Mechanisms Wittig reaction

Trialkyl phosphorus ylides, R3P=CH2, are strong nucleophiles and react with organic ketones and aldehydes to form alkenes via a [2+2] cycloaddition mechanism (Wittig Reaction). We have discovered recently that such ylides can also react with free carbon dioxide at one atmosphere and room temperature. When Me3P=CH2 in THF was bubbled with carbon dioxide gas for 30 minutes, free ketene, CH2=C=0, was produced. The formed... 

To understand the mechanism of the Wittig reaction we need to examine the struc... 

To understand the mechanism of the Wittig reaction, we need to exanine the structure and properties of ylides. Ylides are neutral molecules that have two oppositely... 

Active Figure 19.13 MECHANISM The mechanism of the Wittig reaction between a phosphorus ylide and an aldehyde or ketone to yield an alkene. Sign in at www.thomsonedu.com fo see a simulation based on this figure and to take a short Quiz. 

The final step in an attempted synthesis of laurene, a hydrocarbon isolated from the marine alga Laurencin glandulifera, involved the Wittig reaction shown. The product obtained, however, was not laurene but an isomer. Propose a mechanism to account for these unexpected results. 

Exanrple Ester ( 2) was needed as a model compound to study mechanisms of biological ester hydrolysis. It is clearly made from acid (33) which could be made from aldehyde (34) by a Wittig reaction. We discussed the synthesis of (3b) on page 21. 

Because the mechanisms of the Peterson reaction, a synthetic alternative to the Wittig reaction, have recently been reviewed [13, 14], this section will try only to summarize recent developments and trends concerning the Peterson reaction. 

Monocyclic Phosphoranes. - Further studies on the mechanism and stereochemistry of the Wittig reaction have been conducted by a combination of 1H, 13C and 3 P n.m.r.2k 25. The results show that at -18°C both ois and trans diastereomeric oxaphosphetans (e.g. 17 and 18) may be observed and their decomposition to alkenes monitored by n.m.r. Evidence was presented to suggest that during this process oxaphosphetan equilibration involving the siphoning of (17) into (18) occurred in competition with alkene formation. 

This accounts for the considerable discrepancy between the alkene Z/E ratio found on work-up and the initial oxaphosphetan ais/trans ratio. By approaching the problem from the starting point of the diastereomeric phosphonium salts (19) and (20), deprotonation studies and crossover experiments showed that the retro-Wittig reaction was only detectable with the erythreo isomer (19) via the cis-oxaphosphetan (17). Furthermore, it was shown that under lithium-salt-free conditions, mixtures of (19) and (20) exhibited stereochemical drift because of a synergistic effect (of undefined mechanism) between the oxaphosphetans (17) and (18) during their decomposition to alkenes. 

The notable increases in theoretical studies of ylides and their reactions reflects similar developments throughout organic chemistry. The renewed interest in the mechanism of the Wittig reaction have continued, but in spite of several years of considerable effort from a number of respected research groups a fully satisfactory explanation is still some way off. 

As mentioned in Sections 1 and 3, one of the most important problems in the study of the Wittig reaction mechanism is the determination of the relative stability of betaines with the open structure and cyclic oxapho-sphetanes as intermediates. The solution to a similar problem in chemistry... 

The thio-Wittig reaction, like the Wittig itself, may involve (thia)phosphetane or betaine-type structures as intermediates. A combined experimental and theoretical study over a wide range of conditions and of substrates (aliphatic vs aromatic, aldehyde- vs ketone-derived) suggests a mechanistic continuity, with solvent polarity and substrate electronic effects being the main influences on the transition from one mechanism to another. ... 

The Wittig reaction begins with the preparation of a phosphorus ylide (a Wittig rec ent). Figure 10-29 illustrates to formation of a typical phosphorus ylide by an Sp 2 mechanism, in this case, methylenetriphenylphosphine. 

E. Vedjs and C.F. Marth, Mechanism of the Wittig reaction the role of substituents at phosphorus,... 

Mechanisms of competing reactions of Wittig reagents with substituted 2-amino- 1,4-naphthoquinones have been discussed73 and a study of the stereoselectivity of the indirect Wittig reaction of a 1,2-hydroxyphosphonium salt has led to the conclusion... 

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