Homolytic reactions of ligated boranes. Part 6. Electron spin resonance studies of dialkyl sulphide–boryl radicals

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Abstract
Hydrogen-atom abstraction by ButO˙ or (Me3Si)2N˙ from dialkyl sulphide–boranes (R2S→BH3; R = Me or Et) gives the corresponding dialkyl sulphide–boryl radicals (R2S→BH2), the structures and reactions of which have been studied in solution using e.s.r. spectroscopy. The equilibrium geometry at the radical centre in R2S→BH2 appears to be planar or very nearly so, as evidenced by the 11B and Hα hyperfine coupling constants and by the effect of deuterium substitution at boron. Sulphide–boryl radicals undergo ready β-scission [equation (A)], in common with the amine–boryls R3N→BH2 but in marked contrast to, R2S→BH2→ R˙+ RS[graphic omitted]BH2(A) the phosphine–boryls R3P→BH2. Sulphide–boryl radicals abstract halogen rapidly from alkyl bromides and, at 197 K, t-butyl bromide reacts with Me2S→BH2 2.6-times more rapidly than n-propyl bromide. Approximate rate coefficients for the formation and transformation of sulphide–boryl radicals have been obtained from competition experiments. Ab initio molecular orbital calculations at the 6-31G** level for sulphide–, phosphine–, and amine–boryl radicals and for related species provide a basis for interpretation of the experimental results.

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