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, But(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.