An electron spin resonance study of the structure and reactivity of aminophosphoranyl radicals in solution

Abstract

The structure and reactivity of a series of aminophosphoranyl radicals, generated by addition of photochemically produced alkoxyl radicals to aminophosphines, have been investigated by e.s.r. spectroscopy in solution. These phosphoranyl radicals possess a trigonal bipyramidal structure and some are stereochemically non-rigid, showing temperature-dependent line-shape effects in their spectra. In many cases, two isomers may be detected differing in the apical or equatorial placement of a dialkylamino-group.

The apicophilicities of alkoxy- and dialkylamino-groups in phosphoranyl radicals are very similar, although the latter generally have a greater preference for equatorial placement. In monocyclic aminophosphoranyl radicals in which two ligands form part of a five-membered ring, the two acyclic ligands appear to exchange position more rapidly than the endocyclic ligands, precluding pseudoratation, with the unpaired electron as pivot, as the mechanism of ligand permutation in such species. Both five- and six-membered ring systems incorporating two P–N bonds preferentially bridge apical and equatorial positions in aminophosphoranyl radicals.

Alkoxy(dialkylamino)phosphoranyl radicals fragment by α- or β-scission, depending upon the structure of the radical, but preliminary kinetic studies indicate that interconversion of isomers differing in the apical or equatorial placement of a dimethylamino-group is rapid in comparison with α-scission with P–N bond cleavage.