Visible Light-Induced Aromatic Difluoroalkylation

Asymmetric Three-Component Radical Cascade Reactions Enabled by Synergistic Photoredox/Brønsted Acid Catalysis: Access to α-Amino Acid Derivatives

Multicomponent reactions (MCRs), highly sought-after methods to produce atom-, step-, and energy-economic organic syntheses, have been developed extensively. However, catalytic asymmetric MCRs, especially those involving radical species, remain largely unexplored owing to the difficulty in stereoselectively regulating the extraordinarily high reactivity of open-shell radical species. Herein, we report a conceptually novel catalytic asymmetric three-component radical cascade reaction of readily accessible glycine esters, α-bromo carbonyl compounds and 2-vinylcyclopropyl ketones via synergistic photoredox/Brønsted acid catalysis, in which three sequential C-C (σ/π/σ) bond-forming events occurred through a radical addition/ring-opening/radical-radical coupling protocol, affording an array of valuable enantioenriched unnatural α-amino acid derivatives bearing two contiguous stereogenic centers and an alkene moiety in moderate to good yield with high diastereoselectivity, excellent enantioselectivity and good E-dominated geometry under mild reaction conditions. The radical relay cascade process, especially a unique proton-coupled electron transfer (PCET)-promoted radical-radical coupling, is supported by mechanistic investigations and quantum mechanics calculations and should garner broad interest and further inspire the development of asymmetric multicomponent radical reactions.

Three-Component Photochemical Cyclization/Dithiocarbamate Formation of gem-Difluoro Quinolin-2(1H)-ones

Herein, a novel visible-light-induced 6-exo-trig difluoromethylation cyclization and subsequent carbo-thioesterification reaction is described. This protocol allows efficient access to valuable gem-difluoro quinolin-2(1H)-ones in moderate to excellent yields under mild conditions. Broad amino sources compatibility, including cyclic morpholine, thiazolidine, thiomorpholine, pyrrolidine, 1,4-oxazepane, 2,6-dimethylmorpholine, tert-butyl piperazine-1-carboxylate and noncyclic diethylamine, N-ethylpropan-1-amine, N-benzylethanamine, N-benzyl-trimethylsilanamine, dibenzylamine, and N-(4-methoxybenzyl)ethanamine, demonstrated the practicability of this strategy. A radical-radical crossover route was proposed on the basis of radical inhibition experiments, visible light irradiation on-off test, apparent quantum efficiency (AQE) calculation, and fluorescence quenching studies.

Review and Theoretical Analysis of Fluorinated Radicals in Direct CAr-H Functionalization of (Hetero)arenes

We highlight key contributions in the field of direct radical CAr- H (hetero)aromatic functionalization involving fluorinated radicals. A compilation of Functional Group Transfer Reagents and their diverse activation mechanisms leading to the release of radicals are discussed. The substrate scope for each radical is analyzed and classified into three categories according to the electronic properties of the substrates. Density functional theory computational analysis provides insights into the chemical reactivity of several fluorinated radicals through their electrophilicity and nucleophilicity parameters. Theoretical analysis of their reduction potentials also highlights the remarkable correlation between electrophilicity and oxidizing ability. It is also established that highly fluorinated radicals (e.g. ⋅OCF3) are capable of engaging in single-electron transfer (SET) processes rather than radical addition, which is in good agreement with experimental literature data. A reactivity scale, based on activation barrier of addition of these radicals to benzene is also elaborated using the high accuracy DLPNO-(U)CCSD(T) method.

Access to CF2COR-Containing Quinazolinones via Visible-Light-Induced Domino Difluoroalkylation/Cyclization of N-Cyanamide Alkenes

A green and highly efficient visible-light-induced radical cascade difluoroalkylation/cyclization reaction of N-cyanamide alkenes has been developed. A variety of CF2COR-containing quinazolinones have been obtained in high yields with cheap non-metallic 4CzIPN as the photocatalyst. This photocatalytic reaction provides rapid, facile, and practical access to valuable polycyclic quinazolinone, and it is amenable to the gram scale.

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.

Synthesis of Selectively gem-Difluorinated Molecules; Chiral gem-Difluorocyclopropanes via Chemo-Enzymatic Reaction and gem-Difluorinated Compounds via Radical Reaction

The incorporation of fluorine atoms into an organic compound can alter the chemical reactivity or biological activity of the resulting compound due to the strong electron withdrawing nature of the fluorine atom. We have synthesized many original gem-difluorinated compounds and described the results in four sections. The first section describes the synthesis of optically active-gem-difluorocyclopropanes via the chemo-enzymatic reaction; we applied these compounds to liquid crystalline molecules, then further discovered a potent DNA cleavage activity for the gem-difluorocyclopropane derivatives. The second section describes the synthesis of selectively gem-difluorinated compounds via a radical reaction; we synthesized fluorinated analogues of a sex pheromone of the male African sugarcane borer, Eldana saccharina, and used the compounds as proof for investigating the origin of pheromone molecule recognition on the receptor protein. The third involves the synthesis of 2,2-difluorinated-esters by visible light-driven radical addition of 2,2-difluoroacetate with alkenes or alkynes in the presence of an organic pigment. The last section describes the synthesis of gem-difluorinated compounds via the ring-opening of gem-difluorocyclopropanes. We further developed a novel method of synthesizing gem-difluorohomoallylic alcohols via the ring-opening of gem-difluorocyclopropane and aerobic oxidation by photo-irradiation in the presence of an organic pigment. Since gem-difluorinated compounds that were prepared by the present method have two olefinic moieties with a different reactivity at the terminal position, we accomplished the synthesis of four types of gem-difluorinated cyclic alkenols via the ring-closing-metathesis (RCM) reaction.

Chemoselective Late-Stage Functionalization of Peptides via Photocatalytic C2-Alkylation of Tryptophan

Across eukaryotic proteomes, tryptophan is the least abundant of the 20 canonical amino acids, which makes it an ideal chemical handle for the late-stage functionalization of peptide and protein scaffolds with minimal production of undesired isoforms. Herein, we report the photocatalytic C2-alkylation of tryptophan using bromodifluoroacetate/acetamide-derived radical precursors. This rapid visible-light-mediated reaction is additive-free, operationally simple, and tolerates diverse functionality. We demonstrate the late-stage modification of a variety of complex peptides, including examples of biological significance.

Visible Light Induced C-H/N-H and C-X Bonds Reactions

Herein, we report efficient visible light-induced photoredox reactions of C–H/N–H and C–X Bonds. These methods have provided access to varied portfolio of synthetically important γ-ketoesters, azaspirocyclic cyclohexadienones spirocyclohexadienones, multisubstituted benzimidazole derivatives, substituted N,2-diarylacetamide, 2-arylpyridines and 2-arylquinolines in good yields and under mild conditions. Moreover, we have successfully discussed the construction through visible light-induction by an intermolecular radical addition, dearomative cyclization, aryl migration and desulfonylation. Similarly, we also spotlight the visible light-catalyzed aerobic C–N bond activation from well-known building blocks through cyclization, elimination and aromatization. The potential use of a wide portfolio of simple ketones and available primary amines has made this transformation very attractive.

Visible-Light-Induced C-F Bond Activation for the Difluoroalkylation of Indoles

An aryl disulfide mediated C-F bond activation of the trifluoromethyl group to generate valuable gem-difluoroalkylindoles is described. This method relies on readily available commodity reagents under mild reaction conditions and represents the first transition-metal-free redox-neutral C-F bond activation strategy. The reaction employs various substituted indoles and α-fluoro-substituted esters. Further, this mode of C-F activation was also amenable to the activation of trifluoromethylated arenes for the preparation of bis-benzylic gem-difluoromethylenes between indole and arene substructures, providing access to a unique chemical space.

Organocatalytic difluorobenzylation of 1,2-diketones via mild cleavage of carbon–carbon bonds

Difluoroacetophenones (DFAPs) are developed as a class of novel and practical reagents for organocatalytic difluorobenzylation reactions.

Copper and Photocatalytic Radical Relay Enabling Fluoroalkylphosphorothiolation of Alkenes: Modular Synthesis of Fluorine-Containing S-Alkyl Phosphorothioates and Phosphorodithioates

A photoredox and copper-catalyzed fluoroalkylphosphorothiolation of activated and unactivated alkenes via a radical relay mechanism is reported. By employing fluoroalkyl halides as radical precursors and P(O)SH or P(S)SH compounds as coupling partners, a wide range of β-monofluoroalkyl-, -difluoroalkyl-, -trifluoromethyl-, or -perfluoroalkyl-substituted S-alkyl phosphorothioates and phosphorodithioates can be easily constructed under mild conditions with good functional group tolerance. Furthermore, this modular reaction system can be successfully applied to late-stage functionalization of bioactive molecules.

Recent Progress on Photocatalytic Synthesis of Ester Derivatives and Reaction Mechanisms

Esters and their derivatives are distributed widely in natural products, pharmaceuticals, fine chemicals and other fields. Esters are important building blocks in pharmaceuticals such as clopidogrel, methylphenidate, fenofibrate, travoprost, prasugrel, oseltamivir, eszopiclone and fluticasone. Therefore, esterification reaction becomes more and more popular in the photochemical field. In this review, we highlight three types of reactions to synthesize esters using photochemical strategies. The reaction mechanisms involve mainly single electron transfer, energy transfer or other radical procedures.

Synthesis of Difluoroglycine Derivatives from Amines, Difluorocarbene, and CO2 : Computational Design, Scope, and Applications

A three-component reaction (3CR) for the synthesis of difluoroglycine derivatives has been achieved by using amines, difluorocarbene (generated in situ), and the abundant, inexpensive, and nontoxic C₁ source CO₂ . Various tert-amines and pyridine, (iso)quinoline, imidazole, thiazole, and pyrazole derivatives were incorporated, and the corresponding products were isolated in solid form without purification by column chromatography on silica gel. Detailed reaction profiles of the 3CR were obtained from computational analysis using DFT calculations, and the results critically suggest that simple ammonia is not applicable to this reaction. In addition, as strongly supported by computational predictions, a new reagent that can generate difluorocarbene at 0 °C without any additives was discovered. Finally, radical substitution reactions of the obtained difluoroglycine derivatives under photoredox conditions, as well as a synthetic application as an N-heterocyclic carbene ligand are shown.

A three-component iodine-catalyzed oxidative coupling reaction: a heterodifunctionalization of 3-methylindoles

A metal-free method for the synthesis of heterodifunctional indole derivatives is developed through TBHP/KI-mediated oxidative coupling. The reaction constructs C-O and C-C bonds in succession with the help of tert-butyl peroxy radicals generated by the TBHP/KI catalytic system, enabling the direct realization of the heterodifunctionalization of indole in one pot. The product of this reaction is a novel heterodifunctional compound. This work might provide a new effective method for the synthesis of polycyclic indole compounds.

Visible-light-induced radical cascade cyclization of 1-(allyloxy)-2-(1-arylvinyl)benzenes with sulfonyl chlorides for the synthesis of sulfonated benzoxepines

A visible light photocatalyzed cascade sulfonylation/cyclization of 1-(allyloxy)-2-(1-arylvinyl)benzenes with sulfonyl chlorides for the construction of sulfonated benzoxepines is developed.

Late-stage difluoromethylation: concepts, developments and perspective

This review describes the conceptual advances that have led to the multiple difluoromethylation processes making use of well-defined CF₂H sources.

Visible light induced deaminative alkylation of difluoroenoxysilanes: a transition metal free strategy

Visible-light-promoted deaminative alkylation of difluoroenoxysilanes utilizing Hantzsch ester as a catalyst or through substrate-induced pathway have been demonstrated.

Regio- and Enantioselective Bromocyclization of Difluoroalkenes as a Strategy to Access Tetrasubstituted Difluoromethylene-Containing Stereocenters

Difluoromethylene-containing compounds have attracted substantial research interest over the past decades for their ability to mimic biological functions of traditional functional groups while providing a wide variety of pharmacological benefits bestowed by the C-F bond. We report a novel strategy to access RCF2Br-containing heterocycles by regio- and enantioselective bromocyclization of difluoroalkenes enabled by chiral anion phase-transfer catalysis. The utility of this methodology was highlighted through a synthesis of an analogue of efavirenz, a drug used for treating HIV. Additionally, the synthetic versatility of the CF2Br intermediates was showcased through functionalization to a variety of enantioenriched α,α-difluoromethylene-containing products.

A general and green fluoroalkylation reaction promoted via noncovalent interactions between acetone and fluoroalkyl iodides

The first example of visible light promoted fluoroalkylation reactions initiated via noncovalent interactions between solvents and R_FI is presented.

Visible-Light-Induced para-Selective C(sp2)-H Difluoroalkylation of Diverse (Hetero)aromatic Carbonyls

An efficient visible-light-induced para-selective C(sp²)-H difluoroalkylation of diverse electron-deficient (hetero)aromatic carbonyls (aldehydes and ketones) at ambient temperature has been developed by employing Ir(ppy)₃ as the catalyst and 1,10-phenanthroline as the additive. This protocol was highlighted by its wide substrate scope, high regioselectivity, low catalyst usage, and operational simplicity.

Recent progress in photochemical radical di- and mono-fluoromethylation

Recently, photoinduced radical difluoromethylation has emerged as a step-economical synthetic method of CHF2-containing compounds. In this article, difluoromethylation of alkenes, isonitriles and aryl bromides promoted by photoredox catalysis is described together with a non-catalytic photoinduced system. Representative reactions will be discussed for each highlighted difluoromethylating reagent. In addition, related monofluoromethylation with their corresponding monofluoromethylating reagents is also discussed.

Photoredox-Catalyzed Remote Difunctionalizations of Alkenes To Synthesize Fluoroalkyl Ketones with Dimethyl Sulfoxide as the Oxidant

Visible light-mediated cascade remote oxyfluoroalkylation of alkenes under mild conditions is developed for the first time. The key point of this transformation is the incorporation of alkene fluoroalkylation-initiated remote benzyl C-H bond activation via a 1,5-H shift in a highly controlled site-selective manner and Kornblum reaction with dimethyl sulfoxide as the oxidant. With this method, a broad array of fluoroalkyl groups were introduced into a double bond to produce 1,6-fluoroalkylated ketones at room temperature.

Progress in Difluoroalkylation of Organic Substrates by Visible Light Photoredox Catalysis

The selective incorporation of fluorinated motifs, in particular CF2FG (FG=a functional group) and CF2H groups, into organic compounds has attrracted increasing attention since organofluorine molecules are of the utmost importance in the areas of nuclear imaging, pharmaceutical, agrochemical, and material sciences. A variety of synthetic approaches has been employed in late-stage difluoroalkylation reactions. Visible light photoredox catalysis for the production of CF2FG and CF2H radicals has provided a more sustainable alternative to other conventional radical-triggered reactions from the viewpoint of safety, cost, availability, and "green" chemistry. A wide range of difluoroalkylating reagents has been successfully implemented in these organic transformations in the presence of transition metal complexes or organic photocatalysts. In most cases, upon excitation via visible light irradiation with fluorescent light bulbs or blue light-emitting diode (LED) lamps, these photocatalysts can act as both reductive and oxidative quenchers, thus enabling the application of electron-donor or electron-acceptor difluoroalkylating reagents for the generation of CF2FG and CF2H radicals. Subsequent radical addition to substrates and additional organic transformations afford the corresponding difluoroalkylated derivatives. The present review describes the distinct strategies for the transition metal- and organic-photocatalyzed difluoroalkylation of a broad range of organic substrates by visible light irradiation reported in the literature since 2014.