fullerenes by solid‐state trifluoromethylation with silver trifluoroacetate at 300 °C [45]. The silver salt could add up to 22 trifluoromethyl groups to C60, although other metal trifluoroacetates such as copper, palladium, and chromium salts could add only less than 8 trifluoromethyl groups. Several multi‐trifluoromethylated fullerenes, including isomers, were independently characterized by means of nuclear magnetic resonance (NMR) and mass spectroscopies by Boltalina and coworkers [46] and Taylor and coworkers [47]. In 2007, Goryunkov and coworkers successfully determined the X‐ray crystal structures of some of them and discussed the observed isomeric distribution in mixtures of C60(CF3)n compounds up to n = 6 [48].
In 2015, Zhang and coworkers reported an electrophilic trifluoromethylation of aromatic compounds with trifluoroacetic acid by using a silver catalyst (Scheme 2.24) [49]. The Ag2CO3 catalyst was considered to facilitate the generation of CF3 radical from trifluoroacetic acid via decarboxylation, and then this radical mediates aromatic trifluoromethylation. The resulting Ag(I) species is reoxidized by K2S2O8, used as an additive.
Zhang's conditions have been applied to several types of fluoroalkylations using fluorine‐containing carboxylic acids. Nielsen and coworkers performed a decarboxylative difluoromethylation of N‐heteroaromatic compounds with difluoroacetic acid (Scheme 2.25a) [50]. Wan, Hao, and coworkers reported an aryldifluoromethylation of isocyanides with potassium difluoroarylacetate, affording phenanthridines bearing an arylated difluoromethene motif (Scheme 2.25b) [51, 52]. Wan, Hao, and coworkers [53] and Deng and coworkers [54] independently reported an oxindole synthesis by the reaction of N‐arylacrylamides with potassium difluoroarylacetate or its acid form in the presence of persulfate salts (Scheme 2.25c). Hashmi and coworker employed ethynyl benziodoxolone, as the coupling partner, in a decarboxylative aryldifluoromethylation with difluoroarylacetic acids (Scheme 2.25d) [55].
Scheme 2.24 Silver‐catalyzed electrophilic trifluoromethylation.
Scheme 2.25 Silver‐catalyzed fluoroalkylations with various fluorinated carboxylic acids. DMSO, dimethylsulfoxide; MS4Å, molecular sieves 4Å.
2.2.4 Photochemical Reactions
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 IrIII* to IrIV 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.
2.2.5 Other Methods
2.2.5.1 Hydro‐Trifluoromethylation of Fullerene
A few exceptional reactions that do not fall into the categories described above (2.1–2.4) have been reported recently and are introduced herein. In fullerene trifluoromethylation reactions, silver trifluoroacetate was reported to show