Reactivity of vinyl sulphonic esters. Part II. Cyclisation of arylthiovinyl sulphonates to benzo[b]thiophen derivatives

Abstract

1,2-Diaryl-2-arylthiovinyl arenesulphonates, dissolved in aprotic solvents, gave arenesulphonic acids and 2,3-diarylbenzo[b]thiophens either spontaneously or with a boron trifluoride catalyst. An unusual rearrangement was observed in the cyclisation of vinyl sulphonates having a para-substituent (Me, MeO, Cl, or Br) in the arylthioresidue, which gave 6-substituted benzo[b]thiophens instead of the expected 5-substituted derivatives. No rearrangement was observed in the cyclisation of vinyl sulphonates having a meta-substituent (MeO or Cl) in the arylthio-residue, which gave a mixture of the expected 4- and 6-substituted benzo[b]thiophens. The structures of the benzo[b]thiophens formed were proved by unambiguous synthesis and in some cases by reductive desulphurisation to 1,1,2-triarylethanes. The mechanism of the cyclisation is discussed in terms of the effect of the substituents. A 1,2-sulphur migration is proposed to explain the rearrangement observed.