New catalysts with unsymmetrical N-heterocyclic carbene ligands

Abstract

The importance of unsymmetrical N-heterocyclic carbenes (uNHCs) as ligands in metal-catalyzed reactions is undeniable. While uNHCs show similar properties as compared with symmetrical NHCs, dissymmetrization allows for further fine-tuning. The introduction of chelatization, hemilability, bifunctionality, shielding effects, and chirality-transfer influences the catalyst's stability, reactivity, and selectivity, thus offering access to tailor-made systems including mono-and multidentate uNHC ligands. Based on selected examples, the structure-reactivity relationship of uNHCs employed in metal catalysts is presented. The focus is on catalytically active complexes, which either offer access to new applications or lead to significantly improved results in metal-catalyzed reactions.

Homo‐ and Heterodinuclear Ir and Rh Imine‐functionalized Protic NHC Complexes: Synthetic, Structural Studies, and Tautomerization/Metallotropism Insights

“…C2 L' and complexes 8 and 9 [8]: C1ÀN1 1.31(1), C1ÀN2 1.38(1), C2ÀN1 1.394(9), C2ÀC3 1.37(1), C3ÀN2 1.41(1),C 4 ÀN3 1.31(1),I r1ÀN1 2.083(7), Ir1ÀN3 2.094(6), Ir1ÀC32 2.154(9), Ir1ÀC332 .137 (8), Ir1ÀC36 2.110(9), Ir1ÀC37 2.123(9), C32ÀC33 1.43(2),C 36ÀC371.38(1);N1-C1-N2 111.1(7), N1-Ir1-N3 78.9(3),N 1-Ir1-C3298.6(3), N1-Ir1-C33 94.2(3), N3-Ir1-C369 7.5(3), N3-Ir1-C37 93.6(3). Chem.E ur.J.2016Chem.E ur.J.…”

An N-heterocyclic carbene-palladium-η3-allyl chloride complex for the Suzuki-Miyaura coupling of aryl halides

“…They have also been used as ligands for ruthenium complexes in ring closing metathesis [14][15][16]. While most of NHC's used in cross-coupling reactions are symmetrically substituted, unsymmetrical ligands allow diversification of steric bulkiness in the vicinity of a metal complex allowing fine tuning of reactivity [17][18][19][20]. The synthesis of nonsymmetrical carbenes precursors can be done by an imidazole alkylation, allowing the synthesis of several NHC precursors using different alkyl chains [18,[21][22][23].…”

Enantiopure C1-symmetric N-Heterocyclic Carbene Ligands from Desymmetrized meso-1,2-Diphenylethylenediamine: Application in Ruthenium-Catalyzed Olefin Metathesis

“…Among them, asymmetric transformations mediated by metal catalysts bearing NHCs with chiral architectures have attracted increasing interest, and intensive research efforts have permitted the obtainment of highly stereoselective catalysts [5,6]. The employment of stable, easy-handling chiral NHC Ru-based catalysts has resulted in significant advancements in asymmetric olefin metathesis, a powerful methodology for carbon-carbon bond formation leading to enantioenriched molecules [7][8][9][10][11]. A number of highly enantioselective catalysts bearing C 2 -and C 1 -symmetric NHC ligands derived from chiral 1,2-diamines were reported by Grubbs [12][13][14] and Collins [15][16][17][18] (Figure 1).…”