Nickela-electrocatalyzed sulfide and phosphine oxygenations with water

Abstract

Molecular electrochemistry has emerged as an environmentally-friendly platform. While major recent momentum was gained in electrocatalyzed bond activations by earth-abundant 3d transition metals [1], cost-effective nickela-electrocatalysis continuous to be underdevelopment for electrocatalysis [2]. Nickel is an essential component of metalloproteins involved in catalytic aerobic oxygenation processes, for which nickel-dioxygen species were proposed as the key catalytic species [3]. Consequently, considerable progress was achieved in the preparation of nickel-dioxygen complexes for biomimetic oxygenation reactions. However, these complexes often require stoichiometric rather than catalytic settings (Figure 1(a)) [4]. A considerable challenge as to their application to aerobic oxygenations is represented by the inertness of nickel(II) complexes towards molecule oxygen as well as a major obstacle towards industrial synthesis with molecule oxygen, particularly when employing flammable agent solvents. A recent report by Jiao and co-workers [5] provides an innovative solution by the merge of electrolysis with nickel-catalysis to enable oxygenations of sulfides using H2O as the oxygen transfer agent (Figure 1(b)). Thus, an elegant strategy was devised by a reduction of the nickel(II) precatalyst at the cathode and the in situ generation of O2 from water at the anode, thereby maximizing the resource economy. Key to the success for the oxygenation was the one-electron reduction of the nickel(II)-bipyridine complex C at the cathode to generate the nickel(I) intermediate D, which in turn activates O2 to afford the nickel(II)-superoxo species A. Then, this nickel(II)-superoxo species A is sequentially activated by further one-electron reduction under electrochemical conditions to provide the catalytically active nickel (II)-peroxo species B. This species facilitates the oxygen transfer to an external substrate. Moreover, O2 is formed by electrolytic water splitting at the anode. The innovative catalyst promoted a range of sulfide oxygenation reactions, featuring mild reaction conditions and good tolerance of functional groups. Further, oxygenations of the phosphines under otherwise identical reaction conditions proved viable. While cost-effective nickela-electrocatalyzed oxidative transformations were recently achieved in molecular synthesis [2], the concept of nickel-dioxygen complexes for catalytic aerobic oxygenations in electrochemical conditions represents a significant advancement in the field of electrosynthesis. Given the topical interest in electrocatalysis, the user-friendly nickel(II) catalysis manifold is exciting, and further findings are expected in this rapidly evolving research field.