Abstract
The metathesis of 1-hexene and (E)-anethole in the presence of Grubbs 2nd generation catalyst was monitored by in situ 1H NMR spectroscopy at different temperatures (15 °C, 25 °C, and 45 °C) and anethole mol fractions (XAnethole ≈ 0.17, 0.29, 0.5, 0.71, 0.83). Time traces confirmed the instantaneous formation of (E)-1-(4-methoxyphenyl)-1-hexene, the cross-metathesis product. A maximum concentration of (E)-1-(4-methoxyphenyl)-1-hexene is reached fairly fast (the time depending on the reaction conditions), and this is followed by a decrease in the concentration of (E)-1-(4-methoxyphenyl)-1-hexene due to secondary metathesis. The maximum concentration of (E)-1-(4-methoxyphenyl)-1-hexene was more dependent on the XAnethole than the temperature. The highest TOF (3.46 min−1) was obtained for the reaction where XAnethole was 0.16 at 45 °C. The highest concentration of the cross-metathesis product was however achieved after 6 min with an anethole mol fraction of 0.84 at 25 °C. A preliminary kinetic study indicated that the secondary metathesis reaction followed first order kinetics.
Highlights
Olefin metathesis is commonly used to convert alkenes into new alkenes with rearranged substituents through the intermediacy of a cyclometallacarbene [1,2,3,4]
The consumption and formation rates were monitored in real time by the intended cross-metathesis reaction catalyzed by the Grubbs 2nd generation catalyst
The Grubbs 2nd generation catalyst [1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene)dichloro(phenylmethylene)(tricyclohexylphosphino)ruthenium], (E)-anethole (99%), 1-hexene (97%) and solvents used in this study were purchased from Sigma-Aldrich
Summary
Olefin metathesis is commonly used to convert alkenes into new alkenes with rearranged substituents through the intermediacy of a cyclometallacarbene [1,2,3,4]. Cross-metathesis is a convenient synthetic approach to introduce a molecular fragment, often with a functional group, to a simple alkene to produce a valueadded compound. Grubbs 2nd generation catalyst (see Figure 1 for the structure) is one of the most popular homogeneous olefin metathesis catalysts [1,3,6,12,13]. The formation of alkenes with asymmetric substitution through CM is not inherently selective due to competing SM of the substrates [2,10,14,15]. A study monitoring the formation and conversion of selected products in real time at different conditions is desirable to determine the ideal conditions for the formation of the target CM product in optimum yield
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