A quantum-chemical analysis on the Lewis acidity of diaryl-halonium ions

Abstract

Cyclic diaryliodonium compounds like iodolium derivatives have increasingly found use as noncovalent Lewis acids in the last years. They are more stable 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.

Orbital analysis of bonding in diarylhalonium salts and relevance to periodic trends in structure and reactivity

“…29 Although the orbital composition does not account for the significant difference in Lewis acidity between cyclic and acyclic diarylhalonium compounds, the increased steric repulsion between the aryl ligands and approaching Lewis bases that is a result of a more obtuse C–X–C bond angle in acyclic compounds 16–18 may be an important factor. 34 Considering the design or selection of Lewis acid catalysts, one might anticipate that a stronger Lewis acid is more advantageous, and therefore the iodoniums would be favoured. However, there is at least one reported case in which a bromonium catalyst resulted in a higher yield of product than the corresponding iodonium catalyst highlighting the complexity of catalytic cycles.…”