Mechanism of Formation of Aromatic Vinylphosphonium Salts via the Peterson Olefination

The mechanism of the reaction of tributyl[(trimethylsilyl)methylene]phosphorane with benzaldehyde and its p-substituted analogues has been examined. It has been found that the electronic nature of the p-substituents in aromatic aldehydes strongly influences the stereochemical and kinetic outcome of the Peterson olefination whereas temperature substantially affects their Hammett correlation. This indicates that the Peterson olefination is a multistep reaction involving the formation of at least an oxyanion/betaine and a carbanion as intermediates. In turn, moderate Z-selectivity might be the result of “steric approach intermediate control”; however, E-selectivity seems to result from the silicon–oxygen interaction and interactions of steric substituents in competing erythro- and threo-betaines.