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.一系列带有三唑（triazole）基团的离子型季铵溴化物，包括QAS-trzBn4、QAS-trzPic4与QAS-trzBn2Pic2，通过炔丙基季铵溴化物与苄基叠氮或2-吡啶甲基叠氮之间的铜催化叠氮-炔环加成（CuAAC）反应合成。对QAS-trzBn4和QAS-trzBn2Pic2的X射线晶体学分析表明，溴阴离子（Br⁻）与三唑环氢原子以及亚甲基氢原子（+NCH2）之间存在强相互作用，这一点可由2.68至3.00埃的短程Br⁻···H接触所证实。这些化合物作为双功能单组分催化剂，用于二氧化碳（CO2）与环氧化物环加成反应的催化活性在常压与加压CO2环境下得到了评价。值得注意的是，相较于苄基三唑基催化剂QAS-trzBn4，含吡啶基三唑基团的催化剂展现出更优异的催化性能。在20 atm CO2、100 ℃条件下以QAS-trzPic4开展底物普适性研究，结果显示缺电子环氧化物底物活性更高，可在6小时内实现88%~100%的良好至优异转化率，生成环状碳酸酯。计算研究揭示了吡啶取代体系中的关键结合模式，该模式可使环氧化物与CO2处于近距离排布状态。尤为关键的是，由于CO2与吡啶取代基之间存在有利相互作用，QAS-trzPic4-CO2-环氧化物复合物的稳定性优于其苄基衍生物QAS-trzBn4-CO2-环氧化物复合物。