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

Abstract

The catalytic activity of the arene–ruthenium(II) complexes [RuCl₂(η⁶-C₆H₅OCH₂CH₂OH)(L)] (L = P(OMe)₃ (1a), P(OEt)₃ (1b), P(OⁱPr)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 η⁶-coordinated arene ligand, while sulfuric acid favours the Ru–Cl bond dissociation and the formation of aquo-derivatives [RuCl(H₂O)(η⁶-C₆H₅OCH₂CH₂OH)(L)][Cl].