## 2 Heterogeneous Supported Olein Metathesis Catalysts
Such immobilized ruthenium catalysts on silica support have less intermolecular activity between the catalysts--the same phenomena reported earlier by [PERSON] group for site isolated DVB-PS supported species [16]. The analysis of the Ru leaching in this case would seem to support this claim. In this example, the ruthenium leaching studied by ICP-MS revealed the contamination level in products to be less than 5 ppb for those prepared by both **13** and **14**. This is especially notable since it is by several orders of magnitude the lowest Ru leaching ever reported. Ying group also described using click chemistry for the immobilization of Hoveyda-Grubbs type complexes on nanoporous silica **15**[24]. The catalyst they prepared exhibits good activity and stability as well as recyclability. In addition, these authors demonstrated that this catalyst can be used in a circulating flow reactor. The catalyst was reused in RCM reaction of diethyl diallylmalonate over 8 times with overall conversions of 90% with Ru leaching levels of 11.3 ppm at the first 60 min and 1.6 ppm at 180 min based on ICP-MS analysis of the isolated products. Yet another example of silica-bound Ru metathesis catalysts was reported by [PERSON] and co-workers who used commercially available molecular sieves as supports for the Ru catalyst **16**[25]. These SBA-15 molecular sieves possess several advantages including high surface area, narrow pore size distribution, and high thermal and mechanical stability. The solid-supported ruthenium complex **16** on such sieves was shown to be competent as an RCM catalyst using diethyllmalonate as a substrate. The leaching of Ru into RCM product was found to be as low as 17 ppm. However, attempts to recycle this catalyst were unsuccessful, with the conversion reaching 90% for only two cycles. More recently, [PERSON] and co-workers also reported a synthesis of recyclable silica-supported Hoveyda-Grubbs type complex using an NHC ligand **17**[26]. This catalyst proved to be recyclable for RCM reaction of diethyl diallylmalonate with Ru contamination of 221 ppm found in the product based on ICP-MS of products from the first cycle. Ru leaching in subsequent cycles was not measured but would likely be less as Ru leaching in the first or second cycles of a catalytic reaction could reflect undetectable issues in the synthesis of a catalyst. If a catalyst were only 99% pure (pure by most analyses used in synthesis), the 1% impurity would represent most or all of the observed Ru leaching but would not be relevant in later cycles. [PERSON]'s molybdenum olefin metathesis catalysts have also been supported on always insoluble supports. Some examples of such silica-supported catalysts supports are shown in Figure 4. For example, [PERSON] and co-workers described the synthesis of well-defined surface immobilized catalysts **18** and **19**[27]. These two silica-supported catalysts showed very similar activities in cross metathesis reaction of ethyl oleate (EO) with TOF (turnover frequency measured after 5 min of reaction expressed in mol of substrate converted per mol of Mo per sec) of 0.04 and 0.03, respectively. However, the silica-supported **18** was reportedly more stable than **19**, a difference that was ascribed to the site isolation of metal complexes on the silica support [28]. The stability of **18** and **19** were determined by the time needed to reach equilibrium in the CM reaction of ethyl oleate (_i.e._, around 50% conversion), which was 1 and 24 h, respectively (the longer the time was presumed to reflect a faster decomposition rate for the catalyst). Shortly later, [PERSON] and co-workers developed more active, stable, and selective silica supported molybdenum olefin metathesis catalyst **20**[29]. The increase in reactivity was achieved by replacing one imido group with a siloxy group from the surface. Keeping one remaining imido ligand enhanced the stability of the molybdenum catalyst. This catalyst was also tested its activity in CM of ethyl oleate.