The possible mechanisms and origin of the enantioselectivity of the reaction between 2H-azirine and an aldehyde catalyzed by an N-heterocyclic carbene (NHC) were theoretically studied and predicted at the M06-2X/6-31G(d,p)/IEF-PCM MTBE //M06-2X-GD3/6-311 ++G(2df, 2pd)/ IEF-PCM MTBE level. The mostf avorable reaction pathway consists of four steps, i.e.,c omplexation of the NHC and the aldehyde, stepwise [1,2]-proton transfer,C ÀCb ond formation coupled with another proton transfer,a nd recycling of the NHC. The computational resultsi ndicate that the stereoselectivity-determining step is also the rate-determinings tep, which is the third step (i.e.,i ntermolecular addition). The calculated 99 % ee is very close to the experimentally observed value of 96 % ee,demonstrating that the calculations are reliable. Twoi mportant roles of the NHC were identifiedb y global reactioni ndex (GRI) analysisa nd natural population analysis( NPA), that is, realizing the umpolung reactivity of the aldehyde and facilitating the deprotonation of aldehyde. Moreover,t he efficiency of different NHC catalysts can be mainly predicted by computing the nucleophilic index of the corresponding Breslow intermediates. Furthermore, distortion/interaction and noncovalenti nteraction (NCI) analyses revealed that the p···p interactions between the NHC and substrates were the key factor in the reaction enantioselectivity.Scheme1.Asymmetric additions to the C=Nb ond of 2H-azirines for the synthesis of chiral aziridines under different transitionmetal-andorganocatalyst-catalyzed conditions.Scheme2.The entire catalytic cycle of the title reaction.