Memory effects and site selectivity in the β-scission of tetra-alkoxyphosphoranyl radicals

Abstract

Cyclic and acyclic phosphoranyl radicals of the type (RO)₂Ṗ(OR¹)OR² have been generated by both the reactions (RO)₂POR¹+ R²O˙ and (RO)₂POR²+ R¹O˙ and shown to undergo β-scission to give R¹˙ and R²˙, such that the relative yields of these alkyl radicals are independent of the source of the phosphoranyl intermediate. The phosphoranyl radical exhibits no ‘memory’ of its origin. E.s.r. line-shape effects and the results of ¹³C-labelling studies are interpreted in terms of rapid exchange of apical and equatorial alkoxy ligands in (RO)₄P˙. It is concluded that, in general, the high rate of alkoxy ligand exchange in the phosphoranyl intermediate will preclude the detection of a memory effect in phosphite oxidation by alkoxyl radicals. Both the formation [from RO˙ and (RO)₃P] and the β-scission of (RO)₄P˙ appear to be site-selective processes. The entering alkoxyl radical becomes an apical ligand and cleavage of a C–O bond occurs more readily in an apical alkoxy group. A scheme is presented to account for these site selectivities and for the mobility of the ligands in phosphoranyl radicals. It is suggested that a species in which the unpaired electron is confirned to one σ*-P–ligand orbital is a common intermediate in these processes.