A Quantum‐chemical Analysis on the Lewis Acidity of Diarylhalonium Ions

Cyclic diaryliodonium compounds like iodolium derivatives have increasingly found use as noncovalent Lewis acids in the last years. They are more stable toward nucleophilic substitution than acyclic systems and are markedly more Lewis acidic. Herein, this higher Lewis acidity is analyzed and explained via quantum‐chemical calculations and energy decomposition analyses. Its key origin is the change in energy levels and hybridization of iodine's orbitals, leading to both more favorable electrostatic interaction and better charge transfer. Both of the latter seem to contribute in similar fashion, while hydrogen bonding as well as steric repulsion with the phenyl rings play at best a minor role. In comparison to iodolium, bromolium and chlorolium are less Lewis acidic the lighter the halogen, which is predominantly based on less favorable charge‐transfer interactions. Halogen‐bond (XB) donors such as diaryliodonium ions are becoming an emergent class of organocatalysts. In this class, cyclic diaryliodonium ions are found to be more Lewis acidic than their acyclic variants. The authors performed a detailed quantum‐chemical analysis to unearth the factors influencing this intriguing enhancement. They also performed a comparative analysis of the iodolium, bromolium, and chlorolium ions.