Chlorination and chloroiodination of acetylenes with copper(II) chloride

Abstract

Reactions of acetylenes (RCCR′; R and/or R′= H, alkyl, or phenyl)(1), with copper(II) chloride–lithium chloride in acetonitrile give the corresponding E-dichloroalkenes in good yields except in the case of (1; R = Ph, R′= Bu^t) where the Z-product is favoured. Reactions with copper(II) chloride–iodine or –potassium iodide proceed more smoothly to afford, completely regiospecifically and highly stereospecifically, E-chloroiodoalkenes in high yields. Reactions with iodine chloride also give the same compounds, but the yield and E-stereospecificity are low in comparison with those in the copper(II) chloride–iodine case. An open vinyl cation intermediate in which copper(I) co-ordinates weakly with both the double bond and the chlorine atom attached to carbon is postulated for chlorination, and a cyclic iodonium ion intermediate is proposed for chloroiodination.