Homolytic reactions of ligated boranes. Part 10. Electron spin resonance studies of radicals derived from ligated arylboranes

Abstract

The ligated arylboryl radicals L→ḂHAr [L = Me₃N, Et₃P, or (MeO)₃P; Ar = Ph or p-Bu^tC₆H₄] have been generated in oxirane solvent by hydrogen atom abstraction from L→BH₂Ar using t-butoxyl radicals produced by u.v. photolysis of di-t-butyl peroxide. The e.s.r. spectra of the phosphine- or phosphite-ligated radicals show that there is substantial conjugative delocalisation of the unpaired electron from boron onto the aromatic rings, although this delocalisation is less extensive than in comparable benzylic carbon-centred radicals. The results of ab initio molecular orbital calculations support the proposal that hyperconjugative delocalisation onto the phosphorus ligand competes with conjugative delocalisation onto the ring in the complexed arylboryl radicals. The e.s.r. spectra of the amine–arylboryl radicals were too weak to detect, although these radicals and Et₃P→ḂHAr abstract halogen atoms readily from alkyl bromides to afford spectra of the corresponding alkyl radicals. The ligated arylboryl radicals are less reactive and more selective in bromine atom abstraction than homoleptic ligated alkylboryl radicals, presumably because the former are appreciably stabilised by conjugative delocalisation of the unpaired electron onto the aromatic rings.