Chiral diazaligands for asymmetricsynthesis

Abstract

The synthetic routes to optically pure N -heterocyclic carbene (NHC) complexes and their application in asymmetric catalysis is reviewed. Chiral N -heterocyclic carbenes which are mainly generated from the corresponding azolium salts can be used as ligands to prepare various complexes with transition metals. However, only a few of them have been used in asymmetric catalysis and reported examples where chiral NHC complexes give good enantioselectivity are still rare even if excellent levels of enantioselectivity have been reached (up to >99% ee). This chapter describes the general properties of this family of ligand, the synthesis of the chiral azolium salts, the preparation of NHC-metal complexes and the asymmetric transformations catalyzed by palladium, rhodium, ruthenium, iridium and copper complexes.