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

In this study, we successfully developed an efficient alkylarene oxygenation process catalyzed by ultrafine transition-metal-containing manganese-based oxides with spinel-like structures (M-MnOx, M = Fe, Co, Ni, Cu). These M-MnOx catalysts were prepared via a low-temperature method in 2-propanol-based solutions using tetra-n-butylammonium permanganate, and they exhibited high BET surface areas (typically >400 m2 g−1). The catalytic activities of M-MnOx were significantly higher than those of Mn3O4, demonstrating that the incorporation of transition metals in manganese oxide was critical. Among the 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 NiO and Mn3O4. Furthermore, Ni-MnOx exhibited a broad substrate scope with respect to various types of structurally diverse di-activated (hetero)alkylarenes (16 examples, up to >99% yield). The observed catalysis was truly heterogeneous, and the Ni-MnOx catalyst was reusable for the oxygenation of fluorene at least three times while preserving its high catalytic performance. Based on several pieces of experimental evidence, a plausible oxygen-transfer mechanism is proposed.