The alkene metathesis reactivity of the PUK-Grubbs 2-precatalyst
Loock, Monique Marié
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Metathesis reactions are of great value for industrial processes e.g. Shell Higher Olefin Process (SHOP) to form new alkenes. A lot of catalytic systems were developed to optimize the activity, stability and selectivity of these precatalysts. In a previous study a Grubbs 2-type precatalyst (Gr2Ph (3)) which had a N^O hemilabile ligand with two phenyl groups coordinated to the Ru-metal was developed. This Grubbs 2-type precatalyst, known as the Puk-Grubbs 2-precatalyst (Gr2Ph (3)) was used for 1-octene metathesis reactions and showed an increase in the selectivity, thermal stability, activity and lifetime In comparison to Grubbs 1 (Gr1 (1)) and Grubbs 2 (Gr2 (2)). In order to determine if the precatalyst is of value to metathesis reactions it was used for different 1-alkenes (1-hexene, 1-heptene, 1-nonene and 1-decene) and the conditions optimized. The Gr2Ph (3) precatalyst was successfully synthesized (43% yield) in this study and used for the metathesis reactions of the different 1-alkenes. The metathesis reactions of each 1-alkene was conducted at different temperatures, 55°C and 60°C for 1-hexene reactions, 60°C, 70°C and 80°C for 1-heptene reactions and 60°C, 80°C and 100°C for 1-decene reactions, except 1-nonene, while the Ru:1-alkene ratios were varied for 1-hexene and 1-heptene (1:7000 and 1:9000). The results of the 1-alkene metathesis reactions showed that the Gr2Ph (3) precatalyst increased the lifetime to 35 days and thermal stability when compared to Gr1 (1) (1 hour lifetime) and Gr2 (2) (3 hours lifetime). The TON, selectivity and activity is comparable to those of the Gr2 (2) reaction results, whilst an improvement on the Gr1 (1) results was obtained. The optimum conditions in terms of TON, PMP formation and selectivity for Gr2Ph (3) was found to be at 80°C with a 1:7000 Ru:1-alkene ratio for 1-heptene and 1-decene, but the ethylene accumulation In the mini reactors had an impact on the results. The ethylene accumulation resulted in the conversion of the already formed PMPs into SMPs and IPs during the course of the reaction. The optimum temperature for the 1-hexene reactions were determined to be at 60°C for Gr2Ph (3) with a Ru:1alkene ratio of 1:7000, with a high PMP formation, selectivity and TON. The 1H-NMR investigation of the metathesis reactions of 1-octene and 1-hexene with Gr2Ph (3) in CDCI3 at 30°C with a Ru:1-alkene ratio of 1:40, showed that only one carbene signal was present. Metathesis products were formed and confirmed with GC-MS and a larger amount of SMPs and IPs were formed by the competing isomerization reactions. These results indicate that metathesis does occur but that another mechanism was present or that the ligand was not hemilabile as suspected.
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