Electrochemical 1,2‐Diarylation of Alkenes Enabled by Direct Dual C–H Functionalizations of Electron‐Rich Aromatic Hydrocarbons

Synthesis of Diarylalkanes by Photoreductive 1,2-Diarylation of Alkenes with Aryl Halides and Cyanoaromatics

We report a visible light-induced photoreductive strategy for three-component diarylation of alkenes with aryl halides and cyanoaromatics. Upon photoredox catalysis and with tertiary alkyl amines as the electron transfer agent, aryl halides selectively undergo halogen atom transfer to generate the aryl radicals and two C(sp2)-C(sp3) bonds between the cabron atoms are created in a radical addition and radical-radical coupling fashion to rapidly assemble diverse functionalized polyarylalkanes with high regio- and chemoselectivity. This method can be applied to broad feedstocks, including terminal alkenes, internal alkenes, aryl iodides, aryl bromides, aryl chlorides, electron-deficient benzonitriles, and isonicotinonitriles.

Electrochemical Oxidation Enables Radical Dearomative Spiroannulation to 2H-Spiro[benzofuran-3,9'-fluoren]-2-one

Developing pragmatic strategies for accessing functional benzofuran-2-ones from 3-([1,1'-biphenyl]-2-yl)benzofuran remains an enduring challenge. Herein, we have achieved a highly discriminating electrochemical oxidative dearomative spiroannulation of 3-([1,1'-biphenyl]-2-yl)benzofuran, culminating in the synthesis of 2H-spiro[benzofuran-3,9'-fluoren]-2-one derivatives. By harnessing the electrophilic intermediates of benzofuryl radical cations supported by DFT calculations, we attain exceptional regioselectivity while eliminating the need for stoichiometric oxidants. Mechanistic investigations reveal a sequence of events involving the benzofuran radical cation, encompassing the capture of H2O, nucleophilic arene attack, and subsequent deprotonation, ultimately yielding the final benzofuran-2-ones.

Visible light-induced chemoselective 1,2-diheteroarylation of alkenes

Visible-light photocatalysis has evolved as a powerful technique to enable controllable radical reactions. Exploring unique photocatalytic mode for obtaining new chemoselectivity and product diversity is of great significance. Herein, we present a photo-induced chemoselective 1,2-diheteroarylation of unactivated alkenes utilizing halopyridines and quinolines. The ring-fused azaarenes serve as not only substrate, but also potential precursors for halogen-atom abstraction for pyridyl radical generation in this photocatalysis. As a complement to metal catalysis, this photo-induced radical process with mild and redox neutral conditions assembles two different heteroaryl groups into alkenes regioselectively and contribute to broad substrates scope. The obtained products containing aza-arene units permit various further diversifications, demonstrating the synthetic utility of this protocol. We anticipate that this protocol will trigger the further advancement of photo-induced alkyl/aryl halides activation.

Electrochemical Nickel-Catalyzed 1,2-Diarylation of 1,3-Dienes

Due to the presence of carbon-carbon double bonds, 1,3-dienes exhibit great reactivity. A protocol for the site-selective diarylation of terminal 1,3-dienes is reported here. The transformation is facilitated by the Ni catalyst without the need for additional ligands, utilizing an electrochemical setup. Preliminary results indicate that by introducing chiral ligands moderate enantioselective diarylation products can be obtained. This method affords diversely substituted diarylated products that occur as structural motifs in various natural products.

Nickel-Catalyzed Regioselective Intermolecular Dialkylation of Alkenylarenes: Generation of Two Vicinal C(sp3 )-C(sp3 ) Bonds Across Alkenes

We disclose a Ni-catalyzed regioselective dialkylation reaction of alkenylarenes with α-halocarbonyls and alkylzinc reagents. The reaction produces γ-arylated alkanecarbonyl compounds with the generation of two new C(sp3 )-C(sp3 ) bonds at the vicinal carbons of alkenes. This reaction is effective for the use of primary, secondary and tertiary α-halocarboxylic esters, amides and ketones in conjunction with primary and secondary alkylzinc reagents as the sources of two C(sp3 ) carbons for the dialkylation of terminal and cyclic internal alkenes.

Electrochemical Syntheses of Polycyclic Aromatic Hydrocarbons (PAHs)

Polycyclic aromatic hydrocarbons (PAHs) have surfaced as increasingly viable components in optoelectronics and material sciences. The development of highly efficient and atom-economic tools to prepare PAHs under exceedingly mild conditions constitutes a long-term goal. Traditional syntheses of PAHs have largely relied on multistep approaches or the conventional Scholl reaction. However, Scholl reactions are largely inefficient with electron-deficient substrates, require stoichiometric chemical oxidants, and typically occur in the presence of strong acid. In sharp contrast, electrochemistry has gained considerable momentum during the past decade as an alternative for the facile and straightforward PAHs assembly, generally via electro-oxidative dehydrogenative annulation, releasing molecular hydrogen as the sole stoichiometric byproduct by the hydrogen evolution reaction. This review provides an overview on the recent and significant advances in the field of electrochemical syntheses of various PAHs until January 2023.

Electrochemically enabled decyanative C(sp3)-H oxygenation of N-cyanomethylamines to formamides

Selective oxygenation of C(sp³)-H bonds adjacent to nitrogen atoms is a highly attractive strategy for synthesizing various formamide derivatives while preserving the substrate skeletons. Herein, an environmentally benign electrochemically enabled decyanative C(sp³)-H oxygenation of N-cyanomethylamines using H₂O as a carbonyl oxygen atom source is described, leading to the synthesis of a large class of formamides in good to excellent yields with a broad substrate scope under metal- and oxidant-free conditions. This electrochemical technology highlights the facile incorporation of N-formyl into some important bioactive molecules.

Electrochemical oxidative difunctionalization of diazo compounds with two different nucleophiles

With the fast development of synthetic chemistry, the introduction of functional group into organic molecules has attracted increasing attention. In these reactions, the difunctionalization of unsaturated bonds, traditionally with one nucleophile and one electrophile, is a powerful strategy for the chemical synthesis. In this work, we develop a different path of electrochemical oxidative difunctionalization of diazo compounds with two different nucleophiles. Under metal-free and external oxidant-free conditions, a series of structurally diverse heteroatom-containing compounds hardly synthesized by traditional methods (such as high-value alkoxy-substituted phenylthioacetates, α-thio, α-amino acid derivatives as well as α-amino, β-amino acid derivatives) are obtained in synthetically useful yields. In addition, the procedure exhibits mild reaction conditions, excellent functional-group tolerance and good efficiency on large-scale synthesis. Importantly, the protocol is also amenable to the key intermediate of bioactive molecules in a simple and practical process.

Electroreduction Enables Regioselective 1,2-Diarylation of Alkenes with Two Electrophiles

Precisely introducing two similar functional groups into bulk chemical alkenes represents a formidable route to complex molecules. Especially, the selective activation of two electrophiles is in crucial demand, yet challenging for cross-electrophile-coupling. Herein, we demonstrate a redox-mediated electrolysis, in which aryl nitriles are both aryl radical precursors and redox-mediators, enables an intermolecular alkene 1,2-diarylation with a remarkable regioselectivity, thereby avoiding the involvement of transition-metal catalysts. This transformation utilizes cyanoarene radical anions for activating various aryl halides (including iodides, bromides, and even chlorides) and affords 1,2-diarylation adducts in up to 83 % yield and >20 : 1 regioselectivity with more than 80 examples, providing a feasible approach to complex bibenzyl derivatives.

Synthesis of Unprotected β-Arylethylamines by Iron(II)-Catalyzed 1,2-Aminoarylation of Alkenes in Hexafluoroisopropanol

β-Arylethylamines are prevalent structural motifs in molecules exhibiting biological activity. Here we report a sequential one-pot protocol for the 1,2-aminoarylation of alkenes with hydroxylammonium triflate salts and (hetero)arenes. Unlike existing methods, this reaction provides a direct entry to unprotected β-arylethylamines with remarkable functional group tolerance, allowing key drug-oriented functional groups to be installed in a two-step process. The use of hexafluoroisopropanol as a solvent in combination with an iron(II) catalyst proved essential to reaching high-value nitrogen-containing molecules.

Unraveling the Structure and Reactivity Patterns of the Indole Radical Cation in Regioselective Electrochemical Oxidative Annulations

Oxidation-induced strategy for inert chemical bond activation through highly active radical cation intermediate has exhibited unique reactivity. Understanding the structure and reactivity patterns of radical cation intermediates is crucial in the mechanistic study and will be beneficial for developing new reactions. In this work, the structure and properties of indole radical cations have been revealed using time-resolved transient absorption spectroscopy, in situ electrochemical UV-vis, and in situ electrochemical electron paramagnetic resonance (EPR) technique. Density functional theory (DFT) calculations were used to explain and predict the regioselectivity of several electrochemical oxidative indole annulations. Based on the understanding of the inherent properties of several indole radical cations, two different regioselective annulations of indoles have been successfully developed under electrochemical oxidation conditions. Varieties of furo[2,3-b]indolines and furo[3,2-b]indolines were synthesized in good yields with high regioselectivities. Our mechanistic insights into indole radical cations will promote the further development of oxidation-induced indole functionalizations.

Site-Selective Electrochemical C-H Carboxylation of Arenes with CO2

Herein, a direct, metal-free, and site-selective electrochemical C-H carboxylation of arenes by reductive activation using CO₂ as the economic and abundant carboxylic source was reported. The electrocarboxylation was carried out in an operationally simple manner with high chemo- and regioselectivity, setting the stage for the challenging site-selective C-H carboxylation of unactivated (hetero)arenes. The robust nature of the electrochemical strategy was reflected by a broad scope of substrates with excellent atom economy and unique selectivity. Notably, the direct and selective C-H carboxylation of various challenging arenes worked well in this approach, including electron-deficient naphthalenes, pyridines, simple phenyl derivatives, and substituted quinolines. The method benefits from being externally catalyst-free, metal-free and base-free, which makes it extremely attractive for potential applications.

Electrochemical radical-mediated selective C(sp3)-S bond activation

Selective C(sp³)-S bond breaking and transformation remains a particularly important, yet challenging goal in synthetic chemistry. Over the past few decades, transition metal-catalyzed cross-coupling reactions through the cleavage of C(sp³)-S bonds provided a powerful platform for the construction of target molecules. In contrast, the selective activation of widespread C(sp³)-S bonds is rarely studied and remains underdeveloped, even under relatively harsh conditions. Herein, a radical-mediated electrochemical strategy capable of selectively activating C(sp³)-S bonds is disclosed, offering an unprecedented method for the synthesis of valuable disulfides from widespread thioethers. Importantly, compared with conventional transition-metal catalyzed C-S bond breaking protocols, this method features mild, catalyst- and oxidant-free reaction conditions, as well excellent chemoselectivity towards C(sp³)-S bonds. Preliminary mechanistic studies reveal that sulfur radical species are involved in the reaction pathway and play an essential role in controlling the site-selectivity.

Cathode enabled high faradaic efficiency: reduction of imines to amines with H2O as a H-source

Benefiting from a high overpotential of the competitive hydrogen evolution reaction with a carbon paper cathode, the desired electrochemical reduction of imines was achieved with high faradaic efficiency by using H₂O as a H-source. With this sustainable atom-economic strategy, a series of potentially versatile amines were obtained in medium-to-high yields (49-86%).

Electro-/photocatalytic alkene-derived radical cation chemistry: recent advances in synthetic applications

Alkene-derived radical cations are versatile reactive intermediates and have been widely applied in the construction of complex functionalized molecules and cyclic systems for chemical synthesis. Therefore, the synthetic application of these alkene-derived radical cations represents a powerful and green tool that can be used to achieve the functionalization of alkenes partially because the necessity of stoichiometric external chemical oxidants and/or hazardous reaction conditions is eliminated. This review summarizes the recent advances in the synthetic applications of the electro-/photochemical alkene-derived radical cations, emphasizing the key single-electron oxidation steps of the alkenes, the scope and limitations of the substrates, and the related reaction mechanisms. Using electrocatalysis and/or photocatalysis, single electron transfer (SET) oxidation of the CC bonds in the alkenes occurs, generating the alkene-derived radical cations, which sequentially enables the functionalization of translocated radical cations to occur in two ways: the first involves direct reaction with a nucleophile/radical or two molecules of nucleophiles to realize hydrofunctionalization, difunctionalization and cyclization; and the second involves the transformation of the alkene-derived radical cations into carbon-centered radicals using a base followed by radical coupling or oxidative nucleophilic coupling.

Electrosynthesis of (hetero)aryl nitriles from α-imino-oxy acids via oxidative decarboxylation/N-O cleavage

A new method for the synthesis of (hetero)aryl nitriles via iminyl radicals has been developed through the electrochemical oxidative decarboxylation of α-imino-oxy acids. This protocol provides an efficient approach to nitriles with a broad range of functional-group tolerance under ambient conditions and can be applied for one-pot gram-scale synthesis.

Electrocatalytic Isomerization of Allylic Alcohols: Straightforward Preparation of β-Aryl-Ketones

Electrochemical synthesis has been rapidly developing over the past few years. Here, we report a practical and eco-friendly electrocatalytic isomerization of allylic alcohols to their corresponding carbonyl compounds. This reaction can be carried out in undivided cells without the addition of external chemical oxidants and metal catalysts. Moreover, this reaction features a broad substrate scope including challenging allylic alcohols bearing tri- and tetra-substituted olefins and affords straightforward access to diverse β-aryl-ketones. Mechanistic investigations suggest that the reactions proceed through a radical process. This study represents a unique example in which electrochemistry enables hydrogen atom transfer in organic allylic alcohol substrates using a simple organocatalyst.

Exploring the regioselectivity of the cyanoalkylation of 3-aza-1,5-dienes: photoinduced synthesis of 3-cyanoalkyl-4-pyrrolin-2-ones

Regioselective cyanoalkylalkenylation of 3-aza-1,5-dienes with oxime esters induced by visible light.

Electrochemical radical C(sp3)–H arylation of xanthenes with electron-rich arenes

An efficient electrochemical C(sp3)–H arylation of xanthenes using a carbon anode and platinum cathode as the electrodes is disclosed.

K2S2O8-mediated acylarylation of unactivated alkenes via acyl radical addition/C–H annulation cascade of N-allyl-indoles with silver cocatalysis

A silver-catalyzed, K2S2O8-mediated protocol to access the regioselective acylarylation of unactivated alkenes was reported.

Electrochemical Regioselective Cross-Dehydrogenative Coupling of Indoles with Xanthenes

An electrochemical cross-dehydrogenative coupling of indoles with xanthenes has been established at room temperature. This coupling reaction could proceed in the absence of any catalyst or external oxidant, and generate the indole derivatives in moderate yields. Mechanistic experiments support that a radical pathway maybe involved in this reaction system.

Intermolecular 1,2-Difunctionalization of Alkenes Enabled by Fluoroamide-Directed Remote Benzyl C(sp3)-H Functionalization

A copper-catalyzed remote benzylic C-H functionalization strategy enabling 1,2-difunctionalization of alkenes with 2-methylbenzeneamides and nucleophiles, including alcohols, indoles, pyrroles, and the intrinsic amino groups, is reported, which is characterized by its redox-neutral conditions, exquisite site-selectivity, broad substrate scope, and wide utilizations of late-stage modifying bioactive molecules. This reaction proceeds through nitrogen-centered radical generation, hydrogen atom transfer, benzylic radical addition across the alkenes, single-electron oxidation, and carbocation electrophilic course cascades. While using external nucleophiles manipulates three-component alkene alkylalkoxylation and alkyl-heteroarylation with 2-methylbenzeneamides to access dialkyl ethers, 3-alkylindoles, and 3-alkylpyrroles, omitting the external nucleophiles results in two-component alkylamidation ([5+2] annulation) of alkenes with 2-methylbenzeneamides to benzo-[f][1,2]thiazepine 1,1-dioxides.

Electrochemical Reductive Arylation of Nitroarenes with Arylboronic Acids

The synthesis of diarylamine is extremely important in organic chemistry. Herein, a novel electrochemical reductive arylation of nitroarenes with arylboronic acids was developed. A variety of diarylamines were synthesized without the need for transition-metal catalysts. The reaction could be scaled up efficiently in a flow cell and several derivatization reactions were carried out smoothly. Cyclic voltammetry experiments and mechanism studies showed that acetonitrile, formic acid, and triethyl phosphite all played a role in promoting this reductive arylation transformation.

Electrochemical oxygenation of sulfides with molecular oxygen or water: switchable preparation of sulfoxides and sulfones

A practical and eco-friendly method for the controllable aerobic oxygenation of sulfides by electrochemical catalysis was developed. The switchable preparation of sulfoxides and sulfones was effectively controlled by reaction time, in which both molecular oxygen and water can be used as the oxygen source under catalyst and external oxidant-free conditions. The electrochemical protocol features a broad substrate scope and excellent site selectivity and is successfully applied to the modification of some sulfide-containing pharmaceuticals and their derivatives.

Electrochemical Desaturative β-Acylation of Cyclic N-Aryl Amines

Herein, we disclose a straightforward, robust, and simple route to access β-substituted desaturated cyclic amines via an electrochemically driven desaturative β-functionalization of cyclic amines. This transformation is based on multiple single-electron oxidation processes using catalytic amounts of ferrocene. The reaction proceeds in the absence of stoichiometric amounts of electrolyte under mild conditions, affording the desired products with high chemo- and regioselectivity. The reaction was tolerant of a broad range of substrates and also enables late-stage β-C(sp³ )-H acylation of potentially valuable products. Preliminary mechanistic studies using cyclic voltammetry reveal the key role of ferrocene as a redox mediator in the reaction.

Electrochemical Difunctionalization of Styrenes via Chemoselective Oxo-Azidation or Oxo-Hydroxyphthalimidation

Atom- and step-economic oxo-azidation and oxo-hydroxyphthalimidation of styrenes have been developed under mild electrolytic conditions, respectively. Various valuable alpha-azido or hydroxyphthalimide aromatic ketones were synthesized efficiently from readily available styrenes, azides, and N-hydroxyphthalimides. Mechanism studies show that two different pathways involved in these two transformations.

Electrochemically selective double C(sp2)-X (X = S/Se, N) bond formation of isocyanides

The construction of C(sp²)-X (X = B, N, O, Si, P, S, Se, etc.) bonds has drawn growing attention since heteroatomic compounds play a prominent role from biological to pharmaceutical sciences. The current study demonstrates the C(sp²)-S/Se and C(sp²)-N bond formation of one carbon of isocyanides with thiophenols or disulfides or diselenides and azazoles simultaneously. The reported findings could provide access to novel multiple isothioureas, especially hitherto rarely reported selenoureas. The protocol showed good atom-economy and step-economy with only hydrogen evolution and theoretical calculations accounted for the stereoselectivity of the products. Importantly, the electrochemical reaction could exclusively occur at the isocyano part regardless of the presence of susceptible radical acceptors, such as a broad range of arenes and alkynyl moieties, even alkenyl moieties.

Photocatalytic Decarboxylative [3 + 2] and [4 + 2] Annulation of Enynals and γ,σ-Unsaturated N-(Acyloxy)phthalimides by NaI/PPh3 Catalysis

A practical and eco-friendly strategy for the radical-mediated decarboxylative [3 + 2] and [4 + 2] annulation of enynals and γ,σ-unsaturated N-(acyloxy)phthalimides through the photoactivation of an electron donor-acceptor (EDA) complex has been developed. A wide range of primary, secondary, and tertiary alkyl N-hydroxyphthalimide (NHP) esters can be used as suitable substrates for the synthesis of fused ketones without any transition-metal catalysts or oxidants. This protocol features a broad substrate scope, excellent selectivity, and clean reaction conditions.

Iminyl-radicals by electrochemical decarboxylation of α-imino-oxy acids: construction of indole-fused polycyclics

Iminyl radicals are reactive intermediates that can be used for the construction of various valuable heterocycles. Herein, the electrochemical decarboxylation of α-imino-oxy acids for the generation of iminyl radicals has been accomplished under exogenous-oxidant- and metal-free conditions through the use of nBu₄NBr as a mediator. The resulting iminyl radicals undergo intramolecular cyclization smoothly with the adjacent (hetero)arenes to afford a series of indole-fused polycyclic compounds.

Electrochemical Oxidative Difunctionalization of Alkenes to Access α-Oxygenated Ketones

Dioxygenation of alkenes was developed by the combination of electrochemical synthesis and aerobic oxidation, leading to easy accessibility of α-oxygenated ketones in an eco-friendly fashion. Using air as the oxygen source and the absence of transition metals were the critical features of this protocol. A wide range of alkenes and N-hydroxyimides were found to be compatible and provided α-oxygenated ketones in moderate to high yields.

Sulfoxide and Sulfone Synthesis via Electrochemical Oxidation of Sulfides

The oxidation of diaryl sulfides and aryl alkyl sulfides to the corresponding sulfoxides and sulfones under electrochemical conditions is reported. Sulfoxides are selectively obtained in good yield under a constant current of 5 mA for 10 h in DMF, while sulfones are formed as the major product under a constant current of 10 or 20 mA for 10 h in MeOH. The oxygen of both the sulfoxide and sulfone function is derived from water.

Photocatalytic intermolecular carboarylation of alkenes by selective C-O bond cleavage of diarylethers

Disclosed herein is a novel radical-mediated intermolecular carboarylation of alkenes by cleaving inert C-O bonds. The strategically designed arylbenzothiazolylether diazonium salts are harnessed as dual-function reagents. A vast array of alkenes are proven to be suitable substrates. The benzothiazolyl moiety in the products serves as the formyl precursor, and the OH residue provides the cross-coupling site for further product elaboration, indicating the robust transformability of the products.

Site-Selective Electrochemical C-H Cyanation of Indoles

An electrochemical approach for the site-selective C-H cyanation of indoles employing readily available TMSCN as cyano source has been developed. The electrosynthesis relies on the tris(4-bromophenyl)amine as a redox catalyst, which achieves better yield and regioselectivity. A variety of C2- and C3-cyanated indoles were obtained in satisfactory yields. The reactions are conducted in a simple undivided cell at room temperature and obviate the need for transition-metal reagent and chemical oxidant.

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.