Herein, a photocatalytic strategy for the synthesis of succinic acids by alkene dicarboxylation with carbon dioxide (CO2) and formate salt as synergistic carbon sources is described. The formate salt acts as both a C1 source and reductant in the reaction with catalytic amount of 1,4-diazabicyclo[2.2.2]octane (DABCO) as a hydrogen atom transfer reagent. Various mono-, di-, and trisubstituted alkenes