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Asymmetric homogeneous hydrogenation of olefinic compounds using a rhodium catalyst with chiral phosphine… Mahajan, Devinder 1976

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'ASYMMETRIC HOMOGENEOUS HYDROGENATION OF OLEFINTC COMPOUNDS USING A RHODIUM CATALYST WITH CHIRAL PHOSPHINE LIGANDS by DEVINDEP. MAHAJAN B.Sc. (Honour School), Panjab University (INDIA), 1972 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of CHEMISTRY We accept this thesis as conforming to the required standard THE UNIVERSITY OF 3RITISH COLUH3IA November, 1976 (^\ Devinder Mahajan, 1977 In 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 u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced d e g r e e at the 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 make 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 and 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 may be g r a n t e d by the Head o f my Depar tment o r by 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 not be 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 . Depar tment o f CHEMISTRY The 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 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date ^> W ; m7 . 4 i i A B S T R A C T The hydridorhodium(l)phosphine complex HRhCdiop^ | j +)-diop = (+)-213i-0-isopropylidene-2,3-dihydroxy-l,^-bis(diphenylphosphino)butane] has been studied for catalytic asymmetric hydrogenation of a number of o l e f i n i c substrates. Certain o l e f i n i c carboxylic acids have been reduced to products with up to about 60^  enantiomeric excess. Some rate data show the importance of the degree of substitution at the o l e f i n i c bond i n terms of steric effects and the -COOH function of the substrate also plays a role. Kinetic and spectrophotometry studies on the hydrogenation of itaconic acid i n n-butanol-toluene (2:1) solutions are best interpreted i n terms of a mechanism involving formation of a metal alkyl via coordination of the o l e f i n i c substrate, followed by reaction with Hg to give saturated product and the starting catalyst complex. The actual catalyst i s thought to be HRh(diop)(diop*) where diop* represents a monodentate diop with one dangling -Cr^PPhg moiety. The catalyst systems are somewhat complicated by the occurrence of a number of slower side-reactions, the extent of these varying from negligible for the more rapidly reducing substrates, to possibly of prime importance i n the slow hydrogenations. The i n i t i a l hydride catalyst, for example, i s slowly decomposed to the cationic^F.h(diop)^ f complex by protons from the ole f i n i c substrate. Under H£ the cation reversibly forms a dihydride. The cation i s also formed from a reaction that appears to involve an intermediate alkyl complex and the butanol. i i i TABLE Or CONTENTS Page ABSTRACT 1 A LIST OF TABLES v LIST OF FIGURES v l ABBREVIATIONS v i i i ACKNOTJLEDGMENTS ^ CHAPTER I. INTRODUCTION 1 1.1 Modes of hydrogen activation. 1 1.2 Homogeneous hydrogenation using rhodium complexes. 3 1.3 Asymmetric hydrogenation. 10 1. k Asymmetric hydrogenation of ketones via hydrosilylation. 16 1.5 Hydroformylation. 16 1.6 Aim of the present work. 18 CHAPTER II. EXPERIMENTAL " x9 2.1 Preparation of HRhCdiop^ complex. 19 2.2 Purification of solvents. 19 2.3 Hydrogenation apparatus. 20 2. k Experimental procedure for a typical gas-uptake 20 experiment. 2.5 Solubility determination. 22 2.6 Isolation of reaction products. 23 2.7 Optical rotation measurements. 23 2.8 Spectroscopic work. 2k i v Page CHAPTER III. RESULTS 25 3.1 General. 25 3.2 Kinetics. 2 7 3.3 Effect of added (+)-diop. 32 3.^  Homogeneous hydroe;enation of other substrates. 32 3.5 Spectrophotometrie studies. kk 3.6 P N'.K.R. measurements. 5^  CHAPTER IV. DISCUSSION. 57 k,l General. . 5 7 k.2 Kinetics and Mechanism. 59 •^3 Spectral data and the catalytic hydrogenation mechanism. 68 k.k Decomposition of the HRh(diop) 2 complex. 69 CHAPTER V. GENERAL CONCLUSIONS. . 7k REFERENCES. 76 V LIST OF TA3LS3 Page I Asymmetric homogeneous hydrogenation of o(,^ unsaturated 26 earboxylic acids using H D.h(diop)2 as c a t a l y s t . I I Homogeneous hydrogenation of various 'oi, $ unsaturated 28 earboxylic acids using '-PhCdiop^ as c a t a l y s t . HRh(diop) 2 catalysed hydrogenation of IA i n n-butanol-toluene (2:1). I I I V a r i a t i o n of maximum rate with JHRh(diop)^]. 33 IV V a r i a t i o n of maximum rate with ^ I A J . 35 37 V V a r i a t i o n of maximum rate with PJJ * VI V a r i a t i o n of maximum rate with change i n temperature. 39 VII E f f e c t of added (+)-diop. 40 VTII Homogeneous hydrogenation of various non-acidic ^3 substrates. v i LIST 0? FIGURES Page 1. Apparatus for constant pressure gas-uptake measurements. 21 2. Rate plots for the hydrogenation of itaconic acid using 29 HRh(diop)2 a s catalyst i n n-butanol-toluene(2:l) at 30°. 3. Solubility of hydrogen i n n-butanol-toluene(2:l) at 30 various pressures at 30°. 4. Solubility of hydrogen i n n-butanol-toluene(2:l) at 31 various temperatures. HRh(diop)2 catalysed hydrogenation of itaconic acid i n n-butanol-toluene(2:l) at 30°. 5. Plot of maximum rate against JjP.hCdiop)^ . 34 6. Dependence of maximum rate on itaconic acid concentration 36 7. Dependence of maximum rate on hydrogen pressure. 38 8. Rate plots for the reaction i n the presence of added (-t-)-diop. 41 9. Plot of induction period versus added (+)-diop concentration. 42 10. Hydrogenation of diethyl itaconate using HRhCdiop^ i n n-butanol-toluene(2:l) at 60°. 45 11. Absorption spectra of HRhCdiop^ i n n-butanol-toluene(2:l) 46 at 30+1°. 12. Absorption spectra of HRhydiop^ i n n-butanol-toluene(2:l) Wy i n the presence of itaconic acid at 30 + 1 ° . 13. Plot of logCA^-A^) versus time for spectral changes i n 48 Figure 12 . 14. Absorption spectra of HT'h(diop)? i n n-butanol-toluene(2:1) 50 i n the presence of succinic acid at 30^1°• v i i Page 15. Hydrogenation of i t a c o n i c a c i d using HRhCdiop^ as c a t a l y s t 51 followed spectrophotometrieally at 30+1°. 16. P l o t o f l o g (A^-A^) versus time f o r s p e c t r a l changes i n 52 Figure 15 . 17. Absorption spectra of HRhCdiop)^ i n n-butanol-toluene ( 2 t l ) 53 i n the presence of d i e t h y l maleate at 30+1°• 18. Hydrogenation of d i e t h y l maleate using HRhCdiop^ as c a t a l y s t 55 followed spectrophotometrically at 30*1°• 31 6 19. P proton decoupled NJyLR. spectrum of HRh(diop) 2 i n benzene-d 56 at room temperature. 20. Arrhenius p l o t f o r the HRh(diop)2 catalysed hydrogenation of 60 i t a c o n i c a c i d . r - f 1 - 1 21. P l o t of I 1A I versus maximum ra t e . 6k 22. P l o t of I Hg I versus maximum rate. 67 23. Absorption spectra of HRhCdiop^ i n toluene i n the presence ?2 of d i e t h y l maleate at 30+1°. The following l i s t of abbreviations, most of which are commonly adopted i n chemical research l i t e r a t u r e , w i l l be employed i n t h i s t hesis A l l temperatures are i n °C unless s p e c i f i c a l l y denoted by °K. (+)-diop or jj!"0-25, 3S-0-isopropylidene-2,3-dihydroxy-l, *4-bis(diphenylphos phinojbutane^) diop* dangling diop' DKA N-N-dimethylacetamide PPh-j triphenylphosphine L lig a n d TH? tetrahydrofuran N.M.P.. nuclear magnetic resonance Me methyl Et e t h y l Ph phenyl COD 1,5 cyclooctadiene (*) c h i r a l carbon atom n-butanol-toluene(2/1 = v/v) H2C = C(P.')C02R i t a c o n i c a c i d (R = Hj R = CHg.COgH) d i e t h y l itaconate (R = C2H^; YL = CHg.CC^CV N-acetamidoacrylie acid (R = H; R' = NH.COCH3) at r o p i c acid (R = H, R' = Ph) su c c i n i c acid H0 2C. C?.2-C\. COjH P. 02C.CH = CH.COgR ( c i s ) maleic acid P. = r\ diethylmaleate R = C 2H^ Ph R 2 ^ C = R 1 ^ C0 2rl o(-methyl cinnamic acid (R 1 = H; R 2 = CH^) P -methylcinnamic acid e i t r a c o n i c a c i d mesaconic acid acrylamide 0^-methyl styrene ^-methoxy styrene e t h y l methyl ketone Hex-l-ene Non-l-er.e • -1 frequency, cm chemical s h i f t , PPM (R 1 = M 3 ; P.2 = H) H R 3 H 0 2 C / X P ^ (R 3 = CH-j; R^ = OOgH) (R 3 - C0 2H; R^ = CH 3) H 2C = CHCONHg H2C = C(R 5)Ph R5 = CH 3 P.5 = OCH3 Et-C-Me . 11 0 H,C-4CH9}-CH=CH -> ^ n 2 (n = 3) (n = 6 ) infrared molar extinction coefficient atmosphere maximum concentration i n %/l00 ml molar x i ACKNOV7LED3MENTS' I wish to thank Dr. 3. R . James f o r h i s valuable suggestions and encouragement throughout the course of t h i s research. I also wish to thank a l l my colleagues and friends f o r t h e i r help i n the preparation of t h i s t h e s i s . My thanks to Deanna Carter f o r typing t h i s t h e s i s . INTRODUCTION There i s no doubt about the in c r e a s i n g study and use o f homogeneous c a t a l y s i s i n chemistry both i n terms o f pure research and i t s subsequent a D o l i c a t i o n . Well established homogeneous i n d u s t r i a l l y - u s e d processes 1 2 3 include the Wacker process, the Oxo orocess and some Ziegler-Natta Systems. k The carbonylation of methanol to a c e t i c acid, and the production of o p t i c a l l y 5 a c t i v e amino acids using a hydrogenation c a t a l y s t , are more r e c e n t l y established i n d u s t r i a l processes both using homogeneous systems. Many reactions such as homogeneous hydrogenation, carbonylation, hydroformylation, etc., have been studied i n d e t a i l and t h e i r chemistry has been developed s i g n i f i c a n t l y over the past 15 years or so to the point where the processes are reasonably well understood. This t h e s i s i s concerned with homogeneous hydrogenation using a rhodium complex containing an o p t i c a l l y a c t i v e phosphine l i g a n d . Thus hydro-genation of s u i t a b l e o l e f i n i c substrates can lead to o p t i c a l l y a c t i v e products e.g., the process shown i n equation (1) H H « ; c 6 H ^ 6 / \ r , "2 V CI) C = CrI 2 ± > > \ , / . Ca t a l y s t HOoC CH, H02C ' ^ - ^ J 0 0 nu 2u v>n.j v i a a c a t a l y t i c process. L i t t l e k i n e t i c data are a v a i l a b l e f o r such asymmetric syntheses and t h i s study was conducted to l e a r n more about these extremely i n t e r e s t i n g and s i g n i f i c a n t processes. 1.1 Modes o f hydrogen a c t i v a t i o n -6-9 From e a r l i e r r e s u l t s only t r a n s i t i o n metal ions that possess e l e c t r o n • 5 10 c o n f i g u r a t i o n i n the d - d range were thought to ac t i v a t e molecular hydrogen, though t h i s i s c l e a r l y not a s u f f i c i e n t c o n d i t i o n because simple s a l t s of Mn(II), F e ( I I I ) , Co(II), N i ( I I ) are a l l i n a c t i v e even though they have 5 10 . electrons i n d - d "range. C a t a l y t i c hydrogenation a c t i v i t y a r i s e s from the l a b i l i t y and s t a b i l i t y of hydrido complexes which are presumably e s s e n t i a l 9 intermediates i n these hydrogenation re a c t i o n s . Many hydrogenation c a t a l y s t s have been complexes of group VIII metals i n lower ox i d a t i o n s t a t e s : Fe, Ru, 0s, Co, Rh, I r , Ni, Pd, and Pt. E s p e c i a l l y e f f e c t i v e combinations have 8 r e s u l t e d from d spin-paired (low spin) configurations a r i s i n g from strong l i g a n d f i e l d s or e l e c t r o n d e l o c a l i z i n g (jT -acceptor) ligands. In general, homogeneous hydrogenation o f an o l e f i n i c compound involves three i n t e r r e l a t e d stages:-(a) The a c t i v a t i o n or s p l i t t i n g of molecular hydrogen, (b) then a c t i v a t i o n o f the. Substrate, (c) followed by t r a n s f e r o f a c t i v a t e d hydrogen to the a c t i v a t e d substrate. The hydrogen a c t i v a t i o n process has been observed to happen v i a three mechanisms -10 F i r s t i s h e t e r o l y t i c s p l i t t i n g which involves a net s u b s t i t u t i o n o f a hydride f o r another ligand, with no change i n the ox i d a t i o n state o f the metal; an important f a c t o r may be the nature o f the base present to s t a b i l i z e the 11 released proton (equations 2 and 3 ) . + ML + H 9 ZT~^ ML ,H + H + L (2) n 2 -= n-1 I I I 3- i n 3- + -Ru Clg + H 2^=^H Ru C l 5 + H + C l (3) An o l e f i n molecule co-ordinated to the metal at some stage may then undergo i n s e r t i o n i n t o the K-H bond y i e l d i n g a <r - a l k y l complex. E l e c t r o -p h i l i c attack by a uroton at the cabon attached to the metal can then release 12 the alkane (Scheme I ) . Jfl _ -^X — / 1 / \ I + ^  ai^Y 1 complex _ . R u _ + - C - C -1 R u — / \ C / | Scheme I The second mechanism involves homolytic sp l i t t i n g of hydrogen which i s accompanied by an increase i n the oxidation state and generally the co-13 ordination number of the metal by one (equation k). The reaction, therefore, depends on the susceptibility of the metal to oxidation and the a b i l i t y to Ik expand i t s co-ordination shell. An example i s shown i n equation 5» 2 MLn + H 2 - 2 ML n - 1H + 2L (k) 2^Co ( C N ) J 3 ~ + H 2-^^2JHCo (CN)^j 3~ (5) Olefin hydrogenation with this catalyst can occur when an alkyl complex formed via o l e f i n insertion reacts subsequently with further hydride 15 complex to produce the saturated product (equation 6); £(NC) 5 Co - a l k y l ] 3 " + [HCo(CN)^" • > 2'Jco(CN)^ + alkane (6) The third p o s s i b i l i t y of hydrogen activation involves oxidative addition of hydrogen to the metal which increases the oxidation state and the 16 co-ordination number by two (equations 7 and 8): M - n * ^ ^ n H 2 t t t <?) I 1 1 1 I r Cl(CO)(PPh3) 2 + H 2 3==^slr (H 2) CL CO(PPh 3) 2 (8) An o l e f i n co-ordinated to the dihydride may be reduced by consecutive transfer 17 of the two hydrogen atoms via a g~- alkyl hydride intermediate . 1,2 Homogeneous hydrogenation using rhodium complexes -Though hydrogen reduction of rhodium trichloride solutions to metal, and reduction of inorganic (e.g. NO •. > NH ) and organic substrates catalyzed by 3 " 18,19 rhodium (III) ammine complexes^were known even back i n 1935 , i t was not 6-9 u n t i l 1956 that Halpern and co-workers i n v e s t i g a t e d the a c t i v a t i o n of molecular I I I hydrogen by various metal ions i n aqueous solutions and found out that Rh and other ions such as Cu(II), H g ( I l ) , C u ( l ) , Ag(l) and Hg(I) were, behaving as 20 homogeneous c a t a l y s t s . Thus hydrogen reduction of F e ( I I l ) was achieved using aqueous a c i d c h l o r i d e solutions o f chlororhodate(III) species, and these 21,22 and l a t e r studies on t h i s system l e d to the following mechanism i n which h e t e r o l y t i c s p l i t t i n g of hydrogen was proposed. ' 3- ' k 3- + ' RhClg + H 2 - H R h C l 5 + H + CI <9) 3- 3- + HRhClj + 2 FeCl3 F a s t ^ RhClg + 2 F e C l 2 + H + C l (10) Studies on the hydrogen reduction of Cu(Il) catalyzed by Cu(l) acetate i n quinoline s o l u t i o n had shown, the system to be homogeneous i n which homolytic 23 s p l i t t i n g of hydrogen was the rate determining step: I k' I I -2 Cu + H ? „ 2 Cu H (11) I I - I I Fast I + Cu H + Cu => 2 Cu + H (12) Thus i n homogeneous systems both homolytic and h e t e r o l y t i c s p l i t t i n g of hydrogen had been r e a l i s e d by I960 f o r the reduction of inorganic substrates. An attempt to hydrogenate organic substrates with chlororhodate(TTl) species i n aqueous media f a i l e d because the rhodium (I) produced by hydrogen reduction of rhodium (III) formed a stable complex with the o l e f i n (equation 13) 21 and t h i s complex showed no r e a c t i v i t y towards hydrogen. I I I -H I I I + I MA I P h + H ^ HRh ^ ~- H + Rh ^ "-.Rh (MA) (13) S u r p r i s i n g l y , a change of medium had a pronounced e f f e c t on the course of reaction, and using polar solvents l i k e N-N-dimethylacetamide (DMA) instead of Zh ac i d i c medium, hydrogenation of o l e f i n s was l a t e r achieved. O l e f i n complexes were f i r s t formed according to reaction steps (9) and (13) but further reaction i n this case resulted i n the production of saturated product and regeneration of the catalyst. Discovery of chlorotris(triphenylphosphine)rhddium (I) , RhCl(PPh3)^, :. 25,26 i n 1965 by Wilkinson s group presented new opportunities for more detailed studies i n the f i e l d of homogeneous catalysis especially hydrogenation. Mason 27 and co-workers have reported that i n the solid state the co-ordination of the 8 usually s qiare planar d rhodium(I) i s distorted toward tetrahedral. This compound i s a versatile catalyst and can effect hydrogen-reduction of many different unsaturated compounds such as simple olefins, cyclic monoenes, dienes, terpenes, cl.^unsaturated carboxylic acids, n i t r i l e s , esters, ketones etc., depending on the conditions, although a high selectivity results depending on the degree of substitution of the ol e f i n i c substrate. I n i t i a l molecular weight studies on the RhCl(PPh3)-5 complex suggested dissociation i n solution but a later 28 note by Shriver and co-workers showed that with rigorous exclusion of oxygen, no dissociation was observed. RhCl(PPh3)^ i s extremely reactive and can form reversibly, complexes with both hydrogen and ethylene. Spectroscopic data i n i t i a l l y indicated that the hydride was present i n solution as a solvated 29 octahedral cis-dihydride complex (A) although a later work shows that the H Ph3P-Ph-,P j H (A) J Cl cis-dihydride complex exists as HgRhCKPPl^) with an extremely la b i l e phosphine. Catalytic hydrogenation by the rhodium(I) systems has been rationalized 2U.30.31 by the following scheme I I : K 2 (c) I I I H 2 Rh (a) o l e f i n || KT — alkane k-(e) I ( o l e f i n ) (d) I I I Rh ( o l e f i n ) k x H"2 Rh ( o l e f i n ) -> H 2 (b) Scheme I I There are two pathways possible f o r the hydrogenation process, namely the Kjkx ('unsaturate') route and the K 2 k 2 ('hydride') route. The 'unsaturate' route involves c i s o x i d a t i v e a d d i t i o n o f hydrogen to a square planar R h ^ ( o l e f i n ) complex to give a H2Rh"^* ( o l e f i n ) complex. A hydridoalkyl i s then formed by t r a n s f e r of one hydrogen, which i s followed by the, second, hydrogen t r a n s f e r to y i e l d , v i a reductive elimination, saturated product and regeneration o f the Rh^ c a t a l y s t ; step (b) i s thought to be the r a t e determining step. The hydride route involves step (d) as a r a t e determining step to the same H ^ R n ^ ^ ( o l e f i n ) intermediate, followed by successive s i n g l e hydrogen atom t r a n s f e r s followed by reductive e l i m i n a t i o n o f the product and c a t a l y s t regeneration. 30,32 Spectroscopic data and k i n e t i c studies together with the e f f e c t o f change of solvent i n d i c a t e that hydrogenation with BhCl^PPhj) ^ proceeds v i a the hydride route, while the chlororhodate and r e l a t e d systems i n DMA appear to go v i a the 'unsaturate' route discussed i n scheme I I . A number o f c a t i o n i c complexes of the type£Rh(diene)l^j* (where L = monodentate or L 2 = bidentate ligand) have also been used f o r hydrogenation. The complexes can be conveniently p r e D a r e d i n s i t u from)PhCl(diene)?|in a l c o h o l i c L J 33 solvents. The hydride route again appears to operate f o r these systems. 8 Many other d metal complexes i n c l u d i n g those of Fe ( o ) , Co(I), N i ( I I ) , P d ( l l ) , I r ( l ) and Pt(II) have been reported to catalyse the hydrogen-reduction o f o l e f i n i c compounds. The l i t e r a t u r e up to 1972 has been covered i n a book by James 3°and some recent reviews by Herm 0n and co-workers?/4' and D o l c e t t i and Hoffmaf More recent developments i n v o l v i n g Rh systems w i l l be discussed below or w i l l be covered under the following s e c t i o n on asymmetric hydrogenation. Useful methods f o r the ^reparation o f triphenylphosphine complexes of the platinum group metals containing hydride, carbonyl or n i t r o s y l groups 36 37 have been reported by Robinson and co-workers. • One such complex, hydridotetrakisCtriphenylphosphine)rhodium(l), HRh(PPh3)^ has been used f o r reduction of o l e f i n s and e.g., the complex i s a very e f f i c i e n t c a t a l y s t f o r the homogeneous hydrogenation of hex-l-ene i n benzene although the hydrogenation was 38 accompanied by isom e r i z a t i o n to 2 - and 3 - hexene-s. Molecular weight studies with k i n e t i c data i n d i c a t e that there are two c a t a l y t i c a l l y a c t i v e species i n s o l u t i o n -PPh 3 -PPh 3 H R h ( P P h 3 ) , ^ - :-?.h(PPh3) =^ = ^ H R h ( P P h 3 ) 2 OA) At lower concentration and/or higher temperature the act i v e c a t a l y s t i s thought to be HRhCPPh})^ while at higher complex concentration and/or lower temperature the a c t i v i t y i s a t t r i b u t e d to HRh(PPh3) 3. On the b a s i s of k i n e t i c data the following mechanism (scheme ITI) was proposed i n which the oxidative a d d i t i o n of hydrogen i s the rate determining step i n hydrogenation. The isome r i z a t i o n process i s also accounted f o r . HRhPj, — alkane H I nr1 < R — C H , — CH ? — Rh ( H 2 ) P _ ^ 2 - a l k e n e 2 1 2 2 (n = 2 or 3) ^ H C - C H — R H 2 (P = PPh 3) \ RhP p . _ C H j C H 2 ' ^ ' n ^ H — C H 2 - C H 2 - R h P n HP,hP„ Scheme I I I The c r y s t a l structure of HRh(PPh3) shows that the phosphorus atoms are a t the v e r t i c e s of a regular tetrahedron around the rhodium atom) the hydride l i g a n d was not seen^9 although presence of the hydride l i g a n d was confirmed by i n f r a r e d spectrum ( 1)(H-Rh) 2140 cm ). The nuclear magnetic resonance spectrum (N.M.R.)of the hydride region of the complex i n tetrahydrofuran (THF) even at -60° gave a broadened s i n g l e t confirming the ra p i d exchange of the phosphine ligands. The f a c t that HRh(PPh3>^ was more active than RRh(C0)(PPh3>j was r a t i o n a l i z e d ^ by assuming that increased electron density on the metal (PPI13 versus CO) increased the tendency toward a c o - o r d i n a t i v e l y unsaturated complsx which f a c i l i t a t e d the hydrogenation process. Other Rh(l) and Rh(III) complexes with phospholes as ligands have also been used as hydrogenation c a t a l y s t s ^ " " ^ RhXL 3 L = /?\ or 1 ! Ph Ph D3P(9-phenyl 9-phospha- PP(l-phenylphosphole) -fluorene) Ph TPP(1,2,5 triphenylphosphole) D3P and PP appear to behave i n a s i m i l a r manner to some other t e r t i a r y a l k y l o r a r y l a l k y i p h o s p h i n e s ^ - ^ (e.g. Et-^P, MegPhP, Bu-j P) which also form stable Rh(IIl) complexes of the type RhX^L^ , and the behavior o f TPP p a r a l l e l s that of more bulky phcsphines^''' and gives complexes such as RhHClpLg t h t (L = 3u'Prg. P. Buj MeP, e t c . ) . A l l these rhodium(I) or rhodium(III) complexes are l e s s e f f i c i e n t f o r the hydrogenation of hex-l-ene than KRh(CO)(PPh3)3 or RhCl(PPh3)3 under corresponding conditions. K i n e t i c studies using HRh(DBP)i4> i n d i c a t e d that i t was about 7 times more a c t i v e than RhCl(PPh3)3 , under s i m i l a r conditions, f o r the hydrogenation of terminal o l e f i n s . The structure o f t h i s complex i s presumably s i m i l a r to that of HRh(PPh3)^. The f a c t that metal complexes could be anchored to the surface of porous organic polymers^ l e d Srubbs and co-workers to use the polymer supported catalyst, ( I I ) , prepared from the R h C K P P l ^ complex and the diphenylpho-sphinomethyl r e s i n (I), f o r s e l e c t i v e hydrogenation o f small o l e f i n s . 49 iPPh* (I) A RhCl(PPh 3? -CH 2— CH-?«2 PPh. (Ph3P)2P.hCl (ID + Ph 3P The c a t a l y s t (II) could be e a s i l y f i l t e r e d o f f a f t e r the hydrogenation r e a c t i o n which was an advantage over the soluble c a t a l y s t s , where separation of the c a t a l y s t from products i s more d i f f i c u l t . Various t r a n s i t i o n metal complexes such as Co2(C0)g , RhCl(CO)(PPh3^ , Fe(CO)^, Ni(CO) 2(PPh3) 2, RhgtCCO^etc, supported on resins o f type (I) have also been tested as hydrogenation and hydroformylation c a t a l y s t s , but only the Rhg(CO) complex supported on the styrene divinylbenzene resins was found to be a good hydrogenation c a t a l y s t ^ The subject has been reviewed by Michalska and Webster?" Further developments i n the hydrogenation and hydroformylation of o l e f i n s using supported complexes such as RhCl(PPh3)3 > RhCl(CO)(PPiv^, RhCl(C0)(AsPh3)2 c 2 and Co (CO) (P3u3) were reported very r e c e n t l y by Rony and Roth. The c a t a l y s t 2 6 2 was prepared by depos i t i o n of the metal complex on a porous s o l i d support such as s i l i c a g e l , alumina, activated carbon,etc., followed by di s p e r s i o n of the c a t a l y s t i n the presence of a r e l a t i v e l y n o n - v o l a t i l e l i q u i d phase. A l l these developments look promising although the mechanistic d e t a i l s are s t i l l uncertain. 10 1.3 Asymmetric hydrogenation -Asymmetric synthesis of a number of natural products (where hydrogen i s a t a c h i r a l center) v i a c a t a l y t i c hydrogenation has been a subject of current i n t e r e s t because t h i s method i s an option to the u s u a l l y required biochemical separation of DL mixtures. T r a n s i t i o n metal complexes having a c h i r a l center within co-ordinated ligands have been found to be p a r t i c u l a r l y e f f e c t i v e i n c e r t a i n cases. 53 Horner and Desner f i r s t reported the use of o p t i c a l l y active t e r t i a r y phosphines as ligands i n complexes of univalent rhodium, and Horner and co-workers pointed out that asymmetric hydrogenation o f unsymmetrically substituted carbon-carbon double bonds should be possible by using these complexes (equation 1.) The e l e c t r o n i c environment and bulk of the c a t a l y s t ligands can be var i e d according to the p a r t i c u l a r unsaturated substrate (at l e a s t e m p i r i c a l l y ) i n order to achieve maximum o o t i c a l y i e l d s . 55 Knowles and Sabacky used trichlorotris(methylpropylphenylphosphine)-rhodium I I I (a) f o r the hydrogenation of at r o p i c a c i d IV(a) and i t a c o n i c acid IV (b) to hydratropic a c i d V (a) and methylsuccinic a c i d V (b) re s p e c t i v e l y , i n 15^  and 3» o p t i c a l y i e l d s . RhCl3L-j I I I (a) L = PPhMePr (* = c h i r a l center) H02C 111(b) L = PPh(CH ?CHKeEt) 2 C02H . I C = CHo RhCl 3L- R — C — H X 2 ii> I H 1  c CH^ IV(a) R = Ph v ( a ) R = P h IV(b) P. = H0 2C.CH 2 V(b) R = H0 2C.CH 2 11 The e f f e c t i v e c a t a l y s t i s presumably the RhClL^ Wilkinson-type c a t a l y s t . A small asymmetric e f f e c t was observed when the asymmetry was on the carbon atom of an a l k y l group rather than on the phosphorus atom, e.g., hydrogenation o f a t r o p i c a c i d IV(a) gave hydratropic acid V(a) i n only 1% o p t i c a l p u r i t y usinsr 111(b) as a c a t a l y s t . About the same time Horner and co-workers reported the hydrogenation at ambient conditions of <s(-ethyl styrene ando(-methoxystyrene to (S)-(+)-2phenylbutane (?-&;£ e.e.) and R-(+)-l-methoxy-l-phenylethane (3-b% e.e.) r e s p e c t i v e l y , using a phosphine-rhodium complex formed i n s i t u from |Rh(l , 5 - hexadiene)Cl ) 2 a n c * ( 3 ) - ( + ) -methylphenyln-propylphosphine i n benzene. The r e s u l t s were r a t i o n a l i s e d i n terms of an intermediate complex ( B ) t i n which methyl and n-propyl groups of both phosphines are i n skew posit i o n s , and the o l e f i n co-ordinates i n such a way that c i s - a d d i t i o n of hydrogen r e s u l t s i n p r e f e r e n t i a l formation of one enantiomorph. Use of rhodium(l) complexes with c h i r a l phosphines was fu r t h e r extended 12 57 by Morrison et a l who used t e r t i a r y phosphines having asyrmetry at a carbon atom instead of phosphorus. Thus reduction of (E)-fl(-methylcinnamic a c i d , (E) -P-methylcinnamic a c i d and a t r o p i c acid to products with 61% e.e., 52/= e.e. and 28* e.e, r e s p e c t i v e l y , was observed when tris(neomenthyldiphenylphosphino)-rhodium(I)chloride was used as a c a t a l y s t . The mechanism of the ?.h-catalyzed 30,31 reactions i s presumably the same as f o r RhCl(PPh3)o (Scheme I I ) . 3 5 8 , 5 9 Very high s t e r e o s e l e c t i v i t y was observed when Dang and Kagan prepared ^Rh T(-)-diop)Cls] i n s i t u ( (-)diop = ( - ) - 2 , 3 - 0-isopropylidene - 2 , 3 -dihydroxy-1,4-bis(diphenylphosphino)butane(C);3-solvent) and used i t as a c a t a l y s t f o r asymmetric hydrogenation of unsaturated earboxylic acids. The high o p t i c a l y i e l d s were thought to be due to the conformational r i g i d i t y of the ch e l a t i n g diphosphine at the rhodium and the p a r t i c i p a t i o n of the a c i d f u nction o f the substrates. The ligand asymmetry was at the carbon atom (*) and not a t the phosphorus atom. Reduction of a t r o p i c a c i d , i n the presence of triethylamine, H H 3 C \ / ° -*c CH 2PPh 2 diop = C j H3<r ^ 0 • *C • CH 2PPh 2 H ( C) gave 'S)-hydratropic a c i d i n 64$ o p t i c a l y i e l d , and more than 70$ o p t i c a l y i e l d s were obtained f o r the reduction of fi(-N-acetamidoacrylic acids. The presence of earboxylic group was not c r u c i a l f o r a good o p t i c a l y i e l d as the enamide ( V i a ) , although l a c k i n g a earboxylic function, was reduced to (S)-N-acetylamino-l-phenyl-l-propane (Vlb) with 78-i o p t i c a l p u r i t y . Improved y i e l d s were reported JJHOOCH3 *\ HoCHC = HoCCOIvH — C — H L (Via) 5 .(Vlb) 13 l a t e r by Knowles and co-workers f o r the asymmetric reduction of o(-acetami-d o a c r y l i c acids using rhodium(I) complexes with LKePPh as c h i r a l phosphine ligands (L = Pr, 0-KeOCgH^, cyclohexyl e t c . ) . Such a system with L = (l,2-bis(o-anisylphenylphosphino)ethane) has been used to produce the 60 important drug L-dopa i n^ 9 0 $ y i e l d , h i n t i n g at almost complete s t e r e o s D e c i f i c i t y . 61 Recently, James and co-workers . reported the synthesis of the c h i r a l hydride complex HRh((+)diop) 2, which was used as a c a t a l y s t to reduce N-acetamidoacrylic a c i d to N-Acetyl-(S)-alanine i n 60% o p t i c a l p u r i t y . To overcome the d i f f i c u l t y of separation of the soluble c a t a l y s t and 62 the saturated product a f t e r the hydrogenation reaction, Kagan and co-workers prepared a rhodium complex i n s i t u by f i x i n g a known c h i r a l phpsphinated u n i t onto a synthetic a c h i r a l r e s i n ( V I I ) , which was then reacted with JR-hCl^Hi,,)^) to give (VIII). Complex (VIII) was used f o r asymmetric hydrogenation, H i A z pph c 6 H 6 r 1 I 2 + [ R h C l ( C 2 % ) 2 ] 2 ^[_P-h,Cl,VII,C6H6J 0 k ^—PPh2 ( v i i ) H ( v i i D although the o p t i c a l y i e l d s were very low, e.g. reduction o f ^ - e t h y l styrene (IXa) gave (P . ) - ( - ) -2-phenylbutane(IXb) i n 1.5% o p t i c a l p u r i t y . H 5 6 \ C = C H 2 ^ H 5C 6 — C C H 3 H 5C 2 C 2 H 5 (IXa) (IXb) 63 More recent l y , complete s t e r e o s e l e c t i v i t y has been achieved f o r N-acetamidoacrylic a c i d d e r i v a t i v e s using a c h i r a l diphosphinite rhodium(I) complex prepared from [P.h(l, 5-hexadiene)Cl]^ i n the presence of (+) - trans BDPCH ( (+)-trans-l,2 b i s (diphenylphosphinoxy)cyclohexane) (D). Good o p t i c a l y i e l d s were a t t r i b u t e d to the chelating power and greater d i s t o r t i o n o f the Ph2PO (D) diphenylphosphino group of the diphosphinite. The involvement of N-N dimethylformamide (dmf) i n the complex Cpy2(dmf)F.hCl2(3HZj.)J , which i s an active c a t a l y s t f o r the hydrogenation of 64 64,65 o l e f i n s , l e d McQullin and co-workers to hydrogenate p r o c h i r a l o l e f i n s i n the presence of d i f f e r e n t o p t i c a l l y a c t i v e amide ligands which i n f a c t were used as solvents; substituted c h i r a l amides such as (-)PhCHMeNHCHO or (-)-KeCH(0H)C0NKe2 were used instead of dmf solvent. Thus methyl -3-phenyl-but-2-enoate (Xa) i n (+) or (-) 1-phenylethylformamide gave (+) or (-) methyl - 3-phenylbutanoate (Xb) i n bett e r than 50/j o p t i c a l y i e l d . The PhCMe= CHC02Me H 2 PhCHMe — • CHgCOgMe Cat a l y s t (Xa) (Xb) development of asymmetry i n the product was discussed i n terms of a reaction complex (E) based on the f a c t that the amide i s co-ordinated so that the smallest substituent group, hydrogen, projects towards the complex to minimise E(a) L = H- or :0oHe E(b) Ph *Ke M e 2 N N 0 L = I -Me 1^ V OH Ph^A Ke 15 the s t e r i c compression and the smaller groups of butanoate i . e . H 2CC0 2Ke or Me, w i l l be projected between the H and Me of amide to make the Ph group remote. In agreement with t h i s , there was a c o r r e l a t i o n between the degree of induced asymmetry and the s i z e of the groups i n the amide. An i n t e r e s t i n g feature here i s that the amide group i s co-ordinated through the carbonyl oxygen which places the c h i r a l center f i v e atoms removed from the center of induced asymmetry. 66 B i o l o g i c a l l y active t e t r a h y d r o f o l i c acid (F)has been formed i n and 283 o p t i c a l p u r i t y using (-) and (+) form of the c a t a l y s t prepared by s t i r r i n g (py^RhCl-j and (+) or (-)-N-l-phenylethylformamide under hydrogen and R = —HN—<2~^>-C0NH CH(C00H)CK 2CrI 2C0 2H 67 In 1972, B o n v i c i n i and co-workers reported the f i r s t asymmetric hydrogenation of simple ketones to o p t i c a l l y active alcohols with a c a t i o n i c c a t a l y s t prepared from j j ^ n b d ) ! ^ ClO^ (nbd = norborna -2 ,5-diene; L = PPh-^Me, PPhKe 2 or FMe^) and (P.). - benzylmethylphenylphosphine. The o p t i c a l y i e l d s were low and the r e s u l t s were not reproducible. An analogous complex with L 2 = (-)-dioo, was l a t e r shown to be u s e f u l f o r asymmetric reduction of carbon-carbon, 68 carbon-oxygen and carbon-nitrogen double bonds. With S c h i f f bases (imines) the reactions were slow and solvent e f f e c t s suggested that solvent was present i n the co-ordination sphere of the rhodium i n such a p o s i t i o n to a f f e c t the geometry of the t r a n s i t i o n state. ' Complexes of Co and Ru have also been used f o r the asymmetric 3-hydrogenation of p r o c h i r a l o l e f i n s . O D t i c a l y i e l d s with CotClOc (used with 69' I I r -, o(- amino acids) as c a t a l y s t were poor; with a Ru-(+)diop J Ru 2 C l ^ ( d i o p ) ^ | 16 70 catalyzed system^ 60 > o p t i c a l p u r i t y has been reported f o r the asymmetric reduction of o(- acetamidoacrylic a c i d . 1.4 Asymmetric hydrogenation of ketones v i a h y d r o s i l y l a t i o n -A novel method f o r the synthesis of o p t i c a l l y a c t i v e alcohols v i a h y d r o s i l y l a t i o n of ketones involves the a d d i t i o n of the silane to the p r o c h i r a l ketone to form a siloxane, which on subsequent treatment withGrignard reagent gives an organosilane and the o p t i c a l l y a c t i v e alcohol e.g.; 1 •• R \ 1 S i " " " + ^ 0 = 0 C a t a l y s t ^ ^ R 2 N H . benzene R X 0 C ^ ' H X P (1) RJMgX (2) H 20 RCH(0H)R • R^RgR^SiH (15) Several platinum and rhodium complexes have been used f o r asymmetric 71,59,62,72,73 h y d r o s i l y l a t i o n . The mechanism proposed f o r these reactions involves oxidative a d d i t i o n of the s i l a n e to the c h i r a l rhodium complex to give f i n a l l y two diastereoisomers which are formed as an eq u i l i b r i u m mixture i n unequal amounts due to s t e r i c e f f e c t s . Alkoxysilanes are then formed which lead to the f i n a l products. 1.5 Hydroformylation -Various Comdexes of cobalt and rhodium have been used f o r 74 hydroformylation. The subject has been reviewed and the r e s u l t s show the greater e f f e c t i v e n e s s of rhodium complexes. The HRh(CO)(PPhO - i complex has been 75-77 found 1D be a very e f f e c t i v e c a t a l y s t e s p e c i a l l y f o r alk-l-enes, and the following mechanism seems w e l l established (Scheme IV) i n which the active species i s RhH(CO) 2(PPh3) 2« 17 RHC I /P + OC — Rh'' I V CO H 2C CO. H CO CO RC 2%CHO RHC*"'/| ^ P 11/ co H 2C H Rh**" P ^ j XCOC„H,,R CO ™2\ OC — Rh* CO P ^ " " r CO --P (P= pph 3) Scheme IV The i n i t i a l step i s a s s o c i a t i v e attack of the alkene on the species RhH(CO) 2(PPh^) 2 which leads to an a l k y l complex. The l a t t e r then undergoes CO i n s e r t i o n to form the ac y l d e r i v a t i v e which subsequently undergoes c i s oxidative a d d i t i o n of molecular hydrogen to give the dihydridoacyl complex. Reductive e l i m i n a t i o n of the product (aldehyde) regenerates the a c t i v e c a t a l y s t . Further i n t e r e s t i n t h i s f i e l d was developed when o p t i c a l l y a c t i v e aldehydes were synthesized c a t a l y t i c a l l y using asymmetric ligands on cobalt 78 and rhodium complexes. The subject has been reviewed by Pino and co-workers. I n i t i a l l y , d i f f e r e n t o p t i c a l l y active t e r t i a r y phosphines were used but the best r e s u l t s have been obtained when c a t a l y s t formed i n s i t u from HRh(CO)(PPh 3) 3 and (-)diop was used; o p t i c a l y i e l d s up to 27^ were obtained from asymmetric hydroformylation of cis-butene. The mechanism i s not yet c l e a r l y understood but 73 from the r e s u l t s i t was postulated that the o v e r a l l mechanism i s very s i m i l a r to that o u t l i n e d f o r hydroformylation (scheme IV). The o l e f i n face attacked 18 by hydrogen and carbon monoxide i s always the same and, at l e a s t i n part, asymmetric i n d u c t i o n occurs i n formation of the a l k y l rhodium complex. The i n t e r a c t i o n between o l e f i n and the ligand i s thought to play a l e s s important r o l e . 1.6 Aim of the present work -Rh-diop complexes have proved to be very promising systems f o r c a t a l y t i c asymmetric syntheses due to conformational r i g i d i t y and bulkiness of the diop l i g a n d but no studies have been reported on attempts to understand the mechanism involved. The present work was aimed to elucidate the mechanism by which such systems operate f o r the hydrogenation of various a c i d i c and non-acidic substrates. C H A P T E R I I EXPERIMENTAL 2.1 Preparation of HRh(diop)? complex -61 The complex was prepared by the procedure of James and co-workers with the s l i g h t m odification of omitting formaldehyde. A s o l u t i o n of rhodium t r i c h l o r i d e t r i h y d r a t e (0.13g, 0 . 5 mmole) i n ethanol (5 ml) and a s o l u t i o n of potassium hydroxide (0.20g, 3*6 mmole) i n ethanol (5 ml) were successively added to a vigorously s t i r r e d s o l u t i o n of (+) - diop (0 .55 g» 1»1 mmole) i n b o i l i n g ethanol (15 ml), and the mixture was refluxed f o r 10 minutes. The yellow s o l i d which p r e c i p i t a t e d on c o o l i n g was f i l t e r e d , washed with cold water and ethanol several times and was d r i e d i n vacuo to y i e l d 0.44g o f HRh(diop) ? (^81^ based on Rh). 79 I n e r t atmosphere techniques were used while handling a l l the s o l u t i o n s . The compound analysed f o r hydridobis(diop)rhodium (I) (Found: C = 67.00$; H = 6.10$. Calculated: C = 67.64$; H = 6.001b. The i r spectrum (both K3r p e l l e t and n u j o l mull) showed a band at 2040 cm assigned toD(H-=Rh). The proton N«M«R» spectrum of a deuterated benzene s o l u t i o n of the complex at room temperature showed a doublet o f quintets centered around7^28.4 confirming the presence of theH-Rh bond and four equivalent phosphorus atoms around the metal. 2.2 P u r i f i c a t i o n of solvents -A.R. grade toluene and n - b u t y l a l c o h o l were p u r i f i e d before use. Toluene was refluxed with calcium hydride, and n-butyl alcohol was refluxed with a magnesium metal and i o d i n e mixture to remove the l a s t traces of water. The solvents were stored under an i n e r t atmosphere. (+) - diop was used as obtained from Strem Chemicals and RhCl^O-gO was a loan from Johnson Matthey Limited (39% Rh), Research grade hydrogen was obtained from Matheson Co. and was passed through a "deoxo" c a t a l y t i c p u r i f i e r before use. 20 2.3 Hydrogenation Apparatus -The hydrogenation apparatus used i n the k i n e t i c studies was a constant pressure gas uptake type apparatus, as shown diagrammatically i n Figure 1. The pyrex r e a c t i o n v e s s e l (A), which could be clip p e d to a metal rod shaken by a motor (I) during a reaction, was connected by a s p i r a l glass arrangement with tap (C) to the o i l manometer (D) through tap (H). The o i l manometer which consisted of a c a p i l l a r y U tube f i l l e d with b u t y l phthalate (a l i q u i d of n e g l i g i b l e vapour pressure) was connected to the gas measuring burette c o n s i s t i n g of a mercury r e s e r v o i r (E) and a p r e c i s i o n bored tube (N) o f known diameter. The gas measuring burette was i n turn connected through an Edward's high vacuum needle valve (M) to the gas-handling part of the apparatus, which consisted of a mercury manometer (F), the gas i n l e t (Y) and connections to the Welch Duo Seal rotary vacuum pump (G). The r e a c t i o n f l a s k (A) was thermostated i n a s i l i c o n o i l (Dow Corning 550 f l u i d ) bath (3). I t consisted of a four l i t r e glass beaker i n s u l a t e d by polystyrene foam on a l l sides and enclosed by a wooden box with a small c i r c u l a r hole f o r observing the colour changes of the reaction mixture. The top of the o i l bath was well covered by stereo foam. The gas burette was immersed i n a thermostated water bath made from a perspex rectangular tank. Both thermostat baths were operated using "Jumo" thermo regulators with "mere to mere" r e l a y c o n t r o l c i r c u i t s and heating provided by 25 watt elongated l i g h t bulbs. These together with mechanical s t i r r i n g ensured temperature c o n t r o l to within about + 0.1°C. A v e r t i c a l l y mounted t r a v e l l i n g microscope was used to follow the gas uptake. A lab-chron 1400 timer was used to record the time during the k i n e t i c experiments. 2.4 Experimental procedure f o r a t y p i c a l gas uptake experiment -For each experiment, the required amount of rhodium complex was weighed i n a bucket which was hung by a hook i n the reaction f l a s k (A). The required amount of o l e f i n i n 5 ml of n-butanol - toluene (2:1) was also put i n the Figure 1 - Constant pressure gas-uptake apparatus r e a c t i o n f l a s k (A). The f l a s k (A) was then connected by the s p i r a l and tap (C) to the gas-handling part of the apparatus at ( 0 ) . The reactant s o l u t i o n was degassed by a l t e r n a t e cooling with pumping and warming. Hydrogen was admitted at a pressure somewhat l e s s than that required f o r the experiment and then taps (C) and (P) were closed. The whole system up to tap (H) was then pumped down with taps (K), (L), (J) and (M) open. The f l a s k and s p i r a l arrangement were disconnected from (0) and t r a n s f e r r e d into the thermostated o i l bath with the s p i r a l connected to the o i l manometer through tap (H). Tap (H) was opened and a f t e r the a i r between tap (H) and (C) was pumped out tap (Q) was closed. Hydrogen was admitted to the r e s t of the gas uptake apparatus up to tap (C) which was then opened so that the pressure i n the whole system was equalised. The r e a c t i o n pressure required was adjusted by using the mercury manometer. Tap (J) and needle valve (M) were closed while the i n i t i a l reading of the mercury l e v e l i n (M) was taken. Taps (K) and (L) were closed and the timer and shaker were s t a r t e d simultaneously. As a r e s u l t of any hydrogen uptake, the o i l l e v e l on the l e f t hand side o f the manometer rose and to maintain zero d i f f e r e n c e i n the l e v e l s , hydrogen was admitted i n t o the gas measuring burette through tap (J) and needle valve (M) to give a corresponding r i s e of mercury i n (N). The change i n height o f the mercury was noted as a f u n c t i o n of time.' Since the diameter of (N) was known, the corresponding N.T.P. volume of hydrogen used was found and an uptake p l o t o f gas consumption i n m o l e s / l i t r e against time could be drawn. The use of a small volume of s o l u t i o n (^5 ml) i n a r e l a t i v e l y l a rge indented v e s s e l (/v'30 ml) and a high shaking rate ensured the absence of d i f f u s i o n c o n t r o l i n the rate of gas consumption. 2.5 S o l u b i l i t y Determination -5 ml of n-butanol-toluene (2:1) mixture was put i n t o the r e a c t i o n f l a s k (A) which was then connected by the s p i r a l and tap (C) to the gas-handling part of the apparatus at (0) . The reactant s o l u t i o n was degassed by al t e r n a t e cooling 23 with pumping and warming and then taps (C) and (P) were closed. The f l a s k and s p i r a l arrangement were disconnected from (0) and transferred i n t o the thermostated o i l bath with the s p i r a l connected to the o i l manometer through tap (H). Needle valve (K) 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. Hydrogen was admitted to the gas uptake apparatus at the desired pressure. Taps (K) and (L) were closed a f t e r c l o s i n g taps (J) and needle valve (M). The timer and the shaker were started simultaneously and the uptake of the gas was measured. 2.6 I s o l a t i o n of r e a c t i o n products -D i f f e r e n t methods were used to i s o l a t e the various hydrogenated products. (1) The solvent mixture a f t e r hydrogenation of i t a c o n i c a c i d was pumped o f f and benzene was added to the residue. The mixture was shaken f o r a while when a white s o l i d separated. The s o l i d was f i l t e r e d and i d e n t i f i e d as methylsuccinic a c i d from i t s melting point and n.m.r. spectrum. The same procedure was used f o r N-acetamidoacrylic a c i d . 80 (2) In the case of a t r o p i c acid the s o l u t i o n was concentrated a f t e r hydrogenation and the r e s i d u a l o i l was extracted wib 5' ; sodium hydroxide (25 ml); the/basic extract was then f i l t e r e d through C e l i t e to remove the c a t a l y s t . The f i l t r a t e was a c i d i f i e d with 10$ hydrochloric acid and extracted with ether. The ethereal extracts were d r i e d (MgSO^) and concentrated to give the saturated acid as a l i q u i d which was i d e n t i f i e d as 2 - phenylpropionic a c i d . Esters were also i s o l a t e d by t h i s procedure except that petroleum ether instead of ether was used. 2.7 O p t i c a l dotation Measurements -O p t i c a l r o t a t i o n s were determined on a Perkin-Elmer model 141 Polarimeter. The r o t a t i o n s were reported at the sodium - D l i n e and were obtained from the equation -2k where \joQ = s p e c i f i c r o t a t i o n = observed r o t a t i o n JL = path length i n decimeters e = concentration of s o l u t i o n i n g/ml 2.8 Spectroscopic work - ' (a) A l l i n f r a r e d spectra were obtained using a Perkin-Elmer Model k5? spectrophotometer. The spectra of s o l i d s were obtained as mulls or K3r d i s c s . (b) A l l nuclear magnetic resonance (n.m.r.) spectra were determined using e i t h e r Varian Model HA100 or T60 spectrophotometers. (c) V i s i b l e studies were done on a Perkin-Elmer Model 202 spectrophotometer. O p t i c a l c e l l s of 1 cm or 1 mm o p t i c a l path length were used. Melting points were determined using a F i s h e r John s Meiting Point apparatus. C H A P T E R I I I RESULTS 3.1 The asymmetric homogeneous hydrogenation of three c(, unsaturated earboxylic acids was effected usine the hydridobis^(+)-diop^Jrhodium (I) complex as c a t a l y s t i n n-butanol-toluene solutions (Table I ) . Thus i t a c o n i c acid (1) was hydrogenated completely under mild conditions (see below) to 2 •? (+)-2-methylsuccinic acid (&QD + 3.38°; C 10 . 5 , absolute ethanol) i n d i c a t i n g 20$ e.e. of the R isomer based on a maximum of JVJ 2 ^ 17.09° f o r 8 l the pure R isomer. Atropic acid (2) afforded (-)-2 phenylpropionic acid 25 o (&0jj - 28.2 5 £.8.06, Chloroform) on hydrogenation. From the o p t i c a l p u r i t y o f pure (-)-2-phenylpropionic acid ( M^-76.1°; C 8.06, c h l o r o f o r m ) 8 2 the o p t i c a l y i e l d corresponded to 37$ e.e. of the R isomer S i m i l a r l y N-acetamidoacrylic a c i d (3_) gave N-acetylalardne [ ° i ] 2 " ' - .37°; C2, water) which i n d i c a t e d 56'$ e.e. o f the S isomer with respect to the o p t i c a l l y pure N-acetyl-(S)- alanine (§(] 2 5 - 6 6 . 5 °; C2, water)?3 C0 2H C0 2H C0,H HRh(diop)? | I H 2 C « C / 2 3 0 0 ,H 2 '. > H » » C ^ CH3 H 3 C ^ C — ?. \ C H 2 - C 0 2 H • CHg-COgH . CH 2-C0 2K 1 R-(+) • S-(-) - 60$ hot C0 2H C0 2H C0 2H H 2C — H R h ( d i o p ) g J H » » C — Ph + P h ^ - C - ^ H P h 50°, H 2 ~* CH 3 CH 3 "S-(+) , R-(-) 2 31.5* . 68.5* C0 2H C0 2H / C 0 2 H H 2C—C ^ T.^roc y HRh(diop) 2 j f^C — i NHCOCH3 + H-COCHN C H ' 3 ?0°.H. * I J I CH 3 0 ' H2 CH. 22$ ' 78$ 3 °"3 R-(+) S-(-) TA3LE I Asymmetric homogeneous hydrogenation of <£, j} unsaturated carboxylic acids using hydridobis(diop)rhodium(l) c a t a l y s t . Substrate Product Configuration % e.e. Itaconic Acid 2-methylsuccinic acid R - (•) 20.0 AtroDic Acid 2-pheriylpropionic acid R - (-) 37.0 N-Acetamidoacrylic acid N-Acetylalanine S - (-) 56.0 ro ON No hydrogenation was observed when other isomers of i t a c o n i c acid, namely c i t r a c o n i c (methyl maleic) acid and mesaconic (methylfumaric) acid were used as substrates; e( - methyl- and ^ - methylcinnamic acids also d i d not hydrogenate (Table I I ) . 3.2 K i n e t i c s -To study the k i n e t i c s , i t a c o n i c a c i d (IA) (1) was taken as the substrate since of a l l the substrates t r i e d , t h i s was found to hydrogenate at the f a s t e s t r a t e . Thus, f o r example, an n-butanol-toluene (2:1) s o l u t i o n -3 of H?.h(dioD) 2 (1.5x10 M) at 30° was found to e f f e c t asymmetric hydrogenation -2 of i t a c o n i c a c i d (2x10 M) under an atmosphere or l e s s pressure of hydrogen i n about 30 minutes. The i n i t i a l yellow colour of the HRhCdiopJg- o l e f i n s o lutions changed to l i g h t yellow during the hydrogenation r e a c t i o n (see section 3»5)^ The hydrogenation r e a c t i o n rates were i n s e n s i t i v e to l i g h t and were not d i f f u s i o n c o n t r o l l e d . The constant pre'ssure gas uptake apparatus (Figure l) was used f o r following the k i n e t i c s of i t a c o n i c acid reduction. S-shaped curves were obtained as shown, f o r example, i n Figure 2. The i n i t i a l uptake of hydrogen was followed by a somewhat f a s t e r uptake which then slowed down near the end point; the t o t a l hydrogen uptake corresponded to about 95£ hydrogenation of the substrate. The curvature, showing increasing rate with time, was more r e a d i l y observed a t lower c a t a l y s t concentrations (compare .curves (a) and (b) i n ^ g u r e 2 ) . The k i n e t i c s f o r the system were inve s t i g a t e d over a concentration -3 -2 range of (0.4 - 3.0) x 1C M i n HRh(diop) 2, (0.5-12.0) x 10 M i n i t a c o n i c acid and (56-740) mm pressure of hydrogen [(0.24-2.88) x l o \ i n hydrogen} to help e s t a b l i s h the mechanism of the reacti o n . The experimentally determined i s o l u b i l i t y data are shown i n Figures 3 and 4, Henry s Law i s approximately obeyed upto 1 atmosphere t o t a l pressure of hydrogen (Figure 3)S the s o l u b i l i t y increases with i n c r e a s i n g temperature from 25-40°C. The rate of hydrogen uptake was obtained by measuring the maximum slope of the pl o t i n a l l the gas TABLE II Homogeneous hydrogenation of various cl, ^ unsaturated carboxylic acids at 30°. -2 -3 ({Substratej = 4 x 101', [HRhtdiop)^ = 1,5 x 10M, 7^ 0 m'p^, n-butanol-toluene (2:1) = 5 ml) Substrate Maximum Rate x 10 (M Sec) Itaconic a c i d N-Acetamidoacrylic a c i d o(-methyl cinnamic a c i d (E & Z) p-methyl cinnamic a c i d (E & Z) (a) Atropic acid C i t r a c o n i c a c i d Mesaconic acid Maleic a c i d 58.3 0.33 No hydrogenation i No hydrogenation 0.28 No hydrogenation No hydrogenation 0.70 (a) Temp. = 50 O E 'o o h-0) I-o CO CN I 200 500 1100 1400 1700 2000 800 Time , Sec Figure 2- Rate plots for the hydrogenation of itaconic acid using HRh(diop)2 as catalyst, (30°,740nun p H 2 , 2.0 x 10""2M itaconic acid, n-butanol-toluene (2:l)=5ml, (a) Rh 1.0 10~3M;(b) Rh = 2.0 x 10~ 3M). CO o X CM" X PH2'I N M 132 360 550 740 120 360 ' p H mm of Hg 600 840 Figure 3- Solubility of hydrogen in n-butanol-toluene (2:1) mixture at various pressures at 30°. [ H 2 ] X 103,M 0.67 1.53 2.11 2.76 o 1 r Temperature. C Figure 4- Solubility of hydrogen in n-butanol- toluene (2:1) mixture at various temperatures. H' uptake experiments. The v a r i a t i o n o f maximum slope with rhodium concentration i s shown i n Table I I I . A f i r s t order dependence of rate on the HRh(diop) 2 complex concentration i s i n d i c a t e d from the p l o t of Figure 5« With i n c r e a s i n g i t a c o n i c acid concentration, the maximum rate also increases (Table IV). The p l o t of these data (Figure 6) show that the r e a c t i o n i s f i r s t order i n i t a c o n i c a c i d at low concentrations (up to 0.02M), but f a l l s o f f to l e s s than one with i n c r e a s i n g concentration; the slope of the curve drawn corresponds fet0.12 M) to an order of about 0.75. The hydrogen dependence (Table V, Figure 7) shows a f i r s t order dependence at low pressures, but the dependence f a l l s o f f markedly i n the 100-250 mm range and reaches zero at pressures above 350 mm. The temperature dependence was measured at an o l e f i n i c a c i d concentration o f 0.04M i n conditions independent of hydrogen pressure (f^l atm). The data are shown i n Table VI. 3.3 E f f e c t of added (+)-diop -Addition of even small amounts of (+)- diop ( l i g a n d : Rh = 1J3) had a remarkably i n h i b i t i n g e f f e c t on the c a t a l y t i c hydrogenation . The uptake curve was e s s e n t i a l l y of the same shape as i n the absence o f added (-f)-diop but the maximum rate was decreased by a f a c t o r of about one hundred (Table VII, Figure 8 (a); (b)); the i n i t i a l rate was so slow that an induction period (zero or immeasurably small rate) of j u s t under 5hr might be i n d i c a t e d . Further addition of excess (+)-diop (up to a 1:1 r a t i o with c a t a l y s t ) had l i t t l e e f f e c t on the eventual maximum rate, but the "induction period" became longer, and indeed the p l o t of Figure 9 could be constructed to i n d i c a t e an inductio n period proportional to the concentration of added (+)- diop. 3.4 Homogeneous hydrogenation of other substrates -Homogeneous hydrogenation of various non-acidic o l e f i n s was also achieved (Table V I I I ) , although the rates were slow compared to t h e i r a c i d i c counterparts. No hydrogenation was observed when e t h y l methyl ketone was used TA3LE I I I Va r i a t i o n of maximum rate with (30°, 7h0 mmp4 , { jAJ = 2 X 10M, n-butanol-toluene (2:1) = 5 ml) £*Rh(diop) 2] x 10? M : • r[ H— Maximum Rate x 10? M Sec M Sec 0.40 1.00 1.50 2.00 3.00 1.00 2.22 3.89 5.19 7.00 1.25 1.11 1.30 1.30 1.17 Average 1.23 [Rh] X 103. M Figure 5- Plot of maximum rate against HRh(diop)2 / ( 30°, 740mm PH,/ 2.0 x 10~2M itaconic acid, n-butanol-toluene (2:l)=5ml). TABLE IV Variation of maximum rate with [i/TJ -3 (30°, 740 mm D u , JjIRhCdiop)^ 1.5 x 10M, n-butanol-toluene (2:1) = 5 ml) [iA] X 10, M 5 -1 Maximum Rate x 10, M Sec 0.50 0.91 1.00 2.26 2.00 3.89 • 3.00 4.83 4.00 5.83 5.00 7.00 6.00 8.23 12.00 13.33 ' VJ1 TABLE V Variation of maximum rate with partial pressure of hydrogen (p,T ). -3 r - -2 '2 (30°, BHh(diop) ~] = 1.5 x 10 M, jlAj = 2 x 10 K, n-butanol-toluene (2il) = 5 ml) (b) pu^ (mm) Maximum Rate x 10^  -1 M :>ec 56 0.23 1.61 170 0.66 3.50 284 1.10 4.38 436 1.69 .^59 588 2.29 4.38 740 3.01 4.52 (b) A solvent p a r t i a l pressure of 20 mm was calculated, assuming that the solvent mixture obeyed Raoult s Law. (c) Calculated from Figure 3. CD CO in o X Q) CO E E "x cc 2 750 P H 2 , mm of Hg Figure 7- Dependence of maximum rate on hydrogen pressure, (30, 2.0 x 1 0 - Z M itaconic acid, 1.5 x 10*"*M HRh(diop)2, n-butanol-toluene (2:1) = 5ml). CO TABLE VI V a r i a t i o n of maximum rate with change i n temperature. -2 -3 ( [JA)= 4 X 10 M, ^ P.h(diop)^Ja 1.5 x 10 M, n-butanol-toluene ( 2 i l ) = 5 ml) t°C p„ (mm) 1 o 3 m Maximum Rate x 1 0 5 k, M Sec M Sec 30 7^0 3.01 2.50 0.42 35 733.5 3.78 4.05 0.63 kQ 726 4.55 6.93 716 5.35 9 . 5 5 1.16 1.59 TA3LE VII E f f e c t of added (+)-dior> on the hydrogenation of i t a c o n i c acid using HRh(diop) 2 " . -2 ' -3 as c a t a l y s t (30°, £EA) = 2 x 10 M, JHRh(diop) 2~J- 1.0 x 10 M, 360 mm p ^ ) 7 j(+)-diop)x 10, M Maximum Rate x 10, Induction period M Sec 0.0 280 0.0 3.0 3.14 4.0 7.0 2.86 8.5 10.0 2.73 12.0 41 300 600 900 Time . Sec 1200 1500 1800 1 0 _9> 7 o E X •o , n w O V) X> a I 0 1 1 J 1 — — * 1 i — — 00/. * / ^/ 1 I 5 15 25 35 45 Time . hr F i g u r e 8- R a t e p l o t s f o r t h e i t a c o n i c a c i d h y d r o g e n a t i o n f u s i n g H R h ( d i o p ) 2 a s c a t a l y s t ) i n t h e p r e s e n c e o f a d d e d ( + ) - d i o p , (30, 360mm p l l 2 , 2 . 0 X 1 0 - 2 M i t a c o n i c a c i d , 1.0 x 10~ 3M H R h ( d i o p ) 2 , (a) n o ( + ) - d i o p ; ( + ) - d i o p = 3 x 10"4M (A) (b) ; 10.0 x 1 0 _ 4 i M Cm) (c) , n - b u t a n o l - t o l u e n e (2:1) = 5 m l ) . TABLE VIII homogeneous hydrogenation of various non-acidic substrates r - -1 ' " 2 • i (JSubstrateJ = 4 x 10 M, jHRhCdiopJgJ - 1.5 x 10 M, ^ = 740 mm) Substrate • - 1 I n i t i a l Rate x 10 , M Sec T°C Solvent Diethyl itaconate 0.63 60 n-butanol-toluene (2»1) Diethyl maleate 0.15 30 II Acrylamide 2.83 30 it Hex - 1 - ene 0.35 30 it Hex - 1 - ene Hydrogenation too slow 30 Toluene Non -1 - ene it 30 it o ( - methyl s t y r e n e ^ ) 2.04 50 No solvent E t h ^ y l methyl ketone No hydrogenation 30 n-butanol-toluene ( 2 i l ) (d) Substrate = 8 M Pg = 360 mm as substrate. The rate of hydrogenation of hex-l-ene was extremely slow when toluene instead of n-butanol-toluene(2:1) was used as solvent. A t y p i c a l gas uptake p l o t i s shown i n Figure 10 f o r the hydrogenation of d i e t h y l itaconate at 60°} the hydrogen uptake rate slowly decreased with time and about B0% reduction was achieved i n 26 hrs. No hydrogenation was observed at 30° even a f t e r 6 hr . 3.5 Spectrophotometric Studies -T h e ' v i s i b l e spectrum of HRh(diop)2 i n n-butanol-toluene (2:1) under argon was recorded at room temperature (Figure 11 (A) ). The absorption peak -1 -1 a t 350 nm ( € = 12300 + 100 M cm) was i n v a r i a n t with time (over 1 day) and was unaffected by the add i t i o n of excess diop; the same spectrum was recorded under a hydrogen atmosphere. The yellow s o l u t i o n (of Figure 11(A))was very a i r - s e n s i t i v e and on exposure to a i r the peak completely disappeared within 5 minutes, the colour changing from yellow to almost c o l o r l e s s (Figure 11 (3) ). '-2 Itaconic a c i d (1.0 x 10 M) added to a s o l u t i o n of rTRh(diop)2 i n n-butanol-toluene (2:1) under argon at 30° gave the s p e c t r a l changes shown i n Figure 12; the absorption around 400 nm decreased with time, and an i s o s b e s t i c point a t 436 nm was apparent over the time period from about h a l f an hour to seven hours. The f i n a l spectrum (10-14 hr) showed an absorption -1 -1 peak at 440 nm (€ = 2300 + 50 K cm). A p l o t of l o g (A^-A^) against time (Figure 13 (A)) over the i s o s b e s t i c region of s p e c t r a l changes ( i n Figure (12) ) - 5 - 1 gives a s t r a i g h t l i n e with slopec£6.0 x 10 sec. The value of t h i s slope decreases with a decrease i n i t a c o n i c concentration; thus pl o t s of l o g r _ -3 -3 (A^-A^,) against time f o r [IAJ = 5.0 x 10 K and 2 .5 x 10 K give s t r a i g h t l i n e s -5 -1 -5 -1 with s l o p e s — 4 , 4 x 10 sec and 2,0 x 10 sec r e s p e c t i v e l y . Log pl o t s over the whole range of s p e c t r a l changes f a l l away from l i n e a r i t y at longer times when production o f the 440 nm band occurs (Figure 13 (b) ). On adding 1 atmosphere hydrogen to the f i n a l orange s o l u t i o n , an instantaneous colour change from orange to l i g h t yellow was observed with disappearance of the peak at 440 nm (Figure 12). This hydrogen uptake was 6 12 18 24 30 Time , hr Figure 10- Hydrogenation of diethyl itaconate at 60°, ([Substrate] » 4.0 x 10~ [lIRh(diop) 2 ] • 1.5 x 10-3 M # p H = 740 mm, n-butanol-toluene C2:l) 15000 | 1 [ | 1 1 1 r o W a v e l e n g t h , nm F i g u r e 11- A b s o r p t i o n s p e c t r u m o f H R h ( d i o p l 2 i n n - b u t a n o l - t o l u e n e (2:1) a t o ± 1 . (a) S o l u t i o n u n d e r a r g o n , (b) A f t e r e x p o s i n g t h e s o l u t i o n a i r f o r 5 min. 400 500 Wavelength, nm 600 700 Figure 12- Absorption spectra of RHh(diop)2 U.2 x 10~ 4M)in n-butanol-toluene (2:1) in the presence of itaconic acid (1.0 x 10~2M) at 30 ± 1 ° . \_  Solution under argon. — ; — Solution under hydrogen. -p-Time.hr Figure 13- Plot of log I A t - ) versus time for spectral changes in Figure 12. (I) Over the isosbestic region. (II) Over the whole range. r e v e r s i b l e , since the 440 nm peak could be slowly regenerated on pumping o f f the hydrogen. Si m i l a r s p e c t r a l changes were observed when maleic acid instead o f i t a c o n i c acid was used as a substrate, although generation of the 440 nm peak . was f a s t e r i n the case of maleic acid. Very i n t e r e s t i n g l y , the addition of s u c c i n i c a c i d to HRh(diop) 2 also showed spectral changes (Figure 14) very s i m i l a r to those of. the i t a c o n i c a c i d system, and again the f i n a l s o l u t i o n absorbed hydrogen r e v e r s i b l y as described above. The value of the slope from the p l o t of l o g ( A t - A j against time f o r - 5 - 1 the i s o s b e s t i c region i s 13.3 x 10 sec. The c a t a l y t i c hydrogenation of i t a c o n i c a c i d was also followed spectrophotometrically a t 30° (Figure 15). The i n i t i a l l y yellow s o l u t i o n changed f i n a l l y to l i g h t yellow at the completion of the experiment. The f i n a l spectrum was s i m i l a r to the spectrum obtained on adding hydrogen to the f i n a l s o l u t i o n of Figure 12. A p l o t of log (A - A ) against time t -5 -1 (Figure 16) gives a s t r a i g h t l i n e with slope — 6 . 7 x 10 sec. Solutions taken a t completion of hydrogenation i n the gas uptake experiment exhibited spectra close to that of Figure 15(3); thus, the majority of s p e c t r a l changes of Figure (15) are concerned with a process subsequent to the c a t a l y t i c hydrogenation. Spectrophotometric studies were also done f o r non-acidic substrates a t 30°. On adding d i e t h y l maleate to a s o l u t i o n o f HRh(diop) 2 i n n-butanol-toluene (2:1) under argon (no hydrogen), s p e c t r a l changes somewhat s i m i l a r but slower +han those f o r the i t a c o n i c acid system were observed (Figure 17 (A) ), and the colour of the s o l u t i o n slowly changed from yellow to orange. The a d d i t i o n of hydrogen to the f i n a l orange s o l u t i o n r e s u l t e d i n an instantaneous colour change to l i g h t yellow. This l i g h t yellow s o l u t i o n changed.under hydrogen to yellow a f t e r a few hours and the f i n a l spectrum of t h i s yellow s o l u t i o n (Figure 1? (3) ) was d i f f e r e n t to that of the i t a c o n i c a c i d or maleic acid systems. Somewhat s i m i l a r s p e c t r a l changes were observed with the d i e t h y l itaconate 400 "500 1 "" 600" Wavelength, n m Figure 14- Absorption spectra of HRh(diop)2 t 1.3 x 10~4M) in n-butanol-toluene C2:l) in the presence of succinic acid (1.0 x 10~2M at 30 ± 1°.(under argon). O 400 500 600 Wavelength, nm Figure 15- Hydrogenation of itaconic acid using HRh(diop)2 as o catalyst followed spectrophotometrically at 30 ± 1 , ( [IA] ) = 1.0 x 10~2M, [HRhCdiop)2j - .1-5 x 1Q-4M, n-butanol-toluene (2:1) » 5ml ). (B)—Final spectrum. 700 - 1 2 0 - C X 8 0 h i U) o - 0 . 4 0 h -2400 12000 7200 Ti me, Sec Figure 16- Plot of log ( At - A^) versus time for spectral changes in Figure 15. 16800 400 500 6 0 0 Wave length, n m Figure 17- Absorption spectra of HRh(diop)2 (1.4 x 10~4M) in n-butanol-toluene (2:1) in the presence'of diethyl maleate ( 1.0 x 10~2M) at 30 ± 1°. (A) Solution under argon. (B) Solution under hydrogen. 700 54 system although they were extremely slow compared to those f o r the i t a c o n i c a c i d system. A p l o t of log (A+-A.) against time f o r s p e c t r a l chanees i n the - 5 - 1 i s o s b e s U c region i n Figure 17 (A) gives a s t r a i g h t l i n e with slope a 1.7x10 sec. I n t e r e s t i n g r e s u l t s were obtained when toluene instead of n-butanol-toleune (2:1) was used as solvent f o r the d i e t h y l maleate - HRh(diop)£ system (under argon). The absorbance again decreased with time but no absorption peak at 440 nm was generated. The colour of the s o l u t i o n remained unchanged throughout the experiment. The hydrogenation of d i e t h y l maleate using the HRh(diop)2 c a t a l y s t i n the mixed solvent was followed speetrophotometrically (Figure 18). The i n i t i a l yellow colour remained unchanged throughout the hydrogenation r e a c t i o n . The s l i g h t decrease i n absorbance observed at the s t a r t of the r e a c t i o n (^3 hr.) were almost reversed to give the o r i g i n a l spectrum of the HRh(diop)2 complex during the 70 hr. r e a c t i o n time. 3.6 -^ "P nmr measurements -31 The proton decoupled J P N.M.R. spectrum of IheHRh(diop)2 complex i n benzene - d^ at room temperature showed a doublet ( J _ , =146H^) positioned Rh—r 90.04 ppm r e l a t i v e to a PjjPg c a p i l l a r y ( f i g u r e 19). No peak f o r free (+)-diop was observed. 400 500 600 7C Wavelength ..nm F i g u r e . 18- H y d r o g e n a t i o n o f d i e t h y l m a l e a t e u s i n g HRh(diop) 2 as o c a t a l y s t f o l l o w e d s p e c t r o p h o t o m e t r i c a l l y a t 30 + 1, . ( [ d i e t h y l m a l e a t e ] = 4 x I0-2M, [ H R h ( d i o p ) 2] - 1.5. x 10"3 n - b u t a n o l - t o l u e n e (2 : 1 ) = 5ml ) . . F i n a l s p e c t r a - „. PPM Figure 19- 3 1P proton decoupled nmr spectrum of HRh(diop)2 in benzene- d 6 at room temperature (under argon). ON C H A P T E R I V D I S C U S S I O N 4.1 General -The r e s u l t s (sections 3.1 and 3.4 and Tables I, I I and VIII), show that the complex, HP.h(diop)2, i s an e f f e c t i v e and e f f i c i e n t c a t a l y s t f o r the hydrogenation of c e r t a i n o(, ^ unsaturated carboxylic acids; terminal o l e f i n s are hydrogenated more slowly. The asymmetric reduction of i t a c o n i c a c i d (1) to ( R ) - ( " 0 - methysuccinic a c i d i n 20% o p t i c a l y i e l d i s quite encouraging since the best reported o p t i c a l p u r i t y f o r the i t a c o n i c a c i d system i s 14$ using HRh(C0)(Ph 2PR)3 c a t a l y s t , where Ph?PR = S-(+)-(2-methylbutyl) diohenyl-61 phosphine. Also the hydrogenation of a t r o p i c acid (2) and N-acetamidoacrylic a c i d (3) gives (P.)-(-)-2 phenylpropionic a c i d and N-acetyl-(S)-alanine i n reasonably good o p t i c a l y i e l d s (37$ and 56'* r e s p e c t i v e l y ) , although the y i e l d s are lower compared to i n s i t u lRhCl(-)dioDS.., S = s o l v e n t ! systems where 59 o p t i c a l y i e l d s > 7 0 * have been obtained f o r o(-N-acetamidoacrylic acids, and 64$ f o r a t r o p i c a c i d i n the presence of triethylamine. High o p t i c a l y i e l d s with the £?.hCl(-)-diop s j system have been a t t r i b u t e d to the conformational r i g i d i t y of the c h e l a t i n g diphosphine at the rhodium and the p a r t i c i p a t i o n of the a c i d function of the substrates i n the hydrogenation reaction. The f a c t that HRh(diop) 2 i s an e f f i c i e n t asymmetric hydrogenation 59 c a t a l y s t might appear to c o n t r a d i c t the previous f i n d i n g by Kagan and Dang since on using adibpiRh r a t i o of 2:1, they observed no hydrogenation. However, t h i s was a t t r i b u t e d to the formation of a supposedly i n a c t i v e b i s ( d i o p ) complex JR.h(-)-^iio^j Cl. T h e i r reasoning i s based on the f a c t that [R h ( d i p h o s ^ C l (diphos = bis(diphenylphosphino)ethane) shows no c a t a l y t i c J " 84 a c t i v i t y f o r the hydrogenation of ethylenic compounds because i n the rhodium complex the two diphosphine molecules are strongly bound to the rhodium, preventing simultaneous coordination of the o l e f i n and the hydrogen. Complexes such as HRhfPPh^) , which i s an e f f i c i e n t hydrogenation c a t a l y s t ^ 38 e s p e c i a l l y f o r terminal o l e f i n s e.g. hex-l-ene, become active a f t e r d i s s o c i a t i o n a phosphine l i g a n d . Since there was only speculation, about the formation ofp h(diop ) ^ J & i n s o l u t i o n , i s o l a t i o n of t h i s complex and i t s use as a hydrogenation c a t a l y s t could help r a t i o n a l i z e the data. In more recent work Knowles group have used the i s o l a t e d c a t i o n i c complex (COD)Rh(diop) 3?u as the 78 hydrogenation c a t a l y s t . Since HRh(diop)2» which i s a f i v e coordinate Rh^ complex, hydrogenates i t a c o n i c acid, maleic a c i d and N-acetamidoacrylic a c i d under mild conditions (30°), atropic acid under more severe conditions (50°) and does not seem to hydrogenate mesaconic acid, c i t r a c o n i c a c i d , oC-methylcinnamic acid and ^-methylcinnamic a c i d under mild conditions (30°), i t can be concluded that s t e r i c f a c t o r s are dominant i n the hydrogenation reaction with t h i s c a t a l y s t . Thus substrates containing a terminal rCHg group are hydrogenated under mild conditions since i t i s presumably e a s i e r f o r such substrates to coordinate to the rhodium complex, which already has bulky and r i g i d ligands. Substituents a t the o(-position of the substrates also a f f e c t these reactions since atropic a c i d which has a bulky phenyl group>at theo(-position hydrogenates slowly although i t has a terminal —CHg group. In cases where the terminal carbon i s completely substituted (Table I I ) , the o l e f i n s do not get hydrogenated. Comparison of Tables I I and I I I shows that the a c i d function of the substrates probably p a r t i c i p a t e s i n these hydrogenation reactions. Thus - 6 - 1 maleic acid (max. rate = 0.70 x 10 K Sec) seems to get reduced f a s t e r than -6 -1 d i e t h y l maleate (max. rate = 0.15 x 10 M Sec). S i m i l a r l y , the hydrogenation of -6 -1 i t a c o n i c acid (max. rate = 58.5'x 10 M Sec at 30°) i s much f a s t e r than - 6 - 1 d i e t h y l itaconate (max. rate = 0.63 x 10 M Sec at 60°). The hydrogenation -6 of a t r o p i c acid ("^-carboxyl styrene'*) i s much slower (max. rate = 0.28 x 10 -1 -6 M Sec at 50°), however, than that ofe<-methyl styrene (max. rate = 2.04 x 10 -1 M Sec) but the solvent media used were d i f f e r e n t and other f a c t o r s are also 59 probably involved (see l a t e r section 4 , 4 ) . The e f f e c t of the a c i d p a r t i c i p a t i o n could be due to some sor t of H-bonding i n t e r a c t i o n s between the carboxylate group of the substrate and 0 and H atoms of the c a t a l y s t . The r e a c t i o n rates measured f o r hex-l-ene i n (2:1) n-butanol-toluene -6 -1 (0.35 x 10 M Sec) and toluene ( r e a c t i o n immeasurably slow) suggest some kind of involvement of n-butanol i n these hydrogenation reactions (see l a t e r section 4 , 4 ) . 4.2 K i n e t i c s and Mechanism -From the observed k i n e t i c s ( i . e . a f i r s t order dependence on fj'-hj, a zero order dependence on CH2^ | a t p r e s s u r e s 4 0 0 mm and f i r s t order i n Qjgl a t pressures <£150 ™*>i between zero and f i r s t order i n [IA]), a general rate law f o r the hydrogenation of i t a c o n i c a c i d can be written as -- d-f - * « a f M 2 t <» At low [IA] and low » n l = n 2 = """ a n t* ^ * s ^ e n a Jrd order rate constant. Since at higher [k ^ j . between (1.50-3.01)xlOM, the rate of hydro-genation i s independent of ( i . e . n i = 0 ) , equation (1) i s modified to equation (2) - d [ H 2 ] = k [ R h ] [ l A ] n 2 (2) dt The values of k were computed f o r p a r t i c u l a r values of (?.h) at {IA) = -2 2 x 10 M where the rate i s f i r s t order i n IA ( i . e . n 2 = l ) . The average value - 1 - 1 o f k thus c a l c u l a t e d was found to be 1.23 M Sec (Table I I I ) : the slope of f i g u r e 5 -1 -1 f o r the same data gave k = 1.21 M Sec. Measurement of k over a temperature range of 30°-45° at conditions i n which the rate i s approximately f i r s t order i n IA yi e l d e d a reasonable Arrhenius p l o t (Table VI, Figure 20). From the slope, the a c t i v a t i o n energy E a was found -1 4 -1 to be 16.4 + l . K c a l mole. A'd" was estimated to be about 15.8 + 1 Kcal mole and AS to be about - 6 e.u. r e s p e c t i v e l y . The mechanism by which the hydrogenation of o l e f i n s takes place can be postulated on the b a s i s of the k i n e t i c s and spectrophotometric studies c a r r i e d F i g u r e 20- A r r h e n i u s p l o t f o r t h e H R h ( d i o p ) 2 c a t a l y s e d h y d r o g e n a t i o n o f i t a c o n i c a c i d ; ( LIA3= 4 - ° x 1 0 " 2 m / Rh = 1.5 X 10~ 3M, n - b u t a n o l - t o l u e n e (2:1) = 5ml ) . 61 out f o r the i t a c o n i c a c i d system. The HRh(diop) 2 complex alone i s unreactive towards H 2, and yet there i s evidence f o r i n t e r a c t i o n with the o l e f i n i c substrates. An "unsaturate route", i . e . i n i t i a l coordination of substrate, i s almost c e r t a i n l y involved and indeed f o r a l l rhodium(l) monohydride c a t a l y s t s 42 studied thus f a r such a mechanism has been invoked. B a s i c a l l y t h i s involves HRh + o l e f i n = ^ - H R h ( o l e f i n ) ^Rh ( a l k y l ) H 2 (3) HRh + saturated Droduct J' For the present system the i n i t i a l Rh^ complex i s 5 _coordinate, and hence f o r o l e f i n coordination, some l i g a n d d i s s o c i a t i o n (which has to be a . phosphine) almost c e r t a i n l y occurs to provide the necessary coordination s i t e . 38 Preliminary data on the corresponding HRhtPPh^)^ system i n d i c a t e p r i o r d i s s o c i a t i o n of a PPh-j l i g a n d . Considering the observed inverse dependence on added diop, an obvious mechanism to postulate i s the follo w i n g : K HRh(diop) 2 HRh(diop) + diop (4) (K very small) ..kx HRh(diop) + IA „ Rh(diop)(alkyl) (5) *1 *2 Rh(diop) ( a l k y l ) + H ? > HRh (diop) +. Saturated product (6) Assuming a steady state f o r the Rh(diop)(alkyl) intermediate species, the rate law i s -Rate = Kkxkg [IA] [H^[HRh(diop) J ( ? ) \ d i o p ] { k 1 + k 2 [ H 2 ] ^ The t y p i c a l S-shaped p l o t f o r the hydrogenation of i t a c o n i c a c i d (Figure 2) could be r a t i o n a l i z e d i n terms of the slow uptake region i n v o l v i n g the l i g a n d d i s s o c i a t i o n step to form HRh(diop) which, once b u i l t up, hydrogenates i t a c o n i c a c i d c a t a l y t i c a l l y v i a equations (5) and (6) . However, t h i s rate law i s not consistent with the observed f i r s t order JjP.h] dependence, since as the i s increased so w i l l the [diop] term i n the denominator. Generally an i n i t i a l ligand d i s s o c i a t i o n from the c a t a l y s t w i l l . g i v e a dependence going from f i r s t to h a l f order with i n c r e a s i n g [ c a t a l y s t ] . F i r s t order i n [P.h] could r e s u l t i f r e a c t i o n (5) l i e s w e ll to the r i g h t but i n t h i s case the o l e f i n dependence has to be zero. Spectrophotometric studies (section 3.5) also show that the IP.h-( d i o p ) 2 complex does not d i s s o c i a t e i n n-butanol-toluene (2:1) a t room temp-erature, and added (+)-dioo does not a f f e c t the v i s i b l e spectrum of t h i s 31 complex. Also, no free (+)-diop s i g n a l was seen i n the P nmr spectrum of the complex i n benzene-d^ (s e c t i o n 3 . 6 ) . A l l these experimental f a c t s show that the e q u i l i b r i u m (4) i s not po s s i b l e or else K i s too small to be detected by the conventional methods employed. The l a t t e r case, however, would lead to a l e s s than f i r s t order i n £p-h], as mentioned above. An a l t e r n a t i v e to the above mechanism i s the one o u t l i n e d by equations (8) , (9) and (10). *1 HRh(diop) 2 + IA k ' . Rh(diop)(alkyl) + diop (8) in. (I) (II) k 2 Rh(diop)(alkyl) + H 2 HRh(diop) + Saturated product (9) Fast HRh(diop) + diop — I ? R h ( d i o p ) 2 (10) . The diop lig a n d i n equation (8) may d i s s o c i a t e during the process of o l e f i n coordination to the metal. For t h i s mechanism, considering a steady state f o r the intermediate Rh(diop)(alkyl) species, the rate law i s -k! k 2 [HP.h(diop) 2][lA)[H 2] Rate = k Jdiop] + k2£:2"] A f u l l e r rate law can be written i f we consider that the rate i s very much dependent on the degree of formation of the a l k y l intermediate (II) in equation (8) and thus, writing the t o t a l rhodium concentration ( Q^hjp) as sum of the two species.(I) and ( I I ) , ~Rh] ? = [ l ] + [ i i ] (12) 63 and assuming a steady state f o r P.h(diop)(alkyl) species as before, we get u + . . . . „ , . , . . , , ( 1 3 a ) or tph> = H r ' k l L d i o p j + k 2 K ] £ s ' h ] , = [ i ] { k 2 ^ 2 ] + '^[diop] + k 1 [ d i o P ] + k 2 [ n 2 ] Thus the f i n a l rate expression i n terms of Rh-p becomes Rate = k x 1 C 2 [ H 2 " ] [ I A ] bi (13b) (14) k 2 [ H 2 " ] + k - l i i 0 ? ] + k l L I A 3 This equation (14) a t l e a s t q u a l i t a t i v e l y accommodates most of the k i n e t i c data. The rate i s between zero and f i r s t order i n both H 2 and o l e f i n ; when the rate i s zero order i n H 2, ^ 2 ^ 2 " J m u s t dominate i n the denominator and under these conditions the rate i s f i r s t order i n o l e f i n (see Figure 6 at 0.02 M o l e f i n ) and i n t o t a l Qttf] since the back re a c t i o n of (8) (the k_^  [diop] term) i s i n s i g n i f i c a n t i . e . r e a c t i o n (9) i s r e l a t i v e l y f a s t . An inverse diop e f f e c t w i l l be observed i f , on adding excess diop, the k_^Jji i o p j term of the denominator becomes dominant. Equation (14) can also be written i n the form -1 Rate k]_ [diop] k nk H-l K 2 L n 2 1 kTMJr" L I A 3 wm (15) and a p l o t of 1 against ^ a t constant [kh}r> and Q i ^ y i e l d s a reasonably rate [ik] good s t r a i g h t l i n e (Figure 21) with values of slope and i n t e r c e p t equal to 440 sec 3 - 1 and 5 x 10"^  M Sec r e s p e c t i v e l y . I t should be noted that as the £jA*] varies, so does the [jiiop] term due to equilibrium (8), However, t h i s term w i l l be very small compared with the k 2 [H 2"] and ^  [IA] terms, and the v a r i a t i o n i n the k^{diop] value w i l l be n e g l i g i b l e , since the free diop w i l l be consumed i n the regeneration of the - l - l " c a t a l y s t . The i n t e r c e p t gives a k 2 value of about 50 M Sec, and, assuming 65 -1 -1 k, [diop] i s n e g l i g i b l e , k-j i s about 1,5 K Sec; the k^ value compares with that of L - 1 - 1 -2 1,23 M Sec estimated f o r the Rh dependence at 2.0 x 10 M added IA and 1 atm. of H 2 ( 3 e c t i o n 4 . 2 ) . The very marked inverse dependence on added diop may be r a t i o n a l i z e d i n terms of the k^ back r e a c t i o n at quite low added diop competing s u c c e s s f u l l y with the kg step. The r e a c t i o n does commence a f t e r long periods i n the au t o c a t a l y t i c fashion observed (Figure 8), but the i n i t i a l hydride c a t a l y s t i s thought to be decomposed over the long i n d u c t i o n period (see section 4 . 4 ) . At the conditions of higher Hg pressures i n the f i r s t order o l e f i n region, the mechanism of equations (8), (9) and (10) as written y i e l d s the l i m i t i n g rate law k^ [Rh}r [JA] and as such does not account f o r the " a u t o c a t a l y t i c " uptake curves obtained (see Figure 2 ) . An a l t e r n a t i v e but c l o s e l y r e l a t e d mechanism to that of equations (8), (9) and (10) considers that one o f the diop ligands of the HRh(diop)2 complex "dangles" i n s o l u t i o n i n order to provide a coordination s i t e f o r the substrate. This can be represented by equation (16) assuming square pyramidal structure f o r the complex. H H {PSKJ ^ s o l u t i o n - v P/ 5 h J p ^ — p . where P P = (+)-diop HRh(diop)(diop) 31 The proton decoupled P N.MRspectrum (Figure 19) shows 4 equivalent phosphorus atoms coupled to the Rh : there i s no evidence f o r the dangling diop 31 i n the P N.M.R. anilhe sharpness and p o s i t i o n of the phosphorus resonances i n d i c a t e no rapid e quilibrium (on the N-M.R.timescale) f o r a process such as equation (16). I f rea c t i o n (16) i s , however, followed by reactions (17) and (18), a slow b u i l d up of the HRh(diop)(diop*) species to a steady state concent-r a t i o n could account f o r the S-shaped uptake curves. HRh(diop)(diop*) + IA k l ^ Rhf rilonValkvl) (diop*) (17) "1 • 66 * k 2 Rh(diop)(diop)(alkyl) + H 2 >. HRh(diop)(diop*) • saturated product (18) The maximum rate would be given by expression (19)» which corresponds to equation (14), but since no diop i s d i s s o c i a t e d the k^ term involves no such concentration term: k i k 2 ft21CiA3M ( 1 9 ) R a t e = k i + k 2 t * 2 ] + k i L I A ] where [j>h] r e f e r s to the concentration of HR.h(diop)(diop*). In terms of t o t a l rhodium and assuming X i s small compared to unity ( i . e . Q{Rh(diop) 2]^> JHRh(diop)(diop*)] ), the rate i s given by the expression -k l k 2 K* [ H 2 " ] [ l A ] [ R h ^ Rate = _ _ _ , ^ 2 0 ' *1 + K2U2] + k l K [ I A ] The i n t e r c e p t of the inverse p l o t of Figure 21 s t i l l corresponds to a - 1 - 1 k 2 value of 50 M Sec. At conditions zero order i n FU, the rate law becomes -1 -1 simply k x K* (pOO^ and the k value of 1.23 — Sec estimated f o r these conditions * -1 . -1 • (section 4.2) i s equal to k-. K . A p l o t of rate versus |Hp"l i s shown i n * - 1 - 1 Figure 22: the s t r a i g h t l i n e drawn y i e l d s a k-|_ K value of ^1.7 M Sec ( i n t e r -cept). The slopes of Figures 21 and 22 correspond to expressions which can give values of k-^ ; however the c a l c u l a t i o n s involve the d i f f e r e n c e of two very s i m i l a r numbers^e.n. Figure 21 gives k^ as the d i f f e r e n c e between 0.153 and 0.150 Sec and hence no meaningful value can be estimated. Thus the mechanism of equations (16), (17) and (18) accounts f o r the k i n e t i c dependence on the Rh, H2 and IA,and can account f o r the a u t o c a t a l y t i c nature of the uptake p l o t s . Addition of diop, even at diop : Rh-O, completely k i l l s the hydrogenation r e a c t i o n at l e a s t i n a p r a c t i c a l sense; a slow r e a c t i o n does return a f t e r a lone induction period, but over long periods the i n i t i a l HRh(diop) 2 c a t a l y s t i s believed to be decomposed by the a c i d substrates (see Section 4 . 4 ) . Such a marked added phosphine i n h i b i t i o n , a l o n g with the f i r s t order dependence on Rh, can i n no way be explained by a p r i o r e quilibrium such 0.5 Figure 22- Plot of [ j ^ ] " " 1 against Maximum rate" 1, (30, QlRh(diop) 2}= 1.5 x 10""3M, [IA] = 2.0 x 10~2M, n-butanol-toluene(2:1) = 5ml). 63 a s equation (4) ('< would have to be very small and t h i s would lead to a h a l f order i n Ph). I t i s not immediately c l e a r how added diop would a f f e c t equilibrium (16) i n a quantitative way but i n t u i t i v e l y a marked inverse dependence seems l i k e l y . The qu a n t i t a t i v e aspect of the inverse diop dependence i s complicated by the accompanying slow decomposition of the hydride c a t a l y s t . 4.3 Spectral Data and the C a t a l y t i c Hydrogenation Mechanism -The above mechanistic considerations were based e n t i r e l y on j u s t the observed k i n e t i c s . The spectrophotometric studies c a r r i e d out to complement the observed k i n e t i c s of the c a t a l y t i c hydrogenation reactions were not p a r t i c u l a r l y f r u i t f u l i n t h i s regard, although the studies (see l a t e r discussion) revealed some-interesting f i n d i n g s . The v i s i b l e and nmr s p e c t r a l data show that HRh(diop)2(I) remains .as such i n the butanol-toluene media under vacuum; there i s no diop d i s s o c i a t i o n , or r e a c t i o n with H 2 (Figure 11(a) ). The rapid a i r ox i d a t i o n has not been studied i n any d e t a i l . The v i s i b l e s p e c t r a l changes noted on adding IA to (I) under Ar (Figure 12) are very much slower than the c a t a l y t i c hydrogenation process, and do not correspond i n any way to the r e a c t i o n as followed by the gas uptake technique. Even the v i s i b l e s p e c t r a l changes followed under the H 2 atm (Figure 15) were again much slower than any measured gas-uptake. The v i s i b l e spectrum measured near completion of a c a t a l y t i c hydrogenation (measured by H 2 uptake) corresponds e s s e n t i a l l y to that of the s t a r t i n g complex ( I ) , so that the s p e c t r a l changes of Figure 12 and 15 are concerned with processes subsequent to the c a t a l y t i c hydrogenation, and t h i s appears to be decomposition of the hydride complex by protons (see section 4.4). The mechanism of equations (16), (17) and (18) implies that throughout the hydrogenation the species predominantly present w i l l be ( I ) ; very small but k i n e t i c a l l y s i g n i f i c a n t amounts of HRh(diop)(diop*) w i l l be present. The amounts of the steady state concentration of a l k y l present w i l l increase 69 with i n c r e a s i n g [ I A ] , but since the dependence on t h i s substrate i s s t i l l close to f i r s t order ^0.75) at even the high £lA] (Figure 6) t h i s means that r e l a t i v e l y l i t t l e of the a l k y l i s present (a zero order dependence on o l e f i n implies the a l k y l i s f u l l y formed). The "spectrophotometric hydrogenation" of d i e t h y l maleate shows that f o r t h i s non acid substrate,• there i s no complicating decomposition of hydride, and the v i s i b l e spectrum of the f i n a l s o l u t i o n ( a f t e r /v70;6 hydrogenation of d i e t h y l maleate) shows the absorbance peak of the complex ( I ) . Although the k i n e t i c s f o r d i e t h y l maleate hydrogenation were not studied, the H 2 uptake was very slow, and the s p e c t r a l changes of Figure 18 are r e a d i l y accounted for by the mechanism of equations (16), (17) and (18)J the s l i g h t decrease i n absorbance corresponds to a steady state concentration of a l k y l ( a f t e r / ^ 2 h r ) ; towards the end of the reaction, r e a c t i o n (1?) i s s h i f t e d to the l e f t and the hydride spectrum i s regenerated. 4.4 Decomposition of the HRh(diop) 2 C a t a l y s t -The r e a c t i o n of HRh(diop) 2 with i t a c o n i c a c i d i n the absence of H 2 and under Ar i s shown i n Figure 12 . The i n i t i a l s p e c t r a l changes (at l e a s t over 30 minutes) are s i m i l a r to those observed under the H 2 atm (Figure 15); these must be concerned with the hydrogenation and could be due to a b u i l d up of some a l k y l species (the changes are s i m i l a r to those obtained i n the ester system, see above, section 4.3). Following these s p e c t r a l changes, f u r t h e r changes occur over the next 5 - 7 hr with production of an i s o s b e s t i c point at 436 nm and f i n a l l y an absorption peak a t A / 4 4 0 nm; analyses of these data show a f i r s t order process (Figure 13). The peak at 440 nm r e s u l t s from formation of the c a t i o n i c ?h(diop) 2+ species; t h i s complex has r e c e n t l y been synthesized as the t e t r a f l u o r o b o r a t e s a l t and the v i s i b l e absorption spectrum measured (/\max -1 -1 442 nm, = 3550 + 20 M Cm). Studies on the cation are being continued and are the subject of the f u r t h e r research. The disappearance of the 440 nm peak on adding H 2, and the r e v e r s a l of t h i s on pumping i s a t t r i b u t e d to formation of a dihydride t 70 + Fast P,h(diop) 2 + H 2 5 l o w ^ H 2?.h(diop) 2 + (21) The s p e c t r a l changes giving r i s e to the i s o s b e s t i c system are thus a t t r i b u t e d to r e a c t i o n (22) HHh(diop) 2 + H* — > R h ( d i o p ) 2 + + H g (22) The observed f i r s t order decay (Figure 13) i s i n f a c t a pseudo f i r s t order r e a c t i o n since the [ H + J must remain e s s e n t i a l l y constant (the concentration of the i t a c o n i c acid i s i n large excess). The rate of reaction (22) increased with increasing i n i t i a l [ i A ] but i n a l e s s than f i r s t order manner as expected since the a c i d i s a weak one. More data would be required f o r a more quant-i t a t i v e study of t h i s i n t e r e s t i n g r e a c t i o n . A few studies (e.g. reference 85) have appeared on protonation of t r a n s i t i o n metal monohydride complexes. S i m i l a r spectral'changes occurred with MA and SA (under Ar) (see Figure 14); again a t t r i b u t e d to r e a c t i o n (22). I t i s i n t e r e s t i n g to note that with SA there are again i n i t i a l s p e c t r a l changes which are not involved with the i s o s b e s t i c system! the reason f o r t h i s i s not c l e a r but could be associated with some co-o r d i n a t i o n of the a c i d to the Rh v i a a carboxylate group. The s p e c t r a l changes of Figure 15 ( a f t e r completion of the c a t a l y t i c hydrogenation) must r e f e r to r e a c t i o n (22) followed by r e a c t i o n (21). The acid now involved i n r e a c t i o n (22) i s methylsuccinic and the measured rate (Figure 16) i s very s i m i l a r to that "of the IA r e a c t i o n (the r e l a t i v e pks are s i m i l a r ; methylsuccinic a c i d = 4.13, IA = 3 .85). V i s i b l e spectra measured (under Ar) f o r the a t r o p i c acid, M-acetarr.ido-a c r y l i c acid and KA systems measured a f t e r ^ 7 0 $ hydrogenation was complete) showed an absorbance peak at 440 nm only and no peak at 350 nm; these hydrogenations are slow and from these r e s u l t s i t appears that i n these cases, the conversion of the neutral monohydride species to the c a t i o n i c dihydride species occurs and the hydrogenation could take place v i a e i t h e r one o r both of the monohydride and dihydride species. The spectrophotometric studies using the non-acid substrates such as 71 d i e t h y l maleate and d i e t h y l itaconate i n the absence of H 2 (under Ar) gave the s u r p r i s i n g data shown i n Figure 17. The r e s u l t s i n d i c a t e an i n i t i a l slow formation of the R h ( d i o p ) 2 + species with an e x c e l l e n t i s o s b e s t i c point at ^4^0 nm. Together with the s p e c t r a l changes recorded under H 2 (Figure 18) which are r e a d i l y accounted f o r (see section 4 . 3 ) , the data show thatthe c a t i o n must be formed i n some way from the a l k y l (under H 2, r e a c t i o n 18 occurs). A reason-able explanation involves r e a c t i o n v i t h the alcohol solvent: R h ( d i o p ) 2 ( a l k y l ) + BuOH > Rh(diop) 2 +03u + Saturated product (23) The metal a l k y l bond could be h i g h l y susceptible to protonolysis and proton abstraction from the solvent seems f e a s i b l e . Further work i s necessary to substantiate such a r e a c t i o n but i n any case the c a t i o n i s undoubtedly formed; addi t i o n of H 2 r e s u l t s i n immediate l o s s of the 440 nm peak and t h i s i s a t t r i b u t e d to r e a c t i o n (21). On standing, the spectrum slowly changes giving increased absorption i n the 350-400 nm region with development of a peak at 350 nm (Figure 17(B) ), c h a r a c t e r i s t i c of HRh(diop) 2. A d i s t i n c t p o s s i b i l i t y i s the slow establishment of an equilibrium such as -H 2 R h ( d i o p ) 2 + ^ ^ -•TRh(diop)2 + H + (24) 86 Such an equilibrium has been suggested r e c e n t l y by Schrock and Osborn f o r some re l a t e d t e r t i a r y phosphine and arsine systems although they were not able to i s o l a t e the monohydride species. In the case of a c i d substrates, e q u i l i b r i u m (24) i s not observed (the spectrum remains as that of the dihydride), + since the H maintains the equilibrium to the l e f t . Thus the various s p e c t r a l data although somewhat complex, can be accounted f o r at l e a s t q u a l i t a t i v e l y at t h i s stage by the various reactions o u t l i n e d i n equations (17), (18), (21), (23) and (24). Some strong evidence f o r the involvement of the butanol i n the production of the c a t i o n using non acid substrates ( r e a c t i o n 23) i s the s p e c t r a l data of Figure (23) which shows the changes obtained on adding d i e t h y l maleate to the r 15 400 A . 1 Time,hr, 0 ) 0 20 C O 5 (min 00 23 5 0 0 JL 6 0 0 Wavelength, nm F i g u r e 2 3 - A b s o r p t i o n s p e c t r a o f H R h ( d i o p ) 2 (1.1 x 10~ 3M) i n t o l u e n e i n the p r e s e n c e o f d i e t h y l maleate ( 4.0 x 10~ 2M) a t 3 0 + 1 . S o l u t i o n under argon. S o l u t i o n under hydrogen 7 0 0 HRh(diop)2 complex i n toluene only. Although not f u l l y studied, the spectra show no sign of a 440 nm band due to the Rh(diop)2 species. Small amounts of a l k y l formation seems l i k e l y . The long induction periods noticed f o r the hydrogenation of IA i n the presence of added diop (Figure 8) may or may not involve decomposition of the hydride to the c a t i o n ; i t may w e l l be that the hydrogenation i n these cases involves the r^RhCdiop^"*" complex. A v i s i b l e spectrum of such a hydrogenating s o l u t i o n a f t e r 80$ hydrogenation, and then placed under Ar, showed a peak at 440 nm c h a r a c t e r i s t i c of Rh(diop)2 + species. Further work i s i n progress on these c a t i o n i c systems. 74 C H A P T E R V General Conclusions. The rhodium (I) hydride complex HRh(diop) ?, o r i g i n a l l y prepared i n t h i s 61 lab o r a t o r y by Dr. A. Ter.ster, has been more f u l l y characterized, and i t s c a t a l y t i c properties f o r hydrogenation studied f u r t h e r . The presence of the c h i r a l diop lig a n d induces asymmetric synthesis, and a number of unsaturated o l e f i n i c acids have been hydrogenated to products with moderate enantiomeric excess ( 2 0 to 5 0 £, depending on the substrate). The hydrogenation rates are s e n s i t i v e to s u b s t i t u t i o n at the o l e f i n i c bond, and s t e r i c f a c t o r s c l e a r l y dominate; acids containing terminal (C=-CH2) groups are r e a d i l y hydrogenated although bulky 0<- substituents hinder the reacti o n . The acid function appears to play a r o l e as yet not. delineated, since the corresponding e s t e r s and terminal o l e f i n s such as hex-l-ene are reduced much more slowly. K i n e t i c and spectrophotometric studies on the i t a c o n i c a c i d system suggest a mechanism i n v o l v i n g formation of a metal a l k y l ( v i a co-ordination of the substrate to the monohydrido c a t a l y s t ) followed by subsequent r e a c t i o n with hydrogen to give the saturated product with regeneration of c a t a l y s t . I n d i r e c t evidence suggests f a i r l y strongly that the a c t i v e c a t a l y s t i s a I I HRh(diop)(diop*) complex where diop* r e f e r s to a monodentate dangling phosphine moiety; HRh(diop) 2 ..-^  HRh(diop)(diop*) HR.h(diop)(diop*) + o l e f i n ^ = ^ H R h ( d i o p ) ( d i o p * ) ( o l e f i n ) hT2 • "f ' Saturated product + Rh(diop)(dioo*)(alkyl) HRh(dioo)(dioo*) 75 V i s i b l e spectroscopy revealed some i n t e r e s t i n g and unexpected slow "side reactions" that occurred a f t e r the c a t a l y t i c hydrogenation reaction, i f the o l e f i n reduction was rapid, and po s s i b l y during hydrogenation i n cases where the substrate system reduced slowly. Evidence i s presented f o r each of the steps i n the following system. HP.h(diop) 2 + H+-^=^= HgP.hCdiopJg \ + - 5-b(diop)J where the H + source i s the o l e f i n i c a c i d . 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