Enhanced CO₂/Epoxide Cycloaddition Catalyzed by Pyridine-Substituted Triazole-Quaternary Ammonium Bromide

A series of ionic quaternary ammonium bromides featuring triazole moieties, QAS-trzBn ₄ , QAS-trzPic ₄ , and QAS-trzBn ₂ Pic ₂ , were synthesized via Cu-catalyzed azide-alkyne cycloaddition (CuAAC) between propargyl-based ammonium bromide and benzyl- or 2-picolylazide. X-ray crystallographic analyses of QAS-trzBn ₄ and QAS-trzBn ₂ Pic ₂ revealed strong interactions between Br^- ions and both triazolyl H and methylene H atoms (⁺NCH₂), as evidenced by short Br^-···H contacts ranging from 2.68 to 3.00 Å. The catalytic activities of these compounds as bifunctional, single-component catalysts for the CO₂/epoxide cycloaddition were evaluated under both atmospheric and elevated CO₂ pressures. Notably, catalysts containing pyridyl-triazole groups exhibited superior catalytic performances compared with the benzyl-triazole-based catalyst, QAS-trzBn ₄ . A substrate scope study using QAS-trzPic ₄ under 20 atm of CO₂ at 100 °C revealed that electron-deficient epoxide substrates were more active, yielding good to excellent conversions (88-100%) to cyclic carbonates within 6 h. Computational studies identified key binding modes in pyridine-substituted systems that position both the epoxide and CO₂ in close proximity. In particular, the QAS-trzPic ₄ -CO ₂ -epoxide complex is more stabilized than its benzyl derivative, QAS-trzBn ₄ -CO ₂ -epoxide, due to favorable interactions of CO₂ with the pyridyl substituents.