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

Abstract

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.

Nanostructured Manganese Oxides within a Ring‐Shaped Polyoxometalate Exhibiting Unusual Oxidation Catalysis

“…These results suggest that the electron transfer from the substrate to Mn18 occurred even under an Ar atmosphere, and then the structure change of Mn18 proceeded in the absence of reoxidation by O 2 . In addition, 9,9′‐bifluorene ( 3 a ) formation from 1 a via radical–radical homocoupling was unobserved when Mn18 was employed under an Ar atmosphere (Figure 4c) [20] . These results agreed with the assumption that the present Mn18 ‐catalyzed system did not involve free radical species.…”

“…To investigate this unusual reaction mechanism, we focused on the first step of oxygenation and considered three possible steps: SET, [21] HAT, [22] and deprotonation steps; [23] thus, the substrate parameters ionization energy (IE), bond dissociation energy (BDE), and p K a , corresponding to each mechanism, were compared with the substrate scope results (Table 1). Typical oxygenations of alkylarenes start from SET or HAT of alkylarenes, and the reactivities were related to IE or BDE [20–22] . Surprisingly, the p K a values were related to whether the oxygenation reaction proceeded or not by Mn18 , whereas there was no correlation between the product yields and other parameters.…”

Rapid room-temperature synthesis of ultrasmall cubic Mg–Mn spinel cathode materials for rechargeable Mg-ion batteries

“…The process is commonly applied for synthesizing metal nanoparticles as a wet-process, and recently our group applied it for various Mn oxides and showed high catalytic activity for aerobic oxidation reactions. [30][31][32] We also reported a high-rate capability of LiMn 2 O 4 nanospinels in a Li-ion battery. 30 In this study, an Mg-Mn spinel oxide (MMO) with approximately 5 nm was prepared by reduction of MnO 4 À in an anhydrous MgCl 2 solution in ethanol within 30 min at room temperature, and its cathode performances were investigated.…”

Precise Cu Localization‐Dependent Catalytic Degradation of Organic Pollutants in Water

“…On the other hand, unsaturated coordination sites appear in Cu‐OMS‐2 and accordingly enhance the amounts of oxygen vacancies. As we know, oxygen vacancies adsorb unsaturated oxygen species, like OH − and O − , and substrates …”