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

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Abstract
The ligated alkylboryl radicals L→Ḃ;HR [L = Me3N, Et3P, or (MeO)3P; R = Me, But, 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→ḂHBut 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 ButO˙. Reaction of ButO˙ 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 But 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.

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