Synthesis of a Silica-Based Heterogeneous Second Generation Grubbs Catalyst

The synthesis of a second generation Grubbs catalyst immobilized onto non-porous silica is described. For this purpose, a polymerizable cationic NHC precursor, 1,3-bis(1-mesityl)-4-{[(bicyclo[2.2.1]hept-5-en-2-ylcarbonyl)oxy]methyl}-4,5-dihydro-1H-imidazol-3-ium tetrafluoroborate (5) was prepared and characterized by X-ray analysis. Oligomers were prepared therefrom using both the well-defined Schrock initiator Mo(N-2,6-i-Pr2C6H3)(CHCMe2Ph)[OCMe(CF3)2]2 and the first generation Grubbs catalyst Cl2Ru(CHPh)(PCy3)2. Ru-initiated oligomerizations were terminated with ethyl vinyl ether, Mo-initiated oligomerizations were terminated by addition of (EtO)3SiCH2CH2CH2NCO. (EtO)3Si-terminated oligomers obtained by the Wittig-like reaction between the Mo-containing oligomer and the isocyanate were used for the immobilization of the NHC-precursor containing oligomers on non-porous silica. Both oligomerizations were characterized by quantitative consumption of the corresponding initiator. This allowed the controlled synthesis of oligomers via stoichiometry. Using both non-porous and porous silica, degrees of derivatization of 0.04 and 0.02 mmol, respectively, of cationic NHC precursor/g silica were obtained. These precursors were converted into the corresponding NHC's by standard procedures and used for the generation of a heterogeneous second-generation Grubbs catalyst. Ruthenium loadings of 5.3 and 1.3 μmol/g, corresponding to 0.5 and 0.1 weight-% of catalyst were realized. Additionally, coating techniques were applied, where C18-derivatized silica-60 was loaded with oligo-5. Conversion into the corresponding heterogeneous catalyst revealed 4.1 μmol/g, corresponding to 0.4 weight-% of catalyst. All supported catalysts prepared by this approach were successfully used in RCM in slurry type reactions.