular hydrogen readily leads to the formation of dihydride species as shown synthetically (chapter VII and ref. 2) and spectroscopically (section VI.3.1)(equation [VI.4]): Rh(diop) 2 + + H 2 u K Rh(diop) 2H 2 + [VI.4] Such reactions are well known; several rhodium(I)-phosphine (or arsine) complexes oxidatively add molecular hydrogen to give the corres-3 4 ponding rhodium(III) dihydrides. ' The H2:Rh stoichiometry (Table VI.1) was always somewhat less than the expected value of unity, and was i n -dependent of hydrogen pressure and so the stoichiometry problem is not simply due to establishment of the equilibrium shown in equation [VI.4]. A further complicating equilibrium [VI.5], that has been established in some rhodium-phosphine complexes^, was ruled out since addition of P.S. or Et^N again did not affect the H2:Rh ratio. Rh(diop) 2H 2 + •» H + + HRh(diop)2 [VI. 5] The spectrophotometric and gas-uptake studies indicate that the hydrogen reaction with Rh(diop) ? + is not a simple pseudo first-order -129-process. Visible and stopped-flow studies show a rapid uptake followed by a slow uptake. Similar visible spectral changes obtained in the presence of excess diop indicate that the complex does not dissociate a diop ligand in solution during the hydrogen-reaction and this is con-sistent with the absence of any free diop or monodentate diop signal in h , 31 the P nmr spectrum of the complex under argon or under H„ (Figure VI.7; also see below). 31 + The P nmr spectrum of Rh(diop) 2 showed a doublet at 25°C charac-t e r i s t i c of four equivalent phosphorus atoms around rhodium (Figure VI.7 (a.l ) ) . Surprisingly, this doublet changed to a complicated pattern that remained unchanged from -50°C to -85°C (Figure VI.7(a.2)). The doublet at -10.6 ppm (J =140 Hz) is presumably due to the Rh(diop)„"^ species; Rh—P Z. since there is no signal due to dissociated diop., the presence of mono(diop)-4 rhodium species can be ruled out. Moreover, recent studies have shown that mono(diop)-rhodium(I) complexes do not form detectable amounts of dihydrides 31 + under H^, and in our system, the P nmr spectrum of Rh(diop) 2 under H 2 gave, a doublet of doublet of triplets at 25°C (Fig.VI.7(b)) characteristic 3 of formation of a pure cis-dihydride complex. Thus the other two multiplets at -15.5 ppm and -16.5 ppm observed in Fig. VI.7(a.2) are possibly associated + 31 with a solvated Rh(diop) 2 species although the P nmr peaks have not been assigned. A distorted trigonal bipyramid structure, comparable to that of HRh(diop)2 (section IV.3) seems plausible, but associated species with bridging diop ligands , or in which one rhodium is bonded to a phenyl ring of a diop ligand of \. ; another rhodium^, cannot be ruled out. Quite recently, evidence for solvated (MeOH) species has been obtained 8 31 by Brown et a l . in a closely related P nmr study done on various bicyclo-[2. 2.l]heptadienebis(phosphine)rhodium(I) complexes. Thus, for example, -130-hydrogenation of complex 1(a) in methanol produced exclusively species 11(a). H H \ I / ( M e H + + H L ^ OMe Rh X " Rh I 0 M e / OMe L H (III) H (a) L = PPh3; X=BF4 (b) L = 0-MeC6H4PPhMe; X=BF4 On removing from solutions of complex 11(a), very small amounts of species III(a) was observed. On the other hand7when was added to complex 1(b), the i n i t i a l product under R.^ w a s exclusively 111(b) which on further hydrogenation was slowly converted to complex 11(b). Thus attainment of equilibrium between species II and III was extremely sensitive to the nature of the phosphine ligand attached to rhodium. The kinetic scheme shown in equation [VI.6] accounts reasonably well for the Rh (diop) 2~|/H2 reaction, and is compatible with the findings of Brown et a l . The stopped-flow data for the DMA system (Figure VI.4) can be analyzed in terms of two consecutive pseudo first-order reactions -2 with rate constants k 2 and k^ (Figure VI.13). The values are 10.2 x 10 -3 -1 and 7.4 x 10 s , respectively, and data in other solvents were obtained similarly (Table VI.4). -131-+ 3 + Rh(diop) +S v , •* Rh (diop)(diop*)S 1 -3 k3 + - g i * Rh (diop) (diop*) ( H ) ^ Rh(diop) 2H 2 + (Reaction B) The faster k 2 step is identified with Reaction A, formation of Rh(diop)(diop*)(H) 2S + via Rh(diop)(diop*)S +, while the slower k 2 step is identified with loss of solvent to give the six-coordinate Rh(diop)^2 ' For Reaction A, assuming a steady-state treatment for the intermediate species, the rate law i s , k k'[Rh(diop) + ] [ H ] tote = k_3 + k-[H2] [ V I - 7 ] and the measured k 2 equals k^k^H^y^k^+k^tH,,]). The k 2 is essentially independent of [H2] from 130-320 mm suggesting that k ^ H ^ ^ k ^ and hence k^k^, referring to dissociation of one P atom of the chelating diop. The independence of k 2 with solvent (Table VI.4) suggests a minor role for the solvent in the transition state for the k^ step. Reaction B(k3^=k2) should be zero-order in H 2 > while the data (Table VI.4) indicate about a l/3rd order. The greater solvent dependence noted for k 2, the step i n -volving dissociation of solvent, adds some support to the tentative explanation outlined in equation [VI.6], although the rate surprisingly seems to increase with coordinating a b i l i t y of the solvent. -0.9 Figure VI.13 Analysis of data from Figure VI.4 in terms of two first-order reactions (A and B). -133-Table VI.4 Analysis of various stopped-flow data at 30°C in terms of two first-order reactions([Rh] ^1 x 10~4M ) . — 2 - 1 — 3 - 1 Solvent P ,mm L x 10 ,s k' x 10 ,s DMA 380 10.2 7.4 n-BuOH 375 9.2 5.1 CH2C*2 280 9.9 3.8 MeOH 320 8.2 4.3 Me OH 225 8.1 4.1 MeOH 130 7.4 3.3 MeOH ~35 ~ 6.6 ~2.2 a_ pseudo first-order rate constants. -134-Th e data on the loss of H 2 from Rh(diop) 2H 2 in different solvents under argon (Table VI.2) suggest that the reaction is first-order in rhodium and is solvent dependent. More strongly coordinating solvents (e.g. DMA) promote dehydrogenation presumably via the reverse of the process outlined in equation [VI.6] (Law of Microscopic Reversibility): the reverse of the step seems likely to be rate-determining, and thus may depend more on solvent polarity than coordinating a b i l i t y . VI.4.2 The catalytic hydrogenation mechanism A general rate law for the hydrogenation of itaconic acid can be written as: d[H„] n n 9 n ^ - = k'[Rh] [TA] [H^] [VI.8] where k' is an overall rate constant. The kinetics at the conditions used showed that n^=l and n3=0, and thus equation [VI..8] modifies to: d[H2] _ ^n 2 dt = k[Rh][IA] [VI.9] Approximate values of k were computed for particular values of [IA] (upto 'vO.LM) where the rate is approximately first-order in IA i.e. n 2=l (Fig. VI.10). The average value of k was calculated to be ^0.33 M _ 1s _ 1 (Tables VI.3), while the slope of Fig. VI. 9 gave k = 0.34 M - 1s _ 1, in good agreement. Thermodynamic parameters were also calculated for the IA hydrogenation reaction, by plotting the data from Table VI.3 (Figure VI.14). The slope -135-Figure VI.14 Arrhenius plot for the RhCdiopKBF.-catalyzed hydrogenation of IA ([IAJ=0.1M, [Rfi]=1.0xlO~3M, 752 mm P , n-butanol-toluene (2:1)). 2 -136-of the Arrhenius plot gave the activation energy, E = 17.0±1 Kcal mole AH^ and AS^ were calculated to be 16.4±1 Kcal mole 1 and -3.8±3 e.u. respectively. The mechanism by which hydrogenation takes place can be postulated on the basis of the kinetic and spectroscopic studies. Since solutions of Rh(diop)2 +BF 4 obey Beer's Law, and the v i s i b l e spectrum is invariant with added diop, and further, since no free (or monodentate) diop signal 31 was detected in the P nmr spectrum of the complex, there is no evidence for any involvement of the equilibria outlined in equation [VI.10] (this equilibrium was proposed in section VI.4.1 to account for the kinetics of the rapid dihydride formation (stopped-flow) but i t is not significant here during the relatively slow catalytic hydrogenation): The behavior of the system is in marked contrast to that shown by the HRh(PPh3)4 system which becomes active after a readily detectable 9 dissociation of one of the PPh^ ligands in solution. Visible spectral studies show that Rh(diop)2 + reacts with 1^ extremely rapidly and completely to give a dihydride; any reaction with IA alone (or SA) (Figure VI.12) is partial and very slow compared to the catalytic hydro-genation. Spectral studies under a l l hydrogenation conditions show instantaneous loss of the cation. These results suggest that the hydro-genation reaction involves mainly the well established 'hydride route' (see Figure 1.4), for example: Rh(diop) + Rh(diop)(diop*) or [VI. 1 0] Rh(diop) + diop Rh(diop) 2 + IV Rh(diop) 9H2 V + [VI.U] -137-+ k4 + Rh(diop)„H + IA .. , > Rh(diop) (H) 9 (IA) + diop [VI.12] 1 1 -4 VI Z k o r ^ k 5 ^ Rh(diop)(diop*) (H) 2(IA) + [VI.13] " 5 VII + ^~r\ + Rh(diop) (H) 2(IA) 2—,. Rh(diop) + alkane [VI.14] or VIII Rh(diop)(diop*)(H) 2(IA) + k ? or """""" * Rh(diop) 2 + + alkane [VI. 15] Rh(diop) + + diop F a S t > Rh(diop) 2 + [VI.16] (diop* = a 'monodentate' or 'dangling* moiety) Thus once Rh(diop) 2H 2 + (a six-coordinate rhodium(III) species is formed, a diop ligand could dissociate completely or partially to become monodentate during the process of olefin coordination to provide the necessary vacant coordination site. The intermediate species thus formed (VI or VII) could release the saturated product by consecutive hydride transfers; a fast reassociation of the diop ligand could then regenerate the cationic Rh(diop) 2 + complex. The rate law for the mechanism outlined by equations [ V I . l l ] , [VI.12], [VI.l|], and [VI.16], assuming a steady state for the Rh(diop)(H) 2(IA) + intermediate, i s given by: Kk 4k 6[lV][IA][H 2] R a t e = k_ 4[dio P] + k 6 I ™ - 1 ? ] In terms of total rhodium concentration [Rh] T, this becomes Kk.lo [RhOlA] [H„] R a t e = ^ 6L T 2 J [VI. 18] (k_ 4[diop]+k 6)(l+K[H 2])+Kk 4[H 2][IA] -138-where [Rh] T = [IV] + [V] + [VI] In the pressure range studied, the rate was found to be independent of [H^] and this is consistent with the finding that K is large, eq u i l i -brium [VI.11] lie s far to the right and the concentration of IV is negligible; i.e. 1+K[H2]^K[H2], and rate law [VI.18] reduces to: k 4k 6[Rh] T[IA] R a t e = 1 TT- T~7i T~\—FT7T [VI. 19] k_ 4[diopj+k^ + k^tlA] where [Rh]^ «.[V] + [VI] Rate law [VI.19] is at f i r s t sight in accordance with the observed first-order dependence on rhodium and the zero- to f i r s t - order depend-ence on itaconic acid. A marked inverse diop dependence observed can be explained qualitatively by equilibrium [VI.12] where added diop success-f u l l y competes with the olefin coordination,but the rhodium dependence at conditions first-order in IA should then become fractional (approaching half-order), since the [diop] term in equation [VI.19] increases with increasing [Rh],^: this is not the case. The rate law for the closely related mechanism (equations [VI.11], [VI.13], and [VI.15]), assuming a steady state for the Rh(diop)(diop*)-(H)2(IA) + species, is given by: k k [Rh] [IA] tote ' k_ 5 +k 7 +k 5[IA] [ V I - 2 0 ] This rate expression [VI.20] now accounts for a l l the observed kinetic and spectroscopic data except again for the inverse diop dependence. Both mechanisms outlined above indicate that at high [IA], the dihydrido itaconic -139-species should be fu l l y formed (zero-order in IA), the rate becoming simply k^[VII] (or kg[VI]). Even under these conditions, however, a small amount of added diop (diop: Rh=l:3) completely inhibits the catalytic hydrogenation. Even i f one-third of the rhodium were completely deactivated by some diop coordination, then two-thirds of the activity should s t i l l remain. Since an equilibrium such as [VI.12] is ruled out, we can only speculate the reason for the very marked inverse diop dependence. One possibility is the formation of an inactive polymeric species, initiated by a small amount of added diop ligand; polymerization via diop bridges could tie up available coordination sites and successfully compete with the IA coordination step [VI.13]: + K* + n Rh(diop) 2H 2 + diop > [Rh(diop)(diop*)H 2 ]^ [VI.21] The validity of rate law [VI.2 0] can be tested by an inverse plot. Rearranging gives 1 k-5 + k7 1 1 " + iW [VI. 22] Rate k 5k ?[Rh] T [IA] k ?[Rh] T The plot of Rate vs [IA] ^ is a straight line with slope and 3 4-1 intercept equal to 2x10 s and 1.3x10 Ms respectively (Figure VI.15). The intercept gives a k 7 value of ^0.08 s The ratio of slope/intercept k-5 + k7 gives — r - 0.15M. The value of k in equation [VI.9] now 5 -1 -1 corresponds t 0 k-jk^/k_-j+ k^ and equals ^0.5 M s . This value of k compares with that of 0.34 M "*"s ^  estimated from the slope of Fig. VI.9. Since a reasonable Arrhenius plot is obtained (Fig. VI.14), i t suggests -140-0.8 Figure VI.15 Plot of [IA] versus linear rate (data taken from Fig. VI.10). -141-that is small, and rate determining. Thus k=K^k^ which gives K5^4M-1. Under the conditions zero-order in IA, the only species present the rate determining step. Unfortunately, the high-field """H nmr data of RhCdiop^H^BF^ in methanol-benzene-d^ under saturated in IA were not conclusive since the conditions necessary to produce detectable amounts of the Rh (diop) (diop*) (H^CIA)"*" species were probably not met at the concentration scales used for the experiment. Moreover, the limited concentration of in the nmr tube also presented d i f f i c u l t i e s in studying the reaction. The i n i t i a l faster part of the uptake plots (Fig. VI.8) has not been considered; the mechanism discussed concerns only the region of linear rates. The i n i t i a l rapid dihydride formation (equation [VI.11]) based on the amount of catalyst used is too small to account for the i n i t i a l curvature; a possibility i s a contribution from an 'unsaturate' route (mechanism [VI.23]): in solution should be Rh(diop)(diop*)(H) 0(IA) +, reaction [VI.15] being Rh(diop) 2(IA) + H 2 [VI.23] Rh(diop)„ + saturated product -142-CHAPTER VII REACTIONS OF BIS(DITERTIARYPHOSPHINE)-RHODIUM(I) COMPLEXES WITH THE GAS MOLE-CULES CO, 0 2 > H 2 > AND HC£ VII.l Introduction There has been recent interest in determining the stereochemistry of various five-and six-coordinate iridium(I) and iridium(III) bis(diphos-1-4 phine) complexes in solution using nmr spectroscopy. These complexes exhibit fluxional behaviour and in some cases static structures could not 3 be achieved even at low temperatures. X-Ray crystal structures of + 5 + 6 Ir(dpe) 2CO and M(dpe) 20 2 (M = Ir,Rh) have been reported, and a l l three complexes have approximately trigonal bipyramidal configurations (considering 0 2 as a monodentate ligand). In contrast to iridium, no systematic solution studies have been reported on analogous rhodium complexes. The few rhodium complexes reported include Rh(dpe) ^ ^ P F ^ which is a reversible 0 2 binder^, HRh (dpe) 2C£ +C£ which is reported to be one of the most active catalysts for hydrogenation of terminal. 7 8 ^ v ^ olefins ' , and [Rh(P P)2]C& species (P P = dpe and dppp) which have 9 been used for catalytic decarbonylation of aldehydes ; the decarbony-lation report appeared after completion of the CO studies described in this chapter. The synthesis and characterization of neutral and cationic rhodium(I)-bis (ditertiary phosphine) (ditertiaryphosphine = cf^P^CH^^Pcj),^ (n=l-4) and (+)-diop) complexes were discussed in chapter TV. In this chapter, the synthesis of several rhodium(I) and rhodium(III) complexes obtained by addition of CO, 0 2 > H2, and HC£ to the [Rh(P^P)2]A (A=C£, BF^) complexes w i l l be described. Such methods have been used e a r l i e r ^ to -143-synthesize some rhodium and related iridium complexes. Spectroscopic studies done on the isolated rhodium complexes to establish their solution structure are presented along with kinetic data on the ease of addition of 0^ , CO, and IL, to the Rh(P P) 2 +BF 4 complexes. VII.2 Experimental Solutions of the starting rhodium(I) complexes were handled using inert atmosphere techniques. Stopped-flow data were obtained on a Durrum 110 spectrophotometer equipped with a 2-cm light path cuvette. Stoichiometric gas-uptake experiments were performed using the ampoule technique described in section II.6. PPh^ in an appropriate solvent 31 was used as a reference standard for a l l the low temperature P nmr spectra. Tables VII.l - VII.5 report analytical, conductivity, reaction 1 31 stoichiometry, infrared, and H and P nmr data for the well charac-terized complexes (see section VII.3). Gas-uptake data are also given for systems for which pure complexes were not isolated (Table VII.2). Preparation of complexes The [Rh(p">P)2]A (P^P = cJ^P^CH^Pc^(n=l-4) and . (+)-diop; A=C£, BF. ) complexes were synthesized from the [RhC£ (C0H., . ) „] „ dimer as 4 O 14 Z Z described in Chapter IV. VII.2.1 CO complexes VII.2.1.1 Rh(P°P) 2CO +BF 4~ (P^P = dpm, dpp) - Through a red solution of Rh(dpm)2+BF4" (0.28 g, 0.29 mmol) or Rh(dpp) 2 +BF 4" (0,30 g, 0.30 mmol) in CH 2C£ 2 (5 ml) was bubbled CO gas. Addition of diethyl ether precipitated a yellow complex that was fil t e r e d after cooling to 0°C, washed with ether a few times, and then dried over an atmosphere of CO. The solids were stor Table VII.1 Analytical and molar conductivity (A) data for the Rh(I) and Rh(III) complexes. Analysis (%) A— Complex Decomposition point-,°C Found C H Calculated C H (mho.cm^mol Rh(dpm)2CO+BF4" Rh(dpp)2CO+BF4 168 61.74 4.44 62.09 4.46 85^ 179-182 63.43 5.13 63.34 4.99 81± Rh(dpm) 20 2 +BF 4 209-211 61.00 4.50 60.62 4.44 83 Rh(dpe) 20 2 +BF~ 164-167 61.26 4.69 61.31 4.72 84 Rh(dpp) 20 2 +BF~ Rh(dpp)2H2+BF~ Rh(diop) 2H 2 +BF 4 HRh(dpm)0C£+C£~ 197-201 86 153 144 61.54 63.74 61.22-58.96^ 5.19 5.22 5.65 4.73 61.97 63.80 62.64 63.64 4.97 5.32 5.56 4.77 81 81* 79* 67 1 + _ HRh(dpe)0C£ Cl 136 60.77f 5.00 64.28 5.05 68 1 + -HRh(dpp)2C£ Cl 152 62.34- 5.43 64.88 5.31 68 HRh(dpm) 2C£ +BF 4 204 59.95 4.56 60.35 4.53 95 HRh(dpe)2C£+BF~ HRh(dpp)2C£+BF~ 184 60.89 4.87 61.06 4.79 96 164-167 59.36^ - 5.01 61.71 5.05 95 a_ In air (uncorrected). b_ In nitromethane (under argon) at 25°C. c^  Under CO atmosphere. d_ Under hydrogen atmosphere. e_ Analysis low due to the presence of solvent in the complex (see discussion). f See text. Table VII.2 Gas-uptake data for various rhodium(I)-bis(ditertiaryphosphine) complexes at 25°C. Complex H2/Rh 02/Rh CO/Rh Rh(dpm)2+BE4 0.9^ 0.4 Rh(dpe) 2 +BF 4 Rh(dpp) 2 +BF 4 b b 0.9f 0.4 l.<£ 1.4 Rh(dpb) 2 +BF 4 0.4 2.8^ 1.4 Rh(diop) 2BF 4 0.8^ 0.8s h-1.3 a In methanol using the ampoule technique; values ±0.1 in error ([Rh]=5xl0 mole). b_ No uptake observed. b_' Immeasurably slow. c In 1,2 C 2H 4C£ 2 d_ In DMA e_ Small uptake observed; see section VII.5. _f In n-butanol. £ In DMA at 12°C. -146-Table VII.3 High-field H nmr and infrared data for the rhodium(I) and rhodium(III) complexes Complex Nmr data— i r data-— '4 H-Rh. (cm 1) Rh(dpm)2CO+BF~ v(CO) = 1948 Rh(dpp)2CO+BF~ " = 1961 Rh(dpm)202+BF~ v(02> = 881 Rh(dpe) 20 2 +BF 4 ' " = 8 8 8 Rh(dpp) 20 2 +BF 4 " = 896 Rh(dpp) 2H 2 +BF 4 18.5 v(H-Rh) = 2052 Rh(diop) 2H 2 +BF~ 20.0 " = 2020,2080 HRh(dpm)2C£+CJl" 22.0 v(H-Rh) = 2064 HRh(dpe)2C£+C£" 25.9 " = 2086 HRh(dpp)2C£+C£" 25.0 " = 2100(br) HRh(dpm)2C£+BF~ 22.0 " = 2065 HRh(dpe)0C£+BF,~ 26.0 " — 2084 HRh(dpp)2C£+BF 25.0 " = 2100(br) a In CDC£ 3 at 25°C. b_ In Nujol; br=broad Table VII.4 H nmr data for various rhodium-bis(ditertiaryphosphine) complexes.— Complex 6, coordinated diphosphine aliphatic phenyl Rh(dpm)2CO+BF~ Rh(dpm)2CO+BF4" Rh(dpp)2CO+BF~ Rh(dpm) 20 2 +BF 4 Rh(dpe) 20 2 +BF 4 Rh(dpp) 20 2 +BF 4 4. 2m, 4. 5m— 1.9m, 4.7nM 2.45m^ 7.5m^ 7.5m^-7.4m^ 3.8m, 4.5m 7. 3m 2. 2m 7. 3m 1. 7m, (2-3)m 7.4m Rh(dpp) 2H 2 +BF 4 Rh(diop) 2H 2 +BF 4 HRh(dpm)2C£+C£~ 1.9m, 1.2s, c e 2.3m, 3,1m—'— 2.3m, 2.9m, 3.3m— 7,0m, 7,3m, 7.5m, 7.9m, 8.1n£'-7.5n£ 4. 6m, 5.0m 7.3m HRh(dpe)2C£+C£~ 2. 3m, 2.8m 7.3m HRh(dpp)2C£+C£" 1.6m, 2.3m, 2.8m 7.4m HRh(dpm) 2C£ +BF 4 HRh(dpe) 2C£ +BF 4 HRh(dpp)2C£+BF~ 4. 5m, 4.9m 7.3m 2. 3m, 2.8m 7.3m 1. 6m, 2.3m, 2.8m 7.4m a_ Measured in ppm (downfield positive) from TMS in CDC£3; s=singlet, m-multiplet. b_ Under argon atmosphere c In acetone-d, — 6 cl Under CO atmosphere e_ Under hydrogen atmosphere -148-Table VII.5 P{ H} nmr data for various rhodium-bis(ditertiaryphosphine) complexes.— Complex .b 6— ,ppm JRh-P' H z JP-P' H Z T,°C Rh(dpm)2CO+BF~- +22.27 102.4 - 25 ii £ +19.38 +20.83 97.5 115 - -50 Rh(dpm)2CO+BF4-ii £ +22.59 +19.80 97.5 97.1 - 25 -50 Rh(dpp) CO+BF~-Rh(dpp)„C0 BF -- 6.80 -15.13 133 122.5 30 25 25 z 4 +12.87 85.1 - 5.14 + 9.94 91.6 114.7 46 -50 Rh(dpm)o0o+BF7 +14.39 112.4 38 25 Z Z 4 +30.80 82 Rh(dpe) 20 2 +BF 4 -57.6 132.7 - 25 Rh(dpe) 20 2 +C£" -51.3 -44.3 126 7.7 - 8 0 * Rh(dpp)„0„+BF ~ -14.90 122.5 30 25 Z Z 4 +12.68 84.9 Rh(dpp)0H„+BFT- -19.96 99.6 30 -50 z z +^ - 9.12 82.2 Rh(diop) 2H 2 +BF 4 - -19.9 - 3.8 103 90 S 25 I -21.40 - 6.4 103 90 19.7 -50 I -21.60 - 7.7 103.6 90 19.2 -85 HRh(dpm)0C£+C£~ +16.70 84.3 — 25 I I 2 +15.40 82.6 - -6Q HRh(dpe)0C£+C£~ -51.80 93.8 — 25 it 1 -56.46 94.2 - -60 HRh(dpp)2C£+C£" - 2.34 91 - 25 I I -14.3 + 5.4 - - -5C£ I I -16.0 + 6.4 - - - 8 5 ± HRh(dpm)2C£+BF~ HRh(dpe) 2C£ +BF 4 HRh(dpp)2C£+BF~ +16.9 -51.5 - 2.56 84.4 94.7 91 _ 25 25 25 contd. -149-a. Measured in ppm (upfield positive) from 85% H^ PO^ . Shifts for the free ligands (CBCl^ or CH 2C£ 2) occur at +22.25 (dpm), +12.51 (dpe), +17.25 (dpp), +15.95 (dpb),and +22.50 (diop). b In CH„CJ> - acetone-d, (2:1 V/V). — Z Z D c^  Under Ar. d. Under CO _e Value from ref. 3. f_ Under H.,. £ See text. h. A broadened doublet. i. A doublet of doublet of triplets was obtained although i t was not well resolved. -150-under CO. VII.2.1.2 - CO was bubbled through a yellow solution of Rh(dpe) 2 +BF 4 (0.20g, 0.20 mmol) in CH 2C£ 2 (5 ml) for ^ h . No change in colour was observed. Slow addition of diethyl ether yielded only yellow crystals of the starting complex. VII.2.1.3 - Reaction of CO with Rh(dpb) 2 +BF 4~ using procedure VII.2.1.1 yielded a yellow complex contaminated with a white powder. The infrared (Nujol mull) showed carbonyl bands at 1962 and 2003 cm 1 . C a l c , for example, for Rh(dpb)(dpb*)(CO)2+BF4~: C = 63.41%; H = 5.10%. Found: C = 57.92%; H = 4.91%. Attempts to synthesize [Rh(dpb)2C0]Ci via an exchange reaction, by sti r r i n g trans - RhCi(CO)(PPh^)2 with 3 moles excess dpb in benzene, similar to the method used for preparing bis-3 (diphosphine)-iridium complexes , also gave a mixture of products, the infrared showing carbonyl bands at 1910 and 1941 cm 1. VII.2.1.4 - Using procedure VII.2.1.1,Rh(diop) 2 + B F4~ reacted with CO to give a yellow solid. The i r (Nujol mull) showed terminal CO bands at 1976 and 2015 cm 1 but a low elemental analysis (calc. for.Rh(diop)(diop*)-(CO) 2 +BF 4 ) suggested that the solid isolated was a mixture of complexes. Reaction of Rh(diop)2C£ with CO in benzene followed by addition of diethyl ether yielded a yellow complex that showed two carbonyl bands (Nujol) at 1926 and 1974 cm 1 . The same bands have been reported for 12 [Rh(diop)(diop*)(C0)2]C£ obtained by a different method. However, an additional i r band at 308 cm 1 assigned to v(Rh-C£), not reported in 12 literature , suggests that the complex is probably contaminated with 12 some neutral rhodium species. Repetition of the literature preparation also yielded a mixture of complexes. -151-VII.2.2 Dioxygen complexes The literature method with a slight modification was used to prepare the dioxygen complexes. VII.2.2.1 Rh(dpm)0„+BF. - Oxygen was bubbled through an orange solution z 4 — of Rh(dpm) 2 + B F4~ (0.22 g, 0.23 mmol) in methanol (20 ml). The solution became cloudy after a few minutes and a dark brown solid separated that was washed with methanol and dried under suction for a few hours. VII.2.2.2 R h ( P ^ P ) 2 0 2 + B F 4 ~ ( P " P = dpe, dpp) - Through a methanol solution of R h ( P ^ P ) 2 + B F 4 " (0.30 g, 0.30 mmol) was bubbled 0 2 gas. The resulting solution was concentrated to a 5 ml volume when a solid separated. The mixture was cooled to 0°C and the solid was fi l t e r e d , washed with diethyl ether and dried under suction. The dpe complex was greenish brown while the dpp analogue was brown. VII.2.2.3 - A deep red solution of Rh(dpb) 2 +BF 4~ (0.25 g, 0.24 mmol) in methanol (20 ml) slowly changed to dark brown under oxygen over a period of one day. Addition of diethyl ether precipitated a brown solid that was a mixture of products; the i r showed bands at 851 and 1120 cm 1 possibly attributable to v(0 2) and v ( P-0) respectively. 1"^ Calculated for Rh(dpb0 2) 20 2 +BF 4": C = 59.07%; H = 4.92%;. Found: C = 52.68%; H = 4.98%. VII.2.2.4 - No vis i b l e colour change was observed on stirring Rh(diop) 2 +BF 4 under an oxygen atmosphere even after four days in methanol. Addition of diethyl ether precipitated a yellow complex that mostly contained the starting material (via ir) and the system was not studied further. VII.2.3 Dihydride complexes VII.2.3.1 - Reaction of H 2 with the R h ( P ^ P ) 2 + B F 4 ~ complexes • ( P ° P = dpm, dpe) in CH 2C£ 2 yielded only the starting complex in both cases. -152-VII.2.3.2 Rh(dpp) 2H 2 +BF 4 - H 2 was bubbled through a deep red solution of Rh(dpp) 2 +BF 4~ (0.36 g, 0.35 mmol) in CH 2C£ 2 (10 ml). Within a few minutes the colour changed to light yellow. Diethyl ether was added to induce crystallization and the solution was cooled to 0°C. The white crystals so obtained were filte r e d , washed with ether, dried over B_2, and fi n a l l y stored under H_2. VII.2.3.3 Rh(dipp) 2H 2 +BF 4~ - When a deep red solution of Rh(diop) 2 +BF 4~ (0.41 g, 0.34 mmol) in CH 2C£ 2 (10 ml) was stirred under R^, the colour slowly changed to light yellow in ^ %h. The off-white crystals, obtained using procedure VII.2.3.2, were stored under R^. VII.2.3.4 - No change in colour was observed when Rh(dpb)^BF^ (0.22 g, 0.21 mmol) in CH2C&2 (5 ml) was stirred under H 2« The solid isolated using procedure VII.2.3.2 was probably contaminated with the starting complex. Calc. for Rh(dpb) 2H 2 +BF 4": C = 64.39%; H = 5.56%.Found: C = 62.73%; H = 5.53%. The i r showed a sharp band at 2072 cm 1 assigned to v(H-Rh), while the high f i e l d 1H nmr in CDC£ 3 at 25°C was a pair of multiplets centered around 20.3x VII.2.4 HRh(P°P)2G£+A~ complexes VII.2.4.1 HRh(dpm)2C£+C£~ - HC£(g) was bubbled through an orange solution of Rh(dpm)2C£ (0.30 g, 0.33 mmol) in methanol (15. ml). The solution turned colourless instantaneously and a white solid separated after a few minutes stir r i n g . The solid was fi l t e r e d , washed with methanol a few times, and dried in vacuo to give white microcrystals. The same product was isolated starting with Rh(dpm)2+BF4 . VII.2.4.2 HRh(P^P)2C£+C£~ (P^P. = dpe and dpp) - Through an ethanol solution (vL5 ml) of Rh(P^P)2C£(P^P = dpe or dpp (0.37 g, /v0.4 mmol)) -153-was bubbled HC£(g). The solution turned light yellow instantaneously. After a few minutes s t i r r i n g , excess HC£(g) was removed by a flow of argon; concentration to ^5 ml followed by addition of 20 ml ether yielded light yellow crystals on cooling to 0°C. The complex was fi l t e r e d , washed with diethyl ether, and dried in vacuo. The HRh(dpe)2CJo+C£ complex has been prepared earlier via a different route. VII.2.4.3 Reaction of HC£(g) with [Rh(P^P)JA (P^P = dpb and diop; A=C£,BF4) Method I - HC£(g) was bubbled through a deep red solution of Rh(dpb)2C£ (0.23 g, 0.23 mmol) in CR^CJl^ (5 ml). The color instantaneously changed to light yellow. After removing excess HC£ by bubbling argon, the solution was concentrated to ^2 ml, and THF (5 ml) was added. Slow addition of diethyl ether precipitated a light yellow compound that was f i l t e r e d , washed with ether a few times, and dried in vacuo; i t appeared, however, to be a mixture of products. Method II - The colour turned instantaneously to light yellow when HC£(g) was bubbled through a deep red solution of Rh(dpb)2C£ in benzene. After a few minutes st i r r i n g , the light yellow solid that separated was fi l t e r e d , washed with benzene and dried in vacuo; again, however, a mixture of products resulted. The i r (Nujol) showed bands at 2150 cm (br), assigned to v(H-Rh), and at 280 and 290(sh) cm ^ corresponding to v(Rh-C£). The high-field ''"H nmr in CDC£ 3 at 25°C showed a broad multiplet centered around 26.5T. Calc. for HRh(dpb)2C£+C£~: C = 65.43%; H =5.45%. Found: C = 61.26%; H = 5.24%. Reaction of HC£(g) with Rh(diop) 2 +BF 4 also gave a light yellow mixture of products by both methods. i r (Nujol): V(H-Rh) = 2130 cm (br); >.v(Rh-C£) = 278 and 290 cm - 1. In addition, broad bands at 3500-3600 cm - 1 - 1 5 4 -and at ^ 1 6 0 0 cm were also observed. The high-field "^H nmr in CDCl^ at 2 5 ° C showed an extremely broad multiplet centered around 2 2 . O T , ( 2 ( Calc. for HRh(diop) C£ +C£ : C = 6 3 . 5 4 % ; H = 5 . 5 5 % . Found: 5 8 . 2 9 % ; H = 5 . 4 6 % . A l l the reactions described in section VII. 2 . 4 gave the same products when HC£.DMA was used instead of HC£(g). V I I . 2 . 4 . 4 HRh(p"p)2C£+A~ (P^P = dpm, dpe, and dpp; A = BF^, PFg, SbFfi) The complexes were prepared in each case from the corresponding chloro complex. In a typical preparation,AgA ( 0 . 1 4 mmol) in acetone ( 5 ml) was added to a CH 2C£ 2 ( 5 ml) solution of HRh(p'~P)2C£+C£~ ( 0 . 1 4 mmol•',.) . A white precipitate of AgC£ formed instantaneously. The solution was filtered and concentrated to a 2 ml volume, followed by addition of THF ( 3 ml). Diethyl ether was added to precipitate the product that was fil t e r e d , washed with ether, and dried in vacuo. Attempts were also made to prepare the HRh(p'^ P) 2C£ +A complexes 3 by a procedure used by Miller and Caulton to prepare the corresponding iridium complexes. Thus in a typical preparation, a slight excess of aqueous HBF. or HPF, was added to an ethanol solution of Rh(P P)~C£ (P P = 4 o Z dpm, dpe, and dpp). The yellow or orange complexes that precipitated were fi l t e r e d , washed with ethanol and dried. The infrared spectra of a l l these complexes showed characteristic bands of the anion (BF^ or PFg) but no hydride band was observed. The complexes were not studied further. When two moles of the AgA salt (A = BF^, PF^ ., ShF^) per mole of rs + -rhodium were stirred overnight in the dark with the HRh(P P) 2C£ C£ complexes, the products were identified as the known HRh(P P) 2C£ +A species rather than expected dications of the type [HRh(P^~P)2J[A]2--155-VII.3 Results and Discussion Oxidative or just simple addition of XY molecules (XY=02, H^ , HC£, and CO) to square-planar rhodium(I)- and iridium(I)-bis(diphosphine) cationic complexes is f a i r l y common1^ 1 3 and a number of iridium(I) and iridium(III)-bis(diphosphine)(diphosphine = dpm, dpe, and dpp) complexes have been prepared using this method. 1^' 1 1 The product from the reaction always incorporates one mole of XY to give complexes of the type Ir(P P)2XY . We have used this method in an attempt to prepare several rhodium(I)- and rhodium(III)-bis(diphosphine) complexes (Table VII.l) starting with four-/->> /-\ and five-coordinate rhodium(I) complexes of the type Rh(P 7)(P P = 16 dpm, dpe, dpp, dpb, and diop; A=C£,BF4) via reaction [VII.l]: [Rh(P'rp)2]A + nXY [Rh(p'"p) (XY) ]A [VII.l] The value of n and the extent of reaction [VII.l] varies with the particular diphosphine used (Table VII.2). Gas-uptake data indicate that the value of n_ is independent of the nature of solvent. Tables VII.l-VII.5 l i s t analytical, conductivity, gas-uptake, and spectroscopic data for the isolated complexes. For the sake of simplicity, the complexes have been classified as five- and six-coordinate, and w i l l be discussed separately in sections VII.3.1 and VII.3.2 respectively. VII.3.1 The five-coordinate carbonyl complexes of rhodium(I) Analytically pure complexes are obtained via reaction [VII.l] tor P^ P = dpm or dpp with XY=CO, n=l, and A=BF^. The Rh(dpe)2 BF, com-12 plex, like the chloride analogue , does not add CO. The method seems simpler than one reported for the preparation of [Rh(dpp)2CO]C& using [RhC£(COD)]2 and a five-fold excess of dpp in benzene under CO which gives -156-12 ~ a mixture of products. The P P = dpb and diop, and A=BF^ complexes react with > 1 mole of CO per rhodium (Table VII.2) to give presumably, some dicarbonyl species (ir data), although a pure complex was not isolated. Any such dicarbonyl could maintain four or five coordination by dissociation of one or two ligating P atoms. Attempts to prepare [Rh(dpb)2^0]Cl via an exchange reaction, used for the preparation of [lr(P P^COJCJ!. (P^P = dpm, dpe, and dpp) complexes3, again yielded a mixture of complexes that were not identified. An unsuccessful attempt 12 has been made by Sanger to prepare pure [Rh(dpb)2CO]CSL via a different 31 method. The P nmr spectra of the carbonyl mixture of dpb and diop were too complicated to yield any useful information. Reaction of CO with RMdiop^CJ!. yields a yellow solid that shows i r bands at 1926 and 1974 cm"1 (v(C0)) and 308 cm_1(w) (v(Rh-C£)). A carbonyl 12 complex prepared by Sanger by a different procedure was reported to have the same i r CO bands but no v(Rh-CJl) was reported, and the complex was thought to be [Rh(diop)(diop*)(00)2]Cl with an ionic chloride. We repeated the literature preparation and isolated a solid with i r bands identical to those of the solid obtained by our method. The Rh-CX. band at 308 cm (that of RhCdiop^CJl is at 284 cm "*") and conductivity data in nitro-2 - 1 2 - 1 methane at 25°C, A = 28 mho.cm mol (under argon) and 37 mho.cm mol (under CO), suggest that the chloride is only partially ionized in solution; 12 mixture of complexes, one with the proposed ionic structure (A), and + 'P OC Rh CO (A) Cl P -157-others such as RhC£(CO)(diop) or RhC£(CO)(diop)(diop*) could account for the data. The structures of the isolated rhodium complexes were determined from the data in Tables VII.l—VII.5. The i r spectra of the dpm and dpp com-plexes show only one terminal CO band, in each case, at 1948 and 1961 cm \ respectively. Conductivity data in nitromethane under CO show that both complexes are 1:1 electrolytes (for 1:1 electrolytes, A=75-95 2 - l 1 7 mho.cm moi ). The complexes are f a i r l y stable in the solid state but in solution the CO is readily lost, as evidenced from spectroscopic data. Thus under argon, methanol solutions of the dpm and dpp complexes show visi b l e absorption maxima at 385 and 446 nm, and at 412 nm, respectively, characteristic of Rh(dpm) 2 + B F4~ a n d RMdpp^BF^" (Table IV.2,chapter IV). Also, for example, the "Hi nmr spectrum of the dpm complex in acetone-d^ under argon shows two multiplets for the methylene protons; under CO a single multiplet is observed, again showing the labile character of the CO ligand (Table VII.4). 31 Variable temperature P nmr data in CR2C!l'2~a'cetone~^6 t* i e c a r b o n y l 31 complexes are given in Table VII.5. The P nmr spectrum of the dpm complex under argon is a broadened doublet at 25°C (Fig.VII.la), but becomes a sharp pair of doublets at -50°C (Fig.VII.lb). Under CO, the spectrum is a sharp doublet both at 25°C and -50°C (e.g. Fig. VII.lc). On the other 31 hand, the P nmr of the dpp complex under argon is a sharp doublet at 25°C (Fig.VII.2a). Under CO at 25°C, the spectrum is a doublet of doublet of triplets slightly broadened at the base, (Fig.VII.2.-b ) but this sharpens at -50°C (Fig.VII.2c). For five-coordinate complexes, there are two possible structures (B) 31 and (C). A sharp doublet in the P nmr spectrum could indicate a l l 4P -158-Figure VII.l JJ"P nmr spectra of Rh(dpm)fCTBF^ in Cr^C^-acetone-d (2/l=V/V) a. Under Ar at 25°C. b. Under Ar at -50°C. c. Under CO at -50°C. - 6.80 -15.13 +12.87 -5.14 +9.94 I I I i 1_ PPM PPM PPM 31 + -Figure VII.2 P nmr spectra of Rh(dpp)oC0 BF in CH0C£„-acetone-d, (2/l=V/V) Z 4 Z Z O a. Under Ar at 25°C. b. Under CO at 25°C. c. Under CO at -50°C. (B) Square pyramid (C) Trigonal bipyramid (SP) (TBP) atoms equivalent corresponding to a SP structure or a fluxional TBP structure, whereas a doublet of doublet of triplets must pertain to a 3 31 TBP structure (M=Rh). Miller and Caulton •>, have done related P nmr studies on analogous iridium complexes; a singlet, measured at -21°C for [Ir (dpe^CO]^, changed at -93°C to a pattern characteristic of the TBP geometry, consistent with the solid state structure reported 5 3 earlier . Structure (C) has also been assigned to the [Ir(dpm^COjCJc. 31 complex, which exhibits a singlet in the P nmr spectrum even at -92°C. Similarly, Fe(P°P)(CO)^ complexes are non-rigid to the lowest accessible r\ 18 19 temperatures when P P = dpm or dpe . A decrease in rearrangement barrier between structures (B) and (C) in the order dpp>dpe>dpm has been - A 3 suggested. 31 The P nmr spectra of our rhodium systems can readily be explained. The Rh(dpp)2CO+BF4 complex loses CO readily under argon at 25°C, and the 31 + -sharp P nmr doublet corresponds to that of the RMdpp^BF^ species (see Table IV.4). Under CO at 25°C, Rh(dpp)2CO+ is s t i l l fluxional but the 31 P nmr pattern at -50°C is consistent with a static TBP structure. With the dpm system under argon at 25°C, the broadened doublet is due t o an equilibrium mixture of Rh(dpm)2+ and Rh(dpm)2C0+ complexes. At -50°C, the doublet at +20.83 ppm (1^^=115 Hz) is due to the Rh(dpm)2+ complex (see Table IV.4), and the second doublet can be assigned to the Rh(dpm)9C0 + -161-complex. The spectrum under CO from 25°C to -50°C shows only the one doublet due to Rh(dpm)2CO+, variation of the chemical shift with 20 21 temperature being well known. ' The complex is considered to be fluxional, the TBP structure not being achieved even at -50°C. VII.3.2 Six-coordinate complexes of rhodium(III) Addition of 0^ to RhCP^P)2+BF4 complexes (V~F = dpm, dpe, and dpp) precipitates analytically pure complexes of the formula Rh(P B F4 4" 6 — via reaction [VII.l]. Rh(dpe)„0o has been obtained earlier as a PF, Z Z o salt using a similar method. A solution of Rh(dpb) 2 +BF 4 slowly absorbs ^3 moles of 0 2 (Table VII.2) but the isolated product is a mixture that has not been separated. The bis(diop) cation reacts extremely slowly with 0 2 (Table VII.2) and no product Incorporating oxygen could be isolated. The Rh(P P ) 2 BF^ (P P = dpm and dpe) complexes do not react with R^, + 22 23 The nonreactivity of Rh(dpe)2 towards H 2 has been reported earlier. ' With P P = dpp and (+)-diop, the corresponding dihydrides were readily isolated; the carbon analysis is somewhat low for the diop complex (Table VII.l) due to the presence of solvent (CH 2C£ 2) in the crystals. A dihydride complex is also formed with P P = dpb, but i t was not possible to isolate this complex in pure form, the impurity probably being the starting complex. A H2:Rh stoichiometry of only ^0.6 was observed'in gas-uptake experiments. Oxidative addition of HC£ to Rh(P P) 2C£ complexes (P P = dpm, dpe, and dpp) results in the formation of corresponding HRh(P P) 2C£ +C£ species, but the carbon analyses for a l l these complexes are low (Table VII.l). The method is similar to the one used for the preparation of analogous iridium(III) complexes where again low carbon analyses were found, and -162-these were attributed to the presence of some chloride containing species. The same explanation seems likely for the rhodium complexes; a chloride analysis of 8.1% found for HRh(dpe)2C£+C£ compares with the theoretical value of 7.30%. The complexes were not further purified even after three r\ recrystallizations. With P P = dpb and diop, the reaction with HC£ appears to give more complex mixtures as judged from i r and other data. In the case of diop, a side reaction involving ligand cleavage by HC£ seems likely as inferred from a broad i r band in the 3500-3600 cm ^ region (v(H20) and v(0H)) and a band at ^1600 cm 1 (v(H 20)). Such acetal cleavage 24 by acids is well known and has also been observed in some related 25 ruthenium-dios complexes , where dios is a sulfoxide analogue of diop (-PPh2 replaced by -S0CH3). The HRh(P/IJ)2C£+A~ complexes (P^P = dpm, dpe, and dpp; A=BF^, PF &, SbF&) are obtained analytically pure by the treatment of HRh(lO?) 2C£ +C£ with AgA, followed by recrystallization from CH2C£2~THF-ether. Details are given only for the BF^ complexes. The low carbon analysis in the case of the dpp complex is due to the presence of solvent in the crystals (Table VII.1). An alternative method, similar to one developed by Miller and Caulton 3 for preparing HIr(P^P)2C£+A complexes,was also used, and involved treatment of Rh(P°P)2C£ with aqueous HA (A=BF 4 >PF 6), but this gave unidentified complexes containing no hydride ligand. 4 Following the procedure in a recent report on the preparation of HIr(PO?) 2^ + (P°P = dpe and dpp) complexes, addition of 2 moles of AgBF^ per mole of HRMP^P) 2C£ +C£~ gave only the known HRh(P^P)2C£+BF~ complexes, as judged from i r and nmr data. No HRh(P P ) 2 (BF 4) 2 complexes were detected. Tables VII.l and VII.3-VII.5 l i s t data for the various rhodium(III) -163-complexes. The molar conductivity data (Table VII.l) show that a l l isolated rhodium(III) complexes are 1:1 electrolytes in nitromethane at 25°C. The spectroscopic data provide useful information about the structures of these complexes. The 0^ ligand in Rh(P°P)2C>2+ complexes is readily detected in the i r region as a peroxide 1^ . (Table VII.3),showing that O2 binds to the metal in a side-on ) fashion to give formally six-coordinate rhodium(III) complexes; Rh(dpe) 0 o +PF has been reported Z Z o earlier^ but the v(0„) band could not be seen due to PF, bands in the z O same region. Visible spectra of the dpm and dpp complexes in CH^C^ under argon at 25°C showed continua (increasing absorption on going from 450-350 nm), whereas the dpe complex showed an absorption maximum at 407 nm, the same as that of Rh(dpe)2+ (see Table IV.2),: this is consistent with other data (see below) showing that O2 binding in the dpm and dpp cases is irreversible but is reversible in the dpe case. This reversible O2 binding by the dpe complex has been observed before. 31 The P nmr spectra of these dioxygen complexes (Fig.VII.3) are consistent with structure (D), which is established in the solid state for the Rh(dpe) 20 2 PF & complex. • Y (X=Y=0) Rh P / (D) 31 The dpm and dpp complexes are nonfluxional upto 25°C. The P nmr 3 spectrum of the dpe complex is a doublet of doublet of triplets at ON I +14.39 + 30.80 -14.90 I + 12.68 I PPM Figure VII.3 3 1P nmr spectra in CH2C£2-acetone-d6 (2/l=V/V) at 25°C of: PPM a. Rh(dpm)202+BF4". b. Rh(dpp) 20 2 +BF 4 < -165--80°C consistent with structure (D); the doublet at 25°C (Table VII.5) is due to Rh(dpe) 2 +. The reversible 0^ binding by the dpe complex w i l l be discussed in more detail in section VII.5. The high-field 1H nmr at 25°C for the Rh(P'~P)2H2+BF4~ (P^P = dpp, diop) complexes is a pair of multiplets (Fig.VII.4, Table VII.3)^ suggesting different environments for the two hydride ligands. The same nmr pattern has been reported for Rh[P(CH^X^H,^" a t 25°C and was attributed to a c i s -2 6 dihydride. Infrared data are consistent with the presence of terminal 31 1 hydrides (Table VII.3). The P{ H} nmr spectra of these complexes show two resonances (Fig.VII.5) as expected for complexes with a cis configu-ration. The resonance at lower f i e l d is attributed to the 2P atoms trans to each other whereas the resonance at higher f i e l d is due to the P atoms 26 trans to hydrides; these assignments are based on literature data. A variable temperature study on the Rh(diop) 2H 2 + complex suggests that the resonance at higher f i e l d is temperature dependent whereas the lower f i e l d resonance is independent of temperature (Fig.VII.5). The presence of two resonances at a l l temperatures rules out the possibility of c i s -trans rearrangement that have been observed in complexes of the type 27 28 ML^ H,^  (M=Fe,Ru:L=phosphine, phosphite etc.) ' . Thus the temperature dependence of one set of resonances can be attributed to mutual exchange of the 2P atoms trans to hydrides. This is not unreasonable because a phosphorus ligand trans to a hydride ligand is not in a particularly stable environment, as judged by the inability of several rhodium(I)-13,29 mono(diphosphine) complexes to add B^-The Rh(dpp) 2H 2 +BF 4 complex loses hydrogen immediately in solution as seen by changes in the visible spectrum. The reversible reaction of B^ with Rh(diop) 2 + has already been described in detail in chapter VI. a h2 b.3 — J J -19.96 -9.12 PPM -19.9 -3.8 _ 1 _ PPM 21.40 - 6 . 4 I L_ 21.60 PPM PPM 31 1 Figure VII. 5 P{ H} nmr spectra in CH„CJt0-acetone-d - (2/l=V/V) of: (a) Rh(dpp) 2H 2 +BF 4" at -50°C. (b) Rh(diop) 2H 2 +BF 4" at (1) 25°C, (2) -50°C, and (3) -85°C. -168-The hydride ligand in the HRh(P^P)2CJc+A (A=C£,BF4) complexes is readily detected by i r (Table VII.3), as well as by high-field nmr at 25°C (Figure VII.6, Table VII.3). The dpe and dpp complexes are stable in solution, but lose HC£ in basic solvents (e.g. DMA) to give Rh(dpe) 2 + and Rh(dpp) 2 +, respectively (readily identified from visible bands on comparison with the known values given in Table IV.2). The dpm complex is different and does not lose HCl even in DMA. The "^ P-t^ H} nmr spectra of the HRh(p'~P)2C£+ complexes are sharp doublets at 25°C (Figs.VII.7 and VII.8). These doublets could be due to 4 equivalent P atoms coupling to rhodium in a trans structure (E). Rh I "P Cl (E) trans H Rh' P-(F) Cl CIS 31 1 However, the P{ H} nmr spectrum of the dpp complex is temperature dependent (Table VII.5) and approximates to a doublet of doublet of triplets at -85°C (Fig. VII.8) characteristic of the cis geometry (this results from the H and Cl site exchange being too fast even at -85°C to 3 be distinguishable ). The spectra of the dpm and dpe complexes do not show any temperature dependence (Figure VII.7), and this could be due to fluxional behaviour even at -60°C, as assumed for the corresponding 3 iridium complexes. Some interesting data have been obtained very recently on a related six-coordinate ruthenium(II) complex^HRuCJl-30 31 31 1 1 (diop) 2. ' The P{ H} and H nmr spectra of this complex were 30 interpreted in terms of a cis geometry , but an x-ray crystal structure -169-25.01 Figure VII.6 High-field H nmr spectra of: (a) HRh(dpm)2C£+CJT, (b) HRh(dpe)2CJl+C£,~, and (c) HRh(dpp)2C£+CX~ in CDC£ 3 at 25°C. -170-a 51.80 - 5 6 . 4 5 _ J 1 PPM PPM Figure VII. 7 ^P^H} nmr spectra of HRh(dpe) „C£+C£ in CH C£ -acetone-d, (2/l=V/V) at a. 25°C and b. -60°C7 o -172-31 determination shows this complex to have trans geometry. The inequivalence of the P atoms arises from a bending of 2P atoms toward the hydrogen, and the other two toward the chlorine: the dioxo'-lane ring of the diop fixes the configuration of the seven-membered chelate ring and the geometry distorts to minimize phenyl-phenyl interactions. Without the dioxolane ring (i.e. in the corresponding dpb complex), the four carbon backbone can twist and can lead to pseudo symmetry, and 31 the hydride did appear as a quintet. Thus i t is not possible to assign unequivocally structures to the rhodium dpm and dpe complexes solely on 31 the basis of the P nmr data. VII.4 Attempted use of HRh(P P)^C£+A complexes in catalytic hydrogenation + — 7 8 An earlier claim that HRh(dpe)2C£ C£ ' is among the most active catalysts known for the hydrogenation of olefins prompted us to test the catalytic activity of the HRh(P^P)2C£+A~(P^P = dpm, dpe, dpp; A=C£, BF^) _3 complexes reported in this chapter. Thus under conditions ([Rh]=2xl0 M, [substrate]=0.IM, 30°C, 2-methoxyethanol=5 ml, P =1 atm) similar to those 2 7 8 used in the literature reports ' , no hydrogenation of cyclohexene or itaconic acid was observed. The complexes were found to be inactive even at 60°C. VII.5 Preliminary kinetic studies From the limited stopped-flow data at 30°C on the addition of H^ , 0^, and CO to the Rh(P P) 2 +BF 4 complexes (Table VII.6), no obvious correlation between the rate of' addition and the size of the diphosphine is suggested. For the dpp complex, the ease of addition decreases in the order CO>H2>02< The rates are independent of the nature of solvent. Since the pseudo first-order rate constant for the reaction of 0^ with -173-Table VII.6 Pseudo first-order rate constant— k at 30°C for the reaction Rh(P^P) +BF ~ + XY — R h ( P ° P ) „XY+BF ~ P°P XY kx,10 3,s 1 Final[C]*xl0 4,M dpm °2 943 0.93 I I CO c 1.16 dpp H2 768 1.23 I I I I 720*-'* 1.23 I I °2 3.9 1.22 it I I 3.7* 1.34 I I I I 2.9*'* 1.34 I I CO 14690 1.22 I I I I 14120* 1.16 dpb H, 13 2.45 diop 1.05 a In CH„C£„; P =1.0 atm. b z z Final [C] = Initial[Rh] 2 ; f i n a l P H„ = 280 mm. c Too fast. z d In methanol; fin a l P = 320 mm. - gas e Final P., = 35 mm. - gas £_ Reaction complicated; see Chapter VI for details • g For p/>*P=dpe and XY=0 , see Table VII.7. -174-the dpp complex does not vary linearly with the gas pressure (Table VII.6), the mechanism of these addition reactions does not seem to be a simple one-step reaction [VII.2]. The fractional order dependence on C>2 can probably be explained by a scheme similar to that suggested for Rhi^Pp)* + XY » Rh(P/"p)2XY+ [VII. 2] the reaction of H,, with the Rh(diop) 2 + cation (section VI.4.1). However, the k^' step for the 0 2 reaction must now be rapid since only one overall pseudo first-order rate process is observed. More data are needed to substantiate such a mechanism. The reversible reaction of 0 2 with Rh(dpe) 2 +BF 4 was studied in more detail using visible spectroscopy. Thus the evolution of 0^ from Rh (dpe) 2^2 + B F4 '""n m e t h a n ° l a t 30°C under argon gave spectral changes that showed an isosbestic point at 372 nm, and the final spectrum corres-ponded to that of the Rh(dpe) 2 + species (Fig.VII.9(A)). The changes analyzed for a first-order reaction (Fig.VII.9(B)) to give k_^(Table VII.7) which was found to be independent of the nature of solvent. Visible studies on the reaction of 0^ with Rh(dpe) 2 +BF 4 in methanol at 30°C,which does not go to completion, yield k^the sum of the pseudo first-order forward rate constant and the deoxygenation rate constant (Fig.VII.10 (A and B); Table VII.7). was calculated from the rate constant data to be ^0.6±0.1 atm \ The value of k determined at 300 mm of 0„ is approximate e 2 . due to the small spectral changes observed. (If the forward reaction were first-order in 0 2 > the k^ value at 300 mm should have been about 3.4 s ^ ) . was also estimated directly (e.g. from the spectral data in Fig.VII. 10.A) using equation [VII.3] to be ^0.4 atm 1 both at 700 mm and 300 mm 0 2 [Rh(dpe) o0 o +] K = 2 2 J [VII.3] 1 [Rh(dpe)2+][02] -175-l±J O z < CD or o LO CO < 1 57 6 -24"2 11 456 15 763 PI9 1444 20 6700 350 400 450 WAVELENGTH, nm 400 450 WAVELENGTH, nm Figure VII.9(A) Spectral changes obtained for O2 loss from , Rh(dpe) 20 2 BF 4 (3.7x10 M) in methanol at 30°C (solution under argon). A.l represents the spectrum of the Rh(dpe) 20 2 +BF 4~ complex. Figure VII.10(A) Spectral changes obtained at 30°C on adding O2 (1 atm) to solutions Rh(dpe) of 2 4 B F4 (2.9x10 M) in methanol. +1.61 2 cn q 400 800 Time.s Figure VII. 9(B) Plot of log(A - A j vs time for spectral changes in Fig.VII.9(A). -1.6 0 100 300 500 Time, s Figure VII.10(B) Plot of log(A - A J vs time for spectral changes in Fig. VII.10(A). -177-Table VII.7 Data at 30°C for the determination of equilibrium constant for the reaction + - K l + -Rh(dpe)2 BF 4 +02 k -•. Rh(dpe) 20 2 BF^ [C]xlO ,M P Q ,mm Solvent k^xlO ,s k e x l ° >s K^atm 3.73 3.63 2.90 3.04 70(£ 30(4 DMA MeOH MeOH MeOH 2.64 2.65 4.2 2.7^  ^0.6(±0.1) a k e = k£+k_ l f k£ = k^O^, K^^/k b_ Error ± 5 0 mm. c^  Value in large error. -178-pressures. The two values are in reasonable agreement with the kinetically determined value of ^0.6±0.1 atm ^. For the closely related reaction of [Rh(2=phos)2]A (2=phos= cis-cJ>2P4;CH=CH}P<f>2; A=BPh4) with 0 2 in chlorobenzene at 25°C, has been estimated previously''"''" to be 350 M \ Using solubility data for 0 2 in methanol 05 x 10~3M atm"1 at 20°C 3 2) yields K± % 100 M _ 1 for the dpe system, quite comparable to the value for the 2=phos system. -179-CHAPTER VIII GENERAL CONCLUSIONS, AND RECOMMENDATIONS FOR FUTURE WORK The aim of the work described in this thesis was to study rhodium catalysts incorporating bis(ditertiaryphosphine) ligands, including chiral ones, and their potential as catalysts, particularly for hydrogenation. Several important findings are summarized below, together with some suggestions for further study. Bis(ditertiaryphosphine)-rhodium(I) complexes have been synthesized by a simple method starting with the cyclooctene dimer [RhC£(C0H.. ,) „] „ o 14 Z z and adding the appropriate ditertiaryphosphine: [RhCiKC gH 1 4) 2] 2 + 4 P°P > 2 Rh(p"p)2C£, + 2CgHl4 [VIII.l] (p"p = cp2P«CH2}nP(|>2 (n=l-4) and (+)-diop) Cationic complexes were then prepared by adding AgA(A=BF4,PF^,SbFg) to the chloro complexes, while use of borohydride yielded some corresponding hydrido complexes. Spectroscopic studies revealed that the dpe and dpp Rh(P P)2CJL complexes contained an ionic chloride, while the analogous dpm, dpb, and diop complexes were neutral in non-polar solvents but ionic in polar solvents. High molar conductivity values in nitromethane for the five-coordinate dpe and dpp hydrides (approaching that for 1:1 electrolyte) suggest a reaction with the nitroalkane solvent. There are only a few other complexes that catalyze the hydrogen reduction of nitro-alkanes to amines (see ref. 30, chapter IV), and further investigation into this possibility seems worthwhile. The solution structures of the chloro, hydrido, and cationic complexes 31 1 were studied using variable P nmr spectroscopy. H nmr data showed a -180-solvent-dependent deshielding of the ortho protons (as compared to meta and para protons) of the phenyl group i n some of the complexes. A ' v i r t u a l coupling' of the ligand CtL^  protons with the rhodium i n the Rh(dpm)2+ complex was observed, the P atom of the ligand also showing an unusual h i g h f i e l d s h i f t on coordination to rhodium. Preliminary k i n e t i c data for the c a t a l y t i c hydrogenation of IA showed that the a c t i v i t y increased with increasing chain length of the diphosphine. and the cationic and hydrido complexes were generally more active than the corresponding chloro complexes. Kinetic and spectroscopic studies on the HRh(diop)2 _catalyzed hydro-genation of IA^ has revealed the importance of a species HRh(diop)(diop*), containing a monodentate diop ligand. The hydrogenation mechanism then follows the f a i r l y common 'unsaturate' route v i a a l k y l formation and sub-sequent reaction with (equations [VIII.2]-[VIII.4]). Some side reactions HRh(diop) 2 •» HRh (diop) (diop*) [VIII.2] HRh(diop)(diop*) + o l e f i n ^ Rh(diop)(diop*)(alkyl) [VIII.3] Rh(diop)(diop*)(alkyl) + H 2——> HRh(diop)(diop*) + S.P. [VIII.4] i n this system involving protonation of the catalyst by the acid substrate to give Rh(diop)2 + and U^, and protonation of an intermediate a l k y l by the alcohol solvent to give Rh(diop)2 + and saturated product, were also i n -dicated. The non-acidic substrate styrene was hydrogenated using HRh(diop)2/H2 v i a the same unsaturate route, but as expected, acid de-composition of the catalyst was no longer observed. =f= Study of th i s p a r t i c u l a r itaconic acid system constituted the bulk of my Master Thesis. -181-Several bis(diop) - rhodium complexes were used effectively for the asymmetric hydrogenation of some prochiral substrates, optical yields of >90% being obtained in the hydrogenation of N-acylaminoacrylic acids. In Hydrogenation of IA, the optical yield of ^50% is the highest yet reported. The results indicated that steric factors at the olefinic bond as well as coordination of the -NHCOR group to rhodium through the ^C=0 moiety were important, whereas the participation of the acid function of substrates did not seem important, at least in terms of hydrogenation rates. The rates were slower in the more strongly coordinating DMA, compared to n-butanol-toluene mixtures. The solvent medium generally did not greatly affect the degree of asymmetric induction using the HRh[(+)-diop]^ or Rh[(+)-diop]^ complexes, but a reversal of configuration was noted when Rh[(+)-diop]fH was used as a catalyst in DMA or in n-butanol-toluene mixtures. The solution chemis associated with the chloride complex is complicated, and a more detailed study on this complex should be undertaken. An unusual increase in optical yield with increasing temperature for the hydrogenation of IA using HRh[ (+)-diop] 2 or Rh[ (+)-diop] ^  i-s possibly due to a different mechanism being operative at the different temperatures (see below). Asymmetric hydrogenation of several substrates was effected using the [RhC£(CO)(+)-diop]2 dimer but this complex is far less active than the bis(diop) systems, or the mono(diop)-rhodium complexes studied by other workers. The Rh(diop)2 +-catalyzed hydrogenation of IA was studied in detail. The kinetic data in n-butanol-toluene (2:1) at 15°C suggested that the hydrogenation proceeded mainly via the 'hydride' route outlined in equations [VIII.5]-[VIII.7]: -182-Rh(diop) 2 + + H 2 Rh(diop) 2H 2 + [VIII.5] Rh(diop) 2H 2 + + IA ? 5 = £ s Rh(diop)(diop*)(H) 2(IA) + [VIII.6] Rh(diop)(diop*)(H) 2(IA) + > Rh(diop) 2 + + S.P. [VIII.7] A complete inhibition of the catalytic hydrogenation in the presence of added diop (diop:Rh>0.2) is d i f f i c u l t to account for, but i t has been tentatively attributed to formation of a polymeric species (equation [VIII.8]): nRh(diop) 2H 2 + + diop > [Rh(diop)(diop*)H 2 +] n [VIII.8] Efforts should be made to substantiate this, and also in view of the unusual increase of asymmetric induction with increasing temperature for the IA system, the mechanism should be reinvestigated at the higher temperature. The reaction of Rh(diop) 2 + with H 2 alone (Reaction VIII.5) was studied in detail. Based on the stopped-flow kinetics, and other spectroscopic data, the series of consecutive reactions outlined in equation [VIII.9] were postulated: Rh(diop) 2 + + S ^ ==i: Rh (diop) (diop*) S + -—> Rh(diop) (diop*) (H) 2 S + -S Rh(diop) 2H 2 + ^ [VIII.9] The reverse dehydrogenation reaction showed a simple first-order kinetic dependence on the Rh(diop) 2H 2 + species. The kinetics may however be de-ceptively simple in terms of the mechanism, i f the reverse of equation [VIII.9] is followed. These reactions and the role of solvated species is c r i t i c a l l y important for a better understanding of asymmetric hydro-genation, and require much more detailed study using a wider range of concentrations, temperatures, solvents, and with the other chelating phosphines. -183-More generally, the reactions of [Rh(P F)^]^ complexes (A=C£,BF4) with CO, 0^, E^, and HC£(g),have yielded isolable complexes of the type [Rh(P°P)2XY]A (p"V = dpm, dpp; XY=CO, P*P = dpm, dpe, dpp; XY=02, P^P=dpp, (+)-diop; XY=H2> and p""p=dpm, dpe, dpp; XY=HC£). Variable 31 temperature P nmr data suggested cis geometries with two sets of equi-valent P atoms for the formally d^ six-coordinate dioxygen and dihydrido complexes. The ECU complexes were more fluxional and cis structures could be assigned only to the dpp system; the dpm and dpe complexes indicated equivalent P atoms (possibly trans structures) even at -60°C. A crystal structure of one of these dpm or dpe complexes would be extremely useful. For the five-coordinate CO complexes, a TBP structure was assigned to the dpp complex; the dpm complex was fluxional even at -60°C. Preliminary stopped-flow data on the addition of CO, 0 2 > and H 2 to the Rh(P P ) 2 complexes have been obtained. In the dpp case, the rate i n -creased in the order CO>H2>02> but some of these reactions may involve multi-stage processes (cf. equat ion [VITI.9]) and these overall correlations have l i t t l e value, unless some rate constants for the individual forward and/or reverse reaction steps can be elucidated. Such data are invaluable in terms of the catalytic potential of such "small gas molecule" systems (e.g. catalytic decarbonylat ion by the Rh(dpe)2CJc. and Rh(dpp)2C£ complexes has very recently been reported (see ref. 9b, chapter IV)), and more detailed kinetic studies on these systems are already in progress in this laboratory. -184-REFERENCES  CHAPTER I 1. L. Pasteur, Ann. Chim. Phys. [3] 24, 442 (1848). 2. R. J. Block, Chem. Rev., 38, 501 (1946). 3. C. W. Huffman and W. G. Shelly, Chem. Rev., j^3, 625 (1963); E. N. Safonova and V. M. Belikov, Russ. Chem. Rev., _3J>> 375 (1967). 4. W. R. Brode and R. W. Van Dolah, Ind. Eng. Chem., 39^ , 1157 (1947). 5. S. Akabori, S. Sakurai, Y. Izumi, and Y. F u j i i , Nature, 178, 323 (1956). 6. S. Akabori, Y. Izumi, Y. F u j i i , and S. Sakurai, J. Chem. Soc. Jap. Pure Chem. Sec. (Nippon Kagaku Zasshi) , 77., 1374 (1956); S. Akabori, Y. Izumi, and Y. F u j i i , ibid..,78^, 886 (1957). 7. Y. Izumi, Angew. Chem. Int. Edit., 10, 871 (1971). 8. Y. Orito, S. Niwa, and S. Imai, Chem. Lett., 1131 (1977). 9. N. Izumiya, S. Lee, T. Kanmera, and H. Aoyagi, J. Am. Chem. Soc, 99, 8346 (1977). 10. J. F. Young, J. A. Osborn, F. H. Jardine, and G. Wilkinson, Chem. Comm., 131 (1965). 11. F. A. Jardine, J. A. Osborn, G. Wilkinson, and J. F. Young, Chem. Ind. (Lond.), 560 (1965). 12. B. R. James, 'Homogeneous Hydrogenation', John Wiley and Sons, New York, 1973. 13. B. Bogdanovic, Angew. Chem. Int. Edit., 12, 954 (1973). 14. L. Marko and B. Heil, Catal. Rev., 8, 269 (1973). 15. H. B. Kagan, Pure Appl. Chem., 43_, 401 (1975). 16. J. D. Morrison, W. F. Masler, and M. K. Newberg, Adv. Catal., 25, 81 (1976); J. D. Morrison, W. F. Masler, and S. Hathaway, in -185-"Catalysis in Organic Syntheses 1976" (P. N. Rylander and H. Greenfield, eds.), Academic Press, New York, 1976, p.203. 17. B. R. James, Adv. Organomet. Chem., 17, 3.19(197.9)..' 18. B. D. Vineyard, W. S. Knowles, M. J. Sabacky, G. L. Bachman, and D. J. Weinkauff, J. Am. Chem. Soc, 99_, 5946 (1977) and references therein. 19. M. D. Fryzuk and B. Bosnich, J. Am. Chem. Soc, '99, 6262 (1977). 20. W. R. Cullen and E.-S. Yeh, J. Organomet. Chem., 139, C13 (1977). 21. T. P. Dang and H. B. Kagan, Chem. Comm., 481 (1971). 22. H. B. Kagan and T. P. Dang, J. Am. Chem. Soc, 94, 6429 (1972). 23. W. Dumont, J. C. Poulin, T. P. Dang, and H. B. Kagan, J. Am. Chem. Soc, 95, 8295 (1973). 24. H. B. Kagan, N. Langlois, and T. P. Dang, J.. Organomet. Chem., 90, 353 (1975). 25. T. P. Dang, J. C. Poulin, and H. B. Kagan, J. Organomet. Chem., 91, 105 (1975). 26. D. Sinou and H. B. Kagan, J. Organomet. Chem., 114, 325 (1976). 27. G. Gelbard and H. B. Kagan, Tetrahedron, 32, 233 (1976). 28. Y. Chauvin, D. Commereuc, and R. Stern, J. Organomet. Chem., 146, 311 (1978). 29. W. R. Cullen, A. Fenster, and B. R. James, Inorg. Nucl. Chem. Lett., 10, 167 (1974). 30. Y. Chevailier, Docteur-Ingeni eur Thesis, Paris, 1970; through reference 22. 31. D. Mahajan, M.Sc Dissertation, U.B.C, Vancouver, B.C., 1976; B. R. James and D. Mahajan, Isr. J. Chem., 15, 214 (1977). -186-32. J. Vilim and J. Hetflejs, Collect. Czech. Chem. Comm., 43_, 122 (1978). 33. J. M. Brown and P. A. Chaloner, J. Chem. Soc. Chem. Comm., 321 (1978). 34. F. Pruchnik, Inorg. Nucl. Chem. Lett., 9_, 1229 (1973). 35. F. Pruchnik, Inorg. Nucl. Chem. Lett., 10, 661 (1974). 36. F. N. Tebbe, 2nd Joint CIC/ACS conference, 1977, Inorg. 087; J. M. Manriquez, D. R. McAlister, R. D. Sanner, and J. E. Bercaw, J. Am. Chem. Soc, 98, 6733 (1976). 37. J. Halpern and B. R. James, Can. J. Chem., 44^ 671 (1966). 38. P. S. Hallman, B. R. McGarvey, and G. Wilkinson, J. Chem. Soc. (A), 3143 (1968). 39. R. Cramer, E. Jenner, R. Lindsay, and U. Stolberg, J. Am. Chem. Soc., 85, 1691 (1963). 40. J. Halpern and J. B. Milne, Proc. Int. Congr. Catal., 2nd, Paris, 445 (1960). 41. J. Halpern, J. Harrod, and B. R. James, J. Am. Chem. Soc, 88^, 5150 (1966). 42. B. Hui, Ph.D. Dissertation, U.B.C, Vancouver, B.C., 1969. 43. J. Kwiatek, I. L. Mador, and J. K. Seyler, Adv. Chem. Ser., 37, 201 (1963). 44. M. G. Burnett, P. J. Connolly, and C. J. Kemball, J. Chem. Soc. (A), 800 (1967). 45. J. Halpern, Adv. Chem. Serv., 7j0, 1 (1968). 46. R. G. Banks and J. M. Pratt, J. Chem. Soc. (A), 854 (1968) and references therein. 47. J. Kwiatek and J. K. Seyler, Adv. Chem. Ser., 70, 207 (1968) -187-and references therein. 48. H. M. Feder and J. Halpern, J. Am. Chem. Soc, 97_, 7186 (1975). 49. R. L. Sweany and J. Halpern, J. Am. Chem. Soc, 9£, 8335 (1977). 50. L. Vaska and J. W. Diluzio, J. Am. Chem. Soc, 83, 2784 (1961). 51. P. B. Chock and J. Halpern, J. Am. Chem. Soc, 88^ , 3511 (1966). 52. L. Vaska, Acc. Chem. Res., 1_, 335 (1968), and references therein. 53. B. R. James, A. D. Rattray, and D. K. W. Wang, J. Chem. Soc. Chem. Comm., 792 (1976). 54. V. V. Ipatieff, Jr. and V. G. Tronev, Compt. Rend. Acad. Sci., U.S.S.R. 1, 629 (1935). 55. M. Iguchi, J. Chem. Soc. Jap., 60, 1287 (1939). 56. J. Halpern, Quart. Rev., 10_, 463 (1956). 57. J. Halpern, Adv. Catal., 9_, 302 (1957). 58. J. Halpern, Adv. Catal., 11, 301 (1959). 59. J. Halpern, Collect. Pap. Symp. Coord. Chem., 351 (1964). 60. J. R. Shapley, R. R. Schrock, and J. A. Osborn, J. Am. Chem. Soc, 91, 2816 (1969). 61. R. R. Schrock and J. A. Osborn, J. Chem. Soc. Chem. Comm., 567 (1970). 62. R. R. Schrock and J. A. Osborn, J. Am. Chem. Soc, 93, 2397 (1971). 63. R. R. Schrock and J. A. Osborn, J. Am. Chem. Soc, 93, 3089 (1971). 64. R. R. Schrock and J. A. Osborn, J. Am. Chem. Soc, 98^ , 2134 (1976). 65. R. R. Schrock and J. A. Osborn, J. Am. Chem. Soc, 98, 2143 (1976). 66. R. R. Schrock and J. A. Osborn, J. Am. Chem. Soc. ,98, 4450 (1976). 67. P. B. Hitchcock, M. PcPartlin, and R. Mason, Chem. Comm., 1366 (1969) . 68. J. A. Osborn, F. H. Jardine, J. F. Young, and G. Wilkinson, J. Chem. Soc. (A), 1711 (1966). -188-69. M. A. Bennett and P. A. Longstaff, Chem. Ind. (Lond.), 846 (1965). 70. J. P. Candlin and A. R. Oldham, Discuss. Faraday Soc, 4_6, 60 (1968). 71. F. H. Jardine, J. A. Osborn, and G. Wilkinson, J. Chem. Soc. (A), 1574 (1967). 72. S. Montelatici, A. van der Ent, J. A. Osborn, and G. Wilkinson, J. Chem. Soc. (A), 1054 (1968). 73. F. H.Jardine and G. Wilkinson, J. Chem. Soc. (C), 270 (1967). 74. D. D. Lehman, D. F. Shriver, and I. Wharf, J. Chem. Soc Chem. Comm., 1486 (1970). 75. C. A. Tolman, P. Z. Meakin, D. L. Linder, and J. P. Jesson, J. Am. Chem. Soc, 94, 2762 (1974). 76. B. R. James and G. L. Rempel, Discuss. Faraday Soc, 46, 48 (1968). 77. J. Hjortkjaer, Adv. Chem. Ser., 132, 133 (1974). 78. C. K. Brown and G. Wilkinson, J. Chem. Soc. (A), 2753 (1970) and references therein. 79. R. E. Hermon, S. K. Gupta and D. J. Brown, Chem. Rev., 1, 21 (1973). 80. G. Dolcetti and N. W. Hoffman, Inorg. Chim. Acta, 9_, 269 (1974). 81. W. 0. Haag and D. D. Whitehurst, Belg. Pat., 721, 686 (1969). 82. R. H. Grubbs, L. C. Kroll, J. Am. Chem. Soc, 93, 3062 (1971). 83. R. H. Grubbs, C. Gibbons, L. C. Kroll, W. C. Bonds, and C. H. Brubaker, J. Am. Chem. Soc, 95_, 2373 (1973). 84. C. U. Pittman, Jr., and G. 0. Evans, Chem. Tech., 560 (1973). 85. J. Manassen, D. D. Whitehurst, J. C. Bailar, Jr., S. Carra, M. Graziani, K. Mosbach, R. C. Pitkethly, E. K. Pye, and J. J. Rooney, in "Catalysis Progress in Research" (F. Basolo and R. L. Burwell, Jr., eds.), Plenum, New York and London, 1973, p.177. 86. J. C. Bailar, Jr., Catal. Rev., 10, 17 (1974). -189-87. Z. M. Michalska and D. E. Webster, Plat. Met. Rev. , 18, 65 (1974). 88. D. Commereuc and G. Martino, Rev. Int. Fr. Pet., 30, 89 (1975). 89. A. L. Robinson, Science, 194, 1261 (1976). 90. Yu. I. Ermakov, Catal. Rev., 13, 77 (1976). 91. E. Bayer and V. Schurig, Chem. Tech., 212 (1976). 92. N. Takaishi, H. Imai, C. A. Bertelo, and J. K. S t i l l e , J. Am. Chem. Soc, 98^ , 5400 (1976). 93. C. U. Pittman, Jr., and R. M. Hanes, J. Am. Chem. Soc, £8, 5402 (1976). 94. R. H. Grubbs, E. M. Sweet, and S. Phisanbut, in "Catalysis in Organic Syntheses 1976" (P. N. Rylander and H. Greenfield, eds.), Academic Press, New York, 1976, p.153. 95. C. U. Pittman, Jr., S. Jacobson, L. R. Smith, W. Clements, and H. Hiramoto, in "Catalysis in Organic Syntheses 1976" (P. N. Rylander and H. Greenfield, eds.), Academic Press, New York, 1976, p.161. 96. J. I. Crowley and H. Rapoport, Acc. Chem. Res., 9_, 135 (1976). 97. F. R. Hartley and P. N. Vezey, Adv. Organomet/. Chem., 15, 189 (1977). 98. M. S. Scurrell, Plat. Met. Rev., 21, 92 (1977). 99. C. U. Pittman, Jr., A. Hirao, C. Jones, R. M. Hanes, and Q. Ng, Ann. N. Y. Acad. Sci., 295_, 15 (1977). 100. J. Falbe, "Carbon Monoxide in Organic Synthesis,"Springer-Verlag, Berlin, 1970; R. Kummer, H. J. Nienburg, H. Hohenschutz, and M. Strohmeyer, Adv. Chem. Ser., 132, 19 (1974). 101. F.'J. Smith, Plat. Met. Rev., 19, 93 (1975); Chem.& Eng. News, April 26, 1976, p.25. -190-102. F. E. Paulik, Catal. Rev., 6., 49 (1972). 103. M. Orchin and W. Pupiluis, Catal. Rev. , 6_, 85 (1972). 104. L. Marko, in "Aspects of Homogeneous Catalysis," Vol. 2, (R. Ugo, ed.), Reidel, Dordrecht, 1974, p.3. 105. M. Tanaka, Y. Watanabe, T. Mitsudo, and Y. Takegami, Bull. Chem. Soc. Jap. 47, 1698 (1974). 106. F. Piacenti, M. Bianchi, and P. Frediani, Adv. Chem. Ser., 132, 283 (1974). 107. P. Pino, G. Consiglio, C. Botteghi, and C. Salomon, Adv. Chem. Ser., 132, 295 (1974). 108. P. Pino, F. Piancenti, and M. Bianchi, in "Organic Synthesis via Metal Carbonyls" (I. Wender and P. Pino, eds.), Vol. 2, Wiley, New York, 1977, p.43. 109. R. L. Pruett, Adv. Organomet. Chem., r7, 1(1979). 110. T. Nishiguchi and K. Fukuzumi, J. Am. Chem. Soc, 96., 1893 (1974). 111. T. Nishiguchi, K. Tachi, and K. Fukuzumi, J. Org. Chem., 40, 237, 240 (1975). 112. C. Masters,.A. A. Kiffen, and J. P. Visser, J. Am. Chem. Soc, 98, 1357 (1976). 113. H. Imai, T. Nishiguchi, and K. Fukuzumi, J. Org. Chem., 39, 1622 (1974). 114. Y. Sasson, P. Albin, and J. Blum, Tetrahedron Lett., 833 (1974). 115. Y. Sasson and J. Blum, J. Org. Chem., 40, 1887 (1975). 116. G. Des cotes and D. Sinou, Tetrahedron Lett., 4083 (1976). 117. H. Imai, T. Nishiguchi, and K. Fukuzumi, Chem. Lett., 807 (1975). -191-118. H. Imai, T. Nishiguchi, M. Kobayashi, and K. Fukuzumi, Bull. Chem. Soc. Jap. 48, 1585 (1975). 119. H. Imai, T. Nishiguchi, and K. Fukuzumi, J. Org. Chem., 4J_, 665, 2688 (1976). 120. Y. M. Y. Haddad, H. B. Henbest, J. Husbands, T. R. B. Mitchell, and J. Trocha-Grimshaw, J. Chem. Soc. Perkin(I), 596 (1974). 121. B. R. James and R. H. Morris, J. Chem. Soc. Chem. Comm., 929(1978). 122. L. K. Freidlin, Y. A. Kopyttsev, N. M. Nazarova, B. L. Lebedev, and B. L. Khusid, Izv. Akad. Nauk. S.S.S.R. Ser. Khim., 1325 (1974) 123. G. Briegen and T. J. Nestrick, Chem. Rev., 567 (1974). 124. I. S. Kolomnikov, V. P. Kukolev, and M. E. Volpin, Russ. Chem. Rev., 43, 399 (1974). 125. K. Ohkubo, T. Aoji, K. Hirata,and K. Yoshinaga, Inorg. Nucl. Chem. Lett., 12, 837 (1976) . 126. K. Ohkubo, K. Hirati, K. Yoshinaga, and M. Okada, Chem. Lett., 183, 577 (1976). 127. K. Ohkubo, T. Shoji, I. Terada, and K. Yoshinaga, Inorg. Nucl. Chem. Lett., 13, 443 (1977). 128. K. Ohkubo, T. Ohgushi, T. Kusaga, and K. Yoshinaga, Inorg. Nucl. Chem. Lett., 13, 631 (1977). 129. K. Ohkubo, T. Ohgushi, and K. Yoshinaga, Chem. Lett., 775 (1976). 130. K. Ohkubo, K. Sugahara, I. Terada, and K. Yoshinaga, Inorg. Nucl. Chem. Lett., 14, 297 (1978). 131. L. Horner and D. Degner, Tetrahedron Lett., 5889 (1968). 132. L. Horner, H. Blithe, and H. Siegel, Tetrahedron Lett., 4023 (1968). 133. L. Horner, H. Siegel, and H. Biithe, Angew. Chem. Int. Edit., ]_, 942 (1968). -192-134. W. S. Knowles and M. J. Sabacky, Chem. Comm., 1445 (1968). 135. I. Jardine and F. J. McQuillin, Chem.- Comm., 477 (1969). 136. P. Abley and F. J. McQuillin, J. Chem. Soc. (C), 844 (1971). 137. J. D. Morrison, R. E. Burnett, A. M. Aguiar, C. J. Morrow, and C. Phil l i p s , J. Am. Chem. Soc, 93, 1301 (1971). 138. W. S. Knowles, M. J. Sabacky, and B. D. Vineyard, J. Chem. Soc Chem. Comm., 10 (1972); Chem.Tech., 590 (1972). 139. W. S. Knowles, M. J. Sabacky, B. D. Vineyard, and D. J. Weinkauff, J. Am. Chem. Soc. , 97^ , 2567 (1975). 140. T. Hayashi, T. Mise, S. Mitachi, K. Yamamoto, and M. Kumada, Tetrahedron Lett., 1133 (1976). 141. W. Beck and H. Menzel, J. Organomet. Chem., 133, 307 (1977); A. M. Aguiar, C. J. Morrow, J. D. Morrison, R. E. Burnett, W. F. Masler, and N. S. Bhacca, J. Org. Chem., 41, 1545 (1976). 142. R. Glaser, J. Blumenfeld, and M. Twaik, Tetrahedron Lett., 4639 (1977); R. Glaser, S. Geresh, J. Blumenfeld, and M. Twaik, Tetrahedron, 34, 2405 (1978). 143. C. Fisher and H. S. Mosher, Tetrahedron Lett., 2487 (1977). 144. K. Achiwa, J. Am. Chem. Soc., 98, 8265 (1976). 145. R. H. Grubbs and R. A. Devries, Tetrahedron Lett., 1879 (1977). 146. T. Hayashi, M. Tanaka, and I. Ogata, Tetrahedron Lett., 295 (1977). 147. M. Tanaka and I. Ogata, J. Chem. Soc Chem. Comm., 735 (1975). 148. M. Fi o r i n i , G. M. Giongo, F. Marcati, and W. Marconi, J. Moi. Catal. 1, 451 (1976); G. Pracejus and H. Pracejus, Tetrahedron Lett., 3497 (1977). 149. W. R. Cullen and Y. Sugi, Tetrahedron Lett., 1635 (1978). -193-150. J. Halpern, D. P. Riley, A. S. C. Chan, and J. J. Pluth, J. Am. Chem. Soc, 99, 8055 (1977). 151. R. Glaser, Tetrahedron Lett., 2127 (1975). 152. R. Glaser and B. Vainas, J. Organomet. Chem., 121, 249 (1976). 153. R. Glaser and J. Blumenfeld, Tetrahedron Lett., 2525 (1977). 154. R. Glaser, S. Geresh, J. Blumenfeld, B. Vainas, and M. Twaik, Isr. J. Chem., 15, 17 (1977). 155. R. Glaser and S. Geresh, Tetrahedron Lett., 2527 (1977). 156. S. Brunie, J. Mazan, N. Langlois, and H. B. Kagan, J. Organomet. Chem., 114, 225 (1976). 157. J. K. Beattie, Ace. Chem. Res., 4-, 253 (1971). 158. R. G. Ball and N. C. Payne, Inorg. Chem., 16, 1187 (1977). 159. K. E. Koenig and W. S. Knowles, J. Am. Chem. Soc, 100, 7561 (1978). 160. T. Hayashi, T. Mise, and M. Kumada, Tetrahedron Lett., 4351 (1976) and references therein. 161. I. Ojima, T. Kogure, M. Kumagai, and K. Achiwa, 26th Int. Congr. Pure Appl. Chem., Tokyo, 1977, Abstract 9F51. 162. I. Ojima, T. Kogure, and K. Achiwa, J. Chem. Soc. Chem. Comm., 428 (1977). 163. K. Achiwa, Tetrahedron Lett., 3735 (1977). 164. A. Levi, G. Modena, and G. Scorrano, J. Chem. Soc. Chem. Comm., 6 (1975) and references therein. 165. I. Ojima, T. Kogure, and M. Kumagai, J. Org. Chem., 4^ 2, 1671 (1977). 166. H. B. Kagan, N. Langlois, and T. P. Dang, Tetrahedron Lett., 4865 (1973). 167. H. Boucher and B. Bosnich, J. Am. Chem. Soc, 99, 6253 (1977). -194-168. J. Ashley-Smith, Z. Douek, B. F. G. Johnson, and J. Lewis, J. Chem. Soc. Dalton Trans., 128 (1974); 1776 (1972); C. E. Holloway, G. Hulley, B. F. G. Johnson, and J. Lewis, J. Chem. Soc. (A), 1653 (1970); 53 (1969). CHAPTER II 1. D. F. Shriver, "Manipulation of Air Sensitive Compounds," McGraw H i l l , N.Y., 1969. 2. R. Rigo, M. Bressan, and A. Turco, Inorg. Chem., _7, 1460 (1968). 3. G. R. Ames and W. Davey, J. Chem. Soc, 1794 (1958). CHAPTER III 1. H. B. Kagan, Pure Appl. Chem., 43, 401 (1975). 2. J. D. Morrison, W. F. Masler, and M. K. Newberg, Adv. Catal.,25, 81 (1976); J. D. Morrison, W. F. Masler, and S. Hathaway in "Catalysis in Organic Syntheses" (P. N. Rylander and H. Greenfield, eds.), Academic Press, New York, 1976, p.203. 3. B. R. James, Adv. Organomet. Chem., 17, 319(1979.) . 4. B. Bosnich and M. D. Fryzuk, 172nd Am. Chem. Soc. Meeting, San Francisco, 1976, Abstract 108; M. D. Fryzuk and B. Bosnich, J. Am. Chem. Soc, 99, 6262 (1977). 5. T. P. Dang and H. B. Kagan, J. Am. Chem. Soc, 94, 6429 (1972). 6. W. Dumont, J. C. Poulin, T. P. Dang, and H. B. Kagan, J. Am. Chem. Soc, 95, 8295 (1973). -195-7. P. Bonvicini, A. Levi, G. Modena, and G. Scorrano, J. Chem. Soc. Chem. Comm., 1188 (1972). 8. W. S. Knowles, M. J. Sabacky, and B. D. Vineyard, Adv. Chem. Ser., 132, 274 (1974). 9. Y. Chauvin, D. Commereuc, and R. Stern, J. Organomet. Chem., 146, 311 (1978). 10. W. R. Cullen, A. Fenster, and B. R. James, Inorg. Nucl. Chem. Lett., 10, 167 (1974); A. Fenster, B. R. James, and W. R. Cullen, Inorg. Syn., 17, 81 (1977). 11. Y. Chevailier, Docteur-Ingenieur Thesis, Paris, 1970; through Ref.5. 12. D. Mahajan, M.Sc. Dissertation, U.B.C, Vancouver, B.C., 1976. 13. B. R. James and D. Mahajan, Isr. J. Chem.,15, 214 (1977). 14. R. R. Schrock and J. A. Osborn, J. Am. Chem. Soc. , '.93, 2397 (1971); ibid, 98, 2134 (1976). 15. D. K. W. Wang, Ph.D. Dissertation, U.B.C, Vancouver, B.C., 1978. 16. A. Sanger, J. Chem. Soc. Dalton Trans., 120 (1977). CHAPTER IV •1. L. Vaska and D. L. Catone, J. Am. Chem. Soc, 88, 5324 (1966). 2. K. A. Taylor, Adv. Chem., 70, 195 (1968). 3. J. S. Miller and K. G. Caulton, J. Am. Chem. Soc, 97, 1067 (1975). 4. M. J. Hopkinson and J. F. Nixon, J. Organomet. Chem., 148, 201 (1978). 5. A. Sacco and R. Ugo, J. Chem. Soc, 3274 (1964). 6. J. A. McGinnety, N. C. Payne, and J. A. Ibers, J. Am. Chem. Soc, 91, 6301 (1969). -196-7. Y. Chevailier, Docteur-Ingenieur Thesis, Paris, 1970; through H. B. Kagan and T. P. Dang, J. Am. Chem. Soc, 94, 6429 (1972). 8. A. Sacco, M. Rossi, and C. F. Nobile, J. Chem. Soc. Chem. Comm., 589 (1966). 9a. M. C. Hall, B. T. Kilbourn, and K. A. Taylor, J. Chem. Soc. (A), 2539 (1970). 9b. D. H. Doughty and L. H. Pignolet, J. Am. Chem. Soc, 100, 7083 (1978). 10. W. R. Cullen, A. Fenster, and B. R. James, Inorg. Nucl. Chem. Lett., 10, 167 (1974). 11. B. R. James and D. Mahajan, Isr. J. Chem., 15_, 214 (1977). 12. J.C. Poulin, T.P. Dang, and H. B. Kagan, J. Organomet. Chem., 84, 87 (1975). 13. B. R. James, Adv. Organomet. Chem., 17,319(1979). 14. D. Sinou and H. B. Kagan, J. Organomet. Chem., 114, 325 (1976). 15. D. A. Slack and M. C. Baird, J. Organomet. Chem., 142, C69 (1977). 16. R. Brady, W. V. Miller, and L. Vaska, J. Chem. Soc. Chem. Comm., 393 (1974). 17. G. L. Geoffroy, M. S. Wrighton, G. S. Hammond, and H. B. Gray, J. Am. Chem. Soc. , 9_6, 3105 (1974). 18. A. Van der Ent. and A. L. Onderdelinden, Inorg. Syn., 14, 92 (1973). 19. A. Fenster, B. R. James, and W. R. Cullen, Inorg. Syn., 17, 81 (1977). 20. D. G. Holah, A. N. Hughes, and B. C. Hui, Can. J. Chem., 55., 4048 (1977). 21. B. R. James, R. H. Morris, and K. J. Reimer, Can. J. Chem., 55, 2353 (1977), and references therein. 22. J. D. Morrison, W. F. Masler, and M. K. Neuberg, Adv. Catal., 25, 81 (1976). -197-23. R. R. Schrock and J. A. Osborn, J. Am. Chem. Soc, 98., 2134, 2143 (1976). 24. W. S. Knowles, M. J. Sabacky, B. D. Vineyard, and D. J. Weinkauff, J. Am. Chem. Soc., 97., 2567 (1975). 25. Y. Chauvin, D. Commereuc, and R. Stern, J. Organomet. Chem., 146, 311 (1978). 26. J. Chatt and B. L. Shaw, J. Chem. Soc. (A), 1437 (1966). 27. N. Ahmad, S. D. Robinson, and M. F. Uttley, J. Chem. Soc. Dalton Trans., 843 (1972). 28. R. Brady, B. R. Flynn, G. L. Geoffroy, H. B. Gray, J. Peone, Jr., and L. Vaska, Inorg. Chem., L5, 1485 (1976). 29. W. J. Geary, Coord. Chem. Rev., 7_, 81 (1971). 30. J. F. Knifton, J. Org. Chem., 40, 519 (1975); ibid. , 41_, 1200 (1976). 31. K. C. Dewhirst, W. Keim, and C. A. Reilly, Inorg. Chem., 7_, 546 (1968) 32. J. P. Jesson, "Stereochemistry and stereochemical nonrigidity in transition metal hydrides," in E. L. Muetterties, ed. Transition metal hydrides. Marcel Dekker, New York, 1971, p.110. 33. B. A. Frenz and J. A. Ibers in "Molecular structures of transition metal hydride complexes," E. L. Muetterties, ed. Transition metal hydrides. Marcel Dekker, New York, 1971, p.47. 34. R. W. Baker and P. Pauling, J. Chem. Soc. Chem. Comm., 1495 (1969). 35. B-K. Teo, A. P. Ginsberg, and J. C. Calabrese, J. Am. Chem. Soc, 98, 3027 (1976). 36. B. A. Frenz and J. A. Ibers in "Transition Metal Hydrides", E. L. Muetterties, Ed., Marcel Dekker, New York, 1971, pp 41-44. 37. S. H. Strauss, S. E. Diamond, F. Mares, and D. F. Shriver, Inorg. Chem., 17, 3064 (1978). -198-38. J. F. Nixon and A. Pidcock, Ann. Rev. NMR Spect., 2, 345 (1969). 39. P. E. Garrou, Inorg. Chem., 14, 1435 (1975). 40. A. Sanger, J. Chem. Soc. Dalton Trans., 120 (1977). 41. J. Halpern, D. P. Riley, A.S.C. Chan, and J. J. Pluth, J. Am. Chem. Soc, 99, 8055 (1977). 42. R. R. Schrock and J. A. Osborn, J. Am. Chem. Soc, 93., 2397 (1971). 43. M. A. Bennett, R. Charles, and T. R. B. Mitchell, J. Am. Chem. Soc., 100, 2737 (1978). 44. A. P. Ginsberg and W. E. Lindsell, Inorg. Chem., JL2, 1983 (1973). 45. S. Brunie, J. Mazan, N. Langlois, and H. B. Kagan, J. Organomet. Chem., 114, 225 (1976). 46. B. D. Vineyard, W. S. Knowles, M. J. Sabacky, G. L. Backman, and D. J. Weinkauff, J. Am. Chem. Soc, 99, 5946 (1977). 47. R. G. Ball and N. C. Payne, Inorg. Chem., 16_, 1187 (1977). 48. M. D. Fryzuk and B. Bosnich, J. Am. Chem. Soc, 99^ , 6262 (1977). 49. B. R. James, Homogeneous Hydrogenation, Wiley, New York, 1973, p.269. 50. B. R. James, R. S. McMillan, R. H. Morris, and D. K. W. Wang, Adv. Chem. Ser., 167, 122 (1978). CHAPTER V 1. T. P. Dang and H. B. Kagan, Chem. Comm., 481 (1971); H. B. Kagan and T. P. Dang, J. Am. Chem. Soc, 94., 6429 (1972). 2. Y. Chauvin, D. Commereuc, and R. Stern, J. Organomet. Chem., 146, 311 (1978) and references therein. 3. B. R. James, Adv. Organomet. Chem. , 17;, 319, (1979) and references therein. -199-4. D. Sinou and H. B. Kagan, J. Organomet. Chem., 114, 325 (1976). 5. H. B. Kagan, V. Langlois, and T. P. Dang, J. Organomet. Chem., 90, 353 (1975). 6. T. P. Dang, J. C. Poulin, and H. B. Kagan, J. Organomet. Chem., 91, 105 (1975). 7. G. Gelbard, H. B. Kagan, and R. Stern, Tetrahedron, 32, 233 (1976). 8. R. Glaser, S. Geresh, and J. Blumenfeld, J. Organomet. Chem., 112, 355 (1976). 9. R. Glaser, Tetrahedron Lett., 2127 (1975). 10. R. Glaser and B. Vainas, J. Organomet. Chem., 121, 249 (1976). 11. R. Glaser and J. Blumenfeld, Tetrahedron Lett., 2525 (1977). 12. R. Glaser, S. Geresh, J. Blumenfeld, B. Vainas, and M. Twaik, Tsr. J. Chem., 15, 17 (1977). 13. R. Glaser and S. Geresh, Tetrahedron Lett., 2527 (1977). 14. W. R. Cullen, A. Fenster, and B. R. James, Inorg. Nucl. Chem. Lett., 10, 167 (1974). 15. D. Mahajan, M.Sc. Dissertation, U.B.C, Vancouver, B.C., 1976; B. R. James and D. Mahajan, Isr. J. Chem., 15_, 214 (1977). 16. J. A. McCleverty and G. Wilkinson, Inorg. Syn., 8_, 211 (1966). 17. A. Sanger, J. Chem.Soc. Dalton Trans., 120 (1977). 18. R. Rossi and P. Diversi, Gazz. Chim. I t a l . , 98, 1391 (1968). 19. S. M. Birbaum, L.Levintow, R. B. Kingsley, and J. P. Greenstein, J. Biol. Chem., 194, 455 (1952). 20. J. P. Bakhshi and E. E. Turner, J. Chem. Soc, 171 (1961). 21. W. S. Knowles, M. J. Sabacky, and B. D. Vineyard, Adv. Chem. Ser., 132, 274 (1974). 22. B. D. Vineyard, W. S. Knowles, M. J. Sabacky, G. L. Bachman, and D. J. Weinkauff, J. Am. Chem. Soc, 99, 5946 (1977). -200-23. J. M. Brown and P. A. Chaloner, J. Chem. Soc. Chem. Comm., 321 (1978). 24. J. Halpern, Symposium on "Rhodium in Homogeneous Catalysis", Veszprem, Hungary, 1978, p.30. 25. R. Glaser, J. Blumenfeld, and M. Twaik, Tetrahedron Lett., 4639 (1977). 26. R. Glaser, S. Geresh, J. Blumenfeld, and M. Twaik, Tetrahedron, 34, 2405 (1978). 27. A. Loser, Dissertation, Universitat Bochum, 1973, through reference: B. Bogdanovic, Angew. Chem. Int. Edit., 12, 954 (1973). 28. J. Hjortkjaer, Adv. Chem. Ser., 132, 133 (1974) and references therein. 29. M. D. Fryzuk and B. Bosnich, J. Am. Chem. Soc, 99, 6262 (1977). 30. W. R. Cullen and E.-S. Yeh, J. Organomet. Chem., 139, C13 (1977). 31. W. S. Knowles, M. J. Sabacky, and B. D. Vineyard, J. Chem. Soc. Chem. Comm., 10 (1972). CHAPTER VI 1. D. Mahajan, M.Sc Dissertation, U.B.C, Vancouver, B.C., 1976; B. R. James and D. Mahajan, Isr. J. Chem., L5, 214 (1977). 2. D. Sinou and H. B. Kagan, J. Organomet. Chem., 114, 325 (1976). 3. R. R. Schrock and J. A. Osborn, J. Am. Chem. Soc, 93, 2397 (1971) 4. D. A. Slack and M. C. Baird, J. Organomet. Chem., 142, C69 (1977). 5. R. R. Schrock and J. A. Osborn, J. Am. Chem. Soc, 9J3, 2134, 2143 (1976). 6. A. R. Sanger, J. Chem. Soc. Dalton Trans., 120 (1977). 7. J. Halpern, D. P. Riley, A. S. C. Chan, and J. J. Pluth, J. Am. Chem. Soc, 99, 8055 (1977). 8. J. M. Brown, P. A. Chaloner, and P. N. Nicholson, J. Chem. Soc. Chem. Comm., 646 (1978). 9. J. Hjortkjaer, Adv. Chem. Ser., 132, 133 (1974). -201-CHAPT-ER VII 1. S. D. Ibekwe and K. A. Taylor, J. Chem. Soc. (A), 1(1970). 2. G. Rouschias and W. Bedford, J. Chem. Soc. Dalton Trans., 2531 (1974). 3. J. S. Miller and K. G. Caulton. J. Am. Chem. Soc. , 97_, 1067 (1975). 4. M. J. Hopkinson and J. F. Nixon, J. Organomet. Chem., 148, 201 (1978). 5. J. A. J. Jarvis, R. H. B. Mais, P. G. Owston, and K. A. Taylor, Chem. Comm., 906 (1966). 6. J. A. McGinnety, N. C. Payne, and,J. A. Ib ers, J. Am. Chem. Soc., 91, 6301 (1969). 7. F. Pruchnik, Inorg. Nucl. Chem. Lett., 9_, 1229 (1973). 8. F. Pruchnik, Inorg. Nucl. Chem. Lett., 10, 661 (1974). 9. D. H. Doughty and L. H. Pignolet, J. Am. Chem. Soc, 100, 7083 (1978). 10. L. Vaska and D. L. Catone,.J. Am. Chem. Soc, 88, 5324 (1966). 11. L. Vaska, L. S. Chen, and W. V. Miller, J. Am. Chem. Soc, 93, 6671 (1971). 12. A. R. Sanger, J. Chem. Soc. Dalton Trans., 120 (1977). 13. D. A. Slack and M.. C. Baird, J. Organomet. Chem., 142, C69 (1977). 14. D. Sinuo and H. B. Kagan, J. Organomet. Chem., 114, 325 (1976). 15. R. L. Augustine, R. J. Pellet, J. F. Van Peppen, and J. P. Mayer, Adv. Chem. Ser., 132, 111 (1974) and references therein; J. '.Lyons, Aspects of Homogeneous Catalysis, Vol. 3, 1 (1977). 16. B. R. James and D. Mahajan, Can. J. Chem., 57, 180 (1979). 17. W. J. Geary, Coord. Chem. Rev., 7_, 81 (1971). 18. F. A. Cotton, K.I. Hardcastle, and G. A.Rusholme, J. Coord. Chem., 2, 217 (1973). 19. M. Akhtar, P. D. E l l i s , A. G. MacDiarmid, and J. D. Odom, Inorg. Chem., 11, 2917 (1972). -202-20. S. H. Strauss, S. E. Diamond, F. Mares, and D. F. Shriver, Inorg. Chem., 17, 3064 (1978). 21. J. P. Jesson and P. Meakin, J. Am. Chem. Soc, 96, 5760 (1974). 22. A. Sacco, M. Rossi, and C. F. Nobile, Chem. Comm., 589 (1966). 23. M. C. Hall, B. T. Kilbourn, and K. A. Taylor, J. Chem. Soc. (A), 2539 (1970). 24. R. T. Morrison and R. N. Boyd, "Organic Chemistry," Allyn and Bacon Inc., Boston, 1959, 1st Ed., p.635. 25. R. S. McMillan, Ph.D. Dissertation, U.B.C, Vancouver, B.C., 1976. 26. R. R. Schrock and J. A. Osborn, J. Am. Chem. Soc. , 93, 2397 (1971). 27. F. N. Tebbe, P. Meakin, J. P. Jesson, and E. L. Muetterties, J. Am. Chem. Soc, 92, 1068 (1970). 28. P. Meakin, L. J. Guggenberger, J. P. Jesson, D. H. Gerlach, F. N. Tebbe, W. G. Peet, and E. L. Muetterties, J. Am. Chem. Soc, ^2, 3482 (1970). 29. J. Halpern, D. P. Riley, A.S.C Chan, and J. J. Pluth, J. Am. Chem. Soc, 99, 8055 (1977) and references therein. 30. D. K. W. Wang, Ph.D. dissertation, U.B.C, Vancouver, B.C., 1978. 31. R. C. Ball, B. R. James, J. Trotter, D. K. W. Wang, and K. R. Dixon, J. Chem. Soc. Chem. Comm., 000 (1979). 32. Internat. C r i t i c a l Tables, Vol. I l l , 262 (1928). 

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