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

HFIP-Assisted Single C-F Bond Activation of Trifluoromethyl Ketones using Visible-Light Photoredox Catalysis

A visible light photoredox catalytic method for the selective cleavage of single strong C-F bond in trifluoromethyl ketones is reported. Single electron reduction of trifluoromethyl ketones generates difluoromethyl radicals which can be engaged in intermolecular C-C bond formation with N-methyl-N-arylmethacrylamides to furnish fluorine-containing oxindole derivatives in good yields. The reaction shows excellent chemoselectivity with good functional group tolerance under mild conditions. 1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) as a solvent plays a critical role for the selective single C-F bond cleavage. High-level DFT calculations are depicted to shed light on the mechanism.

Recent Advances in C-F Bond Cleavage Enabled by Visible Light Photoredox Catalysis

The creation of new bonds via C-F bond cleavage of readily available per- or oligofluorinated compounds has received growing interest. Using such a strategy, a myriad of valuable partially fluorinated products can be prepared, which otherwise are difficult to make by the conventional C-F bond formation methods. Visible light photoredox catalysis has been proven as an important and powerful tool for defluorinative reactions due to its mild, easy to handle, and environmentally benign characteristics. Compared to the classical C-F activation that proceeds via two-electron processes, radicals are the key intermediates using visible light photoredox catalysis, providing new modes for the cleavage of C-F bonds. In this review, a summary of the visible light-promoted C-F bond cleavage since 2018 was presented. The contents were classified by the fluorosubstrates, including polyfluorinated arenes, gem-difluoroalkenes, trifluoromethyl arenes, and trifluoromethyl alkenes. An emphasis is placed on the discussion of the mechanisms and limitations of these reactions. Finally, my personal perspective on the future development of this rapidly emerging field was provided.

Recent advances in radical-based C-F bond activation of polyfluoroarenes and gem-difluoroalkenes

The direct employment of polyfluoroarenes and gem-difluoroalkenes as building blocks is regarded as one of the most effective and straightforward strategies for the introduction of fluorine-containing moieties into organic skeletons. Accordingly, radical chemistry has gradually become a mild and powerful method for the activation of their C-F bonds. The radical-based transformations of polyfluoroarenes and gem-difluoroalkenes can be primarily categorized into three types based on the specific intermediates involved: (1) multifluoroaryl radical anions, (2) monofluoroalkenyl radicals and (3) other radicals. Compared with the more established multifluoroaryl radical anion pathway, the monofluoroalkenyl radical-involved cross-coupling reaction can proceed through C-radical cross-coupling, radical addition/elimination or the hydrogen atom transfer process. For the presented examples in this review, the typical reaction modes, substrate scope, radical-involved mechanisms, and late-stage applications in the modification of bioactive molecules are discussed, aiming to provide a comprehensive overview of the recent advances of the radical-based transformations of polyfluoroarenes and gem-difluoroalkenes.

Silylboronate-Mediated Defluorosilylation of Aryl Fluorides with or without Ni-Catalyst

The defluorosilylation of aryl fluorides to access aryl silanes was achieved under transition-metal-free conditions via an inert C-F bond activation. The defluorosilylation, mediated by silylboronates and KOtBu, proceeded smoothly at room temperature to afford various aryl silanes in good yields. Although a comparative experiment indicated that Ni catalyst facilitated this transformation more efficiently, the transition-metal-free protocol is advantageous from a green chemistry perspective.

Advances in silylation and borylation of fluoroarenes and gem-difluoroalkenes via C-F bond cleavage

Organoboron and organosilane compounds are widely used in organic synthesis and pharmaceuticals. In addition, the C-F bond functionalization is a useful tool for the construction of carbon-carbon and carbon-heteroatom bonds. In particular, the late-stage functionalization of bioactive molecules through defluoroborylation and defluorosilylation reactions will provide good opportunities for the development and diversification of new medicinal compounds. Thus, this feature article summarized the methods for the defluorosilylation and defluoroborylation of unreactive monofluoroarenes and gem-difluoroalkenes from 2000 to 2021, which might create some new ideas and will be helpful for further research in this field. These defluoroborylation and defluorosilylation strategies can be applied to synthesize silylated arenes, borylated arenes, silylated fluoroalkenes, and borylated fluoroalkenes, thus providing impressive advantages over traditional methods for the synthesis of organoboron and organosilane compounds in terms of divergent structures.

Electroreductive Cross-Coupling of Trifluoromethyl Alkenes and Redox Active Esters for the Synthesis of Gem-Difluoroalkenes

An electroreductive access to gem-difluoroalkenes has been developed through the decarboxylative/defluorinative coupling of N-hydroxyphtalimides esters and α-trifluoromethyl alkenes. The electrolysis is performed under very simple reaction conditions in an undivided cell using cheap carbon graphite electrodes. This metal-free transformation features broad scope with good to excellent yields. Tertiary, secondary as well as primary alkyl radicals could be easily introduced. α-aminoacids L-aspartic and L-glutamic acid-derived redox active esters were good reactive partners furnishing potentially relevant gem-difluoroalkenes. In addition, it has been demonstrated that our electrosynthetic approach toward the synthesis of gem-difluoroalkenes could use an easily prepared Kratitsky salt as alkyl radical precursor via a deaminative/defluorinative carbofunctionalization of trifluoromethylstyrene.

Photocatalytic Decarboxylative Coupling of Aliphatic N-hydroxyphthalimide Esters with Polyfluoroaryl Nucleophiles

Polyfluoroarenes are an important class of compounds in medical and material chemistry. The synthesis of alkylated polyfluoroarenes remains challenging. Here we describe a decarboxylative coupling reaction of N-hydroxyphthalimide esters of aliphatic carboxylic acids with polyfluoroaryl zinc reagents (Zn-ArF ) via synergetic photoredox and copper catalysis. This method readily converts primary and secondary alkyl carboxylic acids into the corresponding polyfluoroaryl compounds, which could have a wide range of F-content (2F-5F) and variable F-substitution patterns on the aryl groups. Broad scope and good functional group compatibility were achieved, including on substrates derived from natural products and pharmaceuticals. Mechanistic study revealed that a [Cu-(ArF )2 ] species could be responsible for the transfer of polyfluoroaryl groups to the alkyl radicals.

Platinum Indolylphosphine Fluorido and Polyfluorido Complexes: An Interplay between Cyclometallation, Fluoride Migration, and Hydrogen Bonding

The reaction of [PtCl2 (COD)] (COD=1,5-cyclooctadiene) with diisopropyl-2-(3-methyl)indolylphosphine (iPr2 P(C9 H8 N)) led to the formation of the platinum(ii) chlorido complexes, cis-[PtCl2 {iPr2 P(C9 H8 N)}2 ] (1) and trans-[PtCl2 {iPr2 P(C9 H8 N)}2 ] (2). The cis-complex 1 reacted with NEt3 yielding the complex cis-[PtCl{κ2 -(P,N)-iPr2 P(C9 H7 N)}{iPr2 P(C9 H8 N)}] (3) bearing a cyclometalated κ2 -(P,N)-phosphine ligand, while the isomer 2 with a trans-configuration did not show any reactivity towards NEt3 . Treatment of 1 or 3 with (CH3 )4 NF (TMAF) resulted in the formation of the twofold cyclometalated complex cis-[Pt{κ2 -(P,N)-iPr2 P(C9 H7 N)}2 ] (4). The molecular structures of the complexes 1-4 were determined by single-crystal X-ray diffraction. The fluorido complex cis-[PtF{κ2 -(P,N)-iPr2 P(C9 H7 N)}{iPr2 P(C9 H8 N)}] ⋅ (HF)4 (5 ⋅ (HF)4 ) was formed when complex 4 was treated with different hydrogen fluoride sources. The Pt(ii) fluorido complex 5 ⋅ (HF)4 exhibits intramolecular hydrogen bonding in its outer coordination sphere between the fluorido ligand and the NH group of the 3-methylindolyl moiety. In contrast to its chlorido analogue 3, complex 5 ⋅ (HF)4 reacted with CO or the ynamide 1-(2-phenylethynyl)-2-pyrrolidinone to yield the complexes trans-[Pt(CO){κ2 -(P,C)-iPr2 P(C9 H7 NCO)}{iPr2 P(C9 H8 N)}][F(HF)4 ] (7) and a complex, which we suggest to be cis-[Pt{C=C(Ph)OCN(C3 H6 )}{κ2 -(P,N)-iPr2 P(C9 H7 N)}{iPr2 P(C9 H8 N)}][F(HF)4 ] (9), respectively. The structure of 9 was assigned on the basis of DFT calculations as well as NMR and IR data. Hydrogen bonding of HF and NH to fluoride was proven to be crucial for the existence of 7 and 9.

Significant Role of Oxygen Dopants in Photocatalytic PFCA Degradation over h-BN

The activation of the C(sp³)-F bond is extremely difficult due to its unreactive nature. The importance of this bond activation is recently highlighted because extensive distribution of perfluorocarboxylic acids (PFCAs) (C_nF_2n+1COOH) has emerged as a challenging environmental issue. Photocatalytic degradation of PFCAs over a few semiconducting light absorbers is known to remove these water and soil resilient contaminants but with limited efficiency. This work reports density functional theory calculations, through which we present a detailed mechanistic study of photocatalytic degradation of CF₃COOH (the shortest member of the PFCA family) over hexagonal boron nitride (h-BN). Our results clearly demonstrate that the existence of point defects is necessary to activate the h-BN plane for photocatalytic dissociation of the C-F bond. Specifically, we show that vacancies create strong Lewis acid or base sites (B or N vacancy, respectively) that facilitate the activation of the C(sp³)-F bond considerably. Furthermore, this study presents vivid theoretical evidence for the significant role of oxygen dopants, which mitigate the strength of the active sites and promote PFCA degradation over h-BN. Our calculations suggest that while the very stable intermediates generated during the reaction, in the case of h-BN with B or N vacancies, practically poison the catalyst, oxygen dopants make the degradation much more plausible and controllable. This work thus provides both an explanation for recently observed photocatalytic activity of h-BN to decompose PFCAs and valuable insights for exploring defected two-dimensional materials for activating and removing the fluorine-containing contaminants from water and soil.

Synthesis of Tailored Perfluoro Unsaturated Monomers for Potential Applications in Proton Exchange Membrane Preparation

The aim of the present work is the synthesis and characterization of new perfluorinated monomers bearing, similarly to Nafion^®, acidic groups for proton transport for potential and future applications in proton exchange membrane (PEM) fuel cells. To this end, we focused our attention on the synthesis of various molecules with (i) sufficient volatility to be used in vacuum polymerization techniques (e.g., PECVD)), (ii) sulfonic, phosphonic, or carboxylic acid functionalities for proton transport capacity of the resulting membrane, (iii) both aliphatic and aromatic perfluorinated tags to diversify the membrane polarity with respect to Nafion^®, and (iv) a double bond to facilitate the polymerization under vacuum giving a preferential way for the chain growth of the polymer. A retrosynthetic approach persuaded us to attempt three main synthetic strategies: (a) organometallic Heck-type cross-coupling, (b) nucleophilic displacement, and (c) Wittig–Horner reaction (carbanion approach). Preliminary results on the plasma deposition of a polymeric film are also presented. The variation of plasma conditions allowed us to point out that the film prepared in the mildest settings (20 W) shows the maximum monomer retention in its structure. In this condition, plasma polymerization likely occurs mainly by rupture of the π bond in the monomer molecule.

Acid-mediated intermolecular C-F/C-H cross-coupling of 2-fluorobenzofurans with arenes: synthesis of 2-arylbenzofurans

Transition-metal-free acid-promoted biaryl construction was achieved via intermolecular C-F/C-H cross-coupling. By treating 2-fluorobenzofurans with arenes in the presence of AlCl₃, 2-arylbenzofurans were obtained. This protocol was successfully applied to the short-step orthogonal synthesis of a bioactive 2-arylbenzofuran natural product, which allows independent transformations of C-F and C-Br bonds. Mechanistic studies indicated that α-fluorine-stabilized carbocations, generated via the protonation of 2-fluorobenzofurans, served as key intermediates. The Friedel-Crafts-type C-C bond formation between the α-fluorocarbocations and arenes, followed by hydrogen fluoride elimination, afforded 2-arylbenzofurans.

Visible-Light-Induced Palladium-Catalyzed Selective Defluoroarylation of Trifluoromethylarenes with Arylboronic Acids

Selective functionalization of inactive C(sp³)-F bonds to prepare medicinally interesting aryldifluoromethylated compounds remains challenging. One promising route is the transition-metal-catalyzed cross-coupling through oxidative addition of the C(sp³)-F bond in trifluoromethylarenes (ArCF₃), which are ideal precursors for this process due to their ready availability and low cost. Here, we report an unprecedented excited-state palladium catalysis strategy for selective defluoroarylation of trifluoromethylarenes with arylboronic acids. This visible-light-induced palladium-catalyzed cross-coupling proceeds under mild reaction conditions and allows transformation of a variety of arylboronic acids and ArCF₃. Preliminary mechanistic studies reveal that the oxidative addition of the C(sp³)-F bond in ArCF₃ to excited-state palladium(0) via a single electron transfer pathway is responsible for the C(sp³)-F bond activation.

C-F bond activation under transition-metal-free conditions

The unique properties of fluorine-containing organic compounds make fluorine substitution attractive for the development of pharmaceuticals and various specialty materials, which have inspired the evolution of diverse C-F bond activation techniques. Although many advances have been made in functionalizations of activated C-F bonds utilizing transition metal complexes, there are fewer approaches available for nonactivated C-F bonds due to the difficulty in oxidative addition of transition metals to the inert C-F bonds. In this regard, using Lewis acid to abstract the fluoride and light/radical initiator to generate the radical intermediate have emerged as powerful tools for activating those inert C-F bonds. Meanwhile, these transition-metal-free processes are greener, economical, and for the pharmaceutical industry, without heavy metal residues. This review provides an overview of recent C-F bond activations and functionalizations under transition-metal-free conditions. The key mechanisms involved are demonstrated and discussed in detail. Finally, a brief discussion on the existing limitations of this field and our perspective are presented.

An Efficient Synthesis of 2-CF3-3-Benzylindoles

The reaction of α-CF₃-β-(2-nitroaryl) enamines with benzaldehydes afforded effectively α,β-diaryl-CF₃-enones having nitro group. Subsequent reduction of nitro group by NH₄HCO₂-Pd/C system initiated intramolecular cyclization to give 2-CF₃-3-benzylindoles. Target products can be prepared in up to quantitative yields. Broad synthetic scope of the reaction was shown. Probable mechanism of indole formation is proposed.

Site-Selective Cross-Coupling of Polyhalogenated Arenes and Heteroarenes with Identical Halogen Groups

Methods to functionalize arenes and heteroarenes in a site-selective manner are highly sought after for rapidly constructing value-added molecules of medicinal, agrochemical, and materials interest. One effective approach is the site-selective cross-coupling of polyhalogenated arenes bearing multiple, but identical, halogen groups. Such cross-coupling reactions have proven to be incredibly effective for site-selective functionalization. However, they also present formidable challenges due to the inherent similarities in the reactivities of the halogen substituents. In this Review, we discuss strategies for site-selective cross-couplings of polyhalogenated arenes and heteroarenes bearing identical halogens, beginning first with an overview of the reaction types that are more traditional in nature, such as electronically, sterically, and directing-group-controlled processes. Following these examples is a description of emerging strategies, which includes ligand- and additive/solvent-controlled reactions as well as photochemically initiated processes.

Versatile Reaction Pathways of 1,1,3,3,3-Pentafluoropropene at Rh(I) Complexes [Rh(E)(PEt3 )3 ] (E=H, GePh3 , Si(OEt)3 , F, Cl): C-F versus C-H Bond Activation Steps

The reaction of the rhodium(I) complexes [Rh(E)(PEt₃)₃] (E=GePh₃ (1), H (6), F (7)) with 1,1,3,3,3‐pentafluoropropene afforded the defluorinative germylation products Z/E‐2‐(triphenylgermyl)‐1,3,3,3‐tetrafluoropropene and the fluorido complex [Rh(F)(CF₃CHCF₂)(PEt₃)₂] (2) together with the fluorophosphorane E‐(CF₃)CH=CF(PFEt₃). For [Rh(Si(OEt)₃)(PEt₃)₃] (4) the coordination of the fluoroolefin was found to give [Rh{Si(OEt)₃}(CF₃CHCF₂)(PEt₃)₂] (5). Two equivalents of complex 2 reacted further by C−F bond oxidative addition to yield [Rh(CF=CHCF₃)(PEt₃)₂(μ‐F)₃Rh(CF₃CHCF₂)(PEt₃)] (9). The role of the fluorido ligand on the reactivity of complex 2 was assessed by comparison with the analogous chlorido complex. The use of complexes 1, 4 and 6 as catalysts for the derivatization of 1,1,3,3,3‐pentafluoropropene provided products, which were generated by hydrodefluorination, hydrometallation and germylation reactions.

Modular Construction of Functionalized 2-CF3-Indoles

The reaction of α-CF3-β-(2-nitroaryl)enamines with benzaldehydes led to nitro-substituted α,β-diaryl-CF₃-enones in high yield. Subsequent reduction of the nitro-group to the amino moiety by the Pd/C-NH₄HCO₂ system resulted in intramolecular cyclization to form a 5-membered hemiaminal which is stabilized by the presence of a CF₃-group. The reaction of this hemiaminal with various nucleophiles afforded functionalized 2-CF₃-indoles isolated in up to quantitative yields. High efficiency and broad synthetic scope of all steps of the sequence were demonstrated. A possible mechanism of the reaction is discussed.

Transition-Metal-Free Coupling of Polyfluorinated Arenes and Functionalized, Masked Aryl Nucleophiles

A chemoselective C(sp²)−C(sp²) coupling of sufficiently electron‐deficient fluorinated arenes and functionalized N‐aryl‐N’‐silyldiazenes as masked aryl nucleophiles is reported. The fluoride‐promoted transformation involves the in situ generation of the aryl nucleophile decorated with various sensitive functional groups followed by a stepwise nucleophilic aromatic substitution (S_NAr). These reactions typically proceed at room temperature within minutes. This catalytic process allows for the functionalization of both coupling partners, furnishing highly fluorinated biaryls in good yields.

Transition Metal Catalyst-Free, Base-Promoted 1,2-Additions of Polyfluorophenylboronates to Aldehydes and Ketones

A novel protocol for the transition metal-free 1,2-addition of polyfluoroaryl boronate esters to aldehydes and ketones is reported, which provides secondary alcohols, tertiary alcohols, and ketones. Control experiments and DFT calculations indicate that both the ortho-F substituents on the polyfluorophenyl boronates and the counterion K+ in the carbonate base are critical. The distinguishing features of this procedure include the employment of commercially available starting materials and the broad scope of the reaction with a wide variety of carbonyl compounds giving moderate to excellent yields. Intriguing structural features involving O-H⋅⋅⋅O and O-H⋅⋅⋅N hydrogen bonding, as well as arene-perfluoroarene interactions, in this series of racemic polyfluoroaryl carbinols have also been addressed.

Three-component reaction of gem-difluorinated cyclopropanes with alkenes and B2pin2 for the synthesis of monofluoroalkenes

Borylative difunctionalization of alkenes has emerged as a powerful approach for synthesizing highly functionalized molecules. Herein, dual Cu/Pd-catalysed borylfluoroallylation of alkenes was smoothly achieved by using gem-difluorinated cyclopropanes and B2pin2, providing the corresponding monofluoroalkene scaffolds in moderate to high yields with excellent stereoselectivity. Moreover, an array of synthetic building blocks can be obtained by downstream transformations.

Metal-catalysed C-Het (F, O, S, N) and C-C bond arylation

The formation of C-aryl bonds has been the focus of intensive research over the last decades for the construction of complex molecules from simple, readily available feedstocks. Traditionally, these strategies involve the coupling of organohalides (I, Br, Cl) with organometallic reagents (Mg, Zn, B, Si, Sn,…) such as Kumada-Corriu, Negishi, Suzuki-Miyaura, Hiyama and Sonogashira cross-couplings. More recently, alternative methods have provided access to these products by reactions with less reactive C-Het (F, O, S, N) and C-C bonds. Compared to traditional methods, the direct cleavage and arylation of these chemical bonds, the essential link in accessible feedstocks, has become increasingly important from the viewpoint of step-economy and functional-group compatibility. This comprehensive review aims to outline the development and advances of this topic, which was organized into (1) C-F bond arylation, (2) C-O bond arylation, (3) C-S bond arylation, (4) C-N bond arylation, and (5) C-C bond arylation. Substantial attention has been paid to the strategies and mechanistic investigations. We hope that this review can trigger chemists to discover more efficient methodologies to access arylation products by cleavage of these C-Het and C-C bonds.

Catalyst-free carbosilylation of alkenes using silyl boronates and organic fluorides via selective C-F bond activation

A regioselective carbosilylation of alkenes has emerged as a powerful strategy to access molecules with functionalized silylated alkanes, by incorporating silyl and carbon groups across an alkene double bond. However, to the best of our knowledge, organic fluorides have never been used in this protocol. Here we disclose the catalyst-free carbosilylation of alkenes using silyl boronates and organic fluorides mediated by tBuOK. The main feature of this transformation is the selective activation of the C-F bond of an organic fluoride by the silyl boronate without undergoing potential side-reactions involving C-O, C-Cl, heteroaryl-CH, and even CF3 groups. Various silylated alkanes with tertiary or quaternary carbon centers that have aromatic, hetero-aromatic, and/or aliphatic groups at the β-position are synthesized in a single step from substituted or non-substituted aryl alkenes. An intramolecular variant of this carbosilylation is also achieved via the reaction of a fluoroarene with a ω-alkenyl side chain and a silyl boronate.

Iron-Catalyzed Halogen Exchange of Trifluoromethyl Arenes*

The facile production of ArCF₂ X and ArCX₃ from ArCF₃ using catalytic iron(III)halides is reported, which constitutes the first iron-catalyzed halogen exchange for non-aromatic C-F bonds. Theoretical calculations suggest direct activation of C-F bonds by iron coordination. ArCX₃ and ArCF₂ X products of the reaction are synthetically valuable due to their diversification potential. In particular, chloro- and bromodifluoromethyl arenes (ArCF₂ Cl, ArCF₂ Br respectively) provide access to a myriad of difluoromethyl arene derivatives (ArCF₂ R). To optimize for mono-halogen exchange, a statistical method called Design of Experiments was used. Optimized parameters were successfully applied to electron rich and electron deficient aromatic substrates, and to the late stage diversification of flufenoxuron, a commercial insecticide. These methods are highly practical, being run at convenient temperatures and using inexpensive common reagents.

Chemoselective catalytic hydrodefluorination of trifluoromethylalkenes towards mono-/gem-di-fluoroalkenes under metal-free conditions

Fluorine-containing moieties show significant effects in improving the properties of functional molecules. Consequently, efficient methods for installing them into target compounds are in great demand, especially those enabled by metal-free catalysis. Here we show a diazaphospholene-catalyzed hydrodefluorination of trifluoromethylalkenes to chemoselectively construct gem-difluoroalkenes and terminal monofluoroalkenes by simple adjustment of the reactant stoichiometry. This metal-free hydrodefluorination features mild reaction conditions, good group compatibility, and almost quantitative yields for both product types. Stoichiometric experiments indicated a stepwise mechanism: hydridic addition to fluoroalkenes and subsequent β-F elimination from hydrophosphination intermediates. Density functional theory calculations disclosed the origin of chemoselectivity, regioselectivity and stereoselectivity, suggesting an electron-donating effect of the alkene-terminal fluorine atom.

Diastereoselectivity of the Addition of Propargylic Magnesium Reagents to Fluorinated Aromatic Sulfinyl Imines

The addition of propargylmagnesium bromide to fluorinated aromatic sulfinyl imines gave homopropargyl amines with total regio- and diastereoselection. Complete reversal of diastereoselectivity can be achieved in some cases using coordinating (THF) or noncoordinating (DCM) solvents. Substituted propargylic magnesium reagents have been also tested toward fluorinated aryl sulfinyl imines affording chiral homoallenyl amines with good yields and selectivity control. DFT calculations helped to rationalize the origin of the experimental regio- and diastereoselectivities observed in each case.

Highly E-Selective Synthesis of α-Fluoro-β-arylalkenyl Sulfones from gem-Difluoroalkenes with Sodium Sulfinates

The straightforward synthesis of α-fluoro-β-arylalkenyl sulfones under transition-metal- and base-free conditions has been described, which displays broad functional group compatibility and high stereoselectivity. In particular, the strategy is also applied to the late-stage modification of complex natural products and drugs.

Decarboxylative Polyfluoroarylation of Alkylcarboxylic Acids

Polyfluoroarenes are useful building blocks in several areas such as drug discovery, materials, and crop protection. Herein, we report the first polyfluoroarylation of aliphatic carboxylic acids via photoredox decarboxylation. The method proceeds with broad substrate scope and high functional group tolerance. Moreover, small complex molecules such as natural products and drugs can be modified by late-stage modification.

Cu/Pd-Catalyzed cis-Borylfluoroallylation of Alkynes for the Synthesis of Boryl-Substituted Monofluoroalkenes

Monofluoroalkenes normally act as metabolically stable bioisosteres for amide groups (-NH-CO-) and have widespread applications in drug discovery. Additionally, they are widely used as building blocks in organic synthesis. In this study, the Cu/Pd-catalyzed cis-borylfluoroallylation of alkynes was achieved, providing a modular and general tactic for the preparation of monofluorinated alkene scaffolds with high regioselectivity and stereoselectivity. Moreover, an array of synthetic building blocks can be generated by downstream transformations.

Silylium Ions: From Elusive Reactive Intermediates to Potent Catalysts

The history of silyl cations has all the makings of a drama but with a happy ending. Being considered reactive intermediates impossible to isolate in the condensed phase for decades, their actual characterization in solution and later in solid state did only fuel the discussion about their existence and initially created a lot of controversy. This perception has completely changed today, and silyl cations and their donor-stabilized congeners are now widely accepted compounds with promising use in synthetic chemistry. This review provides a comprehensive summary of the fundamental facts and principles of the chemistry of silyl cations, including reliable ways of their preparation as well as their physical and chemical properties. The striking features of silyl cations are their enormous electrophilicity and as such reactivity as super Lewis acids as well as fluorophilicity. Known applications rely on silyl cations as reactants, stoichiometric reagents, and promoters where the reaction success is based on their steady regeneration over the course of the reaction. Silyl cations can even be discrete catalysts, thereby opening the next chapter of their way into the toolbox of synthetic methodology.

Recent Advances in Transition-metal-catalyzed C-C Bond Formation via C(sp2 )-F Bond Cleavage

The activation of a carbon-fluorine bond is one of the most challenging topics in modern synthetic organic chemistry due to their low reactivity compared to other carbon-halogen bonds. In this review, we present the recent developments since 2015 on cross-coupling reactions that form C-C bonds via cleavage of C(sp² )-F bonds. Not only the conventional activation of C(sp² )-F bonds, but also decarbonylative or carbonyl-retentive cleavage of C(acyl)-F bonds will be introduced. This paper mainly describes new protocols for the formation of C(sp² )-C(sp³ ), C(sp² )-C(sp² ), and C(sp² )-C(sp) bonds via transition-metal-catalyzed cleavage of C(sp² )-F bonds.

Pd/Cu-Catalyzed Defluorinative Carbonylative Coupling of Aryl Iodides and gem-Difluoroalkenes: Efficient Synthesis of α-Fluorochalcones

An unprecedented and challenging defluorinative carbonylation was achieved. Enabled by a Pd/Cu cooperative catalyst system, the first example of defluorinative carbonylative coupling has been established. With gem-difluoroalkenes and aryl iodides as the substrates, this methodology offers flexible and facile access to privileged α-fluorochalcones under mild reaction conditions in moderate-to-excellent yields. Mechanistic studies indicated transmetalation between palladium and copper intermediates as a crucial step of the catalytic cycle.

A Tutorial on Selectivity Determination in C(sp2)-H Oxidative Addition of Arenes by Transition Metal Complexes

A Tutorial on factors that determine the selectivity in C(sp²)–H activation and functionalization reactions involving two-electron oxidative addition processes with transition metals is presented. The interplay of the thermodynamics of C(sp²)–H oxidative addition and kinetic influences upon regioselectivity are presented alongside pedagogically valuable experimental and computational results from the literature. Mechanisms and energetics of chelate-assisted C(sp²)–H oxidative addition are examined, as are concepts related to chemoselectivity in the oxidative addition of C(sp²)–H or C(sp²)–X (X = F, Cl, Br, I) bonds with aryl halide substrates.