1,2-Sulphur shift in the acid-catalysed cyclisation of o-arylthiophenyl-substituted carbinols to thioxanthen derivatives

Abstract

During the acid-catalysed cyclisation of some o-arylthiophenyl-substituted carbinols to thioxanthen derivatives, arearrangement was observed with carbinols having a para-methyl substituentin thearylthio-group. Thus, 1-[o-(p-tolylthio)phenyl]ethanol gave 3,9-dimethylthioxanthen, while α-[o-(p-tolylthio)phenyl]benzyl alcohol gave a mixture of 3-methyl- and 2-methyl-9-phenylthioxanthen. The mechanism of the cyclisation is discussed in terms of a competition between ring-closure at the position ortho to the sulphur (ortho-substitution) and ring-closure at the position bearing the sulphur followed by a 1,2-sulphur shift (ipso-substitution).