This chapter covers developments that pertain to the use of monodentate chiral N-heterocyclic carbenes (NHCs) ligands in metal-catalyzed reactions. The use of chiral NHCRu complexes in asymmetric catalysis has long been overshadowed by asymmetric metathesis. In 2003, Andrus and coworkers reported the first and only example of monodentate chiral NHCRh-catalyzed asymmetric 1,4-addition of boron reagents to enones. Montgomery's group reported the application of an achiral NHCNi(0) catalytic system in reductive couplings of aldehydes and alkynes and macrocyclization of ynals. This catalytic system showed advantages over systems without NHC ligands, including a broad substrate scope with both internal and terminal alkynes, direct incorporation of a silyl protecting group, and the ability to tune alkyne regioselection in macrocyclizations. Silver carbene complexes act as very efficient NHC transfer reagents for the synthesis of different metalNHC complexes. The development of enantioselective gold(I) catalysis using chiral NHCs as the ligands is still in its infancy.
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.