Dense CO2 expanded methanol solvent system for synthesis of naproxen via enantioselective hydrogenation

Hydrogenation reactions catalysed by the asymmetric ruthenium [2,2′-bis(diphenylphosphino)-1,1′-binapthyl] catalyst have been studied in both organic and supercritical solvents. These reactions have been investigated in biphasic solvent systems and supercritical CO2 media. The current work forms the first study of the homogeneously catalysed naproxen synthesis reaction in a CO2 expanded methanol solvent system. It has been demonstrated that the reaction proceeded in a CO2 expanded methanol environment, although conversions were not high. Naproxen synthesis studies in dense CO2 expanded methanol exhibited a decreased product enantioselectivity compared to that achieved under similar conditions in neat methanol. UV–vis spectroscopy provided evidence of localised density enhancements which partly explained the observed retarded reaction rate in the CO2/methanol medium, although residual oxygen in the solvent solution was concluded to be the major factor in the retarded reaction rates and low conversions.