Photocatalyst plays a very important role in organic photochemistry. The catalyst has aroused great interest of chemists as it offers a powerful tool to organic synthesis. More and more transition-metal complexes have been proved to catalyze light-driven organic reactions. These photocatalysts can be excited in the wavelength range of visible light and have higher efficiency for light absorption than organic small molecules. Photocatalysts used in organic synthesis are mainly divided into transition metal complex photocatalysts and metal-free organic photocatalysts. Among them, acridine, carbazole, ruthenium and iridium photocatalysts have redox activities and have been widely used in photocatalysis. In addition, other photocatalysts also showed good photocatalytic activities.
Eosin Y, as a cheaper metal-free dye, refers to the 2',4',5',7'-tetrabromo derivative of fluorescein, has been most widely employed. A large number of mechanistic studies have been carried out on the reactions of organometallic photocatalysts, and the oxidative quenching pathway of eosin Y-catalyzed reactions has mostly been interpreted as similar to the related [Ru(bpy)3Cl2]-catalyzed reactions. The catalytic mechanism is also similar to that of ruthenium. Take arene diazonium salts as an example, the reaction mechanism of eosin Y-catalyzed substitutions is shown in the Figure 1.
Fig.1 Chemical structure of eosin Y and proposed general reaction mechanism
The synergistic merger of eosin Y and Cu(OTF)2 is employed to achieve the bond-forming reaction of alkynes with weakly nucleophilic reagents and it is an efficient reaction for arene-ynes. Alkynes have been widely used in organic synthesis, and copper can activate the C-C triple bond as an efficient Lewis acid catalyst. However, copper does not have sufficient Lewis acidity for the typical weakly nucleophilic reagents, so the photocatalyst eosin Y and lewis acid copper salts are combined to form a dual catalytic system for the activation of alkyne, enabling attack from weak nucleus.
Fig.2 The activation of C-C triple bond catalyzed by eosin Y and Cu(OTF)2 
Chiral α-branched amines and their derivatives are common in pharmaceuticals, agrochemicals and biologically relevant natural products. Therefore, it is very important to develop new synthetic approaches for this type of compounds. Under the irradiation of green LED light, a (hetero)aromatic amine, a (hetero)aromatic aldehyde and an electron-deficient olefin undergo a three-component reductive coupling reaction in the presence of eosin Y as photocatalyst and Hantzsch ester as the terminal reducing agent, generating γ-amino acid derivatives. Among them, reactive nucleophilic α-amino radical species with high nucleophilicity are generated, which are capable of carbon-carbon bond formation. The reaction is efficient, simple and broad in scope, providing a complementary strategy to existing synthetic methods.
Fig.3 The three-component reductive coupling reaction catalyzed by eosin Y
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