Theoretical insight into the mechanism and selectivity of the [3 + 2] cycloaddition reaction of N-methyl-1-phenylmethanimine oxide and bicyclopropylidene from the MEDT perspective

The mechanism and regioselectivity of [3 + 2] cycloaddition (32CA) reactions of N-methyl-1-phenylmethanimine oxide nitrone 1 and bicyclopropylidene 2 are analyzed using molecular electron density theory (MEDT) at the B3LYP/6–311 + + G(d,p) level. A study of the electron localisation function (ELF) predicts the zwitter-ionic nature of the nitrone, allowing its participation in zw-type 32CA reactions with a high energy barrier that must be surmounted by suitable electrophilic–nucleophilic interactions. The global electronic flux from the strong nucleophilic bicyclopropylidene 2 to the electrophilic nitrone 1 is predicted by an analysis of the CDFT indices. In this 32CA reaction, no new covalent bonds are generated at the TSs, and the mechanism is one-step and kinetically controlled with low asynchronicity in bond formation. The Gibbs free energy of this 32CA reaction in the gas phase is −9.88 and −15.01 kcal.mol −1 for exo and endo path, respectively. The increased thermodynamic stability of the cycloadducts 4 favors the endo regiochemical route. The ELF topological examination at the transition states is in agreement with the predictions of bonding evolution theory (BET) for the endo and exo routes, which point to a one-step process including early transition states.