Synthesis of C3-Fluorinated Oxindoles through Reagent-Free Cross-Dehydrogenative Coupling

Current-Controlled Electrochemical Approach Toward Mono- and Trifluorinated Isoindolin-1-one Derivatives

An electrochemical intramolecular 5-exo-dig aza-cyclization of 2-alkynylbenzamides and subsequent nucleophilic fluorination have been developed to afford the highly selective synthesis of mono- and trifluorinated isoindolin-1-one derivatives. This work demonstrates the unique capability of synthetic electrochemistry in controlling reaction selectivity through the applied electrolytic parameters. In addition, the obtained monofluorinated 3-methyleneisoindolin-1-one (19) displays interesting photophysical properties that are not observed in its nonfluorinated analog.

Highly efficient synthesis of C3-heteroaryl 3-fluorooxindoles via a one-pot stepwise Ce(iii)/photoassisted cross-dehydrogenative coupling/fluorooxidation process

A one-pot stepwise strategy has been developed to afford C3-heteroaryl 3-fluorooxindoles via a Ce(iii)/photoassisted cross-dehydrogenative coupling/fluorooxidation process in moderate-to-good yields with excellent functional group compatibility.

Electrochemical Migratory Cyclization of N-Acylsulfonamides

Benzoxathiazine dioxide, as a bioisostere of the clinically widely used diazoxide, exhibits interesting biological activity. However, limited success has been achieved in terms of its concise and direct synthesis. We report herein a facile electrochemical migratory cyclization of N-acylsulfonamides to access a diverse array of benzoxathiazine dioxides. The inclusion of electrochemistry is crucial for realizing such a novel transformation, which is substantiated both by the experiments and density-functional-theory calculations.

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.

Mediated Inner-Sphere Electron Transfer Induces Homogeneous Reduction of CO2 via Through-Space Electronic Conjugation

The electrocatalytic reduction of CO₂ is an appealing method for converting renewable energy sources into value-added chemical feedstocks. We report a co-electrocatalytic system for the reduction of CO₂ to CO comprised of a molecular Cr complex and dibenzothiophene-5,5-dioxide (DBTD) as a redox mediator, which achieves high activity (TOF=1.51-2.84×10⁵  s^-1 ) and quantitative selectivity. Under aprotic or protic conditions, DBTD produces a co-electrocatalytic response with 1 by coordinating trans to the site of CO₂ binding and mediating electron transfer from the electrode with quantitative efficiency for CO. This assembly is reliant on through-space electronic conjugation between the π frameworks of DBTD and the bpy fragment of the catalyst ligand, with contributions from dispersive interactions and weak sulfone coordination.

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 Generation and Use in Organic Synthesis of C-, O-, and N-Centered Radicals

During the last decade several research groups have been developing electrochemical procedures to access highly functionalized organic molecules. Among the most exciting advances, the possibility of using free radical chemistry has attracted the attention of the most important synthetic groups. Nowadays, electrochemical strategies based on these species with a synthetic purpose are published continuously in scientific journals, increasing the alternatives for the synthetic organic chemistry laboratories. Free radicals can be obtained in organic electrochemical reactions; thus, this review reassembles the last decade's (2010-2020) efforts of the electrosynthetic community to generate and take advantage of the C-, O-, and N-centered radicals' reactivity. The electrochemical reactions that occur, as well as the proposed mechanism, are discussed, trying to give clear information about the used conditions and reactivity of these reactive intermediate species.

Electrochemical Dehydrogenative C(sp2 )-H Amination

A transition-metal-free direct electrolytic C-H amination involving an electrochemically generated nitrenium ion intermediate has been developed. The electrosynthesis takes place in the absence of any organoiodine catalysts and is enabled by an in situ generated electrolyte. A novel, efficient intramolecular and intermolecular C-H amination has been demonstrated using a simple reaction setup.

Electrochemically Driven Radical Reactions: From Direct Electrolysis to Molecular Catalysis

Organic radicals are versatile synthetic intermediates that provide reactivities and selectivities complementary to ionic species. Despite its long history, electrochemically driven radical reactions remain limited in scope. In the past few years, there have been dramatic increase in research activity in organic electrochemistry. We have been developing electrochemical and electrophotocatalytic methods for the generation and synthetic utilization of organic radicals. In our studies, various radical species such as alkene and arene radical cations and carbon- and heteroatom-centered radicals are generated from readily available precursors through direct electrolysis, molecular electrocatalysis or molecular electrophotocatalysis. These radical species undergo various inter- and intramolecular oxidative transformations to rapidly increase molecular complexity. The simultaneous occurrence of anodic oxidation and cathodic proton reduction allows the oxidative reactions to proceed through H₂ evolution without external chemical oxidants.

Electrophotocatalytic Undirected C-H Trifluoromethylations of (Het)Arenes

Electrophotochemistry has enabled arene C-H trifluoromethylation with the Langlois reagent CF3 SO2 Na under mild reaction conditions. The merger of electrosynthesis and photoredox catalysis provided a chemical oxidant-free approach for the generation of the CF3 radical. The electrophotochemistry was carried out in an operationally simple manner, setting the stage for challenging C-H trifluoromethylations of unactivated arenes and heteroarenes. The robust nature of the electrophotochemical manifold was reflected by a wide scope, including electron-rich and electron-deficient benzenes, as well as naturally occurring heteroarenes. Electrophotochemical C-H trifluoromethylation was further achieved in flow with a modular electro-flow-cell equipped with an in-operando monitoring unit for on-line flow-NMR spectroscopy, providing support for the single electron transfer processes.

Electrochemical Cross-Dehydrogenative Coupling between Phenols and β-Dicarbonyl Compounds: Facile Construction of Benzofurans

Preparative electrochemical synthesis is an ideal method for establishing green, sustainable processes. The major benefits of an electro-organic strategy over that of conventional chemical synthesis are the avoidance of reagent waste and mild reaction conditions. Here, an intermolecular cross-dehydrogenative coupling between phenols and β-dicarbonyl compounds has been developed to build various benzofurans under undivided electrolytic conditions. Neither transition metals nor external chemical oxidants are required to facilitate the dehydrogenation and dehydration processes. The key factor in success was the use of nBu₄ NBF₄ as the electrolyte and hexafluoroisopropanol as the solvent, which play key roles in the cyclocondensation step. This electrolysis is scalable and can be used as a key step in drug synthesis. On the basis of several experimental results, the mechanism, particularly of the remarkable anodic oxidation and cyclization process, was illustrated.

The Fascinating Chemistry of α-Haloamides

The aim of this review is to highlight the rich chemistry of α-haloamides originally mainly used to discover new C-N, C-O and C-S bond forming reactions, and later widely employed in C-C cross-coupling reactions with C(sp3), C(sp2) and C(sp) coupling partners. Radical-mediated transformations of α-haloamides bearing a suitable located unsaturated bond has proven to be a straightforward alternative to access diverse cyclic compounds by means of either radical initiators, transition metal redox catalysis or visible light photoredox catalysis. On the other hand, cycloadditions with α-halohydroxamate-based azaoxyallyl cations have garnered significant attention. Moreover, in view of the important role in life and materials science of difluoroalkylated compounds, a wide range of catalysts has been developed for the efficient incorporation of difluoroacetamido moieties into activated as well as unactivated substrates.

Hai-Chao Xu

"My worst nightmare is forgetting to prepare for a class. The most exciting thing about my research is discovering new reactions …" Find out more about Hai-Chao Xu in his Author Profile.

Suspending Ion Electrocatalysts in Charged Metal-Organic Frameworks to Improve the Conductivity and Selectivity in Electroorganic Synthesis

Electroorganic synthesis is an environmentally friendly alternative to traditional synthetic methods; however, the application of this strategy is heavily hindered by low product selectivity. Metal-organic frameworks (MOFs) exhibit high selectivity in numerous catalytic reactions; however, poor conductivity heavily limits the application of MOFs in electroorganic synthesis. To realize the electrocatalytic application of MOFs in selective electroorganic synthesis, a practically applicable strategy by suspending ion electrocatalysts in charged MOFs is herein reported. This approach could markedly improve the product selectivity in electroorganic synthesis. In the electrocatalytic oxidative self-coupling of benzylamine experiments, the imine product selectivity is markedly improved from 61.3 to 94.9 %, when the MOF-based electrocatalyst is used instead of the corresponding homogeneous electrocatalyst under the identical conditions. Therefore, this work opens a new route to improve the product selectivity in electroorganic synthesis.

Electrochemical Difluoromethylation of Electron-Deficient Alkenes

Electrochemical 1,2-hydroxydifluoromethylation and C-H difluoromethylation of acrylamides were developed by using CF₂ HSO₂ NHNHBoc as the source of the CF₂ H group. These electricity-powered oxidative alkene functionalization reactions do not need transition-metal catalysts or chemical oxidants. The reaction outcome, 1,2-difuntionalization or C-H functionalization, is determined by the substituents on the amide nitrogen atom of the acrylamides instead of by the reaction conditions.

Shielding Effect of Micelle for Highly Effective and Selective Monofluorination of Indoles in Water

Highly selective direct monofluorination of indoles and arenes was developed through an approach that allows site-specific solubility of substrate and fluorine source in the micelle. This approach was highly selective for a broad range of substrates with excellent functional group tolerance. Differences in binding constant and solubility of indoles and arenes in the micelle allowed the fine-tuning of selectivity. Control experiments suggested a radical pathway and provided insight into the role of micelles of the environmentally benign amphiphile PS-750-M. Dynamic light scattering experiments strongly indicated the site-specific solubility of the substrate and fluorine source. The methodology was successfully adapted to gram scale, and the E-factor established from a recycle study indicated that the process is environmentally responsible and sustainable.

Catalyst- and Reagent-Free Electrochemical Azole C-H Amination

Catalyst- and chemical oxidant-free electrochemical azole C-H aminations were accomplished via cross-dehydrogenative C-H/N-H functionalization. The catalyst-free electrochemical C-H amination proved feasible on azoles with high levels of efficacy and selectivity, avoiding the use of stoichiometric oxidants under ambient conditions. Likewise, the C(sp³ )-H nitrogenation proved viable under otherwise identical conditions. The dehydrogenative C-H amination featured ample scope, including cyclic and acyclic aliphatic amines as well as anilines, and employed sustainable electricity as the sole oxidant.

Electrifying Organic Synthesis

The direct synthetic organic use of electricity is currently experiencing a renaissance. More synthetically oriented laboratories working in this area are exploiting both novel and more traditional concepts, paving the way to broader applications of this niche technology. As only electrons serve as reagents, the generation of reagent waste is efficiently avoided. Moreover, stoichiometric reagents can be regenerated and allow a transformation to be conducted in an electrocatalytic fashion. However, the application of electroorganic transformations is more than minimizing the waste footprint, it rather gives rise to inherently safe processes, reduces the number of steps of many syntheses, allows for milder reaction conditions, provides alternative means to access desired structural entities, and creates intellectual property (IP) space. When the electricity originates from renewable resources, this surplus might be directly employed as a terminal oxidizing or reducing agent, providing an ultra-sustainable and therefore highly attractive technique. This Review surveys recent developments in electrochemical synthesis that will influence the future of this area.

Use of Electrochemistry in the Synthesis of Heterocyclic Structures

The preparation and transformation of heterocyclic structures have always been of great interest in organic chemistry. Electrochemical technique provides a versatile and powerful approach to the assembly of various heterocyclic structures. In this review, we examine the advance in relation to the electrochemical construction of heterocyclic compounds published since 2000 via intra- and intermolecular cyclization reactions.