Homolytic reactions of ligated boranes. Part 9. Overall addition of alkanes to electron-deficient alkenes by a radical chain mechanism

Abstract

Methyl bromoacetate and ethyl 2-bromopropanoate are reduced by Buⁿ₃P→BH₃ or Buⁿ₃P→BH₂Ph to methyl acetate and ethyl proponoate, respectively, in chlorobenzene at 80–110 °C in the presence of dibenzoyl peroxide or t-butyl perbenzoate. Amine complexes of borane or phenylborane are much less effective reducing agents. The reductions may also be initiated photochemically and are inhibited by a phenolic radical scavenger. A homolytic chain mechanism is proposed in which the phosphine–boryl radical abstracts halogen from the bromo ester and is subsequently regenerated by reaction of an α-(alkoxycarbonyl) alkyl radical with the phosphine–borane. The latter propagation step, together with halogen abstraction from Rl and addition of the derived alkyl radical to the CC bond, is also involved in the chain reaction between Buⁿ₃P→BH₂Ph, an alkyl iodide, and ethyl acrylate according to equation (A); Buⁿ₃P→BH₃ reacts similarly but gives lower yields of ester. Reaction (A) proceeds smoothly at 110 °C Buⁿ₃P→BH₂Ph + Rl + CH₂CHCO₂Et→RCH₂CH₂CO₂Et + Buⁿ₃P→BHIPh (A) when initiated by t-butyl perbenzoate and moderate yields of isolated esters were obtained from n-butyl iodide, cyclohexyl iodide, and 3β-iodocholest-5-ene. This last iodide gives an epimeric mixture of 3α-and 3β-esters in total isolated yield of ca. 50%. Similar addition reactions take place between Buⁿ₃P→BH₂Ph, Buⁿl, and diethyl vinylphosphonate or phenyl vinyl sulphone. It is concluded that Buⁿ₃P→BH₃ and particularly Buⁿ₃P→BH₂Ph offer promise as alternatives to tin, mercury, and germanium hydrides in radical chain reactions of synthetic value.