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.