Understanding the molecular mechanism of the [3 + 2] cycloaddition reaction of benzonitrile oxide toward electron‐rich N ‐vinylpyrrole: a DFT study

The [3 + 2] cycloaddition (32CA) reaction of benzonitrile oxide, BNO 2 , with an electron‐rich N ‐vinylpyrrole derivative, NVP 3a , in the presence of dichloromethane, has been theoretically studied using density functional theory (DFT) methods at the B3LYP/6‐31G(d) level. This 32CA reaction presents a relatively high activation Gibbs free energy as a result of the low polar character of this zwitterionic‐type ( zw‐type ) reaction. Analyses of the calculated relative Gibbs free energies and transition state geometries indicate that the studied 32CA reaction, in excellent agreement with experimental outcomes, takes place in a complete regioselective manner as a consequence of the steric repulsions that appear at the most unfavorable transition state. An electron localization function (ELF) topological analysis of the bonding changes along this 32CA reaction supports a non‐concerted two‐stage one‐step molecular mechanism in which the formation of the O3 ‐ C5 single bond takes place at the end of the reaction after the complete formation of the C1‐C4 one. Copyright © 2016 John Wiley & Sons, Ltd.