Selective Defluoroallylation of Trifluoromethylarenes

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.

General Defluoroalkylation of Trifluoromethylarenes with Both Electron-Donating and -Withdrawing Alkenes

The incorporation of gem-difluoromethylene units into organic molecules remains a formidable challenge. Conventional methodologies for constructing aryldifluoromethyl derivatives relied on the use of high-functional fluorinating regents under harsh conditions. Herein, we report general and efficient photoredox catalytic systems for defluoroalkylation of readily available trifluoromethylarenes through selective C-F cleavage to deliver gem-difluoromethyl radicals which proceed through reductive addition to both electron-donating and withdrawing alkenes under transition-metal free conditions. Mechanistic studies reveal that thiol serves as both photocatalyst and HAT reagent under visible light irradiation. This synergistic photocatalysis and HAT catalysis protocol exhibits ample and salient features such as high chemo- and regioselectivity, broad substrate scope, amenable gram-scale synthesis and late-stage modification of bioactive molecules.

Visible-Light-Driven Iron-Catalyzed 1,2-Difluoroalkylthiolation of Alkenes

The synthesis of medicinally interesting aryldifluoromethylated compounds has drawn significant research attention in recent years. Herein, we report an unprecedented iron-mediated process for the selective defluorination of trifluoromethylarenes to achieve the 1,2-difluoroalkylthiolation of alkenes. Preliminary mechanistic studies revealed that thiolate anion, trifluoromethylarene, and iron cation could form an electron donor-acceptor (EDA) complex, which induced selective defluorination and then difunctionalization of alkenes to obtain aryldifluoromethylated products. The generated aryldifluoromethylated compounds make it difficult to form an EDA complex again, thus avoiding excessive defluorination. This conversion has concise and ambient reaction conditions and provides an alternative solution for obtaining difluorobenzylic intermediates.

Influence of chemical structures on reduction rates and defluorination of fluoroarenes during catalytic reduction using a rhodium-based catalyst

Continuous growth in fluoroarene production has led to environmental pollution and health concerns owing to their persistence, which is attributed to the stable C-F bond in their structures. Herein, we investigated fluoroarene decomposition via hydrodefluorination using a rhodium-based catalyst, focusing on the effects of the chemical structure and functional group on the defluorination yield. Most compounds, except (pentafluoroethyl)benzene, exhibited full or partial reduction with pseudo-first-order rate constants in the range of 0.002-0.396 min-1 and defluorination yields of 0%-100%. Fluoroarenes with hydroxyl, methyl, and carboxylate groups were selected to elucidate how hydrocarbon and oxygen-containing functional groups influence the reaction rate and defluorination. Inhibition of the reaction rate and defluorination yield based on functional groups increased in the order of hydroxyl < methyl < carboxylate, which was identical to the order of the electron-withdrawing effect. Fluoroarenes with polyfluoro groups were also assessed; polyfluoro groups demonstrated a different influence on catalyst activity than non-fluorine functional groups because of fluorine atoms in the substituents undergoing defluorination. The reaction kinetics of (difluoromethyl)fluorobenzenes and their intermediates suggested that hydrogenation and defluorination occurred during degradation. Finally, the effects of the type and position of functional groups on the reaction rate and defluorination yield were investigated via multivariable linear regression analysis. Notably, the electron-withdrawing nature of functional groups appeared to have a greater impact on the defluorination yield of fluoroarenes than the calculated C-F bond dissociation energy.

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.

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'.

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.

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.

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.

Visible-Light-Promoted C(sp3)-C(sp3) Cross-Coupling of Amino Acids and Aryl Trifluoromethyl Ketones Through Simultaneous Decarboxylation and Defluorination

A photoredox-catalyzed approach for the difluoroalkylation of amino acids was achieved through simultaneous decarboxylation and defluorination processes. This innovative protocol employs commonly available amino acids and trifluoroacetophenones as the primary starting materials, eliminating the necessity for preactivation. This strategy has enabled the synthesis of several difluoroketone functionalized amines in moderate to impressive yields. These synthesized compounds are presented as foundational molecules for subsequent modification. The underlying mechanism for the transformation is anchored in a single electron transfer (SET) radical pathway.

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.

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.

Difluoromethylene insertion into fluoroalkyl copper complexes

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

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.

Access to germasiloxanes and alkynylgermanes mediated by earth-abundant species

The reactions between silanols or terminal acetylenes with alkynylgermanes have been accomplished using potassium bis(trimethylsilyl)amide as the catalyst. This strategy has provided an entry point into various organogermanes including germasiloxanes and alkynylgermanes. Remarkably, not only KHMDS but also simple bases such as KOH can serve as efficient catalysts in this process.

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.

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

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

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.

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

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

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.

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.

Theoretical Insight into the Multiple Roles of LiHMDS in Pd-Catalyzed Borylation of Fluorobenzene

Pd-catalyzed borylation of fluorobenzene was theoretically studied. DFT calculations revealed that the reaction occurs through an unprecedented 3 + 6-membered ring transition state, in which one LiHMDS (HMDS = hexamethyldisilazane) acts as a ligand and another LiHMDS is essential to provide Li···N and Li···F interactions, overcoming the large destabilization of the strong phenyl-F bond distortion. The characteristic feature of LiHMDS was elucidated by comparing it with HMDS and NaHMDS analogues.

1,2-Radical Shifts in Photoinduced Synthetic Organic Transformations: A Guide to the Reactivity of Useful Radical Synthons

The exploration of 1,2-radical shift (RS) mechanisms in photoinduced organic reactions has provided efficient routes for the generation of important radical synthons in many chemical transformations. In this Review, the basic concepts involved in the traditional 1,2-spin-center shift (SCS) mechanisms in recently reported studies are discussed. In addition, other useful 1,2-RSs are addressed, such as those proceeding through 1,2-group migrations in carbohydrate chemistry, via 1,2-boron shifts, and by the generation of α-amino radicals. The discussion begins with a general overview of the basic aspects of 1,2-RS mechanisms, followed by a demonstration of their applicability in photoinduced transformations. The sections that follow are organized according to the mechanisms operating in combination with the 1,2-radical migration event. This contribution is not a comprehensive review but rather aims to provide an understanding of the topic, focused on the more recent advances in the field, and establishes a definition for the nomenclature that has been used to describe such mechanisms.

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.

Carbodefluorination of fluoroalkyl ketones via a carbene-initiated rearrangement strategy

The C-F bond cleavage and C-C bond formation (i.e., carbodefluorination) of readily accessible (per)fluoroalkyl groups constitutes an atom-economical and efficient route to partially fluorinated compounds. However, the selective mono-carbodefluorination of trifluoromethyl (CF3) groups remains a challenge, due to the notorious inertness of C-F bond and the risk of over-defluorination arising from C-F bond strength decrease as the defluorination proceeds. Herein, we report a carbene-initiated rearrangement strategy for the carbodefluorination of fluoroalkyl ketones with β,γ-unsaturated alcohols to provide skeletally and functionally diverse α-mono- and α,α-difluoro-γ,δ-unsaturated ketones. The reaction starts with the formation of silver carbenes from fluoroalkyl N-triftosylhydrazones, followed by nucleophilic attack of a β,γ-unsaturated alcohol to form key silver-coordinated oxonium ylide intermediates, which triggers selective C-F bond cleavage by HF elimination and C-C bond formation through Claisen rearrangement of in situ generated difluorovinyl ether. The origin of chemoselectivity and the reaction mechanism are determined by experimental and DFT calculations. Collectively, this strategy by an intramolecular cascade process offers significant advances over existing stepwise strategies in terms of selectivity, efficiency, functional group tolerance, etc.

Nickel-catalyzed cross-electrophile allylation of vinyl bromides and the modification of anti-tumour natural medicine β-elemene

Herein, we present a facile and efficient allylation method via Ni-catalyzed cross-electrophile coupling of readily available allylic acetates with a variety of substituted alkenyl bromides using zinc as the terminal reductant. This Ni-catalyzed modular approach displays excellent functional group tolerance and a broad substrate scope, which the creation of a series of 1,4-dienes including several structurally complex natural products and pharmaceutical motifs. Moreover, the coupling strategy has the potential to realize enantiomeric control. The practicality of this transformation is demonstrated through the potent modification of the naturally antitumor active molecule β-elemene.

Herein, we present a facile and efficient allylation method via Ni-catalyzed cross-electrophile coupling of readily available allylic acetates with a variety of substituted alkenyl bromides using zinc as the terminal reductant.

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.

Mechanochemical Conversion of Aromatic Amines to Aryl Trifluoromethyl Ethers

Increased interest in the trifluoromethoxy group in organic synthesis and medicinal chemistry has induced a demand for new, selective, general, and faster methods applicable to natural products and highly functionalized compounds at a later stage of hit-to-lead campaigns. Applying pyrylium tetrafluoroborate, we have developed a mechanochemical protocol to selectively substitute the aromatic amino group with the OCF₃ functionality. The scope of our method includes 31 examples of ring-substituted anilines, including amides and sulfonamides. Expected S_NAr products were obtained in excellent yields. The presented concise method opens a pathway to new chemical spaces for the pharmaceutical industry.

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.

Recent advances in radical enabled selective Csp3-F bond activation of multifluorinated compounds

Fluorine-containing molecules have found broad applications in pharmaceutical and agrochemical industries as introducing fluorine into a molecule could significantly tune the biological activities of parent molecules. Thus, the synthesis of fluorine-containing molecules has received substantial attention over the past few decades. As a complementary strategy for the synthesis of fluorinated compounds through new C_sp³-F bonds formation, selective cleavage of inert C_sp³-F bonds from easily-available and cost-effective multifluorinated molecules, such as fluoroalkylaromatics, α-trifluoromethyl alkenes and α-multifluorinated carbonyl compounds, has been emerging as an attractive alternative to access fluorine-containing molecules. Moreover, the inherent nature of radical reactions offers the opportunity for the selective C_sp³-F functionalizations to occur under mild conditions. In this regard, the development of photoredox catalysis, transition-metal catalysis, or electrochemistry to enable radical species generation via selective C_sp³-F cleavage has gained broad attention and substantial progress has been made over recent years. This highlight summerizes the recent advances in the single-electron-transfer enabled selective functionalizations of C_sp³-F bonds in multifluorinated compounds via radical pathways.

Hydride reduction of o-(fluorosilyl)benzodifluorides for subsequent C–F transformations

An efficient method for sequential C–F transformations of o-hydrosilyl-substituted benzotrifluorides is disclosed. A key to the success is hydride reduction of o-fluorosilyl-substituted difluoromethylenes prepared by a single C–F transformation of...

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

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

Organocatalytic difluorobenzylation of 1,2-diketones via mild cleavage of carbon–carbon bonds

Difluoroacetophenones (DFAPs) are developed as a class of novel and practical reagents for organocatalytic difluorobenzylation reactions.

Design, synthesis and biological evaluation of 4-phenoxy-pyridine/pyrimidine derivatives as dual VEGFR-2/c-Met inhibitors

A class of 4-phenoxy-pyridine/pyrimidine derivatives (23a–23p and 24a–24h) were designed, synthesized and evaluated as potent dual VEGFR-2/c-Met inhibitors.

Photoredox-Catalyzed Defluorinative Functionalizations of Polyfluorinated Aliphatic Amides and Esters

Selective C-F bond functionalization of perfluoalkyl units has huge potential towards accessing functionalized organofluorinated compounds, but remains chanllenging due to the high C-F bond strength and inherent selectivity challenges. We report a new catalytic approach for the selective functionalization of the strong C-F bonds in polyfluorinated aliphatic esters and amides. This simple reaction proceeds in mild and operationally fashion with divergent conversions, including hydrodefluorination, defluoroalkylation, and defluoroalkenlylation, affording a diverse array of important partially fluorinated motifs. Straightforward downstream chemistry towards fluorinated alcohols, amines and drug derivatives highlights the potential of the protocol.

Photochemical C-F Activation Enables Defluorinative Alkylation of Trifluoroacetates and -Acetamides

The installation of gem-difluoromethylene groups into organic structures remains a daunting synthetic challenge despite their attractive structural, physical, and biochemical properties. A very efficient retrosynthetic approach would be the functionalization of a single C-F bond from a trifluoromethyl group. Recent advances in this line of attack have enabled the C-F activation of trifluoromethylarenes, but limit the accessible motifs to only benzylic gem-difluorinated scaffolds. In contrast, the C-F activation of trifluoroacetates would enable their use as a bifunctional gem-difluoromethylene synthon. Herein, we report a photochemically mediated method for the defluorinative alkylation of a commodity feedstock: ethyl trifluoroacetate. A novel mechanistic approach was identified using our previously developed diaryl ketone HAT catalyst to enable the hydroalkylation of a diverse suite of alkenes. Furthermore, electrochemical studies revealed that more challenging radical precursors, namely trifluoroacetamides, could also be functionalized via synergistic Lewis acid/photochemical activation. Finally, this method enabled a concise synthetic approach to novel gem-difluoro analogs of FDA-approved pharmaceutical compounds.