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Synthesis of a Bioxazoline‐Derived Ru Metathesis Catalyst

Research in catalyst design has been essential in the progress of olefin metathesis. Since the development of Grubbs’ firstgeneration catalyst (Grubbs I) and Grubbs’ second-generation catalyst (Grubbs II), with strongly s-donating N-heterocyclic carbene (NHC) ligands, a vast amount of modified NHC-based Ru metathesis catalysts has emerged. Phosphine ligands have been successfully substituted by pyridines, imines, and thio derivatives, and monoor bisubstitution of the anionic ligands has given perfluorinated alkoxy, aryloxy, and carboxylicderived complexes. Modification of the NHC frame remains one of the areas of most intensive investigation, since it allows a fine tuning of the stereoelectronic properties of the precatalyst and of the active propagating species. Structures with different ring sizes have been studied, with reports on fourand six-membered NHC catalysts (1, Figure 1), as well as unsymmetrically substituted imidazole and imidazolin-2-ylidene NHCs, and other heterocyclic structures (2–4, Figure 1).