Anion-Suppression Strategy Enables Palladium Metallaphotoredox-Catalyzed Desymmetric Carboxylation with CO 2

Asymmetric synthesis of axially chiral biaryl carboxylic acids using CO2 typically requires multistep procedures and the use of air-sensitive organometallic reagents. Desymmetric carboxylation offers a direct route to axially chiral biaryl carboxylic acids yet remains largely unexplored due to the uncontrollable stoichiometry of CO2 gas, leading to complicated chemo- and enantioselectivity issues. Herein, we report a palladium metallaphotoredox catalysis that unlocks desymmetric carboxylation of prochiral biaryl triflates with CO2, affording a wide range of axially chiral biaryl carboxylic acids in good to high yields with excellent enantioselectivities. The synergistic photoredox/palladium catalysis enables single-electron reduction under mild conditions, avoiding strong reductants and minimizing side reactions. The synthetic utility of this protocol was showcased by efficient preparation of various axially chiral scaffolds, including a novel chiral monophosphine ligand, as well as the enantiodivergent synthesis of axially chiral dicarbonyl compounds without changing the ligand configuration. Mechanistic studies reveal that the in situ-generated carboxylate anion in the first carboxylation serves as an electron-donating group, suppressing carboxylation and enabling high desymmetrization efficiency.