Electrophilic additions to acetylenes. Part VI. Addition of t-butyl chloride and benzyl chloride to alkynes

Abstract

The zinc chloride catalysed addition of t-butyl and benzyl chloride to some mono- and di-substituted alkynes has been studied. The reactions afforded the 1 : 1 addition products, with predominant formation of the isomers of E-configuration. This suggested the intervention of a stepwise electrophilic addition mechanism, with the formation of α-alkyl-substituted vinyl cations and the subsequent preferential attack of the nucleophilic Cl^– from the less hindered side of the intermediates. However, some discrepancy in the E : Z ratios of the addition products and comparison with analogous stereochemical data obtained in the addition of acids to the same substrates suggested that other mechanisms could be involved, though to a small extent. Discussion of the competition among these mechanisms is given.