Formation of naphthodithiophene isomers by flash vacuum pyrolysis of 1,6-di(2-thienyl)- and 1,6-di(3-thienyl)-1,5-hexadien-3-ynes

To develop a synthetic method of thiophene-based polycyclic aromatic compounds, we investigated the tandem cyclization of 1,6-dithienyl-1,5-hexadien-3-ynes under flash vacuum pyrolysis (FVP) conditions at high temperatures (850 or 1050 °C). As a result, several isomeric naphthodithiophene derivatives were obtained as mixtures, from which a few isomers were isolated. The structural assignments of the products were performed on the basis of combination of the experimental and calculated 1H NMR chemical shifts of either purified products or mixtures of them. Plausible mechanisms for the formation of the products are proposed.