Photoredox/Cobalt-Catalyzed C(sp3)–H Bond Functionalization toward Phenanthrene Skeletons with Hydrogen Evolution

The first example of photoredox strategy for synthesis of phenanthrene skeletons through C(sp3)-H functionalization under external oxidant-free conditions is achieved. This transformation relies on the keto-enol tautomerism of 1,3 dicarbonyl moiety, i.e., the enol form of 1,3-dicarbonyl derivatives with relatively lower oxidation potential can be activated by the excited acridinium photocatalyst. The electron and proton eliminated from the substrate are immediately captured by a cobaloxime catalyst to exclusively afford a-carbonyl radical for highly substituted 10-phenanthrenols in good to excellent yields.