Metathesis of alkenes using ruthenium carbene complexes
Mtshatsheni, Kgomotso Ntombizodwa Gina
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In this study different ruthenium carbene complexes, Grubbs 1 and 2 were investigated for the reactions of 1-, 2-, 3-octene and 1-tetradecene. All reactions were conducted in glass mini reactors equipped with Mininert® valves under a dry nitrogen atmosphere. A heating block was used to heat the reactors. Gas chromatography was used to determine the product composition of reaction mixtures. The influence of reaction temperature, solvents and catalyst concentration on the reactions of 1-octene were investigated. The optimum reaction conditions for activity and selectivity with Grubbs 1 and 2 were determined. Increased temperature for Grubbs 2 results in decrease in activity in the homometathesis reaction of 1-octene while the product yield increased as a result of isomerisation. Grubbs 2 catalyst displayed a higher metathesis product yield (65%) at lower temperatures (25°C) with 1-octene and lower metathesis products (less than 10%) at higher temperatures (100°C). Grubbs 1 displayed higher metathesis products (55-64%) at both lower (25°C) and higher temperatures (100°C). Secondary metathesis products due to isomerisation were less than 20% with Grubbs 2 the more active isomerization catalyst. Grubbs 1 and 2 were active for the metathesis of both 2- and 3-octene. With 2-octene high yields of secondary metathesis products were observed. The optimum reaction conditions for the metathesis of 1-octene were used for the reaction of 2-, 3-octene and 1-tetradecene. A higher metathesis yield (70%) was obtained with 2-octene. Secondary metathesis products (Cg - C13) and oligomers (C16 and longer) were obtained in the presence of Grubbs 2. Grubbs 1 and 2 also catalysed the metathesis of 1-tetradecene. The reaction is very slow in the presence of Grubbs 1 and little metathesis products are observed (less than 20%). In the presence of Grubbs 2 40% primary and less than 10% secondary metathesis products were observed. The reaction mixture of 1- and 2-octene in the presence of Grubbs 1 and 2 produced metathesis products less than 20%. Cross metathesis was observed from the mixture, C9 and C13 were produced. Both Grubbs 1and 2 showed a high activity and selectivity towards the metathesis reaction of 1-octene. These catalysts showed that they are still active at high alkene/Ru molar ratios and in different solvents that were used. Relatively high turnover numbers (TON) were obtained for both Grubbs 1 and 2. NMR studies confirmed the existence of the original benzylidene complex and the formation of the active heptylidene and methylidene complexes.
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