Keywords
Chapter 6 Conclusions and recommendations 6.1. Conclusions
6.1.1. Synthesis of propargyl alcohols
The ligands were synthesised according to the literature methods (1) (2). The synthesis of the 1,1-Diphenyl-2-propyn-1-ol ligand did not yield the quantity of ligand as reported in the literature method. The method had to be adapted from literature and a good yield with good purity could be synthesised. Diphenyl-[2]-pyridyl methanol resulted in pure white crystals; the yield was lower as expected because of the process of recrystallization for a ligand with high purity. All of the ligands were analysed with FT-IR, MS and NMR.
6.1.2. Synthesis of the Gr2Ph derivative of ruthenium- indenylidene
An attempt was made to synthesise (3) a Gr2Ph derivative of the ruthenium-indenylidene precatalyst. The hemilabile ligand was coordinated to the ruthenium-indenylidene precatalyst, forming the GrPh2-Ind precatalyst. This was evident after interpretation of the FT-IR spectra and MS spectra displaying the correlation to the wanted functional group vibrations and the expected molar mass. However, no metathesis activity was observed.
On closer inspection of the spectra, it seemed that the precatalyst contained contaminants.
This could be the reason for the precatalyst not having any metathesis activity.
Unfortunately NMR spectra could not be obtained. The precatalyst does not have a proton on the carbene that can be observered with NMR. Also, the precatalyst dissociates before any meaningful spectra can be obtained with longer NMR methods.
6.1.3. Metathesis
The Umicore-M2 (ruthenium-indenylidene) precatalyst which was commercially obtained had good activity for metathesis with 1-octene. It was established that the optimum conditions for metathesis of 1-octene with ruthenium-indenylidene were with a catalyst to alkene ratio of 1:9000 and a reaction temperature of 45°C.. Under these conditions the highest conversion to primary metathesis products are found with the least number of side reactions relative to the amount of 1-octene reacted. Compared to Grubbs 2 investigated in a previous study (3) (4), ruthenium-indenylidene had a slower activation towards metathesis.
The comparison of the metathesis results for Grubbs 2 with ruthenium-indenylidene, shows similar product distributions of the amounts of PMPs, SMPs and IPs. This is irrespective of the fact that ruthenium-indenylidene had a slower activation than Grubbs 2 and that the reaction times were about the same length.
From the experimental data of the cis/trans relationship, it is observed that cis/trans relationship of 7-tetradecene is about 20/80 throughout the reactions. This means that the 2nd pathway in the catalytic cycle (Scheme 2.9) is the dominant pathway. This is also supported by the findings reported by Adlhart (5) and Jordaan, (6) which are discussed in Chapter 2.1.
The metathesis product distribution of PMPs, IPs and SMPs with the ruthenium- indenylidene complex differs notably from 45°C to 60°C. The number of PMPs is the same for both of the temperatures where the number of IPs is double at 60°C and SMPs are 12% more at 60°C. 12% more of the 1-octene substrate was used at 60°C than at 45°C. This confirms that the catalyst performs best at a lower temperature than at a higher temperature. At both temperatures we see the favoured SMPs are the C11, C12 and C13 alkenes. From the extended reaction pathways in chapter 4 we can conclude that these SMPs were formed via the Ru=C5, Ru=C6 and Ru=C7 metal carbenes. These carbenes can only be present after the isomerisation of the 1-octene substrate.
After the kinetic study it can be concluded that the metathesis reaction is pseudo first order in the decrease of 1-octene. This makes sense when we look back at the metathesis result where we see that out of the two factors, concentration and temperature, only temperature had a real influence on the metathesis reactions.
Where concentration did have an effect on the reactions, it was so small that it cannot be seen as a major factor.
6.1.4. Computational
The result of the calculated Gibbs free energies for the dissociation of the PCy3 ligand and the opening of the hemilabile ligand indicates that the Grubbs 2 precatalysts will have a faster activation than Ru-Ind and Ru-Ind derivatives. This is in agreement with the observations from experimental results as discussed in Chapter 4. This faster activation results in Grubbs 2 and Gr2Ph having faster product formation during metathesis thus a higher turnover number in the same reaction time period.
The internal HOMO-LUMO energy gaps in the precatalyst complexes show smaller gaps for the ruthenium-indenylidene precatalyst as compared to Grubbs 2. These results suggest higher activation for ruthenium-indenylidene. This does not correlate with literature and experimental results where Grubbs 2 has faster activation and it also does not correlate with the previously mentioned Gibbs free energy results. For the complexes with the hemilabile ligand the same conclusion can be made.
Results for the HOMO-LUMO energy gaps between precatalyst and catalyst complexes with 1-octene show that the complexes favour the dissociation mechanism. The dissociative mechanism is slowed by the competing PCy3 ligand with 1-octene as the LUMO-HOMO energy gaps is lower between the catalysts and PCy3.
The substituents on the derivatives of the indenylidene ligand has an effect on the HOMO- LUMO energy values with the electron withdrawing groups F and NO2 decreasing the energy gaps, resulting in higher reactivity with substrate. The electron donating group CH3 increases the energy gap thus lowering the reactivity with substrate.
6.2. Recommendations
Other methods for the synthesis of the alkyne ligands should be investigated, focusing on the synthesis of precatalyst with variations of the alkyne ligands (Figure 6.1).
Ru Cl Cl
L1
L2
Grubbs 1: L1 and L2 = PCy3
Grubbs 2: L1 = NHC group and L2 = PCy3 R3
R3 = (CH3, halogens and NO2)
Figure 6.1 : Rutheniun-indenylidene precatalysts with different alkyne ligands.
Although it was found that a catalyst to 1-octene ratio of 1:9000 is the ideal for the reactions, the minimum and maximum catalyst loads should also be obtained. The reason for this is that at the maximum and minimum catalyst loads there will be noticeable differences in the percentages of products formed.
Using the isomerisation products of 1-octene, namely 4-octene, 3-octene and 2-octene in the metathesis reactions will give better insight into the formation of the metathesis products originating from only a specific isomer. Combinations of the isomerisation product should also be considered with the metathesis products. For a better understanding of how the products form, metathesis with selected secondary products should also be done to be able to identify possible new products.
Computationally the selectivity towards the isomers of 1-octene (2-octene, 3-octene and 4- octene) can be investigated focusing on energy to coordinate to the active catalyst in both cis and trans configurations of the octenes. The entire catalyst activation cycles should be modelled and energy profiles compiled in order to compare the activation cycle for
Grubbs 2 and Gr2Ph.
6.3. References
1. Lichtenheldt, M. Entwicklung neuer Ruthenium-Metathese-Katalysatoren und ihre Anwendung in der alternierenden ROMP; PhD Thesis; Technischen Universität Berlin: Berlin, 2008.
2. Herrmann, W. A.; Lobmaier, G. M.; Priermeier, T.; Mattner, M. R.; Scharbert, B. J. Mol. Cat. A 1997, 117, 455.
3. Jordaan, M. Experimental and Theoretical investigation of New Grubbs-type Catalysts for the Metathesis of Alkenes; PhD Thesis; North-West University: Potchefstroom, 2007.
4. Loock, M. M. The alkene metathesis reactivity of the PUK-Grubbs 2-prectatlyst; MSc dissetation;
North-West University: Potchefstroom, 2009.
5. Adlhart, C.; Chen, P. J. Am. Chem. Soc. 2004, 126, 3496.
6. Jordaan, M.; Van Helden, P.; Van Sittert, C. G. C. E.; Vosloo, H. C. M. J. Mol. Cat. A. 2006, 254, 145.