Condensed thiophen ring systems. Part XIV. Photolysis of azidobenzo[b]thiophens in secondary amines

Photolysis of 4-azidobenzo[b]thiophen in diethylamine gave a mixture of starting material, 4-aminobenzo[b]thiophen, 4,4′-azobenzo[b]thiophen (5), tar, and a trace of an unidentified compound. In contrast, 5-azidobenzo[b]thiophen gave 4-amino-5-diethylaminobenzo[b]thiophen (6) as the only isolable product. The 4-amino-5-dialkylaminobenzo[b]thiophens (7)–(9) were prepared similarly by photolysis of a 5-azidobenzo[b]thiophen in the appropriate secondary amine. The mechanism of formation of these products is discussed. Ethyl 5-azidobenzo[b]thiophen-2-carboxylate was photolysed and thermolysed in various other nucleophilic solvents to give products derived from a triplet nitrene only. Ethyl 4-amino-5-diethylaminobenzo[b]thiophen-2-carboxylate was converted into ethyl 3-ethyl-2-methylthieno[3,2-e]benzimidazole-7-carboxylate with a mixture of formic acid and hydrogen peroxide.