Alkene Carboxy-Alkylation via CO₂^•–

Herein, we introduce a new platform for alkene carboxy-alkylation. This reaction is designed around CO₂^•- addition to alkenes followed by radical polar crossover, which enables alkylation through carbanion attack on carbonyl electrophiles. We discovered that CO₂^•- adds to alkenes faster than it reduces carbonyl electrophiles and that this reactivity can be exploited by accessing CO₂^•- via hydrogen atom transfer from formate. This photocatalytic system transforms vinylarenes and carbonyl compounds into a diverse array of substituted γ-lactone products. Furthermore, indoles can be engaged through dearomative carboxy-alkylation, delivering medicinally relevant C(sp³)-rich heterocyclic scaffolds. Mechanistic studies reveal that the active photocatalyst is generated <i>in situ</i> through a photochemically induced reaction between the precatalyst and DMSO. Overall, we have developed a three-component alkene carboxy-alkylation reaction enabled by the use of formate as the CO₂^•- precursor.