Comparative study of the reaction of 2-mercaptobenzimidazole with 2-bromo-1,3-diketones under conventional and green conditions: regioselective access to N/S-difunctionalized benzimidazoles and benzimidazo[2,1-b]thiazoles

Abstract

The development of eco-friendly and energy-efficient synthetic methods remains a priority in modern organic chemistry. In this study, we investigated the reaction of 2-mercaptobenzimidazole and 1,3-diketones in the presence of N-bromosuccinimide (NBS) under conventional and sustainable conditions. While the DCM-mediated and solvent-free approaches yielded 2-((1-acetyl-1H-benzo[4,5]imidazole-2-yl)thio)-1-arylethan-1-one derivatives, the visible-light irradiation strategy promoted a distinct regioselective [3 + 2] cyclo-condensation, affording corresponding cyclized products, i.e., 6-substituted-2-aroyl-3-methylbenzimidazo[2,1-b]thiazole derivatives in high yields. These one-step protocols feature an eco-friendly nature, short reaction times, operational simplicity, high efficiency, and excellent selectivity. Structures of the synthesized compounds were confirmed via comparative analysis of their R_f values, IR, NMR, and HRMS data, with further structural validation via detailed 2D NMR studies, and plausible mechanisms have been proposed for both reaction pathways. Overall, the work highlights condition-driven regioselectivity and the potential of energy-dependent strategies in benzimidazole-based heterocycle synthesis.