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

Abstract

Complexes of palladium and nickel with Nheterocyclic 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.

Porous organic polycarbene nanotrap for efficient and selective gold stripping from electronic waste

“…S24-S25), in good agreement with their XRD analyses. These results confirm that Ptriaz-CN-A participated in a chemical redox reaction to reduce Au 3+ to Au 1+ and Au 0 during the adsorption process, and similar findings were previously reported for N-heterocyclic carbene units by other groups [40][41][42][43] Hence, to better understand the interaction between Au 3+ and Ptriaz-CN-A, the XPS C 1s spectra of Ptriaz-CN-A and Ptriaz-CN-A-Au were compared to study the actual active adsorption site of Ptriaz-CN-A for gold. As shown in Fig.…”

A Simple Protocol for the C−N Cross‐Coupling of Aryl Chlorides with Amines Applying Ni/NHC Catalysis

“…Compounds 6 and 7 could not be isolated, and their quantitative yields could not be determined due to their low stability in the quite drastic reaction conditions [24–25] . However, isolation of more stable benzimidazole‐2‐ones [24] and benzimidazole‐2‐imines [25a] similar to compounds 6 and 7 formed from Ni(II)/NHC complexes under the action of bases and amines in analogous conditions but at lower temperature (100 °C) was reported in the literature [24–25] …”

Selective Buchwald–Hartwig arylation ofC-amino-1,2,4-triazoles and other coordinating aminoheterocycles enabled by bulky NHC ligands and TPEDO activator

“…In addition, fast formation of Pd-black was observed in the reaction mixtures containing precatalysts 1a-c. Apparently, complexes 1a-c suffer facile decomposition via O-NHC, 52,67 N-NHC, 68 H-NHC and C-NHC coupling reactions. [69][70][71][72] More stable complexes 1d and 1e with bulkier NHC ligands provided 6% and 19% yields of 4a, respectively (Table 1, entries 1 and 2).…”