N-Heterocyclic Carbene-Ruthenium Complexes: A Prominent Breakthrough in Metathesis Reactions

The olefin metathesis reaction is considered to be an indispensable synthetic tool for both organic and polymer chemists. This chapter presents a brief survey of some interesting and useful modifications of N-heterocyclic carbene (NHC) ligands in ruthenium metathesis complexes, illustrated with their activity in various metathesis reactions using standard substrates. Among the NHCs used in the preparation of ruthenium complexes so far, the most successful and frequently studied complexes were those bearing imidazole or imidazolin-2-ylidene ligands. Initially, ruthenium complexes bearing unsymmetrically substituted NHCs were developed in order to improve the catalytic activity through increased basicity of the carbene. Unsymmetric diaryl substitution of the NHC backbone can promote new catalytic pathways leading to stereoselectivities not achievable by the standard symmetric substitution. Altering the NHC ring size may also bring about changes in the steric environment around the metal.