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- <i>b</i> ]thiazoles

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 <i>N</i>-bromosuccinimide (NBS) under conventional and sustainable conditions. While the DCM-mediated and solvent-free approaches yielded 2-((1-acetyl-1<i>H</i>-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>i.e.</i>, 6-substituted-2-aroyl-3-methylbenzimidazo[2,1-<i>b</i>]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 <i>via</i> comparative analysis of their <i>R</i> _f values, IR, NMR, and HRMS data, with further structural validation <i>via</i> 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.