Towards cleaner olefin metathesis: tailoring the NHC ligand of second generation ruthenium catalysts to afford auxiliary traits

The increasing use of olefin metathesis in practical syntheses led to a greater demand for tailored catalysts that offer not only high reactivity and stability in the presence of impurities, but also improved handling characteristics, such as custom solubility profiles, simpler post-reaction workup and easier removal of ruthenium residues from the products. Due to mechanistic considerations, the most appropriate way to create such compounds is to decorate known catalysts with utility related tags placed in nonlabile ligands. This critical review summarizes how the NHC ligand can be modified to confer novel auxiliary traits and examines the practical consequences of these modifications.