Mechanistic Study of Ni and Cu Dual Catalyst for Asymmetric C-C Bond Formation; Asymmetric Coupling of 1,3-Dienes with C-nucleophiles to Construct Vicinal Stereocenters

We report details of the reaction mechanism for a coupling reaction of 1,3-dienes with C-nucleophiles that was catalyzed by a Ni/Cu cooperative catalyst system using Ni(cod)(2) and [Cu(CH3CN)(4)]PF6 in the presence of a chiral JOSIPHOS-type bisphosphine ligand and (Pr2NEt)-Pr-i, providing direct access to highly valuable vicinal quaternary and tertiary stereocenters with high enantio- and diastereoselectivity. The bimetallic cooperative catalyst system exhibited a broad substrate scope, including both cyclic/acyclic stabilized nucleophiles and aryl-/alkyl-substituted 1,3-dienes. The bimetallic cooperative catalyst mechanism was elucidated in depth by isolating and characterizing four key complexes of nickel and copper and conducting deuterium labeling experiments, kinetic studies, and density functional theory calculations. The turnover-limiting step of this reaction is the proton-transfer step to diene-coordinated Ni complex 6 from cationic Cu complex 8 to yield p-allyl Ni complex 7 and Cu enolate complex 9, respectively. The stereoselectivity of the reaction was also clarified according to single-point calculations of the key intermediates 7 and 9.