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

Abstract

A series of ionic quaternary ammonium bromides featuring triazole moieties, QAS-trzBn<inf>4</inf>, QAS-trzPic<inf>4</inf>, and QAS-trzBn<inf>2</inf>Pic<inf>2</inf>, 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<inf>4</inf>and QAS-trzBn<inf>2</inf>Pic<inf>2</inf>revealed strong interactions between Br^–ions and both triazolyl H and methylene H atoms (⁺NCH<inf>2</inf>), 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 CO<inf>2</inf>/epoxide cycloaddition were evaluated under both atmospheric and elevated CO<inf>2</inf>pressures. Notably, catalysts containing pyridyl-triazole groups exhibited superior catalytic performances compared with the benzyl-triazole-based catalyst, QAS-trzBn<inf>4</inf>. A substrate scope study using QAS-trzPic<inf>4</inf>under 20 atm of CO<inf>2</inf>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 CO<inf>2</inf>in close proximity. In particular, the QAS-trzPic<inf>4</inf>-CO<inf>2</inf>-epoxide complex is more stabilized than its benzyl derivative, QAS-trzBn<inf>4</inf>-CO<inf>2</inf>-epoxide, due to favorable interactions of CO<inf>2</inf>with the pyridyl substituents.