The demanding of discovering new pharmaceuticals, agrochemicals and advanced functional materials have triggered extensive efforts on efficient synthesis of fluorinated compounds. Over the past decade, the transition-metal-catalyzed fluoroalkylation has emerged as an efficient and straightforward strategy for the synthesis of organofluorine compounds. Despite the importance of the reported synthetic methods, the development of environmentally benign and cost-efficient fluoroalkylation reactions with base metals as catalysis and widely available fluoroalkyl halides as fluoroalkyl sources continues to attract great interest. Here, we reported the first example of iron-catalyzed cross-coupling of diarylzinc reagents with gem-difluoropropargyl bromides. The reaction proceeds under mild reaction conditions and provides a facile access to gem-difluoropropargyl arenes. Additionally, this iron-catalytic system can also be applied to the cross-coupling of aryl Grignard reagents with difluoroalkyl bromides. Applications of the method led to modified bioactive molecules efficiently, offering potential opportunities in medicinal chemistry. Preliminary mechanistic studies reveal that a single electron transfer pathway is involved in the reaction. A representative procedure for iron-catalyzed cross-coupling of diarylzincs with gem-difluoropropargyl bromide is as following: Fe(acac)₃ (10 mol%) was added to a 25 mL of Schlenck tube, the tube was then evacuated and backfilled with Ar (3 times). gem-Difluoropropargyl bromide 2 (0.3 mmol, 1.0 equiv.), TMEDA (0.45 mmol, 1.5 equiv.) and THF (1 mL) were then added, the reaction mixture was stirred at room temperature for 10 min and cooled to -20 ℃. A solution of diarylzinc reagent (0.45 mmol in 1.5 mL of THF, 1.5 equiv.) was added dropwise. After stirring for 4 h at -20 ℃, the reaction mixture was quenched with saturated NH₄Cl solution. The yield was determined by ¹⁹F NMR before working up. If necessary, the reaction mixture was diluted with EtOAc and filtered with a pad of cellite. The filtrate was concentrated, and the residue was purified with silica gel chromatography to give product 3.

Allylic-substituted compounds serve as versatile building block or the primer for many metal-catalyzed reactions. The introduction of fluorine into a drug molecule will change its pharmacokinetic and pharmacodynamic properties. Therefore, a new method of allylic fluorination would uncover novel synthetic approaches towards highly valuable fluorinated compounds such as inhibitors or fluorine-containing polypropylene. To date, the most reported methods for the synthesis of allylic fluoride involve the use of p-nitrobenzoate or trimethylsilyl as leaving group, or cleavage of tertiary cyclopropyl silyl ethers. In the past decades, the research of fluorodecarboxylation has made great progress. The most reported fluorodecarboxylations involving XeF₂, AgF or AgF₂, Selectfluor, N-fluorodibenzenesulfonimide (NFSI) are often accompanied by the occurrence of oxidation or free radical reactions, which may destroy the terminal olefin structure. The use of the fluoride ion (fluoride salt or hydrofluoric acid) as the nucleophilic component presents a series of challenges, including the low intrinsic nucleophilicity which was demonstrated by its frequent use as an additive to modulate catalytic reactivity or product distribution. However, with the assistance of transition-metal catalyst or organocatalysts, fluoride ion often serve as fluorine source for fluorination of C(sp²)―H and C(sp³)―H. Hypervalent iodine reagents, which have ability to activate a C―C multiple bond, have been recognized as an alternative to noble metal catalyst. Inspired by the pioneering exploration, we sought the possibility of achieving allylic fluorination through simple protocol of fluorodecarboxylation with cheap nucleophilic fluorination reagents and mild oxidant. In this work, a new strategy is introduced for the synthesis of allylic fluorides via decarboxylative fluorination of β,γ-unsaturated carboxylic acids using PhI(OAc)₂ and TEA·3HF. The best result was achieved by using 1.2 equiv. of PhI(OAc)₂ and 5 equiv. of TEA·3HF in CH₂Cl₂ at 75 ℃ for 12 h, giving allylic fluoride 2a in 76% yield. The versatile synthetic utilities of the allylic fluorides were also developed through cycloaddition, oxidation, reduction, substitution involving formation of C―O, C―S, C―Se and C―N bond via activation of C―F bond.

The incorporation of fluorine atoms or fluorine-containing fragments to specifical sites of organic compounds would result in unique diversifications in biological or physical properties, such as, significantly regulate the lipid solubility or metabolic stability, and promote specific binding ability to biological targets of target compounds. Monofluoroalkenes are ideal amide bond mimics, and have been widely used in the research field of pharmaceutical chemistry and drug discovery. Previously, we reported the nickel-catalyzed reductive cross coupling of gem-difluoroalkenes with unactivated secondary alkyl iodides and tertiary alkyl bromides. However, only medium yield can be obtained with primary alkyl halides, which might be caused by the lower stability and nucleophilic activity of these substrates. Herein, we report the nickel-catalyzed Suzuki-type cross coupling of fluorinated alkenyl boronates with alkyl halides for the synthesis of primary alkyl group substituted monofluoroalkenes. By using NiBr₂(diglyme) (10 mol%) and 4,4'-di-tert-butyl-2,2'-bipyridine (15 mol%) as catalytic systems, Na₂CO₃ (2 equiv.) as base, N,N-dimethylacetamide as solvent, we achieved the cross coupling of a variety of fluorinated alkenyl boronates with primary alkyl iodides (e.g., 5), bromides (e.g., 9) and relatively inert secondary alkyl bromide (20). Under the mild reaction conditions, this reaction performed smoothly with good isolated yields and well functional group toleration. Many synthetically useful functional groups could survive during the transformation, such as, ether (6, 7), trifluoromethyl (8), cyano (10), ester (11), and even unprotected alcohol hydroxyl group (13). In addition, heterocycles such as tetrahydrofuran (14), phthalimide (15), dioxane (16), indole (17), pyridine (27) and quinoline (35) also posed no problem for this reaction. It should be pointed out that, this reaction is applicable not only to non-activated alkyl halides, but also to the conversion of activated allyl bromides (18, 19). For the fluorinated alkenyl boronates, this reaction also exhibited good functional group compatibility and wide substrate scope, and conducted successfully with both electron-rich (e.g., 4, 24), electron-neutral (e.g., 21), or electron-deficient (e.g., 27, 31) aromatic rings. Finally, the toleration of aryl sulfonate (30) provided further opportunities for subsequent modification through transition-metal-catalyzed cross coupling reactions. Radical clock experiment with (Z)-8-iodooct-3-ene (36) provided a mixture of linear product (37a) and ring-cyclized product (37b). (Bromomethyl)cyclopropane (38) was also subjected to the standard reaction conditions, only ring-opening product (39a) was obtained. In addition, this reaction was significantly inhibited with the addition of TEMPO (2,2,6,6-tetramethylpiperidinooxy). These results indicated a radical-type reaction mechanism for the cross coupling of fluorinated alkenyl boronates with alkyl halides. Further efforts would be devoted to develop one-pot synthesis of monofluoroalkenes through in-situ borylation of gem-difluoroalkenes and subsequent Suzuki-type cross coupling with alkyl halides.

It is known that fluorine is the strongest in electronegativity and a peculiar element. Fluorinated compounds are extensively applied in the areas of pharmaceuticals, agrochemical, materials, life sciences, etc., due to the unique chemical, physical and biological properties of fluorine-containing compounds. Therefore, the development of expedient synthetic strategies for the introduction of —F, —CF₂H and —CF₃ into organic compounds has attracted much attentions of chemists. Although trifluoromethylation has been well developed, difluoromethylation has been less reported. We found that difluorocarbene (:CF₂) could be generated in situ from ethyl bromodifluoroacetate (BrCF₂COOEt) in the presence of Na₂CO₃, which could go through N—H, O—H difluoromethylation smoothly. The scope of substrates was broad, and various functional groups, such as halogen, formyl group, nitro-group, nitrile and so on could be tolerated well. This would be a potential and practical reaction in modification of various bioactive drugs beause benzimidazole, indazole and pyridine are the skeleton of medicine and nature molecule. In addition, a representative procedure for this reaction is as following: An oven-dried Schlenk tube (10 mL) was equipped with a magnetic stir bar, the substrates of nitrogen-containing or oxygen-containing (0.3 mmol), the base (Na₂CO₃,2 equiv., 0.6 mmol), ethyl bromodifluoroacetate (1.2 equiv., 0.36 mmol). The flask was evacuated and backfilled with N₂ for 3 times, acetone or acetonitrile as a solvent for 24 h under N₂ atmosphere. Where after the solvent concentrated in vacuo and the residue was purified by chromatography on silica gel with ethyl acetate:petroleum ether (EA:PE＝1:30) to afford the corresponding products.

α,α-Difluoroketones represent an important subclass of organofluorine compounds, and have been widely applied in medicinal chemistry, particularly as enzyme inhibitors. Efficient use of organofluorine reagents plays a key role for the synthesis of fluorine-containing organic compounds. As an environmental and efficient difluorocarbene reagent, TMSCF₂Br has been well utilized in synthetic applications. In 2013, Hu first utilized TMSCF₂Br as a general difluorocarbene source for the difluoromethylenation of alkenes/alkynes as well as the difluoromethylation of O-, S-, N-, and P-nucleophiles. Moreover, Dilman realized the rapid assembly of various CF₂-containing products by using TMSCF₂Br as a difluorocarbene source, which depended on the concept of three independent components: difluorocarbene, nucleophile, and electrophile. Compared with the previous works, we recently reported a catalytic difluorocyclopropanation of enolizable ketones by using TMSCF₂Br reagent, which acts as not only the difluorocarbene source but also the TMS transfer agent. The in situ generated siloxydifluorocyclopropanes were used for the synthesis of α-fluoroenones, o-fluoronaphthols, α,α-difluorocyclopentenones and α,α-difluorocyclopentanones compounds. Here, we report a simple and effective method for the conversion of enolizable ketones to α,α-difluoro-β-halo-substituted ketones. The whole process involves the in situ formation and regioselective ring opening halogenation of siloxydifluorocyclopropanes. The reaction features easily available raw materials, simple operation and practical method. A representative procedure for this reaction is as following: To a dried polytetrafluoroethene (PTFE) sealed pressure tube were added ketone 1 (0.5 mmol), n-Bu₄NBr (0.05 mmol, 10 mol%), TMSCF₂Br (0.75 mmol) and toluene (2.5 mL) in sequence. The reaction mixture was stirred at 110 ℃ for 2 h, followed by adding an additional amount of TMSCF₂Br (0.5 mmol) for another 4 h. Removal of toluene under reduced pressure delivered a mixture mainly containing 2. The reaction system was allowed to cool to room temperature followed by adding NBS/NIS (0.75 mmol) and CH₃CN (2 mL). The resulting mixture was stirred at room temperature for 2 h to consume 2 and then poured into saturated NaCl solution (30 mL), extracted with CH₂Cl₂ (10 mL×3). The combined organic extracts were dried over anhydrous MgSO₄, filtered and concentrated under reduced pressure to yield the crude product, which was purified by silica gel chromatography (petroleum ether/ethyl acetate: 100/1, V/V) to afford the pure product 4/5.

Although the debate on whether or not C―F bonds can function as H-bond acceptors lasted for tens of years, dating back to 1939 when Pauling pointed out in The Nature of the Chemical Bond that C-F bonds do not have significant power to act as proton acceptors in the formation of hydrogen bonds, more and more evidences support the existence of C―F…H―X interactions, and in particular, C―F…H―O and C―F…H―N interactions cannot be ignored.Because the sum of the van der Waals radii of hydrogen and fluorine atoms is reported to be around as 2.55 Å, C―F…H―X interactions may exist if the calculated distance of F…H is less than 2.50 Å. Strong C―F…H―X interactions may occur if the calculated distance is less than 2.30 Å and the F…H―X angle is greater than 120°.In 2011, we observed strong fluorine effects on the Strecker reaction of ketimines: while Schreiner's thiourea could catalyze the Strecker reaction of acetophenone derived ketimine using TMSCN, it was unable to mediate the corresponding reaction of analogy α-CF₃ or α-CF₂H ketimines. Theoretical calculations revealed that the C―F…H―N interactions between the C―F bond of fluorinated ketimines and thiourea played the key role. This is the first report on the influence of such subtle interactions on organic reactions. Since then, reports from our and other groups revealed various types of C―F…H―X interactions that may be present in the reaction course, to strongly influence the reactivity and selectivity. Although successful examples are still limited, these achievements have suggested that C―F…H―X interactions may exist between the substrate and the catalyst; the substrate and the solvent; different reaction partners, or engender in the transition state with the reaction intermediate. Importantly, known examples demonstrate it possible to harness C―F…H―X interactions to tune reactivity and/or selectivity, which are useful for new reaction development, as well as for the design of new catalysts. To provide reference and inspiration for researchers engaged in organic synthesis, especially the organic fluorine chemistry, we summarize in this review the recent advances in the study of the influences of C―F…H―X interactions on organic reactions.

2-Aminopyrazines are widely found in naturally occurring compounds, drugs and biologically active ingredients. Especially, the compounds containing a fluorinated aminopyrazine have been applied in the pharmaceutical industry. The introduction of a fluorine atom into organic compounds generally leads to a significant change in the chemical, physical and biological properties. Therefore, new method for introducing a fluorine atom into the aminopyrazine ring is highly desirable. Traditional Balz-Schiemann reaction is difficult to employ in the preparation of fluorinated aminopyrazines because of the decomposition of pyrazine derivatives under strong acidic conditions. In general, pyrazines can take place nucleophilic fluorination; aminopyrazines, which is activated by an amino group, can occur electronphilic halogenation; the radical fluorination of pyrazine derivatives has not reported yet. We envisage a direct fluorination of 2-aminopyrazines with Selectfluor may proceed under mild conditions. In this paper, the fluorination of 2-aminopyrazine derivatives with Selectfluor in aqueous phase was studied, and a transition-metal free fluorination of 2-aminopyrazine derivatives was developed. The method affords 5-fluoro-2-aminopyrazines in good yield with excellent chemoselectivity and high regioselectivity. The results suggested that the fluorination may undergo a radical process. Using this method, an enzyme inhibitor having a certain inhibitory effect on analog of B-Raf enzyme was synthesized. The synthesis was as follows: 6-phenyl-2-aminopyrazine (1a, 0.2 mmol), selectfluor (2a, 0.1 mmol), toluene:water [V(toluene):V(water)＝1:1, 2 mL] in a reaction tube. The reaction was carried out at room temperature, monitoring by ¹⁹F NMR. After the completion of the reaction, the reaction mixture was cooled, diluted with ethyl acetate, washed with saturated brine, and dried over anhydrous sodium sulfate. The mixture was filtered, and the filtrate was concentrated and purified by silica gel column chromatography (petroleum ether/ethyl acetate) to give 5-fluoro-2-aminopyrazines 3 and 3-fluoro-2-aminopyrazines 3'.

Organic compounds containing trifluoromethyl (CF₃) group(s) are widely prevalent in biochemical and medicinal science. This is mainly due to the fact that the trifluoromethyl group often improves the metabolic stability and lipophilicity of biologically active compounds. The need of efficient methods for the incorporation of this group into target molecules has spurred research to discover new, practical CF₃ sources. Among various CF₃ sources, the radical trifluoromethylating reagents has provided a strong driving force for the discovery of the novel trifluoromethylation reactions, and contributed enormously to the efficient synthesis of various CF₃-containing compounds. Although a wide variety of radical CF₃ sources are now available to organic chemists, little attention has been paid to assess their trifluoromethyl radical donor abilities (TR^·DA). Moreover, the available radical reagents show a very rich and diverse reactivity. The establishment of an extensive scale to quantify their CF₃ radical donating abilities should be of great value for both the rational design of novel reagents and the judicious selection of appropriate reagent to explore new radical reactions. Herein, we present a systematic computational study of the homolytic X―CF₃ bond dissociation enthalpies of 35 radical trifluoromethylating reagents by using the SMD-M06-2X/[6-311++G(2df, 2p)-Def2-QZVPPD]//SMD-M06-2X/[6-31+G(d)-LANL2DZ] method, aiming to provide an energetic guide for estimating their trifluoromethyl radical donor abilities. A comprehensive TR^·DA scale was constructed, which covers a range from -21.5 to 95.2 kcal·mol^-1. The effects of the frequently used activators including single electron transfer reagents and halogen/chalcogen-bond donors on trifluoromethyl radical donor abilities were investigated. The results show that single electron transfer is the most efficient way to promote the CF₃ radical release. We expect that the results of this study could be highly valuable for the mechanistic understanding and the rational design of novel CF₃ sources and new radical trifluoromethylation reactions.

Fluorinated heterocycles represent a ubiquitous structural motif found in numerous pharmaceuticals, agrochemicals, and functional materials. This is especially true for fluorine-containing five-membered heteroaromatic compounds that have been widely investigated in various fields for a long time. In this context, fluorinated isoxazoles have emerged as valuable scaffolds owing to their diverse biological properties. Among various approaches that have been developed for the synthesis and functionalization of isoxazoles, efficient and modular route to fluorine-substituted isoxazoles are still limited. Traditional methods include the condensation of 2-fluoro-1,3-dicarbonyl derivatives with hydroxylamine, Au-catalyzed fluorocyclization of 2-alkyne O-methyloximes, and direct fluorination of isoxazoles. However, the wide applicability of these approaches often suffers from low chemical yields, harsh reaction conditions, and limited substrate scope. Herein, we describe a one-pot protocol for the construction of fluorinated isoxazoles from CF₃-containing precursors with hydroxylammonium chloride. Typical features of this reaction include mild conditions, simple operations, and good functional group compatibility. This method provides facile access to a series of 3-F-5-aryl-isoxazoles in moderate to good yields from easily available α-CF₃-β-keto esters. Moreover, further synthetic transformations of obtained isoxazoles to important bio-active molecular derivatives have also been demonstrated. A representative procedure for this reaction is as following: α-CF₃-β-keto ester 1 (0.2 mmol, 1.0 equiv.), HONH₂·HCl (46 mg, 0.66 mmol), pyridine (71 μL, 0.88 mmol), and CH₃CN (3.0 mL) were added into an oven-dried vial equipped with a magnetic stir bar. The mixture was stirred at 75 ℃ for 12 h and monitored by thin-layer chromatography (TLC). After completion, 10 mL of water was added and the mixture was extracted with EtOAc for three times. The combined organic layers were washed with saturated NaCl and dried over Na₂SO₄. The mixture was evaporated under reduced pressure and residue was purified by flash chromatography on silica gel eluting with petroleum ether/ethyl acetate (V:V＝30:1) to afford the 3-F-5-aryl-isoxazole 2.

Nitriles are important structural motifs found in agrochemicals, pharmaceuticals, and natural products. Furthermore, nitriles are versatile synthetic precursors for organic synthesis because they can be easily converted into various functionalities, such as amides, ketones, esters, primary amines, aldehydes, carboxylic acids, and nitrogen-containing heterocycles. Therefore, the development of efficient methods for the synthesis of nitrile compounds has attracted much attention from synthetic chemists. Cyanomethylation of various substrates is a synthetically useful reaction because a variety of diversely cyano-containing compounds could be readily prepared. Acetonitrile is the simplest commercially available alkyl nitrile, which can act as the cyanomethyl carbanion source. The traditional method for the cyanomethylation of organic molecules is deprotonation of acetonitrile in the presence of strong base. Alternatively, transition-metal-catalyzed C—H bond activation of acetonitrile represents an attractive approach to cyanomethylated compounds due to its atom and step economy. In this communication, we developed a simple and highly efficient method for the synthesis of cyanated difluorostyrene derivatives by cyanomethylation of α-(trifluoromethyl)styrenes using cheap and commercially available acetonitrile as the CH₂CN^- source. The reaction proceeded smoothly in the presence of LiHMDS at room temperature and was finished within 1 h, affording the cyanated gem-difluoroalkenes in moderate to good yields. Furthermore, the cyanomethylation reaction exhibited good substrate scope and functional group compatibility. A general procedure for the cyanomethylation of α-(trifluoromethyl)styrenes with acetonitrile is as following: α-(trifluoromethyl)styrenes 1 (0.5 mmol) was dissolved in acetonitrile 2a (4 mL) at room temperature under argon atmosphere. Subsequently, a solution of the LiHMDS in THF (1.5 mL, 1.0 mol/L, 1.5 mmol, 3.0 equiv.) was added dropwise within 50 min and stirring was continued for further 10 min (monitored by TLC). After completion of the reaction, the reaction mixture was quenched with saturated aqueous solution of NH₄Cl (15 mL) and extracted with ethyl acetate (5 mL×3). The combined organic layer was dried over anhydrous Na₂SO₄, filtered, and concentrated under vacuum. The crude residue was then purified by column chromatography on silica gel [(V(hexane)/V(ethyl acetate)＝10:1～6:1] directly to afford the pure target compounds.

Fluorine-containing groups can modulate the physicochemical and biological properties of organic molecules. Consequently, the synthesis of fluorinated organic molecules has attracted considerable attention in the field of pharmaceuticals, agrochemicals and material sciences. Among fluorine-containing groups, the trifluoromethylthio group has the highest Hansch’s hydrophobicity parameter and remarkable electron-withdrawing character. The incorporation of a trifluoromethylthio group into organic molecules can significantly enhance their membrane permeability and metabolic stability because of its high lipophilicity and strong electron-withdrawing effect. As a result, various methods have been involved to synthesize SCF₃-containing compounds using electrophilic or nucleophilic trifluoromethylthio reagents. On the other hand, the chirality of pharmaceutical molecules has an important effect on their properties, and different stereoisomers of a pharmaceutical molecules always have dramatically different pharmaceutical activities. Thus, the asymmetric trifluoromethylthiolation of organic molecules is of growing interest in recent years. Up to now, this field is still in the stage of initial development. In this perspective article, we will briefly summarize the methods of asymmetric trifluoromethylthiolation of organic molecules that have been reported so far. Two different strategies including the use of electrophilic trifluoromethylthiolating reagents and the use of trifluoromethylthio-containing building blocks will be introduced. Employing electrophilic trifluoromethylthiolating reagents, the enantioselective trifluoromethylthiolation of β-ketoesters, oxindoles as well as alkenes have been developed using Cinchona alkaloid, copper(Ⅱ) or indane-based chiral sulfide/selenide as the catalyst. Alternatively, using trifluoromethylthiolated building blocks is another approach to establish chiral centers bearing the trifluoromethylthio group. In this approach, an asymmetric trifluoromethylthiolation via enantioselective [2,3]-sigmatropic rearrangement of a sulfonium ylide generated from SCF₃-containing sulfide and metal carbene has been disclosed using chiral Rh(Ⅱ) and Cu(I) as the catalyst. Finally, we will discuss the challenges of the asymmetric trifluoromethylthiolation of organic molecules in the future.

Radical-mediated C-SCF₃ bond formation via the addition of SCF₃ radical to alkenes has become an efficient strategy for the construction of alkyl trifluoromethylthioethers. However, the scope of alkenes is largely limited to activated alkenes in which the presence of adjacent carbonyl or aryl group is required to stabilize the alkyl radical intermediates by p-π conjugation. A few cases involving trifluoromethylthiolation of unactivated olefins have been reported, but in these reactions only a single functional group is incorporated to alkenes. The radical difunctionalization of unactivated olefins remains challenging and has received less attention. Recently, we established a new protocol to realize the radical difunctionalization of alkenes through intramolecularly distal functional group migration. This tactic provides a useful and elegant tool for the elusive functionalization of unactivated olefins. A portfolio of groups such as cyano, heteroaryl, imino, aldehyde, and alkynyl can be readily migrated in the transformation. Herein, we disclose an efficient and practical approach for the trifluoromethylthiolation of unactivated olefins based on the intramolecular migration of heteroaryl and imino groups. The migration is triggered by the addition of SCF₃ radical, which is generated from the mixture of AgSCF₃ and K₂S₂O₈at room temperature, to alkenes. The reaction demonstrates a high functional group compatibility and broad substrate scope. A variety of nitrogen-containing five- and six-membered heteroaryl as well as imino groups are readily migrated, affording the synthetically valuable alkyl trifluoromethylthioether compounds in good yields. The typical procedure is as follows: a mixture of tertiary alcohol (0.2 mmol), AgSCF₃(0.3 mmol), and K₂S₂O₈(0.6 mmol) is loaded in a flame-dried reaction vial which is subjected to evacuation/flushing with nitrogen three times. Dry DMF (2.0 mL) is added to the mixture via syringe, and the mixture is then stirred at room temperature until the starting material is consumed which is determined by TLC. The mixture is extracted with ethyl acetate (10 mL×3). The combined organic extracts are washed with brine, dried over Na₂SO₄, filtered, concentrated, and purified by flash column chromatography on silica gel (eluent: petroleum ether/ethyl acetate) to give the desired product.