Visible-Light Photoredox-Catalyzed\nDirect Carboxylation\nof Tertiary C(sp<sup>3</sup>)–H Bonds with CO<sub>2</sub>:\nFacile Synthesis of All-Carbon Quaternary Carboxylic Acids

Direct carboxylation of C–H bonds with CO<sub>2</sub> represents\nan attractive strategy to synthesize valuable carboxylic acids with\nhigh atom, step, and redox economy. Although great progress has been\nachieved in this field, catalytic carboxylation of tertiary C(sp<sup>3</sup>)–H bonds still remains challenging due to their inherent\ninertness and significant steric hindrance. Herein, we report a direct\ncarboxylation of tertiary benzylic C(sp<sup>3</sup>)–H bonds\nwith CO<sub>2</sub> via visible-light photoredox catalysis. Various\nall-carbon quaternary carboxylic acids, which are of significant importance\nin medicinal chemistry, are successfully obtained with high yields.\nThis direct carboxylation is characterized by good functional group\ntolerance, broad substrate scope, and mild operational conditions.\nFurthermore, our methodology enables the efficient and rapid synthesis\nof key drug or bioactive molecules, such as carbetapentane, caramiphen,\nand PRE-084 (σ1 receptor agonist), and facilitates various functionalizations\nof C(sp<sup>2</sup>)–H bonds using the directing ability of\ntarget carboxylic acids, thus highlighting its practical applications.\nMechanistic studies indicate that a carbanion, which serves as the\nkey intermediate to react with CO<sub>2</sub>, is catalytically generated\nvia a single electron reduction of a benzylic radical through a consecutive\nphotoinduced electron transfer process.