Radical-Promoted Distal C-H Functionalization of C(sp3 ) Centers with Fluorinated Moieties

Due to their unique properties, fluorinated scaffolds are pivotal compounds in pharmaceuticals, agrochemicals, and materials science. Over the last years, the development of versatile strategies for the selective synthesis of fluorinated molecules by direct C-H bond functionalization has attracted a lot of attention. In particular, the design of novel transformations based on a radical process was a bottleneck for distal C-H functionalization reactions, offering synthetic solutions for the selective introduction of fluorinated groups. This Minireview highlights the major contributions in this blossoming field. The development of new methodologies for the remote functionalization of aliphatic derivatives with various fluorinated groups based on a 1,5-hydrogen atom transfer process and a β-fragmentation reaction will be showcased and discussed.

Using the Thiyl Radical for Aliphatic Hydrogen-Atom Transfer: Thiolation of Unactivated C-H Bonds

A metal- and catalyst-free thiyl-radical-mediated activation of alkanes is described. Tetrafluoropyridinyl disulfide is used to perform thiolation of the C-H bonds under irradiation with 400 nm light-emitting diodes. The key C-H activation step is believed to proceed via hydrogen-atom abstraction effected by the fluorinated thiyl radical. Secondary, tertiary, and heteroatom-substituted C-H bonds can be involved in the thiolation reaction. The resulting sulfides have wide potential as photoredox-active radical precursors in reactions with alkenes and heteroarenes.

Novel Bifunctionalization of Activated Methylene: Base-Promoted Trifluoromethylthiolation of β-Diketones with Trifluoromethanesulfinyl Chloride

A novel bifunctionalization of activated methylene was achieved successfully through the base-promoted trifluoromethylthiolation of β-diketones or β-ketoesters with trifluoromethanesulfinyl chloride. A series of α-trifluoromethylthiolated α-chloro-β-diketones and α-chloro-β-ketoesters were obtained in moderate to good yields under mild conditions. When β-diketones containing a phenyl group with a hydroxyl or amino substituent at the ortho position were used as substrates, intramolecular trifluoromethylthiolation/cyclization reaction took place to give the corresponding cyclic products. Furthermore, the protocol could be extended to perfluoroalkylthiolation with the sodium perfluoroalkanesulfinate/POCl₃ system. On the basis of experimental results, plausible mechanisms are proposed.

Late stage trifluoromethylthiolation strategies for organic compounds

Substitution by the CF3S group allows for an increase in lipophilicity and electron-withdrawing properties along with an improvement in the bioavailability of medicinal targets; consequently, the late stage introduction of CF3S moieties into medicinal scaffolds is a sought-after strategy in synthetic organic chemistry. Different newly-developed electrophilic and nucleophilic reagents are used to effect the trifluoromethylthiolation of (hetero)aromatic compounds, aliphatic compounds (alkyl, alkenyl, alkynyl substrates), the trifluoromethylthiolation at the α- and β-carbonyl positions, and heteroatoms (N- and S-). Such reactions can involve homolytic substitutions, or functional-group substitutions (ipso). Addition reactions of electrophilic reagents to double and triple bonds followed by ring-cyclizations will be shown to yield relevant CF3S-substituted heteroaromatic compounds with relevant pharmacological action.

Enantioselective Trifluoromethylthiolating Lactonization Catalyzed by an Indane-Based Chiral Sulfide

Enantioselective trifluoromethylthiolation, especially of alkenes, is a challenging task. In this work, we have developed an efficient approach for enantioselective trifluoromethylthiolating lactonization by designing an indane-based bifunctional chiral sulfide catalyst and a shelf-stable electrophilic SCF3 reagent. The desired products were formed with diastereoselectivities of >99:1 and good to excellent enantioselectivities. The transformation represents the first enantioselective trifluoromethylthiolation of alkenes and the first enantioselective trifluoromethylthiolation that is enabled by a catalyst with a Lewis basic sulfur center.

Visible Light-Promoted Decarboxylative Di- and Trifluoromethylthiolation of Alkyl Carboxylic Acids

Described herein is a new and straightforward decarboxylative di- and trifluoromethylthiolation of alkyl carboxylic acids promoted by visible light. This approach enables the synthesis of biologically relevant alkyl SCF2H and SCF3 compounds from cheap and abundant carboxylic acids. The method is operationally simple, using irradiation from household light sources, and its mild reaction conditions make it tolerant of a range of functional groups. The strategy employs electrophilic phthalimide-derived di- and trifluoromethylthiolation reagents and exploits the ability of the imidyl radical to carry a radical chain.

Visible-Light-Promoted Trifluoromethylthiolation of Styrenes by Dual Photoredox/Halide Catalysis

Herein, we report a new visible-light-promoted strategy to access radical trifluoromethylthiolation reactions by combining halide and photoredox catalysis. This approach allows for the synthesis of vinyl-SCF3 compounds of relevance in pharmaceutical chemistry directly from alkenes under mild conditions with irradiation from household light sources. Furthermore, alkyl-SCF3-containing cyclic ketone and oxindole derivatives can be accessed by radical-polar crossover semi-pinacol and cyclization processes. Inexpensive halide salts play a crucial role in activating the trifluoromethylthiolating reagent towards photoredox catalysis and aid the formation of the SCF3 radical.

Difluorocarbene-Derived Trifluoromethylthiolation and [(18)F]Trifluoromethylthiolation of Aliphatic Electrophiles

The first trifluoromethylthiolation and [(18)F]trifluoromethylthiolation of alkyl electrophiles with in situ generated difluorocarbene in the presence of elemental sulfur and external (radioactive) fluoride ion is described. This transition-metal-free approach is high yielding, compatible with a variety of functional groups, and operated under mild reaction conditions. The conceptual advantage of this exogenous-fluoride-mediated transformation enables unprecedented syntheses of [(18)F]CF3S-labeled molecules from most commonly used [(18)F]fluoride ions. The rapid radiochemical reaction time (≤1 min) and high functional-group tolerance allow access to a variety of aliphatic [(18)F]CF3S compounds in high yields.