Electrochemically Driven Radical Reactions: From Direct Electrolysis to Molecular Catalysis

Ruthenaelectro-catalyzed C-H phosphorylation: ortho to para position-selectivity switch

The position-selective C-H bond activation of arenes has long been a challenging topic. Herein, we report an expedient ruthenium-electrocatalyzed site-selective ortho-C-H phosphorylation of arenes driven by electrochemical hydrogen evolution reaction (HER), avoiding stoichiometric amounts of chemical redox-waste products. This strategy paved the way to achieve unprecedented ruthenaelectro-catalyzed para-C-H phosphorylation with excellent levels of site-selectivity. This electrocatalytic approach was characterized by an ample substrate scope with a broad range of arenes containing N-heterocycles, as well as several aryl/alkylphosphine oxides were well tolerated. Moreover, late-stage C-H phosphorylation of medicinal relevant drugs could also be achieved. DFT mechanistic studies provided support for an unusual ruthenium(iii/iv/ii) regime for the ortho-C-H phosphorylation.

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.

Selective Oxidation of Alcohols to Carbonyls Under Decatungstate-Mediated Photoelectrochemical Conditions

The oxidation of alcohols to the corresponding carbonyl derivatives has been realized under photoelectrochemical conditions in the presence of tetrabutylammonium decatungstate (TBADT) as the homogeneous photocatalyst. The protocol can be applied to both primary and secondary, benzylic and aliphatic alcohols. The desired products are obtained selectively, skipping the need for purposely added chemical oxidants. An in-depth study of photoelectrochemical conditions revealed that the protocol works best under amperostatic conditions in an undivided electrochemical cell irradiated with a 390 nm LED lamp. The comparison with analogous electrochemical and chemical oxidant-promoted photocatalytic transformations demonstrates the superior efficiency and selectivity of the hereby reported photoelectrochemical conditions.

Regioselective Electrooxidative [3+2] Annulation between Indole and Aniline Derivatives to Construct Functionalized Indolo[2,3-b]indoles

A protocol for the electrooxidative [3+2] annulation to generate indolo[2,3-b]indoles in an undivided cell is reported. It exhibits good yields with excellent regioselectivities and tolerates various functional groups without external chemical oxidants. Cyclic voltammetry and density functional theory calculations indicate that the [3+2] annulation is initiated by the simultaneous anodic oxidation of indole and aniline derivatives, and the step to determine the rate relies on the combination of radical cations.

Electrochemical dual oxidative C(sp3)-H amination: switchable synthesis of imidazo-fused quinazolinones

An efficient electrochemical dual C(sp3)-H amination was developed under metal-free and chemical oxidant-free conditions. A series of imidazo[1,5-a]quinazolin-5(4H)-ones and 5-oxo-4,5-dihydroimidazo[1,5-a]quinazoline-3-carbonitriles can be obtained in high yields and the product distribution can be modulated by virtue of this method. The reaction mechanism was investigated and the corresponding intermediates were studied. The reaction features a broad substrate scope, regulation of the product distribution, mild conditions and scalable preparation.

Electrochemical Oxidative (4 + 2) Cyclization of Anilines and o-Phenylenediamines for the Synthesis of Phenazines

Phenazines, crucial constituents of nitrogen-containing heterocycles, widely exist in functional compounds. Herein, we report an anodic oxidative (4 + 2) cyclization between anilines and o-phenylenediamines for the uniform construction of phenazines in a simple undivided cell. Dual C-H amination followed by oxidation represents an outstanding step and atom efficiency. A sequence of phenazines is produced with excellent functional group tolerance at room temperature.

Electro-oxidative three-component cascade coupling of isocyanides with elemental sulfur and amines for the synthesis of 2-aminobenzothiazoles

2-Aminobenzothiazoles are commonly encountered in various functional compounds. Herein, we disclose an electro-oxidative three-component reaction for the effective synthesis of 2-aminobenzothiazoles under mild conditions, utilizing non-toxic and abundant elemental sulfur as the sulfur source. Both aliphatic amines and aryl amines demonstrate good compatibility at room temperature, highlighting the broad functional group tolerance of this approach. Additionally, elemental selenium demonstrated reactivities comparable to those of elemental sulfur.

Electrochemical Multicomponent Cascade Reaction for the Synthesis of Selenazol-2-amines with Elemental Selenium

The first example of an electrochemical multicomponent synthesis of selenium-containing compounds with inexpensive and abundant elemental selenium as the selenating reagent was developed. A variety of selenazol-2-amines were constructed in high yields with good functional group tolerance under metal-free and chemical oxidant-free conditions.

Transition-Metal-Free, Reductive Csp2-Csp3 Bond Constructions via Electrochemically Induced Alkyl Radicals

Construction of the Csp2-Csp3 bond without the aid of transition metal catalysts has been achieved by coupling the electrogenerated alkyl radicals with electron deficient (hetero)arenes in an undivided cell. Simultaneous cathodic reduction of both unactivated alkyl halides and cyanobenzenes under high potential enables radical-radical cross-coupling to deliver alkylarenes in the absence of transition metals. Depending on the coupling partner, the electrogenerated alkyl radicals can also proceed the Minisci-type reaction with N-heteroarenes without redox agents.

Cathodically Coupled Electrolysis to Access Biheteroaryls

An electrochemical approach to biheteroaryls through the coupling of diverse N-heteroarenes with heteroaryl phosphonium salts is reported. The reaction features pH and redox-neutral conditions and excellent regioselectivity, as well as exogenous air or moisture tolerance. Additionally, a one-pot, two-step protocol can be established to realize formal C-H/C-H coupling of heteroarenes, thereby greatly expanding the substrate availability. The utility of this method is demonstrated through late-stage functionalization, the total synthesis of nitraridine, and antifungal activity studies.

Electrochemical Arene Radical Cation Promoted Spirocyclization of Biaryl Ynones: Access to Alkoxylated Spiro[5,5]trienones

Herein, a novel electrochemical arene radical cation promoted dearomative spirocyclization of biaryl ynones with alcohols is described, providing a conceptually novel transformation mode for producing diverse alkoxylated spiro[5,5]trienones. The catalyst- and chemical-oxidant-free spirocyclization protocol features broad substrate scope and high functional group tolerance. Mechanistic studies reveal that the generation of arene radical cation via anodic single-electron oxidation is crucial, with sequential 6-endo-dig cyclization, dissociation of hemiketal, anodic oxidation, and nucleophilic attack of alcohols.

Photoelectrocatalyzed undirected C-H trifluoromethylation of arenes: catalyst evaluation and scope

During the last few years, photoelectrocatalysis has evolved as an increasingly viable tool for molecular synthesis. Despite several recent reports on the undirected C-H functionalization of arenes, thus far, a detailed comparison of different catalysts is still missing. To address this, more than a dozen different mediators were employed in the trifluoromethylation of (hetero-)arenes to compare them in their efficacies.

Paraformaldehyde as C1 Synthon: Electrochemical Three-Component Synthesis of Tetrahydroimidazo[1,5-a]quinoxalin-4(5H)-ones in Aqueous Ethanol

A green and practical method for the electrochemical synthesis of tetrahydroimidazo[1,5-a]quinoxalin-4(5H)-ones through the three-component reaction of quinoxalin-2(1H)-ones, N-arylglycines and paraformaldehyde was reported. In this strategy, EtOH played dual roles (eco-friendly solvent and waste-free pre-catalyst) and the in situ generated ethoxide promoted triple sequential deprotonations.

The electrochemically enabled α-C(sp3)-H azolation of ketones

C-H/N-H cross-coupling has become a key technology for the selective conjugation of azole drug molecules. However, the development of new synthetic models and green chemical methods is imperative to enhance the construction of multi-functional compounds and compounds with unique functional groups. We herein reported an electrochemical synthesis of α-tetrazolyl ketones with excellent yields and broad substrate scope, encompassing electron-donating and electron-withdrawing groups of aryl ketones, heterocycles, and alkyl and various ketone drugs. It was further proved that α-iodoketone was involved in this transformation of the reaction as a critical intermediate.

Electrochemical Difunctionalization of gem-Difluoroalkenes: A Metal-Free Synthesis of α-Difluoro(alkoxyl/azolated) Methylated Ethers

A scalable electrochemical difunctionalization of gem-difluoroalkenes to structurally versatile difluoro motifs was achieved. This methodology features reagent-free conditions, good functional group tolerance, and a relatively broad substrate scope. Meanwhile, the electrolysis protocol is easy to handle, and the products show good regio- and chemoselectivity. The reaction mechanism was also preliminarily studied.

Paired Electrolysis Enabled Cyanation of Diaryl Diselenides with KSCN Leading to Aryl Selenocyanates

The first example of paired electrolysis-enabled cyanation of diaryl diselenides, with KSCN as the green cyanating agent, has been developed. A broad range of aryl selenocyanates can be efficiently synthesized under chemical-oxidant- and additive-free, energy-saving and mild conditions.

Recent Advances in the Electrochemical Functionalization of Isocyanides

Isocyanides are well-known as efficient CO surrogates and C1 synthons in modern organic synthesis. Although tremendous efforts have been devoted to fully exploiting the reactivity of isocyanides, these transformations are primarily limited by their utilization of stoichiometric toxic chemical oxidants. With the recent resurgence of organic electrochemistry, which has considerably laid dormant over the past several decades, electrolysis has been identified as a green and powerful tool to enrich structural diversity by solely utilizing electric current as clean and inherently safe redox equivalents of stoichiometric chemical oxidants. In this regard, the unique reactivity of isocyanides has been studied in numerous electrochemical transformations. This review comprehensively highlights the most relevant progress in electrochemical strategies towards the functionalization of isocyanides up until June of 2022, with a focus on reaction outcomes and mechanisms.

Redox-active molecules as organocatalysts for selective oxidative transformations - an unperceived organocatalysis field

Organocatalysis is widely recognized as a key synthetic methodology in organic chemistry. It allows chemists to avoid the use of precious and (or) toxic metals by taking advantage of the catalytic activity of small and synthetically available molecules. Today, the term organocatalysis is mainly associated with redox-neutral asymmetric catalysis of C-C bond-forming processes, such as aldol reactions, Michael reactions, cycloaddition reactions, etc. Organophotoredox catalysis has emerged recently as another important catalysis type which has gained much attention and has been quite well-reviewed. At the same time, there are a significant number of other processes, especially oxidative, catalyzed by redox-active organic molecules in the ground state (without light excitation). Unfortunately, many of such processes are not associated in the literature with the organocatalysis field and thus many achievements are not fully consolidated and systematized. The present article is aimed at overviewing the current state-of-art and perspectives of oxidative organocatalysis by redox-active molecules with the emphasis on challenging chemo-, regio- and stereoselective CH-functionalization processes. The catalytic systems based on N-oxyl radicals, amines, thiols, oxaziridines, ketone/peroxide, quinones, and iodine(I/III) compounds are the most developed catalyst types which are covered here.

Photocatalytic Modular Cyanoalkylamination of Alkenes Involving Two Different Iminyl Radicals

The modular cyanoalkylamination of alkenes using bench-stable and easy-to-handle α-imino-oxy acid oxime esters as difunctional reagents creates new synthetic avenues. A metal-free photosensitization protocol for the installation of both amino and cyanoalkyl functionalities onto alkene feedstocks in a single step via two differently reactive nitrogen-centered radicals was developed via energy-transfer catalysis. Excellent functional group tolerance and mild reaction conditions also render this protocol suitable for the cyanoalkylamination of pharmaceutically relevant molecule-derived alkenes.

Electrochemical collective synthesis of labeled pyrroloindoline alkaloids with Freon-type methanes as functional C1 synthons

Utilization of Freon-type methanes as functional one-carbon synthons in the synthesis of various deuterated indoline alkaloids was demonstrated here. A series of halomethyl radicals were generated from electro-reductive C-X cleavage of Freon-type methanes and captured efficiently by acrylamides to provide various halogenated oxindoles via radical cyclization. This reaction features good functional group tolerance, and deuterium and fluorine atoms could be introduced facilely from Freon-type methanes. Further transformation of halogenated oxindoles enabled the synthesis of many (labeled) bioactive drug molecules and skeletons, such as deuterated (±)-physostigmine, deuterated (±)-esermethole and deuterated (±)-lansai B.

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.

Electroediting of Soft Polymer Backbones

Synthetic methods that edit soft polymer backbones are critical technologies for tailoring the structures and properties of macromolecules. Developing strategies that leverage underexplored reaction manifolds are vital for accessing new chemical (and functional) space in soft materials. Here, we report a mild electrochemical approach that enables both degradation and functionalization of synthetic polymers. We found that bulk electrolysis (under either homogeneous or heterogeneous conditions) promoted facile, chemoselective chain scission in a variety of olefin-containing materials. Polymer degradation could also be coupled with functionalization (e.g., azidation) to afford new species that could serve as macromonomers.

Electrochemical Cage Activation of Carboranes

Carboranes are boron-carbon molecular clusters that possess unique properties, such as their icosahedron geometry, high boron content, and delocalized three-dimensional aromaticity. These features render carboranes valuable building blocks for applications in supramolecular design, nanomaterials, optoelectronics, organometallic coordination chemistry, and as boron neutron capture therapy (BNCT) agents. Despite tremendous progress in this field, stoichiometric chemical redox reagents are largely required for the oxidative activation of carborane cages. In this context, electrosyntheses represent an alternative strategy for more sustainable molecular syntheses. It is only in recent few years that considerable progress has been made in electrochemical cage functionalization of carboranes, which are summarized in this Minireview. We anticipate that electrocatalysis will serve as an increasingly powerful stimulus within the current renaissance of carborane electrochemistry.

Synthetic Molecular Photoelectrochemistry: New Frontiers in Synthetic Applications, Mechanistic Insights and Scalability

Synthetic photoelectrochemistry (PEC) is receiving increasing attention as a new frontier for the generation and handling of reactive intermediates. PEC permits selective single-electron transfer (SET) reactions in a much greener way and broadens the redox window of possible transformations. Herein, the most recent contributions are reviewed, demonstrating exciting new opportunities, namely, the combination of PEC with other reactivity paradigms (hydrogen-atom transfer, radical polar crossover, energy transfer sensitization), scalability up to multigram scale, novel selectivities in SET super-oxidations/reductions and the importance of precomplexation to temporally enable excited radical ion catalysis.

Electrocatalytic three-component synthesis of 4-halopyrazoles with sodium halide as the halogen source

The first example of the electrocatalytic multicomponent synthesis of 4-chloro/bromo/iodopyrazoles from hydrazines, acetylacetones and sodium halides under chemical oxidant- and external electrolyte-free conditions has been developed.

NHPI-catalyzed electrochemical C–H alkylation of indoles with alcohols to access di(indolyl)methanes via radical coupling

The present indirect electrochemically mediated radical protocol outperforms the traditional Friedel–Crafts route with a broad substrate scope and functional group tolerance, as well as facile gram-scale synthesis without metal contamination.

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.

Synthesis of 1H-indazoles by an electrochemical radical Csp2-H/N-H cyclization of arylhydrazones

The development of efficient and sustainable C-N bond-forming reactions to N-heterocyclic frameworks has been a long-standing interest in organic synthesis. In this work, we develop an electrochemical radical C_sp²-H/N-H cyclization of arylhydrazones to 1H-indazoles. The electrochemical anodic oxidation approach was adopted to synthesize a variety of 1H-indazole derivatives in moderate to good yields. HFIP was not only employed as a solvent or the proton donor, but also can promote the formation of N free radicals. This synthetic methodology is operationally simple, and less expensive electrodes would be suitable for this chemistry.

Metal-Free Hydropyridylation of Thioester-Activated Alkenes via Electroreductive Radical Coupling

An electrochemical hydropyridylation of thioester-activated alkenes with 4-cyanopyridines has been developed. The reactions experience a tandem electroreduction of both substrates on the cathode surface, protonation, and radical cross-coupling process, resulting in a variety of valuable pyridine variants, which contain a tertiary and even a quaternary carbon at the α-position of pyridines, in high yields. The employment of thioesters to the conjugated alkenes enables no requirement of catalyst and high temperature, representing a highly sustainable synthetic method.

Electrodimerization of N-Alkoxyamides for the Synthesis of Hydrazines

An efficient and valuable N-N dimerization reaction of N-alkoxyamides is reported under undivided electrolytic conditions. This electrochemical strategy provides a powerful way to access a wide range of advanced, highly functionalized hydrazines. Remarkably, an N-centered radical generated from the cleavage of the N-H bond under electrolytic conditions plays a crucial role in this transformation. Furthermore, various N-alkoxyamides bearing different substituents are suitable in this transformation, furnishing the corresponding hydrazines in up to 92% yield.

Discovery of a tetraarylhydrazine catalyst for electrocatalytic synthesis of imidazo-fused N-heteroaromatic compounds

The development of electrocatalytic synthetic methods hinges on efficient molecular catalysts. Triarylamines are well-known redox catalysts because of the good stability of their corresponding amine radical cations. Herein we show that tris(4-(tert-butyl)phenyl)amine decomposes unexpectedly during electrolysis in MeOH/THF to afford a tetraarylhydrazine, 1,1,2,2-tetrakis(4-(tert-butyl)phenyl)hydrazine. In addition, we have applied this tetraarylhydrazine, which is either preprepared or formed in situ from tris(4-(tert-butyl)phenyl)amine, as an electrocatalyst for the synthesis of imidazopyridines and related N-heteroaromatic compounds through intramolecular [3 + 2] annulation. This metal-free electrocatalytic method provides straightforward access to the N-heteroaromatic compounds from readily available materials without the need for external chemical oxidants.

Electrochemical Palladium-Catalyzed Oxidative Sonogashira Carbonylation of Arylhydrazines and Alkynes to Ynones

Oxidative carbonylation using carbon monoxide has evolved as an attractive tool to valuable carbonyl-containing compounds, while mixing CO with a stoichiometric amount of a chemical oxidant especially oxygen is hazardous and limits its application in scale-up synthesis. By employing anodic oxidation, we developed an electrochemical palladium-catalyzed oxidative carbonylation of arylhydrazines with alkynes, which is regarded as an alternative supplement of the carbonylative Sonogashira reaction. Combining an undivided cell with constant current mode, oxygen-free conditions avoids the explosion hazard of CO. A diversity of ynones are efficiently obtained using accessible arylhydrazines and alkynes under copper-free conditions. A possible mechanism of the electrochemical Pd(0)/Pd(II) cycle is rationalized based upon cyclic voltammetry, kinetic studies, and intermediates experiments.

Electrocatalytic Dehydrogenative Cyclization of 2-Vinylanilides for the Synthesis of Indoles

Indole is prevalent in bioactive compounds and natural products. The development of efficient and sustainable methods to access this privileged structural scaffold has been a long-standing interest of synthetic chemists. Herein, we report an electrocatalytic method for the synthesis of indoles through dehydrogenative cyclization of 2-vinylanilides. The reactions employ an organic redox catalyst and do not require any external chemical oxidant, providing speedy and efficient access to 3-substituted and 2,3-disubstituted indoles.