Olefin Metathesis [Catalysis]

　　The word "metathesis" generally means redistribution, but in organic chemistry, it is expressed as molecular regenerations by the redistribution of bonds. More specifically, it is widely used as a word meaning the redistribution reactions of two unsaturated molecules bearing carbon-carbon double and/or triple bonds. In the study of metathesis, a large part of time has been spent for development of carbene complexes effectively catalyzing the metathesis reactions. However, the reactions are essentially reversible, therefore, the development of catalysts shifting the equilibrium of reactions forward have been vigorously studied.
　　In the 1990s, it had been shown that the ruthenium complexes developed by Grubbs et al. (1st-generation Grubbs’ catalyst) effectively catalyze olefin metathesis. Furthermore, Grubbs et al. had developed another catalyst with N-heterocyclic carbene (NHC) as a ligand (2nd-generation Grubbs’ catalyst) and successfully controlled the reactivity and stability of the catalyst.
　　On the other hand, Hoveyda et al. have developed improved ruthenium complexes compared to Grubbs’ catalysts. These complexes can form intramolecular chelation from the alkoxy group at an appropriate position of a vinylidene group to a ruthenium metal. These types of catalysts are called “Hoveyda-type catalysts” and are widely known as highly active olefin metathesis catalysts, same as the 2nd-generation Grubbs’ catalyst.^1-2)