Alkenes from halides and epoxides by reductive eliminations with Cr II complexes

Abstract

A variety of β-substituted alkyl halides and epoxides are reductively eliminated in high yields to alkenes by Cr IIen at room temperature. The reducing agent is easily prepared in situ from chromous salts and ethylenediamine. Reductive elimination and protolytic reduction of β-substituted alkyl halides by chromous reagent proceed by similar mechanisms, and involve free radicals and alkylchromium species as intermediates. In the former process, β-substituents such as hydroxy, phenoxy, acetoxy, tosyloxy, bromo, chloro, amino and trimethylammonium groups are readily eliminated from the β-substituted alkylchromium intermediate to generate alkene. Protolytic reduction of the alkylchromium species is a significant competing reaction only with such β-substituents as cyano, benzamido and phthalimido groups. Factors which determine the effect of groups on the relative rates of reductive elimination and protolytic reduction of the alkylchromium species are discussed. The formation of a free radical by an initial ligand transfer of halogen from the alkyl halide to chromous ion is enhanced by β-bromo and trimethylammonium groups. Neighboring group participation by these groups in homolytic processes to form bromine or trimethylammonium-bridged alkyl radicals as intermediates is postulated. Cr IIen is a more powerful reductant than Cr II and the scale of relative reactivity of various halides is compressed in the former relative to the latter. o-Diiodobenzene is reduced by Cr IIen stepwise to benzene, and benzyne does not appear to be an intermediate in the reaction.