Synthetic Organic Electrochemistry: Calling All Engineers

Paired Electrosynthesis at Interdigitated Microband Electrodes: Exploring Diffusion and Reaction Zones in the Absence of a Supporting Electrolyte

Electrosynthesis traditionally requires dedicated reactor systems and an added electrolyte, although some paired electrosynthesis processes are possible at interdigitated microband electrodes simply immersed in solution and without an intentionally added electrolyte. Here, 1,1'-ferrocenedimethanol oxidation and activated olefin electro-hydrogenation reactions are investigated as model processes at a Pt-Pt interdigitated microband array electrode with 5 μm width and with 5 μm interelectrode gap. Voltammetric responses for electro-hydrogenation are discussed, and product yields are determined in methanol (MeOH) in the presence/absence of an added electrolyte (LiClO4). An isotope effect is observed in CH3OD solvent, leading to olefin monodeuteration linked to a fast EC-type process close to the cathode surface (in the cathode reaction zone) rather than to charge annihilation in the interelectrode zone. A finite element simulation is employed to visualize/discuss reaction zones and to contrast the rate of charge annihilation processes with/without a supporting electrolyte.

Electro/Ni Dual-Catalyzed Decarboxylative C(sp3)-C(sp2) Cross-Coupling Reactions of Carboxylates and Aryl Bromide

Paired redox-neutral electrolysis offers an attractive green platform for organic synthesis by avoiding sacrificial oxidants and reductants. Carboxylates are non-toxic, stable, inexpensive, and widely available, making them ideal nucleophiles for C-C cross-coupling reactions. Here, we report the electro/Ni dual-catalyzed redox-neutral decarboxylative C(sp3)-C(sp2) cross-coupling reactions of pristine carboxylates with aryl bromides. At a cathode, a NiII(Ar)(Br) intermediate is formed through the activation of Ar-Br bond by a NiI-bipyridine catalyst and subsequent reduction. At an anode, the carboxylates, including amino acid, benzyl carboxylic acid, and 2-phenoxy propionic acid, undergo oxidative decarboxylation to form carbon-based free radicals. The combination of NiII(Ar)(Br) intermediate and carbon radical results in the formation of C(sp3)-C(sp2) cross-coupling products. The adaptation of this electrosynthesis method to flow synthesis and valuable molecule synthesis was demonstrated. The reaction mechanism was systematically studied through electrochemical voltammetry and density functional theory (DFT) computational studies. The relationships between the electrochemical properties of carboxylates and the reaction selectivity were revealed. The electro/Ni dual-catalyzed cross-coupling reactions described herein expand the chemical space of paired electrochemical C(sp3)-C(sp2) cross-coupling and represent a promising method for the construction of the C(sp3)-C(sp2) bonds because of the ubiquitous carboxylate nucleophiles and the innate scalability and flexibility of electrochemical flow-synthesis technology.

Metal free cross-dehydrogenative N-N coupling of primary amides with Lewis basic amines

Hydrazides, N-N containing structural motifs, are important due to their presence in a wide variety of biologically significant compounds. While the homo N-N coupling of two NH moieties to form the hydrazide N-N bond is well developed, the cross-dehydrogenative hetero N-N coupling remains very unevolved. Here we present an efficient intermolecular N-N cross-coupling of a series of primary benzamides with broad range of Lewis basic primary and secondary amines using PhI(OAc)2 as both a terminal oxidant and a cross-coupling mediator, without the need for metal catalysts, high temperatures, and inert atmospheres, and with substantial potential for use in the late-stage functionalization of drugs.

How to rationally design homogeneous catalysts for efficient CO2 electroreduction?

Electrified converting CO2 into valuable fuels and chemicals using a homogeneous electrochemical CO2 reduction (CO2ER) approach simplifies the operation, providing a potential option for decoupling energy harvesting and renewable chemical production. These merits benefit the scenarios where decentralization and intermittent power are key factors. This perspective aims to provide an overview of recent progress in homogeneous CO2ER. We introduce firstly the fundamentals chemistry of the homogeneous CO2ER, followed by a summary of the crucial factors and the important criteria broadly employed for evaluating the performance. We then highlight the recent advances in the most widely explored transition-metal coordinate complexes for the C1 and multicarbon (C2+) products from homogeneous CO2ER. Finally, we summarize the remaining challenges and opportunities for developing homogeneous electrocatalysts for efficient CO2ER. This perspective is expected to favor the rational design of efficient homogeneous electrocatalysts for selective CO2ER toward renewable fuels and feedstocks.

Synthesis of Partially Fluorinated Alkyl Triflates by Electrochemical Fluorination (Simons Process)

A scalable straightforward synthesis of monofluoro- and difluoromethyl triflate CF3 SO2 OCH2 F (MH2F ) and CF3 SO2 OCHF2 (MHF2 ) through electrochemical fluorination (ECF, Simons process) of methyl triflate MH3 in anhydrous hydrogen fluoride at nickel anodes is presented. The ECF method is also feasible for the preparation of the deuterated analogues CF3 SO2 OCD2 F (MD2F ) and CF3 SO2 OCDF2 (MD2F ). Surprisingly, no H/D exchange occurs during ECF of CF3 SO2 OCD3 (MD3 ); this provides further evidence for a NiF3 /NiF4 -mediated ECF mechanism. The ECF of selected partially fluorinated ethyl triflates is described, and electrochemical fluorination of CF3 SO2 OCH2 CF3 (EH2F3 ) leads to the until now unknown chiral CF3 SO2 OCHFCF3 (EHFF3 ). The analogous fluoromethyl and fluoroethyl nonaflates are also accessible by ECF. This study contains detailed spectroscopic, structural, and thermal data on (fluoro)methyl and fluoro(ethyl) triflates.

A breath of sunshine: oxygenic photosynthesis by functional molecular architectures

The conversion of light into chemical energy is the game-changer enabling technology for the energetic transition to renewable and clean solar fuels. The photochemistry of interest includes the overall reductive/oxidative splitting of water into hydrogen and oxygen and alternatives based on the reductive conversion of carbon dioxide or nitrogen, as primary sources of energy-rich products. Devices capable of performing such transformations are based on the integration of three sequential core functions: light absorption, photo-induced charge separation, and the photo-activated breaking/making of molecular bonds via specific catalytic routes. The key to success does not rely simply on the individual components' performance, but on their optimized integration in terms of type, number, geometry, spacing, and linkers dictating the photosynthetic architecture. Natural photosynthesis has evolved along this concept, by integrating each functional component in one specialized "body" (from the Greek word "soma") to enable the conversion of light quanta with high efficiency. Therefore, the natural "quantasome" represents the key paradigm to inspire man-made constructs for artificial photosynthesis. The case study presented in this perspective article deals with the design of artificial photosynthetic systems for water oxidation and oxygen production, engineered as molecular architectures then rendered on electrodic surfaces. Water oxidation to oxygen is indeed the pervasive oxidative reaction used by photosynthetic organisms, as the source of reducing equivalents (electrons and protons) to be delivered for the processing of high-energy products. Considering the vast and abundant supply of water (including seawater) as a renewable source on our planet, this is also a very appealing option for photosynthetic energy devices. We will showcase the progress in the last 15 years (2009-2023) in the strategies for integrating functional building blocks as molecular photosensitizers, multi-redox water oxidation catalysts and semiconductor materials, highlighting how additional components such as redox mediators, hydrophilic/hydrophobic pendants, and protective layers can impact on the overall photosynthetic performance. Emerging directions consider the modular tuning of the multi-component device, in order to target a diversity of photocatalytic oxidations, expanding the scope of the primary electron and proton sources while enhancing the added-value of the oxidation product beyond oxygen: the selective photooxidation of organics combines the green chemistry vision with renewable energy schemes and is expected to explode in coming years.

Sustainable Photo- and Electrochemical Transformation of White Phosphorous (P4 ) into P1 Organo-Compounds

Elemental white phosphorous (P4 ) is a crucial feedstock for the entire phosphorus-derived chemical industry, serving as a common precursor for the ultimate preparation of high-grade monophosphorus (P1 ) fine chemicals. However, the corresponding manufacturing processes generally suffer from a deep reliance on hazardous reagents, inputs of immense energy, emissions of toxic pollutants, and the generation of substantial waste, which have negative impacts on the environment. In this context, sustainability and safety concerns provide a consistent impetus for the urgent overall improvement of phosphorus cycles. In this Concept, we present an overview of the most recent growth in photo- and electrochemical synthesis of P1 organo-compounds from P4 , with special emphasis on sustainable features. The key aspects of innovations regarding activation mode and mechanism have been comprehensively analyzed. A preliminary look at the possible future direction of development is also provided.

Electrosynthesis and Microanalysis in Thin Layer: An Electrochemical Pipette for Rapid Electrolysis and Mechanistic Study of Electrochemical Reactions

Electrochemistry represents unique approaches for the promotion and mechanistic study of chemical reactions and has garnered increasing attention in different areas of chemistry. This expansion necessitates the enhancement of the traditional electrochemical cells that are intrinsically constrained by mass transport limitations. Herein, we present an approach for designing an electrochemical cell by limiting the reaction chamber to a thin layer of solution, comparable to the thickness of the diffusion layer. This thin layer electrode (TLE) provides a modular platform to bypass the constraints of traditional electrolysis cells and perform electrolysis reactions in the timescale of electroanalytical techniques. The utility of the TLE for electrosynthetic applications benchmarked using NHPI-mediated electrochemical C-H functionalization. The application of microscale electrolysis for the study of drug metabolites was showcased by elucidating the oxidation pathways of the paracetamol drug. Moreover, hosting a microelectrode in the TLE, was shown to enable real-time probing of the profiles of redox-active components of these rapid electrosynthesis reactions.

Electrochemical carbonyl reduction on single-site M-N-C catalysts

Electrochemical conversion of organic compounds holds promise for advancing sustainable synthesis and catalysis. This study explored electrochemical carbonyl hydrogenation on single-site M-N-C (Metal Nitrogen-doped Carbon) catalysts using formaldehyde, acetaldehyde, and acetone as model reactants. We strive to correlate and understand the selectivity dependence on the nature of the metal centers. Density Functional Theory calculations revealed similar binding energetics for carbonyl groups through oxygen-down or carbon-down adsorption due to oxygen and carbon scaling. Fe-N-C exhibited specific oxyphilicity and could selectively reduce aldehydes to hydrocarbons. By contrast, the carbophilic Co-N-C selectively converted acetaldehyde and acetone to ethanol and 2-propanol, respectively. We claim that the oxyphilicity of the active sites and consequent adsorption geometry (oxygen-down vs. carbon-down) are crucial in controlling product selectivity. These findings offer mechanistic insights into electrochemical carbonyl hydrogenation and can guide the development of efficient and sustainable electrocatalytic valorization of biomass-derived compounds.

Electrochemical Gold-Catalyzed 1,2-Difunctionalization of C-C Multiple Bonds

Herein, we disclose the first report of 1,2-difunctionalization of C-C multiple bonds using electrochemical gold redox catalysis. By adopting the electrochemical strategy, the inherent π-activation and cross-coupling reactivity of gold catalysis are harnessed to develop the oxy-alkynylation of allenoates under external-oxidant-free conditions. Detailed mechanistic investigations such as 31 P NMR, control experiments, mass studies, and cyclic voltammetric (CV) analysis have been performed to support the proposed reaction mechanism.

Acetamido-TEMPO mediated electrochemical oxidation of alcohols to aldehydes and ketones

A protocol for the oxidation of alcohols to aldehydes and ketones employing an electrochemical aminoxyl-mediated reaction is presented. The approach employs a catalytic amount of the radical and the use of a base is not required. It is performed using readily available electrodes in a commercially available electrochemistry apparatus and does not require a reference electrode. The methodology is applicable to a range of structurally and electronically diverse substrates, including the oxidation of primary alcohols to aldehydes rather than the more commonly formed carboxylic acids.

Unveiling catalyst-free electro-photochemical reactivity of aryl diazoesters and facile synthesis of oxazoles, imide-fused pyrroles and tetrahydro-epoxy-pyridines via carbene radical anions

Herein, we report a reagent-less (devoid of catalyst, supporting electrolyte, oxidant and reductant) electro-photochemical (EPC) reaction [electricity (50 μA) and blue LED (5 W)] of aryl diazoesters to generate radical anions which are subsequently reacted with acetonitrile or propionitrile and maleimides to generate diversely substituted oxazoles, diastereo-selective imide-fused pyrroles and tetrahydroepoxy-pyridines in good to excellent yield. Thorough mechanistic investigation including a 'biphasic e-cell' experiment supports the reaction mechanism involving a carbene radical anion. The tetrahydroepoxy-pyridines could be fluently converted to fused pyridines resembling vitamin B₆ derivatives. The source of the electric current in the EPC reaction could be a simple cell phone charger. The reaction was efficiently scaled up to the gram level. Crystal structure, 1D, 2D NMRs and HRMS data confirmed the product structures. This report demonstrates a unique generation of radical anions via electro-photochemistry and their direct applications in the synthesis of important heterocycles.

Electrochemically Enabled Total Syntheses of Natural Products

Electrochemical techniques have helped to enable the total synthesis of natural products since the pioneering work of Kolbe in the mid 1800's. The electrochemical toolset grows every day and these new possibilities change the way chemists look at and think about natural products. This review provides a perspective on total syntheses wherein electrochemical techniques enabled the carbon─carbon bond formations in the skeletal assembly of important natural products, discussion of mechanistic details, and representative examples of the bond formations enabled over the last several decades. These bond formations are often distinctly different from those possible with conventional chemistries and allow assemblies complementary to other techniques.

Fine-Tuning Electroauxiliary-Mediated Peptide Modifications Using Second-Generation Electroactive Amino Acids

Arylthioether functional groups serve as effective electroauxiliaries for tunable oxidations. Herein, we disclose the synthesis of second-generation glutamine building blocks bearing 2,4-dimethoxythiophenyl and 2,4-dichlorothiophenyl-derived electroauxiliaries. These building blocks improve SPPS efficiency and enable fine-tuning of the electrochemical window for selective anodic oxidation reactions in comparison to first-generation 4-methoxythiophenyl- and 4-nitrothiophenyl-substituted variants. Installation onto a segment of involucrin, a protein component of human skin, emphasizes the practical application of the new building blocks for iterative functionalizations.

Electrochemical generation and utilization of alkoxy radicals

This highlight summarises electrochemical approaches for the generation and utilization of alkoxy radicals, predominantly focusing on recent advances (2012-present). The application of electrochemically generated alkoxy radicals in a diverse range of transformations is described, including discussion on reaction mechanisms, scope and limitations, in addition to highlighting future challenges in this burgeoning area of sustainable synthesis.

Electro-Triggered Cascade Cyclization to Access Phosphinyl-Substituted N-Containing Heterocycles

An electro-triggered cascade cyclization strategy was disclosed with concomitant phosphinylation and N-heterocycle construction. It provides a novel and environmentally friendly approach to access phosphinyl-substituted N-heterocycles with no external metal catalyst, oxidant, or heating. Mechanistic studies have revealed that anodic oxidation of H-phosphorus compounds occurs first to generate the key P-centered radical directly and cathodic reduction leads to the concurrent release of molecular hydrogen or methane. This protocol features simple operation, broad substrate scope, clean and mild conditions, and atom and step economy to form heterocycle-containing organophosphorus scaffolds.

Electrochemical Cycloaddition Reactions of Alkene Radical Cations: A Route toward Cyclopropanes and Cyclobutanes

Herein, we describe a mild and efficient electrochemical method for cycloaddition reactions of alkene radical cations. Anodic oxidation of olefins produces electrophilic alkene radical cations, which further react with either diazo compounds in a [2 + 1] cycloaddition toward cyclopropane synthesis, or styrene derivatives in a [2 + 2] cycloaddition producing cyclobutanes. Both processes are green, metal- and catalyst-free, and scalable and tolerate a broad range of electron-rich olefins.

Electrochemically Enabled Direct C3-Formylation of Imidazopyridines with Me3 N as a Carbonyl Source

A metal-free and oxidant-free electrochemically enabled strategy for C-3 formylation of imidazopyridines using trimethylamine as a one-carbon source has been established. This conversion has high functional group compatibility under mild conditions and ensures late-stage functionalization of pharmaceutical molecules. Furthermore, unexpected hexafluoroisopropoxylation products have been observed in some cases. Mechanistic studies using cyclic voltammetry and control experiments reveal the key intermediate of the formylation process.

Nonaqueous Li-Mediated Nitrogen Reduction: Taking Control of Potentials

The performance of the Li-mediated ammonia synthesis has progressed dramatically since its recent reintroduction. However, fundamental understanding of this reaction is slower paced, due to the many uncontrolled variables influencing it. To address this, we developed a true nonaqueous LiFePO₄ reference electrode, providing both a redox anchor from which to measure potentials against and estimates of sources of energy efficiency loss. We demonstrate its stable electrochemical potential in operation using different N₂- and H₂-saturated electrolytes. Using this reference, we uncover the relation between partial current density and potentials. While the counter electrode potential increases linearly with current, the working electrode remains stable at lithium plating, suggesting it to be the only electrochemical step involved in this process. We also use the LiFePO₄/Li⁺ equilibrium as a tool to probe Li-ion activity changes in situ. We hope to drive the field toward more defined systems to allow a holistic understanding of this reaction.

Stirring-Free Scalable Electrosynthesis Enabled by Alternating Current

Alternating current (AC) electrolysis is receiving increased interest as a versatile tool for mild and selective electrochemical transformations. This work demonstrates that AC can enable the concept of a stirring-free electrochemical reactor where the periodic switch of electrode polarity, inherent to AC, provides uniform electrolysis across the whole volume of the reactor. Such design implies a straightforward approach for scaling up electrosynthesis. This was demonstrated on the range of electrochemical transformations performed in three different RVC-packed reactors on up to a 50-mmol scale. Redox-neutral, oxidative, and reductive processes were successfully implemented using the suggested design and the applicable frequency ranges were further investigated for different types of reactions. The advantages of the AC-enabled design - such as the absence of stirring and a maximized surface area of the electrodes - provide the possibility for its universal application both for small-scale screening experimentation and large-scale preparative electrosynthesis without significant optimization needed in between.

Two-Step Synthesis, Structure, and Optical Features of a Double Hetero[7]helicene

A novel double aza-oxa[7]helicene was synthesized from the commercially available N¹,N⁴-di(naphthalen-2-yl)benzene-1,4-diamine and p-benzoquinone in two steps. Combining the acid-mediated annulation with the electrochemical sequential reaction (oxidative coupling and dehydrative cyclization) afforded this double hetero[7]helicene. Moreover, the structural and optical features of this molecule have been studied using X-ray crystallographic analysis, and the absorption and emission behaviors were rationalized based on DFT calculations.

Divergent C-H Amidations and Imidations by Tuning Electrochemical Reaction Potentials

Electrochemical C-H functionalizations are attractive transformations, as they are capable of avoiding the use of transition metals, pre-oxidized precursors, or suprastoichiometric amounts of terminal oxidants. Herein an electrochemically tunable method was developed that enabled the divergent formation of cyclic amines or imines by applying different reaction potentials. Detailed cyclic voltammetry analyses, coupled with chronopotentiometry experiments, were carried out to provide insight into the mechanism, while atom economy was assessed through a paired electrolysis. Selective C-H amidations and imidations were achieved to afford five- to seven-membered sulfonamide motifs that could be employed for late-stage modifications.

Hetero-Coupling of Bio-Based Medium-Chain Carboxylic Acids by Kolbe Electrolysis Enables High Fuel Yield and Efficiency

Mixtures of n-carboxylic acids (n-CA) as derived from microbial conversion of waste biomass were converted to bio-fuel using Kolbe electrolysis. While providing full carbon and electron balances, key parameters like electrolysis time, chain length of n-CA, and pH were investigated for their influence on reaction efficiency. Electrolysis of n-hexanoic acid showed the highest coulombic efficiency (CE) of 58.9±16.4 % (n=4) for liquid fuel production among individually tested n-CA. Duration of the electrolysis was varied within a range of 0.27 to 1.02 faraday equivalents without loss of efficiency. Noteworthy, CE increased to around 70 % by hetero-coupling when electrolysing n-CA mixtures regardless of the applied pH. Thus, 1 L of fuel could be produced from 12.4 mol of n-CA mixture using 5.02 kWh (<1 € L^-1 ). Thus, a coupling with microbial processes producing n-CA mixtures from different organic substrates and waste is more than promising.

Scalable Microreactor Concept for the Continuous Kolbe Electrolysis of Carboxylic Acids Using Aqueous Electrolyte

The Kolbe electrolysis is a promising reaction to combine the usage of electrons as reagents and the application of renewable generated carboxylic acids as raw materials producing value added chemicals. Within this study, the electrolysis was conducted in a specially developed concept electrochemical microreactor and draws the particular attention to continuous operation and reuse of the aqueous electrolyte as well as of the dissolved unreacted feedstock. The electrolysis was conducted in alkaline aqueous solution using n‐octanoic acid as model substance. Platinized titanium as anode material in an undivided cell setup was shown to give Kolbe selectivity above 90 %. During the technically relevant conditions of current densities up to 0.6 A cm⁻² and overall electrolysis times of up to 3 h, a high electrode stability was observed. Finally, a proof‐of‐concept continuous operation and the numbering up potential of the ECMR could be demonstrated.

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.

Atom Transfer Radical Polymerization: A Mechanistic Perspective

Since its inception, atom transfer radical polymerization (ATRP) has seen continuous evolution in terms of the design of the catalyst and reaction conditions; today, it is one of the most useful techniques to prepare well-defined polymers as well as one of the most notable examples of catalysis in polymer chemistry. This Perspective highlights fundamental advances in the design of ATRP reactions and catalysts, focusing on the crucial role that mechanistic studies play in understanding, rationalizing, and predicting polymerization outcomes. A critical summary of traditional ATRP systems is provided first; we then focus on the most recent developments to improve catalyst selectivity, control polymerizations via external stimuli, and employ new photochemical or dual catalytic systems with an outlook to future research directions and open challenges.

Electrochemical Benzylic C(sp3)-H Acyloxylation

The development of sustainable C(sp³)-H functionalization methods is of great interest to the pharmaceutical and agrochemical industries. Anodic oxidation is an efficient means of producing benzylic cations that can undergo subsequent in situ nucleophilic attack to afford functionalized benzylic products. Herein, we demonstrate the suitability of carboxylic acids as nucleophiles to yield benzylic esters. This method employs a series of secondary benzylic substrates and functionalized carboxylic acids and is demonstrated on a gram scale in flow.

Demetallation of organometallic and metal-mediated reactions

The use of stoichiometric organometallic reagents and stoichiometric metals formed the basis of vast majority of classical reactions for constructing carbon-carbon bonds. The indispensable requirement of stoichiometric metals for such reactions constitutes significant challenges in terms of resource sustainability, operational safety, and chemical-waste management. The recent developments in C-H functionalizations, hydrogenative alkene/alkyne addition to electrophiles, the hydrazone umpolung chemistry, and other emerging fields such as the electrosynthesis and photoredox chemistry provide potential solutions to overcome these inherent challenges.

Electrosynthesis of Stabilized Diazo Compounds from Hydrazones

An electrochemical synthesis of diazo compounds from hydrazones in yields as high as 99% was performed. This method was elaborated as a useful synthetic method and demonstrated on various diazo compounds (24 examples). Apart from exhibiting an efficiency that matched that of commonly used harsh and toxic chemical oxidants, this reaction is practically simple to set up, requires mild conditions, and is highly electron efficient (3 F/mol).

Electrocatalysis with Molecular Transition-Metal Complexes for Reductive Organic Synthesis

Electrocatalysis enables the formation or cleavage of chemical bonds by a genuine use of electrons or holes from an electrical energy input. As such, electrocatalysis offers resource-economical alternative pathways that bypass sacrificial, waste-generating reagents often required in classical thermal redox reactions. In this Perspective, we showcase the exploitation of molecular electrocatalysts for electrosynthesis, in particular for reductive conversion of organic substrates. Selected case studies illustrate that efficient molecular electrocatalysts not only are appropriate redox shuttles but also embrace the features of organometallic catalysis to facilitate and control chemical steps. From these examples, guidelines are proposed for the design of molecular electrocatalysts suited to the reduction of organic substrates. We finally expose opportunities brought by catalyzed electrosynthesis to functionalize organic backbones, namely using sustainable building blocks.

Multicomponent Electrocatalytic Selective Approach to Unsymmetrical Spiro[furo[3,2-c]pyran-2,5′-pyrimidine] Scaffold under a Column Chromatography-Free Protocol at Room Temperature

Electrochemical synthesis suggested a mild, green and atom-efficient route to interesting and useful molecules, thus avoiding harsh chemical oxidizing and reducing agents used in traditional synthetic methods. Organic electrochemistry offers an excellent alternative to conventional methods of organic synthesis and creates a modern tool for carrying out organic synthesis, including cascade and multicomponent ones. In this research, a novel electrocatalytic multicomponent transformation was found: the electrochemical multicomponent assembly of arylaldehydes, N,N′-dimethylbarbituric acid and 4-hydroxy-6-methyl-2H-pyran-2-one in one pot reaction was carried out in alcohols in an undivided cell in the presence of alkali metal halides with the selective formation of substituted unsymmetrical 1′,3′,6-trimethyl-3-aryl-2′H,3H,4H-spiro[furo[3,2-c]pyran-2,5′-pyrimidine]-2′,4,4′,6′(1′H,3′H)-tetraones in 73–82% yields. This new electrocatalytic process is a selective, facile and efficient way to obtain spiro[furo[3,2-c]pyran-2,5′-pyrimidines]. According to screening molecular docking data using a self-made Python script in Flare, all synthesized compounds may be prominent for different medical applications, such as breast cancer, neurodegenerative diseases and treatments connected with urinary tract, bones and the cardiovascular system.

Isolation and Characterization of a Highly Reducing Aqueous Chromium(II) Complex

The highly reducing Cr^II-(1,3-propylenediaminetetraacetate) (CrPDTA) complex (-1.1 V vs SHE) has been isolated from aqueous solution and the solid-state structure is described. The reduced Cr^IIPDTA complex is characterized by single-crystal X-ray diffraction, elemental analysis, infrared spectroscopy, UV-vis spectroscopy, magnetic moment, and density functional theory calculations. The concentration profile, state of charge, and pH of CrPDTA electrolyte were monitored in a flow battery system in situ by absorption spectroscopy and a pH probe. The stability of Cr^IIPDTA in aqueous environments is demonstrated by the ability to isolate CaCrPDTA, despite the common misconception that water spontaneously evolves hydrogen at such potentials. The reduced Cr^IIPDTA prevents water from coordinating to the metal center by maintaining the same coordinatively saturated pseudo-octahedral structure as the oxidized Cr^IIIPDTA, despite experiencing an increased geometric strain from a Jahn-Teller distortion of the high-spin Cr^II ion. The important difference between solvent reactivity and solvent thermodynamic window is examined by comparing the electrochemical behavior of the reduced species of CrPDTA in various organic solvents to its behavior in aqueous solution. When examined in tetrahydrofuran (THF), the reduction potential of CrPDTA is observed to be -1.19 V vs cobaltocene (-2.52 V vs ferrocene). Reduced CrPDTA in aqueous solution is also exposed to atmospheric O₂ without exhibiting any decomposition of the Cr(III) or Cr(II) species. The techniques detailed provide a higher standard method of characterization for flow battery electrolyte species.

Controlling One- or Two-Electron Oxidation for Selective Amine Functionalization by Alternating Current Frequency

Here, we report a unique electrosynthetic method that enables the selective one-electron oxidation of tertiary amines to generate α-amino radical intermediates over two-electron oxidation to iminium cations, providing easy access to arylation products by simply applying an optimal alternating current (AC) frequency. More importantly, we have discovered an electrochemical descriptor from cyclic voltammetry studies to predict the optimal AC frequency for various amine substrates, circumventing the time-consuming trial-and-error methods for optimizing reaction conditions. This new development in AC electrolysis provides an alternative strategy to solving challenging chemoselectivity problems in synthetic organic 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.

Alternating Current Electrolysis Enabled Formal C-O/O-H Cross-Metathesis of 4-Alkoxy Anilines with Alcohols

While multiple bond metathesis reactions, for example olefin metathesis, have seen considerable recent progress, direct metathesis of traditionally inert C-O single bonds is extremely rare and particularly challenging. Undoubtedly, metathesis reaction of C-O bonds is one of the most ideal routes for the value-added upgrading of molecules involving C-O bonds. Reported here is a new protocol to achieve the formal C-O/O-H cross-metathesis via alternating current electrolysis. Featuring mild reaction conditions, the protocol allows readily available 4-alkoxy anilines and alcohols to be converted into a wide range of valuable products in highly regioselective and chemoselective manner. Moreover, the present strategy can be used in the late-stage modification of pharmaceuticals as well as biologically active compounds, which demonstrated the potential application.

Recent Advances in C(sp3)-C(sp3) and C(sp3)-C(sp2) Bond Formation through Cathodic Reactions: Reductive and Convergent Paired Electrolyses

The formation of C(sp³)-C(sp³) and C(sp³)-C(sp²) bonds is one of the major research goals of synthetic chemists. Electrochemistry is commonly considered to be an appealing means to drive redox reactions in a safe and sustainable fashion and has been utilized for C-C bond-forming reactions. Compared to anodic oxidative methods, which have been extensively explored, cathodic processes are much less investigated, whereas it can pave the way to alternative retrosynthetic disconnections of target molecules and to the discovery of new transformations. This review provides an overview on the recent achievements in the construction of C(sp³)-C(sp³) and C(sp³)-C(sp²) bonds via cathodic reactions since 2017. It includes electrochemical reductions and convergent paired electrolyses.

Electrochemically Mediated Oxidation of Sensitive Propargylic Benzylic Alcohols

The electrochemical oxidation of sensitive propargylic benzylic alcohols having varying substituents is reported. We describe the preparation and characterization of N-hydroxytetrafluorophthalimide (TFNHPI) and pseudo-high-throughput development of a green electrochemical oxidation protocol for sensitive propargylic benzylic alcohols that employs TFNHPI as a stable electrochemical mediator. The electrochemical oxidation of propargylic benzylic alcohols was leveraged to develop short synthetic pathways for preparing gram quantities of resveratrol natural products such as pauciflorols.

Electroreductive synthesis of polyfunctionalized pyridin-2-ones from acetoacetanilides and carbon disulfide with oxygen evolution

A chemical reductant or a sacrificial electron donor is required in any reduction reactions, generally resulting in undesired chemical waste. Herein, we report a reductant-free reductive [3 + 2 + 1] annulation of β-keto amides with CS₂ enabled by the synergy of electro/copper/base using water as an innocuous anodic sacrifice with O₂ as a sustainable by-product. This electrochemical protocol is mild and provides access to polyfunctionalized pyridin-2-ones from simple starting materials in a single step.

Photons or Electrons? A Critical Comparison of Electrochemistry and Photoredox Catalysis for Organic Synthesis

Redox processes are at the heart of synthetic methods that rely on either electrochemistry or photoredox catalysis, but how do electrochemistry and photoredox catalysis compare? Both approaches provide access to high energy intermediates (e.g., radicals) that enable bond formations not constrained by the rules of ionic or 2 electron (e) mechanisms. Instead, they enable 1e mechanisms capable of bypassing electronic or steric limitations and protecting group requirements, thus enabling synthetic chemists to disconnect molecules in new and different ways. However, while providing access to similar intermediates, electrochemistry and photoredox catalysis differ in several physical chemistry principles. Understanding those differences can be key to designing new transformations and forging new bond disconnections. This review aims to highlight these differences and similarities between electrochemistry and photoredox catalysis by comparing their underlying physical chemistry principles and describing their impact on electrochemical and photochemical methods.

Picomole-Scale Transition Metal Electrocatalysis Screening Platform for Discovery of Mild C-C Coupling and C-H Arylation through in Situ Anodically Generated Cationic Pd

Development of new transition-metal-catalyzed electrochemistry promises to improve overall synthetic efficiency. Here, we describe the first integrated platform for online screening of electrochemical transition-metal catalysis. It utilizes the intrinsic electrochemical capabilities of nanoelectrospray ionization mass spectrometry (nano-ESI-MS) and picomole-scale anodic corrosion of a Pd electrode to generate and evaluate highly efficient cationic catalysts for mild electrocatalysis. We demonstrate the power of the novel electrocatalysis platform by (1) identifying electrolytic Pd-catalyzed Suzuki coupling at room temperature, (2) discovering Pd-catalyzed electrochemical C-H arylation in the absence of external oxidant or additive, (3) developing electrolyzed Suzuki coupling/C-H arylation cascades, and (4) achieving late-stage functionalization of two drug molecules by the newly developed mild electrocatalytic C-H arylation. More importantly, the scale-up reactions confirm that new electrochemical pathways discovered by nano-ESI can be implemented under the conventional electrolytic reaction conditions. This approach enables in situ mechanistic studies by capturing various intermediates including transient transition metal species by MS, and thus uncovering the critical role of anodically generated cationic Pd catalyst in promoting otherwise sluggish transmetalation in C-H arylation. The anodically generated cationic Pd with superior catalytic efficiency and novel online electrochemical screening platform hold great potential for discovering mild transition-metal-catalyzed reactions.

Selectfluor-mediated construction of 3-arylselenenyl and 3,4-bisarylselenenyl spiro[4.5]trienones via cascade annulation of N-phenylpropiolamides with diselenides

A cascade annulation of N-phenylpropiolamides with diselenides leading to the construction of 3-arylselenenyl spiro[4.5]trienones was realized under mild conditions with Selectfluor as the sole oxidant.