“All-water” chemistry of tandem N-alkylation–reduction–condensation for synthesis of N-arylmethyl-2-substituted benzimidazoles

A water-assisted tandem N-alkylation–reduction–condensation process has been devised as a new synthetic route for the one-pot synthesis of N-arylmethyl-2-substituted benzimidazoles. Water plays the crucial and indispensable role through hydrogen bond mediated ‘electrophile–nucleophile dual activation’ in promoting selective N-monobenzylation of o-nitroanilines as an alternative to the transition metal-based chemistry for C–N bond formation (amination) and forms the basis of disposing the substituents on the benzimidazole moiety in regiodefined manner. Water also exerts a beneficial effect in the condensation of N-monobenzylated o-phenylenediamines with aldehydes. The water-assisted C–N bond formation chemistry led to metal/base-free synthesis of N-monobenzylated o-nitroanilines and N-monobenzylated o-phenylenediamines. The indispensable/advantageous role of water in the various stage of the N-alkylation–reduction–condensation process exemplifies an ‘all-water’ chemistry for the synthesis of N-arylmethyl-2-substituted benzimidazoles.