In Situ Generation and Synthetic Application of 2-Phenylbenzimidazoline to the Selective Reduction of Carbon–Carbon Double Bonds of Electron-Deficient Olefins

Abstract

Abstract 2-Phenylbenzimidazoline (PBI) as a mild, selective, and convenient reducing agent was efficiently generated in situ from o-phenylenediamine and benzaldehyde in alcohols. A generally applicable method for the selective reduction of carbon–carbon double bonds of a variety of electron-deficient olefins with an alcoholic solution of PBI is described. The reduction of α,β-unsaturated ketones to the corresponding saturated ketones could also be accomplished (but, less effectively) with PBI with the aid of a Lewis-acid catalyst. 1-Methyl-2-(o-nitrophenyl)benzimidazoline prepared and isolated by the reaction of o-nitrobenzaldehyde with N-methyl-o-phenylenediamine reduced benzylidenemalononitrile to give benzylmalononitrile and 1-methyl-2-(o-nitrophenyl)benzimidazole in high yields. This shows the validity of PBI to be the actual reducing species in the present reduction system. From a mechanistic study, the present reductions could be interpreted in terms of a mechanism involving a synchronous transport of a pair of hydrogens or a sequential transfer of a hydride and a proton from PBI to the olefins.