Nickel-Catalyzed Monosubstitution of Polyfluoroarenes with Organozinc Reagents Using Alkoxydiphosphine Ligand

Site-Selective C-H Alkenylation of N-Heteroarenes by Ligand-Directed Co/Al and Co/Mg Cooperative Catalysis

We report herein the design and development of Co/Al and Co/Mg bimetallic catalysts, supported by a phosphine/secondary phosphine oxide (PSPO) bifunctional ligand, for the site-selective C-H alkenylation of nitrogen-containing heteroarenes with alkynes. These catalysts enable the alkenylation of pyridine, pyridone, and imidazo[1,2-a]pyridine derivatives at the C-H site proximal to the Lewis basic nitrogen or oxygen atom, which represents a selectivity profile distinct from that of the previously developed cobalt-diphosphine/aluminum catalyst. The alkenylated products were obtained in moderate to good yields using various heterocycles and differently substituted internal alkynes. Kinetic isotope effect experiments suggest the irreversibility of the C-H activation step, the relevance of which to the rate-limiting step depends on the reaction conditions. Density functional theory calculations indicate that ligand-to-ligand hydrogen transfer is the common mechanism of C-H activation.

Direct C(sp3)-H Polyfluoroarylation: Access to Polyfluoroaryl Amino Acids via Rh-Catalyzed Selective C-F Bond Cleavage

A catalytic selective C-F bond alkylation method for polyfluoroarene with glycinates and derivatives in the presence of a DavePhos-ligated Rh catalyst was developed. This method avoids the preactivation of alkylating reagents and provides an efficient and straightforward route to synthesize a series of polyfluoroaryl amino acids via C(sp³)-H functionalization. This reaction proceeds under mild conditions and exhibits high reactivity and excellent chemoselectivities. Meanwhile, the synthetic potential of this method was demonstrated by gram-scale synthesis, and further transformations proved the application value of the products as well.

Nickel-Catalyzed Defluorophosphonylation of Aryl Fluorides

A Ni-catalyzed cross-coupling reaction between aryl fluorides and dialkyl phosphonates [HP(O)(OR)₂] (R = secondary alkyl groups) in the presence of potassium tert-butoxide as a base is reported. The reaction converted various aryl fluorides into the corresponding aryl phosphonates even when electron-donating substituents were present on the aromatic ring. The combined experimental and computational studies suggested Ni-K⁺ cooperative action of a Ni(0) complex chelated with a strongly electron-donating ion-bridged dimeric phosphite ligand system [P(OR)₂O^-K⁺]₂ that facilitates turnover-limiting C-F bond oxidative addition of aryl fluorides.

Photoredox Polyfluoroarylation of Alkyl Halides via Halogen Atom Transfer

Polyfluoroarene moieties are of interest in medicinal chemistry, agrochemicals, and material sciences. Herein, we present the first polyfluoroarylation of unactivated alkyl halides via a halogen atom transfer process. This method converts primary, secondary, and tertiary alkyl halides into the respective polyfluoroaryl compounds in good yields in the presence of amide, carbamate, ester, aromatic, and sulfonamide moieties, including derivatives of complex bioactive molecules. Mechanistic work revealed that this transformation proceeds through an alkyl radical generated after the halogen atom transfer.

Regioselective and late-stage polyfluoroarylation of arenes with diverse fluoroaryl nucleophiles via Pd-catalysis

Dehydrogenative coupling of arenes with fluoroaryl nucleophiles enables the efficient synthesis of fluoroaromatic compounds from readily available starting materials.

Palladium-catalyzed nucleomethylation of alkynes for synthesis of methylated heteroaromatic compounds

Herein, we disclosed a novel and efficient palladium-catalyzed nucleomethylation of alkynes for the simultaneous construction of the heteroaromatic ring and methyl group. The 3-methylindoles, 3-methylbenzofurans and 4-methylisoquinolines were obtained in moderate to excellent yields. Notably, this methodology was employed as a key step for synthesis of a pregnane X receptor antagonist, zindoxifene, bazedoxifene and AFN-1252. The kinetic studies revealed that reductive elimination might be the rate-determining step.

A novel palladium-catalyzed nucleomethylation of alkynes is developed, affording 3-methylindoles, 3-methylbenzofurans and 4-methylisoquinolines in moderate to excellent yields.

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.

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.

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.

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.

Palladium-Catalysed C-H Bond Zincation of Arenes: Scope, Mechanism, and the Role of Heterometallic Intermediates

Catalytic methods that transform C-H bonds into C-X bonds are of paramount importance in synthesis. A particular focus has been the generation of organoboranes, organosilanes and organostannanes from simple hydrocarbons (X=B, Si, Sn). Despite the importance of organozinc compounds (X=Zn), their synthesis by the catalytic functionalisation of C-H bonds remains unknown. Herein, we show that a palladium catalyst and zinc hydride reagent can be used to transform C-H bonds into C-Zn bonds. The new catalytic C-H zincation protocol has been applied to a variety of arenes-including fluoroarenes, heteroarenes, and benzene-with high chemo- and regioselectivity. A mechanistic study shows that heterometallic Pd-Zn complexes play a key role in catalysis. The conclusions of this work are twofold; the first is that valuable organozinc compounds are finally accessible by catalytic C-H functionalisation, the second is that heterometallic complexes are intimately involved in bond-making and bond-breaking steps of C-H functionalisation.

Nickel-Catalyzed Cross-Coupling of Aryl 2-Pyridyl Ethers with Organozinc Reagents: Removal of the Directing Group via Cleavage of the Carbon-Oxygen Bonds

Reaction of aryl 2-pyridyl ethers with arylzinc reagents under catalysis of NiCl₂(PCy₃)₂ affords aryl-aryl cross-coupling products via selective cleavage of C_Ar-OPy bonds. The reaction features a wide substrate range and good compatibility of functional groups. β-H-free alkylzinc reagents are also applicable as the nucleophiles in the transformation, whereas β-H-containing alkylzinc reagents lead to a mixture of cross-coupling and hydrogenation products.

Cobalt-Catalyzed Defluorosilylation of Aryl Fluorides via Grignard Reagent Formation

Transition-metal-catalyzed transformations of the carbon-fluorine bond not only tackle an interesting problem of challenging bond activation but also offer new synthetic strategies where the relatively inert C-F bond is converted to versatile functional groups. Herein we report a practical cobalt-catalyzed silylation of aryl fluorides that uses a cheap electrophilic silicon source with magnesium. This method is compatible with various silicon sources and can be operated under aerobic conditions. Mechanistic studies support the in situ formation of a Grignard reagent, which is captured by the electrophilic silicon source.

C–F bond arylation of fluoroarenes catalyzed by Pd0 phosphine complexes: theoretical insight into regioselectivity, reactivity, and prediction of ligands

C3-Regioselective arylation of pentafluorobenzene arises from a lower σ* orbital energy of C3–F that stabilizes the transition state further.

Dehalogenative Deuteration of Unactivated Alkyl Halides Using D2O as the Deuterium Source

The general dehalogenation of alkyl halides with zinc using D₂O or H₂O as a deuterium or hydrogen donor has been developed. The method provides an efficient and economic protocol for deuterium-labeled derivatives with a wide substrate scope under mild reaction conditions. Mechanistic studies indicated that a radical process is involved for the formation of organozinc intermediates. The facile hydrolysis of the organozinc intermediates provides the driving force for this transformation.

Umpolung cross-coupling of polyfluoroarenes with hydrazones via activation of C–F bonds

An umpolung strategy for the cross-coupling of polyfluoroarenes with various substituted hydrazones to construct C(sp²)–C(sp³) bonds is developed.

Synthesis, structure and palladium coordination of ambiphilic, pyridine- and phosphine-tethered N-boryl imine ligands

New classes of ambiphilic ligands incorporating N-boryl imines can be generated in two steps from aldehydes, and their structure modulated by the ligand motif.

Cobalt-Catalyzed C-F Bond Borylation of Aryl Fluorides

A mild and practical cobalt-catalyzed defluoroborylation of fluoroarenes is presented for the first time. The method permits straightforward functionalization of fluoroarenes, with high selectivity for borylation of C-F over C-H bonds, and a tolerance for aerobic conditions. Furthermore, two-step ¹⁸F-fluorination was achieved for expanding the scope of ¹⁸F-positron emission tomography probes.

Nickel-Catalyzed Csp2-Csp3 Bond Formation via C-F Bond Activation

A nickel-catalyzed cross coupling of aryl fluorides via C-F bond activation has been developed. The alkylation method allows selective replacement of aryl fluorides by alkyl groups and enables the synthesis of diverse and otherwise difficult to access scaffolds in good yields.

Nickel-catalyzed amination of aryl fluorides with primary amines

The Ni-catalyzed cross-coupling reaction between aryl fluorides and primary amines was enabled by the 1,2-bis(dicyclohexylphosphino)benzene (DCYPBz) or 1,2-bis(dicyclohexylphosphino)ethane (DCYPE) ligands.

Photoredox-Controlled Mono- and Di-Multifluoroarylation of C(sp3 )-H Bonds with Aryl Fluorides

A controllable mono- and di-multifluoroarylation of acyclic and cyclic N-aryl amines with aryl fluorides by photocatalyzed dual C(sp³ )-H/C(sp² )-F functionalization has been developed, providing new access to a wide array of valuable α-fluoroarylated amines. In addition, the one-pot consecutive hetero-di-multifluoroarylation of N-aryl pyrrolidines and N,N-dimethylanilines was achieved with high to excellent diastereoselectivity. This new defluorinative C(sp³ )-C(sp² ) coupling is distinguished by a broad scope, good regioselectivity, and mild conditions as well as gram-scale and late-stage applicability, and thus constitutes a significant advance in the arylation of unactivated C(sp³ )-H bonds with aryl fluorides.