A synthetic route to oxindole-3-acetic acid derivatives is disclosed through a transition-metal free carbo-carboxylation reaction with CO₂˙⁻ as the C1 source in situ generated from formate under photo-induced conditions. In this study, air was utilized as the oxidant in the presence of highly reductive CO₂˙⁻ species to realize the direct aryl C–H activation, preventing the use of unfriendly organo (pseudo)halides as the substrates.