Imidazolium-2-carboxylates derived from N-heterocyclic carbenes (NHCs) and CO₂ serve as efficient catalysts for CO₂-carboxylation of tertiary aziridines bearing various substituents such as halogens, ether, olefin, ester, acetal, and nitro groups on the aziridine ring in 2-propanol, leading to 3-substituted-2-oxazolidones in good to excellent yields and with high selectivity. The NHC–CO₂ adducts facilitate nucleophilic attack of the CO₂ moiety on the aziridines, in which the substituents are intact during the carboxylation. The catalyst is successfully recycled up to five times with no apparent loss in activity.