Synthesis of Transition-metal NHC Complexes using “Protected” NHC Adduct

Abstract

Since the successful isolation of N-heterocyclic carbenes (NHCs), NHCs are now fully established as an important class of ligands for the homogeneous catalysis. The most common method for the preparation of NHC complexes is direct complexation of the free NHC with the metal, in which the free NHC, either isolated or generated in situ, is produced by deprotonation of the corresponding salt as the NHC precursor. However manipulation of the free NHCs is often difficult due to their highly reactive nature toward air and moisture. The reaction of the imidazolium salt with metal precursors bearing the basic ligand such as acetate, acetylacetonate, or alkoxide is also effective for the preparation of NHC complexes, whereas this method is limited to the synthesis of the late transition metal complexes. Recently, the employment of NHC adducts as “protected” forms of the free carbene attracted much attention for new methods of the preparation of the NHC complexes. In this review, the recent developments of “protected” NHC adducts as versatile synthons for the synthesis of NHC complexes are described.