Approaches to benzo[b]thiophenes by gas-phase pyrolysis of methyl 2-(alkylthio)cinnamates

Flash vacuum pyrolysis (FVP) of 3-[2-(t-butylthio)phenyl]propenoate 7 at 700 °C (0.01 Torr) unexpectedly gave a mixture of benzo[b]thiophene derivatives 5 (15%) and 8 (21%), and thiocoumarin 9 (28%). Control experiments show that thiophenoxyls (e.g. 15) cyclise efficiently to benzo[b]thiophene 5 under similar conditions. It follows that FVP of S-t-butyl derivatives of thiophenols, is not an efficient means of generating thiophenoxyl radicals owing to competing hydrogen capture processes.