Riboflavin Photocatalyzed Synthesis of 1,2‐diarylbenzimidazoles From N , N ’‐diarylbenzimidamides

ABSTRACT

A metal‐free, visible‐light‐mediated strategy for the synthesis of 1,2‐diarylbenzimidazoles is reported. The method uses riboflavin tetraacetate as an organic photocatalyst, enabling efficient intramolecular C─N bond formation from N , N ′‐diarylbenzimidamides under mild conditions at room temperature. A broad range of substrates bearing electron‐donating and electron‐withdrawing groups, including halogens, are compatible with the reaction, affording structurally diverse benzimidazoles. Mechanistic studies indicate that the transformation involves radical intermediates generated through the interaction of the excited photocatalyst with the substrate. Photophysical experiments identify the substrate as the primary quencher of the photocatalyst's excited state, while computational analysis supports a hydrogen atom transfer pathway as the key step governing cyclization. This work provides a sustainable and mechanistic approach to benzimidazole synthesis, expanding the scope of metal‐free photocatalysis for the construction of heteroaromatic compounds.