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.

中文翻译：

---

NHC 有机催化/钯催化系统的机理研究：揭示混合手性 Pd(NHC)(PR3) 催化剂设计的重要经验

对高度对映选择性 Pd(PPh3)4/NHC 催化的乙烯基苯并恶嗪酮和烯醛环化的机理进行了全面研究。对反应顺序的研究支持假定的协同催化。有趣的是，对催化活性钯物种的详细研究表明，NHC 具有双重作用，既可作为有机催化剂，又可形成新型混合配体 Pd/NHC/膦复合物。具有催化活性的 Pd/NHC/膦配合物代表了一类新的手性钯催化剂。值得注意的是，膦在这种转变中起着至关重要的作用。