Aerobic Oxidation of Amines to Imines by Cesium-Promoted Mesoporous Manganese Oxide

Selective oxidation of amines to imines is one of the most studied reactions in the field of heterogeneous catalysis. Cs ion-promoted mesoporous manganese oxide (meso Cs/MnO_x) was synthesized using an inverse surfactant micelle as a soft template. The chemical and structural properties of the mesoporous manganese oxide material were characterized by powder X-ray diffraction, nitrogen sorption, electron microscopy, and X-ray photoelectron spectra. The meso-Cs/MnO_x material presented an aggregated nanocrystalline nature with monomodal mesoporous size distributions. The catalyst was found to be effective in the oxidation of amines to imines under aerobic conditions. The meso Cs/MnO_x exhibited oxidation of primary, secondary, cyclic, aromatic, and aliphatic amines to imines, and the conversions reached as high as >99%. The catalyst was also effective in oxidative cross-condensation of two different amines to produce asymmetrically substituted imines. Surface-active Mn³⁺ species along with labile lattice oxygen were found to play an important role in the catalytic activity. Mild reaction conditions (air atmosphere and absence of any oxidative or basic promoters), ease of product separation by simple filtration, and significant reusability make this mesoporous manganese oxide material an economical and ecofriendly catalyst for the syntheses of versatile imine derivatives.