Homolytic reactions of ligated boranes. Part 8. Electron spin resonance studies of radicals derived from ligated alkylboranes

Abstract

The ligated alkylboryl radicals L→Ḃ;HR [L = Me₃N, Et₃P, or (MeO)₃P; R = Me, Bu^t, or Me₂CHCME₂] have been generated in fluid solution, by hydrogen atom abstraction from L→BH₂R using photochemically produced t-butoxyl radicals, and studied by e.s.r. spectroscopy. The boron centre in the amine–alkylboryl radicals is pyramidal whilst that in the phosphine or phosphite complexes appears to be planar or nearly so. Unlike the isoelectronic di-t-butylmethyl radical, Me₃N→ḂHBu^t is a transient species which dimerises at close to the diffusion-controlled rate at 235K. Both amine– and phosphine–alkylboryl radicals abstract halogen rapidly from alkyl bromides, although the latter radicals are less reactive and more selective than the former. Amine–alkylboryl radicals react similarly, but more slowly, with alkyl chlorides, and halogen abstraction by the phosphine analogues was not detected by e.s.r. spectroscopy. Amine–alkylboryl radicals are highly nucleophilic and rapidly abstract an electron deficient α-hydrogen atom from a nitrile or an ester. Because of this property shown by the derived boron radicals, the amine–t-alkylboranes function as polarity-reversal catalysts for the net abstraction of electron deficient hydrogen atoms by t-butoxyl radicals. Thus, photolysis o di-t-butyl peroxide in the presence of Me₃N→BH₂CMe₂CHMe₂ and a substrate affords radical products from the latter which are determined by the reactivity and regioselectivity of Me₃N→Ḃ HCMe₂CHMe₂ rather than of Bu^tO˙. Reaction of Bu^tO˙ with Me₂NH→BH₂R yields Me₂NH→Ḃ HR as the product of kinetic control, but this is converted into the more stable Me₂Ṅ→BH₂R by abstracting hydrogen from the amine–alkylborane. When R is Bu^t or Me₂CHCMe₂, Me₂Ṅ→BH₂R breaks down very rapidly by β-scission to give the corresponding t-alkyl radical. The aziridine–(1,1,2-trimethylpropyl)boryl radical undergoes rapid ring opening.