Theoretical investigation of the regioselective ring opening of 2-methylaziridine. Lewis acid effect

The formation of substituted 1,2-diamines via the regiospecific nucleophilic ring opening of 2-methylaziridine with methylamine was performed by nucleophilic attack at aziridine carbon atoms. A detailed theoretical study was investigated by density functional theory (DFT) at the B3LYP level and second order Moller Plesset perturbation theory (MP2) by using the 6-311G(d,p) basis set. The third Grimme correction term (D3) was used to take into account weak interactions. Solvent effects were computed in methanol and dimethylsulfoxide using the polarizable continuum model (PCM). Emphasis was placed on the ring opening mechanisms of neutral aziridines and aziridinium ions obtained through N-complexation with the BF₃ Lewis acid. Moreover, the effect of substituent groups on the regioselectivity of the ring opening was investigated. The nucleophilic attack was carried out via two pathways (frontside attack M1 and backside attack M2) where activation barriers proved the preference for ring opening through the backside attack at the C3 aziridine carbon atom. The obtained results showed that the frontside attack with methylamine takes place along a concerted mechanism that leads to formation of products through one transition state. However, the backside attack is carried via a stepwise process in which the methylamine attack takes place in an S_N2 fashion where the leaving group is the ring nitrogen. It first conduces a ring opening considered as the rate-determining step followed by formation of a zwitterionic intermediate. This latter undergoes a rotation to allow the proton transfer step and finally leads to formation of the thermodynamic products.