Hajime Okazaki

Chlorination and chloroiodination of acetylenes with copper(II) chloride

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′= But) 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.

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

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′= But) 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 But) 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.

Chlorination and interhalogenation of alkylphenylacetylenes with antimony pentachloride

Reactions of alkylphenylacetylenes with antimony pentachloride in carbon tetrachloride give the corresponding dichloroalkenes in 20–55% yields, (Z)-addition predominating. In the presence of iodine or lead(II) thiocyanate, interhalogenation occurs to give the chloroiodo- or chlorothiocyanato-alkenes respectively as the sole or the main products in fair to good yields, the (E)-isomer being predominant except the case of phenylacetylenes where the stereospecificity is low. A concerted or near-concerted molecular addition of antimony pentachloride or its dimer to triple bond is postulated for the chlorination with very high (Z)-stereospecificity, and the stepwise (E)-attack of iodine monochloride or chlorothiocyanogen activated by antimony pentachloride is proposed for the interhalogenation. Replacement of an iodine, a bromine, or a chlorine atom in vic-dihalogenoalkenes by a chlorine atom of antimony pentachloride has been shown to occur through a vinyl cation intermediate by product analysis as well as a study of the isomerization of the starting dihalogenoalkenes.