Nucleophilic attack on halogeno(phenyl)acetylenes by halide ions

Abstract

Nucleophilic reactions between halogeno(phenyl)acetylenes and halide ions, ArCCX + Y^–, where Ar = C₆H₅ or p-ClC₆H₄, X = Cl or Br, and Y = Cl or Br, have been examined. Halogen exchange of the Finkelstein type was observed for the first time in acetylene halides in anhydrous dimethyl sulphoxide when X = Br and Y = Cl. This exchange did not occur with other X–Y combinations. In the presence of up to 20% water in dimethyl sulphoxide, or under aqueous–organic phase-transfer catalytic conditions, nucleophilic addition (formally of HY) took place for all the X–Y combinations studied. In the additions, the nucleophile Y^– invariably attacked the carbon to which the phenyl group was bound. The mode of HY addition was stereospecifically trans; accordingly, the resulting dihalogenostyrenes always had the (Z)-1,2-dihalogeno-configuration.