Keywords
Chapter 4 Results and discussion
4.1 Synthesis of the Gr2Ph-Ind precatalyst
4.1.3 Investigation of product formation
The product formation during metathesis is usually simplified to primary and secondary metathesis products. By simplifying the product formation valuable information with regard to the mechanism is lost. Relating experimental results to the mechanism of the reaction becomes tedeous. However, it is important that we go deeper into the metathesis reaction as it could possibly give a better insight into the metathesis mechanism during the reaction.
The metathesis products that have formed need to be inspected closer and questions asked such as: What mechanisms are followed for the formation of each alkene? Are only some of all the possible mechanistic routes responsible for the observed products? How much exactly does the formation of products differ from a lower temperature and a high temperature?
With the next set of results, the relationship between the isomers of each product formed and the mechanistic pathway will be investigated.
At 25 °C (Figure 4.20) no secondary metathesis products (SMPs) (retention time 7.5 min to 17.5 min) that formed during the metathesis reaction are observed. From the first sample taken for GC analysis there are indications of products between retention times of 10 to 15 min (SMP range). According to Figure 4.20 these products did not undergo any change during the whole period during which the reaction was monitored. This indicates that the observed products in the SMP range are not products formed during the reaction. Primary
metathesis products are observed for the first time at 20 min, and the amount of products increase with time. Metathesis at 25 °C is slow with primary metathesis products being the only alkenes formed during the reaction.
Figure 4.20 : Stacked GC spectra of a 1-octene metathesis reaction with ruthenium- indenylidene precatalyst at 25 °C
From Figure 4.21, which is the results of a metathesis reaction done at 35 °C, it can be seen that products other than the primary metathesis products have formed. It means that at this temperature the catalyst is no longer 100 % selective for the formation of the primary metathesis products. The first metathesis results, the SMPs and PMPs, observed at 35 °C (Figure 4.21) is at 40 min. Just after a retention time of 10 min, doublet peaks start to show;
however, at reaction time of 640 min they start to converge to one peak as seen at reaction time 1260 min. This is because one of the isomers forms dominantly and base of the peak of this isomer overlaps with the other isomer. The GC is no longer able to pick up separate products and only one integration value is given for this product.
Figure 4.21 : Stacked GC spectra of a 1-octene metathesis reaction with ruthenium- indenylidene precatalyst at 35 °C
From the comparison of the metathesis reactions at 35 °C and 25 °C, it can be seen that there is a definite change in the amount of products in the reaction. This is due to SMPs being observed for the first time, meaning that at 35 °C secondary metathesis takes place.
For the metathesis reaction at 45 °C (Figure 4.22), the products form faster than at 25 °C and 35 °C. At a time of 40 min there are already ample PMPs presents and the reaction rate for the formation of SMPs also dramatically increased from 35 °C. At a time of 160 min tridecene, with a retention time of about 15 min is already present in a sizeable quantity, where at 35 °C, tridecene can only be observed in a sizeable quantity at 640 min. Between 10 min and 12.5 min the cis and trans isomers of dodecene are visible at 40 and 80 min, but from 160 min there is only one product peak visible because of overlapping. From the results we see that a lower temperature would be more favourable, but Figure 4.22 shows that a temperature of 45 °C leads to better product distribution if the ratios of all the formed products and the amount of 1-octene used are taken into account. To clearly quantify the results seen in the spectra, the calculated results for the metathesis reactions are to follow.
Figure 4.22 : Stacked GC spectra of a 1-octene metathesis reaction with ruthenium- indenylidene precatalyst at 45 °C
Figure 4.23 : Stacked GC spectra of a 1-octene metathesis reaction with ruthenium- indenylidene precatalyst at 60 °C
At 60 °C (Figure 4.23) and 80 °C (Figure 4.24) it is clear that the rate of formation of primary and other products is faster than at the lower temperatures. Once again we can see for dodecene, at retention time of 11 min, initially two adjacent peaks which are the cis and
trans isomers, but at 120 min for 60 °C and at 15 min for 80 °C the two peaks overlap and only one peak is observed.
Figure 4.24 : Stacked GC spectrums of a 1-octene metathesis reaction with ruthenium- indenylidene precatalyst at 80 °C
Combining the results for each metathesis reaction into tables, we are able to quantify each of the peaks visible in the stacked GC spectra above.
During the 45 °C metathesis reaction 7-tetradecene is the most abundant product throughout the reaction which keeps on increasing as the reaction progresses. 5-decene has the lowest abundance during the reaction especially after 80 min (Table 4.3). With some close inspection we can see that 4-decene fluctuates during the length of the reaction by constantly increasing and decreasing in percentage. Of the formed alkenes, the longer chain alkenes seem to be the most abundant of the formed products. This could be attributed to shorter chain alkenes being converted to longer chain alkene, which are stable and reaction with these alkenes is not favoured.
Table 4.3:Distribution of observed substrate and metathesis products for the 45 °C metathesis reaction for ruthenium-indenylidene .
As shown in Table 4.4 below, from 40 to 80 minutes 4-octene is dominant, but from 160 min 4-octene takes second place and 3-octene is now the dominant isomer. 3-nonene is the most abundant isomer of the nonenes throughout the reaction. 4-decene stays the dominant isomer from 40 to 320 minutes where after 3-decene takes the lead. For undecene, dodecene, tridecene and tetradecene the trans isomer stays dominant during the entire length of the reaction.
Table 4.4: Ratios of isomers formed for each of the different alkenes at 45 °C for ruthenium- indenylidene.
45 °C reaction 40 min 80 min 160 min 320 min 640 min 1280 min ratio % ratio % ratio % ratio % ratio % ratio %
4-octene 86 84 35 39 29 22
3-octene 12 14 56 51 55 61
2-octene 2 2 9 10 16 17
2-nonene 23 24 2 2 3 3
3-nonene 73 72 93 93 92 92
4-nonene 4 4 5 5 5 5
5-decene 3 4 5 1 6 6
4-decene 83 79 71 91 37 23
3-decene 14 17 24 8 57 71
5-undecene (trans) 52 59 100 91 100 100
5-undecene (cis) 48 41 0 9 0 0
5-dodecene (trans) 51 50 76 75 91 100
5-dodecene (cis) 49 50 24 25 9 0
6-tridecene (trans) 100 100 82 83 84 84
6-tridecene (cis) 0 0 18 17 16 16
7-tetradecene (cis) 18 17 18 17 16 17
7-tetradecene (trans) 82 83 82 83 84 83
In comparison with 45 °C, 7-tetradecene is also the most abundant metathesis product at 60 °C (Table 4.5), however 3-decene has the lowest abundance at 60 °C. At this higher temperature the metathesis products start to reach some point of equilibrium. For example, after 320 min most of the longer chain alkenes are reaching equilibrium.
Table 4.5: Distribution of observed substrate and metathesis products for the 60 °C metathesis reaction for ruthenium-indenylidene.
Again the isomers of each product are grouped together in Table 4.6 for the 60 °C metathesis reaction and the results changes very little. The results for octene follow the same trend as for 45 °C, but the actual percentages are different since the catalyst is more active at the higher temperature and thus the reaction is faster. 3-nonene is still the most of the nonenes throughout the reaction, quickly reaching 90 % early in the reaction. Of the decenes, 5-decene has the highest percentage value where at 45 °C 4-decene is higher. For undecene, dodecene and tridene the trans isomers are dominant, which is the same trend as for 45 °C.
Table 4.6: Ratios of isomers formed for each of the different alkenes at 60 °C for ruthenium-indenylidene.
60 °C reaction
30 min 90 min 180 min 300 min 660 min 1260 min Ratio % Ratio % Ratio % Ratio % Ratio % Ratio %
4-octene 47 42 20 13 8 11
3-octene 45 49 62 67 48 68
2-octene 8 9 18 20 44 21
2-nonene 1 0 2 2 2 2
3-nonene 91 93 93 92 92 92
4-nonene 8 7 5 6 6 6
5-decene 82 83 64 59 59 59
4-decene 18 17 28 32 32 32
3-decene 0 0 8 9 9 9
5-undecene (trans) * * 100 100 100 100
5-undecene (cis) * * 0 0 0 0
5-dodecene (trans) 42 51 62 66 66 66
5-dodecene (cis) 58 49 38 34 34 34
6-tridecene (trans) * 96 96 97 100 97
6-tridecene (cis) * 4 4 3 0 3
7-tetradecene (cis) 19 19 18 18 18 22
7-tetradecene (trans) 81 81 82 82 82 78
* Products can be present but the amounts present are almost zero.
The 80 °C reaction (Table 4.7) has similarities to 45 °C and 60 °C, but now the amount of secondary metathesis products are definitely more and 6-tridecene is close to the amount of 7-tetradecene. In this reaction the result that stands out the most is the decrease in the amount of 7-tetradecene, where for the lower temperature an increase is mostly observed with a slight decrease at 60 °C (300 min). This indicate that at this high temperature, 7-tetradecene is used as substrate in the metathesis reaction. If 7-tetradecene is to be used as a substrate in the metathesis reaction, 2-nonene, 3-decene, 5-dodecene and 6-tridecene are possible products that can form. When studying the table of results, from 180 min where 7-tertradecene starts to decrease, 3-decene is observed for the first time and 6-tridecene does increase more readily than the other formed alkenes.
Table 4.7: Distribution of substrate and metathesis products for the 80 °C metathesis reaction for ruthenium-indenylidene.
The above results give considerable information on the isomerisation between the formed metathesis products and how the product ratios change with the change in temperature.
To summarise the results: It was found that at the low temperature of 25 °C the metathesis reaction is slow to initiate and primarily forms PMPs with very little SMPs. The rate of the reaction increases with the increase in temperature, so does the amount of SMPs that form.
The 45 °C metathesis reaction gives the most PMPs, and the SMPs are below 20 %.
Although the amount of SMPs at the lower temperatures are below 10 %, the amount of unreacted 1-octene at the lower temperatures are more compared to the amount of 1- octene left at 45°C. For example, the amount of SMPs at 35 °C are below 10 % and PMPs are about 50% with unreacted 1-octene ranging around 40 %. Thus, the conclusion is that a reaction temperature of 45°C with a catalyst load of 1 to 9000, is the optimum conditions for the metathesis of 1-octene with ruthenium-indenylidene. Although the reactions are faster as the temperature increases and at the end of the reactions the amount of 1-octene left at 80 °C is the least, the amount of formed 7-tetradecene is the most at 45 °C. This is because the secondary metathesis products are easily formed as the temperature is
throughout all the reaction is about 20 % cis and 80 % trans. These ratios also compare excellently with literature values for the metathesis of 1-octene with 1st and 2nd generation Grubbs catalysts at a low and high temperatures. (6) According to the catalytic cycle discussed in Chapter 2 Scheme 2.10, this means that for all reactions the second pathway is the most active. The ratio distribution of the observed metathesis products are shown in graphs in the Appendix in Figure 1 to Figure 14.
From the observations of the different alkenes that formed in the metathesis reactions, Scheme 4.1 was put together showing which reaction steps are responsible for the formation of the observed alkenes. These proposed reaction pathways are drawn according to possibilities from Scheme 2.11.
Scheme 4.1: Proposed reaction pathways of formed products
From Scheme 4.1 we see that self-metathesis of 1-octene has to take place first to give the primary metathesis products, and with this the isomerisation of 1-octene also takes place.
Now 1-octene, 2-octene, 3-octene and 4-octene can undergo cross-metathesis to form the secondary metathesis products. The isomers can also undergo self-metathesis, where in this case 3-octene reacted with itself to give 5-decene.
As Scheme 4.1 shows only simplified pathways for the metathesis reaction of 1-octene, Scheme 4.2 is extended, showing multiple routes with different possible active complexes present during the reaction. The reaction pathways include reactions with only the different octenes, because to include all the possible reactions will make the scheme very
any of the formed products, resulting in the formation of other possible alkenes and other possible active carbene species. Not all the possible products are shown as the focus is to try and explain the formation of the products observed with the GC and GC-MS.
Scheme 4.2:Extended reaction pathways for the 1-octene metathesis reaction
Initially the octenes react with the heptalidene (Ru=CHC6) carbene species forming the primary product 7-tetradecene and some of the secondary metathesis products. In this process the methylidene (Ru=CH2), ethylidene (Ru=CHC1), propylidene (Ru=CHC2), buthylidene (Ru=CHC3) and pentylidene (Ru=CHC4) carbene species are formed. These newly formed carbene species again react with 1- and 2-octene forming hexylidene (Ru=CHC5) and heptylidene (Ru=CHC6). All of these carbenes react with the octenes to give other secondary metathesis products and also some of the products that have originally formed.
From Scheme 4.2 it is clear that the mechanism of alkene metathesis is complex, and at any given time all or some of the pathways are producing products. As the metathesis reactions
more stable than others. The result is that there are products that in turn gets used again in the metathesis reactions and also products that are so stable that they do not change much as the reactions progress.
Together with Scheme 4.2 and the metathesis results of this study it quickly became clear that the mechanistic pathways for the formation of the different observed alkenes are not as simple as originally proposed in Schemes 2.11 and 4.2.