Electrophilic additions to acetylenes. Part II. Addition of alkyl chlorides to diphenylacetylene

Abstract

The addition of t-butyl, diphenylmethyl, and benzyl chlorides to diphenylacetylene in dichloromethane in the presence of a Lewis acid has been investigated. With t-butyl chloride the expected 1 : 1 addition products are formed, while with diphenylmethyl chloride cyclisation to indene derivatives is observed. In the case of benzyl chloride both addition and cyclisation products are obtained. The configuration of the 1 : 1 addition products, assigned by ¹H n.m.r. spectroscopy, shows that in the case of t-butyl chloride the anti addition product is predominant, while with benzyl chloride the syn addition product is preferentially formed. The results are discussed in terms of linear vinyl cation intermediates, which may give either the cyclisation or addition products. The apparent contradiction between the results with t-butyl chloride and with benzyl chloride is resolved by considering preferential attack of the nucleophile Cl^– from the less hindered side of the cationic intermediate.