Activating harmful small molecules under mild conditions: theoretical insights into cinchonine-based valorization of CO2, CS2, and COS

DFT calculations have been employed to deeply investigate the mechanism of CO2 cycloaddition to aziridines catalyzed by cinchonine hydrochloride salt, forming oxazolidin-2-ones under ambient conditions (room temperature, 0.1 MPa CO2). Computed energy barriers align with experimental observations and support a dual activation mechanism involving hydrogen bonding and nucleophilic attack at the aziridine carbon atom. The theoretical study also accounts for the observed regioselectivity, rationalizing the preference for nucleophilic attack at the more substituted aziridine carbon atom. Consistent with experimental findings, the calculations reveal that the reaction efficiency is influenced by the nature of the substituent at the aziridine nitrogen atom, explaining the lack of reactivity observed with N-aryl aziridines due to steric and electronic factors that hamper the reaction. Furthermore, the DFT study suggests that COS and CS2 can be activated for analogous cycloaddition reactions. Although these transformations involve higher energy barriers compared to CO2 cycloaddition, the formation of oxazolidin-2-thiones and thiazolidin-2-thiones is predicted to be feasible under slightly elevated temperatures (for CS2) or near-ambient conditions (for COS). These findings highlight the potential of cinchonine hydrochloride salt as an efficient, biocompatible and cost-effective catalyst for the sustainable valorization of small harmful molecules under mild conditions.