Chiral N-heterocyclic carbene ligands with additional chelating group(s) applied to homogeneous metal-mediated asymmetric catalysis

Since their early development, N-heterocyclic carbene (NHC) ligands and their related metal complexes have attracted much attention, due to their high performances in many catalytic processes. Structural variations on the heterocycle allowed to fine tune their steric and electronic properties. Introduction of one (or more) additional donor group(s) on the N-side arm(s) opened the possibility to transform classical monodentate NHCs into bi- or tridentate ligands with different metal-donor affinities for binding. The additional donor may be a neutral donor, e.g. phosphine, (thio)ether, or anionic, e.g. alkoxide, offering a myriad of possibilities for the resulting polyfunctional NHC ligands. Another development of importance in the field of the NHC chemistry was the introduction of chirality, which can be either 1) central, located on an N-side arm and/or on the ligand backbone, 2) axial or 3) planar, opening the door to the application of NHC metal complexes to asymmetric catalysis. The present review summarizes the examples of chiral NHC ligands, functionalized by an additional donor group, which were successfully applied to metal-mediated asymmetric catalysis. When possible, the beneficial (or detrimental) effect of the NHC-functionalization will be discussed, along with the influence of the nature of the additional function, e.g. -OH vs. -OMe. (C) 2019 Elsevier B.V. All rights reserved.