Chiral NHC–metal-based asymmetric catalysis

The strong σ-donor but poor π-acceptor abilities of N-heterocyclic carbenes (NHCs) result in chemical stability and coordination versatility, thus allowing them to coordinate with many metals ranging from alkaline-earth metals to rare-earth metals. Moreover, the easy introduction of chiral elements into NHCs and the facile preparation of their precursors have made chiral NHCs promising chiral ligands in metal-based asymmetric catalysis. Therefore, strategies for the design of efficient NHC–metal catalysts and novel asymmetric synthetic methodologies have dramatically increased over the past 10 years. Herein, the growing field of chiral NHC–metal-based asymmetric catalysis is comprehensively described.

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► The design concepts for chiral N-heterocyclic carbenes (NHC) ligands have been demonstrated proof-of-concept asymmetric catalysis.
► The easy introduction of chiral elements into NHCs and their facile preparations have made chiral NHCs as the promising alternative ligands to the more commonly used phosphines and phosphites for various asymmetric catalytic reactions. More importantly, chiral NHC–metal complexes also presented special catalytic properties in asymmetric catalyses under the atmosphere of oxygen and moisture such as asymmetric oxidations.
► Novel asymmetric synthetic methodologies achieved by NHC–metal complexes were disclosed, and asymmetric catalytic activity of NHC–metal complexes in asymmetric catalysis was discussed.
► The NHC–metal-based catalytic mechanisms were presented.