Site selectivity in the β-scission of alkoxyphosphoranyl radicals. A reinterpretation

Abstract

The rate constants for β-scission of cyclic and acyclic trigonal bipyramidal (TBP) phosphoranyl radicals [graphic omitted]Ṗ(OBut)X (A) and (EtO)2Ṗ(OBut)X (B)[X = EtO or (Me3Si)2N] to give t-butyl radicals have been measured by kinetic e.s.r. spectroscopy. The phosphoranyl radicals exist as equilibrium mixtures of isomers which differ in the occupancy of apical and equatorial sites and which interconvert much more rapidly than they undergo β-scission. Radicals for which access of the ButO group to an equatorial site is restricted undergo β-scission relatively slowly. The relative rates of t-butyl radical-forming β-scission at 233 K in hydrocarbon solution are shown to be approximately proportional to the equilibrium mole fraction of the isomer in which the ButO group occupies an equatorial site, provided that allowance is made for the retarding effect of the five-membered ring. β-Scission of TBP alkoxyphosphoranyl radicals thus involves equatorial site selectivity. Previous results for phosphoranyl radicals (A) and (B)(X = Me3SiO) are reinterpreted in terms of equatorial site selectivity rather than the apical site selectivity originally proposed. The apicophilicity of Me3SiO is almost certainly lower than that of ButO, whilst in the previous work the reverse order of apicophilicity was assumed.