Molybdenum-doped alpha -MnO sub(2) 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 sub(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 -MnO sub(2), beta -MnO sub(2), gamma -MnO sub(2), and delta -MnO sub(2), and their structure-activity relationships were examined for the aerobic oxygenation of thioanisole. Amongst them, alpha -MnO sub(2) showed the best catalytic performance for the oxygenation. Moreover, alpha -MnO sub(2) was highly stable during the catalytic oxygenation possibly due to the tunnel K super(+) ions. In order to further improve the catalytic performance of alpha -MnO sub(2), substitutional doping of transition metal cations, such as Mo super(6+), V super(5+), Cr super(3+), and Cu super(2+), into the framework was carried out. Undoped alpha -MnO sub(2) possessed a fibrous morphology. When high-valent transition metal cations were doped, especially Mo super(6+), 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 Mo super(6+)-doped alpha -MnO sub(2) (Mo-MnO sub(2)), various kinds of sulfides could efficiently be oxidized to the corresponding sulfoxides as the major products. The observed catalysis was truly heterogeneous, and Mo-MnO sub(2) could repeatedly be reused while keeping its high catalytic performance. Besides sulfide oxygenation, Mo-MnO sub(2) 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.