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

Abstract

Hydrogen-atom abstraction by Bu^tO˙ 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 H_α 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.