Synthesis, Characterization, and Catalytic Studies of Unsymmetrical Chiral NCC Pincer Pd(II) and Ni(II) Complexes Bearing (Imidazolinyl)aryl NHC Ligands

Abstract

A series of palladium(II) and nickel(II) complexes based on unsymmetrical chiral (imidazolinyl)aryl NHC ligands are reported. The new ligand presursors 3a−g were prepared from commercially available 3-bromobenzoic acid, in which the carboxyl and bromo functionalities were converted to chiral imidazoline and NHC moieties, respectively, in three steps. Treatment of 3a−g with Pd(OAc) 2 in dimethylacetamide in the presence of NaOAc, followed by addition of LiCl, gave the seven palladium(II) pincer complexes 4a−g via direct aryl C−H activation. Likewise, the six nickel(II) pincer complexes 5a−f were obtained by reacting 3a−f with anhydrous NiCl 2 . All of the ligand precursors and pincer complexes are bench-stable and were fully characterized. The absolute configurations of palladium complexes 4a,c and 5e were further determined by X-ray diffraction. Finally, Pd pincer complex catalyzed asymmetric Friedel−Crafts alkylation and aza-Morita−Baylis−Hillman reactions were briefly assessed.

Transition Metal Complexes Supported by N‐Heterocyclic Carbene‐Based Pincer Platforms: Synthesis, Reactivity and Applications

“…Song and others reported a series of palladium (II) and nickel (II) complexes 107 and 108 bearing non-symmetric C NHC C aryl N pincer ligands (Scheme 35). [51] Some of the Pd pincer complexes were also utilized to catalyze asymmetric aza-Friedel À Crafts and aza-Morita-Baylis-Hillman reactions, exhibiting moderate stereo-selectivity. Singh et al prepared a series of Pd II C NHC NE (E = S/Se) pincers of type 110 (Scheme 36).…”

NCC Pincer Ni (II) Complexes Catalyzed Hydrophosphination of Nitroalkenes with Diphenylphosphine

“…Pincer Pd and Ni complexes 1 and 2 were prepared according to previous report by our group. [ 22a ] In addition, complex 2 g was new compound and fully characterized. Our initial investigation started with hydrophosphination of trans‐ β ‐nitrostyrene 3a with diphenylphosphine 4a in the presence of Pd and Ni complexes (Table 1).…”

“…[ 17c,21 ] Recently, we have also developed imidazoline‐ and imidazopyridine‐based pincer complexes, which exhibited good activity in Friedel‐Crafts alkylation, Suzuki‐Miyaura reaction, transfer hydrogenations, and other applications. [ 22,23 ] As the continuation of our previous work, we herein firstly developed NCC pincer Ni (II) complex 2a catalyzed hydrophosphination of nitroalkenes (Scheme 1).…”

Chiral Imidazoline Ligands and Their Applications in Metal‐Catalyzed Asymmetric Synthesis†

“…Catalysts 64 (Figure 16) with an imidazoline and an NHC on a phenyl ring were reported in 2018 by Song, Zhu and co‐workers. [ 131 ] These two catalysts were employed in asymmetric aza‐F—C alkylation of indoles with imines, and asymmetric aza‐Moris‐Baylis‐Hillman reaction of acrylonitriles and imines for preliminary screening. The highest ee value for these reactions is respectively 28% and 66%.…”