Alkene\nCarboxy-Alkylation via CO<sub>2</sub><sup>•–</sup>

Herein,\nwe introduce a new platform for alkene carboxy-alkylation.\nThis reaction is designed around CO<sub>2</sub><sup>•–</sup> addition to alkenes followed by radical polar crossover, which enables\nalkylation through carbanion attack on carbonyl electrophiles. We\ndiscovered that CO<sub>2</sub><sup>•–</sup> adds to\nalkenes faster than it reduces carbonyl electrophiles and that this\nreactivity can be exploited by accessing CO<sub>2</sub><sup>•–</sup> via hydrogen atom transfer from formate. This photocatalytic system\ntransforms vinylarenes and carbonyl compounds into a diverse array\nof substituted γ-lactone products. Furthermore, indoles can\nbe engaged through dearomative carboxy-alkylation, delivering medicinally\nrelevant C(sp<sup>3</sup>)-rich heterocyclic scaffolds. Mechanistic\nstudies reveal that the active photocatalyst is generated <i>in situ</i> through a photochemically induced reaction between\nthe precatalyst and DMSO. Overall, we have developed a three-component\nalkene carboxy-alkylation reaction enabled by the use of formate as\nthe CO<sub>2</sub><sup>•–</sup> precursor.