Synthesis, Stability, and Photoreactivity of Diazirinyl-Substituted N-Heterocycles Based on Indole, Benzimidazole, and Imidazole

The synthesis, thermal stability, and photoreactivity of trifluoromethyl diazirines installed at the heterocyclic section of N-methylindole, N-methylbenzimidazole, and at N-methylimidazole were investigated. N-Tosyl-3-diazirinylindole and N-methyl-2-diazirinylbenzimidazole proved to be thermally stable, whereas the corresponding 2-diazirinylindole was not. The least stable was 2-diazirinylimidazole, which underwent rapid decomposition. Quenching with ethanol indicated that the corresponding carbene was formed. Decomposition is rationalized by exothermic coarctate ring-opening of the carbene. Quantum mechanical calculations [B3LYP/6-311G (2d,2p)] predict singlet ground states of all carbenes. Accordingly, Friedel-Crafts alkylation products were formed on irradiation (350 nm, Rayonet) of the N-methylbenzimidazole-based diazirine in the presence of phenol.