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 H-1 NMR spectroscopy. The reaction kinetics of the transformations have been studied by H-1 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.