Catalytic Defluoroalkylation of Trifluoromethylaromatics with Unactivated Alkenes

Photoreduction of Trifluoromethyl Group: Lithium Ion Assisted Fluoride-Coupled Electron Transfer from EDA Complex

Photoinduced single-electron reduction is an efficient method for the mono-selective activation of the C-F bond on a trifluoromethyl group to construct a difluoroalkyl group. We have developed an electron-donor-acceptor (EDA) complex mediated single-electron transfer (EDA-SET) of α,α,α-trifluoromethyl arenes in the presence of lithium salt to give α,α-difluoroalkylarenes. The C-F bond reduction was realized by lithium iodide and triethylamine, two common feedstock reagents. Mechanistic studies revealed the generation of a α,α-difluoromethyl radical by single-electron reduction and defluorination, followed by the radical addition to alkenes. Lithium salt interacted with the fluorine atom to promote the photoinduced reduction mediated by the EDA complex. Computational studies indicated that the lithium-assisted defluorination and the single-electron reduction occurred concertedly. We call this phenomenon fluoride-coupled electron transfer (FCET). FCET is a novel approach to C-F bond activation for the synthesis of organofluorine compounds.

Hybrid Palladium-Catalyzed Intramolecular Carboamination of Conjugated Dienes: Synthesis of Functionalized Pyrrolidines via Selective Trifluoromethylarene Defluorination

The incorporation of difluoromethylene groups into aza-heterocycles represents a compelling yet underexplored avenue in contemporary chemical research. In this study, we unveil a hybrid palladium-catalyzed intramolecular gem-difluoroalkylamination of conjugated dienes, providing a versatile approach to the synthesis of diverse functionalized pyrrolidines. Noteworthy features include mild reaction conditions and a remarkable tolerance toward various functional groups. Additionally, the use of alkyl iodides as electrophiles facilitates the generation of the corresponding alkylamination products. Control experiments support a proposed hybrid palladium-catalyzed radical-polar crossover pathway, offering insights into the underlying chemical processes governing this transformation.

Strongly reducing helical phenothiazines as recyclable organophotoredox catalysts

Recyclable phenothiazine organophotoredox catalysts (PTHS 1-3, E1/2ox* = -2.34 to -2.40 V vs. SCE) have been developed. When the recycling performance was evaluated, PTHS-1 could be recovered at least four times without loss of its catalytic activity. These recyclable organophotoredox catalysts represent a promising tool for sustainable organic synthesis.

Visible-light-driven alkene dicarboxylation with formate and CO2 under mild conditions

Low-cost formate salt was used as the reductant and part of the carboxyl source in a visible-light-driven dicarboxylation of diverse alkenes, including simple styrenes. The highly competing hydrocarboxylation side reaction was successfully overridden. Good yields of products were obtained under mild reaction conditions at ambient temperature and pressure of CO2. The dual role of formate salt may stimulate the discovery of a range of new transformations under mild and friendly conditions.

Selective Mono-Defluorinative Cross-Coupling of Trifluoromethyl arenes via Multiphoton Photoredox Catalysis

A new cross-coupling of trifluoromethyl arenes has been realized via multiphoton photoredox catalysis. Trifluoromethyl arenes were demonstrated to undergo selective mono-defluorinative alkylation under mild reaction conditions providing access to a series of valuable α,α-difluorobenzylic compounds. The reaction shows broad substrate scope and general functional group tolerance. In addition to the electron-deficient trifluoromethyl arenes that are easily reduced to the corresponding radical anion, more challenging electron-rich substrates were also successfully applied. Steady-State Stern-Volmer quenching studies indicated that the trifluoromethyl arenes were reduced by the multiphoton excited Ir-based photocatalyst.

Sequential C-F Bond Transformation of the Difluoromethylene Unit in Perfluoroalkyl Groups: A Combination of Fine-Tuned Phenothiazine Photoredox Catalyst and Lewis Acid

A sequential process via photoredox catalysis and Lewis acid mediation for C-F bond transformation of the CF2 unit in perfluoroalkyl groups has been achieved to transform perfluoroalkylarenes into complex fluoroalkylated compounds. A phenothiazine-based photocatalyst promotes the defluoroaminoxylation of perfluoroalkylarenes with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) under visible light irradiation, affording the corresponding aminoxylated products. These products undergo a further defluorinative transformation with various organosilicon reagents mediated by AlCl3 to provide highly functionalized perfluoroalkyl alcohols. Our novel phenothiazine catalyst works efficiently in the defluoroaminoxylation. Transient absorption spectroscopy revealed that the catalyst regeneration step is crucial for the photocatalytic aminoxylation.

A review of frustrated Lewis pair enabled monoselective C-F bond activation

Frustrated Lewis pair (FLP) bond activation chemistry has greatly developed over the last two decades since the seminal report of metal-free reversible hydrogen activation. Recently, FLP systems have been utilized to allow monoselective C-F bond activation (at equivalent sites) in polyfluoroalkanes. The problem of 'over-defluorination' in the functionalization of polyfluoroalkanes (where multiple fluoro-positions are uncontrollably functionalized) has been a long-standing chemical problem in fluorocarbon chemistry for over 80 years. FLP mediated monoselective C-F bond activation is complementary to other solutions developed to address 'over-defluorination' and offers several advantages and unique opportunities. This perspective highlights some of these advantages and opportunities and places the development of FLP mediated C-F bond activation into the context of the wider effort to overcome 'over-defluorination'.

C-F bond activation enables synthesis of aryl difluoromethyl bicyclopentanes as benzophenone-type bioisosteres

Bioisosteric design has become an essential approach in the development of drug molecules. Recent advancements in synthetic methodologies have enabled the rapid adoption of this strategy into drug discovery programs. Consequently, conceptionally innovative practices would be appreciated by the medicinal chemistry community. Here we report an expeditous synthetic method for synthesizing aryl difluoromethyl bicyclopentane (ADB) as a bioisostere of the benzophenone core. This approach involves the merger of light-driven C-F bond activation and strain-release chemistry under the catalysis of a newly designed N-anionic-based organic photocatalyst. This defluorinative coupling methodology enables the direct conversion of a wide variety of commercially available trifluoromethylaromatic C-F bonds (more than 70 examples) into the corresponding difluoromethyl bicyclo[1.1.1]pentanes (BCP) arenes/difluoromethyl BCP boronates in a single step. The strategy can also be applied to [3.1.1]and [4.1.1]propellane systems, providing access to analogues with different geometries. Moreover, we have successfully used this protocol to rapidly prepare ADB-substituted analogues of the bioactive molecule Adiporon. Biological testing has shown that the ADB scaffold has the potential to enhance the pharmacological properties of benzophenone-type drug candidates.

Photo- and electro-chemical strategies for the activations of strong chemical bonds

The employment of light and/or electricity - alternatively to conventional thermal energy - unlocks new reactivity paradigms as tools for chemical substrate activations. This leads to the development of new synthetic reactions and a vast expansion of chemical spaces. This review summarizes recent developments in photo- and/or electrochemical activation strategies for the functionalization of strong bonds - particularly carbon-heteroatom (C-X) bonds - via: (1) direct photoexcitation by high energy UV light; (2) activation via photoredox catalysis under irradiation with relatively lower energy UVA or blue light; (3) electrochemical reduction; (4) combination of photocatalysis and electrochemistry. Based on the types of the targeted C-X bonds, various transformations ranging from hydrodefunctionalization to cross-coupling are covered with detailed discussions of their reaction mechanisms.

Photoinduced copper-catalyzed C-N coupling with trifluoromethylated arenes

Selective defluorinative functionalization of trifluoromethyl group (-CF3) is an attractive synthetic route to the pharmaceutically privileged fluorine-containing moiety. Herein, we report a strategy based on photoexcited copper catalysis to activate the C-F bond of di- or trifluoromethylated arenes for divergent radical C-N coupling with carbazoles and aromatic amines. The use of different ligands can tune the reaction products diversity. A range of substituted, structurally diverse α,α-difluoromethylamines can be obtained from trifluoromethylated arenes via defluorinative C-N coupling with carbazoles, while an interesting double defluorinative C-N coupling is ready for difluoromethylated arenes. Based on this success, a carbazole-centered PNP ligand is designed to be an optimal ligand, enabling a copper-catalyzed C-N coupling for the construction of imidoyl fluorides from aromatic amines through double C-F bond functionalization. Interestingly, a 1,2-difluoroalkylamination strategy of styrenes is also developed, delivering γ,γ-difluoroalkylamines, a bioisostere to β-aminoketones, in synthetically useful yields. The DFT studies reveal an inner-sphere electron transfer mechanism for Cu-catalyzed selective activation of C(sp3)-F bonds.

Defluorinative Alkylboration of Alkenes Enabled by Dual Photoredox and Copper Catalysis

A regioselectivity reversed three-component defluorinative alkylboration of alkenes with trifluoromethyls and bis(pinacolato)diboron via dual photoredox/copper catalysis is reported. The mild conditions are compatible with a wide array of nonactivated trifluoromethyl aromatics bearing electron-donating or electron-neutral substituents, trifluoroacetamides, and various nonactivated terminal and internal alkenes, enabling straightforward access to synthetically valuable γ-gem-difluoroalkyl boronates with high efficiency. Furthermore, this protocol is applicable to alkene-tethered trifluoromethyl aromatics to furnish gem-difluoromethylene-containing cyclic compounds. Synthetic applications and preliminary mechanistic studies are also presented.

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.

Programmable synthesis of difluorinated hydrocarbons from alkenes through a photocatalytic linchpin strategy

The introduction of difluoromethylene moieties into organic molecules has garnered significant attention due to their profound influence on the physicochemical and biological properties of compounds. Nonetheless, the existing approaches for accessing difluoroalkanes from readily available feedstock chemicals remain limited. In this study, we present an efficient and modular protocol for the synthesis of difluorinated compounds from alkenes, employing the readily accessible reagent, ClCF2SO2Na, as a versatile "difluoromethylene" linchpin. By means of an organophotoredox-catalysed hydrochlorodifluoromethylation of alkenes, followed by a ligated boryl radical-facilitated halogen atom transfer (XAT) process, we have successfully obtained various difluorinated compounds, including gem-difluoroalkanes, gem-difluoroalkenes, difluoromethyl alkanes, and difluoromethyl alkenes, with satisfactory yields. The practical utility of this linchpin strategy has been demonstrated through the successful preparation of CF2-linked derivatives of complex drugs and natural products. This method opens up new avenues for the synthesis of structurally diverse difluorinated hydrocarbons and highlights the utility of ligated boryl radicals in organofluorine chemistry.

α-Difluoroalkylation of Benzyl Amines with Trifluoromethylarenes

An α-difluoroalkylation of benzyl amines with trifluoromethylarenes is disclosed herein. This protocol is characterized by its operational simplicity, excellent chemoselectivity and broad scope-even with advanced synthetic intermediates-, thus offering a new entry point to medicinally-relevant α-difluoroalkylated amines from simple, yet readily accessible, precursors.

C-F Activation of Fluorinated Esters Using Photocatalytically Generated Diaminoalkyl Radical

A method for hydrofluoroalkylation of alkenes with trifluoroacetic esters under visible light irradiation affording difluorinated products is described. The reaction involves readily available trimethyltriazinane as a shoichiometric reducing agent, which generates a diamino-substituted alkyl radical serving as a strong electron donor. It is believed that the cleavage of the C-F bond by a triazinane-derived radical involves single electron reduction coupled with fluoride transfer.

Iodine(III)-Mediated Migratory gem-Difluorinations: Synthesis of β Transformable Functionality Substituted gem-Difluoroalkanes

Geminal-difluoroalkanes featuring intriguing steric and electronic properties are of great significance in medicinal chemistry, and great progresses have been achieved for their synthesis. In recent years, iodine(III) reagent-mediated migratory gem-difluorination of alkenes has proved to be an efficient and powerful strategy to access to diverse gem-difluoroalkanes, especially those bearing a readily transformable functionality (TF), which are important for rapid assembly of complex gem-difluorinated molecules in a modular and diverse manner. In this review, we systematically summarize the recent development of iodine(III)-mediated migratory gem-difluorination reactions for the synthesis of gem-difluoroalkanes bearing a synthetically versatile TF at the β position. The reaction mechanism and the utilities of the products are also discussed. This review is presented and grouped basically according to the types of transformable functionalities within the products.

C-F Transformations of Benzotrifluorides by the Activation of Ortho-Hydrosilyl Group

Single C-F transformations of aromatic trifluoromethyl compounds are challenging issues due to the strong C-F bond. We have recently developed selective methods for single C-F transformations such as allylation of o-hydrosilyl-substituted benzotrifluorides through the hydride abstraction with trityl cations. Single C-F thiolation and azidation of o-(hydrosilyl)benzotrifluorides were achieved using trityl sulfides and trityl azide catalyzed by Yb(OTf)3 . Treatment of o-(hydrosilyl)benzotrifluorides with trityl chloride resulted in single C-F chlorination. The resulting fluorosilyl group served in further transformations including protonation, halogenation, and Hiyama cross-coupling with C-Si cleavage. We also synthesized benzyl fluorides by LiAlH4 -reduction of the resulting fluorosilanes and further C-F transformations. These methods enabled us to prepare a broad range of organofluorines from simple benzotrifluorides through C-F and C-Si transformations.

Photocatalytic C-F alkylation of trifluoromethyls using o-phosphinophenolate: mechanistic insights and substrate prediction

Density functional theory computations reveal the radical mechanism of photocatalytic defluoroalkylation and hydrodefluorination of N-phenyl-2,2,2-trifluoromethylacetamide with o-phosphinophenolate (PO) cooperative catalysis. The energy gaps between the singlet substrate LUMOs and triplet photocatalyst SOMOs can be used as an effective "chemical descriptor" for predicting catalyst activity. Cesium formate assisted C-F bond activation is the most favorable path. A series of available organic structures are computationally predicted as potential substrates.

Recent Advances in C-H Functionalisation through Indirect Hydrogen Atom Transfer

The functionalisation of C-H bonds has been an enormous achievement in synthetic methodology, enabling new retrosynthetic disconnections and affording simple synthetic equivalents for synthons. Hydrogen atom transfer (HAT) is a key method for forming alkyl radicals from C-H substrates. Classic reactions, including the Barton nitrite ester reaction and Hofmann-Löffler-Freytag reaction, among others, provided early examples of HAT. However, recent developments in photoredox catalysis and electrochemistry have made HAT a powerful synthetic tool capable of introducing a wide range of functional groups into C-H bonds. Moreover, greater mechanistic insights into HAT have stimulated the development of increasingly site-selective protocols. Site-selectivity can be achieved through the tuning of electron density at certain C-H bonds using additives, a judicious choice of HAT reagent, and a solvent system. Herein, we describe the latest methods for functionalizing C-H/Si-H/Ge-H bonds using indirect HAT between 2018-2023, as well as a critical discussion of new HAT reagents, mechanistic aspects, substrate scopes, and background contexts of the protocols.

Defluoroalkylation of Trifluoromethylarenes with Hydrazones: Rapid Access to Benzylic Difluoroarylethylamines

Here, we report an efficient and modular approach toward the formation of difluorinated arylethylamines from simple aldehyde-derived N,N-dialkylhydrazones and trifluoromethylarenes (CF₃-arenes). This method relies on selective C-F bond cleavage via reduction of the CF₃-arene. We show that a diverse set of CF₃-arenes and CF₃-heteroarenes react smoothly with a range of aryl and alkyl hydrazones. The β-difluorobenzylic hydrazine product can be selectively cleaved to form the corresponding benzylic difluoroarylethylamines.

Transition metal-free photochemical C-F activation for the preparation of difluorinated-oxindole derivatives

The development of strategies for single and selective C-F bond activation represents an important avenue to overcome limitations in the synthesis of valuable fluorine-containing compounds. The synthetic and medicinal research communities would benefit from new routes that access such relevant molecules in a simple manner. Herein we disclose a straightforward and mechanistically distinct pathway to generate gem-difluoromethyl radicals and their installation onto N-arylmethacrylamides for the preparation of valuable difluorinated oxindole derivatives. To achieve operational simplicity, the use of a readily available benzenethiol as a photocatalyst under open-to-air conditions was developed, demonstrating the facile multigram preparation of the targeted fluorinated molecules. Additionally, dispersion-corrected density functional theory (DFT) and empirical investigations provide a new basis to support the proposed reaction pathway, indicating that arene thiolate is an efficient organophotocatalyst for this transformation.

Photoredox Catalyzed Single C-F Bond Activation of Trifluoromethyl Ketones: A Solvent Controlled Divergent Access of gem-Difluoromethylene Containing Scaffolds

Selective defluorinative functionalization of trifluoromethyl ketones is a long-standing challenge owing to the exhaustive mode of the process. To meet the demands for the installation of the gem-difluoromethylene unit for the construction of the molecular architectures of well-known pharmaceuticals and agrochemicals, a distinct pathway is thereby highly desirable. Here, a protocol is introduced that allows the divergent synthesis of gem-difluoromethylene group containing tetrahydrofuran derivatives and linear ketones via single C-F bond activation of trifluoromethyl ketones using visible-light photoredox catalysis in the presence of suitable olefins as trapping partner. The choice of appropriate solvent and catalyst plays a significant role in controlling the divergent behavior of this protocol. Highly reducing photo-excited catalysts are found to be responsible for the generation of α,α-difluoromethyl ketone (DFMK) radicals as the key intermediate via a SET process. This protocol also results in a high diastereoselectivity towards the formation of partially fluorinated cyclic ketal derivatives with simultaneous construction of one C-C and two C-O bonds. State-of-the-art DFT calculations are performed to address the origin of diastereoselectivity as well as the divergence of this protocol.

Organoboron Reagent-Controlled Selective (Deutero)Hydrodefluorination

(Deuterium-labeled) CF₂ H- and CFH₂ -moieties are of high interest in drug discovery. The high demand for the incorporation of these fluoroalkyl moieties into molecular structures has witnessed significant synthetic progress, particularly in the (deutero)hydrodefluorination of CF₃ -containing compounds. However, the controllable replacement of fluorine atoms while maintaining high chemoselectivity remains challenging. Herein, we describe the development of a selective (deutero)hydrodefluorination reaction via electrolysis. The reaction exhibits a remarkable chemoselectivity control, which is enabled by the addition of different organoboron sources. The procedure is operationally simple and scalable, and provides access in one step to high-value building blocks for application in medicinal chemistry. Furthermore, density functional theory (DFT) calculations have been carried out to investigate the reaction mechanism and to rationalize the chemoselectivity observed.

Defluorinative Transformation of (2,2,2-Trifluoroethyl)arenes Catalyzed by the Phosphazene Base t-Bu-P2

In this study, we demonstrated that 1-tert-butyl-2,2,4,4,4-pentakis(dimethylamino)-2λ5,4λ5-catenadi(phosphazene) (t-Bu-P2) catalyzes the defluorinative functionalization reactions of (2,2,2-trifluoroethyl)arenes with alkanenitriles to produce monofluoroalkene products. The reaction proceeds through HF elimination from a (2,2,2-trifluoroethyl)arene to form a gem-difluorostyrene intermediate, which is followed by nucleophilic addition of an alkanenitrile and elimination of a fluoride anion. The catalysis is compatible with a variety of functional groups.

Experimental and computational insights into the mechanism of FLP mediated selective C-F bond activation

Frustrated Lewis pairs (FLP) comprising of B(C₆F₅)₃ (BCF) and 2,4,6-triphenylpyridine (TPPy), P(o-Tol)₃ or tetrahydrothiophene (THT) have been shown to mediate selective C-F activation in both geminal and chemically equivalent distal C-F sites. In comparison to other reported attempts of C-F activation using BCF, these reactions appear surprisingly facile. We investigate this reaction through a combination of experimental and computational chemistry to understand the mechanism of the initial C-F activation event and the origin of the selectivity that prevents subsequent C-F activation in the monoactivated salts. We find that C-F activation likely occurs via a Lewis acid assisted S_N1 type pathway as opposed to a concerted FLP pathway (although the use of an FLP is important to elevate the ground state energy), where BCF is sufficiently Lewis acidic to overcome the kinetic barrier for C-F activation in benzotrifluorides. The resultant intermediate salts of the form [ArCF₂(LB)][BF(C₆F₅)₃] (LB = Lewis base) are relatively thermodynamically unstable, and an equilibrium operates between the fluorocarbon/FLP and their activation products. As such, the use of a fluoride sequestering reagent such as Me₃SiNTf₂ is key to the realisation of the forward C-F activation reaction in benzotrifluorides. Selectivity in this reaction can be attributed to both the installation of bulky Lewis bases geminal to residual C-F sites and from electronic re-ordering of kinetic barriers (of C-F sites in products and starting materials) arising from the electron withdrawing nature of the pyridinium, phosphonium and sulfonium groups.

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

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

Late-Stage Modification of Drugs via Alkene Formal Insertion into Benzylic C-F Bond

C-F insertion of carbon-atom units is underdeveloped although it poses significant potential applications in both drug discovery and development. Herein, we report a photocatalytic protocol for late-stage modification of trifluoromethyl aromatic drugs involving formal insertion of abundant alkene feedstocks into a benzylic C-F bond selectively. This redox-neutral transformation features mild conditions and extraordinary functional group tolerance. Preliminary studies are consistent with this transformation involving a radical-polar crossover pathway. Additionally, it offers an alternative strategy for difunctionalization of alkenes via quenching of the carbocation intermediate with nucleophiles other than external fluoride.

Controlled monodefluorination and alkylation of C(sp3)-F bonds by lanthanide photocatalysts: importance of metal-ligand cooperativity

The controlled functionalization of a single fluorine in a CF3 group is difficult and rare. Photochemical C-F bond functionalization of the sp3-C-H bond in trifluorotoluene, PhCF3, is achieved using catalysts made from earth-abundant lanthanides, (CpMe4)2Ln(2-O-3,5- t Bu2-C6H2)(1-C{N(CH)2N(iPr)}) (Ln = La, Ce, Nd and Sm, CpMe4 = C5Me4H). The Ce complex is the most effective at mediating hydrodefluorination and defluoroalkylative coupling of PhCF3 with alkenes; addition of magnesium dialkyls enables catalytic C-F bond cleavage and C-C bond formation by all the complexes. Mechanistic experiments confirm the essential role of the Lewis acidic metal and support an inner-sphere mechanism of C-F activation. Computational studies agree that coordination of the C-F substrate is essential for C-F bond cleavage. The unexpected catalytic activity for all members is made possible by the light-absorbing ability of the redox non-innocent ligands. The results described herein underscore the importance of metal-ligand cooperativity, specifically the synergy between the metal and ligand in both light absorption and redox reactivity, in organometallic photocatalysis.

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

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

Alternatives to Iridium: A Polyaza[7]helicene as a Strongly Reductive Visible Light Photoredox Catalyst

The use of a readily accessible polyazahelicene as a strongly reducing metal-free alternative to the commonly used precious metal based photoredox catalysts is demonstrated. An improved two-step synthesis of the catalyst is described, and its photophysical properties with respect to its use as a photoredox catalyst are evaluated. Its activity under visible light irradiation is proven by application in two double radical light-driven multicomponent reactions. The azahelicene gave comparable results to an iridium-based catalyst originally used for the same transformations.

Modular synthesis of α-arylated carboxylic acids, esters and amides via photocatalyzed triple C-F bond cleavage of methyltrifluorides

α-Arylated carboxylic acids, esters and amides are widespread motifs in bioactive molecules and important building blocks in chemical synthesis. Thus, straightforward and rapid access to such structures is highly desirable. Here we report an organophotocatalytic multicomponent synthesis of α-arylated carboxylic acids, esters and amides from exhaustive defluorination of α-trifluoromethyl alkenes in the presence of alkyltrifluoroborates, water and nitrogen/oxygen nucleophiles. This operationally simple strategy features a unified access to functionally diverse α-arylated carboxylic acids, esters, and primary, secondary, and tertiary amides through backbone assembly from simple starting materials enabled by consecutive C–F bond functionalization at room temperature. Preliminary mechanistic investigations reveal that the reaction operates through a radical-triggered three-step cascade process, which involves distinct mechanisms for each defluorinative functionalization of the C–F bond.

Here we report an organophotocatalytic synthesis of α-arylated carboxylic acids, esters and amides from exhaustive defluorination of α-trifluoromethyl alkenes in the presence of alkyltrifluoroborates, water and nitrogen/oxygen nucleophiles.

Effects of the Chalcogenide Identity in N‐Aryl Phenochalcogenazine Photoredox Catalysts

Phenochalcogenazines such as phenoxazines and phenothiazines have been widely employed as photoredox catalysts (PCs) in small molecule and polymer synthesis. However, the effect of the chalcogenide in these catalysts has not been fully investigated. In this work, a series of four phenochalcogenazines is synthesized to understand how the chalcogenide impacts catalyst properties and performance. Increasing the size of the chalcogenide is found to distort the PC structure, ultimately impacting the properties of each PC. For example, larger chalcogenides destabilize the PC radical cation, possibly resulting in catalyst degradation. In addition, PCs with larger chalcogenides experience increased reorganization during electron transfer, leading to slower electron transfer. Ultimately, catalyst performance is evaluated in organocatalyzed atom transfer radical polymerization and a photooxidation reaction for C(sp²)−N coupling. Results from these experiments highlight that a balance of PC properties is most beneficial for catalysis, including a long‐lived excited state, a stable radical cation, and a low reorganization energy.

Defluorinative Alkylation of Trifluoromethylbenzimidazoles Enabled by Spin-Center Shift: A Synergistic Photocatalysis/Thiol Catalysis Process with CO2•

We describe a catalytic strategy for direct single C(sp³)-F bond alkylation of trifluoromethylbenzimidazoles under a photoinduced thiol catalysis process. The CO₂ radical anion (CO₂^•-) proved to be the most efficient single-electron reductant to realize such a transformation. The spin-center shift of the generated radical anion intermediate is the key step in realizing C-F bond activation under mild conditions with high efficiency.

Photocatalytic C(sp3) radical generation via C-H, C-C, and C-X bond cleavage

C(sp³) radicals (R˙) are of broad research interest and synthetic utility. This review collects some of the most recent advancements in photocatalytic R˙ generation and highlights representative examples in this field. Based on the key bond cleavages that generate R˙, these contributions are divided into C–H, C–C, and C–X bond cleavages. A general mechanistic scenario and key R˙-forming steps are presented and discussed in each section.

C(sp³) radicals (R˙) are of broad research interest and synthetic utility.

Asymmetric Defluoroallylation of 4-Trifluoromethylpyridines Enabled by Umpolung C-F Bond Activation

Carbon-fluorine bond activation of the trifluoromethyl group represents an important approach to fluorine-containing molecules. While selective defluorinative functionalization reactions of CF₃ -containing substrates have been achieved by invoking difluorocarbocation, difluorocarboradical, or difluoroorganometallic species as the key intermediates, the transformations via fluorocarbanion mechanism only achieved limited success. Furthermore, the enantioselective defluorinative transformation of the CF₃ group remained a formidable challenge. Here we report a defluorinative functionalization reaction of 4-trifluoromethylpyridines involving difluoro(pyrid-4-yl)methyl anion as the key intermediate, which was developed based upon our previous studies on the N-boryl pyridyl anion chemistry. In particular, asymmetric defluoroallylation of 4-trifluoromethylpyridines and -pyrimidines could be achieved by using Ir-catalysis to forge a difluoroalkyl-substituted chiral center.

Cu catalyzed [4 + 2] cycloaddition for the synthesis of highly substituted 3-fluoropyridines

A copper catalyzed annulation-aromatization of benzyl trifluoromethyl ketimines with 3-acryloyloxazolidin-2-ones for the synthesis of 3-fluoropyridines through double C-F bond cleavages has been developed. In this approach, the annulation occurred between the in situ formed dienes from trifluoromethyl ketimines via the first C-F bond cleavage and 3-acryloyloxazolidin-2-ones. Then the aromatization afforded 3-fluoropyridines in moderate yields through the second C-F bond cleavage. The 3-fluoropyridine products could be further hydrolyzed to multi-substituted 3-pyridinecarboxylic acids.