Molybdenum-doped α-MnO2 as an efficient reusable heterogeneous catalyst for aerobic sulfide oxygenation

Oxygenation of sulfides to sulfoxides and/or sulfones is an important transformation, and the development of efficient heterogeneous catalysts for oxygenation, which can utilize O-2 as the terminal oxidant, is highly desired. In this study, we have successfully developed manganese oxide-based efficient heterogeneous catalysts for aerobic oxygenation of sulfides. Firstly, we prepared four kinds of manganese oxides possessing different crystal structures, such as alpha-MnO2, beta-MnO2, gamma-MnO2, and delta-MnO2, and their structure- activity relationships were examined for the aerobic oxygenation of thioanisole. Amongst them, alpha-MnO2 showed the best catalytic performance for the oxygenation. Moreover, alpha-MnO2 was highly stable during the catalytic oxygenation possibly due to the tunnel K+ ions. In order to further improve the catalytic performance of alpha-MnO2, substitutional doping of transition metal cations, such as Mo6+, V5+, Cr3+, and Cu2+, into the framework was carried out. Undoped alpha-MnO2 possessed a fibrous morphology. When high-valent transition metal cations were doped, especially Mo6+, the lengths of the fibers drastically shortened to form grain-like aggregates of ultrafine nanocrystals, resulting in an increase in specific surface areas and the numbers of catalytically active surface sites. In the presence of Mo6+-doped alpha-MnO2 (Mo-MnO2), various kinds of sulfides could efficiently be oxidized to the corresponding sulfoxides as the major products. The observed catalysis was truly heterogeneous, and Mo-MnO2 could repeatedly be reused while keeping its high catalytic performance. Besides sulfide oxygenation, Mo-MnO2 could efficiently catalyze several aerobic oxidative functional group transformations through single-electron transfer oxidation processes, namely, oxygenation of alkylarenes, oxidative alpha-cyanation of trialkylamines, and oxidative S-cyanation of benzenethiols.