Charge-recombinative triplet sensitization of alkenes for DeMayo-type [2 + 2] cycloaddition

Synthetic photochemistry has undergone significant development, largely owing to the development of visible-light-absorbing photocatalysts (PCs). PCs have significantly improved the efficiency and precision of cycloaddition reactions, primarily through energy or electron transfer pathways. Recent research has identified photocatalysis that does not follow energy- or electron-transfer formalisms, indicating the existence of other, undiscovered photoactivation pathways. This study unveils an alternative route: a charge-neutral photocatalytic process called charge-recombinative triplet sensitization (CRTS), a mechanism with limited precedents in synthetic chemistry. Our investigations revealed CRTS occurrence in DeMayo-type [2 + 2] cycloaddition reactions catalyzed by indole-fused organoPCs. Our mechanistic investigations, including steady-state and transient spectroscopic analyses, electrochemical investigations, and quantum chemical calculations, suggest a mechanism involving substrate activation through photoinduced electron transfer, followed by charge recombination, leading to substrate triplet state formation. Our findings provide valuable insights into the underlying photocatalytic reaction mechanisms and pave the way for the systematic design and realization of innovative photochemical processes.

Fluoroalkyl Sulfoximines for Versatile Photocatalytic Radical Fluoroalkylations

Fluoroalkyl sulfoximines, which serve as electron-accepting fluoroalkyl radical sources, are easy-to-handle, solid, and bench-stable chemicals. Fluoroalkyl radicals can be generated from sulfoximine reagents using strong one-electron injectors, such as a highly reducing photoredox catalyst in the excited state. Our group has developed photocatalytic radical di- and mono-fluoromethylation and α-monofluoroalkylation of olefins with the corresponding fluoroalkyl sulfoximines. In this personal account, appropriate combinations of fluoroalkyl sulfoximines and photoredox catalysts, leading to successful radical fluoroalkylation, have been discussed.

Visible Light-Induced Metal-Free Fluoroalkylations

Fluoroalkylation is a crucial synthetic process that enables the modification of molecules with fluoroalkyl groups, which can enhance the properties of compounds and have potential applications in medicine and materials science. The utilization of visible light-induced, metal-free methods is of particular importance as it provides an environmentally friendly alternative to traditional methods and eliminates the potential risks associated with metal-catalyst toxicity. This Account describes our studies on visible light-induced, metal-free fluoroalkylation processes, which include the use of organic photocatalysts or EDA complexes. We have utilized organophotocatalysts such as Nile red, tri(9-anthryl)borane, and an indole-based tetracyclic complex, as well as catalyst-free EDA chemistry through photoactive halogen bond formation or an unconventional transient ternary complex formation with nucleophilic fluoroalkyl source. A variety of π-systems including arenes/heteroarenes, alkenes, and alkynes have been successfully fluoroalkylated under the developed reaction conditions.

N-O Bond Activation by Energy Transfer Photocatalysis

A radical shift toward energy transfer photocatalysis from electron transfer photocatalysis under visible-light photoirradiation is often due to the greener prospects of atom and process economy. Recent advances in energy transfer photocatalysis embrace unique strategies for direct small-molecule activation and sometimes extraordinary chemical bond formation in the absence of additional/sacrificial reagents. Selective energy transfer photocatalysis requires careful selection of substrates and photocatalysts for a perfect match with respect to their triplet energies while having incompatible redox potentials to prevent competitive electron transfer pathways. Substrates containing labile N-O bonds are potential targets for generating reactive key intermediates via photocatalysis to access a variety of functionalized molecules. Typically, the differential electron densities of N and O heteroatoms have been exploited for generation of either N- or O-centered radical intermediates from the functionalized substrates by the electron transfer pathway. However, the latest developments involve direct N-O bond homolysis via energy transfer to generate both N- and O-centered radicals for their subsequent utilization in diverse organic transformations, also in the absence of sacrificial redox reagents. In this Account, we highlight our key contributions in the field of N-O bond activation via energy transfer photocatalysis to generate reactive radical intermediates, with coverage of useful mechanistic insights. More specifically, well-designed N-O bond-containing substrates such as 1,2,4-oxadiazolines, oxime esters, N-indolyl carbonates, and N-enoxybenzotriazoles were successfully utilized in versatile transformations involving selective energy transfer over electron transfer from photocatalysts with high triplet state energy. Direct access to reactive N-, O-, and C-centered (if decarboxylation follows) radical intermediates was achieved for diverse cross-couplings and rearrangement processes. In particular, a variety of open-shell nitrogen reactive intermediates, including N(sp²) and N(sp³) radicals and nitrenes, have been utilized. Notably, diversified transformations of identical substrates have been achieved through careful control of the reaction conditions. 1,2,4-Oxadiazolines were converted into spiro-azolactams through iminyl intermediates in the presence of ¹O₂, benzimidazoles, or sulfoximines with external sulfoxide reagent through triplet nitrene intermediates under inert conditions. Besides, oxime esters underwent either intramolecular C(sp³)-N radical-radical coupling or intermolecular C(sp³)-N radical-radical coupling by a combined energy transfer-hydrogen atom transfer strategy. Furthermore, a series of electrochemical and photophysical experiments as well as computational studies were performed to substantiate the proposed selective energy-transfer-driven reaction pathways. We hope that this Account will serve as a guide for the rational design of selective energy transfer processes through the activation of further labile chemical bonds.

Modern methods for the synthesis of perfluoroalkylated aromatics

Perfluoroalkyl-containing substances (PFAS), have become omnipresent materials in the modern world for both commercial and research applications. Compounds such as perfluoroalkylated arenes and heteroarenes have found uses in surfactants, lubricants, and flame retardants as a result of their astonishing chemical stability. Consequently, the synthesis of such compounds encompasses a large body of scientific articles and patents developed in the previous century. Most recent reviews on this subject have thus focused on summarizing this traditional literature, and have thereby spurred the development of a new wave of reaction manifolds employing modern synthesis principles. This new generation of methodologies focuses on the greener synthesis of perfluoroalkylated aromatic scaffolds, through the use of more efficient organometallic reactions, as well as by photochemical and electrochemical strategies. Herein, we will summarize this cohort of reactions while highlighting current challenges and future desirable outcomes for their environmentally friendly synthesis.

Towards a practical perfluoroalkylation of (hetero)arenes with perfluoroalkyl bromides using cobalt nanocatalysts

This paper report a convenient methodology for perfluoroalkylation including trifluoromethylation of (hetero)arenes with perfluoroalkyl bromides using a specific cobalt-based nanocatalyst.

Radical fluoroalkylation reactions of (hetero)arenes and sulfides under red light photocatalysis

Fluoroalkylation reactions of (hetero)aromatics have been accomplished through the low-power illumination from red LEDs (λmax = 635 nm) of commercially available perfluoroalkyl iodides RF-I and phthalocyanine zinc salt as photocatalyst in MeCN : DMF solvent mixture. This methodology has been extended to the perfluorobutylation of sulfides. As far as we are concerned, this is the first report on a perfluoroalkylation reaction of (hetero)aromatics and sulfides under red-light photocatalysis.

Progress and prospects in the use of photocatalysis for the synthesis of organofluorine compounds

Data on the synthesis of fluorinated organic compounds by photocatalysis are systematically considered and analyzed. The attention is focused on the mechanisms of photocatalytic reactions and the selectivity problem.

The bibliography includes 173 references.

Visible-light-mediated difluoromethylphosphonation of alkenes for the synthesis of CF2P-containing heterocycles

Novel synthesis of CF₂P-containing heterocycles has been developed via visible-light-mediated nucleophilic cyclization of unsaturated carboxylic acids, alcohol and sulfonamides.

Photocatalyzed Intermolecular Aminodifluoromethylphosphonation of Alkenes: Facile Synthesis of α,α-Difluoro-γ-aminophosphonates

An efficient and practical method for the synthesis of α,α-difluoro-γ-aminophosphonates through photocatalyzed intermolecular aminodifluoromethylphosphonation of alkenes has been developed. In this reaction, difluoromethylphosphonate is used as an important fluorinated reagent. Furthermore, the mild reaction conditions, simple operation, and broad substrate scope make this protocol very practical and attractive. The derivatization reaction in the synthesis of difluoromethylphosphonated chiral binaphthylamine ligands and α,α-difluoro-γ-aminophosphoric acid highlight the applicability of this method.

Copper-catalyzed remote oxidation of alcohols initiated by radical difluoroalkylation of alkenes: facile access to difluoroalkylated carbonyl compounds

A Cu-catalyzed oxidation of alcohols triggered by the radical difluoroalkylation of alkenes has been developed.

Trifluoroethoxy-Coated Subphthalocyanine affects Trifluoromethylation of Alkenes and Alkynes even under Low-Energy Red-Light Irradiation

Photoredox chemical reactions induced by visible light have undergone a renaissance in recent years. Polypyridyl dyes such as Ir(ppy)3 and Ru(bpy)3 are key catalysts in this event, and blue- or white-light irradiation is required for the chemical transformations. However, it remains a challenge to achieve reactions under the lower energy of red light. We disclose, herein, that trifluoroethoxy-coated subphthalocyanine realizes the red-light-driven trifluoromethylation of alkenes and alkynes with trifluoromethyl iodide in good-to-high yields. Perfluoroalkylations were also achieved under red light. The reaction mechanism is discussed with the support of UV/Vis spectroscopy and cyclic voltammetry of trifluoroethoxy-coated subphthalocyanine. Light irradiation/dark study also supports the proposed mechanism.

The direct decarboxylative allylation of N-arylglycine derivatives by photoredox catalysis

The direct decarboxylative allylation of N-arylglycine derivatives has been accomplished via visible-light photoredox catalysis.

Visible-light-induced installation of oxyfluoroalkyl groups

(Hetero)aryloxytetrafluoroethylation of heteroaromatics and alkenes has been achieved by visible-light photocatalysis utilizing readily synthesized oxyfluoroalkyl reagents.

Controlled Fluoroalkylation Reactions by Visible-Light Photoredox Catalysis

Owing to their unique biological, physical, and chemical properties, fluoroalkylated organic substances have attracted significant attention from researchers in a variety of disciplines. Fluoroalkylated compounds are considered particularly important in pharmaceutical chemistry because of their superior lipophilicity, binding selectivity, metabolic stability, and bioavailability to those of their nonfluoroalkylated analogues. We have developed various methods for the synthesis of fluoroalkylated substances that rely on the use of visible-light photoredox catalysis, a powerful preparative tool owing to its environmental benignity and mechanistic versatility in promoting a large number of synthetically important reactions with high levels of selectivity. In this Account, we describe the results of our efforts, which have led to the development of visible-light photocatalytic methods for the introduction of a variety of fluoroalkyl groups (such as, -CF₃, -CF₂R, -CH₂CF₃, -C₃F₇, and -C₄F₉) and arylthiofluoroalkyl groups (such as, -CF₂SPh, -C₂F₄SAr, and -C₄F₈SAr) to organic substances. In these studies, electron-deficient carbon-centered fluoroalkyl radicals were successfully generated by the appropriate choice of fluoroalkyl source, photocatalyst, additives, and solvent. The redox potentials of the photocatalysts and the fluoroalkyl sources and the choice of sacrificial electron donor or acceptor as the additive affected the photocatalytic pathway, determining whether an oxidative or reductive quenching pathway was operative for the generation of key fluoroalkyl radicals. Notably, we have observed that additives significantly affect the efficiencies and selectivities of these reactions and can even change the outcome of the reaction by playing additional roles during its course. For instance, a tertiary amine as an additive in the reaction medium can act not only as a sacrificial electron donor in photoredox catalysis but also as a hydrogen atom source, an elimination base for dehydrohalogenation of the intermediate, and also a Brønsted base for deprotonation. In the same context, the selection of solvent is also critical since it affects the rate and selectivity of reactions depending upon its polarity and reagent solubilizing ability and plays additional roles in the process, for example, as a hydrogen atom source. By clearly understanding the roles of additives and solvent, we designed several controlled fluoroalkylation reactions where different products were formed selectively from the same starting substrates. In addition, we could exploit one of the most important advantages of radical reactions, that is, the use of unactivated π-systems such as alkenes, alkynes, arenes, and heteroarenes as radical acceptors without prefunctionalization. Furthermore, fluoroalkylation processes under mild room-temperature reaction conditions tolerate various functional groups and are therefore easily applicable to late-stage modifications of highly functionalized advanced intermediates.

Fine Design of Photoredox Systems for Catalytic Fluoromethylation of Carbon-Carbon Multiple Bonds

Trifluoromethyl (CF3) and difluoromethyl (CF2H) groups are versatile structural motifs, especially in the fields of pharmaceuticals and agrochemicals. Thus, the development of new protocols for tri- and difluoromethylation of various skeletons has become a vital subject to be studied in the field of synthetic organic chemistry. For the past decades, a variety of fluoromethylating reagents have been developed. In particular, bench-stable and easy-to-use electrophilic fluoromethylating reagents such as the Umemoto, Yagupolskii-Umemoto, Togni, and Hu reagents serve as excellent fluoromethyl sources for ionic and carbenoid reactions. Importantly, the action of catalysis has become a promising strategy for developing new fluoromethylations. For the past several years, photoredox catalysis has emerged as a useful tool for radical reactions through visible-light-induced single-electron-transfer (SET) processes. Commonly used photocatalysts such as [Ru(bpy)3](2+) and fac-[Ir(ppy)3] (bpy = 2,2'-bipyridine; ppy = 2-pyridylphenyl) have potential as one-electron reductants strong enough to reduce those fluoromethylating reagents, resulting in facile generation of the corresponding fluoromethyl radicals. Therefore, if we can design proper reaction systems, efficient and selective radical fluoromethylation would proceed without any sacrificial redox agents, i.e., via a redox-neutral process under mild reaction conditions: irradiation with visible light, including sunlight, below room temperature. It should be noted that examples of catalytic fluoromethylation of compounds with carbon-carbon multiple bonds have been limited until recent years. In this Account, we will focus on our recent research on photoredox-catalyzed fluoromethylation of carbon-carbon multiple bonds. First, choices of the photocatalyst and the fluoromethylating reagent and the basic concept involving a redox-neutral oxidative quenching cycle are explained. Then photocatalytic trifluoromethylation of olefins is discussed mainly. Trifluoromethylative difunctionalization reactions, i.e., simultaneous introduction of the CF3 group and a different functional group across carbon-carbon double bonds, are in the middle of the discussion. Oxy-, amino-, and ketotrifluoromethylation allow us to synthesize various organofluorine compounds bearing C(sp(3))-CF3 bonds. In addition, the synthesis of valuable trifluoromethylated alkenes is also viable when the olefins have an appropriate leaving group or undergo deprotonation. The present reaction system features high functional group compatibility and high regioselectivity. Furthermore, future prospects, especially trifluoromethylative difunctionalization of alkynes and difluoromethylation of alkenes, are also discussed.

Diastereoselective Synthesis of CF3- and CF2H-Substituted Spiroethers from Aryl-Fused Cycloalkenylalkanols by Photoredox Catalysis

Simple synthesis of CF3- and CF2H-spiroethers from aryl-fused cycloalkenylalkanols by photoredox catalysis has been developed. Modification of the fluoromethylating reagents and the photoredox catalysts leads to both CF3- and CF2H-spiroetherification. The present photocatalytic system allows us to access a variety of new anti-fluoromethylated spiroethers in a highly selective manner.

Operationally simple hydrotrifluoromethylation of alkenes with sodium triflinate enabled by Ir photoredox catalysis

We report herein a single component Ir photoredox catalyst which is capable of catalyzing the hydrotrifluoromethylation of terminal alkenes and Michael acceptors with sodium triflinate (Langlois reagent) in methanol under irradiation at room temperature. Various synthetically useful functional groups, including ester, amide, ether, aldehyde, sulfone, ketone and aryl boronate, are well tolerated in this reaction.

Visible-light-induced three-component 1,2-difluoroalkylarylation of styrenes with α-carbonyl difluoroalkyl bromides and indoles

A novel visible light photoredox catalysis three-component 1,2-difluoroalkylarylation of styrenes was disclosed, and two new C–C bonds were generated in a single step through regioselective incorporation of a CF₂ group and a variety of indoles to CC bonds.