## 2 Heterogeneous Supported Olein Metathesis Catalysts
This was a quite different scheme for use of a heterogeneous-supported catalyst. In this case a pre-catalyst 7 was synthesized by a metathesis reaction, shaking [PERSON] 1 st generation catalyst and a vinyl-substituted DVB-PS derivative for 2 h in dichloromethane. The resulting benzylidene-immobilized pre-catalyst **7** is immobilized on the insoluble resin by a strong bond and the immobilized pre-catalyst was isolated by filtration. According to their report, **7** was indefinitely stable under normal atmospheric conditions with no loss of activity. In the application of **7**, a so-called boomerang reaction was involved. In this scheme, the Ru initially undergoes a metathesis reaction with an alkene of a ring-closing metathesis (RCM) substrate like diethyl diallymlanolate. The resulting metallocycle then forms a new Ru alkylidene and the original vinyl-substituted DVB-PS. The alkylidene complex goes into solution, becoming a homogeneous catalyst effecting ring-closing metathesis (RCM) on the rest of the substrate. Then, when the substrate is consumed, the authors postulate that the remaining soluble alkylidene reacts with the vinyl-substituted DVB-PS. This leads to a recapture of the Ru complex by the resin by a strong bond after the completion of the reaction. This behavior of the ruthenium catalyst where it comes off the resin and returns later was likened to the action of a \"boomerang\". In this work, the catalyst was recycled up to three times in an RCM reaction of diethyl diallymlanolate, with addition of 3,3-dimethyl-1-butene (9 mol % based on diethyl diallymlanolate), with Ru contamination in the product of 500 ppm. Catalyst recycling was accomplished by simple filtration of the solid-supported catalyst and the product was isolated from the filtrate by evaporation of the solvent. Following [PERSON] and [PERSON]'s work on polymer-supported catalysts and the other studies of homogeneous Ru metathesis catalysts that had shown the utility of Ru-based alkylidene catalysts that contained \(N\)-heterocyclic carbene (NHC) ligands, [PERSON] and co-workers described the synthesis of [PERSON] 2 nd generation catalysts like **8** that were immobilized on DVB-PS via an \(N\)-heterocyclic carbene ligand [18]. An advantage of this approach using \(N\)-heterocyclic carbene (NHC) ligands to immobilize a Ru complex is that an NHC ligand is a stronger sigma donor than phosphine ligands. Unlike phosphine ligands like those in **7** that are thought to dissociate from the ruthenium center to initiate catalysis [19], the NHC ligand should remain bound to the ruthenium center before, during, and after the metathesis reaction. Thus, leaching of Ru from the resin was expected to be reduced [18]. In the first example of this chemistry, the solid-supported catalyst **8** was prepared by ligand exchange between a DVB-PS-supported \(N\)-heterocyclic carbene ligand and the phosphine ligand on Grubbs 1 st generation catalyst **1**. The resulting DVB-PS supported heterogeneous ruthenium catalyst was characterized by \({}^{31}\)P NMR and IR spectroscopy. The general reactivity of **8** was demonstrated by the use of **8** in various metathesis reactions including RCM and yne-ene metathesis reactions. In the case of ring-closing metathesis reaction of diethyl diallylmalonate, up to four cycles of cyclization of diethyl diallylmalonate to 4,4-dicarboethoxycyclopentene could be effected using **8**. These reports stated that the products were obtained as colorless solids or oils. While this visual assay of Ru leaching is promising, a more quantitative assay of the amount of Ru leaching was not reported. This work was subsequently extended in a report where the [PERSON] group reported solid-supported Ru metathesis catalysts like **9** and **10** that had high stability, activity, and recyclability [20]. In recycling experiments using these two catalysts, both catalysts were recycled up to four times in RCM reaction of diallyltyosylamide.