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

A series of ionic quaternary ammonium bromides featuring triazole moieties, QAS-trzBn4, QAS-trzPic4, and QAS-trzBn2Pic2, 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-trzBn4 and QAS-trzBn2Pic2 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-trzBn4. A substrate scope study using QAS-trzPic4 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-trzPic4-CO2-epoxide complex is more stabilized than its benzyl derivative, QAS-trzBn4-CO2-epoxide, due to favorable interactions of CO2 with the pyridyl substituents.