The self-metathesis of methyl oleate was carried out in the presence of ruthenium alkylidene catalysts 1–4 at 40 °C. The Hoveyda-Grubbs catalysts 3 and 4 were evaluated in 1-butyl 3-methylimidazolium (bmim) and 1-butyl 2,3-dimethylimidazolium (bdmim) based ionic liquids with different anions (PF₆⁻, BF₄⁻ and NTf₂⁻) (Scheme 1). Figure 1 shows the activity of 3 in [bmim] and [bdmim] type ionic liquids. For catalyst 3, similar conversions were observed in [bmim] and [bdmim] type ionic liquids (ILs). As there was not much difference in the catalytic activity, the benefit of selecting a particular IL (among PF₆⁻, BF₄⁻ and NTf₂⁻) was found to be of less importance. For catalyst 4, the highest activity was found in [bdmim][BF₄] with a conversion of 85% (Figure 2). Comparing the cations of the ionic liquids, the C2-methylated imidazolium cation [bdmim] led to a slightly better conversion. The benefit of using [bdmim] cation in the ethenolysis of methyl oleate has been previously reported by Thurier et al. [16].

To study the effect of temperature on metathesis activity, the reactions were conducted at temperatures 40 °C and 60 °C. The results are summarized in Table 1. An increase in substrate conversion was observed with increase in temperature. At both the temperatures, the highest substrate conversion was seen with catalyst 4. Generally for all the runs, Hoveyda-Grubbs catalyst 4 exhibited good activity, although isomerization and formation of secondary metathesis products [17] seems to be a drawback. Hoveyda-Grubbs catalyst 3 and 4 showed good solubility in ionic liquid compared to Grubbs catalyst 1 and 2. The reason for the high activity of the Hoveyda-Grubbs second generation catalyst can be attributed to the absence of phosphine ligand. It has been proved that phosphine ligand suppresses the catalyst activity in Ru-catalyzed olefin metathesis [18]. The presence of free phosphine in solution can inhibit coordination of olefins to the transition metal centre by re-association with the active Ru complex. With the non-phosphine Ru complex (catalyst 4), activation occurs through the loss of O→Ru chelation. The styrenyl ether ligand completes less effectively with olefin substrates for Ru chelation [19]. [bdmim] type ILs can prevent the carbene formation of catalyst, which is believed to be formed by the deprotonation of imidazolium cation or from the oxidative addition of imidazolium to Ru centre [20].

The self-metathesis of methyl ricinoleate was carried out in the presence of catalysts 1 and 2 in [bmim][X] type ILs. Table 2 summarizes the activity of catalysts 1–4 for the self-metathesis of methyl ricinoleate. For catalyst 1, the highest activity was shown in 5a with a substrate conversion of 45% and the activity was found to be in the order PF₆ > BF₄ > NTf₂ (Figure 3). For catalyst 2, the same trend was followed, with the reaction in 5a yielding the highest conversion of 58% (Figure 4). The primary metathesis products (PMP) obtained from the metathesis of methyl ricinoleate (A) were 9-octadecene- 7,12-diol (B) and dimethyl 9-octadecenedioate (C), as illustrated in Scheme 2 [21].

As solvents, ionic liquids are more advantageous than conventional organic solvents, which make them recyclable and environmentally friendly. It has been reported that the ionic liquids could be reused for at least three runs [3,16,22,23]. In a typical experiment, metathesis reaction was carried out in ionic liquids, and, after extraction of reaction products with heptane (2 × 3 mL), the glass reactor was loaded with fresh methyl oleate and was introduced for a new run.

To study the recyclability, Ru catalysts 1–4, in different ionic liquids, were subjected to consecutive runs (Table 3). Grubbs catalyst 1 proved to be stable in three consecutive runs, with the third run resulting in a decreased conversion. Grubbs catalyst 2 showed an increased conversion in the third cycle when compared with catalyst 1. Comparing the Hoveyda-Grubbs catalysts 3 and 4, the best result was obtained with catalyst 3. The results prove that catalyst 3 can be reused for three runs without loss of activity. In spite of good activity in the first run, the second generation Hoveyda-Grubbs catalyst 4 showed low catalytic activity during the second run.

1-Butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]), 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF₄]), 1-butyl-3-methylimidazolium bis(trifluoromethylsulphonyl)imide ([bmim][NTf₂]), 1-butyl-2,3-dimethylimidazolium hexafluorophosphate ([bdmim][PF₆]), 1-butyl-2,3- dimethylimidazolium tetrafluoroborate ([bdmim][BF₄]), were all reagent grade chemicals from Sigma-Aldrich. Methyl oleate (≥99%) and methyl ricinoleate (>99%) were obtained from Sigma-Aldrich and were treated with activated alumina and stored under N₂ atmosphere at a subzero temperature. Ethyl vinyl ether was purchased from Fluka. Nonadecane purchased from Fluka was used as the internal standard (IS). Ruthenium catalysts 1–4 were stored under N₂ and used as purchased from Sigma-Aldrich. Chromatograms were obtained using Varian Star 3400 CX GC equipped with a DB-624 capillary column (J&W Scientific, 30 m × 0.53 mm) and a flame ionization detector (FID). The oven temperature was held at 200 °C and then increased to 270 °C at a rate of 20 °C min⁻¹. The injector temperature was set at 270 °C and the detector temperature at 300 °C with N₂ as carrier gas.

All the reactions were performed under a N₂ atmosphere in a glass reactor fitted with a thermometer and a rubber septum. For the reaction in RTILs, 0.5 mL of the substrate was added to 1 mL of ionic liquid and stirred for 10 min to attain the reaction temperature. An internal standard (0.05 g) was added followed by the addition of 12.4 mg of catalyst. All the catalysts were soluble in ILs with the substrate forming a biphasic mixture with ILs. Samples were withdrawn by a syringe at regular time intervals for up to 4 hours. The reaction was terminated by immediately quenching with a few drops of ethyl vinyl ether [22]. The quenched sample was diluted with solvent and analyzed by GC.