The acid-catalysed rearrangements of 4,5-bis(2-thienylhydroxymethyl)-1,3-dithiole-2-thione

Under strongly acidic conditions, the title compound 1 readily participates in several possible rearrangement pathways, affording a product distribution which is relative to the choice of solvent and acid catalyst. Thus, using chloroform or acetone as the solvents and HBr or HClO4 as the catalysts, compounds 2–4 have been isolated and fully characterised; in addition, compound 5 was identified in the reaction mixture and characterised by 1H NMR spectroscopy. The reaction kinetics of the transformations have been studied by 1H NMR spectroscopy, using deuterated chloroform or acetone as the NMR solvents. A key intermediate in the reaction mechanisms is the allylic carbocation 6, which rearranges to give the fused system 3; in the presence of bromide anions, the carbocation forms an ion-pair intermediate 7, leading to the formation of compounds 2, 4 and/or 5, depending on the solvent.