Privileged chiral N-heterocyclic carbene ligands for asymmetric transition-metal catalysis

Chiral ligands play a central role in enantioselective transition-metal catalysis. The success of achiral N-heterocyclic carbenes (NHCs) as stable electron-rich neutral ligands in homogeneous catalysis led to the development of a manifold of chiral NHCs as stereodirecting ancillary ligands for various enantioselective transformations. Due to the modular design of NHCs and the ease of access to their azolium salt precursors, tailor-made NHCs are readily available. Many chiral NHC scaffolds have been synthesised and tested in catalysis. Herein, we highlight only those NHC structures which have enabled high degrees of enantioselectivity in transition-metal catalysis. Following a brief introduction to the field of chiral NHCs, this tutorial review introduces different categories of chiral NHCs and provides a guide to the structural fine-tuning of ligand requirements and stereochemical models.