Wittig reaction definition

The latest proposal by Vedejs (3d) is that the Wittig reaction proceeds via a concerted but asynchronous puckered 4-center cycloaddition pathway in which the stereoselectivity is determined by multiple steric effects and varying degrees of rehybridization at the phosphorus atom in the transition state. At present, there is not definitive evidence to prove that the reaction must proceed in this manner. Recent MNDO-PM3 computations by us (7,8) and somewhat related MNDO computations by Yamataka et al, (9) do not support the puckered 4-center cycloaddition hypothesis for the reactions of unstabilized ylides with aldehydes (3d), Instead, the MNDO-PM3 computations indicate that such Wittig reactions proceed through an essentially planar transition state (TS) with respect to the four central atoms, P-C-C-O. This process is... 

If ET intermediates play any role in representative aldehyde or ketone Wittig reactions, they are too short-lived for detection by the fastest available radical or radical anion clocks. This is conceivable if the geometry of the radical ion pair resembles that of an oxaphosphetane with partially developed bonds (223c). Such a scenario fits within the broad definition of a four-center mechanism and allows little (if any) distinction between the geometry of stereochemistry-determining TS that invoke ET versus those that do not. More precise distinctions may have theoretical significance, but they will not influence the stereochemical issues that are of concern in this review. 

This is obviously a Wittig reaction and we should expect a Z-alkene as the ylid is not stabilized by further conjugation. The evidence is plain the signals at 5.10 and 5.35 are the alkene hydrogens and the coupling constant between them is 10 Hz. This is definitely a Z-alkene. 

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