N-Heterocyclic carbene (NHC) catalysis can access umpolung reactivity of carbonyl compounds as acyl anions, enolates and homoenolates. Considerable progress has been made in asymmetric transformation by the introduction of chirality to the NHC scaffold. However, the single catalyst system has limitations in chemo-, regio- and stereoselectivity. A solution to these problems has recently emerged with the synergistic combination of NHC catalysis and metal catalysis. A number of NHC/metal combinations have been found not only to improve reactivity and stereoselectivity, but also to enable access to unprecedented transformations. This paper highlights examples of successful asymmetric NHC/metal cooperative catalysis.
Keywords: Cooperative catalysis; Lewis acid; Metal catalysis; N-Heterocyclic carbene; Umpolung.