Eugenol isomerization promoted by arene-ruthenium(II) complexes in aqueous media: influence of the pH on the catalytic activity

The catalytic activity of the arene-ruthenium(II) complexes [RuCl2(eta(6)-C6H5OCH2CH2OH)(L)] (L = P(OMe)(3) (1a), P(OEt)(3) (1b), P((OPr)-Pr-i)(3) (1c), P(OPh)(3) (1d), PPh3 (1e)) in the isomerization of eugenol into isoeugenol has been evaluated. Best results in terms of activity and selectivity were observed with those catalysts containing an aliphatic P-donor ligand (1a-c). Under optimized conditions, full conversions in extremely short reaction times (5 min), and with high levels of trans-selectivity (up to 98%), could be achieved. Addition of both NaOH or H2SO4 to the aqueous media resulted in rate enhancements, suggesting two different activation pathways of the pre-catalysts. We have evidenced that sodium hydroxide promotes the release of the eta(6)-coordinated arene ligand, while sulfuric acid favours the Ru-Cl bond dissociation and the formation of aquo-derivatives [RuCl(H2O)(eta(6)-C6H5OCH2CH2OH)(L)][Cl].