Elucidating the Mechanism of Aryl Aminations Mediated by NHC-Supported Nickel Complexes: Evidence for a Nonradical Ni(0)/Ni(II) Pathway

Nickel catalysis
is gaining in popularity in recent years, mostly
within the area of cross-coupling. However, unlike Pd, the mechanisms
of Ni-catalyzed C–C and C–heteroatom bond forming reactions
have been much less studied, in particular when N-heterocyclic carbenes
are used as ligands. Here, we present a thorough study of the mechanism
of C–N cross-coupling reactions catalyzed by an NHC-Ni complex.
Focusing on the coupling of 2-chloropyridines with indole catalyzed
by [(IPrNi­(styrene)2] (IPr = N,N′-bis­(2,6-diisopropylphenyl)­imidazol-2-ylidene),
we have examined each of the elementary steps: i.e., oxidative addition,
ligand substitution, and reductive elimination. All relevant catalytic
intermediates have been isolated and structurally characterized by
both spectroscopic and crystallographic methods. Kinetic studies have
revealed that the reductive elimination is the rate-limiting step.
Catalyst deactivation is related to the formation of unproductive
dinuclear pyridyl-bridged NHC-NiII species, which can be
prevented by increasing the size of the heteroaryl chloride. These
investigations support a neutral Ni(0)/Ni­(II) catalytic cycle. Calculations
corroborate the experimental evidence and confirm the influence exerted
by the ligands in each of the elementary steps.