Discovery of the N–NHC Coupling Process under the Conditions of Pd/NHC- and Ni/NHC-Catalyzed Buchwald–Hartwig Amination

Complexes of palladium and nickel with N-heterocyclic carbene ligands (M/NHC, M = Pd, Ni) are widely used as effective catalysts for various amination reactions. A previously unaddressed transformation of M/NHC complexes under typical conditions of the Buchwald–Hartwig amination is disclosed. M^II/NHC complexes react with primary aromatic and aliphatic amines in the presence of strong bases to give azol-2(5)-imines and M(0) species via a reductive elimination of NHC and azanide (N-deprotonated amine) ligands. Depending on the structures of the NHC and azanide, the N–NHC coupling can make a significant contribution to the M/NHC catalyst decomposition in the Buchwald–Hartwig and other amination reactions conducted in the presence of strong bases. The discovery of the N–NHC coupling reaction has been shown to be critically influenced by the steric bulkiness of N-substituents on the NHC ligand. The high steric bulkiness of the NHC is an important factor in suppressing the N–NHC coupling deactivation pathway.