The steric course of the formation of benzocyclobutene by thermal desulphonylation of 1,3-dihydrobenzo[c]thiophen 2,2-dioxide

Abstract

Thermolysis of cis-[1,3-²H₂]-dihydrobenzo[c]thiophen 2,2-dioxide (1b) gives trans-[1,2-²H₂]-benzocyclobutene (3b); in conjunction with the Woodward–Hoffmann rules and the stereochemistry of closely related reactions, this provides strong evidence for a mechanism involving disrotatory formation of o-quinodimethane (2) with extrusion of sulphur dioxide, followed by conrotatory closure of (2) to benzocyclobutene.