Mechanism of hydrolysis of phosphonium salts and ylides

Abstract

Hydrolysis of (+)-benzylethylmethylphenylphosphonium iodide via the corresponding ylide in a low polarity medium gives (±)-ethylmethylphenylphosphine oxide. This racemisation is in contrast to the inversion of configuration that accompanies alkaline hydrolysis of the salt in highly aqueous media. The stereochemical difference must be due to the change in medium because the mechanisms are basically the same. It is concluded that the decrease in the polarity of the medium increases the lifetimes of the five-co-ordinate intermediate phosphoranes, permitting extensive pseudorotation. Furthermore, the inversion in the hydrolyses in highly aqueous media can be seen to be due to the short lifetimes of the intermediate phosphoranes, which inhibits pseudorotation, and blocks the reaction pathways leading to products with retention of configuration.