Unsupported nanoporous palladium-catalyzed chemoselective hydrogenation of quinolines: Heterolytic cleavage of H2 molecule

An efficient and highly chemoselective heterogeneous catalyst system for quinoline hydrogenation was developed using unsupported nanoporous palladium (PdNPore). The PdNPore-catalyzed chemoselective hydrogenation of quinoline proceeded smoothly under mild reaction conditions (low H2 pressure and temperature) to yield 1,2,3,4-tetrahydroquinolines (py-THQs) in satisfactory to excellent yields. Various synthetically useful functional groups, such as halogen, hydroxyl, formyl, ethoxycarbonyl, and aminocarbonyl groups, remained intact during the quinoline hydrogenation. No palladium was leached from PdNPore during the hydrogenation reaction. Moreover, the catalyst was easily recovered and reused without any loss of catalytic activity. The results of kinetic, deuterium-hydrogen exchange, and deuterium-labeling experiments indicated that the present hydrogenation involves heterolytic H2 splitting on the surface of the catalyst.