Unsupported Nanoporous Gold-Catalyzed Chemoselective Reduction of Quinolines Using Formic Acid as a Hydrogen Source

The unsupported nanoporous gold (AuNPore) is demonstrated to be an active and stable heterogeneous catalyst for chemoselective transfer hydrogenation of quinolines using bio-renewable formic acid (HCO2H) as hydrogen source. The AuNPore-catalyzed transfer hydrogenation of quinolines proceeded smoothly in the presence of low amounts of HCO2H at a relatively low temperature to produce the desired 1,2,3,4-tetrahydroquinolines (py-THQs) in good to excellent yields. Mechanistic studies revealed that the AuNPore-catalyzed transfer hydrogenation of quinolines proceeded through a sequence of 1,4-hydride addition, isomerization, and 1,2-hydride addition. This operationally simple protocol offers a practical alternative to the currently known methods for chemoselective quinoline reduction.