Ligated boryl radicals. Part 2. Electron spin resonance studies of trialkylamin–boryl radicals

Abstract

Transient amine–boryl radicals R₃N → ḂH₂(1) have been generated in solution by hydrogen-atom abstraction from a variety of amine–borane complexes R₃N → BH₃, and studied by e.s.r. spectroscopy. The e.s.r. spectrum of (1; R = Me), and not of [Me₃N → BH₃]^–˙ as previously proposed, is also detected after γ-irradiation of polycrystalline Me₃N → BH₃ at 77 K. The hyperfine splitting constants for (1) indicate a pyramidal arrangement of ligands about the boron centre, in contrast to the effectively planar geometry at the radical centre in isoelectronic primary alkyl radicals. In solution, (1; R = Et) rapidly abstracts halogen from alkyl bromides and less readily from alkyl chlorides; at 246 K Bu^tBr is only 1.2 times more reactive than PrⁿBr. The radical (1; R = Et) adds to MeCN to give an iminyl radical and displaces a methyl radical from MeNC; it fails to add to ethylene, but adds readily to the more electron-deficient double bond in trimethyl(vinyl) silane. Amine–boryls undergo β-scission much more readily than their isoelectronic alkyl radical counterparts; this difference is attributable to the greater exothermicity of the former cleavage. The aziridine– and azetidine–boryl radicals undergo rapid ring opening; C–N cleavage in the cis-isomer of the 2-methylaziridine–boryl radical gives mainly the secondary alkyl radical, while the trans-isomer gives mainly the less stable primary alkyl radical. Substituted but-3-enylamine–boryl radicals, analogues of the hex-5-enyl radical, undergo regioselective 1,5-exo-cyclisation to give analogues of cyclopentylmethyl. Relative rates of β-scission and cyclisation of amine-boryl radicals have been determined in competition with their abstraction of bromine from ethyl bromide.