Heterogeneously catalyzed aerobic oxidative C-H functionalization under mild conditions is a chemical process to obtain desired oxygenated products directly. Nanosized murdochite-type oxide Mg6MnO8 (Mg6MnO8-MA) was successfully synthesized by the sol-gel method using malic acid. The specific surface area reached up to 104 m2 g-1, which is about 7 times higher than those (2-15 m2 g-1) of Mg6MnO8 synthesized by previously reported methods. Mg6MnO8-MA exhibited superior catalytic performance to those of other Mn- and Mg-based oxides, including manganese oxides with Mn-O-Mn active sites for the oxidation of fluorene with molecular oxygen (O2) as the sole oxidant under mild conditions (40 °C). The present catalytic system was applicable to the aerobic oxidation of various substrates. The catalyst could be recovered by simple filtration and reused several times without obvious loss of its high catalytic performance. The correlation between the reactivity and the pKa of the substrates, basic properties of catalysts, and kinetic isotope effects suggest a basicity-controlled mechanism of hydrogen atom transfer. The 18O-labeling experiments, kinetics, and mechanistic studies showed that H abstraction of the hydrocarbon proceeds via a mechanism involving O2 activation. The structure of Mg6MnO8 consisting of isolated Mn4+ species located in a basic MgO matrix plays an important role in the present oxidation.
Keywords: C−H oxidation; molecular oxygen; murdochite-type oxide; nanoparticle.