Homolytic reactions of ligated boranes. Part 4. Electron spin resonance studies of radicals derived from secondary phosphine–boranes

Abstract

Radicals produced by reaction of photochemically or thermally generated t-butoxyl radicals with dimethyl-, diethyl-, and diphenyl-phosphine–boranes have been studied in solution by e.s.r. spectroscopy. Hydrogen abstraction from R2PH→BH3 appears to yield the phosphinyl–borane radical R2Ṗ→BH3 as the major product, although its e.s.r. spectrum is difficult to detect. Evidence for the competitive formation of an isomeric phosphoranyl radical [R2P(BH2)H]˙ was found for dimethylphosphine–borane. No phosphine–boryl radical R2PH→ḂH2, analogous to R3P→ḂH2 obtained from the reaction of ButO˙ with a trialkylphosphine–borane, was detected. Like the isoelectronic silyl and phosphonyl radicals, R2Ṗ→BH3 abstracts halogen from alkyl bromides and adds readily to alkenes and isocyanides to give alkyl and imidoyl radical adducts, respectively. Spin-trapping of R2Ṗ→BH3 with 2-methyl-2-nitrosopropane or phenyl-N-t-butylnitrone affords the appropriate nitroxides, whilst with 1-nitroso-2,4,6-tri-t-butylbenzene addition takes place exclusively at oxygen to give an oxyaminyl radical.