Electron spin resonance study of the reaction of hydroxyl radicals with pyrrole, imidazole, and related compounds

Radicals produced by the reaction of OH with pyrrole, 2-pyrrolecarboxylic acid, N-methylpyrrole, N-methyl-2-pyrrolecarboxylic acid, imidazole, 2-methylimidazole, 4,5-imidazoledicarboxylic acid, isoxazole, pyrazole, 3,5-pyrazoledicarboxylic acid, maleimide, and N-ethylmaleimide have been studied by the in situ radiolysis steady-state esr technique. In all cases OH has been found to add to a carbon rather than to abstract H. With pyrroles addition takes place at the position adjacent to the nitrogen and with imidazoles addition to both positions 2 and 5 has been found. In contrast to the furans the OH adducts do not undergo ring opening in alkaline solutions. In the case of pyrrole, imidazole, and 2-methylimidazole the OH adducts have been found to undergo a base-catalyzed water elimination involving the OH and the H from NH. In the carboxy derivatives water elimination is a slow process and was not detected. In the latter radicals a small splitting by the OH proton has been observed in neutral solution but disappeared at pH >10 for imidazoles and >11 for pyrroles. The disappearance of the OH proton splitting is a result of rapid proton exchange. Furthermore, dissociation of the OH proton occurs with pK = 12 for the 4,5-imidazoledicarboxylate adduct and pK = 13.5 for the adducts of 2-pyrrolecarboxylate and N-methyl-2-pyrrolecarboxylate. Hydroxyl radicals add to isoxazole at the 5 position adjacent to the oxygen rather than that adjacent to the nitrogen. Addition to pyrazole also takes place at the 5 position, next to the NH group, and in both cases of isoxazole and pyrazole comparable allylic type radicals are observed. With 3,5-pyrazoledicarboxylic acid, however, OH adds to the 4 position probably because the 5 position is sterically hindered by the carboxyl group.