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Investigation of Factors Influencing the Stereochemistry of the Wittig Reaction

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Investigation of Factors Influencing the Stereochemistry of the Wittig Reaction THE INVESTIGATION OF FACTORS INFLUENCING THE STEREOCHEMISTRY OF THE WITTIG REACTION By JEROME THOMAS KRESSE A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA December, 1965 ACKNOWLEDGMENTS The author wishes to express his appreciation to his research director, Dr. George B. Butler, for his guidance and encouragement during the execution of this work. The author also expresses his gratitude to his fellow graduate students and associates for their helpful suggestions and criticisms. Particular thanks are due Mrs. Frances Kost and Mrs. Thyra Johnston for their conscientious typing of this dissertation. The author also thanks his wife for her patience, encouragement and under- standing. Without her cooperation this work would not have been possible. The financial support of the Petroleum Research Fund is also gratefully acknowledged. TABLE OF CONTENTS PAGE ACKNOWLEDGMENTS ii LIST OF TABLES v LIST OF FIGURES vi CHAPTER I INTRODUCTION 1 Historical Background 1 Stereochemistry 9 Statement of the Problem 19 Method of Attack 20 H DISCUSSION AND RESULTS 22 Solvents 22 Temperature Effects 27 Reaction Times 33 Concentration Effects 34 Substituent Effects 37 Anion Effects 49 1, 4 -Addition and Isomerization 51 TABLE OF CONTEXTS (Continued) CHAPTER PAGE EI EXPERIMENTAL 54 Equipment and Data 54 Source and Purification of Materials 54 Solvents 55 Aldehydes " 57 Miscellaneous Chemicals 57 Preparation and Purification of Phosphonium Salts ... 59 Apparatus 61 Reactants 61 65 The Reaction • • Analysis 67 Calibration of the Internal Standard 67 Qualitative Analysis 69 Precision of Chromatographic Analysis 72 IV SUMMARY 75 LIST OF REFERENCES 76 BIOGRAPHICAL SKETCH 79 LIST OF TABLES TABLE PAGE 1 Solvent and Halide Ion Effects 11 2 Combination Effects on Stereochemistry 13 3 Solvent Effects 23 4 Temperature Effects 28 5 Reaction Times 33 6 Concentration Effects 35 7 Substituent and Anion Effects 38 8 Ring Opening Reactions of 4-Octene Oxide and Stilbene Oxide 47 9 Anion Effects 50 10 Reactions of Methyltriphenylphosphoranes with Crotonaldehyde 52 11 Physical Constants of Phosphonium Salts 62 12 Infrared Absorption Phosphonium Salts 73 LIST OF FIGURES FIGURE PAGE 8 1 Energy Profile for the Reaction of a Stable Ylide 2 Postulated Reaction Paths of the Wittig Reaction 17 41 3 Possible Betaine Configurations 63 4 Reaction Manifold 70 5 Calibration Curve for trans -1, 3 -pentadiene 71 6 Chromatogram of Reaction Products vi CHAPTER I INTRODUCTION Historical Background Since Wittig and Geissler first reported the olefin synthesis which has be- come known as the Wittig- synthesis literally hundreds of papers have been published concerning the synthetic and mechanistic aspects of the reaction. This synthesis in its general form involves the reaction of a phosphorane derived from a phosphonium salt with an aldehyde or ketone. The phosphonium suits are usually prepared from triphenylphosphine and an alkyl or arylalkyl halide. The phosphorane is produced from the phosphonium salt by removal of an a-hydro- gen by a suitable base. Addition of a carbonyl containing compound to a solution of the phosphorane forms an intermediate betaine which then collapses to produce the olefin and triphenylphosphine oxide. + P: + R R CHX->[e P-CHR R ] X" 9 o i 9 I 1 + B:—>BK+ + P-C-R R 1 9 II II + RR,CO-»RRC-CRR, 3 4 1 2 3 4 | + j C.. P O in m->R, R ,C = CR R + ClP->0 x 2 o 4 J Although the reaction appears straightforward synthetically, mechanistically the reaction is exceedingly complex. The structure of the phosphorane is apparent- ly a resonance hybrid of canonical forms IV and V. Form IV is also referred to as an ylene and form V as an ylide. Form IV requires the assumption of d ir bonding, CR R " °R R V= 1 2 ~ ^ 1 2 IV V between carbon and phosphorus. That such bonding exists for phosphorus and 2 ofeerelements of the third period has been demonstrated by base catalyzed a deuterium exchange studies. Form V is that of a carbanion, the extremely strong conjugate base of the phosphonium salt. limiting forms of a If we assume that structures IV and V approximate the phosphorane we can see that the degree to which this structure resembles either IV and carbon. or V will be dependent on the nature of the substitutents on phosphorus 3 existence of a Jaffe has calculated that one of the criteria for d ir bonding is the groups positive charge on phosphorus in its singly bonded structure. It follows that lessen the on phosphorus which would be electron donating, e.g. alkyl groups would the incorporation importance of the contribution of form IV to the hybrid. Likewise would tend to make form of electron withdrawing groups on carbon, e. g. carbethoxy that groups IV more important. Regarding the reactivity of the phosphorane we find reactive species. increasing the contribution of form IV to the hybrid produce a less 4 triphenylfluorenylidenephosphorane This can be seen in comparing the properties of groups capable of delocal- and tributylethylidenephosphorane. The former contains positive charge on the phosphorus. izing the charge on the carbon and intensifying the 3 It is extremely unreactive. It can only be hydrolyzed by refluxing with strong base. The latter compound contains groups which tend to diminish the positive charge on phosphorus and increase the carbanion character of the structure. It is hydrolyzed rapidly on exposure to the atmosphere. Both compounds will react with carbonyl compounds but the conditions for carrying out the reaction are much more vigorous for the fluorenylidenephosphorane. Between these two extremes lie almost a continuum of compounds with vary- ing reactivities. It is this range of reactivity which has made the study of the Wittig reaction both interesting and challenging. Work done on one system cannot be directly correlated with a more or less reactive system. This is especially true of the stereochemistry of the reaction. 5 The mechanism of the reaction as postulated by Wittig involves an attack by form V of the phosphorane on the electrophilic carbon of the carbonyl to give a betaine. P+ o P+ o- I II II c- + .c —»- .c -c / N / \ / \ RS X R R R R R R R l 2 3 4 l 2 3 4 V VI The betaine then may collapse via a four membered cyclic transition state to olefin and triphenylphosphine oxide. In attempting to elucidate the mechanism Wittig studied the reaction between methylenetriphenylphosphorane and benzalde- — K><? P- -> o 3 c- -> = c V V ./ \v 7: R' R R R. 2 3 1 2 3 4 VI hyde. During the reaction Wittig was able to trap the betaine by the addition of hydrogen iodide giving 2-phenyl-2-hydroxyethyltriphenylphosphonium iodide which (£lP= CH + 0CHO —> 0CH— CH o 2 I 2 O PG^ VII k&CH - CH ! 2 vn OH P0 3 VIII on heating gave styrene and (LP —>0. He also refluxed benzophenone with the betaine hydroiodide, but was unable to obtain 1, 1-diphenylethylene, the product one might expect if betaine formation were reversible. Wittig therefore concluded that betaine formation is irreversible. 7 However, Filszar, Hudson and Salvadori found that the lithium bromide ether complex of the betaine isolated by Wittig yielded no olefin products on heating but rather benzaldehyde. These workers concluded that betaine formation is rever- sible and that decomposition of betaine to olefin and phosphine oxide is rate deter- . 0-CH— CH, pCH=CH + QP—>0 E:A + I c=o c9 ^r + bh r c=cn OH Pep, e _ 2 2 vm Q Speziale and Bissing have demonstrated the reversibility of betaine forma- tion by reacting triphenyl and tributylphosphine with cis_ and trans -ethyl phenyl- glycidate in the presence of m eta -chlor obenzaldehyde O /\ R P + 0CH—CHCO C H_ > R P —CHCO C H O" —CH0 IX x X > R P >0 + OCH = CHC0 C H 3 2 2 5 XI X —> R P = CHCO C H + 0CHO XII XII + m-CIC H CHO > B^—^O + m-ClC^CH^CHCO^H. xni They found using a 1:1:1 molar ratio of triphenylphosphine: trans-ethyl obtained 48.7 per cent cis phenylglyeidate: i .eta-chlorobenzaldehyde that they -chloro- ethyl cinnamate; 17.4 per cent trans ethyl cinnamate; 3. per cent cis meta cinnamate and 30.9 per cent trans meta-chiorocinnamate. These workers further showed through a kinetic study of the reaction between a series of substituted benzaldehydes with carbomethoxymethylenetriphenylphos- order, first order in phorane (a stable ylide) that the rate of reaction is second G ylide and first order in aldehyde. They found that the reaction rate is increased by increases in solvent polarity, substitution of butyl for phenyl on the phosphorus of the ylide and by electron withdrawing substitutents on the aldehyde. The mechanism they postulated based on kinetics and the epoxide ring open- ing reactions mentioned previously is slow, reversible betaine formation followed by rapid decomposition of the betaine to olefin and phosphine oxide. 9 House and Rasmusson had previously suggested a mechanism for the reaction of acetaldehyde with carbomethoxymethylenetriphenylphosphorane to give methyl tiglate and methyl angelate in which there is a rapid reversible formation of betaines with slow preferential decompositions of the stereoisomeric betaines to products. Concerning reactive ylides, Wittig, Weizmann and Schlosser had observed that though betaine formation with reactive ylides is rapid, taking place within a few minutes, the decomposition of betaine requires standing at room temperature or heating for several hours. They concluded that betaine decomposition is the slow step in this type of reaction. Bergelson and Shemyakin interpreted the stereochemistry observed in the reaction of stable ylides (giving mostly trans olefin) as opposed to unstable or reactive ylides (giving mostly cis olefin) by using two different reaction paths.
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