Stereochemistry of the bromination of acetylenes with bromine and copper(II) bromide

Abstract

Reactions of acetylenes (RCCR′: R and/or R′= H, alkyl, or phenyl)(1) with molecular bromine in chloroform give a mixture of the corresponding E- and Z-dibromoalkenes in high yields, the E-isomer being predominant under kinetically controlled conditions except in the case of (1; R = Ph, R′= Bu^t) where the Z-isomer is the sole product. From the reaction with an excess of bromine or under u.v. irradiation a thermodynamic equilibrium mixture of both isomers was obtained, the same mixture being readily formed by u.v. irradiation of a solution of any mixture of isomers in chloroform. Reactions with copper(II) bromide in acetonitrile at 20–25 °C also give the same products, the rates of reacton being much less and the isomerization of the products much slower, and the stereospecificity being much higher than for bromination with molecular bromine. The kinetically controlled product ratios between E- and Z-dihalogenoalkenes are obtained from the reactions of four alkylphenylacetylenes (R = Ph, R′= H, Me, Prⁱ, or Bu^t) with iodine, iodine monochloride, bromine, and chlorine. The results are explained in terms of a cyclic iodonium ion intermediate for the former two reactions and an open vinyl cation intermediate for the last; for bromination, an open vinyl cation intermediate in which bromine may interact weakly with the benzylic carbon is postulated.