Analogies between silicon and phosphorus stereochemistry—II : Influence of the attacking anion upon the stereochemistry of nucleophilic displacement at tetrahedral phosphorus

Abstract

The stereochemistry of reactions of cyclic halogenophosphates with a representative series of p-substituted aryloxides emphasizes the relative influence of the attacking anion. For a given leaving group, the stereochemistry depends essentially upon the electronic character of the p-substituent and the ion-pair dissociation of the nucleophile. Both stereochemical and kinetic data rule out a cation-assisted mechanism to explain retention at phosphorus. Meanwhile, a comparison between S N2(P) and S N2(Si) suggests similar mechanisms in the two series. Retention and/or inversion are the consequence of two competing reactions of similar energies. The stereochemistry is determined by the factors which affect the approach of the nucleophile to give two initial intermediates of different geometries.