Ceria-supported ruthenium catalysts for the synthesis of indole via dehydrogenative N-heterocyclization

Simple heterogeneous Ru/CeO2 catalysts as well as Ru/ZrO2 catalysts were found to be quite effective for the selective direct synthesis of indole via intramolecular dehydrogenative N-heterocyclization of 2-(2-aminophenyl) ethanol, while catalysts supported on SiO2, Al2O3, TiO2, and MgO were less effective. Ru/CeO2 catalysts that were calcined at a relatively low temperature, 200 degrees C, showed excellent activity and gave indole in a yield over 99% by the reaction at 140 degrees C for 24 h (Ru catalyst 2.5 mol%). Spectroscopic studies of the Ru/CeO2 catalysts indicated the formation of Ru-IV-QO species on the surface, which is considered to be transformed into the catalytically-active species at the initial stage of the reaction. Hot filtration tests and an ICP-AES analysis indicated that these Ru/CeO2 catalysts act heterogeneously and that the leaching of ruthenium species into the solution is negligible. These catalysts could be recycled without a significant loss of activity, which suggests that the present oxide-supported catalysts are promising alternatives to conventional homogeneous catalysts.