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

Abstract

Oxygenation of sulfides to sulfoxides and/or sulfones is an important transformation, and the development of efficient heterogeneous catalysts for oxygenation, which can utilize O₂ 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 α-MnO₂, β-MnO₂, γ-MnO₂, and δ-MnO₂, and their structure–activity relationships were examined for the aerobic oxygenation of thioanisole. Amongst them, α-MnO₂ showed the best catalytic performance for the oxygenation. Moreover, α-MnO₂ was highly stable during the catalytic oxygenation possibly due to the tunnel K⁺ ions. In order to further improve the catalytic performance of α-MnO₂, substitutional doping of transition metal cations, such as Mo⁶⁺, V⁵⁺, Cr³⁺, and Cu²⁺, into the framework was carried out. Undoped α-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 α-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 α-cyanation of trialkylamines, and oxidative S-cyanation of benzenethiols.