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Aerobic Oxygenation of Alkylarenes over Ultrafine Transition‐Metal‐Containing Manganese‐Based Oxides

The oxygenation of alkylarenes to the corresponding aryl ketones is an important reaction, and the development of efficient heterogeneous catalysts that can utilize O2 as the sole oxidant is highly desired. In this study, we developed an efficient alkylarene oxygenation process catalyzed by ultrafine transition‐metal‐containing Mn‐based oxides with spinel or spinel‐like structures (M‐MnOx, M=Fe, Co, Ni, Cu). These M‐MnOx catalysts were prepared by a low‐temperature reduction method in 2‐propanol‐based solutions using tetra‐n‐butyl ammonium permanganate (TBAMnO4) as the Mn source, and they exhibited high Brunauer–Emmett–Teller surface areas (typically >400 m2 g−1). Using fluorene as the model substrate, the catalytic activities of M‐MnOx and Mn3O4 were compared. The catalytic activities of M‐MnOx were significantly higher than that of Mn3O4, which demonstrates that the incorporation of transition metals in manganese oxide was critical. Among the series of M‐MnOx catalysts examined, Ni‐MnOx exhibited the highest catalytic activity for the oxygenation. In addition, the catalytic activity of Ni‐MnOx was higher than that of a physical mixture of Mn3O4 and NiO. Furthermore, Ni‐MnOx exhibited a broad substrate scope with respect to various types of structurally diverse (hetero)alkylarenes (16 examples). The observed catalysis was truly heterogeneous, and the Ni‐MnOx catalyst was reusable for the oxygenation of fluorene at least three times and its high catalytic performance was preserved, for example, the reaction rate, final product yield, and product selectivity. The present Ni‐MnOx‐catalyzed oxygenation process is possibly initiated by a single‐electron oxidation process, and herein a plausible oxygen‐transfer mechanism is proposed based on several pieces of experimental evidence.