The Difluoromethyl Group as a Masked Nucleophile: A Lewis Acid/Base Approach

Capturing Unstable Carbanionic Intermediates via Halogen Transfer: Base-Promoted Oxidative Coupling Reactions of α,α-Difluoromethylarenes

We describe how the merger of deprotonation, halogenation, and substitution into compatible processes enables the productive functionalization of traditionally unstable carbanionic intermediates. This strategy enables the first oxidative coupling protocol of α,α-difluorobenzylic C─H bonds with heteronucleophiles. Here, transiently generated α,α-difluorobenzylic carbanionic intermediates undergo halogen transfer from 2-bromothiophenes to form electrophilic ArCF2Br compounds for in situ nucleophilic substitution, thereby avoiding α-fluoride elimination pathways that typically plague α-fluorocarbanions. This method streamlines the modular synthesis of α,α-difluorobenzyl(thio)ethers and led to the broader realization that halogen transfer to unstable carbanions is an enabling principle across diverse C(sp2)─H and C(sp3)─H systems.

Diversifying fluoroalkanes: light-driven fluoroalkyl transfer via vinylboronate esters

We outline a new synthetic strategy to prepare tertiary difluoromethylene-containing molecules from fluoroalkane precursors and vinyl-pinacol boronic ester (vinyl-BPin) reagents. Under irradiation, fluoroalkyl(vinyl)pinacol boronate esters [vinyl-BPin-CF2R]- undergo a conjugate radical addition process to form new C-C bonds, which does not require air-free conditions and tolerates oxygen and nitrogen-containing heterocycles as well as many classical functional groups. We demonstrate the versatility of this method through a one-pot synthetic protocol using RCF2H precursors and vinyl-BPin reagents in the presence of a Brønsted base. Widely available fluoroalkanes (HFC-23 and HFC-32) and difluoromethyl heteroarenes are used in this protocol, representing distinct strategies to generate tertiary -CF2H, -CF3 and -CF2-heteroarene molecules. Experimental and theoretical mechanistic investigations reveal a reaction sequence involving radical initiation followed by an ionic 1,2-boronate rearrangement.

New synthetic approaches for the construction of difluoromethylated architectures

Fluorinated compounds play a vital role in the fields of agrochemicals, pharmaceuticals, and materials science because of their unique lipophilicity, permeability, and metabolic stability. Among all such appealing fluorine-containing functional groups, the difluoromethyl group has attracted considerable attention owing to its outstanding chemical and physical properties. It has been used as a lipophilic hydrogen bond donor and a bioisostere of thiol, hydroxy, or amino groups. The excellent properties of the CF2H group have motivated many chemists to develop effective strategies for the selective incorporation of the CF2H group into target molecules. Over the past decades, a variety of efficient, atom-economical, and facile methods have been discovered for the difluoromethylation of organic substrates. This review summarizes the developments in different types of difluoromethylations, which could be classified into the following categories: radical difluoromethylation, transition metal-catalyzed difluoromethylation, and nucleophilic and electrophilic difluoromethylation.

Monodefluorinative Halogenation of Perfluoroalkyl Ketones via Organophosphorus-Mediated Selective C-F Activation

Through the prosperity of organofluorine chemistry in modern organic synthesis, perfluorinated organic compounds are now abundant and widely available. Consequently, these substances become attractive starting materials for the production of complex, multifunctional fluorinated molecules. However, the inherent challenges associated with the activation and discrimination of the C-F bonds typically lead to overdefluorination as well as functional group incompatibility. To address these problems, our group utilized a rationally designed organophosphorus reagent that promoted mild and selective manipulation of a single C-F bond in trifluoromethyl and pentafluoroethyl ketones via an interrupted Perkow-type reaction, which allowed the replacement of fluorine with more labile and synthetically versatile congeners such as chlorine, bromine, and iodine. The resulting α-haloperfluoroketones have two reactive units with orthogonal properties that would be suitable for the subsequent structural diversification. DFT calculations identified the favorable P-F interaction as the crucial factor from both thermodynamic and kinetic viewpoints.

New Opportunities to Access Fluorinated Molecules Using Organophotoredox Catalysis via C(sp3)-F Bond Cleavage

Fluorinated molecules are of paramount importance because of their unique properties. As a result, the search for innovative approaches to the synthesis of this class of compounds has been relentless over the years. Among these, the combination of photocatalysis and organofluorine chemistry turned out to be an effective partnership to access unattainable fluorinated molecules. This Perspective provides an overview of the recent advances in synthesizing fluorinated molecules via an organophotoredox-catalyzed defluorination process from trifluoromethylated compounds. It encompasses the preparation of difluoromethylated (hetero)arenes, amides, and esters as well as gem-difluoroalkene derivatives using C(sp3)-F bond activation or β-fragmentation. This Perspective will highlight remaining challenges and discuss future research opportunities.

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.

Cyclometalated N-Difluoromethylbenzimidazolylidene Platinum(II) Complexes with Built-in Secondary Coordination Spheres: Photophysical Properties and Bioimaging

Bidentate Pt(II) complexes with cyclometalated N-heteroarene or N-heterocyclic carbene (NHC) ligands have been extensively studied as phosphorescent emitters over the past two decades. Herein, we introduce a difluoromethyl group (CF2H) into the wingtip of NHCs, where CF2H acts as a lipophilic hydrogen bond (HB) donor. Their cyclometalated Pt(II) complexes show excellent PLQYs (up to 93%) and phosphorescence lifetimes mainly due to the rigid structure with hydrogen bonding between the CF2H group and the adjacent O atom at the β-diketonate ligand. Bioimaging studies demonstrate high cellular uptake efficiency and deep tumor penetration capability of complex 7 in HeLa cells and multicellular tumor spheroids, highlighting their potential as bioimaging probes.

N-Heteroaromatic Fluoroalkylation through Ligand Coupling Reaction of Sulfones

Ligand coupling on hypervalent main group elements has emerged as a pivotal methodology for the synthesis of functionalized N-heteroaromatic compounds in recent years due to the avoidance of transition metals and the mildness of the reaction conditions. In this direction, the reaction of N-heteroaryl sulfur(IV) and N-heteroaryl phosphorus(V) compounds has been well studied. However, the ligand coupling of sulfur(VI) is still underdeveloped and the reaction of alkyl N-heteroarylsulfones is still elusive, which does not match the high status of sulfones as the chemical chameleons in organic synthesis. Here we present a ligand coupling-enabled formal SO2 extrusion of fluoroalkyl 2-azaheteroarylsulfones under the promotion of Grignard reagents, which not only enriches the chemistry of sulfones, but also provides a novel and practical synthetic tool towards N-heteroaromatic fluoroalkylation.

A metal-free strategy to construct fluoroalkyl-olefin linkages using fluoroalkanes

We present a metal-free strategy to access fluoroalkyl-olefin linkages from fluoroalkane precursors and vinyl-pinacol boronic ester (BPin) reagents. This reaction sequence is templated by the boron reagent, which induces C-C bond formation upon oxidation. We developed this strategy into a one-pot synthetic protocol using RCF2H precursors directly with vinyl-BPin reagents in the presence of a Brønsted base, which tolerated oxygen- and nitrogen-containing heterocycles, and aryl halogens. We also found that HCF3 (HCF-23; a byproduct of the Teflon industry) and CH2F2 (HCF-32; a low-cost refrigerant) are amenable to this protocol, representing distinct strategies to generate RCF2H and RCF3 molecules. Finally, we demonstrate that the vinyldifluoromethylene products can be readily derivatized, representing an avenue for late-stage modification after installing the fluoroalkyl unit.

Ir-Catalyzed Ortho-Selective C-H Borylation of Difluoromethyl Arenes

The difluoromethyl group (CF2H) has received great attention due to its distinct properties in recent years. Herein, we report a new strategy for postmodification of difluoromethyl compounds. Ortho-selective C-H borylation of difluoromethyl arenes is achieved by a cyclometalated mesoionic carbene-Ir complex. The regioselectivity is controlled by a hydrogen bond between CF2H and the boryl group via the outer-sphere direction.

Difluoromethylene insertion into fluoroalkyl copper complexes

Herein, we report the insertion of a difluoromethylene into 1,1,2,2-tetrafluoro-2-arylethyl copper complexes to synthesize extended perfluoroalkyl-bridged compounds that have various functional groups on each edge (ArCF2CF2(CF2)nR, R = arenes, halogens, alkyl, alkenyl, and benzyloxycarbonyl). Further, the one-pot syntheses of perfluoroalkyl-bridged compounds from aryl boronic acid esters were carried out.

Room temperature deoxofluorination of aromatic aldehydes with XtalFluor-E under highly concentrated conditions

The synthesis of difluoromethyl-containing compounds exploiting the deoxofluorination reaction of aromatic aldehydes using XtalFluor-E is described. This transformation occurs at room temperature under highly concentrated conditions, i.e., with no added solvent. A wide range of difluoromethyl-containing compounds was obtained in 21 to 87% isolated yields.

3-Nitroindoles Serving as N-Centered Nucleophiles for Aza-1,6-Michael Addition to para-Quinone Methides

An unprecedented N-alkylation of 3-nitroindoles with para-quinone methides was developed for the first time. Using potassium carbonate as the base, a wide range of structurally diverse N-diarylmethylindole derivatives were obtained with moderated to good yields via the protection group migration/aza-1,6-Michael addition sequences. The reaction process was also demonstrated by control experiments. Different from the previous advances where 3-nitrodoles served as electrophiles trapping by various nucleophiles, the reaction herein is featured that 3-nitrodoles is defined with latent N-centered nucleophiles to react with ortho-hydrophenyl p-QMs for construction of various N-diarylmethylindoles.

Preparation and Functionalization of Mono- and Polyfluoroepoxides via Fluoroalkylation of Carbonyl Electrophiles

We outline a new synthetic method to prepare mono- and polyfluoroepoxides from a diverse pool of electrophiles (ketones, acyl chlorides, esters) and fluoroalkyl anion equivalents. The initially formed α-fluoro alkoxides undergo subsequent intramolecular ring closure when heated. We demonstrated the versatility of the method through the isolation of 16 mono- and polyfluoroepoxide products. These compounds provide unique entry points for further diversification via either fluoride migration coupled with ring opening, or defluorinative functionalization reactions, the latter of which can be used as a late-stage method to install select bioactive moieties. The reaction sequences described herein provide a pathway to functionalize the commonly observed products formed from 1,2-addition into carbonyl electrophiles.

Transition State Stabilization by SCF2 -H⋅⋅⋅O Bifurcated Hydrogen Bond

The difluoromethylthio group (SCF₂ H), which is generally considered a highly lipophilic weak hydrogen bonding donor, has attracted special interest from the pharmaceutical and agrochemical industry. Remarkably, there have been relatively few literature investigations of SCF₂ H hydrogen bonding interactions. Here, we report the determination of the hydrogen bond acidity parameter A of the SCF₂ H in the most popularly used electrophilic difluoromethylthiolating reagent. We present kinetic and computational evidence of the RSCF₂ -H⋅⋅⋅O2 bifurcated hydrogen bond for stabilizing the SCF₂ H-transferring transition state, which could cause a reversal of apparent electrophilic reactivity of difluoromethylthiolating and trifluoromethylthiolating reagents. Solvent effects on the RSCF₂ -H⋅⋅⋅O2 bifurcated hydrogen bonds will also be discussed.

Synthesis of Chiral Hypervalent Trifluoromethyl Organobismuth Complexes and Enantioselective Olefin Difluorocarbenation Screenings

Two hypervalent trifluoromethyl organobismuth complexes were prepared from commercially available chiral amines, (R)-1-cyclohexylethylamine and (1R, 2R, 3R, 5S)-(-)-isopinocampheylamine; however, only the complex from the latter amine was prepared as a single stereoisomer. Both organobismuth complexes were fully characterized by NMR spectroscopy and single-crystal X-ray crystallography, revealing that the structures were similar to previously reported complexes with a hypervalent Bi-N bond. The complexes were catalytically active in olefin difluorocarbenation with Ruppert-Prakash reagent (TMS-CF₃ ) used as a terminal source of CF₂ . The catalyst derived from isopinocampheylamine was screened with three prochiral olefins of various reactivity in DCM and toluene. All reactions afforded the 1,1-difluorocyclopropanes in good yields, but no enantiomeric excess was observed.

Electronically Ambivalent Hydrodefluorination of Aryl‐CF3 groups enabled by Electrochemical Deep‐Reduction on a Ni Cathode

We report a general procedure for the direct mono- and di-hydrodefluorination of ArCF3 compounds. Exploiting the tunability of electrochemistry and the selectivity enabled by a Ni cathode, the deep reduction garners high selectivity with good to excellent yields up to gram scale. The late-stage peripheral editing of CF3 feedstocks to construct fluoromethyl moieties will aid the rapid diversification of lead-compounds and compound libraries.

Rapid 18 F- and 19 F-Difluoromethylation through Desulfurative Fluorination of Transient N-, O-, and C-Linked Dithioles

The difluoromethyl group plays an important role in modern medicinal and agrochemistry. While several difluoromethylation reagents have been reported, these typically rely on difluoromethyl carbenes or anions, or target specific processes. Here, we describe a conceptually unique and general process for O-H, N-H and C-H difluoromethylation that involves the formation of a transient dithiole followed by facile desulfurative fluorination using silver(I) fluoride. We also introduce the 5,6-dimethoxy-1,3-benzodithiole (DMBDT) function, which undergoes sufficiently rapid desulfurative fluorination to additionally support ¹⁸ F-difluoromethylation. This new process is compatible with the wide range of functional groups typically encountered in medicinal chemistry campaigns, and the use of Ag¹⁸ F is demonstrated in the production of ¹⁸ F-labeled derivatives of testosterone, perphenazine, and melatonin, 58.0±2.2, 20.4±0.3 and 32.2±3.6 MBq μmol^-1 , respectively. We expect that the DMBDT group and this ¹⁸ F/¹⁹ F-difluoromethylation process will inspire and support new efforts in medicinal chemistry, agrochemistry and radiotracer production.

Palladium-Catalyzed Aryldifluoromethylation of Aryl Halides with Aryldifluoromethyl Trimethylsilanes

Diaryl difluoromethanes are valuable targets for medicinal chemistry because they are bioisosteres of diaryl ethers and can function as replacements for diaryl methane, ketone, and sulfone groups. However, methods to prepare diaryl difluoromethanes are scarce, especially methods starting from abundant aryl halides. We report the Pd-catalyzed aryldifluoromethylation of aryl halides with aryldifluoromethyl trimethylsilanes (TMSCF2 Ar). The reaction occurs when the catalyst contains a simple, but unusual, dialkylaryl phosphine ligand that promotes transmetallation of the silane. Computational studies show that reductive elimination following transmetallation occurs with a low barrier, despite the fluorine atoms on the α-carbon, due to coordination of the difluorobenzyl π-system to palladium. The co-development of a cobalt-catalyzed synthesis of the silanes broadened the scope of the process including several applications to the synthesis biologically relevant diaryl difluoromethanes.

AlCl3-Mediated CHF2 Transfer and Cyclocondensation of Difluoromethoxy Functionalized o-Phenylenediamines to Access N-Substituted Benzimidazoles

Herein, we report for the first time a transition-metal-free frustrated Lewis pair (FLP) catalyzed CHF₂ group migration from an oxygen atom to the neighboring nitrogen atom, which led to the synthesis of N-substituted benzimidazoles at room temperature with excellent yields, broad functional group tolerance, and a short reaction time. The oxygen-attached difluoromethane acted as a C1 source in the synthesis of N-substituted benzimidazoles in the presence of AlCl₃ by cleaving one C-O bond and two C-F bonds, resulting in formation of two new C-N bonds.

A Base-Promoted Reductive Coupling Platform for the Divergent Defluorofunctionalization of Trifluoromethylarenes

We report a trifluoromethylarene reductive coupling method that dramatically expands the scope of difluorobenzylic substructures accessible via C-F bond functionalization. Catalytic quantities of a Lewis base, combined with a disilane reagent in formamide solvent, promotes the replacement of a single trifluoromethyl fluorine atom with a silylated hemiaminal functional group. The reaction proceeds through a difluorobenzyl silane intermediate that can also be isolated. Together, these defluorinated products are shown to provide rapid access to over 20 unique difluoroalkylarene scaffolds.

From Styrenes to Fluorinated Benzyl Bromides: A Photoinduced Difunctionalization via Atom Transfer Radical Addition

An operationally simple and practical method is disclosed to achieve the difunctionalization of styrenes, generating fluorinated benzyl bromides via a photoinduced atom transfer radical addition process. The developed method is mild, atom-economical, cost-effective, employs very low photocatalyst loading (1000 ppm), and is highly compatible with a broad range of functional groups on styrene. The versatility of the fluorinated benzyl bromides is demonstrated through their derivatization to a variety of valuable compounds.

Nucleophilic strategies to construct –CF2– linkages using borazine-CF2Ar reagents

Using nucleophilic, boron-based –CF2Ar reagents, we demonstrate three methods to form C–CF bonds: (1) nucleophilic aromatic substitution, (2) palladium catalyzed cross-coupling, and (3) nucleophilic substitution.

Efficient enantioselective synthesis of CF2H-containing dispiro[benzo[b]thiophene-oxindole-pyrrolidine]s via organocatalytic cycloaddition

A novel kind of CF2H-containing dispiro[benzo[b]thiophene-oxindole-pyrrolidine] has been achieved via an organocatalyzed 1,3-dipole reaction.

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.

Synthesis of ArCF2X and [18F]Ar-CF3 via Cleavage of the Trifluoromethylsulfonyl Group

A versatile synthesis of ArCF₂X and [¹⁸F]Ar-CF₃ type compounds from readily available ArCF₂SO₂CF₃ has been developed. Diverse nucleophiles, including weak nucleophiles such as halides (¹⁸F^-, Cl^-, Br^-, and I^-), RSH, and ROH, could react with ArCF₂SO₂CF₃ efficiently to give the corresponding difluoromethylene products. The control experiments and the Hammett plot indicated that the reaction might proceed through a difluorocarbocation intermediate generated from the steric hindrance-assisted cleavage of the trifluoromethylsulfonyl group.

Biomimetic enantioselective synthesis of β,β-difluoro-α-amino acid derivatives

Although utilization of fluorine compounds has a long history, synthesis of chiral fluorinated amino acid derivatives with structural diversity and high stereoselectivity is still very appealing and challenging. Here, we report a biomimetic study of enantioselective [1,3]-proton shift of β,β-difluoro-α-imine amides catalyzed by chiral quinine derivatives. A wide range of corresponding β,β-difluoro-α-amino amides were achieved in good yields with high enantioselectivities. The optically pure β,β-difluoro-α-amino acid derivatives were further obtained, which have high application values in the synthesis of fluoro peptides, fluoro amino alcohols and other valuable fluorine-containing molecules.

Transition Metal-Catalyzed Cross-Couplings of Benzylic Sulfone Derivatives

In recent years, the use of organosulfones as a new class of cross-coupling partner in transition-metal catalyzed reactions has undergone significant advancement. In this personal account, our recent investigations into desulfonylative cross-coupling reactions of benzylic sulfone derivatives catalyzed by Pd, Ni, and Cu catalysis is described. Combined with the facile α-functionalizations of sulfones, our methods can be used to form valuable multiply-arylated structures such as di-, tri-, and, tetraarylmethanes from readily available substrates. The reactivity of sulfones can be increased by introducing electron-withdrawing substituents such as 3,5-bis(trifluoromethyl)phenyl and trifluoromethyl groups, which enable more challenging cross-coupling reactions. Reactive intermediates including Cu-carbene complexes were identified as key intermediates in sulfone activation, representing new types of C-SO₂ bond activation processes. These results indicate sulfones are powerful functional groups, enabling new catalytic desulfonylative transformations.

Synthesis of Difluoroalkylated Heteroarenes via Difluorocarbene

A diversity-oriented and C2-selective synthesis of difluoroalkyl-substituted heteroarenes from three fragments, N-methoxyazinium salts, difluorocarbene, and electrophilic or radical reagents, is described. The reaction proceeds via the addition of difluorinated phosphorus ylide to in situ methylated heteroarene N-oxides, leading to phosphonium salts, which can undergo further transformations under basic or photoinduced conditions.

Base-Catalyzed H/D Exchange Reaction of Difluoromethylarenes

The budding deuteriodifluoromethyl group (CF₂D) is a potentially significant functional group in medicinal chemistry. Herein, we investigated t-BuOK-catalyzed H/D exchange reaction of difluoromethylarenes in DMSO-d₆ solution. The method provides excellent deuterium incorporation at the difluoromethyl group. Meanwhile, the effect of a trace amount of D₂O in DMSO-d₆ solution on the deuteration reaction was also investigated.

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.

Ruthenium-Catalyzed Meta-Selective C-H Difluoromethylation of Phenol Derivatives

With pyrimidine as the directing group, we achieved the meta-selective difluoromethylation of phenol derivatives using ruthenium as a catalyst. This synthetic scheme provided an efficient method for the syntheses of fluorine-containing phenol derivatives. A wide variety of phenol derivatives were well-suited, affording the corresponding products in moderate-to-good yields.

Deprotonative Functionalization of the Difluoromethyl Group

The functionalization of 3-(difluoromethyl)pyridine has been developed via direct deprotonation of -CHF₂ with a lithiated base and subsequent trapping with various electrophiles in THF. In situ quenching gives access to 3-pyridyl-CF₂-SiMe₂Ph as a new silylated compound, which can be postfunctionalized with a fluoride source to obtain a larger library of 3-(difluoroalkyl)pyridines that could not be accessed via direct deprotonation.

Catalytic Enantioselective Synthesis of Difluorinated Alkyl Bromides

We report an iodoarene-catalyzed enantioselective synthesis of β,β-difluoroalkyl bromide building blocks. The transformation involves an oxidative rearrangement of α-bromostyrenes, utilizing HF-pyridine as the fluoride source and m-CPBA as the stoichiometric oxidant. A catalyst decomposition pathway was identified, which, in tandem with catalyst structure-activity relationship studies, facilitated the development of an improved catalyst providing higher enantioselectivity with lower catalyst loadings. The versatility of the difluoroalkyl bromide products was demonstrated via highly enantiospecific substitution reactions with suitably reactive nucleophiles. The origins of enantioselectivity were investigated using computed interaction energies of simplified catalyst and substrate structures, providing evidence for both CH-π and π-π transition state interactions as critical features.