Intramolecular cyclization to 1-phenyl-1-benzothiophenium salts by electrophilic addition of o-(phenylsulfanyl)phenylalkynes

Electrophilic addition of 1-[o-(phenylsulfanyl)phenyl]-2-(p-methoxyphenyl)ethyne with electrophiles such as perchloric acid, tetrafluoroboric acid, bromine, and benzenesulfenyl chloride gave 1-phenyl-1 -benzothiophenium salts exclusively. The substituent effect on the intramolecular cyclization with electrophiles has been examined. The aryl-substituted alkynes afforded predominantly the cyclized 1-phenyl-1 -benzothiophenium salts but methyl-substituted alkynes yielded a mixture of the cyclized salt and the 1,2-addition product. In addition to the intramolecular cyclization at the intermediate vinyl cation or bridged ion, it is proposed, on the basis of the product from the reaction of methyl-substituted alkyne, that the π complex partly participates in the intramolecular cyclization.