Ring cleavage of a 3-azidothiophene: novel extrusion of acetylene

Abstract

Mild thermal decomposition of ethyl 2-azido-3-(3-azido-2-thienyl)propenote (8) results in the cleavage of the thiophene ring with extrusion of acetylene and formation of ethyl 5-cyanoisothiazole-3-carboxylate (10), together with ethyl thieno[3,2-c]pyridazine-3-carboxylate (9). The former reaction represents the first fragmentation of a five-membered heteroaromatic β-nitrene to extrude an acetylene, and the latter reaction is a rare example of formal intramolecular coupling of nitrenes in solution thermolysis. The vinyl azide group is necessary for this cleavage of the thiophene ring and a mechanism is proposed in which the derived nitrene co-ordinates with the thiophene sulphur atom to faciliate ring fragmentation. The analogous furan does not fragment similarly. Diels–Alder cycloadditions of 4-phenyl-1,2,4-triazole-3,5-dione to 2-vinylthiophenes were investigated as an independent approach to the thieno[3,2-c]pyridazine system, but the fused 4-phenyl-1,2,4-triazoline-3,5-dione ring proved resistant to hydrolytic cleavage.