## 3 Homogeneous Supported Olein Metathesis Catalysts
Examples of PEG-bound Ru metathesis catalysts are shown in Figure 8. Although PEG and PEG-bound catalysts are soluble in  
Figure 7: Fluorous-tagged ruthenium catalysts **37–41**. many organic solvents including water, they are insoluble in solvents like hexane, diethyl ether, and cold ethanol. Thus, these PEG derivatives can be utilized to separate catalysts and products for catalyst recovery and recycling by either solvent precipitation or liquid/liquid extraction. An early example of the synthesis and application of a PEG\({}_{5000}\)-bound ruthenium metathesis catalyst **42** was reported by [PERSON] in 2000 [49]. This soluble polymer-supported catalyst **42** was recycled 8 times in the RCM reaction of allyl(4-pentenyl)tosylamide with more than 92% conversion in each cycle. The catalyst was recovered by precipitation with diethyl ether. Although the author reported the success in catalyst recycling, the analysis of Ru leaching was not reported. It also should be noted that this type of process for catalyst recovery requires a large excess of the ether solvent--a possible issue in green chemistry terms. In 2003, [PERSON] and co-workers described the study of a PEG\({}_{3400}\)-bound Hoveyda-Grubbs 2 nd generation catalyst **43**[48]. The soluble support was attached to the benzylidene ligand _ortho_ to the metal carbene. This catalyst can catalyze RCM reaction of diallytosylamide for 5 cycles at room temperature. The recovery of this catalyst from RCM reaction was also carried out by precipitation in diethyl ether and filtration. In this work, the authors carried out some additional characterization of the recovered PEG-bound Ru complex. The precipitate was analyzed by \({}^{1}\)H NMR spectroscopy, measuring the intensity of the vinyl proton of the alkylidene group in the catalyst at 16.50 \(\delta\). The results showed that only 57% of **43** was recovered after the first cycle of RCM reaction. The [PERSON] and [PERSON] groups have also described the use soluble polymer supports for ruthenium olefin metathesis catalysts. In their work, they focused on soluble polyolefin oligomers that can be more efficiently separated as an alternative to PEG supports whose separation typically generates large volumes of solvents waste during the polymer precipitation step. These alternative polymers are polyethylene (PE\({}_{\text{Olig}}\)) and polyisobutylene oligomers (PIB). Several techniques for separation of these types of polymer-supported species including liquid/liquid separation and solid/liquid separation shown in Figure 9 can be used. The first synthesis and application of PIB-supported Hoveyda-Grubbs type catalyst using such polymers was described in 2007 [50]. In this case, the PIB (\(M_{\text{n}}=1000\)) was attached to the catalyst as a benzylidene ligand following a prior strategy described by [PERSON] [17]. This pre-catalyst **44** was then used to catalyze RCM reactions in heptane. The products were then separated from the catalyst solution by an acetonitrile extraction. In some cases, the RCM product self-separated from the catalyst. In such cases where the product precipitated from the heptane solution, a filtration was only needed to isolate the product. This PIB-supported ruthenium complex **44** was reused for at least five cycles. The Ru leaching into the product was measured and varied from 20 to 1000 ppm, suggesting that the \"boomerang\" scheme for recycling the catalyst was not as effective as desired. In order to improve the recoverability of the PIB-supported ruthenium catalyst, an alternative type of catalyst was prepared where the PIB (\(M_{\text{n}}=1000\)) chains were attached to the non-dissociating \(N\)-heterocyclic carbene ligands of a Hoveyda-Grubbs catalyst as in complex **45**[51]. This catalyst design led to improvements in both the consistency and effectiveness of Ru catalyst/product separation. Leaching levels dropped to values as low as 73 ppm (0.37% of the charged Ru catalyst). Moreover, catalyst **45** could be reused for 20 cycles.