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 Bu^tO˙ 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.