Proton transfer from imidazole, benzimidazole, and their 1-alkyl derivatives. FMO analysis of the effect of methyl and benzo substitution

The rate–pH profile for detritiation from the C-2 position of 1-methylimidazole has been determined in aqueous solution at 85 °C. The profile is consistent with a mechanism involving attack by hydroxide ion on the conjugate acid of the substrate to give an ylid intermediate in the rate-determining step. At higher pH, hydroxide-catalyzed exchange of the neutral species becomes increasingly important. Comparison of the second-order rate constants derived from the rate–pH profiles of imidazole, 1-methylimidazole, benzimidazole, and 1-methylbenzimidazole showed that methyl substitution caused the rate to increase by 2-to 3-fold while benzo annelation increased the rate by 10- to 20-fold. Frontier molecular orbital (FMO) analysis of the reaction scheme for proton transfer from imidazole, benzimidazole, and their 1-alkyl derivatives has been used to explain the rate-accelerating effect of methyl substitution and benzo annelation in these processes.