Metal-Free Direct CH Perfluoroalkylation of Arenes and Heteroarenes Using a Photoredox Organocatalyst

Site-Selective Radical Aromatic C-H Functionalization of Alloxazine and Flavin through Ground-State Single Electron Transfer

Flavins and their alloxazine isomers are key chemical scaffolds for bioinspired electron transfer strategies. Their properties can be fine-tuned by functional groups, which must be introduced at an early stage of the synthesis as their aromatic ring is inert towards post-functionalization. We show that the introduction of a remote metal-binding redox site on alloxazine and flavin activates their aromatic ring towards direct C-H functionalization. Mechanistic studies are consistent with a synthetic sequence involving ground-state single electron transfer (SET) with an electrophilic source followed by radical-radical coupling. This unprecedented reactivity opens new opportunities in molecular editing of flavins by direct aromatic post-functionalization and the utility of the method is demonstrated with the site-selective C6 functionalization of alloxazine and flavin with a CF3 group, Br or Cl, that can be further elaborated into OH and aryl for chemical diversification.

Controllable Double Difluoromethylene Insertions into S-Cu Bonds: (Arylthio)tetrafluoroethylation of Aryl Iodides with TMSCF2Br

A new method of constructing "ArSCF2CF2Cu" from ArSCu and TMSCF2Br (TMS=trimethylsilyl) has been developed. The cross-coupling reactions of the obtained "ArSCF2CF2Cu" with diverse aryl iodides (Ar'I) provide an efficient access to Ar'CF2CF2SAr. Mechanistic studies demonstrate that the "ArSCF2CF2Cu" species were generated through controllable double difluoromethylene insertions into ArS-Cu bonds rather than the 1,2-addition of ArSCu to tetrafluoroethylene.

Substituent-Determined Intramolecular Hydrogen Transfer for Photopromoted Intermolecular Cycloaddition of Anthraquinones with Aryl Olefins

The formation of intramolecular hydrogen bonds in anthraquinones makes them inert to photoinduced reactions; therefore, it is a great challenge to phototransform these compounds. Herein, we reported a formal visible-light-induced [4 + 2] cycloaddition of both 1-hydroxyanthraquinones and 1-aminoanthraquinones with olefins under external photocatalyst-free conditions with high regioselectivity. More than 60 substrates are disclosed, demonstrating the reliability of this protocol to construct diverse functionalized anthraquinone derivatives.

Visible-Light-Induced DDQ-Catalyzed Fluorocarbamoylation Using CF3SO2Na and Oxygen

The synthesis of carbamoyl fluorides via visible-light induced DDQ catalysis of secondary amines is described. This protocol employs sodium trifluorosulfinate and molecular oxygen for the in situ generation of carbonyl difluoride, which is reacted with amines to afford the corresponding carbamoyl fluorides efficiently. Moreover, carbamoyl fluorides are easily transformed to synthetically useful carbonyl compounds under mild reaction conditions.

Enroute sustainability: metal free C-H bond functionalisation

The term "C-H functionalisation" incorporates C-H activation followed by its transformation. In a single line, this can be defined as the conversion of carbon-hydrogen bonds into carbon-carbon or carbon-heteroatom bonds. The catalytic functionalisation of C-H bonds using transition metals has emerged as an atom-economical technique to engender new bonds without activated precursors which can be considered as a major drawback while attempting large-scale synthesis. Replacing the transition-metal-catalysed approach with a metal-free strategy significantly offers an alternative route that is not only inexpensive but also environmentally benign to functionalize C-H bonds. Recently metal free synthetic approaches have been flourishing to functionalize C-H bonds, motivated by the search for greener, cost-effective, and non-toxic catalysts. In this review, we will highlight the comprehensive and up-to-date discussion on recent examples of ground-breaking research on green and sustainable metal-free C-H bond functionalisation.

Metal-free visible-light-induced hydroxy-perfluoroalkylation of conjugated olefins using enamine catalyst

We developed a simple and sustainable method for the hydroxy-perfluoroalkylation of electron-deficient conjugated olefins and styrenes. In this protcol, in situ generated enamine forms electron-donor-accepter (EDA) complexes with perfluoroalkyl iodide, and reaction proceed with visible-light irradiation. Tertiary amine also interacts with perfluoroalkyl iodide via halogen-bonding, promoting the perfluoroalkyl radical generation. This reaction does not require any transition-metal or photoredox catalyst, and gaseous oxygen is used as the green hydroxy source. Moreover, various commercially available substrates and perfluoroalkyl iodides were tolerated, affording the desired hydroxy-perfluoroalkylated products in good to moderate yields (>50 examples, up to 90%).

Eosin Y-Catalyzed Visible-Light-Induced Hydroperfluoroalkylation of Electron-Deficient Alkenes

The eosin Y-catalyzed hydroperfluoroalkylation of electron-deficient alkenes is described herein. The reaction proceeded smoothly under visible light irradiation and selectively afforded a hydroperfluoroalkylated product. Various perfluoroalkyl bromides and electron-deficient olefins can be used in this reaction, and all chemicals required for this reaction are safe and readily available.

Solution-processable microporous polymer platform for heterogenization of diverse photoredox catalysts

In contemporary organic synthesis, substances that access strongly oxidizing and/or reducing states upon irradiation have been exploited to facilitate powerful and unprecedented transformations. However, the implementation of light-driven reactions in large-scale processes remains uncommon, limited by the lack of general technologies for the immobilization, separation, and reuse of these diverse catalysts. Here, we report a new class of photoactive organic polymers that combine the flexibility of small-molecule dyes with the operational advantages and recyclability of solid-phase catalysts. The solubility of these polymers in select non-polar organic solvents supports their facile processing into a wide range of heterogeneous modalities. The active sites, embedded within porous microstructures, display elevated reactivity, further enhanced by the mobility of excited states and charged species within the polymers. The independent tunability of the physical and photochemical properties of these materials affords a convenient, generalizable platform for the metamorphosis of modern photoredox catalysts into active heterogeneous equivalents.

Heterogeneous photocatalysts are easily separable and recyclable, but homogeneous ones are precisely tunable. Here, the authors merge the benefits of these two catalyst types by incorporating diverse dyes into solution-processable, porous polymers.

Copper-mediated pentafluoroethylation of organoboronates and terminal alkynes with TMSCF3

The TMSCF₃-derived CuCF₂CF₃ species has been successfully applied in pentafluoroethylation of organoboronates and terminal alkynes. By using 1,10-phenanthroline as a ligand, a broad range of (hetero)arylboronates and alkenylboronates were smoothly pentafluoroethylated under aerobic conditions. Furthermore, terminal alkynes can undergo aerobic cross-coupling with the TMSCF₃-derived CuCF₂CF₃ species in the absence of additional ligands.

Ligand-Promoted Nickel-Catalyzed para-Selective Carboxylation of Anisoles

It has always been a challenge in free radical chemistry to control site selectivity during the reaction of free radicals with aromatic rings. Herein, we report the site-selective carboxylation of anisoles through the direct reaction of the bromoform radical with a benzene ring at the para position under the assistance of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline with nickel(II) as the catalyst. A wide variety of anisoles were compatible, leading to para-carboxylated products in moderate to good yields. A preliminary mechanistic study suggested that the Ni(II) complex coordinates with the methoxyl group of the aromatic ring, which may have increased the steric hindrance at the ortho and meta positions, while this weak interaction reduces the aromaticity of the aromatic ring, affording an activated phenyl ring, thereby leading to highly para-selective carboxylation.

Ru-Bipyridine Entrapped in the Supercages of EMC-1 Faujasite as Catalyst for the Trifluoromethylation of Arenes

Trifluoromethyl (CF₃) groups are versatile structural motifs especially in the field of agrochemicals and pharmaceuticals. However, current trifluoromethylation reactions are generally associated with stoichiometric amounts of transition metals/metal oxidants, homogeneous catalysts, high temperatures, and expensive trifluoromethylating agents. In this work, the homogeneous photocatalyst Ru(bipy)₃²⁺ is entrapped in the pores of a faujasite support (EMC-1) via a "ship-in-a-bottle" strategy. The formation of the coordination compound was confirmed by Fourier transform infrared (FTIR), UV-Vis spectroscopy, and X-ray absorption spectroscopy (XAS). Due to its high stability toward acidified environments, this single-site heterogeneous catalyst is suitable for the trifluoromethylation of synthetically interesting (hetero)arenes under visible-light irradiation at room temperature. Furthermore, the heterogeneous catalyst could efficiently be reused for at least three times with minimal catalyst leaching/deactivation.

Visible light mediated C-H trifluoromethylation of (hetero)arenes

A protocol on visible light mediated C-H trifluoromethylation of unactivated (hetero)arenes under blue LED irradiation has been developed. The reaction enables the rapid construction of a range of CF3-containing (hetero)arenes...

A straightforward access to trifluoromethylated natural products through late-stage functionalization

This review summarizes the applications of late-stage strategies in the direct trifluoromethylation of natural products in the past ten years, with particular emphasis on the reaction model of each method.

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.

Radical Perfluoroalkylation of Arenes via Carbanion Intermediates

The use of sodium dithionite with perfluoroalkyl iodides under basic conditions facilitates the direct perfluoroalkylation of arenes with pendant benzylic electron-withdrawing groups. This occurs via attack of the arene on the electrophilic perfluoroalkyl radical, through the donation of electron density from a benzylic anion. The substrate scope was expanded beyond benzylic nitriles with cyclic substrates bearing electron-withdrawing groups at the benzylic position-enforcing donation of electron density to the aromatic ring and enabling attack on the perfluoroalkyl radical.

Visible light-catalyzed fluoroalkylation reactions of free aniline derivatives

The electron-rich nature of aminoaromatic compounds and the electrophilic character of fluoroalkyl R_F radicals allow for a special match in substitution reactions. We herein present visible light photocatalyzed fluoroalkylation reactions of aniline derivatives, with a study of the reaction mechanisms. The examples evaluated make use of different photocatalysts, such as polypyridyl complexes of Ir or Ru transition metals, organic dyes such as Rose Bengal, phthalocyanine-metal organocatalysts, or visible-light activated complexes. Different visible light sources that span from the blue region of the electromagnetic spectrum to low power red light irradiation sources deliver the excited photocatalysts that ensue into the production of fluoroalkyl R_F radicals. In turn, many sources of R_F radicals can be employed, such as fluoroalkyl halides, Togni's reagents, Umemoto's reagent, etc. All these protocol variants demonstrate the expansion of the methodology and the versatility of photocatalytic techniques applied to a special family of organic compounds such as aminoaromatic substrates, which has been studied by different groups. Contributions from our own laboratory will be given.

[Basic Study on Organic Electron Transfer Reactions by Electrochemistry Combined with Quantum Chemical Calculations]

This review summarizes the results of the author's basic research on organic electrochemistry conducted at Gifu Pharmaceutical University over about 40 years. After completing graduate school, the author became a research associate in Prof. Tanekazu Kubota's laboratory and started research on molecular spectroscopy in 1983. After Prof. Kubota retired in 1989, the author continued investigations in the field of organic electrochemistry as an independent researcher. At that time, a research environment in which ab initio molecular orbital calculations can be used as an analytical tool for experimental research was developed, and the author commenced research on organic electrochemistry combined with quantum chemical calculations as a lifework. The author's research topics were basic research on the molecular theory of redox potentials of organic molecules, molecular design of functional molecules, intermolecular interactions of organic molecules involving electron transfers and electron transfer systems composed of bioactive quinones, and analytical application research based on the basic electrochemistry. In this review article, the essence of the research results is introduced while reflecting on the existing situation at the time of the research. The author concludes the review by expressing gratitude to all colleagues for supporting the research in the author's laboratory.

Regioselective C-H Trifluoromethylation of Aromatic Compounds by Inclusion in Cyclodextrins

A regioselective radical C-H trifluoromethylation of aromatic compounds was developed using cyclodextrins (CDs) as additives. The C-H trifluoromethylation proceeded with high regioselectivity to afford the product in good yield, even on the gram scale. In the presence of CDs, some substrates underwent a single trifluoromethylation selectively, whereas mixtures of single- and double-trifluoromethylated products were formed in the absence of the CD. ¹H NMR experiments indicated that the regioselectivity was controlled by the inclusion of a substrate inside the CD cavity.

Light-driven, heterogeneous organocatalysts for C-C bond formation toward valuable perfluoroalkylated intermediates

The favorable exploitation of carbon nitride (CN) materials in photocatalysis for organic synthesis requires the appropriate fine-tuning of the CN structure. Here, we present a deep investigation of the structure/activity relationship of CN in the photocatalytic perfluoroalkylation of organic compounds. Four types of CN bearing subtle structural differences were studied via conventional characterization techniques and innovative nuclear magnetic resonance (NMR) experiments, correlating the different structures with the fundamental mechanistic nexus and especially highlighting the importance of the halogen bond strength between the reagent and the catalyst surface. The optimum catalyst exhibited an excellent performance, with a very wide reaction scope, and could prominently trigger the model reaction using natural sunlight. The work lays a platform for establishing a new approach in the development of heterogeneous photocatalysts for organic synthesis related to medical, agricultural, and material chemistry.

Ruthenium-Catalyzed Site-Selective Trifluoromethylations and (Per)Fluoroalkylations of Anilines and Indoles

Introducing (per)fluoroalkyl groups into arenes continues to be an interesting, but challenging area in organofluorine chemistry. We herein report an ortho-selective C-H perfluoroalkylation including trifluoromethylations of anilines and indoles without the need of protecting groups using Rf I and Rf Br as commercially available reagents. The availability and price of the starting materials and the inherent selectivity make this novel methodology attractive for the synthesis of diverse (per)fluoroalkylated building blocks, for example, for bioactive compounds and materials.

Organic semiconductor photocatalyst can bifunctionalize arenes and heteroarenes

Photoexcited electron-hole pairs on a semiconductor surface can engage in redox reactions with two different substrates. Similar to conventional electrosynthesis, the primary redox intermediates afford only separate oxidized and reduced products or, more rarely, combine to one addition product. Here, we report that a stable organic semiconductor material, mesoporous graphitic carbon nitride (mpg-CN), can act as a visible-light photoredox catalyst to orchestrate oxidative and reductive interfacial electron transfers to two different substrates in a two- or three-component system for direct twofold carbon-hydrogen functionalization of arenes and heteroarenes. The mpg-CN catalyst tolerates reactive radicals and strong nucleophiles, is straightforwardly recoverable by simple centrifugation of reaction mixtures, and is reusable for at least four catalytic transformations with conserved activity.

Synthesis of Fluorinated Amide Derivatives via a Radical N-Perfluoroalkylation-Defluorination Pathway

A one-pot approach to fluorinated hydroxamic acid, amide, and thioamide derivatives is reported. The reaction proceeds via an N-perfluoroalkylation of nitrosoarenes with perfluoroalkanesulfinates, resulting in labile N-perfluoroalkylated hydroxylamines. By the addition of suitable additives, a controllable oxy/thiodefluorination of the fluorinated hydroxylamine intermediates was achieved. The method highlights N-perfluoroalkylated amines as versatile intermediates for further synthesis.

Electrophotocatalytic Undirected C-H Trifluoromethylations of (Het)Arenes

Electrophotochemistry has enabled arene C-H trifluoromethylation with the Langlois reagent CF3 SO2 Na under mild reaction conditions. The merger of electrosynthesis and photoredox catalysis provided a chemical oxidant-free approach for the generation of the CF3 radical. The electrophotochemistry was carried out in an operationally simple manner, setting the stage for challenging C-H trifluoromethylations of unactivated arenes and heteroarenes. The robust nature of the electrophotochemical manifold was reflected by a wide scope, including electron-rich and electron-deficient benzenes, as well as naturally occurring heteroarenes. Electrophotochemical C-H trifluoromethylation was further achieved in flow with a modular electro-flow-cell equipped with an in-operando monitoring unit for on-line flow-NMR spectroscopy, providing support for the single electron transfer processes.

Synthesis of an anthraquinone-containing polymeric photosensitizer and its application in aerobic photooxidation of thioethers

Work on the synthesis of a polymeric photosensitizer and its application in the photooxidation of thioethers is reported herein. Firstly, the polymeric photosensitizer was designed and synthesized by the reaction of anthraquinone-2-carbonyl chloride (AQ-2-COCl) with poly(2-hydroxyethyl methacrylate) (PHEMA). Then, the visible light-induced photooxidation of thioethers under aerobic conditions was investigated. The results revealed that the reaction yielded sulfoxides highly chemoselectively in excellent yields with good substrate tolerance. Importantly, AQ-PHEMA could be easily recovered and reused more than 20 times without significant loss of the catalytic activity.

Work on the synthesis of a polymeric photosensitizer and its application in the photooxidation of thioethers is reported herein.

Recent developments in photoredox-catalyzed remote ortho and para C-H bond functionalizations

In recent years, the research area of direct C-H bond functionalizations was growing exponentially not only due to the ubiquity of inert C-H bonds in diverse organic compounds, including bioactive natural and nonnatural products, but also due to its impact on the discovery of pharmaceutical candidates and the total synthesis of intricate natural products. On the other hand, more recently, the field of photoredox catalysis has become an indispensable and unparalleled research topic in modern synthetic organic chemistry for the constructions of challenging bonds, having the foremost scope in academia, pharmacy, and industry. Therefore, the development of green, simpler, and effective methodologies to accomplish direct C-H bond functionalization is well overdue and highly desirable to the scientific community. In this review, we mainly highlight the impact on, and the utility of, photoredox catalysts in inert ortho and para C-H bond functionalizations. Although a surge of research papers, including reviews, demonstrating C-H functionalizations have been published in this vital area of research, to our best knowledge, this is the first review that focuses on ortho and para C-H functionalizations by photoredox catalysis to provide atom- and step-economic organic transformations. We are certain that this review will act as a promoter to highlight the application of photoredox catalysts for the functionalization of inert bonds in the domain of synthetic organic chemistry.

An efficient and facile strategy for trifluoromethylation and perfluoroalkylation of isoquinolines and heteroarenes

An effective and regioselective strategy for trifluoromethylation and perfluoroalkylation of isoquinolines and heteroarenes was developed.

Substrate-directed chemo- and regioselective synthesis of polyfunctionalized trifluoromethylarenes via organocatalytic benzannulation

Highly chemo- and regioselective substrate-directed benzannulation of trisubstituted CF₃-alkenes and 2-benzylidenemalononitriles or 2-nitroallylic acetates has been achieved via Michael-initiated [4 + 2] or Rauhut–Currier-initiated [3 + 3] annulation.

Aerobic photooxidative hydroxylation of boronic acids catalyzed by anthraquinone-containing polymeric photosensitizer

We report herein the synthesis of a polymeric photosensitizer and its application in aerobic photooxidative hydroxylation of boronic acids.

Electrocatalytic reactivity of imine/oxime-type cobalt complex for direct perfluoroalkylation of indole and aniline derivatives

Imine/Oxime-type cobalt complex-catalyzed direct perfluoroalkylation of indole and aniline derivatives with nonafluorobutyl iodide (n-C₄F₉I) under mild electrochemical method was demonstrated.