Mechanistic insight into C−S bond construction using CO2 as a carbon source: Synthesis of benzothiazoles from 2-aminothiophenols in imidazolium acetate

DFT calculations on synthesis of benzothiazoles via cyclization of 2-aminothiophenols with CO₂ and triethoxysilane have revealed a novel mechanism that fundamentally differs from previously proposed mechanisms. In this new mechanism, the acetate anion plays a pivotal role in both stages of the reaction: first, in the formation of the formoxysilane intermediate via CO₂ reduction, and second, in its subsequent transformation to the product, benzothiazoles. The acetate anion acts as a nucleophile to activate the Si−H bond of triethoxysilane in the CO₂ reduction stage and as a base, deprotonating 2-aminothiophenol and generating a HOAc molecule, which then acts as a proton shuttle in the subsequent transformations leading to the final product. Throughout the whole reaction process, it is the acetate anion that plays a substantial role in catalyzing the reaction by activating the Si–H bond of triethoxysilane, contrasting with the previous notion that the imidazolium cation activates CO₂ through the formation of a NHC-CO₂ adduct. Furthermore, the proposed mechanism offers a rational explanation for the observed inefficiency of imidazolium trifluoromethansulfonate as a catalyst for this reaction. The elucidation of this new mechanism sheds light on the intricate details of the benzothiazole synthesis and may inspire further investigations in this field.