Chlorination and interhalogenation of alkylphenylacetylenes with antimony pentachloride
Abstract
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.