Single C(sp3)–F Bond Activation in a CF3 Group: Ipso-Defluorooxylation of (Trifluoromethyl)alkenes with Oximes

Transition-Metal-Free Hydrodefluoroamination of Trifluoromethyl Enones for the Synthesis of α-Fluoroenamides

A three-component strategy was developed to enable hydrodefluoroamination of β-trifluoromethyl enones by selectively activating two C(sp3)-F bonds in the trifluoromethyl group. The method involved a sequence of carbonyl reduction, hydrodefluorination, and defluoroamination under transition-metal-free conditions. Synthetically useful (E)-stereospecific α-fluoroenamides were obtained in good yields with diverse functional group tolerance, which could be easily transformed into valuable organofluorides and heterocycles. The carbonyl auxiliary exerts both electronic and steric impacts on the CF3-alkenes, allowing for controllable and selective defluorination.

Nickel-catalyzed hydrodefluorination/deuterodefluorination of CF3-alkenes with formic acid

The synthesis of deuterated gem-difluoroalkenes via selective deuterodefluorination of β-CF3-cinnamates using a nickel catalyst has been reported for the first time. Commercially available deuterated formic acid is a cheap and convenient deuterium source. The nickel-catalyst showed high selectivity for monodefluorination and avoided competitive reactions such as multiple defluorination or hydrogenation.

Picking Two out of Three: Defluorinative Annulation of Trifluoromethyl Alkenes for the Synthesis of Monofluorinated Carbo- and Heterocycles

The increasing demand of organofluorine compounds in medicine, agriculture, and materials sciences makes sophisticated methods for their synthesis ever more necessary. Nowadays, not only the C-F bond formation but also the selective C-F bond cleavage of readily available poly- or perfluorine-containing compounds have become powerful tools for the effective synthesis of organofluorine compounds. The defluorinative cross-coupling of trifluoromethyl alkenes with various nucleophiles or radical precursors in an SN 2' manner is a convergent route to access gem-difluoroalkenes, which in turn react with nucleophiles or radical precursors via an SN V-type reaction. If the SN V reactions occur intramolecularly, the dual C-F bond cleavage of trifluoromethyl alkenes allows facile assembly of monofluorinated cyclic skeletons with structural complexity and diversity. In this personal account, we summarized the advances in this field on the basis of coupling and cyclization partners, including binucleophiles, alkynes, diradical precursors and radical precursors bearing a nucleophilic site. Accordingly, the annulation reactions can be achieved by base-mediated sequential SN 2'/SN V reactions, transition metal catalyzed or mediated reactions, photoredox catalysis, and the combination of photocatalytic reactions with SN V reaction. In the context of seminal works of others in this field, a concise summary of the contributions of the authors is also offered.

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.

Base-Controlled Divergent Synthesis of Fluorine-Containing Benzo[4,5]imidazo[2,1-b][1,3]thiazines from 2-Mercaptobenzimidazoles and β-CF3-1,3-Enynes

A base-controlled divergent cyclization between 2-mercaptobenzimidazoles and β-CF3-1,3-enynes providing either trifluoromethylated or fluorinated benzo[4,5]imidazo[2,1-b][1,3]thiazines has been developed. The β-CF3-1,3-enyne, as a three-carbon synthon, underwent a 1,8-diazabicyclo[5.4.0] undec-7-ene (DBU)-catalyzed tandem hydroamination/intramolecular hydrothiolation to give CF3-substituted 3,4-dihydro-2H-benzo[4,5]imidazo[2,1-b][1,3]thiazine, whereas reaction with KOH afforded fluorinated 4H-benzo[4,5]imidazo[2,1-b][1,3]thiazine exclusively. In addition, the synthetic utility of this methodology was showcased through a variety of downstream derivatizations.

Base-Mediated α-gem-Difluoroalkenylations of Aldehydes and Ketones

Herein, we present a base-mediated nucleophilic substitution reaction of α-trifluoromethylstyrenes with simple silyl enol ethers, enabling the efficient synthesis of carbonyl-substituted gem-difluoroalkenes. The merit of this protocol is exhibited by its mild reaction conditions, broad substrate scope, and scalable preparation. Notably, this method demonstrates its applicability for late-stage functionalization of structurally complex molecules. Moreover, we illustrate that the resulting products can serve as valuable precursors for the synthesis of diverse medicinally relevant compounds.

Controlled Synthesis of α-CF2H or α-CF2Cl Styrenes from the Same Precursors: Dehydrazinative Hydrogenation or Chlorination of 3,3-Difluoroallyl Hydrazines

By carefully choosing the reaction conditions, we have developed the controllable FeCl₃- or CuCl₂-mediated dehydrazinative hydrogenation or chlorination of 3,3-difluoroallyl hydrazines to access α-CF₂H or α-CF₂Cl styrenes. The current reaction provides for the first time a facile method for the direct and selective synthesis of α-CF₂H and α-CF₂Cl styrenes starting from the same precursors, which is easy to scale up and displays a broad substrate scope and good functional group tolerance. Moreover, product derivatization experiments demonstrated that the resulting α-CF₂Cl styrenes are practical and versatile building blocks for the diversified synthesis of fluorinated molecules.

Defluorinative Esterification and 1,3-Dietherification of (Trifluoromethyl)alkenes with Alcohols: Controlled Synthesis of α-Arylacrylates and 1,3-Diethers

Purification-controlled defluorinative esterification and 1,3-dietherification of (trifluoromethyl)alkenes with alcohols are achieved, delivering various useful α-arylacrylates and 1,3-diethers in high yields. Remarkably, this reaction enables the cleavage of three C-F bonds in a CF₃ group, and it is transition-metal free and catalyst-free, has simple operation, features mild conditions, is gram-scalable, and has broad substrate scope and valuable functional group tolerance. Mechanism studies indicated that the isolated monofluoroalkene-decorated 1,3-diethers are the intermediates, and the acidic property of silica gel assisted the defluorinative transformation of these 1,3-diethers to access α-arylacrylates with H₂O as the oxygen source.

Advances in Catalytic C–F Bond Activation and Transformation of Aromatic Fluorides

The activation and transformation of C–F bonds in fluoro-aromatics is a highly desirable process in organic chemistry. It provides synthetic methods/protocols for the generation of organic compounds possessing single or multiple C–F bonds, and effective catalytic systems for further study of the activation mode of inert chemical bonds. Due to the high polarity of the C–F bond and it having the highest bond energy in organics, C–F activation often faces considerable academic challenges. In this mini-review, the important research achievements in the activation and transformation of aromatic C–F bond, catalyzed by transition metal and metal-free systems, are presented.

Gem-Difluoroallylation of Aryl Sulfonium Salts

The unprecedented photochemical late-stage defluorinative gem-difluoroallylation of aryl sulfonium salts, which are formed site-selectively by direct C(sp²)─H functionalization, is herein disclosed. This method is distinguished by its mild reaction conditions, wide scope, and excellent site-selectivity. As showcase examples, a Flurbiprofen and Pyriproxyfen derivatives could be late stage C(sp²)─H gem-difluoroallylated with high yields. Experimental and computational investigations were conducted.

Divergent Synthesis of Fluorinated Alkenes, Allenes, and Enynes via Reaction of 2-Trifluoromethyl-1,3-enynes with Carbon Nucleophiles

Herein, inorganic base K₃PO₄ promoted divergent synthesis of CF₃-substituted allenes, cyclopentenes, alkynes, and fluorinated enynes via regioselective nucleophilic addition of carbon nucleophiles to 2-trifluoromethyl-1,3-enynes was developed. With the choice of different carbon nucleophiles, various fluorinated compounds could be obtained under K₃PO₄/DMF reaction system. When malononitriles were used as nucleophiles, CF₃-substituted allenes, cyclopentenes, and alkynes could be obtained, respectively. By using 1,3-dicarbonyl compounds as nucleophiles, ring-monofluorinated 4H-pyrans could be prepared, and 1,1-difluoro-1,3-enynes could be furnished with the participation of diethyl malonate. Moreover, these five kinds of fluorinated allenes, alkenes, and enynes are valuable building blocks.

Superbase-Mediated gem-Difluoroalkenylations of Sulfoximines

At ambient temperature, deprotonated sulfoximines react with 1-trifluoromethylalkenes to provide either N- or C-gem-difluoroalkenylated products. The reaction site depends upon the N substituent of the starting material. The optimal conditions involve the use of a superbasic system NaOH in dimethyl sulfoxide. The reactions are characterized by a broad substrate scope and medium to high yields. Scale-up experiments of both the N- and C-gem-difluoroalkenylations proceeded well. Treatment of a N-difluoroallyl sulfoximine with an aryl thiol under dioxygen afforded the corresponding oxygenated addition product.

Selective C-F Bond Etherification of Trifluoromethylalkenes with Phenols

Selective functionalization of a single sp³ C-F bond of the CF₃ group represents an appealing way to generate the pharmaceutically privileged difluoromethylene moiety. Herein, a novel protocol for the C-F bond etherification of trifluoromethylalkenes and phenols is reported. A variety of alkenyl difluoroaryloxy ethers are obtained with good chemoselectivity and functional group tolerance. DFT calculation results and control experiments suggest that metal ion Cs⁺ might be involved in this defluorooxylation. Furthermore, the desired product exhibits a good insecticidal activity.

DBN-Mediated Addition Reaction of α-(Trifluoromethyl)styrenes with Diazoles, Triazoles, Tetrazoles, and Primary, Secondary, and Secondary Cyclic Amines

A mild and efficient DBN-mediated addition reaction of α-(trifluoromethyl)styrenes with diazoles, triazoles, tetrazoles, and primary, secondary, and secondary cyclic amines was developed. This practical protocol provided a robust method for the synthesis of various β-trifluoromethyl nitrogen-containing heterocycles and β-trifluoromethyl amines.

Chemo- and regioselective defluorinative annulation of (trifluoromethyl)alkenes with pyrazolones: synthesis and insecticidal activity of 6-fluoro-1,4-dihydropyrano[2,3-c]pyrazoles

The chemo- and regioselective defluorinative [3 + 3] annulation of (trifluoromethyl)alkenes and pyrazolones gives useful 6-fluoro-1,4-dihydropyrano[2,3-c]pyrazoles.

Bond Energy Enabled Amines Distinguishing: Chemo-, Regioselective 1,3-Diamination of (Trifluoromethyl)alkenes with Different Amines by Two C(sp3)-F Bonds Cleavage

The methods to distinguish different amines are rarely investigated. In this manuscript, a bond energy enabled amines distinguishing strategy is reported. With (trifluoromethyl)alkenes as linchpins, a chemo-, regioselective three-component defluorinative...

Selective C-F Bond Allylation of Trifluoromethylalkenes

Selective C-F bond functionalization of CF₃ group represents an appealing strategy for the incorporation of pharmaceutically privileged difluoromethylene moiety. Despite the recent significant advancement attained in the functionalization of Ar-CF₃ molecules, prescriptions amenable for alkenyl-CF₃ congeners remain sufficiently inadequate. Herein, we report a strategically novel protocol for the C-F bond elaboration of trifluoromethylalkene derivatives. By using readily available allyl metallics as nucleophilic coupling partner, the present reaction enables an expedient construction of structurally diversified CF₂ -bridged 1,5-dienes. Furthermore, the exquisite selectivity observed in this transformation is revealed to be based on the underlying mechanism that consists of a cascade of nucleophilic S_N 2' defluorinative allylation and electronically promoted Cope rearrangement.

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

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

Synthesis of CF3-Containing Linear Nitriles from α-(Trifluoromethyl)styrenes

Four unprecedented base-catalyzed/mediated nucleophilic additions of TMSCN to α-(trifluoromethyl)styrenes and 2-trifluoromethyl enynes were developed. The reaction proceeded smoothly at room temperature under mild and transition-metal-free conditions without affecting the trifluoromethyl group and afforded the corresponding CF₃-containing alkyl, alkynyl, and butadienyl nitriles in moderate to excellent yields in a highly regioselective manner, respectively.

Regioselective Synthesis of 5-Trifluoromethylpyrazoles by [3 + 2] Cycloaddition of Nitrile Imines and 2-Bromo-3,3,3-trifluoropropene

A general and practical method for the synthesis of 5-trifluoromethylpyrazoles is reported that occurs by the coupling of hydrazonyl chlorides with environmentally friendly and large-tonnage industrial feedstock 2-bromo-3,3,3-trifluoropropene (BTP). This exclusively regioselective [3 + 2] cycloaddition of nitrile imines and with BTP is catalyst-free and operationally simple and features mild conditions, high yields, gram-scalable, a broad substrate scope, and valuable functional group tolerance. Significantly, our method has been applied for the synthesis of the key intermediate of an active agonist of sphingosine 1-phosphate receptor.

Two C(sp3)-F Bond Activation in a CF3 Group: ipso-Defluorinative Amination Triggered 1,3-Diamination of (Trifluoromethyl)alkenes with Indoles, Carbazoles, Pyrroles, and Sulfonamides

A novel strategy enabled cleavage of two C(sp³)-F bonds in a CF₃ group is reported. Triggered by ipso-defluorinative amination, this 1,3-diamination of (trifluoromethyl)alkenes with indoles, carbazoles, pyrroles, and sulfonamides gave acyclic 1,3-diamine products bearing a monofluoroalkene moiety in high yields with good to excellent Z/E selectivities. Preliminary mechanistic studies enable the isolation of the reaction intermediate and indicate that a unique sequential ipso-/γ-selective defluorinative amination pathway is involved in this transformation.

Synthesis of α-aminooxy amides through [3 + 3] cycloaddition and Sc(OTf)3-catalyzed double C-N bond cleavage in a one-pot reaction

Various α-aminooxy amides bearing a quaternary carbon at the α-position were prepared in good to excellent yields under mild reaction conditions from N-vinyl nitrones and α-bromohydroxamates. The N-vinyl nitrones tolerate a wide range of N-vinyl fluorenone nitrones and N-vinyl isatin nitrones. Mechanistic studies show that the reaction initially proceeds through [3 + 3] cycloaddition between N-vinyl nitrones and aza-oxyallyl cations generated from α-bromohydroxamates to afford six-membered N,O-heterocycles, followed by double C-N bond cleavage in the presence of the Sc(OTf)3 catalyst. A selective N-O bond cleavage of the obtained α-aminooxy amides is also realized under Fe/NH4Cl conditions. Furthermore, gram-scalable preparations of α-aminooxy amides are easily achieved.

Functionalization of a Single C-F Bond of Trifluoromethylarenes Assisted by an ortho-Silyl Group Using a Trityl-Based All-in-One Reagent with Ytterbium Triflate Catalyst

Catalytic thiolation and azidation of a single C-F bond of trifluoromethylarenes were achieved assisted by an ortho-silyl group with all-in-one reagents to generate a trityl cation and nucleophiles. The reactions catalyzed by ytterbium triflate efficiently afforded a wide variety of difluoromethylenes avoiding the further C-F bond cleavage, by virtue of the mild conditions without the generation of a Brønsted acid.

Defluoroalkylation of sp3 C-F Bonds of Industrially Relevant Hydrofluoroolefins

A simple, one-pot procedure is reported for the selective defluoroalkylation of trifluoromethyl alkene derivatives with aldehydes and ketones. The reaction sequence allows construction of a new C-C bond in a highly selective manner from a single sp³ C-F bond of a CF₃ group in the presence of sp² C-F bonds. The scope incorporates industrially relevant fluorocarbons including HFO-1234yf and HFO-1234ze. No catalyst, additives or transition metals are required, rather the methodology relies on a recently developed boron reagent. Remarkably, the boron site of this reagent plays a dual role in the reaction sequence, being nucleophilic at boron in the C-F cleavage step (S_N 2') but electrophilic at boron en route to the carbon-carbon bond-forming step (S_E 2'). The duplicitous behaviour is underpinned by a hydrogen atom migration from boron to the carbon atom of a carbene ligand.

Three component hydroxyletherification and hydroxylazidation of (trifluoromethyl)alkenes: access to α-trifluoromethyl β-heteroatom substituted tertiary alcohols

The three component hydroxyletherification and hydroxylazidation reactions of (trifluoromethyl)alkenes are reported, providing various useful α-trifluoromethyl β-heteroatom substituted tertiary alcohols in high yields.

Ligand-controlled copper-catalyzed 1,2 or 1,4-protoborylation of 2-trifluoromethyl-1,3-conjugated enynes

A novel ligand-controlled highly regioselective synthesis of CF₃-substituted homopropargylboronates and homoallenylboronates via copper-catalyzed 1,2 or 1,4-protoborylation of 2-trifluoromethyl-1,3-conjugated enynes with B₂pin₂ was developed.

Double allylic defluorinative alkylation of 1,1-bisnucleophiles with (trifluoromethyl)alkenes: construction of all-carbon quaternary centers

Synthesis of symmetric gem-difluoroalkene substituted products bearing all-carbon quaternary centers via double allylic defluorinative alkylation of 1,1-bisnucleophiles with (trifluoromethyl)alkenes.

Palladium-catalyzed selective defluorinative sulfenylation for the synthesis of fluorovinylthioethers

An efficient transformation of trifluoromethylated benzyl bromides to monofluoroalkenes through Pd-catalyzed selective defluorinative sulfenylation is described.

Recent advances in the synthesis and applications of α-(trifluoromethyl)styrenes in organic synthesis

α-Trifluoromethylstyrene derivatives are versatile synthetic intermediates for the preparation of more complex fluorinated compounds.

An efficient tandem synthesis of chromones from o-bromoaryl ynones and benzaldehyde oxime

A transition-metal-free approach was developed to synthesize chromones from o-bromoaryl ynones and benzaldehyde oxime by sequential C–O bond formation.