Pyridyl-Supported Pyrazolyl−N-Heterocyclic Carbene Ligands and the Catalytic Activity of Their Palladium Complexes in Suzuki−Miyaura Reactions

Abstract

Palladium complexes of two new types of unsymmetrical pyridyl-supported pyrazolyl-N-heterocyclic carbene ligands were synthesized and structurally characterized. A strategy to release the steric strain of the ligand was realized by the introduction of methylene linkers to the ligand molecule. All the palladium complexes exhibited good to excellent catalytic activity in Suzuki-Miyaura reactions of phenyl or p-tolylboronic acid with aryl halides including iodobenzene, aryl bromides, and activated aryl chlorides under mild conditions, revealing that the new ligands are promising for the construction of highly active transition-metal catalysts.

Synthesis and catalytic activity of N-heterocyclic carbene metal carbonyl complexes based on 1-[2-(pyrazol-1-yl)phenyl]imidazole

“…The solid was characterized by 1 H and 13 C NMR spectroscopy. For further separation, the filtrate was concentrated to dryness under reduced pressure, and the residue was purified by column chromatography on silica using CH 2 Cl 2 /hexane (1/1 v/v) as the eluent to give the corresponding derivatives.…”

“…Recent investigations show that donor-functionalized NHC-metal complexes can generate the active catalytic species through the reversible coordination/dissociation process of the donor functions from the metal centers, which accelerate the rapid development of various donor-functionalized NHC ligands, such as N-, P-, O-, S-functionalized NHCs [5][6][7][8][9][10]. In the family of N-functionalized NHCs, pyrazolylfunctionalized NHCs have drawn much attention owing to the coordination versatility of pyrazole-based ligands and the catalytic activity of their complexes [11][12][13][14][15][16][17][18]. For example, some transition metal carbonyl complexes bearing pyrazolyl ligands [19][20][21][22], such as bis(pyrazol-1-yl)methane, tris(pyrazol-1-yl)methane and pyrazolylpyridine have been used as catalyst precursors for olefin epoxidation.…”

Fe3O4 Nanoparticles-Supported Palladium-Bipyridine Complex: Effective Catalyst for Suzuki Coupling Reaction

“…The most frequently used catalyst for the Suzuki coupling reaction is triphenylphosphine/Pd complexes but often a number of by-products are formed in the reaction [3,4]. Recently, new types of ligands, such as carbenes [5,6], pincer ligands [7], and palladacycles [8,9], etc., all binding to palladium have emerged as catalyst for the Suzuki reaction with excellent catalytic results in terms of yield and selectivity. However, these catalysts, in general, suffer from the deficits in recovery and recycling of homogeneous palladium catalysts and the separation of catalysts and products.…”

Ru(II) pyridyl-based NNN complex catalysts for (asymmetric) transfer hydrogenation of ketones at room temperature

“…Following the procedure previously reported from our laboratory [21][22][23][24][25][26][27][28][29][30][31][32], complex 6a was tested as a catalyst for the transfer hydrogenation of acetophenone in the presence of i-PrOK. To our surprise, 6a exhibited excellent catalytic activity for the reduction of acetophenone in 2-propanol even at room temperature (28 °C).…”

“…Although new ligands and their transition metal complexes have been established in this area [17][18][19][20], the desire for more highly active catalytic systems with better stereoselectivity and broader substrate scopes under mild conditions is still strong. We have recently documented that highly active transition metal complex catalysts could be rationally constructed by introducing a benzimidazolyl or pyrazolyl coordinating arm on a pyridyl-based ligand framework [21][22][23][24][25][26][27][28][29][30]. A series of highly active Ru(II) complexes have been obtained for the transfer hydrogenation of ketones [31,32], achieving >99% yield and final TOFs up to 720 000 h -1 at 82 °C, and 99% ee and final TOFs up to 55 800 h -1 at 28 °C in ATH (Scheme 1).…”