Hydrodifluoromethylation of Alkenes with Difluoroacetic Acid

Expanding tracer space for positron emission tomography with high molar activity 18F-labeled α,α-difluoromethylalkanes

Positron emission tomography (PET) is an advanced biomedical imaging modality that relies on well-designed radiotracers to report on specific protein targets and processes occurring in living animals and humans. Cyclotron-produced short-lived fluorine-18 (t1/2 = 109.8 min) is widely used to radiolabel tracers for PET. Herein we aim to expand the chemical space available for PET tracer development to include structures with 18F-labeled α,α-difluoromethylalkyl groups. We report an efficient and broad-scope method for labeling such groups with high molar activities based on a single-step radiofluorination of α-bromo-α-fluoroalkanes. The method is applicable to bioactive compounds and drug-like molecules, and is readily automated for radiotracer production. The unique physical and biochemical features of the α,α-difluoromethyl group can now be exploited in the design of new PET tracers.

Diversified Fluoroalkylation of Alkenes Using Quaternary Fluoroalkyl Alcohols as the Fluoroalkylating Reagents

Given the widespread presence of fluoroalkyl functionalities in bioactive molecules, the development of fluoroalkylation reactions with bench-stable and easy-to-use fluoroalkylating reagents is highly desirable. In addition, realization of mono-, di-, tri-, or polyfluoroalkyation usually requires distinct types of fluoroalkylating reagents under different or even harsh reaction conditions, and a universal method to accomplish different hydrofluoroalkylation of alkenes is lacking. Herein, the use of quaternary fluoroalkyl alcohols is reported as the universal fluoroalkylating reagents to readily facilitate mono-, di-, tri-, or polyfluoroalkylation of a wide range of alkene substrates in high yields. Moreover, a cascade reaction of hydrofluoroalkylation followed by intramolecular fluoroalkylation facilitates the construction of a variety of high-value complex heterocycles bearing diverse fluoroalkyl functionalities from alkenes. Mechanistic studies suggest that a proton-coupled electron transfer (PCET) process may be involved through a radical-generating pathway. The utility of this method is showcased by the late-stage fluoroalkylation of various high-value complex molecules derived from either natural products or drug-like compounds. Of note is that a continuous-flow system is amenable to this homogeneous photoredox conditions, thereby opening up a possibility of using this protocol to realize large-scale manufacturing of fluoroalkyl products with industrial interests.

Visible-light-promoted radical cyclisation of unactivated alkenes in benzimidazoles: synthesis of difluoromethyl- and aryldifluoromethyl-substituted polycyclic imidazoles

An efficient and eco-friendly approach for synthesizing difluoromethyl- and aryldifluoromethyl-substituted polycyclic imidazoles was established via a visible-light-promoted radical cyclization reaction. This method employed the readily accessible and inexpensive CF2HCO2H or PhCF2COOH, along with benzimidazoles bearing unactivated alkenes and PhI(OAc)2 as substrates, and proceeded without the need of any base, metal catalyst, photocatalyst or additive. In total, 24 examples were examined, and all of them successfully underwent cyclization reaction to produce the target products in good to excellent yields. Mechanistic studies revealed that the reaction proceeds via a radical pathway.

Organic Photoredox Catalytic Difluoroalkylation of Unactivated Olefins to Access Difluoro-Containing Tetrahydropyridazines

Herein, we disclose a readily available phenothiazine derivative as an organocatalyst, which upon excitation with 371 nm light acquires a strongly reducing power and serves to induce the radical cascade difluoromethylation/cyclization reaction of N-homoallylacetohydrazides. A variety of CF2COR-tetrahydropyridazines have been obtained in moderate to excellent yields. This catalytic platform proceeds under metal-free conditions with a wide substrate scope and broad functional group compatibility, which unlocks the new reactivity of phenothiazine derivatives and adds significant synthetic value to N-heterocycles.

Synthesis of Alkyl Bis(trifluoromethyl)carbinols via Fe-LMCT-Enabled Hydrobis(trifluoromethyl)carbinolation of Alkenes

Bis(trifluoromethyl)carbinols are valuable pharmacophores, yet their synthesis is challenging, largely due to a scarcity of safe and effective bis(trifluoromethyl)carbinolation reagents. Here, we realized the hydrobis(trifluoromethyl)carbinolation of alkenes, utilizing stable and readily accessible 2,2-bis(trifluoromethyl)glycolic acid as a source of both the bis(trifluoromethyl)carbinol unit and a hydrogen atom. This process leverages a photoinduced Fe-LMCT-enabled radical decarboxylation process that generates a key bis(trifluoromethyl)carbinol radical intermediate. Our mild protocol facilitates the synthesis of a diverse range of alkyl bis(trifluoromethyl)carbinols, including structurally complex molecules with pharmaceutical relevance.

Diastereoselective Hydrodifluoromethylation of Alkenyl N-Heterocycles via Photocatalytic Radical-Polar Crossover

A diastereoselective hydrodifluoromethylation of N-heteroaryl alkenes was successfully established. This method was applicable to an array of N-heteroaryl substrates with both cyclic and acyclic alkenes while displaying tolerance to a variety of functional groups. The conditions were also expanded to obtain hydrotrifluoromethylated products with similar results. Initial mechanistic studies suggest that the final protonation step is accessed through a radical-polar crossover process.

Recent Progress in Synthetic Applications of Hypervalent Iodine(III) Reagents

Hypervalent iodine(III) compounds have found wide application in modern organic chemistry as environmentally friendly reagents and catalysts. Hypervalent iodine reagents are commonly used in synthetically important halogenations, oxidations, aminations, heterocyclizations, and various oxidative functionalizations of organic substrates. Iodonium salts are important arylating reagents, while iodonium ylides and imides are excellent carbene and nitrene precursors. Various derivatives of benziodoxoles, such as azidobenziodoxoles, trifluoromethylbenziodoxoles, alkynylbenziodoxoles, and alkenylbenziodoxoles have found wide application as group transfer reagents in the presence of transition metal catalysts, under metal-free conditions, or using photocatalysts under photoirradiation conditions. Development of hypervalent iodine catalytic systems and discovery of highly enantioselective reactions using chiral hypervalent iodine compounds represent a particularly important recent achievement in the field of hypervalent iodine chemistry. Chemical transformations promoted by hypervalent iodine in many cases are unique and cannot be performed by using any other common, non-iodine-based reagent. This review covers literature published mainly in the last 7-8 years, between 2016 and 2024.

2,2-Difluoroethylation of Heteroatom Nucleophiles via a Hypervalent Iodine Strategy

The 2,2-difluoroethyl group is an important lipophilic hydrogen bond donor in medicinal chemistry, but its incorporation into small molecules is often challenging. Herein, we demonstrate electrophilic 2,2-difluoroethylation of thiol, amine and alcohol nucleophiles with a hypervalent iodine reagent, (2,2-difluoro-ethyl)(aryl)iodonium triflate, via a proposed ligand coupling mechanism. This transformation offers a complementary strategy to existing 2,2-difluoroethylation methods and allows access to a wide range of 2,2-difluoroethylated nucleophiles, including the drugs Captopril, Normorphine and Mefloquine.

1,3-Hydro-di/monofluoromethylation of N,N'-Cyclic Azomethine Imines with HCF2SO2Na/H2CFSO2Na via Photocatalytic Radical Addition

The radical 1,3-hydro-di/monofluoromethylation of N,N'-cyclic azomethine imines with HCF2SO2Na/H2CFSO2Na via photoredox catalysis is described. This reaction exhibits broad functional group compatibility, providing the desired products in good yields. However, CF3SO2Na failed to produce the trifluoromethyl product. DFT calculations revealed that the transition state activation energy for radical trifluoromethylation was significantly higher and the isotropic charge repulsion makes it difficult for the CF3 radical to transfer electrons.

Cu-Electrocatalysis Enables Vicinal Bis(difluoromethylation) of Alkenes: Unraveling Dichotomous Role of Zn(CF2H)2(DMPU)2 as Both Radical and Anion Source

The difluoromethyl group (CF2H) serves as an essential bioisostere in drug discovery campaigns according to Lipinski's Rule of 5 due to its advantageous combination of lipophilicity and hydrogen bonding ability, thereby improving the ADME properties. However, despite the high prevalence and importance of vicinal hydrogen bond donors in pharmaceutical agents, a general synthetic method for doubly difluoromethylated compounds in the vicinal position is absent. Here we describe a copper-electrocatalyzed strategy that enables the vicinal bis(difluoromethylation) of alkenes. By leveraging electrochemistry to oxidize Zn(CF2H)2(DMPU)2-a conventionally utilized anionic transmetalating source, we paved a way to utilize it as a CF2H radical source to deliver the CF2H group in the terminal position of alkenes. Mechanistic studies revealed that the interception of the resultant secondary radical by a copper catalyst and subsequent reductive elimination is facilitated by invoking the Cu(III) intermediate, enabling the second installation of the CF2H group in the internal position. The utility of this electrocatalytic 1,2-bis(difluoromethylation) strategy has been highlighted through the late-stage bioisosteric replacement of pharmaceutical agents such as sotalol and dipivefrine.

Metal- and additive-free β-C(sp2)-H decarboxylative alkylsulfonylation of enamides from phenyliodine(III) dicarboxylates and sulfur dioxide

A green process for the direct C(sp2)-H decarboxylative alkylsulfonylation of enamides under metal- and additive-free conditions is reported. This reaction employs phenyliodine(III) dicarboxylates as the alkyl radical precursors and DABCO·(SO2)2 as the sulfur dioxide surrogate. Diverse (E)-alkylsulfonyl enamides are generated in moderate to good yields with high stereoselectivity under extremely mild conditions via a radical process. A broad substrate scope and excellent functional group tolerance are presented. Moreover, a cascade alkylsulfonylation/cyclization reaction of N-methacryloyl enamides occurs smoothly, giving rise to various alkylsulfonylated pyrrolidones.

The 18F-Difluoromethyl Group: Challenges, Impact and Outlook

The difluoromethyl functionality has proven useful in drug discovery, as it can modulate the properties of bioactive molecules. For PET imaging, this structural motif has been largely underexploited in (pre)clinical radiotracers due to a lack of user-friendly radiosynthetic routes. This Minireview provides an overview of the challenges facing radiochemists and summarises the efforts made to date to access 18F-difluoromethyl-containing radiotracers. Two distinct approaches have prevailed, the first of which relies on 18F-fluorination. A second approach consists of a 18F-difluoromethylation process, which uses 18F-labelled reagents capable of releasing key reactive intermediates such as the [18F]CF2H radical or [18F]difluorocarbene. Finally, we provide an outlook for future directions in the radiosynthesis of [18F]CF2H compounds and their application in tracer radiosynthesis.

Visible-Light-Induced Hydrodifluoromethylation of Unactivated Alkenes with Difluoroacetic Anhydride

We herein described a practical and efficient protocol for hydrodifluoromethylation of unactivated alkenes using readily available difluoroacetic anhydride as a difluoromethyl source by merging photocatalysis and N-hydroxyphthalimide activation. This method features a wide substrate scope and excellent compatibility with various functional groups, as demonstrated by more than 50 examples, including bioactive molecules and pharmaceutical derivatives. Mechanism investigation indicated that N-hydroxyphthalimide may also serve as the hydrogen atom donor.

Promoting Photocatalytic Direct C-H Difluoromethylation of Heterocycles using Synergistic Dual-Active-Centered Covalent Organic Frameworks

Difluoromethylation reactions are increasingly important for the creation of fluorine-containing heterocycles, which are core groups in a diverse range of biologically and pharmacologically active ingredients. Ideally, this typically challenging reaction could be performed photocatalytically under mild conditions. To achieve this separation of redox processes would be required for the efficient generation of difluoromethyl radicals and the reduction of oxygen. A covalent organic framework photocatalytic material was, therefore, designed with dual reactive centers. Here, anthracene was used as a reduction site and benzothiadiazole was used as an oxidation site, distributed in a tristyryl triazine framework. Efficient charge separation was ensured by the superior electron-donating and -accepting abilities of the dual centers, creating long-lived photogenerated electron-hole pairs. Photocatalytic difluoromethylation of 16 compounds with high yields and remarkable functional group tolerance was demonstrated; compounds included bioactive molecules such as xanthine and uracil. The structure-function relationship of the dual-active-center photocatalyst was investigated through electron spin resonance, femtosecond transient absorption spectroscopy, and density functional theory calculations.

Visible-Light-Induced PhI(OAc)2-Mediated Alkylation of Heteroarenes with Simple Alkanes and Ethers

The direct alkylation of heteroarenes with alkanes has been successfully achieved through visible-light-induced hypervalent iodine-mediated C-H functionalization of both coupling partners at ambient temperatures. This reaction proceeds via the in situ generation of nucleophilic alkyl radicals from alkanes through hydrogen atom transfer (HAT), followed by a Minisci-type reaction with heteroarenes. These mild reaction conditions have demonstrated their suitability for the alkylation of a wide range of heterocycles, including azoles, pyridines, quinolines, isoquinolines, and quinoxalinones.

Difluoroacetic acid: an alternative acid in the detritylation reaction for the solid-phase synthesis of oligonucleotides

Dichloroacetic acid or trichloroacetic acid are commonly used in the detritylation reaction of the automated solid-phase synthesis of oligonucleotides. Dichloroacetic acid, however, is often contaminated with trichloroacetaldehyde (chloral), leading to the formation of inseparable impurities in the final oligonucleotide product. In this work, three different sequences, namely T18, d(TAA)6, and an 18-mer mixed sequence, were used as models to compare the deprotection efficiency of three acids: trichloroacetic acid, dichloroacetic acid, and difluoroacetic acid. Comparable purities of full-length products were obtained for the synthesis of the three model sequences when dichloroacetic acid or difluoroacetic acid were used during the detritylation reaction, however, conditions need to be optimized for the synthesis of purine-rich sequences. Therefore, difluoroacetic acid is a potential alternative to dichloroacetic acid in the solid-phase synthesis of oligonucleotides to avoid the impurity formation due to presence of chloral.

Difluoromethylated Difunctionalization of Alkenes under Visible Light

Difluoromethylated compounds usually act as bioisosteres for alcohol functional groups and show unique physicochemical and biological properties. The cyano-difluoromethylation of alkenes using 5-((difluoromethyl)sulfonyl)-1-phenyl-1H-tetrazole as a CF2H radical difluoromethyl precursor was developed to afford nitriles including a CF2H group. A low-cost, stable, easily handled 5-((difluoromethyl)sulfonyl)-1-methyl-1H-tetrazole (DFSMT) was synthesized and applied as the radical CF2H reagent. Using DFSMT as the radical CF2H precursor, the oxyl-difluoromethylation of alkenes was developed to obtain difluoromethylated ether products. All of the reactions showed good functional group tolerability. Initial mechanistic experiments indicated that the CF2H radical was involved as the key active intermediate.

Photochemical Difluoromethylation of Quinoxalin-2(1H)-ones with Difluoroacetic Anhydride and Pyridine N-Oxide

A novel photochemical difluoromethylation of quinoxalin-2(1H)-ones under catalyst-free conditions was achieved with difluoroacetic anhydride and pyridine N-oxide. The green and mild reaction conditions as well as readily attainable difluoroacetic anhydride provide a useful protocol to prepare C3-difluoromethylated quinoxalin-2(1H)-ones.

Photocatalytic hydrofluoroalkylation of alkenes with carboxylic acids

Incorporation of fluoroalkyl motifs in pharmaceuticals can enhance the therapeutic profiles of the parent molecules. The hydrofluoroalkylation of alkenes has emerged as a promising route to diverse fluoroalkylated compounds; however, current methods require superstoichiometric oxidants, expensive/oxidative fluoroalkylating reagents and precious metals, and often exhibit limited scope, making a universal protocol that addresses these limitations highly desirable. Here we report the hydrofluoroalkylation of alkenes with cheap, abundant and available fluoroalkyl carboxylic acids as the sole reagents. Hydrotrifluoro-, difluoro-, monofluoro- and perfluoroalkylation are all demonstrated, with broad scope, mild conditions (redox neutral) and potential for late-stage modification of bioactive molecules. Critical to success is overcoming the exceedingly high redox potential of feedstock fluoroalkyl carboxylic acids such as trifluoroacetic acid by leveraging cooperative earth-abundant, inexpensive iron and redox-active thiol catalysis, enabling these reagents to be directly used as hydroperfluoroalkylation donors without pre-activation. Preliminary mechanistic studies support the radical nature of this cooperative process.

Oxidant- and Base-Free, Copper-Catalyzed Difluoromethylation of Haloalkynes

We report herein a highly efficient copper-catalyzed protocol for the transformation of haloalkynes to the corresponding difluoromethylated alkynes. This scalable protocol exhibits a broad substrate scope and excellent functional group tolerance, enabling the late-stage difluoromethylation of bioactive molecules. Additionally, the strategy of utilizing the difluoromethylalkynes in gram-scale reactions and multiple transformations has proven to be highly valuable in synthetic chemistry.

Manganese/Cobalt Bimetallic Relay Catalysis for Divergent Dehydrogenative Difluoroalkylation of Alkenes

The involvement of manganese radical for halogen atom transfer (XAT) reactions has been esteemed as one reliable method but encountered with limited catalytic models. In this paper, a novel bimetallic relay catalysis of Mn2 (CO)10 and cobaloxime has been developed for divergent dehydrogenative difluoroalkylation of alkenes using commercially available difluoroalkyl bromides. A wide range of structurally diverse terminal, cyclic and internal alkenes as well as tetrasubstituted alkenes are found to be good coupling partners to deliver difluoroalkylated allylic products and difluoromethylated cyclic products, accompanied with the production of H2 as the by-product. This bimetallic relay strategy features broad substrate scope, mild reaction conditions and excellent functional group compatibility. Its success represents an important step-forward to expedite the construction of a rich library of difluoroalkylated products.

Structural insights into hydrolytic defluorination of difluoroacetate by microbial fluoroacetate dehalogenases

Fluorine forms the strongest single bond to carbon with the highest bond dissociation energy among natural products. However, fluoroacetate dehalogenases (FADs) have been shown to hydrolyze this bond in fluoroacetate under mild reaction conditions. Furthermore, two recent studies demonstrated that the FAD RPA1163 from Rhodopseudomonas palustris can also accept bulkier substrates. In this study, we explored the substrate promiscuity of microbial FADs and their ability to defluorinate polyfluorinated organic acids. Enzymatic screening of eight purified dehalogenases with reported fluoroacetate defluorination activity revealed significant hydrolytic activity against difluoroacetate in three proteins. Product analysis using liquid chromatography-mass spectrometry identified glyoxylic acid as the final product of enzymatic DFA defluorination. The crystal structures of DAR3835 from Dechloromonas aromatica and NOS0089 from Nostoc sp. were determined in the apo-state along with the DAR3835 H274N glycolyl intermediate. Structure-based site-directed mutagenesis of DAR3835 demonstrated a key role for the catalytic triad and other active site residues in the defluorination of both fluoroacetate and difluoroacetate. Computational analysis of the dimer structures of DAR3835, NOS0089, and RPA1163 indicated the presence of one substrate access tunnel in each protomer. Moreover, protein-ligand docking simulations suggested similar catalytic mechanisms for the defluorination of both fluoroacetate and difluoroacetate, with difluoroacetate being defluorinated via two consecutive defluorination reactions producing glyoxylate as the final product. Thus, our findings provide molecular insights into substrate promiscuity and catalytic mechanism of FADs, which are promising biocatalysts for applications in synthetic chemistry and bioremediation of fluorochemicals.

Recent Advances in Visible Light-Mediated Radical Fluoro-alkylation, -alkoxylation, -alkylthiolation, -alkylselenolation, and -alkylamination

In the last few years, many reagents and protocols have been developed to allow for the efficient fluorofunctionalization of a diverse set of scaffolds ranging from alkanes, alkenes, alkynes, and (hetero)arenes. The concomitant rise of organofluorine chemistry and visible light-mediated synthesis have synergistically expanded the fields and have mutually benefitted from developments in both fields. In this context, visible light driven formations of radicals containing fluorine have been a major focus for the discovery of new bioactive compounds. This review details the recent advances and progress made in visible light-mediated fluoroalkylation and heteroatom centered radical generation.

Photocatalytic Hydro Tri/Difluoromethylation of Alkenes with Bench Stable Tri/Difluoromethylating Reagents

Herein, we demonstrate the synthesis and characterization of bench stable tri/difluoromethylating reagents and their potential applications in redox neutral hydro tri/difluoromethylation of alkenes enabled by visible light. The new tri/difluoromethylating reagents are obtained on a gram-scale through simply cyclocondensation of commercially available anthranilamide with phenyltrifluoro or difluoromethyl ketone. Preliminary mechanistic studies indicated that a canonical photoredox catalytic cycle is being operative. DFT studies support this and further reveal that deprotonation occurs before radical cleavage. DFT studies also show that the better yield with HCF2 reagent is attributed to the favorable expulsion of the corresponding radical moiety.

Nickel-Catalyzed Markovnikov-Selective Hydrodifluoromethylation of Alkynes Using BrCF2 H

A Markovnikov-selective hydrodifluoromethylation of alkynes with BrCF2 H via nickel catalysis is described. This protocol proceeds via a migratory insertion of nickel hydride to alkyne followed by a CF2 H-coupling, enabling straightforward access to diverse branched CF2 H-alkenes with high efficiency and exclusive regioselectivity. The mild condition applies to a wide array of aliphatic and aryl alkynes with good functional group compatibility. Mechanistic studies are presented to support the proposed pathway.

Merging Copper(I) Photoredox Catalysis and Iodine(III) Chemistry for the Oxy-monofluoromethylation of Alkenes

A simple process for the oxy-monofluoromethylation of alkenes is described. In combination with visible-light copper(I) photoredox catalysis, an easily accessible iodine(III) reagent containing monofluoroacetoxy ligands serves as a powerful source of a monofluoromethyl (CH₂ F) radical, enabling the step economical synthesis of γ-fluoro-acetates from a broad range of olefinic substrates under mild conditions. Applications to late-stage diversification of alkenes derived from complex molecules, amino acids and the synthesis of fluoromethylated heterocycles are also demonstrated.

Copper-Catalyzed Visible-Light-Induced Allylic Difluoromethylation of Unactivated Alkenes Using Difluoroacetic Acid

We herein describe a straightforward allylic difluoromethylation reaction of unactivated alkenes. Compared to cross-couplings of prefunctionalized allylic substrates for the construction of allylic CF₂H bonds, this reaction employs readily available alkenes as substrates under mild conditions. Difluoroacetic acid is used as an inexpensive and easy-to-handle source of CF₂H radical under visible light irradiation with PIDA. The copper catalyst plays an important role of diverting the reaction pathway toward allylic difluoromethylation as opposed to previously found hydrodifluoromethylation.

Organoboron Reagent-Controlled Selective (Deutero)Hydrodefluorination

(Deuterium-labeled) CF₂ H- and CFH₂ -moieties are of high interest in drug discovery. The high demand for the incorporation of these fluoroalkyl moieties into molecular structures has witnessed significant synthetic progress, particularly in the (deutero)hydrodefluorination of CF₃ -containing compounds. However, the controllable replacement of fluorine atoms while maintaining high chemoselectivity remains challenging. Herein, we describe the development of a selective (deutero)hydrodefluorination reaction via electrolysis. The reaction exhibits a remarkable chemoselectivity control, which is enabled by the addition of different organoboron sources. The procedure is operationally simple and scalable, and provides access in one step to high-value building blocks for application in medicinal chemistry. Furthermore, density functional theory (DFT) calculations have been carried out to investigate the reaction mechanism and to rationalize the chemoselectivity observed.

Metal-Free C-H Difluoromethylation of Imidazoles with the Ruppert-Prakash Reagent

The reaction of trimethyl(trifluoromethyl)silane-tetrabutylammonium difluorotriphenylsilicate (CF₃SiMe₃-TBAT) with a series of imidazoles gives products of the formal difluorocarbene insertion into the C-H bond at the C-2 position (i.e., C-difluoromethylation). According to NMR spectra, the corresponding 2-(trimethylsilyl)difluoromethyl-substituted derivatives are likely formed as the intermediates in the reaction, and then, they slowly convert to 2-difluoromethyl-substituted imidazoles. Quantum chemical calculations of two plausible reaction mechanisms indicate that it proceeds through the intermediate imidazolide anion stabilized through the interaction with solvent molecules and counterions. In the first proposed mechanism, the anion reacts with difluorocarbene without an activation barrier, and then, the CF₂ moiety of the adduct attacks the CF₃SiMe₃ molecule. After the elimination of the CF₃ anion, 2-(trimethylsilyl)difluromethyl-substituted imidazole is formed. Another possible reaction pathway includes silylation of imidazolide anion at the N-3 atom, followed by the barrierless addition of difluorocarbene at the C-2 atom and then by 1,3-shift of the SiMe₃ group from N-3 to the carbon atom of the CF₂ moiety. Both proposed mechanisms do not include steps with high activation barriers.

Visible-Light-Induced Difluoroalkylation of Alkenes and Alkynes with Fluoro-Containing Hypervalent Iodane (III) Reagents Under Photo-Catalyst-Free Conditions

A visible-light-induced difluoroalkylation of unactivated alkenes by fluoro-containing hypervalent iodine-based difluoroalkylation reagent was achieved for the first time under photo-catalyst-free conditions. Moreover, the same reaction conditions were applicable to the difluoroalkylation of alkynes to give the hydrodifluoroalkylation products in moderate to excellent yields. The readily available reagent, broad substrate scope, and photo-catalyst-free conditions make this protocol an efficient and environmental friendly method for the hydrodifluoroalkylation of alkenes and alkynes.

Phenyliodine(III)diacetate (PIDA): Applications in Organic Synthesis

One of the hypervalent iodines most widely used as an oxidizing agent in organic chemistry is (diacetoxyiodo)benzene (PhI(OAc)2), also known as (DAIB), phenyliodine(III) diacetate (PIDA). In this septennial mini-review, the authors have concisely and systematically presented representative applications of PIDA in organic synthesis involving C-H functionalization, hetero-hetero bond formations, heterocyclic ring construction, rearrangements or migrations and miscellaneous reactions along with their interesting mechanistic aspects starting from the summer of 2015 to the present.

Difluoromethylsulfonyl Imidazolium Salt for Difluoromethylation of Alkenes

Herein, we describe the design and synthesis of a difluoromethylsulfonyl imidazolium salt, which can act as a radical difluoromethylation reagent to achieve the challenging amino- and oxy-difluoromethylation of alkenes. Notably, the three steps for the synthesis of the imidazolium salt do not require any tedious distillation or column chromatography purification process, and the amino- and oxy-difluoromethylation paths are simply determined by the selection of reaction solvents.

Radical hydrodifluoromethylation of unsaturated C-C bonds via an electroreductively triggered two-pronged approach

Due to its superior ability in controlling pharmaceutical activity, the installation of difluoromethyl (CF₂H) functionality into organic molecules has been an area of intensive research. In this context, difluoromethylation of C-C π bonds mediated by a CF₂H radical have been pursued as a central strategy to grant access to difluoromethylated hydrocarbons. However, early precedents necessitate the generation of oxidative chemical species that can limit the generality and utility of the reaction. We report here the successful implementation of radical hydrodifluoromethylation of unsaturated C-C bonds via an electroreductively triggered two-pronged approach. Preliminary mechanistic investigations suggest that the key distinction of the present strategy originates from the reconciliation of multiple redox processes under highly reducing electrochemical conditions. The reaction conditions can be chosen based on the electronic properties of the alkenes of interest, highlighting the hydrodifluoromethylation of both unactivated and activated alkenes. Notably, the reaction delivers geminal (bis)difluoromethylated products from alkynes in a single step by consecutive hydrodifluoromethylation, granting access to an underutilized 1,1,3,3-tetrafluoropropan-2-yl functional group. The late-stage hydrodifluoromethylation of densely functionalized pharmaceutical agents is also presented.

Difluoromethylation of Unactivated Alkenes Using Freon-22 through Tertiary Amine-Borane-Triggered Halogen Atom Transfer

The application of abundant and inexpensive fluorine feedstock sources to synthesize fluorinated compounds is an appealing yet underexplored strategy. Here, we report a photocatalytic radical hydrodifluoromethylation of unactivated alkenes with an inexpensive industrial chemical, chlorodifluoromethane (ClCF₂H, Freon-22). This protocol is realized by merging tertiary amine-ligated boryl radical-induced halogen atom transfer (XAT) with organophotoredox catalysis under blue light irradiation. A broad scope of readily accessible alkenes featuring a variety of functional groups and drug and natural product moieties could be selectively difluoromethylated with good efficiency in a metal-free manner. Combined experimental and computational studies suggest that the key XAT process of ClCF₂H is both thermodynamically and kinetically favored over the hydrogen atom transfer pathway owing to the formation of a strong boron-chlorine (B-Cl) bond and the low-lying antibonding orbital of the carbon-chlorine (C-Cl) bond.

Organocatalytic Three-Component Acyldifluoromethylation of Vinylarenes via N-Heterocyclic Carbene-Catalyzed Radical Relay

We herein describe an N-hetercyclic carbene-catalyzed three-component acyldifluoromethylation of vinylarenes, aldehydes, and NaSO₂CF₂H. This organocatalytic approach provides a practical route for the synthesis of pharmaceutically relevant α-aryl-β-difluormethyl ketones without the need for transition metals or photocatalysts. The late-stage acyldifluoromethylation of drug analogues was also demonstrated. The reaction design employs NaSO₂CF₂H as the source of the CF₂H radical in the presence of an oxidant for the radical relay.

Straightforward Synthesis of Alkyl Fluorides via Visible-Light-Induced Hydromono- and Difluoroalkylations of Alkenes with α-Fluoro Carboxylic Acids

We herein report the first visible-light-induced hydromono- and difluoroalkylations of alkenes with inexpensive and easily accessible α-fluoro carboxylic acids. This metal-free protocol exhibits mild conditions, high efficiency, and excellent functional-group tolerance, providing a straightforward approach to mono- and difluoroalkylated alkanes. Moreover, the fluorine effect on the hydrofluoroalkylation reaction is discussed in detail.

Photoinduced Three-Component Difluoroalkylation of Quinoxalinones with Alkenes via Difluoroiodane(III) Reagents

An environmentally friendly strategy for the photocatalyzed three-component reaction between quinoxalinones, alkenes, and hypervalent iodine(III) reagents is disclosed. The new designed difluoroiodane(III) reagent shows excellent reactivity, providing a wide range of difluoroalkyl-substituted quinoxaline-2(1H)-ones in moderate to excellent yields under mild conditions. Experimental studies demonstrated that a difluoroalkyl radical intermediate was involved in this reaction.

Acyldifluoromethylation Enabled by NHC-Photoredox Cocatalysis

With the continued interest in the properties of the difluoromethyl (CF₂H) group, small molecules with alkyl-CF₂H motifs have gained increasing attention. However, concise and efficient synthetic protocols to achieve these structures are still urgently needed. Herein, we report a new acyldifluoromethylation of inert alkenes via a synergistic NHC-photoredox catalysis featuring broad substrate scope and good functional group tolerance.

Difluoromethylarylation of Alkynes from [Bis(difluoroacetoxy)iodo]benzene: Access to CF2H-Containing Dibenzazepines

A photoinduced radical difluoromethylarylation via tandem addition-cyclization of alkynes with easily available [bis(difluoroacetoxy)iodo]benzene is accomplished, providing a straightforward and practical route for the construction of difluoromethylated dibenzazepines. Various sensitive functional groups can be compatible under photoinduced conditions. Mechanism investigation reveals that this transformation is initiated by the addition of alkyne with difluoromethyl radical, generated in situ from [bis(difluoroacetoxy)iodo]benzene.

Catalytic alkene skeletal modification for the construction of fluorinated tertiary stereocenters

Herein we describe the first construction of fluorinated tertiary stereocenters based on an alkene C(sp2)-C(sp2) bond cleavage. The new process, that takes advantage of a Rh-catalyzed carbyne transfer, relies on a branched-selective fluorination of tertiary allyl cations and is distinguished by a wide scope including natural products and drug molecule derivatives as well as adaptability to radiofluorination.

Synthesis of Visible-Light-Activated Hypervalent Iodine and Photo-oxidation under Visible Light Irradiation via a Direct S0→Tn Transition

Heavy atom-containing molecules cause a photoreaction by a direct S₀ → T_n transition. Therefore, even in a hypervalent iodine compound with a benzene ring as the main skeleton, the photoreaction proceeds under 365-400 nm wavelength light, where UV-visible spectra are not observed by usual measurement method. Some studies, however, report hypervalent iodine compounds that strongly absorb visible light. Herein, we report the synthesis of two visible light-absorbing hypervalent iodines and their photooxidation properties under visible light irradiation. We also demonstrated that the S₀ → T_n transition causes the photoreaction to proceed under wavelengths in the blue and green light region.