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In 1993, Mallouk and Lai developed a photochemical approach for trifluoromethylation with silver trifluoroacetate, in which TiO₂ was used as a photocatalyst for trifluoromethylation of aromatic compounds (Scheme 2.26a) [56]. The TiO₂ catalyst promotes photolysis of trifluoroacetate to generate trifluoromethyl radical. In 2017, Su, Li, and coworkers developed a new catalytic system using Rh‐modified TiO₂ nanoparticles for photochemical trifluoromethylation with trifluoroacetic acid in the presence of Na₂S₂O₈ as an additive (Scheme 2.26b) [57].

Scheme 2.26 Photochemical trifluoromethylation using TiO₂.

Not only simple arenes but also N‐containing heteroarenes were available, affording the desired products in modest to good yields. Very recently, Hosseini‐Sarvari and Bazyar achieved a photocatalytic trifluoromethylation using sodium trifluoroacetate under blue LED irradiation in the presence of Au‐modified ZnO catalyst (Au@ZnO core–shell nanoparticles) (Scheme 2.27) [58]. Au@ZnO could catalyze not only aromatic trifluoromethylation but also coupling‐type trifluoromethylations of aryl halides as well as boronic acids under appropriate conditions.

Scheme 2.27 Au@ZnO‐catalyzed photochemical trifluoromethylations.

Qing and coworkers developed a homogeneous photocatalytic system for hydro‐aryldifluoromethylation of alkenes with difluoroarylacetic acids by using an iridium photoredox catalyst, Ir[dF(CF₃)ppy]₂(dtbpy)]BF₄ (Scheme 2.28) [59]. Methoxybenziodozole (BIOMe) as an additive plays a crucial role in the catalytic cycle; it accelerates photolysis of the carboxylic acid by forming a hypervalent iodine intermediate possessing carboxylate as a ligand, and promotes turnover of the photoredox catalytic cycle by oxidizing excited‐state Ir^III* to Ir^IV species. The CF₃ radical generated by photolysis reacts with alkene and the resulting alkyl radical affords the hydro‐aryldifluoromethylated product via hydrogen abstraction from N‐methylpyrrolidone (NMP).

Zhu and coworkers also developed a photocatalytic fluoroalkylation of alkenes bearing benzaldehyde or propenal functionalities on the side chain with fluorinated carboxylic acids by using the combination of Ir‐catalyst and the oxidant PhI(OAc)₂ (Scheme 2.29) [60]. Cyclic ketones bearing fluoroalkyl groups, such as CF₂Ar, CF₂H, and CF₂Me groups, were obtained up to 90% yield.

Scheme 2.28 Ir‐photocatalyzed aryldifluoromethylation.

Scheme 2.29 Carbo and heterocyclic ketone synthesis by Ir‐photocatalyzed carbo‐fluoroalkylation of alkenyl aldehydes.

Recently, Gouverneur and coworkers developed a photocatalyst‐free hydro‐difluoromethylation [61a] as well as ‐chlorofluoromethylation [61b] of simple alkenes under blue LED irradiation (Scheme 2.30); the reactions were accomplished by the use of a combination of fluorine‐containing carboxylic acid and iodobenzene diacetate.

Scheme 2.30 Metal‐free photochemical hydro‐fluoroalkylations.