Direct C–H Multifluoroarylation of Ethers through Hydrogen Atom Transfer Using 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.

Visible Light-Induced 1,2-Diphenyldisulfane-Mediated Defluoroborylation of Polyfluoroarenes

A green and practical protocol of defluoroborylation of polyfluoroarenes with stable and readily accessible NHC-borane was developed, using 1,2-diphenyldisulfane as a hydrogen atom transfer (HAT) and single electron transfer (SET) reagent precursor under visible-light irradiation, leading to the concise formation of value-added fluorinated organoboron scaffolds. Mechanism studies revealed the method underwent a boryl radical addition reaction with polyfluoroarene, followed by successive single electron transfer pathways and defluorination of the C-F bond to offer the targeted product. This unprecedented platform relies on 1,2-diphenyldisulfane and base without using expensive photocatalysts, highlighting the methodology has promising application value to prepare borylated polyfluoroarene compounds.

Photocatalytic C-O Coupling Enzymes That Operate via Intramolecular Electron Transfer

Efficient and environmentally friendly conversion of light energy for direct utilization in chemical production has been a long-standing goal in enzyme design. Herein, we synthesized artificial photocatalytic enzymes by introducing an Ir photocatalyst and a Ni(bpy) complex to an optimal protein scaffold in close proximity. Consequently, the enzyme generated C-O coupling products with up to 96% yields by harvesting visible light and performing intramolecular electron transfer between the two catalysts. We systematically modulated the catalytic activities of the artificial photocatalytic cross-coupling enzymes by tuning the electrochemical properties of the catalytic components, their positions, and distances within a protein. As a result, we discovered the best-performing mutant that showed broad substrate scopes under optimized conditions. This work explicitly demonstrated that we could integrate and control both the inorganic and biochemical components of photocatalytic biocatalysis to achieve high yield and selectivity in valuable chemical transformations.

Transition metal-free photochemical C-F activation for the preparation of difluorinated-oxindole derivatives

The development of strategies for single and selective C-F bond activation represents an important avenue to overcome limitations in the synthesis of valuable fluorine-containing compounds. The synthetic and medicinal research communities would benefit from new routes that access such relevant molecules in a simple manner. Herein we disclose a straightforward and mechanistically distinct pathway to generate gem-difluoromethyl radicals and their installation onto N-arylmethacrylamides for the preparation of valuable difluorinated oxindole derivatives. To achieve operational simplicity, the use of a readily available benzenethiol as a photocatalyst under open-to-air conditions was developed, demonstrating the facile multigram preparation of the targeted fluorinated molecules. Additionally, dispersion-corrected density functional theory (DFT) and empirical investigations provide a new basis to support the proposed reaction pathway, indicating that arene thiolate is an efficient organophotocatalyst for this transformation.

Photoredox-Catalyzed C-H Functionalization Reactions

The fields of C-H functionalization and photoredox catalysis have garnered enormous interest and utility in the past several decades. Many different scientific disciplines have relied on C-H functionalization and photoredox strategies including natural product synthesis, drug discovery, radiolabeling, bioconjugation, materials, and fine chemical synthesis. In this Review, we highlight the use of photoredox catalysis in C-H functionalization reactions. We separate the review into inorganic/organometallic photoredox catalysts and organic-based photoredox catalytic systems. Further subdivision by reaction class─either sp2 or sp3 C-H functionalization─lends perspective and tactical strategies for use of these methods in synthetic applications.

Photochemical and electrochemical strategies in C–F bond activation and functionalization

The recent advances in photochemical or electrochemical C–F bond activation and functionalization have been summarized and discussed.

Diazaphospholene-Catalyzed Hydrodefluorination of Polyfluoroarenes with Phenylsilane via Concerted Nucleophilic Aromatic Substitution

The metal-free catalytic C-F bond activation of polyfluoroarenes was achieved with diazaphospholene as the catalyst and phenylsilane as the terminal reductant. Density functional theory calculations suggested a concerted nucleophilic aromatic substitution mechanism.

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

The creation of new bonds via C-F bond cleavage of readily available per- or oligofluorinated compounds has received growing interest. Using such a strategy, a myriad of valuable partially fluorinated products can be prepared, which otherwise are difficult to make by the conventional C-F bond formation methods. Visible light photoredox catalysis has been proven as an important and powerful tool for defluorinative reactions due to its mild, easy to handle, and environmentally benign characteristics. Compared to the classical C-F activation that proceeds via two-electron processes, radicals are the key intermediates using visible light photoredox catalysis, providing new modes for the cleavage of C-F bonds. In this review, a summary of the visible light-promoted C-F bond cleavage since 2018 was presented. The contents were classified by the fluorosubstrates, including polyfluorinated arenes, gem-difluoroalkenes, trifluoromethyl arenes, and trifluoromethyl alkenes. An emphasis is placed on the discussion of the mechanisms and limitations of these reactions. Finally, my personal perspective on the future development of this rapidly emerging field was provided.

Recent advances in radical-based C-F bond activation of polyfluoroarenes and gem-difluoroalkenes

The direct employment of polyfluoroarenes and gem-difluoroalkenes as building blocks is regarded as one of the most effective and straightforward strategies for the introduction of fluorine-containing moieties into organic skeletons. Accordingly, radical chemistry has gradually become a mild and powerful method for the activation of their C-F bonds. The radical-based transformations of polyfluoroarenes and gem-difluoroalkenes can be primarily categorized into three types based on the specific intermediates involved: (1) multifluoroaryl radical anions, (2) monofluoroalkenyl radicals and (3) other radicals. Compared with the more established multifluoroaryl radical anion pathway, the monofluoroalkenyl radical-involved cross-coupling reaction can proceed through C-radical cross-coupling, radical addition/elimination or the hydrogen atom transfer process. For the presented examples in this review, the typical reaction modes, substrate scope, radical-involved mechanisms, and late-stage applications in the modification of bioactive molecules are discussed, aiming to provide a comprehensive overview of the recent advances of the radical-based transformations of polyfluoroarenes and gem-difluoroalkenes.

Diastereoselectivity of the Addition of Propargylic Magnesium Reagents to Fluorinated Aromatic Sulfinyl Imines

The addition of propargylmagnesium bromide to fluorinated aromatic sulfinyl imines gave homopropargyl amines with total regio- and diastereoselection. Complete reversal of diastereoselectivity can be achieved in some cases using coordinating (THF) or noncoordinating (DCM) solvents. Substituted propargylic magnesium reagents have been also tested toward fluorinated aryl sulfinyl imines affording chiral homoallenyl amines with good yields and selectivity control. DFT calculations helped to rationalize the origin of the experimental regio- and diastereoselectivities observed in each case.

Synthesis and docking calculations of tetrafluoronaphthalene derivatives and their inhibition profiles against some metabolic enzymes

Syntheses of tetrahydroepoxy, O-allylic, O-prenylic, and O-propargylic tetrafluoronaphthalene derivatives, starting from 1-bromo-2,3,4,5,6-pentafluorobenzene, are reported here for the first time. The O-substituted tetrafluoronaphthalene derivatives were designed and also synthesized via a one-pot nucleophilic substitution reaction in excellent yields, whereas the tetrafluorotetrahydroepoxynaphthalene derivate was synthesized via a reduction reaction in excellent yield. The chemical structures of all the synthesized molecules were characterized by nuclear magnetic resonance, infrared spectroscopy, and high-resolution mass spectrometry techniques. In this study, a series of novel tetrafluoronaphthalene derivatives (2, 2a, 4-6) was tested toward several enzymes including α-glucosidase, acetylcholinesterase (AChE), and human carbonic anhydrase I and II (hCA I/II). The tetrafluoronaphthalene derivatives 2, 2a, and 4-6 showed IC₅₀ and K_i values in the range of 0.83-1.27 and 0.71-1.09 nM against hCA I, 1.26-1.85 and 1.45-5.31 nM against hCA II, 39.02-56.01 and 20.53-56.76 nM against AChE, and 15.27-34.12 and 22.58-30.45 nM against α-glucosidase, respectively. Molecular docking calculations were made to determine the biological activity values of the tetrafluoronaphthalene derivatives against the enzymes. After the calculations, ADME/T analysis was performed to examine the effects on human metabolism. Finally, these compounds had antidiabetic and anticholinesterase potentials.

Benzylic Hydroperoxidation via Visible-Light-Induced Csp3-H Activation

A highly efficient benzylic hydroperoxidation has been realized through a visible-light-induced Csp³-H activation. We believe that this reaction undergoes a direct HAT mechanism catalyzed by eosin Y. This approach features the use of a metal-free catalyst (eosin Y), an energy-economical light source (blue LED), and a sustainable oxidant (molecular oxygen). Primary, secondary, and tertiary hydroperoxides as well as silyl, benzyl, and acyl peroxides were successfully prepared with good yields and excellent functional group compatibility.