The ‘trimer’ and ‘pentamer’ from the polymerisation of thiophen by polyphosphoric acid

Polymerisation of thiophen with phosphoric acid gave two major products, 2,4-di-2-thienyltetrahydrothiophen (I) as cis- and trans-isomers, and 4,7-di-2-thienyl-1,2,3,4-tetrahydrobenzo[b]thiophen (IV). The structures of the isomers of (I) were established by dehydrogenation to 2,2′:4′,2″-terthienyl (II) and a detailed analysis of the 1H n.m.r. spectrum. Physical methods were used to elucidate the tetrahydro[b]benzothiophen structure (IV) from which 4,7-di-2-thienylbenzo[b]thiophen (VI) was obtained by dehydrogenation.