Development of Ceria-Supported Ruthenium Catalysts Effective for Various Synthetic Reactions

Abstract

Our recent results on organic transformations such as C–C bond formation via the activation of stable C–C or C–H bonds and aerobic oxidation of alcohols catalyzed by CeO2-supported ruthenium are reviewed. A simple, recyclable heterogeneous Ru/CeO2 catalyst showed excellent activity for sequential transfer-allylation/isomerization of homoallyl alcohols with aldehydes to saturated ketones via the C–C bond activation. While homogeneous ruthenium and rhodium complex catalysts require additives and/or pressurized CO, the reaction with Ru/CeO2 smoothly proceeded in the absence of any additives. The Ru/CeO2 catalyst also showed excellent activity for the addition of sp2 C–H bonds of aromatic ketones to vinylsilanes. The Ru/CeO2 catalyst realized the chelation-assisted arylation of stable aromatic C–H bonds with aryl chlorides. The activity of the catalyst was greatly improved by the PPh3-modification under hydrogen atmosphere prior to the reactions. The catalyst acts heterogeneously without a significant leaching of ruthenium species, indicating that the Ru/CeO2 catalyst has an advantage over homogeneous catalysts from practical and environmental points of view. The effects of chemical and physical properties of CeO2 on the activity of CeO2-supported noble metal catalysts were examined. Porous CeO2 powders were prepared by the coagulation of solvothermally synthesized colloidal ceria nanoparticles, and the thus-prepared CeO2 powders showed an oxygen migration ability far superior to the CeO2 samples prepared by the usual precipitation method. The ruthenium catalysts supported on the former CeO2 powders showed a high activity for the aerobic oxidation of benzyl alcohol. The effects of the pore structure of CeO2 powders on the activity of the Ru/CeO2 catalysts are also discussed.