The thermolysis of five-membered heteroaromatic anhydrides as a route to arynes and fused cyclopropenones

Abstract

In order to investigate if the mechanism of the flash vacuum thermolysis (FVT) of 2,3-thiophenedicarboxylic anhydride proceeds via a thiophenocyclopropenone intermediate, the reaction was carried out in the presence of several trapping agents. The results obtained by the use of nucleophiles such as ammonia and water were inconclusive. Trapping agents such as hexafluoroacetone (HFA), pentafluoroacetone (PFA) and methyl trifluoroacetate (MTFA) led to the isolation of y-lactone adducts which were proven to be the derivative of gamma-thenoic acid and not of 2-thenoic acid by an independent synthesis of the two possible PFA-derived gamma-lactones. Control experiments demonstrated that neither thiophene anhydride nor thenoic acids reacted directly with HFA thereby leaving the thiophenocyclopropenone as the most likely species being trapped. The formation of the gamma-lactone adducts indicates a significant contribution to the structure of thiophenocyclopropenone of a 1,3-dipolar form with the anion at the 2-position. Trapping with fluorinated carbonyl compounds was also successful for cyclopropenones fused to 1-phenylpyrrole and benzene, generated from the FVT of 1-phenylpyrrole-2,3-dicarboxylic anhydride and the oxidation of J-aminobenzotriazin-4(3H)-one, respectively. An attempt to trap 1-phenyl-2,3-dehydropyrrole from the FVT of 1-phenylpyrrole-2,3-dicarboxylic anhydride in the presence of dienes failed indicating that 2,3-dehydropyrrole did not form and hence might be less stable than dehydrothiophene. Further evidence for the existence of 2,3-dehydrothiophene was also provided from the FVT of 2,3-thiophenedicarboxylic anhydride in the presence of benzene. The chemistry of benzyne-diene (furan, cyclopentadiene and thiophene) adducts was studied in order to establish the feasibility of the postulated modes of cleavage of the corresponding thiophyne-diene adducts.