Spiroketal-Based Diphosphine Ligands in Pd-Catalyzed Asymmetric Allylic Amination of Morita–Baylis–Hillman Adducts: Exceptionally High Efficiency and New Mechanism

Exceptionally high activity (with a TON up to 4750) of the palladium complexes of SKP ligand was discovered in the catalysis of asymmetric allylic amination of MBH adducts with aromatic amines. A comprehensive mechanistic study indicates that the unique structural features of the SKP ligand, with a long P···P distance in its solid-state structure, were favorable for allowing two P atoms to play a bifunctional role in the catalysis. Herein, one of the P atom forms a C–P σ-bond with the terminal carbon atom of allyl moiety as a Lewis base, and an alternative P atom coordinates to Pd atom. The cooperative action of organo- and organometallic catalysis discovered in the present catalytic system is most likely responsible for its high activity, as well as excellent regio- and enantioselectivities. The mechanism disclosed in the present catalytic system is distinct from most of the currently recognized mechanisms for Pd-catalyzed allylic substitutions.研究发现，SKP配体（SKP ligand）的钯配合物用于催化MBH加合物与芳香胺的不对称烯丙基胺化反应时，展现出极高的催化活性，其周转数（TON）可达4750。全面的机理研究表明，SKP配体具备独特的结构特征：其固态结构中存在较长的P···P间距，这使得两个磷原子能够在催化过程中发挥双功能作用。具体而言，其中一个磷原子作为路易斯碱（Lewis base），与烯丙基片段的末端碳原子形成C-P σ键，另一个磷原子则与钯原子配位。本催化体系中所发现的有机催化与有机金属催化协同作用，极有可能是其兼具高催化活性与优异区域选择性和对映选择性的关键原因。该催化体系所揭示的反应机理，与当前主流的钯催化烯丙基取代反应机理均存在显著差异。