3.1 Optimisation of reaction conditions
The optimum reaction conditions for Grubbs precatalysts 1 and 2 were determined using methyl oleate as the model substrate and were subsequently applied on all substrates except where otherwise indicated. The parameters in the optimisation of the Grubbs pre-catalysts were (1) solvent polarity (2) reaction temperature (3) reaction atmosphere and (4) substrate:Ru molar ratio. Scheme 2 illustrates the formation of the primary metathesis products (PMP), B and C, from the self-metathesis of the oleate-type fatty compounds.
To investigate the influence of solvent polarity on catalyst activity the solvents Hex, PhH, PhCl, DCM and DCE were used, having polarities of 9, 11.1, 18.8, 30.9 and 32.7, respectively, according to the Reichardt's normalised solvent polarity scale (ETN) [14]. Figure 1 shows the influence of solvent polarity on catalyst activity expressed as % conversion with MO/Ru molar ratio of 100. The highest activity was obtained in DCM or DCE both of which have moderate polarities relative to Hex, the solvent of lowest polarity. The poor solubility of precatalysts in acetone, ethanol, methanol and water, which have relatively higher polarities, was a limitation that led to the exclusion of these solvents in further experiments. The activity of 1 increased with increasing solvent polarity in the order Hex < PhH < PhCl < DCM ~ DCE. The observed order of activity showed a positive correlation between solvent polarity and catalyst activity with halogenated solvents (DCM and DCE) providing the best reaction conditions.
From varying the reaction temperature, the activity of 1 showed no significant change over the temperature range 20–60°C. However, a sharp increase in activity was observed at 80 °C along with the formation of secondary metathesis products (SMP) resulting from double bond isomerisation. A further increase in reaction temperature led to a decrease in catalyst activity.Figure 2 illustrates the influence of reaction temperature on the activity and selectivity of the precatalysts 1 and 2 and Scheme 3 shows possible SMP (F–J) from the cross-metathesis of the substrate olefin A with isomerisation products D and E.
Under air atmosphere a slightly lower activity was observed with MO conversion of 46.7% compared to 49% under N2. The deactivation of 1 under air atmosphere was nevertheless insignificant compared with the extent to which the ill-defined metathesis catalysts deactivate under similar conditions [7].
A steady increase in the turnover number (TON) was observed for MO/1 molar ratios from 100 to 1000. Thereafter a sharp decrease in the TON occurred (Table 1) after reaching a maximum of 525 at a MO/1 molar ratio of 1000 with a selectivity of 100%. Generally the optimum conditions obtained for 1 using MO compared favourably with those previously obtained by Buchowicz and Mol [10].
When MO underwent self-metathesis in the presence of 2, solvent polarity had very little influence on catalyst activity, in contrast to the results obtained with 1. From Figure 1 we see no significant difference in catalyst activity with the change in solvent polarity.
From varying the reaction temperature, the activity of 2 showed no significant change within the temperature range 20–60 °C but then a sharp increase in activity occurred at 80 °C and beyond with the formation of SMP. Catalyst 2 showed stability even at 120 °C whereas 1 showed some deactivation at 100 °C already. However, a comparatively low selectivity for PMP at elevated reaction temperatures implied that the reaction conditions were strongly acting in favour of double bond isomerisation. Figure 2 compares the activities and selectivities of 1 and 2 on MO self-metathesis. It can be seen from the results obtained that the activity and selectivity towards the PMP in both cases was only comparable within the temperature range 20–60 °C.
A slightly lower MO conversion (50.5%) was obtained in air compared to 51.5% under N2 atmosphere. A steady increase in TON up to a maximum of 675 corresponding to MO/2 molar ratio of 1500 was observed (Table 1). The observed decrease in TON after reaching maximum MO loading for both 1 and 2 was consistent with the results obtained by Jordaan and Vosloo [15] using a Ru catalyst with a chelating pyridinyl-alcoholato ligand.
3.2 Influence of oxygen containing functional groups on catalyst activity and selectivity
The metathesis activity and selectivity of 1 and 2 were examined using MO, MHO, MEO and OA as substrates with substrate/Ru molar ratio of 100 and DCM as solvent. The influence of the different organic functional groups on catalyst activity and selectivity is discussed in this section.
3.2.1 Influence of oxygen containing functional groups on the activity and selectivity of <bold>1</bold>
The products and the yields obtained from the metathesis of MO, MHO, MEO and OA in the presence of 1 are given in Table 2. The typical gas chromatograms of reaction mixtures under typical reaction conditions of these fatty compounds are given in Figure 3. For MO and MHO a close to 2:1:1 statistical product distribution was attained at steady state with conversions of 49 and 39.6% respectively and a selectivity of > 99% towards PMP. The MO conversion was slightly higher than that reported in the literature [10] under similar conditions. MHO conversion was, on the other hand, comparable to the reported conversions for MO and methyl erucate. In the case of MEO and OA, conversions of 23.5 and 24.5% were obtained with selectivities of 57 and 88% respectively. The presence of SMP in the MEO and OA product streams suggested the occurrence of double bond isomerisation in the presence of 1. Table 3 compares substrate conversions, selectivities and the yields obtained for PMP and SMP. The decreasing order of substrate reactivity was MO > MHO > OA > MEO, thus indicating that 1 is less tolerant towards the epoxy and the carboxylic acid groups compared to the hydroxyl and the ester groups. The lower reactivity of 1 in the presence of MHO, OA and MEO seems to suggest phosphine displacement by oxygen-bearing nucleophiles, a side reaction competing with dissociative substitution of a phosphine ligand with an olefinic substrate [9], to form four coordinate Ru(OR)2L(=CHPh) complexes in accordance to the route proposed by Coalter et al. [16]. In addition, a β-hydride elimination reaction occurs in which the metallocycle intermediate once formed gives the allyl hydride intermediate which finally forms a coordinated alkene [17].
3.2.2 Influence of oxygen containing functional groups on the activity and selectivity of <bold>2</bold>
Table 2 shows the products and yields obtained from the self-metathesis of MO, MHO, MEO and OA in the presence of 2. The selectivity towards the PMP was > 99% for MO and MHO metathesis with conversions of 51.5 and 48.4% respectively. On the other hand MEO and OA metathesis gave conversions of 58 and 67%, respectively, accompanied by higher isomerisation activity judging from the high SMP yields obtained in this instance. Table 4 compares substrate conversions, selectivities and the yields obtained for PMP and SMP. The observed substrate reactivity in decreasing order was OA > MEO > MO > MHO, thus indicating that the tolerance of complex 2 towards the epoxy and the carboxylic acid groups is much higher compared to that of complex 1. These results further indicate that polar organic functional groups possess a significant influence on both substrate reactivity and catalyst selectivity. Indeed 2 showed an improved activity with double bond isomerisation more pronounced than was the case with 1. The higher activity of 2 had been attributed to the stabilisation of the intermediate metallacycle by the more bulky and basic N-heterocyclic carbene ligand [6,8–9,18–19]. Indeed the results obtained with and 1 and 2 served to further demonstrate that these catalysts are highly resistant to deactivation by olefins containing polar functional groups compared to the ill-defined metathesis catalysts [20].