Tailored cobalt-salen complexes enable electrocatalytic intramolecular allylic C-H functionalizations

Electrochemically promoted tandem cyclization of functionalized methylenecyclopropanes: synthesis of tetracyclic benzazepine derivatives

In this study, an electrocatalytic tandem cyclization reaction of amide-tethered methylenecyclopropanes has been developed, which can realize the rapid construction of tetracyclic benzazepine derivatives in moderate yields with good functional group compatibility under relatively mild conditions. In this transformation, the catalytic amount of ferrocene serves as the electrocatalytic medium, and electron transfer on electrodes can replace oxidants or reducing agents, which is more environmentally friendly than and economically comparable to traditional photocatalysis or metal catalysis. Moreover, the origin of the regiochemistry is well elucidated through density functional theory (DFT) calculations.

Electrochemical Synthesis of γ-Lactones from the Intermolecular Oxidative Coupling between Malonates and Styrenes

We report a ferrocene-mediated electrochemical intermolecular oxidative annulation between malonates and styrenes that avoids the use of excess oxidants such as Mn(OAc)3. The reaction proceeds via presumably the generation of a malonyl radical that adds to the styrene. After further anodic oxidation, the resulting benzylic carbocation is intercepted by one of the esters to deliver the desired γ-lactones.

Correspondence on "Organo-Mediator Enabled Electrochemical Deuteration of Styrenes"

The recently reported electrochemical, organo-mediator enabled deuteration of styrenes, a reaction referred to as "electrochemical deuterium atom transfer", differs mechanistically from reported direct electrochemical hydrogenations/deuterations only by a mediated, homogeneous SET to the substrates. By comparing direct vs. mediated processes in general and for styrene reduction, we display that Qiu's work does not change the concept of this chemistry. Experiments with mediators and the direct reduction of examples from the reported scope show that even electron-rich substrates can be reduced when our direct protocol, published six months before Qiu's work, is applied.

Strategies and Mechanisms of First-Row Transition Metal-Regulated Radical C-H Functionalization

Radical C-H functionalization represents a useful means of streamlining synthetic routes by avoiding substrate preactivation and allowing access to target molecules in fewer steps. The first-row transition metals (Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) are Earth-abundant and can be employed to regulate radical C-H functionalization. The use of such metals is desirable because of the diverse interaction modes between first-row transition metal complexes and radical species including radical addition to the metal center, radical addition to the ligand of metal complexes, radical substitution of the metal complexes, single-electron transfer between radicals and metal complexes, hydrogen atom transfer between radicals and metal complexes, and noncovalent interaction between the radicals and metal complexes. Such interactions could improve the reactivity, diversity, and selectivity of radical transformations to allow for more challenging radical C-H functionalization reactions. This review examines the achievements in this promising area over the past decade, with a focus on the state-of-the-art while also discussing existing limitations and the enormous potential of high-value radical C-H functionalization regulated by these metals. The aim is to provide the reader with a detailed account of the strategies and mechanisms associated with such functionalization.

Ferrocene-Mediated Electrochemical Polycyclization of Malonates

We report access to the core of biologically relevant aromatic abietane diterpenoids and to the formal synthesis of podocarpic and lambertic acids or γ-lactones via an electrochemical bicyclization process initiated by the ferrocene-mediated anodic oxidation of a malonate via single electron-transfer. This approach permits escaping the use of excess of oxidants such as Mn(OAc)3 and the associated complicated purification.

Electrocatalytic dehydrogenative and defluorinative coupling between aldehyde-derived N,N-dialkylhydrazones and fluoromalonates: synthesis of 2-pyrazolines

An electrocatalytic synthesis of 2-pyrazolines via dehydrogenative and defluorinative cross-coupling reactions between (hetero)arylaldehyde-derived N,N-dialkylhydrazones and fluoromalonates is disclosed. Salient features of this work include (i) readily available starting materials, (ii) practical reaction conditions, and (ii) a formal oxidative (4 + 1)-cycloaddition via triple C-H bond functionalization. Cyclic voltammetry analyses support the electrocatalytic formation of an α-fluoromalonyl radical.

Mechanistic Analysis Reveals Key Role of Interchalcogen Multicatalysis in Photo-Aerobic 3-Pyrroline Syntheses by Aza-Wacker Cyclizations

A light-driven dual and ternary catalytic aza-Wacker protocol for the construction of 3-pyrrolines by partially disulfide-assisted selenium-π-acid multicatalysis is reported. A structurally diverse array of sulfonamides possessing homopolar mono-, di- and trisubstituted olefinic double bonds is selectively converted to the corresponding 3-pyrrolines in up to 95 % isolated yield and with good functional group tolerance. Advanced electrochemical mechanistic investigations of the protocol suggest a dual role of the disulfide co-catalyst. On the one hand, the disulfide serves as an electron hole shuttle between the excited photoredox catalyst and the selenium co-catalyst. On the other hand, the sulfur species engages in the final, product releasing step of the catalytic cycle by accelerating the β-elimination of the selenium moiety, which was found in many cases to lead to considerably improved product yields.

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.

Electrocatalytic Access to Azetidines via Intramolecular Allylic Hydroamination: Scrutinizing Key Oxidation Steps through Electrochemical Kinetic Analysis

Azetidines are prominent structural scaffolds in bioactive molecules, medicinal chemistry, and ligand design for transition metals. However, state-of-the-art methods cannot be applied to intramolecular hydroamination of allylic amine derivatives despite their underlying potential as one of the most prevalent synthetic precursors to azetidines. Herein, we report an electrocatalytic method for intramolecular hydroamination of allylic sulfonamides to access azetidines for the first time. The merger of cobalt catalysis and electricity enables the regioselective generation of key carbocationic intermediates, which could directly undergo intramolecular C-N bond formation. The mechanistic investigations including electrochemical kinetic analysis suggest that either the catalyst regeneration by nucleophilic cyclization or the second electrochemical oxidation to access the carbocationic intermediate is involved in the rate-determining step (RDS) of our electrochemical protocol and highlight the ability of electrochemistry in providing ideal means to mediate catalyst oxidation.

Electrochemical monofluoroalkylation cyclization of N-arylacrylamides to construct monofluorinated 2-oxindoles

An electrochemical monofluoroalkylation cyclization of N-arylacrylamides to synthesize monofluorinated 2-oxindoles has been developed, which employs common dimethyl 2-fluoromalonate as a monofluoroalkyl radical precursor and obviates the use of prefunctionalized monofluoroalkylation reagents and sacrificial oxidants. A variety of monofluorinated nitrogen-containing heterocyclic compounds were efficiently obtained with satisfactory yields from readily available materials.

The recent advances in cobalt-catalyzed C(sp3)-H functionalization reactions

Over the past decades, reactions involving C-H functionalization have become a hot theme in organic transformations because they have a lot of potential for the streamlined synthesis of complex molecules. C(sp³)-H bonds are present in most organic species. Since organic molecules have massive significance in various aspects of life, the exploitation and functionalization of C(sp³)-H bonds hold enormous importance. In recent years, the first-row transition metal-catalyzed direct and selective functionalization of C-H bonds has emerged as a simple and environmentally friendly synthetic method due to its low cost, unique reactivity profiles and easy availability. Therefore, research advancements are being made to conceive catalytic systems that foster direct C(sp³)-H functionalization under benign reaction conditions. Cobalt-based catalysts offer mild and convenient reaction conditions at a reasonable expense compared to conventional 2^nd and 3^rd-row transition metal catalysts. Consequently, the probing of Co-based catalysts for C(sp³)-H functionalization is one of the hot topics from the outlook of an organic chemist. This review primarily focuses on the literature from 2018 to 2022 and sheds light on the substrate scope, selectivity, benefits and limitations of cobalt catalysts for organic transformations.

A Liquid-Liquid-Solid System to Manipulate the Cascade Reaction for Highly Selective Electrosynthesis of Aldehyde

Electrocatalytic synthesis of aldehydes from alcohols exhibits unique superiorities as a promising technology, in which cascade reactions are involved. However, the cascade reactions are severely limited by the low selectivity resulting from the peroxidation of aldehydes in a traditional liquid-solid system. Herein, we report a novel liquid-liquid-solid system to regulate the selectivity of benzyl alcohol electrooxidation. The selectivity of benzaldehyde increases 200-fold from 0.4 % to 80.4 % compared with the liquid-solid system at a high current density of 136 mA cm^-2 , which is the highest one up to date. In the tri-phase system, the benzaldehyde peroxidation is suppressed efficiently, with the conversion of benzaldehyde being decreased from 87.6 % to 3.8 %. The as-produced benzaldehyde can be in situ extracted to toluene phase and separated from the electrolyte to get purified benzaldehyde. This strategy provides an efficient way to efficiently enhance the selectivity of electrocatalytic cascade reactions.

Synthesis of amphiphilic chiral salen complexes and their conformational manipulation at the air-water interface

A series of amphiphilic salen complexes, [L1a,bM] and [L2a,bM], were designed and synthesized. These complexes consist of two or four hydrophilic triethylene glycol (TEG) chains and a hydrophobic π-extended metallosalen core based on naphthalene or phenanthrene. The obtained amphiphilic complexes, [L1bM] (M = Ni, Cu, Zn), formed a monolayer at the air-water interface, while the monocationic [L1bCo(MeNH₂)₂](OTf) did not form a well-defined monolayer. The number of hydrophilic TEG chains also had an influence on the monolayerformation behavior; the tetra-TEG derivatives, [L1bNi] and [L2bNi], showed a pressure rise at a less compressed region than the bis-TEG derivatives, [L1aNi] and [L2aNi]. In addition, the investigation of their compressibility and compression modulus suggested that the tetra-TEG derivatives, [L1bNi] and [L2bNi], are more flexible than the corresponding bis-TEG analogues, [L1aNi] and [L2aNi], and that the phenanthrene derivatives [L1a,bNi] were more rigid than the corresponding naphthalene analogues, [L2a,bNi]. The Langmuir-Blodgett (LB) films of one of the complexes, [L1bNi], showed CD spectra slightly different from that in solution, which may originate from the unique anisotropic environment of the air-water interface. Thus, we demonstrated the possibility of controlling the chiroptical properties of metal complexes by mechanical compression.

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.

Intercepting Hydrogen Evolution with Hydrogen-Atom Transfer: Electron-Initiated Hydrofunctionalization of Alkenes

Hydrogen-atom transfer mediated by earth-abundant transition-metal hydrides (M-Hs) has emerged as a powerful tool in organic synthesis. Current methods to generate M-Hs most frequently rely on oxidatively initiated hydride transfer. Herein, we report a reductive approach to generate Co-H, which allows for canonical hydrogen evolution reactions to be intercepted by hydrogen-atom transfer to an alkene. Electroanalytical and spectroscopic studies provided mechanistic insights into the formation and reactivity of Co-H, which enabled the development of two new alkene hydrofunctionalization reactions.

imethyl 2-Benzylmalonate

An electrochemical dearomatizing spirocyclization of alkynes with dimethyl 2-benzylmalonates for the preparation of spiro[4.5]deca-trienones has been developed. This approach adopts ferrocene (Cp₂Fe) as an electrocatalyst to produce carbon-centered radical intermediates from C-H-based malonates, which obviates the forthputting of noble-metal reagents, sacrificial chemical oxidants and 2-bromomalonates. A wide variety of spiro compounds are efficiently prepared with satisfactory results under mild conditions.

Electrochemical 5-exo-dig aza-cyclization of 2-alkynylbenzamides toward 3-hydroxyisoindolinone derivatives

Preparation of biologically relevant 3-hydroxyisoindolinones from readily available 2-alkynylbenzamides is an appealing synthetic approach. However, such kinds of compounds preferably undergo O-attacked 5-exo-dig/6-endo-dig cyclizations. Herein, we report an electrochemically generated amidyl radical proceeding via a highly selective N-attacked 5-exo-dig radical cyclization to form 3-hydroxyisoindolinone derivatives. This reaction features simple operation, good selectivity, and broad substrate scope. Moreover, gram-scale preparation and synthetic elaborations imply the potential applicability of this protocol for the synthesis of diverse isoindolinone derivatives.

Facilitating Rh-Catalyzed C-H Alkylation of (Hetero)arenes and 6-Arylpurine Nucleosides (Nucleotides) with Electrochemistry

An electrochemical approach to promote the ortho-C-H alkylation of (hetero)arenes via rhodium catalysis under mild conditions is described. This approach features mild conditions with high levels of regio- and monoselectivity that tolerate a variety of aromatic and heteroaromatic groups and offers a widely applicable method for late-stage diversification of complex molecular architectures including tryptophan, estrone, diazepam, nucleosides, and nucleotides. Alkyl boronic acids and esters and alkyl trifluoroborates are demonstrated as suitable coupling partners. The isolation of key rhodium intermediates and mechanistic studies provided strong support for a rhodium(III/IV or V) regime.

Electrocatalytic Allylic C-H Alkylation Enabled by a Dual-Function Cobalt Catalyst

The direct functionalization of allylic C-H bonds with nucleophiles minimizes pre-functionalization and converts inexpensive, abundantly available materials to value-added alkenyl-substituted products but remains challenging. Here we report an electrocatalytic allylic C-H alkylation reaction with carbon nucleophiles employing an easily available cobalt-salen complex as the molecular catalyst. These C(sp³ )-H/C(sp³ )-H cross-coupling reactions proceed through H₂ evolution and require no external chemical oxidants. Importantly, the mild conditions and unique electrocatalytic radical process ensure excellent functional group tolerance and substrate compatibility with both linear and branched terminal alkenes. The synthetic utility of the electrochemical method is highlighted by its scalability (up to 200 mmol scale) under low loading of electrolyte (down to 0.05 equiv) and its successful application in the late-stage functionalization of complex structures.

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.

Efficient one-step synthesis of 3-(indol-2-yl)quinoxalin-2(1H)-ones via electrochemical oxidative cross-dehydrogenative coupling

A novel and efficient synthetic strategy for 3-(indol-2-yl)quinoxalin-2(1H)-ones was developed via electrochemical oxidative cross-dehydrogenative coupling of indoles and quinoxalin-2(1H)-ones.

Electrochemical Intermolecular Monofluoroalkylation of α,β-Unsaturated Carboxylic Acids and Heteroaromatics with 2-Fluoromalonate Esters

An electrochemical approach for the preparation of monofluorides from α,β-unsaturated carboxylic acids and heteroaromatics with readily available 2-fluoromalonate esters as monofluoroalkyl radical precursors has been developed. The electrosynthesis employs ferrocene (Cp₂Fe) as a catalyst in a simple undivided cell with a broad substrate scope, which obviates the need for sacrificial oxidizing reagents.

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.