Effects of Lewis Acids on Photoredox Catalysis

The reactivity of organic photoredox catalysts in oxidation reactions is enhanced by the addition of Lewis acids, which bind to the catalyst to increase its oxidizing ability. Redox-inactive metal ions, such as Sc³⁺, also act as Lewis acids and can bind to flavins to increase their one-electron reduction potentials and photocatalytic stability. The Sc³⁺ ion can bind to acridine and pyrene to enhance their photoredox catalytic activity in the oxidation of substrates by dioxygen. The binding of Lewis acids to radical anions enhances their overall photocatalytic activity by prohibiting a back electron transfer from the radical anions of substrates to the oxidized photocatalysts. The photocatalytic asymmetric oxidation of terminal olefins by using water as the oxygen source with a mononuclear nonheme chiral manganese complex has been made possible by the addition of acetic acid, which is essential to afford the products with high enantioselectivity.