Recent Advances in C-F Bond Cleavage Enabled by Visible Light Photoredox Catalysis

Radical reactions enabled by polyfluoroaryl fragments: photocatalysis and beyond

Polyfluoroarenes have been known for a long time, but they are most often used as fluorinated building blocks for the synthesis of aromatic compounds. At the same time, due to peculiar fluorine effect, they have unique properties that provide applications in various fields ranging from synthesis to materials science. This review summarizes advances in the radical chemistry of polyfluoroarenes, which have become possible mainly with the advent of photocatalysis. Transformations of the fluorinated ring via the C-F bond activation, as well as use of fluoroaryl fragments as activating groups and hydrogen atom transfer agents are discussed. The ability of fluoroarenes to serve as catalysts is also considred.

Sequential C-F Bond Transformation of the Difluoromethylene Unit in Perfluoroalkyl Groups: A Combination of Fine-Tuned Phenothiazine Photoredox Catalyst and Lewis Acid

A sequential process via photoredox catalysis and Lewis acid mediation for C-F bond transformation of the CF2 unit in perfluoroalkyl groups has been achieved to transform perfluoroalkylarenes into complex fluoroalkylated compounds. A phenothiazine-based photocatalyst promotes the defluoroaminoxylation of perfluoroalkylarenes with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) under visible light irradiation, affording the corresponding aminoxylated products. These products undergo a further defluorinative transformation with various organosilicon reagents mediated by AlCl3 to provide highly functionalized perfluoroalkyl alcohols. Our novel phenothiazine catalyst works efficiently in the defluoroaminoxylation. Transient absorption spectroscopy revealed that the catalyst regeneration step is crucial for the photocatalytic aminoxylation.

Picking Two out of Three: Defluorinative Annulation of Trifluoromethyl Alkenes for the Synthesis of Monofluorinated Carbo- and Heterocycles

The increasing demand of organofluorine compounds in medicine, agriculture, and materials sciences makes sophisticated methods for their synthesis ever more necessary. Nowadays, not only the C-F bond formation but also the selective C-F bond cleavage of readily available poly- or perfluorine-containing compounds have become powerful tools for the effective synthesis of organofluorine compounds. The defluorinative cross-coupling of trifluoromethyl alkenes with various nucleophiles or radical precursors in an SN 2' manner is a convergent route to access gem-difluoroalkenes, which in turn react with nucleophiles or radical precursors via an SN V-type reaction. If the SN V reactions occur intramolecularly, the dual C-F bond cleavage of trifluoromethyl alkenes allows facile assembly of monofluorinated cyclic skeletons with structural complexity and diversity. In this personal account, we summarized the advances in this field on the basis of coupling and cyclization partners, including binucleophiles, alkynes, diradical precursors and radical precursors bearing a nucleophilic site. Accordingly, the annulation reactions can be achieved by base-mediated sequential SN 2'/SN V reactions, transition metal catalyzed or mediated reactions, photoredox catalysis, and the combination of photocatalytic reactions with SN V reaction. In the context of seminal works of others in this field, a concise summary of the contributions of the authors is also offered.

Exhaustive Hydrodefluorination or Deuterodefluorination of Trifluoromethylarenes via Metal-Free Photoredox Catalysis

Perfluoroalkyl compounds are persistent environmental pollutants due to their chemical and thermal stability. Hydrodefluorination is one of the most promising strategies for the disposal of fluorine-containing compounds, which has attracted much attention from a broad spectrum of scientific communities. Herein, we disclose a metal-free, visible-light-promoted protocol for the exhaustive hydrodefluorination of a wide variety of trifluoromethylarenes with up to 95% yields. Moreover, methyl-d3 groups can be obtained via deuterium water with a D ratio of up to 94%.

Monofluoromethylation of N-Heterocyclic Compounds

The review focuses on recent advances in the methodologies for the formation or introduction of the CH2F moiety in N-heterocyclic substrates over the past 5 years. The monofluoromethyl group is one of the most versatile fluorinated groups used to modify the properties of molecules in synthetic medical chemistry. The review summarizes two strategies for the monofluoromethylation of N-containing heterocycles: direct monofluoromethylation with simple XCH2F sources (for example, ICH2F) and the assembly of N-heterocyclic structures from CH2F-containing substrates. The review describes the monofluoromethylation of pharmaceutically important three-, five- and six-membered N-heterocycles: pyrrolidines, pyrroles, indoles, imidazoles, triazoles, benzothiazoles, carbazoles, indazoles, pyrazoles, oxazoles, piperidines, morpholines, pyridines, quinolines and pyridazines. Assembling of 6-fluoromethylphenanthridine, 5-fluoromethyl-2-oxazolines, C5-monofluorinated isoxazoline N-oxides, and α-fluoromethyl-α-trifluoromethylaziridines is also shown. Fluoriodo-, fluorchloro- and fluorbromomethane, FCH2SO2Cl, monofluoromethyl(aryl)sulfoniummethylides, monofluoromethyl sulfides, (fluoromethyl)triphenylphosphonium iodide and 2-fluoroacetic acid are the main fluoromethylating reagents in recent works. The replacement of atoms and entire functional groups with a fluorine atom(s) leads to a change and often improvement in activity, chemical or biostability, and pharmacokinetic properties. The monofluoromethyl group is a bioisoster of -CH3, -CH2OH, -CH2NH2, -CH2CH3, -CH2NO2 and -CH2SH moieties. Bioisosteric replacement with the CH2F group is both an interesting task for organic synthesis and a pathway to modify drugs, agrochemicals and useful intermediates.

Selective Defluoroalkylation and Hydrodefluorination of Trifluoromethyl Groups Photocatalyzed by Dihydroacridine Derivatives

The selective functionalization of trifluoromethyl groups through C-F cleavage poses a significant challenge due to the high bond energy of the C(sp3)-F bonds. Herein, we present dihydroacridine derivatives as photocatalysts that can functionalize the C-F bond of trifluoromethyl groups with various alkenes under mild conditions. Mechanistic studies and DFT calculations revealed that upon irradiation, the dihydroacridine derivatives exhibit high reducibility and function as photocatalysts for reductive defluorination. This process involves a sequential single-electron transfer mechanism. This research provides valuable insights into the properties of dihydroacridine derivatives as photocatalysts, highlighting the importance of maintaining a planar conformation and a large conjugated system for optimal catalytic activity. These findings facilitate the efficient catalytic reduction of inert chemical bonds.

Regioselective Synthesis of 3-Trifluoromethylpyrroles by [3 + 2] Cycloaddition of N-Acyl α-Amino Acids and 2-Bromo-3,3,3-trifluoropropene

A mild and concise method for the synthesis of 3-trifluoromethylpyrroles via base-mediated [3 + 2] cycloaddition of N-acyl α-amino acids and 2-bromo-3,3,3-trifluoropropene is described. N-Acyl α-amino acids serve as 1,3-dipole precursors without additional activating agents directly. A high level of regioselectivity was observed, regardless of the electronic nature and size of the substituents on 1,3-dipoles.

Photoredox radical/polar crossover enables C-H gem-difunctionalization of 1,3-benzodioxoles for the synthesis of monofluorocyclohexenes

A photocatalytic C-H gem-difunctionalization of 1,3-benzodioxoles with two different alkenes for the synthesis of highly functionalized monofluorocyclohexenes is described. Using 4CzIPN as the photocatalyst, the direct single electron oxidation of 1,3-benzodioxoles allows their defluorinative coupling with α-trifluoromethyl alkenes to produce gem-difluoroalkenes in a redox-neutral radical polar crossover manifold. The C-H bond of the resultant γ,γ-difluoroallylated 1,3-benzodioxoles was further functionalized via radical addition to electron-deficient alkenes using a more oxidizing iridium photocatalyst. The capture of in situ generated carbanions by an electrophilic gem-difluoromethylene carbon and consecutive β-fluoride elimination afford monofluorocyclohexenes. The synergistic combination of multiple termination pathways of carbanions enables rapid incorporation of molecular complexity via stitching simple and readily accessible starting materials together.

Nickel-Catalyzed Exhaustive Hydrodefluorination of Perfluoroalkyl Arenes

Perfluoroalkyl compounds are persistent environmental pollutants due to their strong C(sp³)-F bonds. Hydrodefluorination has emerged as a potential alternative disposal method for perfluoroalkyl compounds. Although the transformation of trifluoromethyl arenes into the corresponding methyl arenes has been studied by several research groups, hydrodefluorination reactions of longer perfluoroalkyl chains remain rare. Herein, we report exhaustive hydrodefluorination reactions of pentafluoroethyl arenes and longer-chain analogues using molecular nickel catalysis. Despite the cleavage of multiple C(sp³)-F bonds, the reaction already proceeds upon gentle heating (60 °C). A mechanistic investigation indicated that the reaction proceeds via benzylic hydrodefluorination reactions followed by homobenzylic ones. We reveal the multiple roles of the Ni catalyst, which include C-F bond cleavage, promotion of HF elimination, and hydrosilylation.

Synthesis of 2-Fluorobenzofuran by Photocatalytic Defluorinative Coupling and 5-endo-trig Cyclization

An alkyl radical-triggered dual C-F bond cleavage of α-CF₃-ortho-hydroxystyrenes for the synthesis of 2-fluorobenzofurans was developed. The visible-light-induced defluorinative cross-coupling reactions of α-CF₃-ortho-hydroxystyrenes with a variety of carboxylic acids produced gem-difluoroalkenes, which underwent S_NV-type 5-endo-trig cyclization to give 2-fluorobenzofurans. Mechanistic studies indicated that the electron transfer between phenoxyl radicals and carboxylates was the major pathway for the generation of alkyl radicals.

Visible-Light-Induced Monofluoroalkenylation and gem-Difluoroallylation of Inactivated C(sp3)-H Bonds via 1,5-Hydrogen Atom Transfer (HAT)

The direct monofluoroalkenylation of C(sp³)-H bonds is of great importance and quite challenging. Current methods have been restricted to the monofluoroalkenylation of activated C(sp³)-H bonds. Here, we reported the photocatalyzed C(sp³)-H monofluoroalkenylation of inactivated C(sp³)-H bonds with gem-difluoroalkenes via 1,5-hydrogen atom transfer. This process shows good functional group tolerance, such as halides (F, Cl), nitrile, sulfone, ester, and pyridine, and good γ-selectivity. Moreover, this method succeeds in the photocatalyzed gem-difluoroallylation of inactivated C(sp³)-H with α-trifluoromethyl alkenes.

Reduction of SF5CF3via iridium catalysis: radical trifluoromethylation of aromatics

The greenhouse gas SF₅CF₃ acts as CF₃ source for the photocatalytic trifluoromethylation of arenes on using [Ir(dtbbpy)(ppy)₂]PF₆ (4,4'-di-tert-butyl-2,2'-dipyridyl, ppy = 2-phenylpyridine) as catalyst. The trifluoromethylation of C₆D₆ in the presence of 1-octanol results in the concomitant generation of 1-fluorooctane, presumably by intermediate SF₄.

Nickel-catalyzed enantioselective α-heteroarylation of ketones via C-F bond activation to construct all-carbon quaternary stereocenters

Nickel-catalyzed asymmetric α-heteroarylation of ketones with fluorinated heteroarenes is reported via C-F bond activation. A series of ketones and 2-fluoropyridine derivatives with different functional groups proceed well to provide the corresponding products containing all-carbon quaternary stereocenters in good yields (up to 99% yield) and high ee values (up to 99% ee). In addition, drug molecule donepezil could also be compatible under the reaction conditions to afford late-stage diversification of pharmaceuticals.

Photocatalytic Defluorinative Three-Component Reaction of α-Trifluoromethyl Alkenes, Alkenes, and Sodium Sulfinates: Synthesis of Monofluorocyclopentenes

A photocatalytic three-component reaction of α-trifluoromethyl alkenes, electron-rich alkenes, and sodium sulfinates for the synthesis of gem-difluoroalkenes in a radical/polar crossover manner was developed. Due to the strong electron-withdrawing nature of the sulfonyl group, the resultant gem-difluoroalkenes can be converted into various monofluorocyclopentenes via intramolecular base-mediated S_NV reactions in one pot.

Relay Photocatalytic Reaction of N-Aryl Amino Acids and 2-Bromo-3,3,3-trifluoropropene: Synthesis of 4-(Difluoromethylidene)-tetrahydroquinolines

The bulk industrial chemical 2-bromo-3,3,3-trifluoropropene (BTP) was first employed as a coupling partner in photocatalytic defluorinative reactions with N-aryl amino acids. Photoredox activation of the C(sp²)-Br bond of the resultant 2-bromo-1,1-difluoroalkenes generates gem-difluoro vinyl radicals for further radical cyclization. Various 4-(difluoromethylidene)-tetrahydroquinolines were assembled in good yields by combining two photoredox cycles with a single photocatalyst.

Synthesis of 5-Fluoro-dihydroindolizines from Pyrrole-2-acetic Acids and Trifluoromethyl Alkenes via Dual C-F Bond Cleavage in a CF3 Group

Herein, we describe the synthesis of 5-fluoro-dihydroindolizines via dual C-F bond cleavage in a trifluoromethyl group. The photocatalytic defluorinative coupling of pyrrole-2-acetic acids and α-trifluoromethyl alkenes cleaved the first C-F bond, providing gem-difluoroalkenes bearing an unprotected pyrrole motif. Subsequently, an intramolecular S_NV reaction closed the ring by forming a C-N bond concomitantly with the cleavage of the second C-F bond. Using indole-2-acetic acids as the substrates, the reactions also allow the assembly of 6-fluoro-dihydropyrido[1,2-a]indoles.

Recent Advances in Visible-Light-Mediated Amide Synthesis

Visible-light photoredox catalysis has attracted tremendous interest within the synthetic community. As such, the activation mode potentially provides a more sustainable and efficient platform for the activation of organic molecules, enabling the invention of many controlled radical-involved reactions under mild conditions. In this context, amide synthesis via the strategy of photoredox catalysis has received growing interest due to the ubiquitous presence of this structural motif in numerous natural products, pharmaceuticals and functionalized materials. Employing this strategy, a wide variety of amides can be prepared effectively from halides, arenes and even alkanes under irradiation of visible light. These methods provide a robust alternative to well-established strategies for amide synthesis that involve condensation between a carboxylic acid and amine mediated by a stoichiometric activating agent. In this review, the representative progresses made on the synthesis of amides through visible light-mediated radical reactions are summarized.

Visible-light-promoted defluorinative ring-opening gem-difluoroallylation of cycloalkanols using 1-trifluoromethyl alkenes

A photocatalytic redox-neutral defluorinative ring-opening coupling of cycloalkanols with α-trifluoromethyl alkenes for the synthesis of gem-difluoroalkenes bearing a distal carbonyl group was developed.

Recent advances in C-F bond activation of trifluoromethylated carbonyl compounds and derivatives

The selective activation of the inert C-F bonds is one of the most challenging topics in modern organic chemistry. Trifluoromethylketones represent a class of versatile building blocks, which are inexpensive...