Aziridination of Aromatic Aldimines Through Stabilized Ammonium Ylides: A Molecular Electron Density Theory Study

Abstract

The domino reaction between ammonium salt AS 4 and aldimine AD 2, in the presence of Cs2CO3, yielding aziridine trans‐AZ 7 was studied within the Molecular Electron Density Theory at the B3LYP (PCM, DCM)/def2‐TZVP//B3LYP (PCM, DCM)/GenECP computational level. This domino reaction begins with the proton abstraction from AS 4 by Cs2CO3 yielding ammonium ylide AY 5. The subsequent nucleophilic attack of AY 5 on AD 2 furnishes the gauche betaine‐like intermediate IN‐2Tg1, which experiences a C–C single bond rotation generating the anti betaine‐like intermediate IN‐2Ta. Finally, IN‐2Ta is converted into trans‐AZ 7 by an SNi process. Analysis of the relative Gibbs free energies indicates that conversion of IN‐2Ta into trans‐AZ 7 is the rate‐determining step. Analysis of the reactivity indices allows characterizing AY 5 as a strong nucleophile and AD 2 as a strong electrophile, in clear agreement with the high polar character of the addition step, while analysis of the electrophilic Parr functions explains the regioselectivity within the C1–C2 single bond formation. A Bonding Evolution Theory study of the bonding changes along the ring‐closure step allows understanding the mechanism of the SNi reaction, which takes place through a non‐concerted two‐stage one‐step mechanism involving zwitterionic species having a carbocationic C1 center.