Electrons and Holes as Catalysts in Organic Electrosynthesis

Anodic Commodity Polymer Recycling: The Merger of Iron-Electrocatalysis with Scalable Hydrogen Evolution Reaction

Plastics are omnipresent in our everyday life, and accumulation of post-consumer plastic waste in our environment represents a major societal challenge. Hence, methods for plastic waste recycling are in high demand for a future circular economy. Specifically, the degradation of post-consumer polymers towards value-added small molecules constitutes a sustainable strategy for a carbon circular economy. Despite of recent advances, chemical polymer degradation continues to be largely limited to chemical redox agents or low energy efficiency in photochemical processes. We herein report a powerful iron-catalyzed degradation of high molecular weight polystyrenes through electrochemistry to efficiently deliver monomeric benzoyl products. The robustness of the ferraelectrocatalysis was mirrored by the degradation of various real-life post-consumer plastics, also on gram scale. The cathodic half reaction was largely represented by the hydrogen evolution reaction (HER). The scalable electro-polymer degradation could be solely fueled by solar energy through a commercially available solar panel, indicating an outstanding potential for a decentralized green hydrogen economy.

The Photoredox Paradox: Electron and Hole Upconversion as the Hidden Secrets of Photoredox Catalysis

Although photoredox catalysis is complex from a mechanistic point of view, it is also often surprisingly efficient. In fact, the quantum efficiency of a puzzlingly large portion of photoredox reactions exceeds 100% (i.e., the measured quantum yields (QYs) are >1). Hence, these photoredox reactions can be more than perfect with respect to photon utilization. In several documented cases, a single absorbed photon can lead to the formation of >100 molecules of the product, behavior known to originate from chain processes. In this Perspective, we explore the underlying reasons for this efficiency, identify the nature of common catalytic chains, and highlight the differences between HAT and SET chains. Our goal is to show why chains are especially important in photoredox catalysis and where the thermodynamic driving force that sustains the SET catalytic cycles comes from. We demonstrate how the interplay of polar and radical processes can activate hidden catalytic pathways mediated by electron and hole transfer (i.e., electron and hole catalysis). Furthermore, we illustrate how the phenomenon of redox upconversion serves as a thermodynamic precondition for electron and hole catalysis. After discussing representative mechanistic puzzles, we analyze the most common bond forming steps, where redox upconversion frequently occurs (and issometimes unavoidable). In particular, we highlight the importance of 2-center-3-electron bonds as a recurring motif that allows a rational chemical approach to the design of redox upconversion processes.

Mechanochemistry Drives Alkene Difunctionalization via Radical Ligand Transfer and Electron Catalysis

A general and modular protocol is reported for olefin difunctionalization through mechanochemistry, facilitated by cooperative radical ligand transfer (RLT) and electron catalysis. Utilizing mechanochemical force and catalytic amounts of 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO), ferric nitrate can leverage nitryl radicals, transfer nitrooxy-functional group via RLT, and mediate an electron catalysis cycle under room temperature. A diverse range of activated and unactivated alkenes exhibited chemo- and regioselective 1,2-nitronitrooxylation under solvent-free or solvent-less conditions, showcasing excellent functional group tolerance. Mechanistic studies indicated a significant impact of mechanochemistry and highlighted the radical nature of this nitrative difunctionalization process.

Overcoming Challenges in Electrosynthesis Using High‐Throughput Electrochemistry: Hypervalent Iodine‐Mediated Phenol Dearomatization, a Case Study

Despite many recent efforts, the field of organic electrosyn‐thesis faces important challenges due to the intricate nature of heterogeneous redox processes, the wide parameter space to be explored and the lack of standardized methods. To overcome these limitations, we developed a cost‐effective high‐throughput electrochemical (HTE) reactor capable of running 24 individually controlled parallel reactions. This system allows the rapid testing of electrochemical parameters on a given reaction, assessing not only yield but also reproducibility. Using the hypervalent iodine‐mediated dearomatization of phloretic acid as a demonstration of HTE capabilities, we ran more than 200 electrosyntheses in different experimental conditions and demonstrate the effect of parameters such as total charge transferred, current, electrode materials, electrolyte formulation and concentration, mediator formulation and concentration and electrochemical technique of oxidation. Notably, this report demonstrates that while catalytic amounts of iodine mediator can be used successfully, the reproducibility may be affected, which calls for a cautious approach when developing similar transformations. Using cyclic voltammetry, density functional theory, chronopotentiometry, and Raman spectroscopy, we shed light on the causes of this issue.

Access to carbonyl compounds via the electroreduction of N-benzyloxyphthalimides: Mechanism confirmation and preparative applications

A method to access carbonyl compounds using reductive conditions was evaluated via electrochemical reduction of their corresponding N-benzyloxyphthalimide derivatives (NBOPIs). The mechanism of this originally reported electrochemical reaction was proposed based on DFT calculation and is experimentally confirmed herein, contrasting simulated and experimental cyclic voltammetry data. The reaction scope studied in a preparative scale and using redox sensitive functional groups showed good selectivity and tolerance toward oxidation under the reaction conditions with a moderate to good yield (50-71%). Nevertheless, some restrictions with reducible functional groups, like benzyl-brominated and nitro-aromatic derivatives, were observed. The present approach can be considered a self-sustainable electrochemical catalysis for the synthesis of aromatic carbonylic compounds passing through anion radical intermediates produced by a cathodic reaction.

Hole Catalysis of Cycloaddition Reactions: How to Activate and Control Oxidant Upconversion in Radical-Cationic Diels-Alder Reactions

In order to use holes as catalysts, the oxidized product should be able to transfer the hole to a fresh reactant. For that, the hole-catalyzed reaction must increase the oxidation potential along the reaction path, i.e., lead to "hole upconversion." If this thermodynamic requirement is satisfied, a hole injected via one-electron oxidation can persist through multiple propagation cycles and serve as a true catalyst. This work provides guidelines for the rational design of hole-catalyzed Diels-Alder (DA) reactions, the prototypical cycloaddition. After revealing the crucial role of hyperconjugation in the absence of hole upconversion in the parent DA reaction, we show how upconversion can be reactivated by proper substitution. For this purpose, we computationally evaluate the contrasting effects of substituents at the three possible positions in the two reactants. The occurrence and magnitude of hole upconversion depend strongly on the placement and nature of substituents. For example, donors at C1 in 1,3-butadiene shift the reaction to the hole-upconverted regime with an increased oxidation potential of up to 1.0 V. In contrast, hole upconversion in C2-substituted 1,3-butadienes is activated by acceptors with the oxidation potential increase up to 0.54 V. Dienophile substitution results in complex trends because the radical cation can be formed at either the dienophile or the diene. Hole upconversion is always present in the former scenario (up to 0.65 V). Finally, we report interesting stereoelectronic effects that can activate or deactivate upconversion via a conformational change.

Research on Electric Field-Induced Catalysis Using Single-Molecule Electrical Measurement

The role of catalysis in controlling chemical reactions is crucial. As an important external stimulus regulatory tool, electric field (EF) catalysis enables further possibilities for chemical reaction regulation. To date, the regulation mechanism of electric fields and electrons on chemical reactions has been modeled. The electric field at the single-molecule electronic scale provides a powerful theoretical weapon to explore the dynamics of individual chemical reactions. The combination of electric fields and single-molecule electronic techniques not only uncovers new principles but also results in the regulation of chemical reactions at the single-molecule scale. This perspective focuses on the recent electric field-catalyzed, single-molecule chemical reactions and assembly, and highlights promising outlooks for future work in single-molecule catalysis.

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.

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

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

Electrosynthetic C-F bond cleavage

Fluorinated organic compounds are common among pharmaceuticals, agrochemicals and materials. The significant strength of the C-F bond results in chemical inertness that, depending on the context, is beneficial, problematic or simply a formidable synthetic challenge. Electrosynthesis is a rapidly expanding methodology that can enable new reactivity and selectivity for cleavage and formation of chemical bonds. Here, a comprehensive overview of synthetically relevant electrochemically driven protocols for C-F bond activation and functionalization is presented, including photoelectrochemical strategies.

Radical cation Diels–Alder reactions of arylidene cycloalkanes

TiO2 photoelectrochemical and electrochemical radical cation Diels–Alder reactions of arylidene cycloalkanes are described, leading to the construction of spiro ring systems. Although the mechanism remains an open question, arylidene cyclobutanes are found to be much more effective in the reaction than other cycloalkanes. Since the reaction is completed with a substoichiometric amount of electricity, a radical cation chain pathway is likely to be involved.

Recent synthetic methods involving carbon radicals generated by electrochemical catalysis

Driven by a resurgence of interest in electrode-driven synthetic methods, this paper covers recent activity in the field of mediated electrochemical and photoelectrochemical bond activation, inclusive of C-H, C-C, C-N, and other C-heteroatom bonds.

Electron-catalysed molecular recognition

Molecular recognition^1-4 and supramolecular assembly^5-8 cover a broad spectrum^9-11 of non-covalently orchestrated phenomena between molecules. Catalysis¹² of such processes, however, unlike that for the formation of covalent bonds, is limited to approaches^13-16 that rely on sophisticated catalyst design. Here we establish a simple and versatile strategy to facilitate molecular recognition by extending electron catalysis¹⁷, which is widely applied^18-21 in synthetic covalent chemistry, into the realm of supramolecular non-covalent chemistry. As a proof of principle, we show that the formation of a trisradical complex²² between a macrocyclic host and a dumbbell-shaped guest-a molecular recognition process that is kinetically forbidden under ambient conditions-can be accelerated substantially on the addition of catalytic amounts of a chemical electron source. It is, therefore, electrochemically possible to control²³ the molecular recognition temporally and produce a nearly arbitrary molar ratio between the substrates and complexes ranging between zero and the equilibrium value. Such kinetically stable supramolecular systems²⁴ are difficult to obtain precisely by other means. The use of the electron as a catalyst in molecular recognition will inspire chemists and biologists to explore strategies that can be used to fine-tune non-covalent events, control assembly at different length scales^25-27 and ultimately create new forms of complex matter^28-30.

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.

Microscopic mechanisms of cooperative communications within single nanocatalysts

Catalysis is an experimental approach to accelerate chemical reactions. It plays a critical role in modern industries. Recent experimental studies uncovered striking observations of cooperative communications for reactions on nanocatalysts. In these experiments, it was shown that the chemical reactions observed at specific active sites might effectively stimulate the same reactions at the neighboring sites. We developed a theoretical model to investigate the microscopic mechanisms of these phenomena. Our idea is that the catalytic communication is the result of the complex dynamics of charged holes. Explicit calculations are able to quantitatively explain all experimental observations, clarifying the molecular origin of cooperative communications. The presented theoretical framework might be utilized for developing efficient catalytic systems with better control over chemical reactions.

Catalysis is a method of accelerating chemical reactions that is critically important for fundamental research as well as for industrial applications. It has been recently discovered that catalytic reactions on metal nanoparticles exhibit cooperative effects. The mechanism of these observations, however, remains not well understood. In this work, we present a theoretical investigation on possible microscopic origin of cooperative communications in nanocatalysts. In our approach, the main role is played by positively charged holes on metal surfaces. A corresponding discrete-state stochastic model for the dynamics of holes is developed and explicitly solved. It is shown that the observed spatial correlation lengths are given by the average distances migrated by the holes before they disappear, while the temporal memory is determined by their lifetimes. Our theoretical approach is able to explain the universality of cooperative communications as well as the effect of external electric fields. Theoretical predictions are in agreement with experimental observations. The proposed theoretical framework quantitatively clarifies some important aspects of the microscopic mechanisms of heterogeneous catalysis.

Atropoenantioselective Palladaelectro-Catalyzed Anilide C–H Olefinations Viable with Natural Sunlight as Sustainable Power Source

Enantioselective electrocatalyzed transformations represent a major challenge. We herein achieved atropoenantioselective pallada-electrocatalyzed C–H olefinations and C–H allylations with high efficacy and enantioselectivity under exceedingly mild reaction conditions. With (S)-5-oxoproline as the chiral ligand, activated and non-activated olefins were suitable substrates for the electro-C–H activations. Dual catalysis was devised in terms of electro-C–H olefination, along with catalytic hydrogenation. Challenging enantiomerically-enriched chiral anilide scaffolds were thereby obtained with high levels of enantio-control in the absence of toxic and cost-intensive silver salts. The resource-economy of the transformation was even improved by directly employing renewable solar energy.

Asymmetric pallada-electrocatalyzed C–H activation of achiral anilides were accomplished by catalyst control with high levels of enantioselectivity. Dual catalysis was devised, while photovoltaic cells could be used to empower the electrocatalysis.

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

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

Electrochemically Catalyzed N–N Coupling and Ring Cleavage Reaction of 1H-Pyrazoles

The electrocatalyzed N–N coupling and ring cleavage reaction of 3-methyl-, 3,5-dimethyl-, 3-methyl-5-phenyl- and 3,5-diphenyl-1H-pyrazole was investigated and led to the electro-organic synthesis of new heterocyclic compounds. The results revealed that electrochemically produced 1H-pyrazoleox plays the role of acceptor in a reaction with the starting molecule via a N–N coupling and ring cleavage reaction of pyrazoles. The proposed reaction sequence consists of anodic oxidation, dimerization, rearrangement and reduction. The electrochemically catalyzed reactions were accomplished under constant-current and constant-potential conditions using an undivided electrochemical cell with the advantages of mild reaction conditions, remarkable yields and environmental compatibility.

The Longer Route can be Better: Electrosynthesis in Extended Path Flow Cells

This personal account provides an overview of work conducted in my research group, and through collaborations with other chemists and engineers, to develop flow electrolysis cells and apply these cells in organic electrosynthesis. First, a brief summary of my training and background in organic synthesis is provided, leading in to the start of flow electrosynthesis in my lab in collaboration with Derek Pletcher. Our work on the development of extended path electrolysis flow reactors is described from a synthetic organic chemist's perspective, including laboratory scale-up to give several moles of an anodic methoxylation product in one day. The importance of cell design is emphasised with regards to achieving good performance in laboratory electrosynthesis with productivities from hundreds of mg h^-1 to many g h^-1 , at high conversion in a selective fashion. A simple design of recycle flow cell that can be readily constructed in a small University workshop is also discussed, including simple modifications to improve cell performance. Some examples of flow electrosyntheses are provided, including Shono-type oxidation, anodic cleavage of protecting groups, Hofer-Moest reaction of cubane carboxylic acids, oxidative esterification and amidation of aldehydes, and reduction of aryl halides.

Electrocatalytic Activation of Donor-Acceptor Cyclopropanes and Cyclobutanes: An Alternative C(sp3 )-C(sp3 ) Cleavage Mode

We describe the first electrochemical activation of D-A cyclopropanes and D-A cyclobutanes leading after C(sp3 )-C(sp3 ) cleavage to the formation of highly reactive radical cations. This concept is utilized to formally insert molecular oxygen after direct or DDQ-assisted anodic oxidation of the strained carbocycles, delivering β- and γ-hydroxy ketones and 1,2-dioxanes electrocatalytically. Furthermore, insights into the mechanism of the oxidative process, obtained experimentally and by additional quantum-chemical calculations are presented. The synthetic potential of the reaction products is demonstrated by diverse derivatizations.

Electrochemical Installation of CFH2 -, CF2 H-, CF3 -, and Perfluoroalkyl Groups into Small Organic Molecules

Electrosynthesis can be considered a powerful and sustainable methodology for the synthesis of small organic molecules. Due to its intrinsic ability to generate highly reactive species under mild conditions by anodic oxidation or cathodic reduction, electrosynthesis is particularly interesting for otherwise challenging transformations. One such challenge is the installation of fluorinated alkyl groups, which has gained significant attention in medicinal chemistry and material science due to their unique physicochemical features. Unsurprisingly, several electrochemical fluoroalkylation methods have been established. In this review, we survey recent developments and established methods in the field of electrochemical mono-, di-, and trifluoromethylation, and perfluoroalkylation of small organic molecules.

Organic Electrochemistry: Expanding the Scope of Paired Reactions

Paired electrochemical reactions allow the optimization of both atom and energy economy of oxidation and reduction reactions. While many paired electrochemical reactions take advantage of perfectly matched reactions at the anode and cathode, this matching of substrates is not necessary. In constant current electrolysis, the potential at both electrodes adjusts to the substrates in solution. In principle, any oxidation reaction can be paired with any reduction reaction. Various oxidation reactions conducted on the anodic side of the electrolysis were paired with the generation and use of hydrogen gas at the cathode, showing the generality of the anodic process in a paired electrolysis and how the auxiliary reaction required for the oxidation could be used to generate a substrate for a non-electrolysis reaction. This is combined with variations on the cathodic side of the electrolysis to complete the picture and illustrate how oxidation and reduction reactions can be combined.

Evolution of Earth-Abundant 3 d-Metallaelectro-Catalyzed C-H Activation: From Chelation-Assistance to C-H Functionalization without Directing Groups

Catalyzed C-H functionalizations have emerged as a transformative platform for molecular syntheses. Despite of indisputable advances, oxidative C-H activations have been largely restricted to precious transition metals and stoichiometric amounts of chemical oxidants. In contrast, we herein discuss the potential of earth-abundant, environmentally-benign 3d transition metals for C-H activation, which has recently gained major momentum. Thus, a strategy for full resource economy has been established in our group, with green electricity as a renewable redox agent, giving valuable hydrogen as the sole byproduct under redox mediator-free conditions. In this account, we detail our accomplishments in 3d metallaelectrocatalysis towards green syntheses until March 2021.

Electricity-Driven Post-Functionalization of Conducting Polymers

Electrochemical doping of conducting polymers (CPs) generates polarons (radical ionic species) and bipolarons (ionic species) in their backbone via multi-electron transfer between an electrode and the CP. In the electrochemical polymer reaction (ePR), these generated ionic species are regarded as reactive intermediates for further transformation of the chemical structures of CPs. This electrochemical post-functionalization can easily be used to control the degree of reactions by turning a power supply on/off, as well as tuning the applied electrode potential, which leads to fine-tuning of the various properties of the CPs, such as the HOMO/LUMO level and PL properties. This Account summarizes recent developments in the electrochemical post-functionalization of CPs. In particular, we focus on reaction design for the ePR, with respect to the preparation and structure of the precursor polymers, applicable functional groups, efficient reaction conditions, and electrolytic methodologies.

Synthetic Semiconductor Photoelectrochemistry

In the field of synthetic organic chemistry, photochemical and electrochemical approaches are often considered to be competing technologies that induce single electron transfer (SET). Recently, their fusion, i. e., the "photoelectrochemical" approach, has become the focus of attention. In this approach, both solar and electrical energy are used in creative combinations. Historically, the term "photoelectrochemistry" has been used in more inorganic fields, where a photovoltaic effect exhibited by semiconducting materials is employed. Semiconductors have also been studied intensively as photocatalysts; however, they recently have taken a back seat to molecular photocatalysts. In this account, we would like to revisit semiconductor photocatalysts in the field of synthetic organic chemistry to demonstrate that semiconductor "photoelectrochemical" approaches are more than mere alternatives to molecular photochemical and/or electrochemical approaches.

Organic Electrochemistry: Molecular Syntheses with Potential

Efficient and selective molecular syntheses are paramount to inter alia biomolecular chemistry and material sciences as well as for practitioners in chemical, agrochemical, and pharmaceutical industries. Organic electrosynthesis has undergone a considerable renaissance and has thus in recent years emerged as an increasingly viable platform for the sustainable molecular assembly. In stark contrast to early strategies by innate reactivity, electrochemistry was recently merged with modern concepts of organic synthesis, such as transition-metal-catalyzed transformations for inter alia C-H functionalization and asymmetric catalysis. Herein, we highlight the unique potential of organic electrosynthesis for sustainable synthesis and catalysis, showcasing key aspects of exceptional selectivities, the synergism with photocatalysis, or dual electrocatalysis, and novel mechanisms in metallaelectrocatalysis until February of 2021.

Electrochemical rearrangement protocols towards the construction of diverse molecular frameworks

Rearrangement reactions constitute a critical facet of synthetic organic chemistry and demonstrate an attractive way to take advantage of existing structures to access various important molecular frameworks. Electroorganic chemistry has emerged as an environmentally benign approach to carry out organic transformations by directly employing an electric current and avoids the use of stoichiometric chemical oxidants. The last few years have witnessed a resurgence of electroorganic chemistry that has promoted a renaissance of interest in the development of novel redox electroorganic transformations. This review manifests the evolution of electrosynthesis in the area of rearrangement chemistry and covers the achievements in the field of migration, ring expansion, and rearrangements along with the mechanisms involved.

Electrochemical Insight into Mechanisms and Metallocyclic Intermediates of C-H Functionalization

Transition metal-catalyzed C-H activation has emerged as a powerful tool in organic synthesis and electrosynthesis as well as in the development of new methodologies for producing fine chemicals. In order to achieve efficient and selective C-H functionalization, different strategies have been used to accelerate the C-H activation step, including the incorporation of directing groups in the substrate that facilitate coordination to the catalyst. In this review, we try to underscore that the understanding the mechanisms of the catalytic cycle and the reactivity or redox activity of the key metal cyclic intermediates in these reactions is the basis for controlling the selectivity of synthesis and electrosynthesis. Combination of the electrosynthesis and voltammetry with traditional synthetic and physico-chemical methods allows one to achieve selective transformation of C-H bonds to functionalized C-C or C-X (X=heteroatom or halogen) bonds which may encourage organic chemists to use it in the future more often. The possibilities and the benefits of electrochemical techniques are analyzed and summarized.

Chemoselective Electrochemical Hydrogenation of Ketones and Aldehydes with a Well-Defined Base-Metal Catalyst

Hydrogenation reactions are fundamental functional group transformations in chemical synthesis. Here, we introduce an electrochemical method for the hydrogenation of ketones and aldehydes by in situ formation of a Mn-H species. We utilise protons and electric current as surrogate for H2 and a base-metal complex to form selectively the alcohols. The method is chemoselective for the hydrogenation of C=O bonds over C=C bonds. Mechanistic studies revealed initial 3 e- reduction of the catalyst forming the steady state species [Mn2 (H-1 L)(CO)6 ]- . Subsequently, we assume protonation, reduction and internal proton shift forming the hydride species. Finally, the transfer of the hydride and a proton to the ketone yields the alcohol and the steady state species is regenerated via reduction. The interplay of two manganese centres and the internal proton relay represent the key features for ketone and aldehyde reduction as the respective mononuclear complex and the complex without the proton relay are barely active.

Facilitating Ir-Catalyzed C-H Alkynylation with Electrochemistry: Anodic Oxidation-Induced Reductive Elimination

An electrochemical approach in promoting directed C-H alkynylation with terminal alkyne via iridium catalysis is reported. This work employed anodic oxidation of Ir(III) intermediate (characterized by X-ray crystallography) to promote reductive elimination, giving the desired coupling products in good yields (up to 95%) without the addition of any other external oxidants. This transformation is suitable for various directing groups with H2 as the only by-product, which warrants a high atom economy and practical oxidative C-C bond formation under mild conditions.

Electrochemically Catalyzed Newman-Kwart Rearrangement: Mechanism, Structure-Reactivity Relationship, and Parallels to Photoredox Catalysis

The facilitation of redox-neutral reactions by electrochemical injection of holes and electrons, also known as "electrochemical catalysis", is a little explored approach that has the potential to expand the scope of electrosynthesis immensely. To systematically improve existing protocols and to pave the way toward new developments, a better understanding of the underlying principles is crucial. In this context, we have studied the Newman-Kwart rearrangement of O-arylthiocarbamates to the corresponding S-aryl derivatives, the key step in the synthesis of thiophenols from the corresponding phenols. This transformation is a particularly useful example because the conventional method requires temperatures up to 300 °C, whereas electrochemical catalysis facilitates the reaction at room temperature. A combined experimental-quantum chemical approach revealed several reaction channels and rendered an explanation for the relationship between the structure and reactivity. Furthermore, it is shown how rapid cyclic voltammetry measurements can serve as a tool to predict the feasibility for specific substrates. The study also revealed distinct parallels to photoredox-catalyzed reactions, in which back-electron transfer and chain propagation are competing pathways.

Electro-organic synthesis - a 21st century technique

The severe limitations of fossil fuels and finite resources influence the scientific community to reconsider chemical synthesis and establish sustainable techniques. Several promising methods have emerged, and electro-organic conversion has attracted particular attention from international academia and industry as an environmentally benign and cost-effective technique. The easy application, precise control, and safe conversion of substrates with intermediates only accessible by this method reveal novel pathways in synthetic organic chemistry. The popularity of electricity as a reagent is accompanied by the feasible conversion of bio-based feedstocks to limit the carbon footprint. Several milestones have been achieved in electro-organic conversion at rapid frequency, which have opened up various perspectives for forthcoming processes.

Insights into Cobalta(III/IV/II)-Electrocatalysis: Oxidation-Induced Reductive Elimination for Twofold C-H Activation

The merger of cobalt‐catalyzed C−H activation and electrosynthesis provides new avenues for resource‐economical molecular syntheses, unfortunately their reaction mechanisms remain poorly understood. Herein, we report the identification and full characterization of electrochemically generated high‐valent cobalt(III/IV) complexes as crucial intermediates in electrochemical cobalt‐catalyzed C−H oxygenations. Detailed mechanistic studies provided support for an oxidatively‐induced reductive elimination via highly‐reactive cobalt(IV) intermediates. These key insights set the stage for unprecedented cobaltaelectro two‐fold C−H/C−H activation.

Metalla-electrocatalyzed C-H Activation by Earth-Abundant 3d Metals and Beyond

To improve the efficacy of molecular syntheses, researchers wish to capitalize upon the selective modification of otherwise inert C-H bonds. The past two decades have witnessed considerable advances in coordination chemistry that have set the stage for transformative tools for C-H functionalizations. Particularly, oxidative C-H/C-H and C-H/Het-H transformations have gained major attention because they avoid all elements of substrate prefunctionalization. Despite considerable advances, oxidative C-H activations have been dominated by precious transition metal catalysts based on palladium, ruthenium, iridium, and rhodium, thus compromising the sustainable nature of the overall C-H activation approach. The same holds true for the predominant use of stoichiometric chemical oxidants for the regeneration of the active catalyst, prominently featuring hypervalent iodine(III), copper(II), and silver(I) oxidants. Thereby, stoichiometric quantities of undesired byproducts are generated, which are preventive for applications of C-H activation on scale. In contrast, the elegant merger of homogeneous metal-catalyzed C-H activation with molecular electrosynthesis bears the unique power to achieve outstanding levels of oxidant and resource economy. Thus, in contrast to classical electrosyntheses by substrate control, metalla-electrocatalysis holds huge and largely untapped potential for oxidative C-H activations with unmet site selectivities by means of catalyst control. While indirect electrolysis using precious palladium complexes has been realized, less toxic and less expensive base metal catalysts feature distinct beneficial assets toward sustainable resource economy. In this Account, I summarize the emergence of electrocatalyzed C-H activation by earth-abundant 3d base metals and beyond, with a topical focus on contributions from our laboratories through November 2019. Thus, cobalt electrocatalysis was identified as a particularly powerful platform for a wealth of C-H transformations, including C-H oxygenations and C-H nitrogenations as well as C-H activations with alkynes, alkenes, allenes, isocyanides, and carbon monoxide, among others. As complementary tools, catalysts based on nickel, copper, and very recently iron have been devised for metalla-electrocatalyzed C-H activations. Key to success were detailed mechanistic insights, prominently featuring oxidation-induced reductive elimination scenarios. Likewise, the development of methods that make use of weak O-coordination benefited from crucial insights into the catalyst's modes of action by experiment, in operando spectroscopy, and computation. Overall, metalla-electrocatalyzed C-H activations have thereby set the stage for molecular syntheses with unique levels of resource economy. These electrooxidative C-H transformations overall avoid the use of chemical oxidants and are frequently characterized by improved chemoselectivities. Hence, the ability to dial in the redox potential at the minimum level required for the desired transformation renders electrocatalysis an ideal platform for the functionalization of structurally complex molecules with sensitive functional groups. This strategy was, inter alia, successfully applied to scale-up by continuous flow and the step-economical assembly of polycyclic aromatic hydrocarbons.

Probing the versatility of metallo-electro hybrid catalysis: enabling access towards facile C–N bond formation

Metallo-electro catalysis has emerged as sustainable alternate to conventional transition metal methodologies. This review highlights the recent advances for the formation of C–N bonds by merging transition metal catalysis with electrosynthesis.

Metal-free chemoselective hydrogenation of unsaturated carbon–carbon bonds via cathodic reduction

A straightforward protocol for efficient and highly selective hydrogenation of unsaturated carbon–carbon bonds via electrochemical reduction has been reported.

Electrochemical sulfonylation of alkenes with sulfonyl hydrazides: a metal- and oxidant-free protocol for the synthesis of (E)-vinyl sulfones in water

An electrochemical sulfonylation of alkenes with sulfonyl hydrazides for the synthesis of (E)-vinyl sulfones in water is reported.

The synthesis of sulfonated 4H-3,1-benzoxazines via an electro-chemical radical cascade cyclization

We achieved sulfonated 4H-3,1-benzoxazines under ambient conditions without any metals and external chemical oxidants via electrochemical radical cascade cyclizations.