Organocatalysis Combined with Photocatalysis

Over the past decade, the combination of visible light photocatalysis and organocatalysis has made remarkable progress in modern chemical synthesis. In these dual catalysis system, photocatalysts or photosensitizers absorb visible light to induce their photoexcited states which can activate unreactive substrates via electron or energy transfer mechanisms, and organocatalysts are usually employed to regulate the chemical reactivity of the other substrates. By doing so, two reactive species react with each in a selective—especially enantioselective—way, to provide the final products. This article summarizes the recent development of cooperative catalysis by the combination of organocatalysis and photocatalysis in asymmetric organic synthesis. These reactions are classified according to the manner of activation of the organocatalysts. Enamine/iminium catalysts are used to activate unreactive carbonyl molecules. Nucleophilic catalysts including nitrogen heterocycle carbene catalysts and tertiary amine catalysts are employed to reverse the reactivity of electrodeficient substrates including aldehydes and enals. Chiral Brønsted acid catalysts are used to activate substrates by forming key H-bonding complexes between substrates and catalysts.