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

Abstract

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 °C, showed excellent activity and gave indole in a yield over 99% by the reaction at 140 °C for 24 h (Ru catalyst 2.5 mol%). Spectroscopic studies of the Ru/CeO₂ catalysts indicated the formation of Ru^IVO 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.