Synthesis of β-Aminoenones via Cross-Coupling of In-Situ-Generated Isocyanides with 1,3-Dicarbonyl Compounds

Difluorocarbene-Enabled Dehydration of Primary Amides To Access Nitriles

A cost-effective and environmentally friendly method for the direct conversion of primary amides to nitriles was developed using commercially available non-toxic ethyl bromodifluoroacetate as a difluorocarbene precursor under metal-free and ligand-free conditions. The reaction features high yields and tolerates various sensitive moieties, including alkyl, alkenyl, ether, sulfone, sulfoxide, heteroaryl, chloro, bromo, iodo, hydroxyl, nitro, and cyano groups, and late-stage modification of complex molecules is also feasible. Moreover, the present method is effective on large scales, showing potential for industrial application.

Palladium-catalyzed ligand-regulated divergent synthesis of pyrrole[2,3-b]indoles and ureas from 2-ethynylanilines and isocyanides

Herein, a palladium-catalyzed and ligand-controlled protocol for the divergent synthesis of pyrrole[2,3-b]indole and urea derivatives has been described. Pyrrole[2,3-b]indoles ("cyclization on" products) via tandem cyclization of o-alkynylanilines with isocyanides in the absence of a ligand and ureas ("cyclization off" products) via oxidative amination of anilines with isocyanides in the presence of a ligand were obtained both in moderate to good yields with high selectivity. In this chemistry, cyclic and acyclic products were easily accessed with the same starting materials under the regulation of the ligand.

Cu-Catalyzed Carbocyclization for General Synthesis of N-Containing Heterocyclics Enabled by BrCF2COOEt as a C1 Source

A practical and efficient copper-catalyzed carbocyclization of 2-functionalized anilines with ethyl bromodifluoroacetate has been developed. Ethyl bromodifluoroacetate is employed as the C1 source via quadruple cleavage in this transformation. This reaction can afford a variety of N-containing heterocyclics with satisfactory yields and excellent functional group compatibility.

Photocatalytic Multicomponent Annulation of Amide-Anchored 1,7-Diynes Enabled by Deconstruction of Bromotrichloromethane

We present the first example of visible-light-mediated multicomponent annulation of 1,7-diynes by taking advantage of quadruple cleavage olf carbon-halogen bonds of BrCCl3 to generate a C1 synthon, which was adeptly applied to the preparation of skeletally diverse 3-benzoyl-quinolin-2(1H)-one acetates in moderate to good yields. Controlled experiments demonstrated that H2O acted as both oxygen and hydrogen sources, and gem-dichlorovinyl carbonyl compound exhibited as a critical intermediate in this process. The mechanistic pathway involves Kharasch-type addition/6-exo-dig cyclization/1,5-(SN")-substitution/elimination/binucleophilic 1,6-addition/proton transfer/tautomerization sequence.

Visible-Light-Induced Desaturative β-Alkoxyoxalylation of N-Aryl Cyclic Amines with Difluoromethyl Bromides and H2O Via a Triple Cleavage Process

A visible-light-driven desaturative β-alkoxyoxalyation of N-aryl cyclic amines with difluoromethyl bromides and H2O has been reported. This tandem reaction is triggered by homolysis of the C-Br bond to produce the difuoroalkyl radical, which undergoes the subsequent defluorinated β-alkoxyoxalylation cascades to afford a wide range of β-ketoester/ketoamides substituted enamines. The prominent feature of this reaction contains photocatalyst-free, transition-metal free, and mild conditions. The 18O labeling experiment disclosed that H2O is the oxygen source of the carbonyl unit.

Unconventional Transformations of Difluorocarbene with Amines and Ethers

ConspectusFluorine-containing compounds are extensively involved in various fields originating from intriguing and unique characteristics of fluorine atom; notably, in pharmaceuticals, the involvement of a fluorine atom or a fluorine-containing group is a chief technique for improving the pesticide effect and developing new drugs. Difluorocarbene, one of the most important and powerful fluorine-containing reagents, is widely employed and studied in many areas mainly to assemble gem-difluoromethyl molecules, including but not limited to the abundant reactions between difluorocarbene with nucleophilic substrates, Wittig reaction with ketones or aldehydes, cascade reaction with both a nucleophile and an electrophile, or [2+1] cycloaddition with alkenes or alkynes. However, its unconventional and intriguing protocols beyond as a difluoromethyl synthon have rarely been studied, and thus, it is highly desired given its abundance, inexpensiveness and peculiar properties. In this Account, we mainly discuss our discovery with unconventional transformations of difluorocarbene, instead of as a sole difluoromethyl source (different from other dihalocarbene), actually can serve as an electron acceptor to activate C-X bonds (X = N and O) and thus promote a myriad of fascinating transformations for the assembly of versatile valuable products with various aza-compounds (primary/secondary/tertiary amines as well as NH₃ and NaNH₂ and so on) and aliphatic ethers in the absence of transition metals and expensive ligands. Inspired by the electron-deficient characteristics of difluorocarbene, we first found that the isocyanides could be readily formed in situ when the unoccupied orbital of difluorocarbene meets the lone-pair of primary amines; in basic condition, a cascade defluorination and cyclizations could afford plethora of valuable N-containing heterocycles. Meanwhile, we disclosed that cyano anion could be accessible in situ as well when difluorocarbene and NaNH₂ or NH₃ were mixed up in suitable basic conditions, and thus a series of aryl nitrile compounds were obtained in the presence of Pd catalysis and ArI. Interestingly, when difluorocarbene encountered secondary amines, formamides were rendered under mild reactions. Of note, concomitant functionalizations of C and N moieties via cleavage of the unstrained C(sp³)-N bond in the absence of metal and oxidant are sparce, which indeed significantly add versatility and diversity to products. Gratifyingly, by uitilizing difluorocarbene and cyclic tertiary amines, we achieved difluorocarbene-mediated deconstructive functionalizations for the first time, showing successive C(sp³)-N bond scission of amines and simultaneous functionalization of C and N atoms which would be introduced into the products in the absence of transition metals and oxidants. This method provides a brand-new while very universal synthetic pathway to selectively cleave inert unactivated Csp³-N bonds, in which halodifluoromethyl reagents act as both C1 synthon and halo (Cl, Br, I) sources. Fascinatingly, nitrogen ylides are generated in situ from difluorocarbene and tertiary amines, and an intriguing and universal approach for deaminative arylation or alkenylation of tertiary amines was disclosed for the first time in appropriate basic conditions, which represents an intriguing reaction mode to lead to a formal transition-metal free Suzuki cross coupling. Besides, we also disclosed that difluorocarbene could proceed novel atom recombination to render meaningful 2-fluoroindoles or 3-(2,2-difluoroethyl)-2-fluoroindoles from ortho-vinylanilines, 3-fluorined oxindoles from 2-aminoarylketones, in which difluorocarbene acts as a C1 synthon and F1 source simultaneously. Last but not the least, we recently found that the lone-pair-electron of oxygen could trap difluorocarbene as well to form oxonium ylide, which eventually leads to C-O bond cleavage with the formation of difluoromethyl ethers.

Difluorocarbene-Enabled Synthesis of 3-Alkenyl-2-oxindoles from ortho-Aminophenylacetylenes

Herein, we report a transition-metal-free [4 + 1] cyclization pathway from difluorocarbene and ortho-amino aryl alkynone, rendering an effective and universal strategy for the construction of 3-alkenyl-2-oxindoles. Our strategy starts from cheap and accessible ortho-amino aryl alkynone instead of the direct indole skeleton; moreover, in situ generated difluorocarbene from commercially available halogenated difluoroalkylative reagents enables the cleavage of a C-N bond and formation of new C-N bonds and C-C bonds.

AlCl3-Mediated CHF2 Transfer and Cyclocondensation of Difluoromethoxy Functionalized o-Phenylenediamines to Access N-Substituted Benzimidazoles

Herein, we report for the first time a transition-metal-free frustrated Lewis pair (FLP) catalyzed CHF₂ group migration from an oxygen atom to the neighboring nitrogen atom, which led to the synthesis of N-substituted benzimidazoles at room temperature with excellent yields, broad functional group tolerance, and a short reaction time. The oxygen-attached difluoromethane acted as a C1 source in the synthesis of N-substituted benzimidazoles in the presence of AlCl₃ by cleaving one C-O bond and two C-F bonds, resulting in formation of two new C-N bonds.

N-Difluoromethylation of N-pyridyl-substituted anilines with ethyl bromodifluoroacetate

A general protocol for N-difluoromethylation of aniline derivatives is developed. Commercially available ethyl bromodifluoroacetate serves as a difluorocarbene source in the presence of a base. This carbene surrogate is attractive owing to its favorable stability, environmental friendliness and inexpensiveness. This reaction system features notable operational simplicity (bench top-grade solvents can be used without any pre-drying and do not require inert atmosphere protection). A wide range of functional groups in aniline derivatives are well-tolerated, and good-to-excellent product yields are generally obtained.

Double Capture of Difluorocarbene by 2-Aminostyrenes Enables the Construction of 3-(2,2-Difluoroethyl)-2-fluoroindoles

We report herein an efficient strategy to construct 3-(2,2-difluoroethyl)-2-fluoroindoles from activated o-aminostyrenes with ethyl bromodi-fluoroacetate as a difluorocarbene source. Through double capture of a difluorocarbene, two different types of fluorine motifs are incorporated into the products with simultaneous construction of one C-N and two C-C bonds, without the need for transition metals. This reaction features high efficiency and excellent functional group compatibility and has great potential in the late-stage modifications of pharmaceutical molecules and natural products.

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.

Difluorocarbene enables to access 2-fluoroindoles from ortho-vinylanilines

2-Fluoroindoles as an important structural scaffold are widely existing in many bioactive or therapeutic agents. Despite their potential usefulness, efficient constructions of 2-fluoroindole derivatives are very sparce. The development of straightforward synthetic approaches to access 2-fluoroindoles is highly desirable for studying their fundamental properties and applications. Herein, we report an efficient and general strategy for the construction of 2-fluoroindoles in which a wide variety of 2-fluoroindoles were accessed with high efficiency and chemoselectivity. Instead of starting from indole skeletons, our strategy constructs indole scaffolds alongside the incorporation of fluorine atom on C2 position in a formal [4+1] cyclization from readily accessible ortho-vinylanilines and difluorocarbene. In our protocol, commercially accessible halodifluoroalkylative reagents provide one carbon and one fluorine atom by cleaving one C-N tertiary bond and forming one C-N bond and one C-C double bond with ortho-vinylanilines. Downstream transformations on 2-fluoroindoles lead to various valuable bioactive molecules which demonstrated significant synthetic advantages over previous reports. And mechanistic studies suggest that the reaction undergoes a cascade difluorocarbene-trapping and intramolecular Michael addition reaction followed by Csp³-F bond cleavage.

Solvent-Dependent Cyclization of 2-Alkynylanilines and ClCF2COONa for the Divergent Assembly of N-(Quinolin-2-yl)amides and Quinolin-2(1H)-ones

Herein, we present an expedient Cu-catalyzed [5 + 1] cyclization of 2-alkynylanilines and ClCF₂COONa to divergent construction of N-(quinolin-2-yl)amides and quinolin-2(1H)-ones by regulating the reaction solvents. Notably, nitrile acts as a solvent and performs the Ritter reactions. ClCF₂COONa is used as a C1 synthon in this transformation, which also represents the first example for utilization of ClCF₂COONa as an efficient desiliconization reagent. The current protocol involves in situ generation of isocyanide, copper-activated alkyne, Ritter reaction and protonation.

Direct Synthesis of N-Difluoromethyl-2-pyridones from Pyridines

A novel method for the synthesis of N-difluoromethyl-2-pyridones was described. This protocol enables the synthesis of N-difluoromethyl-2-pyridones from readily available pyridines using mild reaction conditions that are compatible with a wide range of functional groups. The preliminary mechanistic study revealed that N-difluoromethylpyridinium salts were the key intermediates to complete this conversion.

Metal-Free Difunctionalization of Pyridines: Selective Construction of N-CF2H and N-CHO Dihydropyridines

The reactivity of N-difluoromethylpyridinium salts is seldom explored because of their instability and low availability. Here we present a novel nucleophilic addition of N-difluoromethylpyridinium salts with nitroalkanes to synthesize N-CF₂H-dihydropyridines and N-CHO-dihydropyridines in a highly efficient and regioselective pathway. This protocol exhibits good functional group tolerance and good to excellent yields.

Elemental-Sulfur-Enabled Divergent Synthesis of Disulfides, Diselenides, and Polythiophenes from β-CF3 -1,3-Enynes

Divergent synthesis for precise constructions of cyclic unsymmetrical diaryl disulfides or diselenides and polythiophenes from CF₃ -containing 1,3-enynes and S₈ was developed when the ortho group is F, Cl, Br, and NO₂ on aromatic rings. Meanwhile, disulfides (diselenides) were also quickly constructed when the ortho group is H. These transformations undergo cascade thiophene construction/selective C3-position thiolation process, featuring simple operations, divergent synthesis, broad substrate scope, readily available starting materials, and valuable products. A novel plausible radical annulation process was proposed and validated by DFT calculations for the first time. A series of derivatizations about the thiophene (TBT) and disulfides were also well-represented.

C(sp3)–H functionalization with isocyanides

This review highlights the state-of-the-art advances in C(sp³)–H functionalization involving isocyanides through the synergistic combination of isocyanide insertion and C(sp³)–H bond activation.

Isocyanide Multicomponent Reactions on Solid Phase: State of the Art and Future Application

Drug discovery efforts largely depend on access to structural diversity. Multicomponent reactions allow for time-efficient chemical transformations and provide advanced intermediates with three or four points of diversification for further expansion to a structural variety of organic molecules. This review is aimed at solid-phase syntheses of small molecules involving isocyanide-based multicomponent reactions. The majority of all reported syntheses employ the Ugi four-component reaction. The review also covers the Passerini and Groebke-Blackburn-Bienaymé reactions. To date, the main advantages of the solid-phase approach are the ability to prepare chemical libraries intended for biological screening and elimination of the isocyanide odor. However, the potential of multicomponent reactions has not been fully exploited. The unexplored avenues of these reactions, including chiral frameworks, DNA-encoded libraries, eco-friendly synthesis, and chiral auxiliary reactions, are briefly outlined.

Visible-Light Photoredox-Catalyzed Formal [5 + 1] Cycloaddition of N-Tosyl Vinylaziridines with Difluoroalkyl Halides

A visible-light photoredox-catalyzed formal [5 + 1] cycloaddition of N-tosyl vinylaziridines with difluoroalkyl halides as unique C1 synthons was developed. The procedure provides an efficient and practical method to synthesize diverse pyridines in moderate to good yields. The reaction underwent a radical-initiated kinetically controlled ring-opening of vinylaziridines and involved a key α,β-unsaturated imine intermediate, followed by an E2 elimination, a 6π electrocyclization, and defluorinated aromatization.

Recent progress on selective deconstructive modes of halodifluoromethyl and trifluoromethyl-containing reagents

Halodifluoromethyl and trifluoromethyl-containing compounds are widely employed in organic chemistry, pharmaceuticals and materials science. Therefore, their applications and transformations have received significant attention during the past few decades. The single, double, triple and quadruple cleavage of halodifluoromethyl compounds and various deconstructive modes of trifluoromethyl-containing compounds could generate a variety of synthons to prepare more valuable products. Herein, we summarize the most significant achievements in this field with an intriguing focus on results from the last decade.

Deconstructive Functionalizations of Unstrained Carbon-Nitrogen Cleavage Enabled by Difluorocarbene

Transition-metal- or oxidant-promoted deconstructive functionalizations of noncyclic carbon-nitrogen bonds are well established, usually only leaving one moiety functionalized toward the final product. In contrast, concomitant C- and N-functionalizations via the unstrained C(sp3)-N bond under metal- and oxidant-free conditions are very rare, which would favorably confer versatility and product diversity. Disclosed herein is the first difluorocarbene-induced deconstructive functionalizations embodying successive C(sp3)-N bond cleavage of cyclic amines and synchronous functionalization of both constituent atoms which would be preserved in the eventual molecular outputs under transition-metal-free and oxidant-free conditions. Correspondent access to deuterated formamides with ample isotopic incorporation was demonstrated by a switch to heavy water which is conceivably useful in pharmaceutical sciences. The current strategy remarkably administers a very convenient, operationally simple and novel method toward molecular diversity from readily available starting materials. Therefore, we project that these findings would be of broad interest to research endeavors encompassing fluorine chemistry, carbene chemistry, C-N bond activation, as well as medicinal chemistry.

Palladium-Catalyzed Imidoylation-Triggered [2 + 2 + 1] Cyclization of Internal Alkyne with Isocyanides

In this work, a palladium-catalyzed [2 + 2 + 1] cyclization of internal alkynes with double isocyanides is described. This facile procedure is efficient for synthesizing various pyrrolo[3,2-c]quinolin-2-amines. The reaction worked well with a broad reaction scope. In the process, it is believed that sequential double isocyanide insertion, 6-exo-dig cyclization of alkyne, and addition of an imino group are involved.

[3+1+1] type cyclization of ClCF2COONa for the assembly of imidazoles and tetrazoles via in situ generated isocyanides

A facile synthesis of imidazoles and tetrazoles via [3+1+1] type cyclization of ClCF₂COONa is developed.

A convenient reagent for the conversion of aldoximes into nitriles and isonitriles

Described herein is the convenient transformation of aldoximes into nitriles or isonitriles by slightly modifying reaction conditions.

Palladium-catalyzed cyanation of aryl halides with in situ generated CN− from ClCF2H and NaNH2

An efficient Pd-catalyzed cyanation of aryl halides with in situ generated CN⁻ anions is described.

Transition metal-free C-F/C-Cl/C-C cleavage of ClCF2COONa for the synthesis of heterocycles

A transition metal-free and external oxidant-free annulation of substrates having two nitrogen-nucleophilic sites with ClCF₂COONa was demonstrated, affording a series of 1,3,5-triazines and quinazolinones in up to 96% yields. Notably, ClCF₂COONa was employed as the C1 synthon for valuable heterocycles. Using this protocol, two C-N bonds were formed in one pot via the cleavage of two C-F bonds, one C-Cl bond and one C-C bond. This method avoided the use of a transition metal and an oxidant and generated low toxicity inorganic waste.

tert-Butyl Nitrite Mediated Synthesis of Fluorinated O-Alkyloxime Ether Derivatives

A tert-butyl nitrite (TBN)-mediated synthesis of fluorinated O-alkyloxime ether derivatives with bromodifluoroalkyl reagents as the fluorine sources has been developed. A variety of halodifluorinated compounds were found compatible, delivering the desired products in moderate to excellent yields. This transformation features a simple operation, can be done in air, and found to involve radicals. This protocol represents a straightforward approach to access various fluorinated O-alkyloxime ether derivatives.

Chlorodifluoromethane as a C1 Synthon in the Assembly of N-Containing Compounds

The development of C1 synthons to afford the products that add one extra carbon has become an important research theme in the past decade, and significant progress has been achieved with CO₂, CO, HCOOH, and others as C1 units. Despite the great advance, the search for new C1 synthons that display unique reactivity, complement to the current C1 sources, and add more value to C1 chemistry is still desirable. Herein, we report a quadruple cleavage of chlorodifluoromethane to yield a C1 source, which was successfully employed in the construction of various N-containing compounds especially with pharmaceutical molecules under mild conditions. This strategy provides a useful method for late-stage modification of pharmaceutical compounds. Four bonds in ClCF₂H were orderly cleaved under basic conditions in the absence of transition metals. Preliminary mechanistic studies revealed that (E)-N-phenylformimidoyl fluoride intermediate is involved in this process by in situ¹H NMR studies and control experiments.

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Quadruple cleavage of ClCF₂H to afford a C1 synthon

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The cleavage of two stable C(sp³)-F bonds in aliphatic gem-difluoroalkanes

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Enrich C1 chemistry, green chemistry, and fluorine chemistry

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Various N-containing compounds were afforded via different role of ClCF₂H

S8-Catalyzed triple cleavage of bromodifluoro compounds for the assembly of N-containing heterocycles

An unprecedented S₈-catalyzed selective triple-cleavage of bromodifluoroacetamides is disclosed for the first time.

An unprecedented S₈-catalyzed selective triple-cleavage of bromodifluoroacetamides is disclosed for the first time. Valuable 2-amido substituted benzimidazoles, benzoxazoles and benzothiazoles were obtained in good to excellent yields in a cascade protocol in this strategy. Mechanistic studies suggested that a C2 source was generated in situ by selective cleavage of three C–X bonds, including two inert C(sp³)–F bonds on bromodifluoroacetamides, while leaving C–C bonds intact. This strategy will undoubtedly further consummate the role of halo difluoro compounds and enrich both fluorine chemistry and pharmaceutical sciences.

Copper-catalyzed regioselective cyclization of vinyl azides by gem-difluoromethylene for trisubstituted pyridines

A novel copper-catalyzed cyclization of readily available vinyl azides with gem-difluoromethylene is described.

Transition metal-free assembly of 1,3,5-triazines using ethyl bromodifluoroacetate as C1 source

Transition-metal and oxidant-free annulation of amidine to access 2,4-disubstituted-1,3,5-triazines using ethyl bromodifluoroacetate as C1 source via quadruple cleavage.