Cycloaddition of CO₂ to challenging N-tosyl aziridines using a halogen-free niobium complex: Catalytic activity and mechanistic insights

An efficient and facile approach to the regioselective synthesis of N-tosyloxazolidinones from the corresponding N-tosylaziridines and CO₂ was developed using dual catalytic systems involving an early transition metal coordination compound as a Lewis acid and a nucleophilic cocatalyst. Among the screened Lewis acids, halogen-free niobium pentaethoxide (Nb(OEt)₅) displayed the best catalytic activity when used in the presence of tetrabutylammonium iodide (TBAI). Systematic DFT calculations, supported by catalytic experiments, demonstrate that CO₂ insertion is the rate determining step for this process and it is highly dependent on the steric hindrance at the niobium center.