The synthesis and pyrolysis of the thiopene cyclic anhydrides

Abstract

2,3-Thiophenedicarboxylic anhydride and 2,3-thianaphthene dicarboxylic anhydride were prepared by sequential metal-halogen exchange reactions using n-butyllithium and carbon dioxide on the corresponding dibromides. 3,4-Thiophenedicarboxylic anhydride was prepared by a previously reported method. Although the sealed tube pyrolysis of these anhydrides gave no indication of the corresponding hetarynes, the Flash Vacuum Thermolysis (FVT) of both 2,3-thiophenedicarboxylic anhydride and 2,3-thianaphthenedicarboxylic anhydride in the presence of diene traps gave products which are best rationalized by the intermediacy of a five-membered hetaryne. Products were isolated which correspond to 4+2 Diels-Alder addition reactions. In one case 2,3-thiophyne appears to undergo an ene-reaction with methylacetylene. The 3,4-anhydride failed to react under FVT conditions.