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
i
Pr2NEt, 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.
Amide bonds are stable due to the resonance between the nitrogen lone pair and the carbonyl moiety, and therefore the chemical transformation of amides, especially tertiary amides, involving C–N bond fission is considered one of the most difficult organic reactions, unavoidably requiring harsh reaction conditions and strong acids or bases.
Amide transformations involving C–N bond cleavage are recognized as difficult reactions owing to the inert nature of amides resulting from resonance. Accordingly, a strong inductive effect and geometrical distortion reasonably decrease the resonance stabilization to attenuate the C–N linkage. Although the conversion of such activated amides has been studied intensively, reaction systems for “unactivated” amides are underdeveloped. We herein report that a zinc(II) trifluoromethanesulfonate [Zn(OTf)2] catalyst achieves the esterification of a wide range of unactivated tertiary amides with the assistance of intramolecular acyl rearrangement. The reaction was applied to the one‐pot removal of various amide‐based directing groups under mild reaction conditions to afford the corresponding esters in high yields.
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