Homolytic reactions of ligated boranes. Part 3. Electron spin resonance studies of radicals derived from dialkylamine–boranes

Abstract

Liquid-phase e.s.r. studies show that photochemically generated t-butoxyl radicals abstract hydrogen from secondary amine–boranes R₂NH→BH₃ to give the corresponding amine–boryl radical R₂NH→ḂH₂ as the kinetically controlled product. Depending on the nature of the N-alkyl groups, the amine–boryl may undergo β-scission or may rapidly abstract hydrogen from the parent amine–borane to give the isomeric aminyl–borane radical R₂Ṅ→BH₃ which is thermodynamically more stable. Deuterium labelling experiments exclude 1,2-hydrogen atom migration as the mechanism of the isomerisation. The less electrophilic cyclopropyl radical reacts with R₂NH→BH₃ to yield the aminyl–borane directly by abstraction of hydrogen from nitrogen. In the reactions with t-butoxyl radicals, intermediate amine–boryl radicals may be intercepted by halogen atom transfer from alkyl bromides or by addition to 2-methyl-2-nitrosopropane; with one amine–borane, Bu^t(Prⁱ)NH→BH₃, the amine–boryl radical was detected directly by e.s.r. spectroscopy. The e.s.r. spectra of the aminyl–borane radicals indicate appreciable hyperconjugative delocalisation of the unpaired electron onto the BH₃ group. Ab initio and/or semi-empirical molecular orbital calculations for H₃N→ḂH₂, H₂Ṅ→BH₃, and their N-methylated derivatives support the conclusions reached by experiment.