Metal-Free Photocatalyst with Redox Center and Lewis Acid Site for Selective Synthesis of Mono- and Disubstituted Benzimidazoles via Alcohol and Aromatic Diamine Cross-Dehydrocoupling

Here we present the first metal- and additive-free bipyridinium-based photocatalyst for selective dehydrocondensation of aromatic diamine with alcohol to mono- and disubstituted benzimidazoles under mild conditions by modulating the composition of the solvent. Notably, the method is versatile and applicable to less reactive substrates such as heterocyclic alcohols and aliphatic alcohols as well as the synthesis of quinoxaline and the pharmaceutically active molecule benzothiazole. In contrast to most other photocatalytic systems for the synthesis of benzimidazole in which aldehydes and o-phenylenediamine are used as substrates, the replacement of alcohols for aldehydes as substrates shows great advantages due to the abundant and cost-effective nature of alcohols as fermentation products of renewable plant-derived materials and the fact that the sole byproduct in the synthesis of benzimidazoles using alcohols and o-phenylenediamine is environmentally friendly water in stoichiometric quantities. Combining the redox activity and Lewis acidity of bipyridinium photocatalyst, benzyl alcohol was oxidized to benzaldehyde by activated singlet oxygen and superoxide free radicals, and then intermediate aldehydes were activated by the Lewis acid sites of bipyridinium catalyst and reacted with o-phenylenediamine to produce mono- and disubstituted benzimidazole by cross-dehydrocoupling.