## 3 Homogeneous Supported Olein Metathesis Catalysts
In contrast, a similar untagged analog of **34** lost its activity in the second and subsequent runs. In 2007, [PERSON] and co-workers reported the synthesis of **35** and **36**, an improved version of **34**[39]. However, while the activities of both catalysts, toward the same substrate mentioned earlier, were good for the first cycle, the catalyst activities significantly dropped in the second cycle. In general, these studies relied on measuring catalyst reactivity as a basis for catalyst recyclability. The amount of Ru leaching was typically not mentioned except in a few cases. An alternative scheme for liquid/liquid separation that has been used in metathesis and in other chemistry is fluorous phase technology. In this scheme, catalysts are modified with fluorinated phase tags to facilitate separation, recovery and recycling in perfluorinated solvents [40]. Examples of  
Figure 6: Ionic liquid-tagged ruthenium catalysts **33–36**. fluorous-tagged Ru catalysts are shown in Figure 7. Since the first report by [PERSON] [41], fluorous biphasic catalysis has greatly expanded and developed into a general strategy for separations [42]. This chemistry typically uses a mixture of organic and fluorous solvents, relying on the fact that fluorous solvents are often immiscible with most organic solvents at room temperature and such solvent mixtures are often biphasic. Such solvent mixtures can be used in two ways. In one type of experiment, a reaction is carried out under biphasic conditions with a biphasic liquid/liquid separation after a reaction. Alternatively, in some cases fluorous and organic solvents become miscible at elevated temperature. In those thermomorphic systems, a reaction can be effected under monophasic conditions and the catalyst and the fluorous phase can be separated from an organic product in an organic phase at room temperature when the biphasic mixture reforms and the phases can be separated one from another by a gravity-based liquid/liquid biphasic separation. In 2004, [PERSON] and [PERSON] reported the immobilization of a Grubbs-type catalyst on poly(fluoroalkyl acrylate) **37**[42]. The catalyst **37** was used in RCM reactions of diallytosylamide in a monophasic PhCF\({}_{3}\)/CH\({}_{2}\)Cl\({}_{2}\) (1:19 \(v/v\)) solvent system. Extraction of the fluorous species using perfluorohexane (FC-72) and EtOAc after each reaction allowed the catalyst to be recovered and reused. The authors were able to recycle this fluorous-tagged ruthenium catalyst for 20 cycles. However, leaching was not described. Inspired by this work, [PERSON] group reported the study of other recoverable metathesis catalysts using lighter fluorous supports, with only 17 fluorine atoms per Ru in **38** and **39**_versus_ 170 fluorine atoms per Ru in **37**[43]. These catalysts **38** and **39** show similar activity to their non-fluorous-bound analogs in RCM reaction of diallytosylamide. However, the separations of catalysts **38** and **39** involved the use of fluorous silica gel rather than a liquid/liquid extraction. Thus, extra solvents including acetonitrile were needed to obtain the product and ether was needed to recover the fluorous-tagged catalyst. The recovered catalyst can be reused for at least five cycles with the average product yield of 97%. Later on, [PERSON] and co-workers reported two other light fluorous-tagged catalysts **40** and **41**[44]. Compared to **39**, fluorous-tagged catalyst **40** had an improvement in activity with similar recyclability (90% recovery of catalyst), while **41** showed higher activity than both **39** and **40** but was not recoverable [44]. The fluorous-tagged catalyst **40** was recycled and reused in an RCM reaction of diethyl diallymalonate for five times. The product yield was 95%-100% in each cycle. Soluble polymeric supports are an alternative to ionic or fluorous tags. Poly(ethylene glycol) (PEG) is one of the most widely used such supports for reagents and organometallic complexes [45; 46; 47]. PEG has been used both to bind catalysts and has been used as a solvent [48].