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.