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

Abstract

The RuCl 2(PPh 3) 3 and RuCl 3(AsPh 3) 2·CH 3OH 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 RuCl 2(PPh 3) 3 catalyst. However, 94.2% conversion of methyl chavicol with 98.6% selectivity of trans-isomer was observed using methanol as a solvent in RuCl 3(AsPh 3) 2·CH 3OH. The kinetics of isomerization of methyl chavicol and eugenol using RuCl 2(PPh 3) 3 and RuCl 3(AsPh 3) 2·CH 3OH 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 RuCl 2(PPh 3) 3 and RuCl 3(AsPh 3) 2·CH 3OH 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 RuCl 2(PPh 3) 3 complex catalyst. The RuCl 2(PPh 3) 3 complex was recycled five times for the isomerization of methyl chavicol and eugenol.