Kinetic (Not Equilibrium) Factors Are Dominant in Wittig Reactions of Conjugated Ylides

Article abstract of DOI:10.1021/ja00197a055

The Wittig reaction of ester-, vinyl-, or benzyl-stabilized ylides is examined in detail.Extensive control experiments have been performed to probe the oxaphosphetane intermediates, and reversal in these systems is ruled out as a significant (>5percent) process.Betaine reversal, on the other hand, can be detected in the control experiments, depending on the conditions.Maximum betaine reversal is associated with formation of the anti betaine rotamers, while syn betaines can be generated in ethanol or THF without significant reversal cases.It is emphasized that betainesare obligatory intermediates in the control experiments, but they are neither obligatory nor likely intermediates in Wittig reactions, especially the E-selective examples conducted in aprotic solvents.Only the oxaphosphetanes are required to describe the overall Wittig process from ylide to alkene.Other intermediates are not necessary, including zwitterions, specific conformers, pseudorotamers, etc.The traditional control experiments are more complex and are shown to involve anti betaines as well as syn betaines (2, 24, 25), and in certain cases, hydroxy ylides (28) derived from the betaines.The E-selective reactions of ester-stabilized ylides are described as asynchronous cycloadditions with a relatively advanced, oxaphosphetane-like transition state.Exceptionally E-selective olefination is achieved using the allylic dibenzophospole ylide 11a.The intermediate oxaphosphetanes 14a, 15a, and 15b are observed for the first time in a conjugated-ylide reaction.