Bulky Aryloxide Ligand Stabilizes a Heterogeneous Metathesis Catalyst

Silica-Supported Molybdenum Oxo Alkylidenes: Bridging the Gap between Internal and Terminal Olefin Metathesis

Grafting a molybdenum oxo alkylidene on silica (partially dehydroxylated at 700 °C) affords the first example of a well-defined silica-supported Mo oxo alkylidene, which is an analogue of the putative active sites in heterogeneous Mo-based metathesis catalysts. In contrast to its tungsten analogue, which shows poor activity towards terminal olefins because of the formation of a stable off-cycle metallacyclobutane intermediate, the Mo catalyst shows high metathesis activity for both terminal and internal olefins that is consistent with the lower stability of Mo metallacyclobutane intermediates. This Mo oxo metathesis catalyst also outperforms its corresponding neutral silica-supported Mo and W imido analogues.

Alkyne Metathesis with Silica-Supported and Molecular Catalysts at Parts-per-Million Loadings

Improvement of the activity, stability, and chemoselectivity of alkyne-metathesis catalysts is necessary before this promising methodology can become a routine method to construct C≡C triple bonds. Herein, we show that grafting of the known molecular catalyst [MesC≡Mo(OtBu_F6 )₃ ] (1, Mes=2,4,6-trimethylphenyl, OtBu_F6 =hexafluoro-tert-butoxy) onto partially dehydroxylated silica gave a well-defined silica-supported active alkyne-metathesis catalyst [(≡SiO)Mo(≡CMes)(OtBu_F6 )₂ ] (1/SiO_2-700 ). Both 1 and 1/SiO_2-700 showed very high activity, selectivity, and stability in the self-metathesis of a variety of carefully purified alkynes, even at parts-per-million catalyst loadings. Remarkably, the lower turnover frequencies observed for 1/SiO_2-700 by comparison to 1 do not prevent the achievement of high turnover numbers. We attribute the lower reactivity of 1/SiO_2-700 to the rigidity of the surface Mo species owing to the strong interaction of the metal site with the silica surface.

Immobilized Grubbs catalysts on mesoporous silica materials: insight into support characteristics and their impact on catalytic activity and product selectivity

The ideal support characteristics for immobilization of the Hoveyda–Grubbs 2 catalyst were defined in the metathesis of cyclooctene and the reaction mechanism to cyclic oligomers was unraveled.

MeReO3/Al2O3 and Me4Sn-activated Re2O7/Al2O3 alkene metathesis catalysts have similar active sites

MeReO₃/Al₂O₃ and Re₂O₇/Al₂O₃ activated with Me₄Sn, which have the same reactivity toward functionalized alkenes and ethylene, share similar surface sites.