## 2 Heterogeneous Supported Olein Metathesis Catalysts
However, the ability to catalyze cross metathesis (CM) reactions of complex **9** was showed to be higher than that of complex **10**. For example, the conversion of CM between acrylonitrile and 4-pentenyl benzoate catalyzed by **9** was 98% in 12 h while the conversion was only 15% in 12 h in the case of **10**. The authors suggested that the superior activity of complex **9** owe to the ability of the catalyst to dissociate in solution, becoming homogeneous active species, unlike complex **10**. The leaching of Ru in the products was not described. The invention of new highly active ruthenium olefin metathesis catalyst pyridine-ligated Grubbs 3 rd generation catalysts has led to a new type of Ru catalyst which can catalyze cross metathesis reactions of a broader range of substrates and afford polymers with narrow polydispersity (PDI) by a ring-opening metathesis polymerization (ROMP). [PERSON] and [PERSON] reported the first example of a solid-supported version of this catalyst in 2005 [21]. Their report noted the possibility of using this supported catalyst in a continuous flow process due to the ability to reload the catalytic species onto the same solid support. Thus, even if leaching were to occur, the catalyst could be easily be regenerated. The recyclability of **11** was tested in a ring-closing metathesis reaction of diethyl diallylmalonate at 110 \({}^{\circ}\)C. This solid-supported catalyst showed activity up to 5 cycles but a decrease in product yield was noted in each subsequent cycle. The authors ascribed this to the thermal instability of **11** or leaching of Ru from the weakly bound pyridine ligands. However, the authors were able to show  
Figure 2: Examples of DVB-PS-supported (® = divinylbenzene cross-linked polystyrene or = divinylbenzene cross-linked 4-vinylpyridine) olefin metathesis catalysts **6–11**. that **11** could indeed be reactivated by washing/Ru re-addition protocol (1 M HCl, 1 M NaOH, H\({}_{2}\)O, MeOH, toluene, then addition of **3**). Ru contamination in the products was not mentioned. The other general strategy for catalyst immobilization on insoluble supports commonly is to use inorganic supports. Silica-based materials are the most common examples of these sorts of solid supports. Select examples of Ru metathesis catalysts immobilized on silica are shown in Figure 3. [PERSON] and co-workers reported the synthesis of an immobilized Grubbs 2 nd generation ruthenium complex on silica gel using hydroxyalkyl groups on \(N\)-heterocyclic carbene ligand 12 [22]. This immobilized catalyst was successfully recycled in RCM reaction of diethyl diallylmalonate through three cycles. The catalyst **12** was separated from the RCM product by simple filtration. The crude product was isolated as colorless liquid. The authors did report a detailed analysis of Ru leaching noting that the products had _ca._ 250 ppm of Ru contamination based on ICP-MS analysis. [PERSON] and co-workers have also described other versions of silica-supported olefin metathesis ruthenium catalysts **13** and **14**[23]. The silica-supported complexes **13** and **14** were competent catalysts in RCM and CM reactions with reactivity that was comparable to that of their homogeneous analogs. The catalytic activity toward an RCM reaction of diethyl diallylmalonate of catalyst **14** (31% conversion after 10 min) was shown to have slightly lower than the catalytic activity of **13** (67% conversion after 10 min). The authors also showed that immobilized ruthenium catalyst **13** can be recycled up to eight times in RCM reaction of allyl(2-methyl-2-propenyl)malonate with conversions of substrate to product in the 60%-80% range when reaction time was 2 h, conversions that could reach 100% with reaction times of 12 h. The authors suggested that these catalysts improve recyclability and eliminate issues associated with the decomposition of the ruthenium complex via bimolecular pathway.