Heterogeneous viologen catalysts for metal-free and selective oxidations

Metal-free oxidation, a green chemistry process, has drawn significant attention from catalysis researchers. However, most oxidation processes are completed by homogeneous metal-free catalysts, whereas heterogeneous metal-free materials have been developed with limited successes. In this study, polymerized ionic networks (PINs) with N,N′-dialkyl-4,4′-bipyridinium units as heterogeneous viologen type of catalysts exhibited high efficiency in the oxidation of aromatic sulfides/alcohols to sulfoxides/aldehydes, respectively (conversion: >90%, selectivity: >95%). The catalytic performance of PINs originates from the electron-accepting ability of the viologen unit, which can reduce H₂O₂ into an active species. Especially, the synthesis of PIN catalysts is a one-step simple polymerization reaction between benzyl bromide and bipyridine in air. The metal-free heterogeneous feature, high selectivity, mild conditions (60 °C, 1 h), and the facile preparation of the catalyst make the current selective oxidation approach attractive.