Development of Quinoline-Derived Chiral Diaminocarbene Ligands and Their Transition Metal Complexes: Synthesis, Structural Characterization, and Catalytic Properties

Abstract

N-Heterocyclic carbenes (NHCs) bearing a fused tricyclic skeleton have recently been developed as a new type of promising ligands for asymmetric transition metal catalysis. In this article, two kinds of chiral tricyclic NHCs featuring partially reduced 2,2′-bisquinoline- and 2-(piperidin-2-yl)­quinoline skeleton have been prepared via a direct asymmetric hydrogenation synthetic strategy. Their Rh- and Pd-complexes were synthesized and characterized by NMR and high-resolution mass spectra as well as X-ray crystallography. The electronic effect and steric environment of these new chiral tricyclic NHCs were systematically evaluated by determining the Tolman electronic parameter (TEP) and the percent buried volumes (%V bur) of the corresponding metalated complexes. It was revealed that these new ligands possess strong electron-donating properties and high steric demands. Additionally, these chiral NHC ligands have been successfully employed in the enantioselective palladium-catalyzed intramolecular α-arylation in high yield with good enantioselectivity (up to 80% ee). It is noteworthy that the rigid fused tricyclic rings and bulky ortho substituents can greatly improve the electron-donating property and chiral induction ability of these chiral NHC ligands.

C 2-Symmetric atropisomeric N-heterocyclic carbene–palladium(ii) complexes: synthesis, chiral resolution, and application in the enantioselective α-arylation of amides

“…Enantiopure complexes were tested in the asymmetric intramolecular α-arylation of amides using amide 5a as the benchmark substrate (Table 3). 8 c ,20 Reactions were performed at 40 °C in DME using potassium tert -butoxide as the base. Neither complex cis - 2b due to its “pseudo meso ” structure nor 2c due to its poor stability was evaluated.…”

C1‐Symmetric Atropisomeric NHC Palladium(II) Complexes: Synthesis, Resolution and Characterization

“…The palladium‐catalyzed intermolecular α‐arylation of amide has been used as benchmark reaction to evaluate to ability of chiral palladium‐based complexes, in particular containing chiral NHC ligands to induce efficiently an enantioselectivity [4b,c,5d,9a,22] . Therefore, we tested enantiopure complexes 9 and 10 in the intermolecular α‐arylation of amide 15 (Scheme 6).…”

Synthesis and comparative study of (NHCF)PdCl2Py and (NHCF)Ni(Cp)Cl complexes: investigation of the electronic properties of NHC ligands and complex characteristics

“…[17][18][19][20][21][22] The properties of NHC ligands with electron acceptor substituents have been most widely studied for palladium complexes in cross-coupling reactions, ruthenium and rhodium complexes in metathesis reactions and gold complexes in various other transformations. [23][24][25][26][27][28][29][30] In the catalytic cycle of cross-coupling reactions, it has been demonstrated that nickel NHC complexes are more active in oxidative addition reactions than palladium. 31 Nickel has been shown to have catalytic properties in a range of reactions, including the activation of the C-F bond, which is a challenging task for researchers.…”