Mechanism of N-heterocyclic carbene-catalyzed chemical fixation of CO2 with aziridines: a theoretical study

Abstract

The reaction mechanism of cycloaddition of CO₂ with N-benzylaziridine catalyzed by N-Heterocyclic Carbenes (NHCs) has been investigated using density functional theory (DFT) at the M06-2X (IEFPCM, 2-propanol)/6-311++G(d,p)//M06-2X/6-31G(d,p) level. The calculations reveal that the reaction prefers to proceed through a three step mechanism mediated by free NHC rather than being catalyzed by the NHC–CO₂ adduct. Free NHC plays a role as the catalyst precursor to promote the initial ring-opening of the aziridine with the incorporation of CO₂ through S_N2 anti nucleophilic attack, leading to the formation of the carboxylate intermediate. Then, the generated carboxylate as an active intermediate can easily react with the excess of N-benzylaziridine and CO₂. Finally, the intramolecular nucleophilic addition allows the release of the cycloaddition product with the recovery of the active intermediate. Compared with background reaction, the higher nucleophilicity of free NHC as well as the stabilization from the t-Bu group on the nitrogen atom of the imidazolium ring help to lower the energy barrier of the ring-opening step, which accelerates the formation of the active intermediate and suppresses the generation of by-product oligomer. In addition, the calculations predict that the NHCs bearing the additional ring fusion beside the C–C and C–N bonds of the imidazolium ring might be more powerful catalysts for chemical fixation of CO₂ with aziridines, owing to the enhancement of the nucleophilicity of the NHCs and the reactivity of the carboxylate intermediate.