The mechanism of the Wittig olefin synthesis

Abstract

The ratios of cis- and trans-stilbene produced from the erythro form of the phosphonium salt MePh₂P⁺CHPh·CHPh(OH) l^– and base in various solvents at different temperatures and in the presence of m-chlorobenzaldehyde have been determined. The results are consistent with a picture of Wittig olefin syntheses using reactive phosphoranes, which involves (a) reversible betaine formation, (b) betaine dissociation at a rate comparable to that of betaine elimination, (c) little double-bond character in the transition states leading to olefin, and (d) a transition state leading to betaine dissociation which involves considerable desolvation. Direct interconversion of diastereoisomeric betaines is of minor importance.