Cobalt‐Catalyzed Oxidative C−H Activation: Strategies and Concepts

Inexpensive cobalt‐catalyzed oxidative C−H functionalization has emerged as a powerful tool for the construction of C−C and C−Het bonds, which offers unique potential for transformative applications to modern organic synthesis. In the early stage, these transformations typically required stoichiometric and toxic transition metals as sacrificial oxidants; thus, the formation of metal‐containing waste was inevitable. In contrast, naturally abundant molecular O2 has more recently been successfully employed as a green oxidant in cobalt catalysis, thus considerably improving the sustainability of such transformations. Recently, a significant momentum was gained by the use of electricity as a sustainable and environmentally benign redox reagent in cobalt‐catalyzed C−H functionalization, thereby preventing the consumption of cost‐intensive chemicals while at the same time addressing the considerable safety hazards related to the use of molecular oxygen in combination with flammable organic solvents. Considering the unparalleled potential of the aforementioned approaches for sustainable green synthesis, this Review summarizes the recent progress in cobalt‐catalyzed oxidative C−H activation until early 2020. Metal‐catalyzed C−H activation has emerged as an increasingly powerful tool for molecular synthesis, with major advances achieved by using Earth‐abundant cobalt catalysts. Recent momentum was gained by oxidative C−H transformations, prominently featuring aerobic and electrochemical approaches. Herein, the findings in this topical area of sustainable synthesis are summarized and critically discussed.