Isomerisation of alkenes using metal carbenes and related transition metal complexes
Mahahle, Mpho Mamojuta Daphne
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Different transition metal complexes such as RuCl2(=CHPh)(PCy3,)2, RuCl2(=CHPh)(PCy3)(IMes), RuCl2(=CH2) (PCy3)2, RuCl2(= CHBu)(PCy3)2, RuCl(H)(PPh3)3(CO), RuCl3, RhCl3-3H20, were used to isomerise alkenes. For the majority of experiments, reactions were performed on 1-octene and 4-octene, gas chromatography was used to monitor the consumption of the starting alkenes and the formation of isomers. The variation of the reaction temperature on the different catalytic systems, the variation of the alkenelcatalyst molar ratio and the variation of the solvents on the isomerisation activity of the different catalytic systems were investigated. The results showed that the best conversion and selectivity are obtained with ruthenium carbenes. lsomerisation was observed at high temperatures ≥ 100°C and at lower temperatures more metathesis products were observed. Short chain alkenes achieved higher isomerisation activity than long chain alkenes: 4-octene > 4-nonene > 4-decene. lsomerisation favoured more stable internal alkenes with the order of activity being trans-4-octene > trans-3-octene > trans-2-octene > cis-2-octene > 1 -octene. From the study it is clear that RuCl2(=CHPh)(PCy3)2 is a capable double bond isornerisation catalyst for internal alkenes because metathesis products were not detected in any of the reactions using it. But when it comes to terminal alkenes, the complex gives a mixture of isomerisation and metathesis products. RuCI2(=CHPh)(PCy3)(lMes) is catalytically active for the isomerisation and metathesis of both the internal and terminal alkenes at high temperatures, at room temperature it is selective towards the isomerisation of internal alkenes and metathesis of terminal alkenes. Although RuCI(H)(PPh3)3(CO)/PhCI is catalytically active for the isomerisation of terminal alkenes at room temperature, it does not isomerise internal alkenes at lower temperatures. Interestingly, at high temperatures, especially after prolonged reaction times, isomerisation products were detected. The rhodium carbene and dimer are also catalytically active and selective for the isomerisation of internal and terminal alkenes, metathesis reactions were not observed in reactions involving these two catalysts. RhCl3-3H2O and RuCl3, cannot isomerise alkenes rapidly; longer reaction hours and high temperatures are required to get the isomerisation products. The isomerisation reaction proceeded with first order kinetics in both catalyst and 4-octene which is in agreement with a simple bimolecular interaction between the octene substrate and the ruthenium center. The kinetics data of 1-octene isomerisation did not fit simple first order or second order rate expressions, presumably due to secondary reactions (metathesis). The data showed a reasonably linear second order fit at the initial stages. The reaction mechanisms can be illustrated by the catalytic cycles.
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