Stable and Versatile Pd Precursors for the Preparation of Robust Pd Catalysts Under Continuous-Flow.

To achieve global sustainability, the chemical and engineering communities require the development of versatile precursors that can be used to synthesize robust catalysts. To meet this demand, we developed a new Pd precursor for highly efficiently incorporating fine Pd-metal into supports. An atmospherically stable Pd precursor (Pd-80) was prepared by thermally promoting the aerobic oxidation of tetrakis(triphenylphosphine)palladium. The physical properties of Pd-80 were investigated by NMR spectroscopy, SEM, XPS, solvent-relaxation NMR spectroscopy, and dynamic light scattering (DLS) experiments. We also prepared a cordierite-supported Pd catalyst (Pd/cordierite) by stirring Pd-80 and cordierite powder in chloroform at room temperature. Pd/cordierite selectively catalyzed the hydrogenation of various reducible functional groups, including alkynes, azides, nitro groups, olefins, CO2Bn, N-Cbz, O-Bn, aromatic ketones, and styrene oxide, in continuous-flow hydrogenation reactions. The Pd/cordierite-catalyzed continuous-flow hydrogenation of nitrobenzene derivatives afforded the corresponding anilines, with catalytic activities maintained for over 250 h of continuous operation with a turnover number (TON) of 61,090.