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 iPr2NEt, 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 π-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.