Electrochemical Decarboxylative N-Alkylation of Heterocycles

Entry to 2-aminoprolines via electrochemical decarboxylative amidation of N‑acetylamino malonic acid monoesters

The electrochemical synthesis of 2-aminoprolines based on anodic decarboxylation-intramolecular amidation of readily available N-acetylamino malonic acid monoesters is reported. The decarboxylative amidation under Hofer-Moest reaction conditions proceeds in an undivided cell under constant current conditions in aqueous acetonitrile and provides access to N-sulfonyl, N-benzoyl, and N-Boc-protected 2-aminoproline derivatives.

Alkylazolation of Alkenes via Photocatalytic Radical Polar Crossover

We present a photocatalytic method for alkylamination of alkenes, enabling efficient C-C and C-N bond formation to construct aza-heterocycles valuable in drug discovery. Using a radical-polar crossover pathway, electron-deficient alkenes are reduced to electrophilic radicals, which react with electron-rich alkenes to form nucleophilic radicals. Oxidation of these intermediates yields carbocations, which are trapped by aza-heteroarenes to afford alkylaminated products. This strategy enhances molecular complexity while suppressing side reactions.

Modular access to saturated bioisosteres of anilines via photoelectrochemical decarboxylative C(sp3)-N coupling

In drug development, the substitution of benzene rings in aniline-based drug candidates with saturated bridged bicyclic ring systems often enhances pharmacokinetic properties while preserving biological activity. However, current efforts predominantly focuses on bicyclo[1.1.1]pentylamines, accessing analogs capable of mimicking ortho- and meta-substituted anilines remains challenging due to the lack of a versatile and modular synthetic methods. Herein, we present a modular approach to access a diverse array of saturated bioisosteres of anilines via photoelectrochemical-induced decarboxylative C(sp3)-N Coupling. The success of this reaction hinges on the merging the cooperative ligand-to-metal charge transfer (LMCT) with copper-catalyzed amination. Notably, this net-oxidative C(sp3)-N forming reaction operates under mild electrode potentials and proceeds through hydrogen evolution, eliminating the need for external chemical oxidants. Our research enables the facile decarboxylative amination of a set of sp3-rich small-ring cage carboxylic acids, thus offering a versatile bioisosteric replacement for ortho-, meta-, and para-substituted anilines and di(hetero)aryl amines.

Electrochemical Decarboxylation Coupling Reactions

Carboxylic acids are attractive synthetic feedstocks with stable, non-toxic, and inexpensive properties that can be easily obtained from natural sources or through synthesis. Carboxylic acids have long been considered environmentally friendly coupling agents in various organic transformations. In recent years, electrochemically mediated decarboxylation reactions of decarboxylic acids and their derivatives (NHPI) have emerged as effective new methods for constructing carbon-carbon or carbon-heterocarbon chemical bonds. Compared with transition metal and photochemistry-mediated catalytic reactions, which do not require the addition of oxidants and strong bases, electrochemically-mediated decarboxylative transformations are considered a sustainable strategy. In addition, various functional groups tolerate the electrochemical decarboxylation conversion strategy well. Here, we summarize the recent electrochemical decarboxylation reactions to better elucidate the advantages of electrochemical decarboxylation reactions.

Electrochemically Generated Carbocations in Organic Synthesis

This Minireview examines a selection of case studies that showcase distinctive and enabling electrochemical approaches that have allowed for the generation and reaction of carbocation intermediates under mild conditions. Particular emphasis is placed on the progress that has been made in this area of organic synthesis and polymer chemistry over the past decade.

Electrochemical Decarboxylative Cross-Coupling with Nucleophiles

Decarboxylative cross-coupling reactions are powerful tools for carbon-heteroatom bonds formation, but typically require pre-activated carboxylic acids as substrates or heteroelectrophiles as functional groups. Herein, we present an electrochemical decarboxylative cross-coupling of carboxylic acids with structurally diverse fluorine, alcohol, H2O, acid, and amine as nucleophiles. This strategy takes advantage of the ready availability of these building blocks from commercial libraries, as well as the mild and oxidant-free conditions provided by electrochemical system. This reaction demonstrates good functional-group tolerance and its utility in late-stage functionalization.

Electrochemistry-Enabled C-Heteroatom Bond Formation of Alkyl Germanes

Because of their robustness and orthogonal reactivity features, alkyl germanes bear significant potential as functional handles for the construction of C(sp3)-rich scaffolds, especially in the context of modular synthetic approaches. However, to date, only radical-based reactivity has been accessible from these functional handles, which limits the types of possible decorations. Here, we describe the first general C(sp3)-heteroatom bond formation of alkyl germanes (-GeEt3) by leveraging electrochemistry to unlock polar reactivity. This approach allowed us to couple C(sp3)-GeEt3 with a variety of nucleophiles to construct ethers, esters, amines, amides, sulfonamides, sulfides, as well as C-P, C-F, and C-C bonds. The compatibility of the electrochemical approach with a modular synthetic strategy of a C1 motif was also showcased, involving the sequential functionalization of Cl, Bpin, and ultimately GeEt3 via electrochemistry.

Electrochemical deoxygenative amination of stabilized alkyl radicals from activated alcohols

Alkylamine structures represent one of the most functional and widely used in organic synthesis and drug design. However, the general methods for the functionalization of the shielded and deshielded alkyl radicals remain elusive. Here, we report a general deoxygenative amination protocol using alcohol-derived carbazates and nitrobenzene under electrochemical conditions. A range of primary, secondary, and tertiary alkylamines are obtained. This practical procedure can be scaled up through electrochemical continuous flow technique.

Electrophotochemical metal-catalyzed synthesis of alkylnitriles from simple aliphatic carboxylic acids

We report a practical and sustainable electrophotochemical metal-catalyzed protocol for decarboxylative cyanation of simple aliphatic carboxylic acids. This environmentally friendly method features easy availability of substrates, broad functional group compatibility, and directly converts a diverse range of aliphatic carboxylic acids including primary and tertiary alkyl acids into synthetically versatile alkylnitriles without using chemical oxidants or costly cyanating reagents under mild reaction conditions.

Electro-oxidative Methylation of 2-Isocyanobiaryls Using N,N-dimethylformamide (DMF) as Carbon Source: Synthesis of 6-Methylphenanthridines

A benign electrochemical method to access 6-methylphenanthridines from 2-isocyanobiaryls using N,N-dimethylformamide (DMF) as a methyl source is reported. The protocol operates at ambient temperature without the need for harmful methylating reagents. Mechanistic studies suggested that DMF delivered a methylene synthon, followed by reduction at the cathode and tautomerization. The method offers environmental benefits by avoiding metal-based reagents and harsh conditions.

Mild Approach to Nucleoside Analogues via Photoredox/Cu-Catalyzed Decarboxylative C-N Bond Formation. Total Synthesis of Oxetanocin A

The conventional N-glycosylation methods for nucleoside synthesis usually require strongly acidic or basic conditions. Here we report the decarboxylative C(sp3)-N coupling of glycosyl N-hydroxyphthalimide esters with nucleobases via dual photoredox/Cu catalysis, which offered a mild approach to nucleoside analogues. A total synthesis of oxetanocin A, an antiviral natural product containing an oxetanose moiety, has been achieved by using this method.

Electrochemical Ni-Catalyzed Decarboxylative C(sp3 )-N Cross-Electrophile Coupling

A new electrochemical transformation is presented that enables chemists to couple simple alkyl carboxylic acid derivatives with an electrophilic amine reagent to construct C(sp3 )-N bond. The success of this reaction hinges on the merging of cooperative electrochemical reduction with nickel catalysis. The chemistry exhibits a high degree of practicality, showcasing its wide applicability with 1°, 2°, 3° carboxylic acids and remarkable compatibility with diverse functional groups, even in the realm of late-stage functionalization. Furthermore, extensive mechanistic studies have unveiled the engagement of alkyl radicals and iminyl radicals; and elucidated the multifaceted roles played by i Pr2 O, Ni catalyst, and electricity.

Electrochemical desulfurative formation of C-N bonds through selective activation of inert C(sp3)-S bonds

In this study, we introduce an efficient, metal-free electrocatalytic desulfurative protocol for forming C-N bonds by selectively activating inert C(sp3)-S bonds of alkyl thioethers. This method offers a straightforward and environmentally friendly approach for modification of heterocyclic compounds from readily accessible thioethers. Preliminary mechanistic investigations suggest that the reaction proceeds via a carbocation intermediate. Furthermore, successful synthesis on a 10-gram scale was achieved in a continuous flow electrochemical reactor.

Late-Stage Isotopic Exchange of Primary Amines

Stable isotopes such as 2H, 13C, and 15N have important applications in chemistry and drug discovery. Late-stage incorporation of uncommon isotopes via isotopic exchange allows for the direct conversion of complex molecules into their valuable isotopologues without requiring a de novo synthesis. While synthetic methods exist for the conversion of hydrogen and carbon atoms into their less abundant isotopes, a corresponding method for accessing 15N-primary amines from their naturally occurring 14N-analogues has not yet been disclosed. We report an approach to access 15N-labeled primary amines via late-stage isotopic exchange using a simple benzophenone imine as the 15N source. By activating α-1 and α-2° amines to Katritzky pyridinium salts and α-3° amines to redox-active imines, we can engage primary alkyl amines in a deaminative amination. The redox-active imines proceed via a radical-polar crossover mechanism, whereas the Katritzky salts are engaged in copper catalysis via an electron donor-acceptor complex. The method is general for a variety of amines, including multiple drug compounds, and results in complete and selective isotopic labeling.

Electrochemical Decarboxylative Elimination of Carboxylic Acids to Alkenes

An electrochemical strategy for the decarboxylative elimination of carboxylic acids to alkenes at room temperature has been developed. This mild and oxidant-free method provides a green alternative to traditional thermal decarboxylation reactions. Structurally diverse aliphatic carboxylic acids, including biologically active drugs, underwent smooth conversion to the corresponding alkenes in good to excellent yields.

Iron-mediated modular decarboxylative cross-nucleophile coupling

Carboxylic acids are valuable building blocks for pharmaceutical discovery because of their chemical stability, commercial availability, and structural diversity. Decarboxylative coupling reactions enable versatile functionalization of these feedstock chemicals, but many of the most general methods require prefunctionalization of carboxylic acids with redox-active moieties. These internal oxidants can be costly, their installation impedes rapid library synthesis, and their use results in environmentally problematic organic byproducts. We report herein a method for the direct decarboxylative cross-coupling of native carboxylic acids with nucleophilic coupling partners mediated by inexpensive, terrestrially abundant, and nontoxic Fe(III) salts. This method involves an initial photochemical decarboxylation followed by radical-polar crossover, which enables the construction of diverse carbon-carbon, carbon-oxygen, and carbon-nitrogen bonds with remarkable generality.

Bismuth radical catalysis in the activation and coupling of redox-active electrophiles

Radical cross-coupling reactions represent a revolutionary tool to make C(sp³)-C and C(sp³)-heteroatom bonds by means of transition metals and photoredox or electrochemical approaches. However, the use of main-group elements to harness this type of reactivity has been little explored. Here we show how a low-valency bismuth complex is able to undergo one-electron oxidative addition with redox-active alkyl-radical precursors, mimicking the behaviour of first-row transition metals. This reactivity paradigm for bismuth gives rise to well-defined oxidative addition complexes, which could be fully characterized in solution and in the solid state. The resulting Bi(III)-C(sp³) intermediates display divergent reactivity patterns depending on the α-substituents of the alkyl fragment. Mechanistic investigations of this reactivity led to the development of a bismuth-catalysed C(sp³)-N cross-coupling reaction that operates under mild conditions and accommodates synthetically relevant NH-heterocycles as coupling partners.

Radical Decarboxylative Carbon-Nitrogen Bond Formation

The carbon-nitrogen bond is one of the most prevalent chemical bonds in natural and artificial molecules, as many naturally existing organic molecules, pharmaceuticals, agrochemicals, and functional materials contain at least one nitrogen atom. Radical decarboxylative carbon-nitrogen bond formation from readily available carboxylic acids and their derivatives has emerged as an attractive and valuable tool in modern synthetic chemistry. The promising achievements in this research topic have been demonstrated via utilizing this strategy in the synthesis of complex natural products. In this review, we will cover carbon-nitrogen bond formation via radical decarboxylation of carboxylic acids, Barton esters, MPDOC esters, N-hydroxyphthalimide esters (NHP esters), oxime esters, aryliodine(III) dicarboxylates, and others, respectively. This review aims to bring readers a comprehensive survey of the development in this rapidly expanding field. We hope that this review will emphasize the knowledge, highlight the proposed mechanisms, and further disclose the fascinating features in modern synthetic applications.

Azolation of Benzylic C-H Bonds via Photoredox-Catalyzed Carbocation Generation

A visible-light photoredox-catalyzed method is reported that enables the coupling between benzylic C-H substrates and N-H azoles. Classically, medicinally relevant N-benzyl azoles are produced via harsh substitution conditions between the azole and a benzyl electrophile in the presence of strong bases at high temperatures. Use of C-H bonds as the alkylating partner streamlines the preparation of these important motifs. In this work, we report the use of N-alkoxypyridinium salts as a critically enabling reagent for the development of a general C(sp3)-H azolation. The platform enables the alkylation of electron-deficient, -neutral, and -rich azoles with a range of C-H bonds, most notably secondary and tertiary partners. Moreover, the protocol is mild enough to tolerate benzyl electrophiles, thus offering an orthogonal approach to existing SN2 and cross-coupling methods.

Decarboxylative Cross-Coupling: A Radical Tool in Medicinal Chemistry

Carboxylic acids, the most versatile and ubiquitous diversity input used in medicinal chemistry for canonical polar bond constructions such as amide synthesis, can now be employed in a fundamentally different category of reaction to make C-C bonds by harnessing the power of radicals. This outlook serves as a user-guide to aid practitioners in both the design of syntheses that leverage the simplifying power of this disconnection and the precise tactics that can be employed to enable them. Taken together, this emerging area holds the potential to rapidly accelerate access to chemical space of value to modern medicinal chemistry.

A Unified Synthetic Strategy to Introduce Heteroatoms via Electrochemical Functionalization of Alkyl Organoboron Reagents

Based on systematic electrochemical analysis, an integrated synthetic platform of C(sp³)-based organoboron compounds was established for the introduction of heteroatoms. The electrochemically mediated bond-forming strategy was shown to be highly effective for the functionalization of sp³-hybridized carbon atoms with significant steric hindrance. Moreover, virtually all the nonmetallic heteroatoms could be utilized as reaction partners using one unified protocol. The observed reactivity stems from the two consecutive single-electron oxidations of the substrate, which eventually generates an extremely reactive carbocation as the key intermediate. The detailed reaction profile could be elucidated through multifaceted electrochemical studies. Ultimately, a new dimension in the activation strategies for organoboron compounds was accomplished through the electrochemically driven reaction development.

Alkylphosphinites as Synthons for Stabilized Carbocations

We present a new acid-free method for the generation of carbocations based on a redox condensation reaction that enables S_N1 reactions with a variety of nucleophiles. We utilize readily synthesized phosphinites that are activated by diisopropyl azodicarboxylate to form betaine structures that collapse upon adding a pronucleophile, thereby yielding reactive carbocation intermediates. We also employ this approach for the alkylation of some bioactive molecules.

Nickel-Catalyzed Decarboxylative Cross-Coupling of Indole-3-acetic Acids with Aryl Bromides by Convergent Paired Electrolysis

Herein, nickel-catalyzed decarboxylative cross-coupling of indole-3-acetic acids with aryl bromides by convergent paired electrolysis was developed in an undivided cell. This protocol features good functional group tolerance, chemical redox agent-...

The Direct Decarboxylative N-Alkylation of Azoles, Sulfonamides, Ureas, and Carbamates with Carboxylic Acids via Photoredox Catalysis

Herein, we describe a method for the direct decarboxylative C-N coupling of carboxylic acids with a range of nitrogen nucleophiles. This platform employs visible-light-mediated photoredox catalysis and an iodine(III) reagent to generate carbocation intermediates directly from aliphatic carboxylic acids via a radical-polar crossover mechanism. A variety of C-N bond-containing products are constructed from a diverse array of nitrogen heterocycles, including pyrazoles, imidazoles, indazoles, and purine bases. Furthermore, sulfonamides, ureas, and carbamates can also be utilized as the nucleophile to generate a selection of N-alkylated products. Notably, a two-step approach to construct free amines directly from carboxylic acids is accomplished using Cbz-protected amine as the nucleophile.

Electrochemical-Promoted Nickel-Catalyzed Oxidative Fluoroalkylation of Aryl Iodides

This work describes a general strategy for metal-catalyzed cross-coupling of fluoroalkyl radicals with aryl halides under electrochemical conditions. The contradiction between anodic oxidation of fluoroalkyl sulfinates and cathodic reduction of low-valent nickel catalysts can be well addressed by paired electrolysis, allowing for direct introduction of fluorinated functionalities into aromatic systems.

Decatungstate-Mediated C(sp3 )-H Heteroarylation via Radical-Polar Crossover in Batch and Flow

Photocatalytic hydrogen atom transfer is a very powerful strategy for the regioselective C(sp3 )-H functionalization of organic molecules. Herein, we report on the unprecedented combination of decatungstate hydrogen atom transfer photocatalysis with the oxidative radical-polar crossover concept to access the direct net-oxidative C(sp3 )-H heteroarylation. The present methodology demonstrates a high functional group tolerance (40 examples) and is scalable when using continuous-flow reactor technology. The developed protocol is also amenable to the late-stage functionalization of biologically relevant molecules such as stanozolol, (-)-ambroxide, podophyllotoxin, and dideoxyribose.

Decarboxylative N-Alkylation of Azoles through Visible-Light-Mediated Organophotoredox Catalysis

An organophotoredox-catalyzed decarboxylative cross-coupling between azole nucleophiles and aliphatic carboxylic acid-derived redox-active esters is demonstrated. This protocol efficiently installs various tertiary or secondary alkyl fragments onto the nitrogen atom of azole nucleophiles under mild and transition-metal-free conditions. The pyridinium additive successfully inhibits the formation of elimination byproducts from the carbocation intermediate. This reaction is applicable to the synthesis of a protein-degrader-like molecule containing an azole and a thalidomide.

Electrochemically facilitated oxidative C–H amination enables access to fluorescent N9-fused tricyclic xanthines

An electrochemically enabled intramolecular C−H amination route for accessing a broad range of fluorescent N9-fused tricyclic xanthines with various substitution patterns under simple, green, and mild condition is developed.

Recent progress on electrochemical synthesis involving carboxylic acids

Carboxylic acids are not only essential sections of medicinal molecules, natural products and agrochemicals but also basic building blocks for organic synthesis. However, high temperature, expensive catalysts and excess oxidants are normally required for carboxylic acid group transformations. Therefore, more eco-friendly and efficient methods are urgently needed. Organic electrochemistry, as an environmentally friendly and sustainable synthetic method, can potentially avoid the above problems and is favored by more and more organic chemists. This review summarized the recent progress on the electrochemical synthesis of carboxylic acids to construct more complex compounds, emphasizing the development of electrosynthesis methodologies and mechanisms in order to attract more chemists to recognize the importance and applications of electrochemical synthesis.