## 3 Homogeneous Supported Olein Metathesis Catalysts  
Up to this point, this discussion has focused on the immobilization of olefin metathesis catalysts on heterogeneous supports. While this has been a common technique to isolate and recycle the catalysts, it is not the only possible scheme for catalyst product separation and catalyst recycling. Soluble phase tag methods used in combinatorial chemistry and in peptide synthesis have also been developed as an alternative tool for separation process between catalysts and the products. In these cases, the strategy is to use two different phases to recover catalysts. The presence of the two phases can result from perturbation of a homogeneous reaction mixture or can involve two liquid phases. Thus, the supports for this technique do not always have to be macromolecules--small molecules like fluorous tags or ionic tags can also be used to effect catalyst/product separation and catalyst recycling. Such soluble supports have potential advantages in that they can avoid some of the problems of heterogeneized catalysts that include more complicated analyses and the observation that the reactivity and selectivity of an immobilized catalyst differ from what is seen with an optimized homogeneous catalyst analog. These problems reflect the fact that the advantage of heterogeneous catalysts, which is their ease of separation from the reaction mixture, is an issue not just at the separation step after the reaction but also during the reaction and during the catalyst synthesis. The strategy in which phase tag that is soluble is used can avoid this issue in that catalyst soluble supports or phase tags allow the catalysts to  
Figure 5: Polystyrene-based-supported Schrock catalysts **22–26** and polynorborene-based-supported Schrock catalysts **27–30**. be synthesized and characterized in homogeneous solution and allow them to carry out their reaction as homogenous catalysts. Such tags are only used to effect separation after the reaction is complete. An example of this approach is the use of ionic liquid (IL) immobilized catalysts (Figure 6). Ionic liquids are alternative solvents that are useful because of their unique properties including non-volatility, high stability, and good recyclability [34]. These alternative solvents are immiscible with most organic solvents. Thus, they can be used in catalytic reactions as a recyclable phase. [PERSON] and co-workers reported the use of Grubbs 1 st generation catalyst **1** and [PERSON] 2 nd generation catalyst in an RCM reaction of 1,5-diallyl-3-benzyl-5-isobutylimidazolidine-2,4-dione in 1-butyl-3-methylimidazolium hexafluorophosphate (BMI PF\({}_{6}\)) as an ionic liquid solvent. These catalysts were recycled up to three times with Ru residues of \(\alpha\). 5000 and 1600 ppm in the product, respectively [35]. One year later, [PERSON] reported the use of BMI PF\({}_{6}\) as solvent in an RCM reaction of diallytosylamide catalyzed by a ruthenium benzylidine salt and was able to recycle the catalyst for two cycles [36]. Following this work, [PERSON] and co-workers introduced the ionic liquid-tagged ruthenium catalyst **33** which was synthesized in order to minimize the leaching of the catalyst from the ionic liquid phase [37]. This ionic liquid-bound catalyst **33** completed an RCM reaction of diallytosylamide with BMI PF6 as a solvent at 60 \({}^{\circ}\)C in 45 min using 2.5 mol % of **33**. The isolation of the product was achieved by extraction with toluene and the ionic liquid phase containing **33** was reused for an RCM reaction of diallytosylamide for 8 cycles. Importantly, this catalyst was stable enough to catalyze the ninth cycle of this RCM reaction without any loss in activity after three months. [PERSON] and [PERSON] reported the synthesis of a similarly stable ionic liquid-tagged catalyst **34**[38]. This catalyst too was recycled very effectively through 17 cycles in the RCM reaction of diallytosylamide without any loss in activity.