Thiophenes as Traps for Benzyne. 2. Cycloadditions and Ene Reactions

Article abstract of DOI:10.1021/jo00270a024

The reactions of 11 mono- and disubstituted thiophenes (1b-1l) with benzyne generated from diphenyliodonium 2-carboxylate (7) were studied under a standard set of conditions.The major products were naphthalenes (3) whose substitution patterns indicated that both <4 + 2> and <2 + 2> cycloaddition occurred, with the former predominating.A significant proportion of <2 + 2> cycloaddition was found only with the 3-halothiophenes 1i and 1k, suggesting enamine-like enhancement of the 2?-character of the thiophene.Benzothiophenes (8) were found in low yields in several of the reactions, and their substitution patterns suggested a <3 + 2> cycloaddition mechanism involving an intermediate betaine 11.Ene (17) and especially double ene (18) products were observed with the alkylthiophenes 1b-d and 1g.The halothiophenes 1h-l underwent substantial carbon-halogen bond cleavage leading to halogenated phenylthiophenes.