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‐1 H ‐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 ‐Pr 2 C 6 H 3 )(CHCMe 2 Ph)[OCMe(CF 3 ) 2 ] 2 and the first generation Grubbs catalyst Cl 2 Ru(CHPh)(PCy 3 ) 2 . Ru‐initiated oligomerizations were terminated with ethyl vinyl ether, Mo‐initiated oligomerizations were terminated by addition of (EtO) 3 SiCH 2 CH 2 CH 2 NCO. (EtO) 3 Si‐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 C 18 ‐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.