A DFT Study on the Mechanism of Selective Formation of Substituted Azepines From 1‐Azabutadienes and Cyclopropanes

Density Functional Theory (DFT) calculations were performed to elucidate the reaction mechanism of the Mg-catalyzed cyclization between an azadiene-type imine and a cyclopropane. The results reveal that the [4+3] cyclization pathway leading to azepine formation is both kinetically and thermodynamically preferred over the intuitively more accessible [3+2] pathway toward pyrrolidine formation. This unusual selectivity arises from the interplay of opposing electronic and steric effects, with steric effects ultimately dominating.