Mechanistic Studies on a Cooperative NHC Organocatalysis/Palladium Catalysis System: Uncovering Significant Lessons for Mixed Chiral Pd(NHC)(PR₃) Catalyst Design

A comprehensive investigation of the mechanism of the highly enantioselective Pd­(PPh3)4/NHC-catalyzed annulation of vinyl benzoxazinanones and enals has been conducted. A study of reaction orders supports the postulated cooperative catalysis. Interestingly, a detailed investigation of the catalytically active palladium species pointed toward a dual role of the NHC acting as an organocatalyst and forming a novel mixed ligand Pd/NHC/phosphine complex. The catalytically active Pd/NHC/phosphine complex represents a new class of chiral palladium catalyst. Remarkably, phosphine plays a crucial role in this transformation. These complexes could be characterized by X-ray crystallographic analysis and employed as catalysts for the enantioselective [4 + 1] annulation reaction of vinyl benzoxazinones and sulfur ylides in good yields and good enantioselectivities.本研究针对高对映选择性Pd(PPh₃)₄/氮杂环卡宾（N-heterocyclic carbene，NHC）催化乙烯基苯并恶嗪啉酮与烯醛的环化反应机制开展了全面探究。反应级数研究验证了此前提出的协同催化假说。值得关注的是，对催化活性钯物种的详细分析揭示，氮杂环卡宾兼具双重作用：既作为有机催化剂，又可参与构建一类新型混合配体钯/氮杂环卡宾/膦配合物。该催化活性钯/氮杂环卡宾/膦配合物代表了一类全新的手性钯催化剂。尤为关键的是，膦配体在该转化反应中发挥了至关重要的作用。此类配合物可通过X射线晶体衍射分析完成表征，并可作为催化剂应用于乙烯基苯并恶嗪啉酮与硫叶立德的对映选择性[4+1]环化反应，且能获得良好的产率与对映选择性。