Photoredox-mediated catalysis enables a variety of challenging synthetic transformations. Iridium complexes have been widely used as typical photocatalysts in photoredoxmediated organic reactions. Among them, iridium polypyridyl catalysts are very effective single electron transfer (SET) reagents due to their ability to engage in single electron transfer (SET) processes with organic substrates under the photoexcitation of visible light[1].
Iridium photocatalysts can be divided into the following two categories according to the ligands.
Homoleptic iridium photocatalysts refer to the same cyclometalated phenylpyridine for each ligand, and the mechanism during the reaction is described as an oxidative quenching mechanism. The excited state of *Ir(ppy)3 is capable of radical-radical coupling reactions, and SET reduction of electron-deficient cyanoarenes to generate arylated products. These reactions include α-arylation of amines, α-arylation of benzyl ethers and olefins, β-arylation of carbonyl groups, etc.
Cationic polypyridyl complexes of iridium(III) refer to Ir(ppy)2(N^N)+ type photocatalysts, containing a bipyridine-type ligand. Ir(ppy)2(bpy)+ is the simplest heteroleptic iridium photocatalytic agent that is used for sulfonamidation of aryl halides. Additionally, Ir(py)2(dtbbpy)+, the di-tert-butyl-substituted analogue, is another heteroleptic iridium photocatalyst. This catalyst not only enables the free radical coupling reaction of α-amino radicals with carbon-centered radicals, but also allows the direct alkylation of various heteroarenes.
Iridium catalysts are widely used in reductive reactions and oxidative reactions.
Iridium catalysts have been used in reductive reactions such as radical cyclization, radical cyclization/rearrangement, and reduction of α-ketoepoxides. The catalyst Ir(ppy)3 has been shown to facilitate photoredox radical cyclizations of various alkyl, alkenyl or aryl iodide starting materials, generating a series of carbocyclic and heterocyclic structures. Besides, these free radical reactions can be capable of proceeding with rearrangements. For example, bromocyclopropane as a substrate for radical cyclization, can be catalyzed by iridium catalysts Ir(ppy)2(dtbbpy)PF6 to generate cyclized products with a tricyclic structure in good yields. And this iridium photocatalyst can also participate in the formation of C-C bonds. Under visible light irradiation, the substrate α-ketoepoxides is reduced to the corresponding radical intermediate by the iridium catalyst, and the intramolecular tandem ring-opening/allylation proceeded to obtain the product β-hydroxy-α-allylketone in good yield[1].
Fig. 1 Reductive reactions catalyzed by Iridium catalysts[1].
Iridium complexes as photocatalysts have also been used for photocatalytic oxidative coupling reactions. For example, the oxidative coupling of nitroalkanes with tertiary N-arylamines under the excitation of visible light is capable of generating imine ions, ultimately giving aza-Henry adducts. The photocatalyst used for this oxidative aza-Henry reaction is the iridium complex Ir(ppy)2(dtbbpy)PF6. What's more, the reaction is able to obtain the oxidative coupling products in high yields using only catalytic amounts of Iridium complexes as catalysts without the need for external oxidants[3].
Fig. 2 The aza-Henry reaction catalyzed by Iridium catalyst[3].
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