Scorpionate Catalysts for Coupling CO₂ and Epoxides to Cyclic Carbonates: A Rational Design Approach for Organocatalysts

Novel scorpionate-type organocatalysts capable of effectively coupling carbon dioxide and epoxides under mild conditions to afford cyclic propylene carbonates were developed. On the basis of a combined experimental and computational study, a precise mechanistic proposal was developed and rational optimization strategies were identified. The epoxide ring-opening, which requires an iodide as a nucleophile, was enhanced by utilizing an immonium functionality that can form an ion pair with iodide, making the ring-opening process intramolecular. The CO2 activation and cyclic carbonate formation were catalyzed by the concerted action of two hydrogen bonds originating from two phenolic groups placed at the claw positions of the scorpionate scaffold. Electronic tuning of the hydrogen bond donors allowed to identify a new catalyst that can deliver >90% yield for a variety of epoxide substrates within 7 h at room temperature under a CO2 pressure of only 10 bar, and is highly recyclable.本研究开发了一类新型蝎型有机催化剂（scorpionate-type organocatalysts），该催化剂可在温和条件下高效催化二氧化碳（CO₂）与环氧化物（epoxides）的偶联反应，生成环状碳酸丙烯酯。基于实验与计算结合的系统性研究，我们提出了精准的反应机理假说，并确定了合理的优化策略。在该催化体系中，以碘离子作为亲核试剂（nucleophile）的环氧化物开环步骤，可通过可与碘离子形成离子对（ion pair）的亚铵官能团（immonium functionality）得到强化，使开环过程成为分子内反应。二氧化碳活化与环状碳酸酯生成步骤，则由源自蝎型骨架（scorpionate scaffold）爪部位点的两个酚羟基基团所形成的双重氢键的协同作用完成催化。通过对氢键给体（hydrogen bond donors）进行电子调控，我们筛选得到一款新型催化剂：在仅10 bar的二氧化碳压力、室温条件下，该催化剂可在7小时内对多种环氧化物底物实现超过90%的产率，且具备优异的可回收性能。