Solar-driven, noble-metal-free one-pot photocatalytic synthesis of 2-methylbenzimidazole over C, N, and S-co-doped TiO2

Developing noble-metal-free photocatalysts for selective solar-driven C–N heterocycle synthesis remains a key challenge in sustainable organic chemistry. Herein, C, N, and S-modified anatase/brookite TiO₂ was prepared through a thiourea-assisted post-treatment and used for the one-pot synthesis of 2-methylbenzimidazole under simulated sunlight. The non-metal modification enhances visible-light response by narrowing the band gap and generating defect-related electronic states. Under simulated solar light, the optimized CNS-TiO₂ achieved complete o-dinitrobenzene conversion with 92% yield and selectivity toward 2-methylbenzimidazole, outperforming both undoped TiO₂ and commercial Au-loaded TiO₂. Surface acid-site analysis, together with p-toluenesulfonic acid-assisted experiments, confirmed that Brønsted acidity strongly promotes the condensation/cyclization steps, raising the yield to 99%. Mechanistic studies based on substrate/donor screening and intermediate detection suggest a coupled photo-redox and acid-assisted pathway involving nitro reduction, alcohol oxidation, C–C bond cleavage, and heterocycle formation. This work highlights a solar-driven strategy for transforming nitroarenes into valuable benzimidazoles using a multifunctional donor and a cocatalyst-free TiO₂ platform.