The chemistry of heteroarylphosphorus compounds. Part IV. The effect of substituents at phosphorus on the fate of phosphobetaines in protic solvents. Studies of the mechanism of the Wittig reaction

Abstract

In order to investigate the effects of strongly electron-withdrawing substituents at phosphorus on the preferred mechanistic course of the Wittig reaction in protic solvents, a study has been made of the reactions of a series of phosphonium salts bearing 2-furyl substituents, with benzaldehyde, in alcoholic alkoxide media. It is shown that the presence of the electron-withdrawing 2-furyl substituent at phosphorus strongly favours intramolecular collapse of a cyclic oxaphosphetan intermediate leading to the ‘normal’ Wittig products. Aryl-migration rearrangement products, derived from vinylphosphonium intermediates, are not observed in the reaction of methylphosphonium salts bearing 2-furyl substituents, in contrast to those of methylphosphonium salts bearing phenyl substituents. The results demonstrate that the preferred course of the reactions between phosphorus ylides and benzaldehyde in protic solvents is also dependent on the nature of the substituents at phosphorus.