t-BuONa-Mediated Redox Condensation between o-Nitroanilines and Benzyl Alcohols towards 2-Phenyl Benzimidazoles under Transition-Metal-Free Conditions

A novel and environmentally benign method was developed for the direct synthesis of 2-substituted benzimidazoles via a transition metal-free redox tandem annulation of benzyl alcohols and ortho-nitroanilines in the presence of a strong base, e.g. alkaline alkoxides or hydroxides. A good tolerance of functional groups was observed, affording an array of 2-aryl benzimidazole derivatives in high yields. Early mechanistic studies showed that sodium tert-butoxide could initiate the hydrogen transfer from benzyl alcohol to the nitro group in ortho-nitroaniline followed by imine condensation, cyclization, and oxidation. This protocol features a convenient tactic that does not require any external oxidants or solvents for the synthesis of bioactive benzimidazole-involving compounds.