The mechanism of hydrolysis of phosphonium ylides

Abstract

The mechanism of hydrolysis of benzylidenetriphenylphosphorane is similar to that of the hydrolysis of the corresponding salt. The much faster rate of hydrolysis of the ylide is due to the different media in which hydrolysis occurs. It has been found that a large reduction in the water content of the media can accelerate the reaction by a factor of >10⁵. It is proposed that the low polarities of the solutions in which ylides are usually hydrolysed shift the pre-equilibrium concentrations extensively away from phosphonium hydroxide and towards the five co-ordinate hydroxyphosphorane which decomposes under the catalytic action of base to give phosphine oxide and hydrocarbon. The rate of hydrolysis of benzyltriphenylphosphonium bromide is also greatly accelerated in media of low polarity, the rate being similar to that of the ylide in the same media.