Analogies between silicon and phosphorus stereochemistry—I: Influence of the leaving group on the stereochemistry of nucleophilic displacement at phosphorus

Abstract

Stereochemistry of cyclic halogenophosphates has been studied by adding aryloxides, in THF. All extra parameters being equal, the fluoro substituent which is of the more electronegative (and the more apicophilic) is displaced with the more predominant retention F>Cl≈Br. Such behaviour cannot be explained with the usual concepts of phosphorus stereochemistry and, in particular, the generally accepted view of apical attack giving the more stable P V intermediate. By contrast, strong analogies with the stereochemistry observed at silicon are clearly evidenced. Like for silicon compounds, front-side attack could explain predominant retention, particularly in P-F bond cleavage. Trigonal bipyramidal intermediates with the entering group in equatorial position and the leaving group in apical position, or square-pyramidal geometries may be envisaged.