Reaction of diphenyl disulfide with alkynes promoted by di-tert-butyl and dibenzoyl peroxide: a useful synthetic route to 3-(and 2,3-) substituted benzo[b]thiophenes

Thermal reactions of diphenyl disulfide 1 with phenylacetylenes 2a–e promoted by di-tert-butyl peroxide (TBP) at 110 or 150 °C provides a useful synthetic route to the (2-substituted) 3-phenylbenzo-[b]thiophenes 4a–e, which result from intramolecular cyclization of 1-phenyl-2-(phenylthio)vinyl radical intermediates 3a–e. Similar reactions of the disulfide 1 with tert-butyl-and trimethylsilylacetylenes 2f and 2g also provide the corresponding benzothiophenes 4f and 4g in satisfactory yields; low yields of the benzothiophene products 4h, i, however, are obtained with hex-1-yne 1h and hex-3-yne 1i. Evidence is presented that the thermal reaction of the disulfide 1 with the alkynes 2 at 100 °C in the presence of dibenzoyl peroxide, while being of little use for the synthesis of benzothiophenes 4, generally leads to the 2-(phenylthio)vinyl benzoates 10; this is ascribed to initial addition of benzoyloxy radicals to alkyne triple bonds.