Difluoroacetic Acid as a New Reagent for Direct C−H Difluoromethylation of Heteroaromatic Compounds

Visible Light-Induced Radical Cascade Difluoromethylation/Cyclization of Unactivated Alkenes: Access to CF2H-Substituted Polycyclic Imidazoles

An efficient and mild protocol for the visible light-induced radical cascade difluoromethylation/cyclization of imidazoles with unactivated alkenes using easily accessible and bench-stable difluoromethyltriphenylphosphonium bromide as the precursor of the -CF2H group has been developed to afford CF2H-substituted polycyclic imidazoles in moderate to good yields. This strategy, along with the construction of Csp3-CF2H/C-C bonds, is distinguished by mild conditions, no requirement of additives, simple operation, and wide substrate scope. In addition, the mechanistic experiments have indicated that the difluoromethyl radical pathway is essential for the methodology.

Selective Defluorination of Trifluoromethyl Substituents by Conformationally Induced Remote Substitution

The selective reduction of an aromatic trifluoromethyl substituent to a difluoromethyl substituent may be achieved by base-promoted elimination to form a difluoro-p-quinomethide which is trapped by an intramolecular nucleophile. High yields are obtained when the nucleophilic trap entails the conformationally favoured cyclisation of an aminoisobutyric acid (Aib) derivative. The resulting cyclised difluoromethyl-substituted arylimidazolidinone products are readily converted to versatile difluoromethyl-substituted aldehydes by reduction and hydrolysis. Defluorination is successful on a range of benzenoid (both para and ortho CF3-substituted) and heterocyclic substrates. Double defluorination may likewise be achieved sequentially, or in a single step, from an Aib dipeptide derivative.

Introduction of the difluoromethyl group at the meta- or para-position of pyridines through regioselectivity switch

Difluoromethyl pyridines have gained significant attention in medicinal and agricultural chemistry. The direct C-H-difluoromethylation of pyridines represents a highly efficient economic way to access these azines. However, the direct meta-difluoromethylation of pyridines has remained elusive and methods for site-switchable regioselective meta- and para-difluoromethylation are unknown. Here, we demonstrate the meta-C-H-difluoromethylation of pyridines through a radical process by using oxazino pyridine intermediates, which are easily accessed from pyridines. The selectivity can be readily switched to para by in situ transformation of the oxazino pyridines to pyridinium salts upon acid treatment. The preparation of various meta- and para-difluoromethylated pyridines through this approach is presented. The mild conditions used also allow for the late-stage meta- or para-difluoromethylation of pyridine containing drugs. Sequential double functionalization of pyridines is presented, which further underlines the value of this work.

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.

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.

Divergent Construction of Thiochromanes and N-Arylbutanamides via Arylthiodifluoromethyl Radical-Triggered Cascade of Alkenes

A strategy utilizing silver-catalyzed oxidative decarboxylation radical cascade cyclization of arylthiodifluoroacetic acids with alkenes for the simple and efficient preparation of difluoromethylated thiochromanes and 2,2-disubstituted-N-arylbutanamides derivatives has been developed. This approach includes good functional group tolerance, easily accessible starting materials, and operational simplicity.

Photoredox Catalysis-Enabled C-H Difluoromethylation of Heteroarenes with Pentacoordinate Phosphorane as the Reagent

Difluoromethylated heterocyclic compounds have found broad applications in numerous bioactive molecules. Herein, we report photoredox catalysis-induced direct C-H difluoromethylation of heterocycles by using bis(difluoromethyl) pentacoordinate phosphorane (PPh3(CF2H)2, 1) as the reagent. A variety of heterocycles, such as quinoxalin-2(1H)-one, thiophene, indole, and coumarin, are readily tailored with a difluoromethyl group. The method is featured as transition-metal-free by using an organic compound Erythrosin B as the catalyst and O2 as the oxidant.

Synthesis of 2-Fluoroalkylated Oxazoles from β-Monosubstituted Enamines via Fluoroacyloxylation and Cyclization Mediated by Fluoroalkyl-Containing Hypervalent Iodine(III) Species Generated In Situ

A metal-free synthesis of a series of fluoroalkyl-containing oxazoles from β-monosubstituted enamines was developed. This fluoroacyloxylation/cyclization cascade process was mediated by fluoroalkyl-containing hypervalent iodine(III) species formed in situ from the reaction of phenyliodine(III) diacetate (PIDA) and RCF₂CO₂H (R = H, Cl, Br, F, CF₃, CH₃, Ph, SAr, OAr).

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.

Synthesis of Alkyl Fluorides and Fluorinated Unnatural Amino Acids via Photochemical Decarboxylation of α-Fluorinated Carboxylic Acids

Leveraging α-fluoroalkyl or fluorobenzyl radicals to introduce fluorinated motifs allows for the rapid preparation of fluorine-containing building blocks. Herein, we report the generation of α-fluoroalkyl or fluorobenzyl radicals from readily available α-fluorocarboxylic acids under mild reaction conditions and their utilization in the Giese-type addition on Michael acceptors and dehydroamino acids, resulting in the preparation of mono- and difluorinated Michael addition adducts and unnatural fluorinated amino acids.

Transition-metal-free radical difluorobenzylation/cyclization of unactivated alkenes: access to ArCF2-substituted ring-fused quinazolinones

A mild and efficient transition-metal-free radical difluorobenzylation/cyclization of unactivated alkenes toward the synthesis of difluorobenzylated polycyclic quinazolinone derivatives with easily accessible α,α-difluoroarylacetic acids has been developed. This transformation has the advantages of wide functional group compatibility, a broad substrate scope, and operational simplicity. This methodology provided a highly attractive access to pharmaceutically valuable ArCF₂-containing polycyclic quinazolinones.

Mechanism-Driven Development of Group 10 Metal-Catalyzed Decarbonylative Coupling Reactions

Transition-metal-catalyzed cross-coupling reactions are widely used in both academia and industry for the construction of carbon-carbon and carbon-heteroatom bonds. The vast majority of cross-coupling reactions utilize aryl (pseudo)halides as the electrophilic coupling partner. Carboxylic acid derivatives (RC(O)X) represent a complementary class of electrophiles that can engage in decarbonylative couplings to produce analogous products. This decarbonylative approach offers the advantage that RC(O)X are abundant and inexpensive. In addition, decarbonylative coupling enables both intramolecular (between R and X of the carboxylic acid derivative) as well as intermolecular bond-forming reactions (in which an exogeneous nucleophile is coupled with the R group derived from RC(O)X). In these intermolecular reactions, the X-substituent on the carboxylic acid can be tuned to facilitate both oxidative addition and transmetalation, thus eliminating the need for an exogeneous base. This Account details our group's development of a diverse variety of base-free decarbonylative coupling reactions catalyzed by group 10 metals. Furthermore, it highlights how catalyst design can be guided by stoichiometric organometallic studies of these systems.Our early studies focused on intramolecular decarbonylative couplings that transform RC(O)X to the corresponding R-X with extrusion of CO. We first identified Pd and Ni monodentate phosphine catalysts that convert aryl thioesters (ArC(O)SR) to the corresponding thioethers (ArSR). We next expanded this reactivity to fluoroalkyl thioesters, using readily available fluoroalkyl carboxylic acids as the fluoroalkyl (RF) source. A Ni-phosphinoferrocene catalyst proved optimal, and the large bite angle bidentate ligand was necessary to promote the challenging RF-S bond-forming reductive elimination step.We next pursued intramolecular decarbonylative couplings of aroyl halides. Palladium-based catalysts bearing dialkylbiaryl ligands (e.g., BrettPhos) were identified as optimal for converting aroyl chlorides (ArC(O)Cl) to aryl chlorides (ArCl). These ligands were selected based on their ability to facilitate the key C-Cl bond-forming reductive elimination step of the catalytic cycle. In contrast, all attempts to convert aroyl fluorides [ArC(O)F)] to aryl fluorides (ArF) were unsuccessful with either Pd- or Ni-based catalysts. Organometallic studies of the Ni-system show that C(O)-F oxidative addition and CO deinsertion proceed smoothly, but the resulting nickel(II) aryl fluoride intermediate fails to undergo C-F bond-forming reductive elimination.In contrast to its inertness to reductive elimination, this nickel(II) aryl fluoride proved highly reactive toward transmetalation. The fluoride ligand serves as an internal base, such that no additional base is required. We leveraged this "transmetalation active" intermediate to achieve base-free Ni-catalyzed intermolecular decarbonylative coupling reactions between aroyl fluorides and boron reagents to access both biaryl and aryl-boronate ester products. By tuning the electrophile, transmetalating reagent, and catalyst, this same approach also proved applicable to base-free intermolecular decarbonylative fluoroalkylation (between difluoromethylacetyl fluoride and arylboronate esters) and aryl amination (between phenol esters and silyl amines).Moving forward, a key goal is to identify catalyst systems that enable more challenging bond constructions via this manifold. In addition, CO inhibition remains a major issue leading to the requirement for high temperatures and high catalyst loadings. Identifying catalysts that are resistant to CO binding and/or approaches to remove CO under mild conditions will be critical for making these reactions more practical and scalable.

Visible-Light-Promoted bis(Difluoromethylation)/Cyclization of 2-Vinyloxy Arylalkynes to Prepare Benzofuran Derivatives

A visible-light-promoted difluoromethylation/cyclization of 2-vinyloxy arylalkynes was developed, providing a variety of bis(difluoromethyl)-substituted benzofurans in moderate to good yields. A plausible mechanism involving difluoromethyl radical cascade cyclization and solvent-promoted ionic addition was proposed. This protocol has the advantages of having mild reaction conditions, simple operation, and good functional group tolerance.

Pd‐Catalyzed Difluoromethylations of Aryl Boronic Acids, Halides, and Pseudohalides with ICF 2 H Generated ex Situ

An expedient ex‐situ generation of difluoroiodomethane (DFIM) and its immediate use in a Pd‐catalyzed difluoromethylation of aryl boronic acids and ester derivatives in a two‐chamber reactor is reported. Heating a solution of bromodifluoroacetic acid with sodium iodide in sulfolane proved to be effective for the generation of near stoichiometric amounts of DFIM for the ensuing catalytic coupling step. A two‐step difluoromethylation of aryl (pseudo)halides with tetrahydroxydiboron as a low‐cost reducing agent, both promoted by Pd catalysis, proved effective to install this fluorine‐containing C₁ group onto several pharmaceutically relevant molecules. Finally, the method proved adaptable to deuterium incorporation by simply adding D₂O to the DFIM‐generating chamber.

Electrochemical-Oxidation-Promoted Direct N-ortho-Selective Difluoromethylation of Heterocyclic N-Oxides

An efficient and green electrochemical N-ortho-selective difluoromethylation method of various quinoline and isoquinoline N-oxides has been developed. In this method, sodium difluoromethanesulfinate (HCF₂SO₂Na) was used as the source of the difluoromethyl moiety, and various N-ortho-selective difluoromethylation quinoline and isoquinoline N-oxides were obtained in good to excellent yields under a constant current. In addition, the reaction was easy to scale up and maintained a good yield. Preliminary mechanism studies suggested that the reaction undergoes a free-radical addition and hydrogen elimination pathway.

Silver-catalyzed monofluoroalkylation of heteroarenes with α-fluorocarboxylic acids: an insight into the solvent effect

A mild and efficient method for direct C-H monofluoroalkylation of heteroarenes with easily accessible and inexpensive α-fluorocarboxylic acids has been developed. This silver-catalyzed reaction affords mono- and bis-monofluoroalkylated heteroarenes in good yields under mild conditions, and the solvent effect on the monofluoroalkylation reaction is discussed in detail.

Difluoromethylation of Heterocycles via a Radical Process

Difluoromethylation is of prime importance for its applicability in functionalizing diverse fluorine-containing heterocycles, which are core moieties of various biologically and pharmacologically active ingredients. Due to their significant biological and...

Regioselective oxidative C–H heptafluoroisopropylation of heteroarenes with heptafluoroisopropyl silver

The regioselective C–H heptafluoroisopropylation of heteroarenes with heptafluoroisopropyl silver provided convenient access to a wide range of CF(CF3)2-containing heteroarenes.

2-Difluoromethylpyridine as a bioisosteric replacement of pyridine-N-oxide: the case of quorum sensing inhibitors

Herein, we demonstrate that 2-difluoromethylpyridine is a bioisosteric replacement of pyridine-N-oxide. Using the quorum sensing inhibitor 4NPO as a model compound, a library of 2-difluoromethylpyridine derivatives was designed, synthesized, and evaluated toward quorum sensing activity, biofilm formation, anti-violacein activity, and protease activity. As a result, compounds 1 (IC50 of 35 ± 1.12 μM), 5 (IC50 of 19 ± 1.01 μM), and 6 (IC50 of 27 ± 0.67 μM) showed a similar or better activity in comparison to 4NPO (IC50 of 33 ± 1.12 μM) in a quorum sensing system of Pseudomonas aeruginosa. In addition, compounds 1, 5, 6, and 4NPO showed good antibiofilm biomass of Pseudomonas aeruginosa and reduced violacein production in Chromobacterium violaceum. In terms of protease activity, compounds 1, 5, and 6 showed significant activity compared to 4NPO. Overall, the replacement of pyridine-N-oxide by 2-difluoromethylpyridine enhances the activity of the model compound, which could open a new path for bioisosteric replacement in drug discovery and development.

Decarbonylative Fluoroalkylation at Palladium(II): From Fundamental Organometallic Studies to Catalysis

This Article describes the development of a decarbonylative Pd-catalyzed aryl-fluoroalkyl bond-forming reaction that couples fluoroalkylcarboxylic acid-derived electrophiles [R_FC(O)X] with aryl organometallics (Ar-M'). This reaction was optimized by interrogating the individual steps of the catalytic cycle (oxidative addition, carbonyl de-insertion, transmetalation, and reductive elimination) to identify a compatible pair of coupling partners and an appropriate Pd catalyst. These stoichiometric organometallic studies revealed several critical elements for reaction design. First, uncatalyzed background reactions between R_FC(O)X and Ar-M' can be avoided by using M' = boronate ester. Second, carbonyl de-insertion and Ar-R_F reductive elimination are the two slowest steps of the catalytic cycle when R_F = CF₃. Both steps are dramatically accelerated upon changing to R_F = CHF₂. Computational studies reveal that a favorable F₂C-H---X interaction contributes to accelerating carbonyl de-insertion in this system. Finally, transmetalation is slow with X = difluoroacetate but fast with X = F. Ultimately, these studies enabled the development of an (SPhos)Pd-catalyzed decarbonylative difluoromethylation of aryl neopentylglycol boronate esters with difluoroacetyl fluoride.

Transition-Metal-Free Decarboxylative Cyclization of N-Arylacrylamides with 2,2-Difluoro-2-(phenylthio)acetic Acid: Synthesis of Thiodifluorooxindole Derivatives

An efficient transition-metal-free decarboxylative cyclization of N-arylacrylamides with 2,2-difluoro-2-(phenylthio)acetic acid for the construction of thiodifluoroindoleone derivatives is described. This strategy features stable and readily available substrates, mild reaction conditions, and transition-metal-free catalysts. Notably, this protocol has successfully applied to synthesis of gem-difluoroalkenes, which exist in numerous biologically active compounds.

Wonderful fusion of organofluorine chemistry and decarboxylation strategy

Decarboxylation strategy has been emerging as a powerful tool for the synthesis of fluorine-containing organic compounds that play important roles in various fields such as pharmaceuticals, agrochemicals, and materials science. Considerable progress in decarboxylation has been made over the past decade towards the construction of diverse valuable fluorinated fine chemicals for which the fluorinated part can be brought in two ways. The first way is described as the reaction of non-fluorinated carboxylic acids (and their derivatives) with fluorinating reagents, as well as fluorine-containing building blocks. The second way is dedicated to the exploration and the use of fluorine-containing carboxylic acids (and their derivatives) in decarboxylative transformations. This review aims to provide a comprehensive summary of the development and applications of decarboxylative radical, nucleophilic and cross-coupling strategies in organofluorine chemistry.

Synthesis of α-Difluoromethyl Aryl Ketones through a Photoredox Difluoromethylation of Enol Silanes

We report here an efficient and highly straightforward access to α-difluoromethylated ketones through a visible light-mediated difluoromethylation of readily available enol silanes. The method, which takes advantage of the polyvalence of Hu's reagent, N-tosyl-S-difluoromethyl-S-phenylsulfoximine, used here as a CHF₂ radical precursor under catalytic photoredox conditions, is practical, scalable, and provides the corresponding α-CHF₂ ketones in good to excellent yields.

Metal-Free Difunctionalization of Pyridines: Selective Construction of N-CF2H and N-CHO Dihydropyridines

The reactivity of N-difluoromethylpyridinium salts is seldom explored because of their instability and low availability. Here we present a novel nucleophilic addition of N-difluoromethylpyridinium salts with nitroalkanes to synthesize N-CF₂H-dihydropyridines and N-CHO-dihydropyridines in a highly efficient and regioselective pathway. This protocol exhibits good functional group tolerance and good to excellent yields.

Modern approaches towards the synthesis of geminal difluoroalkyl groups

This review will cover the importance of and most recent approaches toward geminal difluoroalkyl groups. Transition metal-mediated, photochemical, organocatalytic, and other methods as well as their mechanistic implications will be discussed, with special emphasis on applications to biologically-relevant compounds.

Synthetic applications of α,α-difluoroarylacetic acids and salts via decarboxylative functionalization

α,α-Difluoroarylacetic acids are stable, inexpensive and readily available building blocks which can be used to access various difluoromethylated aryl motifs via decarboxylative functionalization for the formation of carbon–carbon and carbon-heteroatom (F, O, S) bonds.

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.

Slicing and Splicing of Bromodifluoro-N-arylacetamides: Dearomatization and Difunctionalization of Pyridines

Copper-catalyzed dearomatization and difunctionalization of pyridines have been disclosed, in which bromodifluoro-N-arylacetamide was sliced into five fragments and three or four of them were transferred to pyridine partners. Through this reaction, novel N-difluoromethyl-2-imine dihydropyridine derivatives can be conveniently accessed from commercially available 4-amino substituted pyridines. This strategy demonstrates a novel fluorination method featuring high atom economy, environmental friendliness, an easily available catalyst, and simple operation.

Nickel-Catalyzed Decarbonylative Synthesis of Fluoroalkyl Thioethers

This report describes the development of a nickel-catalyzed decarbonylative reaction for the synthesis of fluoroalkyl thioethers (R_FSR) from the corresponding thioesters. Readily available, inexpensive, and stable fluoroalkyl carboxylic acids (R_FCO₂H) serve as the fluoroalkyl (R_F) source in this transformation. Stoichiometric organometallic studies reveal that R_F-S bond-forming reductive elimination is a challenging step in the catalytic cycle. This led to the identification of diphenylphosphinoferrocene as the optimal ligand for this transformation. Ultimately, this method was applied to the construction of diverse fluoroalkyl thioethers (R_FSR), with R = both aryl and alkyl.

Silver-Catalyzed Decarboxylative Allylation of Difluoroarylacetic Acids with Allyl Sulfones in Water

A practical silver-catalyzed decarboxylative allylation of α,α-difluoroarylacetic acids with allyl sulfones is described, which provides a variety of β,β-difluorinated alkenes in good yields. Notably, the reaction proceeds smoothly in water with good functional group tolerance. The practicality and synthetic value of this process was demonstrated by scaled-up experiment and elaboration of the products via reduction or Heck reaction. Primary mechanism investigations suggest that a radical process might be involved.

Direct C-H difluoromethylation of heterocycles via organic photoredox catalysis

The discovery of modern medicine relies on the sustainable development of synthetic methodologies to meet the needs associated with drug molecular design. Heterocycles containing difluoromethyl groups are an emerging but scarcely investigated class of organofluoro molecules with potential applications in pharmaceutical, agricultural and material science. Herein, we developed an organophotocatalytic direct difluoromethylation of heterocycles using O₂ as a green oxidant. The C–H oxidative difluoromethylation obviates the need for pre-functionalization of the substrates, metals and additives. The operationally straightforward method enriches the efficient synthesis of many difluoromethylated heterocycles in moderate to excellent yields. The direct difluoromethylation of pharmaceutical moleculars demonstrates the practicability of this methodology to late-stage drug development. Moreover, 2′-deoxy-5-difluoromethyluridine (F₂TDR) exhibits promising activity against some cancer cell lines, indicating that the difluoromethylation methodology might provide assistance for drug discovery.

Heterocycles containing difluoromethyl groups are molecules with potential application in pharmaceutical, agricultural and materials science. Here, the authors show an organophotocatalytic difluoromethylation of heterocycles using O₂ as green oxidant and preliminarily study the products’ bioactivity.

Ag-Catalyzed minisci C–H difluoromethylarylation of N-heteroarenes

A mild silver-catalyzed decarboxylative C–H difluoromethylarylation of electron-deficient N-heteroarenes.

Practical fluorothiolation and difluorothiolation of alkenes using pyridine-HF and N-thiosuccinimides

Fluorothiolation and difluorothiolation of alkenes using pyridine-HF and N-thiosuccinimides.

Reaction of Papaverine with Baran DiversinatesTM

The reaction of papaverine with a series of Baran Diversinates^TM is reported. Although the yields were low, it was possible to synthesize a small biodiscovery library using this plant alkaloid as a scaffold for late-stage C–H functionalization. Ten papaverine analogues (2–11), including seven new compounds, were synthesized. An unexpected radical-induced exchange reaction is reported where the dimethoxybenzyl group of papaverine was replaced by an alkyl group. This side reaction enabled the synthesis of additional novel fragments based on the isoquinoline scaffold, which is present in numerous natural products. Possible reasons for the poor yields in the Diversinate^TM reactions with this particular scaffold are discussed.

Late-Stage 18 F-Difluoromethyl Labeling of N-Heteroaromatics with High Molar Activity for PET Imaging

Despite a growing interest in CHF2 in medicinal chemistry, there is a lack of efficient methods for the insertion of CHF18 F into druglike compounds. Herein described is a photoredox flow reaction for 18 F-difluoromethylation of N-heteroaromatics that are widely used in medicinal chemistry. Following the two-step synthesis for a new 18 F-difluoromethylation reagent, the photoredox reaction is completed within two minutes and proceeds by C-H activation, circumventing the need for pre-functionalization of the substrate. The method is operationally simple and affords straightforward access to radiolabeled N-heteroaromatics with high molar activity suitable for biological in vivo studies and clinical application.