Functionalization of fluorinated molecules by transition-metal-mediated C-F bond activation to access fluorinated building blocks

Mild [3 + 3] Annulation of (Trifluoromethyl)alkenes with Thioureas Enabled by Chemoselective Defluorinative Amination: Synthesis of 6-Fluoro-3,4-dihydropyrimidine-2(1H)-thiones

The chemoselective defluorinative [3 + 3] annulation of (trifluoromethyl)alkenes with thioureas is reported. This protocol affords various attractive 6-fluoro-3,4-dihydropyrimidine-2(1H)-thiones in high yields, features transition-metal free, mild conditions, efficient, is operationally simple and gram-scalable, tolerates diverse useful functional groups.

Electrosynthesis of Fluoroalkenes from Alpha-CF3 and Alpha-CF2H Benzyl Halides

We have developed an efficient synthesis of fluoroalkenes via tandem electrochemical dehalogenation-elimination protocol. The key step is the generation of carbon anion by electrochemical reductive dehalogenation of alkyl halides. Various gem-difluoroalkenes and monofluoroalkenes were prepared in moderate to good yields from α-difluoromethylated/α-trifluoromethylated benzyl halides.

Zirconium-mediated carbon-fluorine bond functionalisation through cyclohexyne "umpolung"

Polarity reversal, or "umpolung", is a widely acknowledged strategy to allow organic functional groups amenable to react in alternative ways to the usual preference set by their electronic features. In this article, we demonstrate that cyclohexyne umpolung, realized through complexation to zirconocene, makes the small strained cycloalkyne amenable to C-F bond functionalisation. Such strong bond activation chemistry is unprecedented in "free" aryne and strained alkyne chemistry. Our study also reveals that the reactivity of the Zr-cyclohexyne complex is highly sensitive to the degree of fluorination of the heteroarene. In addition, parasitic reactions of the ancillary ligand PMe3 were observed when pentafluoropyridine was the substrate.

Nickel-catalyzed cross-coupling of 2-fluorobenzofurans with arylboronic acids via aromatic C-F bond activation

2-Fluorobenzofurans underwent efficient nickel-catalyzed coupling with arylboronic acids through the activation of aromatic C-F bonds. This method allowed us to successfully synthesize a range of 2-arylbenzofurans with various substituents. The reaction, which proceeded under mild conditions, involved β-fluorine elimination from nickelacyclopropanes formed by the interaction of 2-fluorobenzofurans with zero-valent nickel species. This protocol facilitates orthogonal coupling reactions of aromatic C-F and C-Br bonds with arylboronic acids.

Defluorinative functionalization approach led by difluoromethyl anion chemistry

Organofluorine compounds have greatly benefited the pharmaceutical, agrochemical, and materials sectors. However, they are plagued by concerns associated with Per- and Polyfluoroalkyl Substances. Additionally, the widespread use of the trifluoromethyl group is facing imminent regulatory scrutiny. Defluorinative functionalization, which converts the trifluoromethyl to the difluoromethyl motifs, represents the most efficient synthetic strategy. However, general methods for robust C(sp3)-F bond transformations remain elusive due to challenges in selectivity and functional group tolerance. Here, we present a method for C(sp3)-F bond defluorinative functionalization of the trifluoromethyl group via difluoromethyl anion in flow. This new approach tames the reactive difluoromethyl anion, enabling diverse functional group transformations. Our methodology offers a versatile platform for drug and agrochemical discovery, overcoming the limitations associated with fluorinated motifs.

Photoredox-Catalyzed 1,4-Dichloromethyldimerization of Alkenes with Chloroform: Access to Polychlorinated Vicinal Diaryl Alkanes

A visible-light-mediated strategy is reported for the direct synthesis of polychlorinated vicinal diaryl alkanes from aryl alkenes and chloroform. In this approach, two haloalkyl radicals generated from chloroform via halogen atom transfer (XAT) and direct single electron transfer (SET) within the same photoredox catalysis cycle enable the 1,4-dichloromethyldimerization of alkenes. Besides chloroform, this strategy is applicable to carbon tetrachloride, bromotrichloromethane, and α-bromo carboxylic esters, yielding corresponding 1,4-disubstituted vicinal diaryl alkanes. Diverse polychlorinated structures containing highly congested vicinal quaternary carbon centers are effectively synthesized by this method. The potential of this reaction in late-stage drug modification is highlighted by the successful transformation of olefins with pharmaceutical structures.

Electrochemical destruction of PFAS at low oxidation potential enabled by CeO2 electrodes utilizing adsorption and activation strategies

The persistence and ecological impact of per- and poly-fluoroalkyl substances (PFAS) in water sources necessitate effective and energy-efficient treatment solutions. This study introduces a novel approach using cerium dioxide (CeO2) electrodes enhanced with oxygen vacancy (Ov) to catalyze the defluorination of PFAS. By leveraging the unique affinity between cerium and fluorine-containing species, our approach enables adsorptive preconcentration and catalytic degradation at low oxidation potentials (1.37 V vs. SHE). Demonstrating high removal and defluorination efficiencies of perfluorooctanoic acid (PFOA) at 94.0 % and 73.0 %, respectively, our approach also proves effective in the environmental matrix. It minimizes the impacts of co-existing natural organic matter and chloride ions, crucial benefits of operating at lower oxidation potentials. The role of Ov in CeO2 is validated by both experimental results and density functional theory modeling, demonstrating that these sites can activate the C-F bond and substantially reduce the energy barriers for defluorination. Consequently, our CeO2-based method not only achieves defluorination efficiencies comparable to more energy-intensive techniques but does so while requiring less than 0.62 kWh/m3 per order. This positions our approach as a promising, cost-effective alternative for the remediation of PFAS-contaminated waters, emphasizing its relevance and effectiveness in environmental remediation scenarios.

Catalytic Concerted SNAr Reactions of Fluoroarenes by an Organic Superbase

We herein propose that the catalytic concerted SNAr reaction is a powerful method to prepare functionalized aromatic scaffolds. Classic stepwise SNAr reactions involving addition/elimination processes require the use of electron-deficient aromatic halides to stabilize Meisenheimer intermediates, despite their widespread use in medicinal chemistry research. Recent efforts have been made to develop concerted SNAr reactions involving a single transition state, allowing the use of electron-rich substrates based on the use of stoichiometric amounts of strong bases or reactive nucleophiles. This study demonstrates that, without the use of such reagents, the organic superbase t-Bu-P4 efficiently catalyzes the concerted SNAr reactions of aryl fluorides regardless of their electronic nature. The key to establishing this system is the dual activation of aryl fluoride and anionic nucleophiles by the t-Bu-P4 catalyst. Furthermore, this catalysis allows excellent functional group tolerance, utilization of diverse nucleophiles, and late-stage functionalization of bioactive compound derivatives. These findings make possible diverse applications in chemical synthesis and pharmaceutical development.

Palladium-Catalyzed Double ipso-Defluoroetherification of Allylic gem-Difluorides with Phenols or Alcohols

Significant progress has been made in the synthesis of diverse ketals through a palladium-catalyzed reaction involving allylic gem-difluorides and various phenols or alcohols. This methodology facilitates double ipso-defluoroetherification of allylic gem-difluorides, resulting in high product yields with excellent regioselectivity. The reactions were conducted under mild conditions and exhibited outstanding tolerance to a wide range of functional groups. Notably, this approach overcomes the traditional limitation of palladium-catalyzed processes, where nucleophiles typically target the common C-1 position of allylic gem-difluorides, enabling selective functionalization at the rare C-3 position instead.

Pd-Catalyzed Site-Selective Defluorinative Etherification Between Unactivated Perfluoroarenes and Hydrobenzoxazoles

Polyfluoroaryl ethers represent an important framework of biologically active molecules and materials. Owing to the strong bond dissociation energy of C-F bond, selectivity and other issues, transition metal-catalyzed synthesis of polyfluoroaryl ethers from perfluoroarenes via the activation of C-F bond is challenging and underdeveloped, as compared to the well-documented C-O bond formation starting from aryl iodides, aryl bromides or aryl chlorides. Herein, an unprecedented Pd-catalyzed defluorinative etherification for the synthesis of polyfluoroaryl ether skeletons using hydrobenzoxazoles as phenol surrogate, has been reported. The substrate scope for this protocol is broad, with respect to hydrobenzoxazoles and perfluoroarenes, under mild reaction conditions. More importantly, challenging alkenyl and alkynyl substituted polyfluoroarenes could be successfully used as the cou-pling component for Pd-catalyzed etherification reaction. Density functional theory (DFT) calculations were employed to investigate the reaction mechanism, which suggested that oxidative addition between polyfluorobenzene and Pd(0) constituted the rate-determining step.

Assembly of functionalized gem-difluoroalkenes via photocatalytic defluorocyanoalkylation and defluoroacylation of α-CF3 styrenes with oxime esters

We report an efficient photocatalytic protocol for the defluorocyanoalkylation and defluoroacylation of α-trifluoromethyl styrenes by utilizing oxime esters as radical donors, allowing for the preparation of diverse gem-difluoroalkenes. The treatment of α-trifluoromethyl styrenes with cyclobutanone oxime esters led to the formation of distal cyano group-anchored gem-difluoroalkenes. Notably, adding K2CO3 as an inorganic base to the photocatalytic system afforded γ,γ-difluoroallylic ketones by utilizing acyl oxime esters as the acylating agents. Preliminary mechanistic investigations into this reaction pathway revealed the involvement of single-electron reduction, C-C bond cleavage initiated by iminyl radicals, radical addition, and β-fluoride elimination steps.

Transition-Metal-Free Late-Stage Decarboxylative gem-Difluoroallylation of Primary Alkyl Acids

A transition-metal-free late-stage decarboxylative gem-difluoroallylation of carboxylic acids with α-trifluoromethyl alkenes has been described by the use of organo-photoredox catalysis. Both primary alkyl and heteroaryl acids were readily incorporated. This approach merits feedstock materials, mild reaction conditions, and wide functionality tolerance. The synthetic utility of this approach has been highlighted by the late-stage functionalization of a variety of acid-containing natural products and drug molecules.

Defluorinative Haloalkylation of Unactivated Alkenes Enabled by Dual Photoredox and Copper Catalysis

A three-component defluorinative haloalkylation of alkenes with trifluoromethyl compounds and TBAX (X = Cl, Br) via dual photoredox/copper catalysis is reported. The mild conditions are compatible with a wide array of activated trifluoromethyl aromatics bearing diverse substituents, and various nonactivated terminal and internal alkenes, enabling straightforward access to synthetically valuable γ-gem-difluoroalkyl halides with high efficiency. Mechanistic studies indicate that the [Cu] complexes not only serve as XAT catalysts but also facilitate the SET reduction of trifluoromethyl groups by photocatalysts. Additionally, the resulting alkyl halide products can serve as versatile conversion intermediates for the synthesis of a diverse range of γ-gem-difluoroalkyl compounds.

Photocatalytic Defluorinative α-Aminoalkylation of Allylic Difluorides

A photocatalytic process was devised to synthesize monofluoroalkenes via defluorinative functionalization of allylic difluorides. N-Alkylanilines are used as precursors to α-aminoalkyl radicals, which undergo regioselective addition to allylic difluorides, and subsequent SET and fluoride elimination produce monofluoroalkenes. C-C bond formation on the aniline is site-selective for the least substituted carbon α to nitrogen.

Base-Promoted Nucleophilic Phosphorylation of Benzyl Fluorides via C(sp3)-F Cleavage

Herein, a transition-metal-free phosphorylation of benzyl fluorides with P(O)-H compounds is disclosed. In the presence of tBuOK, various benzyl fluorides react with P(O)-H compounds to produce the corresponding benzyl phosphine oxides, phosphinates, and phosphonates in good to high yields. This base-promoted phosphorylation reaction offers a facile and general strategy for the construction of a C(sp3)-P bond.

Chemoselective Thioacylation of Amines Enabled by Synergistic Defluorinative Coupling

A mild and chemoselective method for the thioacylation of amines, including amino acids and peptides, using gem-difluoroalkenes and sulfide, is reported. The distinguishing of the different nucleophilic sites (S-site and diverse N-sites) by the chemoselective C-F bond functionalization of gem-difluoroalkenes enables the unique synergistic defluorinative coupling reaction. This reaction features mild conditions, is operationally simple, efficient, and gram-scalable, tolerates various functional groups, and is activator-free and without racemization. Thioamide moieties were incorporated site-specifically into bioactive compounds. The proposed mechanism is illustrated by a DFT calculation.

Stereodivergent Palladium-Catalyzed C-F Bond Functionalization of gem-Difluoroalkenes

We herein describe a stereodivergent C-F bond functionalization of gem-difluoroalkenes. Using trisubstituted β,β-difluoroacrylates, both E and Z monofluoroalkene products can be obtained with excellent diastereoselectivities. The design of two different reaction manifolds, i.e., Pd(II)- versus Pd(0)-catalyzed cross-coupling of boronic acids, is the key to stereocontrol.

Borane-Catalyzed Coupling of Diazooxindoles and Difluoroenoxysilanes to Tetrasubstituted Monofluoroalkenes

A highly stereoselective coupling reaction of diazooxindoles with difluoroenoxysilanes catalyzed by Lewis acidic boranes has been developed. The reaction proceeded at ambient temperature under transition metal-free conditions with wide functional group tolerance. By using this simple procedure, a series of tetrasubstituted monofluoroalkenes can be accessed in good yield with high selectivity.

C-H and C-F bond activation of fluorinated propenes at Rh: enabling cross-coupling reactions with outer-sphere C-C coupling

The reaction of [Rh{(E)-CF[double bond, length as m-dash]CHCF3}(PEt3)3] with Zn(CH3)2 results in the methylation of the alkenyl ligand to give [Rh{(E/Z)-C(CH3)[double bond, length as m-dash]CHCF3}(PEt3)3]. Variable temperature NMR studies allowed the identification of a heterobinuclear rhodium-zinc complex as an intermediate, for which the structure [Rh(CH3)(ZnCH3){(Z)-C(CH3)[double bond, length as m-dash]CHCF3}(PEt3)2] is proposed. Based on these stoichiometric reactions, unique Negishi-type catalytic cross-coupling reactions of fluorinated propenes by consecutive C-H and C-F bond activation steps at room temperature were developed. The C-H bond activation steps provide a fluorinated ligand at Rh and deliver the fluorinated product, whereas the C-F bond activation and C-C coupling occur via outer-sphere nucleophilic attack at the fluorinated alkenyl ligand.

Photoredox Catalyzed Synthesis of gem-Difluoroalkenes and Monofluorinated Cyclooctenes via 1,5-HAT Process

gem-Difluoroalkenes and monofluorinated cycloalkenes have emerged as basic structural units in a variety of bioactive molecules and natural products. Thus, developing straightforward and efficient methods for synthesizing fluorinated alkene compounds is of considerable significance. Herein, we disclose a visible-light-induced defluorination of 2-trifluoromethyl-1-alkene via a 1,5-HAT process using N-alkoxyphtalimides as both radical precursor and potential nucleophile. The mild and stepwise reaction leads to a variety of structurally diverse gem-difluoroalkenes and monofluorinated cyclooctenes with high efficiency, respectively.

Unveiling the Selectivity of Ru(II)-Catalyzed C-H Activation for Defluorinative Cyclization of 2-Arylbenzimidazole and Trifluoromethyl Diazo: A DFT Study

The synthesis of monofluorinated heterocyclic compounds by C-H activation combined with defluorination is useful. Studies on the reaction mechanism and selectivity have shown that these processes play a positive role in promoting the development of monofluorinated reactions. Density functional theory (DFT) calculations were performed to investigate the mechanism and selectivity of Ru(II)-catalyzed 2-arylbenzimidazole with trifluoromethyl diazo. DFT calculations showed that C-H activation occurs through a concerted metalation/deprotonation (CMD) mechanism. After that, deprotonation and defluorinative cyclization are assisted by acetate and trifluoroethanol (TFE). Further mechanistic insights through noncovalent interaction (NCI) analysis were also obtained to elucidate the origin of the selectivity in the defluorination process.

Rhodium-Catalyzed Enantio- and Regioselective Allylation of Indoles with gem-Difluorinated Cyclopropanes

The use of gem-difluorinated cyclopropanes (gem-DFCPs) as fluoroallyl surrogates under transition-metal catalysis has drawn considerable attention recently but such reactions are restricted to producing achiral or racemic mono-fluoroalkenes. Herein, we report the first enantioselective allylation of indoles under rhodium catalysis with gem-DFCPs. This reaction shows exceptional branched regioselectivity towards rhodium catalysis with gem-DFCPs, which provides an efficient route to enantioenriched fluoroallylated indoles with wide substrate scope and good functional group tolerance.

Regio- and Stereoselective Hexafluoroisopropoxylation and Trifluoroethoxylation of Allenamides

Incorporating fluorinated moieties into organic molecules is an attractive strategy to enhance drug-like properties. Herein, we have developed a simple and self-promoted protocol for hexafluoroisopropoxylation and trifluoroethoxylation of allenamides with fluorinated alcohols such as HFIP and TFE. The reaction provided the fluoroalkoxylated products in a regio- and stereoselective manner in good to moderate yields under mild conditions.

Radical reactions enabled by polyfluoroaryl fragments: photocatalysis and beyond

Polyfluoroarenes have been known for a long time, but they are most often used as fluorinated building blocks for the synthesis of aromatic compounds. At the same time, due to peculiar fluorine effect, they have unique properties that provide applications in various fields ranging from synthesis to materials science. This review summarizes advances in the radical chemistry of polyfluoroarenes, which have become possible mainly with the advent of photocatalysis. Transformations of the fluorinated ring via the C-F bond activation, as well as use of fluoroaryl fragments as activating groups and hydrogen atom transfer agents are discussed. The ability of fluoroarenes to serve as catalysts is also considred.

Palladium-Catalyzed Defluorinative Alkylation of gem-Difluoroalkenes with Cyclopropanols: Stereoselective Synthesis of γ-Fluorinated γ,δ-Unsaturated Ketones

A palladium-catalyzed defluorinative alkylation of gem-difluoroalkenes with cyclopropyl alcohols was developed. A range of γ-fluorinated γ,δ-unsaturated ketones were constructed in good yields with excellent stereoselectivities. In addition, by base-mediated intramolecular nucleophilic vinylic substitution (SNV), the products could be further transformed to 2,5-dimethylenetetrahydrofurans and analogues with excellent stereoselectivities.

Controllable chemical redox reactions to couple microbial degradation for organic contaminated sites remediation: A review

Global environmental concern over organic contaminated sites has been progressively conspicuous during the process of urbanization and industrial restructuring. While traditional physical or chemical remediation technologies may significantly destroy the soil structure and function, coupling moderate chemical degradation with microbial remediation becomes a potential way for the green, economic, and efficient remediation of contaminated sites. Hence, this work systematically elucidates why and how to couple chemical technology with microbial remediation, mainly focused on the controllable redox reactions of organic contaminants. The rational design of materials structure, selective generation of reactive oxygen species, and estimation of degradation pathway are described for chemical oxidation. Meanwhile, current progress on efficient and selective reductions of organic contaminants (i.e., dechlorination, defluorination, -NO2 reduction) is introduced. Combined with the microbial remediation of contaminated sites, several consideration factors of how to couple chemical and microbial remediation are proposed based on both fundamental and practical points of view. This review will advance the understanding and development of chemical-microbial coupled remediation for organic contaminated sites.

Applications of Transition Metal-Catalyzed ortho-Fluorine-Directed C-H Functionalization of (Poly)fluoroarenes in Organic Synthesis

The synthesis of organic compounds efficiently via fewer steps but in higher yields is desirable as this reduces energy and reagent use, waste production, and thus environmental impact as well as cost. The reactivity of C-H bonds ortho to fluorine substituents in (poly)fluoroarenes with metal centers is enhanced relative to meta and para positions. Thus, direct C-H functionalization of (poly)fluoroarenes without prefunctionalization is becoming a significant area of research in organic chemistry. Novel and selective methodologies to functionalize (poly)fluorinated arenes by taking advantage of the reactivity of C-H bonds ortho to C-F bonds are continuously being developed. This review summarizes the reasons for the enhanced reactivity and the consequent developments in the synthesis of valuable (poly)fluoroarene-containing organic compounds.

Palladium-catalyzed alkynylation of allylic gem-difluorides

Herein, a palladium-catalyzed regioselective alkynylation, esterification, and amination of allylic gem-difluorides via C-F bond activation/transmetallation/β-C elimination or nucleophilic attack has been achieved. This innovative protocol showcases an extensive substrate range and operates efficiently under mild reaction conditions, resulting in high product yields and Z-selectivity. Particularly noteworthy is its exceptional tolerance towards a wide array of functional groups. This developed methodology provides effective and convenient routes to access a diverse array of essential fluorinated enynes, esters and amines.

Pd-Catalyzed Oxidative C-H Arylation of (Poly)fluoroarenes with Aryl Pinacol Boronates and Experimental and Theoretical Studies of its Reaction Mechanism

We report the synergistic combination of Pd(OAc)2 and Ag2O for the oxidative C-H arylation of (poly)fluoroarenes with aryl pinacol boronates (Ar-Bpin) in DMF as the solvent. This procedure can be conducted easily in air, and without using additional ligands, to afford the fluorinated unsymmetrical biaryl products in up to 98 % yield. Experimental studies suggest that the formation of [PdL2(C6F5)2] in DMF as coordinating solvent does not take place under the reaction conditions as it is stable to reductive elimination and thus would deactivate the catalyst. Thus, the intermediate [Pd(DMF)2(ArF)(Ar)] must be formed selectively to give desired arylation products. DFT calculations predict a low barrier (5.87 kcal/mol) for the concerted metalation deprotonation (CMD) process between C6F5H and the Pd(II) species formed after transmetalation between the Pd(II)X2 complex and aryl-Bpin which forms a Pd-Arrich species. Thus a Pd(Arrich)(Arpoor) complex is generated selectively which undergoes reductive elimination to generate the unsymmetrical biaryl product.

Stoichiometric and Catalytic Lithium Nickelate-Mediated C-F Bond Alkynylation of Fluoroarenes

Low-valent nickelates have recently been shown to be key intermediates that facilitate challenging cross-coupling reactions under mild conditions. Expanding the synthetic potential of these heterobimetallic complexes, herein we report the success of trilithium nickelate Li3(TMEDA)3Ni(C≡C-Ph)3 in promoting stoichiometric C-F activation of assorted aryl fluorides furnishing novel mixed Li/Ni(0) or Li/Ni(II) species depending on the substrate and conditions employed. These stoichiometric successes can be upgraded to catalytic regimes to enable the atom-efficient alkynylation of aryl fluorides and polyfluoroarenes with lithium acetylides and precatalyst Ni(COD)2, which operates without the intervention of external ligands, Cu cocatalysts, or additives.

Photouranium-Catalyzed C-F Activation Hydroxylation via Water Splitting

The C-F bond is the strongest covalent single bond (126 kcal/mol) in carbon-centered bonds, in which the highest electronegativity of fluorine (χ = 4) gives rise to the shortest bond length (1.38 Å) and the smallest van der Waals radius (rw = 1.47 Å), resulting in enormous challenges for activation and transformation. Herein, C-F conversion was realized via photouranium-catalyzed hydroxylation of unactivated aryl fluorides using water as a hydroxyl source to deliver multifunctional phenols under ambient conditions. The activation featured cascade sequences of single electron transfer (SET)/hydrogen atom transfer (HAT)/oxygen atom transfer (OAT), highly integrated from the excited uranyl cation. The *UO22+ prompted water splitting under mild photoexcitation, caging the active oxygen in a peroxo-bridged manner for the critical OAT process and releasing hydrogen via the HAT process.

C-F Bond Insertion: An Emerging Strategy for Constructing Fluorinated Molecules

C-F Insertion reactions, where an organic fragment formally inserts into a carbon-fluorine bond in a substrate, are highly attractive, yet largely unexplored, methods to prepare valuable fluorinated molecules. The inherent strength of C-F bonds and the resulting need for a large thermodynamic driving force to initiate C-F cleavage often leads to sequestering of the released fluoride in an unreactive by-product. Recently, however, several groups have succeeded in overcoming this challenge, opening up the study of C-F insertion as an efficient and highly atom-economical approach to prepare fluorinated compounds. In this article, the recent breakthroughs are discussed focusing on the key conceptual advances that allowed for both C-F bond cleavage and subsequent incorporation of the released fluoride into the product.

Cross-coupling of organic fluorides with allenes: a silyl-radical-relay pathway for the construction of α-alkynyl-substituted all-carbon quaternary centres

Controlling the transformation of versatile and reactive allenes is a considerable challenge. Herein, we report an efficient silylboronate-mediated cross-coupling reaction of organic fluorides with allenes to construct a series of sterically demanding α-ethynyl-containing all-carbon quaternary centers (ACQCs), using catalyst-free silyl-radical-relay reactions to selectively functionalize highly inert C-F bonds in organic fluorides. The key to the success of this transformation lies in the radical rearrangement of an in situ-generated allenyl radical to form a bulky tertiary propargyl radical; however, the transformation does not show efficiency when using the propargyl isomer directly. This unique reaction enables the cross-coupling of a tertiary carbon radical center with a C(sp2)-F bond or a benzylic C(sp3)-F bond. α-Ethynyl-containing ACQCs with (hetero)aromatic substituents and benzyl were efficiently synthesized in a single step using electronically and sterically diverse organic fluorides and allenes. The practical utility of this protocol is showcased by the late-stage functionalization of bioactive molecules and the modification of a liquid crystalline material.

Picking Two out of Three: Defluorinative Annulation of Trifluoromethyl Alkenes for the Synthesis of Monofluorinated Carbo- and Heterocycles

The increasing demand of organofluorine compounds in medicine, agriculture, and materials sciences makes sophisticated methods for their synthesis ever more necessary. Nowadays, not only the C-F bond formation but also the selective C-F bond cleavage of readily available poly- or perfluorine-containing compounds have become powerful tools for the effective synthesis of organofluorine compounds. The defluorinative cross-coupling of trifluoromethyl alkenes with various nucleophiles or radical precursors in an SN 2' manner is a convergent route to access gem-difluoroalkenes, which in turn react with nucleophiles or radical precursors via an SN V-type reaction. If the SN V reactions occur intramolecularly, the dual C-F bond cleavage of trifluoromethyl alkenes allows facile assembly of monofluorinated cyclic skeletons with structural complexity and diversity. In this personal account, we summarized the advances in this field on the basis of coupling and cyclization partners, including binucleophiles, alkynes, diradical precursors and radical precursors bearing a nucleophilic site. Accordingly, the annulation reactions can be achieved by base-mediated sequential SN 2'/SN V reactions, transition metal catalyzed or mediated reactions, photoredox catalysis, and the combination of photocatalytic reactions with SN V reaction. In the context of seminal works of others in this field, a concise summary of the contributions of the authors is also offered.

Nickel-catalysed chelation-assisted reductive defluorinative sulfenylation of trifluoropropionic acid derivatives

Herein we reported a directing-group assisted strategy for nickel-catalysed reductive defluorinative sulfenylation of trifluoropropionic acid derivatives with disulfides in the presence of Zn, involving triple C-F bond cleavage. This process yielded a diverse array of carbonyl-sulfide di-substituted alkenes in moderate to good yields with good functional group tolerance. Specifically, the reactions exhibited high E-selectivity with E/Z ratio up to >99 : 1.

Enantioselective nickel-catalyzed Mizoroki-Heck cyclizations of amide electrophiles

Amide cross-couplings that rely on C-N bond activation by transition metal catalysts have emerged as valuable synthetic tools. Despite numerous discoveries in this field, no catalytic asymmetric variants have been disclosed to date. Herein, we demonstrate the first such transformation, which is the Mizoroki-Heck cyclization of amide substrates using asymmetric nickel catalysis. This proof-of-concept study provides an entryway to complex enantioenriched polycyclic scaffolds and advances the field of amide C-N bond activation chemistry.

Metallomimetic C-F Activation Catalysis by Simple Phosphines

Delivering metallomimetic reactivity from simple p-block compounds is highly desirable in the search to replace expensive, scarce precious metals by cheap and abundant elements in catalysis. This contribution demonstrates that metallomimetic catalysis, involving facile redox cycling between the P(III) and P(V) oxidation states, is possible using only simple, cheap, and readily available trialkylphosphines without the need to enforce unusual geometries at phosphorus or use external oxidizing/reducing agents. Hydrodefluorination and aminodefluorination of a range of fluoroarenes was realized with good to very good yields under mild conditions. Experimental and computational mechanistic studies show that the phosphines undergo oxidative addition of the fluoroaromatic substrate via a Meisenheimer-like transition state to form a fluorophosphorane. This undergoes a pseudotransmetalation step with a silane, via initial fluoride transfer from P to Si, to give experimentally observed phosphonium ions. Hydride transfer from a hydridosilicate counterion then leads to a hydridophosphorane, which undergoes reductive elimination of the product to reform the phosphine catalyst. This behavior is analogous to many classical transition-metal-catalyzed reactions and so is a rare example of both functional and mechanistically metallomimetic behavior in catalysis by a main-group element system. Crucially, the reagents used are cheap, readily available commercially, and easy to handle, making these reactions a realistic prospect in a wide range of academic and industrial settings.

Enzyme Engineering Renders Chlorinase the Activity of Fluorinase

Organofluorine compounds have attracted substantial attention owing to their wide application in agrochemistry. Fluorinase (FlA) is a unique enzyme in nature that can incorporate fluorine into an organic molecule. Chlorinase (SalL) has a similar mechanism as fluorinase and can use chloride but not fluoride as a substrate to generate 5'-chloro-deoxyadenosine (5'-ClDA) from S-adenosyl-l-methionine (SAM). Therefore, identifying the features that lead to this selectivity for halide ions is highly important. Here, we engineered SalL to gain the function of FlA. We found that residue Tyr70 plays a key role in this conversion through alanine scanning. Site-saturation mutagenesis experiments demonstrated that Y70A/C/S/T/G all exhibited obvious fluorinase activity. The G131S mutant of SalL, in which the previously thought crucial residue Ser158 for fluoride binding in FlA was introduced, did not exhibit fluorination activity. Compared with the Y70T single mutant, the double mutant Y70T/W129F increased 5'-fluoro-5-deoxyadenosine (5'-FDA) production by 76%. The quantum mechanics (QM)/molecular mechanics (MM) calculations suggested that the lower energy barriers and shorter nucleophilic distance from F- to SAM in the mutants than in the SalL wild-type may contribute to the activity. Therefore, our study not only renders SalL the activity of FlA but also sheds light on the enzyme selectivity between fluoride versus chloride.

Photoinduced Dual C-F Bond Activation of Hexafluorobenzene Mediated by Boron Atom

The reaction of laser-ablated boron atoms with hexafluorobenzene (C6 F6 ) was investigated in neon and argon matrices, and the products are identified by matrix isolation infrared spectroscopy and quantum-chemical calculations. The reaction is triggered by a boron atom insertion into one C-F bond of hexafluorobenzene on annealing, forming a fluoropentafluorophenyl boryl radical (A). UV-Vis light irradiation of fluoropentafluorophenyl boryl radical causes generation of a 2-difluoroboryl-tetrafluorophenyl radical (B) via a second C-F bond activation. A perfluoroborepinyl radical (C) is also observed upon deposition and under UV-Vis light irradiation. This finding reveals the new example of a dual C-F bond activation of hexafluorobenzene mediated by a nonmetal and provides a possible route for synthesis of new perfluorinated organo-boron compounds.

Copper-Catalyzed Protoarylation of gem-Difluoroallenes

A copper-catalyzed protoarylation of gem-difluoroallenes with aryl boronic esters has been developed, enabling highly regioselective synthesis of gem-difluoroalkenes in high yields. The mild reaction conditions allow for a variety of functional groups to be tolerated, and the reaction can be extended to protoalkenylation of gem-difluoroallenes. The synthetic utility of this method has been demonstrated in gram-scale operation as well as synthesis of chiral gem-difluoroalkenes bearing γ-carbon stereogenic centers in moderate enantioselectivity using a chiral bidentate phosphine ligand with a copper catalyst.

Palladium-Catalyzed Diversified Synthesis of Monofluorinated Alkenes from Allylic gem-Difluorides through Pd-OH Intermediate

Significant advancements in synthesis of monofluoroalkenes via palladium-catalyzed reactions involving allylic gem-difluorides and diverse nucleophiles have been achieved. This method allows regioselective arylation, alkylation, allylation, alkenylation, and hydrogenation of allylic gem-difluorides, yielding high Z-selectivity and favorable product yields under mild conditions. Tolerating various functional groups, these transformations utilize a common Pd-OH intermediate. Additionally, employing triple Pd-catalyzed cross-coupling yields diverse trisubstituted alkenes efficiently.

Selective Defluoroalkylation and Hydrodefluorination of Trifluoromethyl Groups Photocatalyzed by Dihydroacridine Derivatives

The selective functionalization of trifluoromethyl groups through C-F cleavage poses a significant challenge due to the high bond energy of the C(sp3)-F bonds. Herein, we present dihydroacridine derivatives as photocatalysts that can functionalize the C-F bond of trifluoromethyl groups with various alkenes under mild conditions. Mechanistic studies and DFT calculations revealed that upon irradiation, the dihydroacridine derivatives exhibit high reducibility and function as photocatalysts for reductive defluorination. This process involves a sequential single-electron transfer mechanism. This research provides valuable insights into the properties of dihydroacridine derivatives as photocatalysts, highlighting the importance of maintaining a planar conformation and a large conjugated system for optimal catalytic activity. These findings facilitate the efficient catalytic reduction of inert chemical bonds.

Synthetic Advantages of Defluorinative C-F Bond Functionalization

Much progress has been made in the development of methods to both create compounds that contain C-F bonds and to functionalize C-F bonds. As such, C-F bonds are becoming common and versatile synthetic functional handles. This review summarizes the advantages of defluorinative functionalization reactions for small molecule synthesis. The coverage is organized by the type of carbon framework the fluorine is attached to for mono- and polyfluorinated motifs. The main challenges, opportunities and advances of defluorinative functionalization are discussed for each class of organofluorine. Most of the text focuses on case studies that illustrate how defluorofunctionalization can improve routes to synthetic targets or how the properties of C-F bonds enable unique mechanisms and reactions. The broader goal is to showcase the opportunities for incorporating and exploiting C-F bonds in the design of synthetic routes, improvement of specific reactions and advent of new methods.

Defluorinative 1,3-Dienylation of Fluoroalkyl N-Triftosylhydrazones with Homoallenols

A silver-catalyzed regioselective defluorinative 1,3-dienylation of trifluoromethyl phenyl N-triftosylhydrazones using homoallenols as 1,3-dienyl sources provides a variety of α-(di)fluoro-β-vinyl allyl ketones with excellent functional group tolerance in moderate to good yields. The reaction proceeds through a silver carbene-initiated sequential etherification and Claisen type [3,3]-sigmatropic rearrangement cascade. The synthetic utility of this protocol was demonstrated through the downstream synthetic elaboration toward diverse synthetically useful building blocks.

Anion-Dependent Reactivity of Mono- and Dinuclear Boron Cations

The dinuclear bis(N-heterocyclic carbene) borane adduct 2 rapidly reacts with tritylium salts at room temperature but the outcome is strongly impacted by the respective counter-ion. Using tritylium tetrakis(perfluoro-tert-butoxy)aluminate affords - depending on the solvent - either the bis(boronium) ion 4 or the hydride-bridged dication 5. In case of tritylium hexafluorophosphate, however, H/F exchange occurs between boron and phosphorus yielding the dinuclear BF3 adduct 3 along with phosphorus dihydride trifluoride. H/F exchange also takes place when using the mononuclear N-heterocyclic carbene BH3 adduct 6 and hence provides a facile route to PH2 F3 , which is usually synthesized in more complex reaction sequences regularly involving toxic hydrogen fluoride. DFT calculations shed light on the H/F exchange between the borenium ion and the [PF6 ]- counter-ion and the computed mechanism features only small barriers in line with the experimental observations.

Bond-Forming Processes Enabled by Silicon-Masked Aryl- and Alkyl-Substituted Diazenes

Aryl- and alkyldiimides (R-N═NH with R = aryl or alkyl) are elusive intermediates of valuable synthetic use, as they are assumed to be transient species in processes involving both carbon (with concomitant loss of N2) and nitrogen nucleophiles (with conservation of the N═N moiety). The actual compounds are fragile and as such not bench stable which is why they have not yet found the attention they deserve. Conversely, early contributions showed that the stability of the parent diimide is significantly increased by replacing the hydrogen atom by a silyl group, but the synthetic applicability of these silicon-protected aryl- and alkyldiazenes has been far less explored, in part due to the absence of general procedures for their preparation. This Perspective provides an overview of the underexplored diazene chemistry that has witnessed considerable progress in recent years and highlights the potential of this motif in a range of synthetically useful (catalytic) transformations. The rediscovered silicon-masked diazenes constitute a versatile platform possessing enhanced stability and tamed reactivity in comparison to the parent hydrogen-substituted diimides. Aryl, diazenyl, and alkyl anionic key intermediates can be selectively generated in situ under Lewis base or transition metal catalysis, giving rise to novel synthetic approaches as viable alternatives to the already existing methodologies.

Anion-Driven C-F Bond Activation of Trifluoromethyl N-Aryl Hydrazones: Application to the Synthesis of 1,3,4-Oxadiazoles

The CF3 group attached to N-aryl hydrazone could be activated upon treatment with a suitable base, thus serving as an excellent C1 unit for the assembly of a series of 1,3,4-oxadiazoles by reaction with hydrazides. The transformation is proposed to proceed via the intermediate formation of a gem-difluorinated azoalkene. Furthermore, this reaction features simple conditions and a broad substrate scope with respect to both trifluoromethyl N-aryl hydrazones and hydrazides.

Main group metal-mediated strategies for C-H and C-F bond activation and functionalisation of fluoroarenes

With fluoroaromatic compounds increasingly employed as scaffolds in agrochemicals and active pharmaceutical ingredients, the development of methods which facilitate regioselective functionalisation of their C-H and C-F bonds is a frontier of modern synthesis. Along with classical lithiation and nucleophilic aromatic substitution protocols, the vast majority of research efforts have focused on transition metal-mediated transformations enabled by the redox versatilities of these systems. Breaking new ground in this area, recent advances in main group metal chemistry have delineated unique ways in which s-block, Al, Ga and Zn metal complexes can activate this important type of fluorinated molecule. Underpinned by chemical cooperativity, these advances include either the use of heterobimetallic complexes where the combined effect of two metals within a single ligand set enables regioselective low polarity C-H metalation; or the use of novel low valent main group metal complexes supported by special stabilising ligands to induce C-F bond activations. Merging these two different approaches, this Perspective provides an overview of the emerging concept of main-group metal mediated C-H/C-F functionalisation of fluoroarenes. Showcasing the untapped potential that these systems can offer in these processes; focus is placed on how special chemical cooperation is established and how the trapping of key reaction intermediates can inform mechanistic understanding.

Photocatalytic redox-neutral selective single C(sp3)-F bond activation of perfluoroalkyl iminosulfides with alkenes and water

Visible-light-promoted site-selective and direct C-F bond functionalization of polyfluorinated iminosulfides was accomplished with alkenes and water under redox-neutral conditions, affording a diverse array of γ-lactams with a fluoro- and perfluoroalkyl-substituted carbon centre. A variety of perfluoroalkyl units, including C2F5, C3F7, C4F9, and C5F11 underwent site-selective defluorofunctionalization. This protocol allows high chemoselectivity control and shows excellent functional group tolerance. Mechanistic studies reveal that the remarkable changes of the electron geometries during the defluorination widen the redox window between the substrates and the products and ensure the chemoselectivity of single C(sp3)-F bond cleavage.

Palladium-Catalyzed Fluorinative Bifunctionalization of Aziridines and Azetidines with gem-Difluorocyclopropanes

An unprecedented Pd-catalyzed fluorinative bifunctionalization of aziridines and azetidines was successfully developed via regioselective C-C and C-F bond cleavage of gem-difluorocyclopropanes, leading to various β,β'-bisfluorinated amines and β,γ-bisfluorinated amines. This reaction was achieved by incorporating a 2-fluorinated allyl group and a fluorine atom scissored from gem-difluorocyclopropane in 100 % atom economy for the first time. The mechanistic investigations indicated that the reaction underwent amine attacking 2-fluorinated allyl palladium complex to generate η2 -coordinated N-allyl aziridine followed by fluoride ligand transfer affording the final β- and γ-fluorinated amines.