Selective double bond isomerization of allyl phenyl ethers catalyzed by ruthenium metal complexes

The RuCl2(PPh3)3 and RuCl3(AsPh3)2·CH3OH complex catalysts were studied for the double bond isomerization of methyl chavicol and eugenol in the polar aprotic (DMSO, acetonitrile), polar protic (ethanol, methanol, n-propanol, iso-propanol, n-butanol, iso-butanol, n-hexanol) and non-polar (benzene, toluene, n-hexane, cyclohexane, tetrahydrofuran) solvents. The highest conversion of methyl chavicol (99.7%) with 95.4% selectivity of trans-anethole was observed in ethanol using RuCl2(PPh3)3 catalyst. However, 94.2% conversion of methyl chavicol with 98.6% selectivity of trans-isomer was observed using methanol as a solvent in RuCl3(AsPh3)2·CH3OH. The kinetics of isomerization of methyl chavicol and eugenol using RuCl2(PPh3)3 and RuCl3(AsPh3)2·CH3OH catalysts in ethanol or methanol involving the effect of substrate concentrations, catalyst amount, solvents and temperature on the initial rate of reaction was studied in details. The initial rates of reaction were observed to increase on increasing the initial concentrations of methyl chavicol, eugenol and catalysts. However, the initial rate of reaction decreased on increasing the solvent concentration. Activation energies for the isomerization of methyl chavicol were calculated using RuCl2(PPh3)3 and RuCl3(AsPh3)2·CH3OH catalysts and were found to be 4.3 and 6.0 kJ/mol, respectively. Activation energy for the isomerization of eugenol was found to be 6.9 kJ/mol using RuCl2(PPh3)3 complex catalyst. The RuCl2(PPh3)3 complex was recycled five times for the isomerization of methyl chavicol and eugenol.

Graphical abstractThe applicability of RuCl2(PPh3)3 and RuCl3(AsPh3)CH3OH metal complexes have been investigated as catalysts for the selective double bond isomerization of methyl chavicol to trans-anethole and eugenol to trans-isoeugenol in polar aprotic, polar protic and non-polar solvents. The highest conversions of, methyl chavicol (99.7%) with 95.4% selectivity of trans-anethole and eugenol (99.8%) with 95.6% selectivity of trans-isoeugenol were observed in ethanol using RuCl2(PPh3)3 catalyst. Additionally, the detail kinetics of isomerization of methyl chavicol and eugenol using RuCl2(PPh3)3 and RuCl3(AsPh3)2·CH3OH catalysts in alcoholic solvents have also been carried out to observe the effect of the various influencing factors on the initial rate of reaction. Figure optionsDownload full-size imageDownload as PowerPoint slide