Contributions of organic electrosynthesis to green chemistry

Electrochemically Enabled One-Pot Multistep Synthesis of C19 Androgen Steroids

The synthesis of many valuable C19 androgens can be accomplished by removal of the C17 side chain from more abundant corticosteroids, followed by further derivatization of the resulting 17-keto derivative. Conventional chemical reagents pose significant drawbacks for this synthetic strategy, as large amounts of waste are generated, and quenching of the reaction mixture and purification of the 17-ketosteroid intermediate are typically required. Herein, we present mild, safe, and sustainable electrochemical strategies for the preparation of C19 steroids. A reagent and catalyst free protocol for the removal of the C17 side chain of corticosteroids via anodic oxidation has been developed, enabling several one-pot, multistep procedures for the synthesis of androgen steroids. In addition, simultaneous anodic C17 side chain cleavage and cathodic catalytic hydrogenation of a steroid has been demonstrated, rendering a convenient and highly atom economic procedure for the synthesis of saturated androgens.

Ni(II) Schiff-Base Complexes as Chiral Electroauxiliaries and Methodological Platform for Stereoselective Electrochemical Functionalization of Amino Acids

The concept of chiral electroauxiliary based on the redox active chiral platform to perform transformations of a redox inactive substrate is suggested and discussed in the context of the targeted stereoselective electrochemical functionalization of amino acids. Tailor-made amino acids are essential structural features of modern medicinal chemistry and drug design; the development of efficient synthetic approaches to these compounds is of topical interest. The modified substrate (an amino acid) is included as a structural motif in the redox active complex (with a possibility to be released after modification) that integrates "a bifunctional linker" (the structural motif capable to "catch" a substrate) and a chiral moiety responsible for asymmetry induction. The amino acid, being included as a part of such ensemble, becomes stable towards redox destruction and its targeted electrochemical modification saving the amino acid skeleton is possible, thus developing new modes of reactivity for well-known compounds.

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.

Electrogenerated NHCs in Organic Synthesis: Ionic Liquids vs Organic Solvents Effects

In the last twenty years, N-heterocyclic carbenes (NHCs) have been extensively studied for their application as organocatalysts in stereoselective synthesis as well as ligands for transition metals-promoted synthetic methodologies. Derived mainly from azolium salts, NHCs have demonstrated exceptional versatility in their generation usually performed by deprotonation or reduction (chemical or electrochemical). In particular, the generation of NHC under electrochemical conditions, starting from azolium-based ionic liquids, has proven to be a successful green approach and demonstrated wide applicability in organic synthesis. In this Personal Account, the application of electrogenerated NHCs in organic synthesis will be discussed, with a particular attention to the different reactivity in ionic liquids compared to classical organic solvents.

Progress and perspectives of electrochemical insights for C–H and N–H sulfonylation

A comprehensive electrosulfonylation study has been carried out via cathodic and anodic approaches for the production of organosulfone and sulfonamide derivatives.

Metal-Free Twofold Electrochemical C-H Amination of Activated Arenes: Application to Medicinally Relevant Precursor Synthesis

The efficient production of many medicinally or synthetically important starting materials suffers from wasteful or toxic precursors for the synthesis. In particular, the aromatic non-protected primary amine function represents a versatile synthetic precursor, but its synthesis typically requires toxic oxidizing agents and transition metal catalysts. The twofold electrochemical amination of activated benzene derivatives via Zincke intermediates provides an alternative sustainable strategy for the formation of new C-N bonds of high synthetic value. As a proof of concept, we use our approach to generate a benzoxazinone scaffold that gained attention as a starting structure against castrate-resistant prostate cancer. Further improvement of the structure led to significantly increased cancer cell line toxicity. Thus, exploiting environmentally benign electrooxidation, we present a new versatile and powerful method based on direct C-H activation that is applicable for example the production of medicinally relevant compounds.

Electrochemical Synthesis of Carbazoles by Dehydrogenative Coupling Reaction

A constant current protocol, employing undivided cells, a remarkably low supporting electrolyte concentration, inexpensive electrode materials, and a straightforward precursor synthesis enabling a novel access to N‐protected carbazoles by anodic N,C bond formation using directly generated amidyl radicals is reported. Scalability of the reaction is demonstrated and an easy deblocking of the benzoyl protecting group is presented.

Electrosynthesis of 3,3′,5,5’-Tetramethyl-2,2′-biphenol in Flow

3,3′,5,5’-Tetramethyl-2,2′-biphenol is well known as an outstanding building block for ligands in transition-metal catalysis and is therefore of particular industrial interest. The electro-organic method is a powerful, sustainable, and efficient alternative to conventional synthetic approaches to obtain symmetric and non-symmetric biphenols. Here, we report the successive scale-up of the dehydrogenative anodic homocoupling of 2,4-dimethylphenol (4) from laboratory scale to the technically relevant scale in highly modular narrow gap flow electrolysis cells. The electrosynthesis was optimized in a manner that allows it to be easily adopted to different scales such as laboratory, semitechnical and technical scale. This includes not only the synthesis itself and its optimization but also a work-up strategy of the desired biphenols for larger scale. Furthermore, the challenges such as side reactions, heat development and gas evolution that arose during optimization are also discussed in detail. We have succeeded in obtaining yields of up to 62% of the desired biphenol.

Electrode Materials in Modern Organic Electrochemistry

The choice of electrode material is critical for achieving optimal yields and selectivity in synthetic organic electrochemistry. The material imparts significant influence on the kinetics and thermodynamics of electron transfer, and frequently defines the success or failure of a transformation. Electrode processes are complex and so the choice of a material is often empirical and the underlying mechanisms and rationale for success are unknown. In this review, we aim to highlight recent instances of electrode choice where rationale is offered, which should aid future reaction development.

Electrochemical N-Demethylation of 14-Hydroxy Morphinans: Sustainable Access to Opioid Antagonists

The most challenging step in the preparation of many opioid antagonists is the selective N-demethylation of a 14-hydroxymorphinan precursor. This process is carried out on a large scale using stoichiometric amounts of hazardous chemicals like cyanogen bromide or chloroformates. We have developed a mild reagent- and catalyst-free procedure for the N-demethylation step based on the anodic oxidation of the tertiary amine. The ensuing intermediates can be readily hydrolyzed to the target nor-opioids in very good yields.

Recent Advances in Asymmetric Catalytic Electrosynthesis

The renewed interest in electrosynthesis demonstrated by organic chemists in the last years has allowed for rapid development of new methodologies. In this review, advances in enantioselective electrosynthesis that rely on catalytic amounts of organic or metal-based chiral mediators are highlighted with focus on the most recent developments up to July 2020. Examples of C-H functionalization, alkene functionalization, carboxylation and cross-electrophile couplings are discussed, along with their related mechanistic aspects.

Electrochemical Oxidation of Organic Molecules at Lower Overpotential: Accessing Broader Functional Group Compatibility with Electron-Proton Transfer Mediators

Electrochemical organic oxidation reactions are highly appealing because protons are often effective terminal electron acceptors, thereby avoiding undesirable stoichiometric oxidants. These reactions are often plagued by high overpotentials, however, that greatly limit their utility. Single-electron transfer (SET) from organic molecules generates high-energy radical-cations. Formation of such intermediates often requires electrode potentials far above the thermodynamic potentials of the reaction and frequently causes decomposition and/or side reactions of ancillary functional groups. In this Account, we show how electrocatalytic electron-proton transfer mediators (EPTMs) address this challenge. EPTMs bypass the formation of radical-cation intermediates by supporting mechanisms that operate at electrode potentials much lower (≥1 V) than those of analogous direct electrolysis reactions.The stable aminoxyl radical TEMPO (2,2,6,6-tetramethylpiperidine N-oxyl) is an effective mediator for electrochemical alcohol oxidation, and we have employed such processes for applications ranging from pharmaceutical synthesis to biomass conversion. A complementary electrochemical alcohol oxidation method employs a cooperative Cu/TEMPO mediator system that operates at 0.5 V lower electrode potential than the TEMPO-only mediated process. This difference, which arises from a different catalytic mechanism, rationalizes the broad functional group tolerance of Cu/TEMPO-based aerobic alcohol oxidation catalysts.Aminoxyl mediators address long-standing challenges in the "Shono oxidation," an important method for α-C-H oxidation of tertiary amides and carbamates. Shono oxidations are initiated by a high-potential SET step that limits their utility. Aminoxyl-mediated Shono-type oxidations have been developed that operate at much lower potentials and tolerate diverse functional groups. Analogous reactivity underlies α-C-H cyanation of secondary cyclic amines, a new method that enables efficient diversification of piperidine-based pharmaceutical building blocks and preparation of non-natural amino acids.Electrochemical oxidations of benzylic C-H bonds are commonly initiated by SET to generate arene radical cations, but such methods are again plagued by large overpotentials. Mediated electrolysis methods that promote hydrogen-atom-transfer (HAT) from benzylic C-H bonds to Fe-oxo species and phthalimide N-oxyl (PINO) support C-H oxygenation, iodination, and oxidative-coupling reactions. A complementary method merges photochemistry with electrochemistry to achieve amidation of C(sp³)-H bonds. This unique process operates at much lower overpotentials compatible with diverse functional groups.These results have broad implications for organic electrochemistry, highlighting the importance of "overpotential" considerations and the prospects for expanding synthetic utility by using mediators to bypass high-energy outer-sphere electron-transfer mechanisms. Principles demonstrated here for oxidation are equally relevant to electrochemical reductions.

Translating batch electrochemistry to single-pass continuous flow conditions: an organic chemist’s guide

The recent renaissance of electrochemical methods for organic synthesis has also attracted increased interest towards flow electrochemistry as the most suitable scale-up strategy. Many electrochemical methods using flow cells are based on recirculation of the electrolyte solution. However, single-pass processing is very attractive as it permits integration of the electrochemical reaction with other synthetic or purification steps in a continuous stream. Translation of batch electrochemical procedures to single-pass continuous flow cells can be challenging to beginners in the field. Using the electrochemical methoxylation of 4-methylanisole as model, this paper provides newcomers to the field with an overview of the factors that need to be considered to develop a flow electrochemical process, including advantages and disadvantages of operating in galvanostatic and potentiostatic mode in small scale reactions, and the effect of the interelectrode gap, supporting electrolyte concentration and pressure on the reaction performance. A comparison of the reaction efficiency in batch and flow is also presented.

Electrochemical formation of N,N′-diarylhydrazines by dehydrogenative N–N homocoupling reaction

A novel electrochemical access to N,N′-diarylhydrazines is developed using commercial anilines, a simple setup, and an ecologically efficient electrolyte system.

Organic electrosynthesis: from academia to industry

The growing impetus to develop greener and more cost-efficient synthetic methods has prompted chemists to look for new ways to activate small organic molecules. In this review, we cover the most recent industrial developments in electrosynthesis.

A review of asymmetric synthetic organic electrochemistry and electrocatalysis: concepts, applications, recent developments and future directions

The direct exploitation of ‘electrons’ as reagents in synthetic organic transformations is on the verge of a renaissance by virtue of its greenness, sustainability, atom economy, step economy and inherent safety. Achieving stereocontrol in such organic electrochemical reactions remains a major synthetic challenge and hence demands great expertise. This review provides a comprehensive discussion of the details of stereoselective organic electrochemical reactions along with the synthetic accomplishments achieved with these methods.

Bipolar Electrochemistry: A Powerful Tool for Electrifying Functional Material Synthesis

Electrosynthesis is a powerful method for the synthesis of organic, inorganic, and polymeric materials based on electron-transfer-driven reactions at the substrate/electrode interface. The use of electricity for synthetic reactions without the need for hazardous chemical oxidants and reductants is recognized as a green and sustainable method. Other advantages include control of the reaction selectivity by tuning the electrode potentials. A different mode for driving electrochemical reactions has recently been proposed, in which bipolar electrodes (BPEs) are available as wireless electrodes that undergo anodic and cathodic reactions simultaneously. Bipolar electrochemistry is an old technology that has recently garnered renewed attention because of the interesting features of BPEs: (i) the wireless nature of a BPE is useful for sensors and material synthesis; (ii) the gradient potential distribution on BPEs is a powerful tool for the preparation of gradient surfaces and materials; and (iii) electrophoresis is available for effective electrolysis. In addition to these unique features, a BPE system only requires a small amount of supporting electrolyte in principle, whereas a large amount of electrolyte is necessary in conventional electrochemistry. Hence, bipolar electrochemistry is an inherently green and sustainable chemical process for the synthesis of materials. In this Account, recent progress in bipolar electrochemistry for the electrosynthesis of functional materials is summarized. The wireless nature of BPEs was utilized for symmetry breaking to produce anisotropic materials based on the site-selective modification of conductive objects by electrodeposition and electropolymerization. Potential gradients on a BPE interface have been successfully used as controllable templates to form molecular or polymeric gradient materials, which are potentially applicable for high throughput analytical equipment or as biomimetic materials. The electric field necessary to drive BPEs is also potentially useful to induce the directed migration of charged species. The synergetic effects of electrophoresis and electrolysis were also successfully demonstrated to obtain various functional materials. These features of bipolar electrochemistry and the various combinations of techniques have the potential to change the methodologies of material synthesis. Furthermore, the fundamental principle of bipolar electrochemistry infers that very small amounts of supporting electrolyte are necessary for an electrode system, which is expected to lead new methods of sustainable organic electrosynthesis.

Recent Advances in Oxidative R1-H/R2-H Cross-Coupling with Hydrogen Evolution via Photo-/Electrochemistry

Photo-/electrochemical catalyzed oxidative R¹-H/R²-H cross-coupling with hydrogen evolution has become an increasingly important issue for molecular synthesis. The dream of construction of C-C/C-X bonds from readily available C-H/X-H with release of H₂ can be facilely achieved without external chemical oxidants, providing a greener model for chemical bond formation. Given the great influence of these reactions in organic chemistry, we give a summary of the state of the art in oxidative R¹-H/R²-H cross-coupling with hydrogen evolution via photo/electrochemistry, and we hope this review will stimulate the development of a greener synthetic strategy in the near future.

Radical-Type Reactivity and Catalysis by Single-Electron Transfer to or from Redox-Active Ligands

Controlled ligand-based redox-activity and chemical non-innocence are rapidly gaining importance for selective (catalytic) processes. This Concept aims to provide an overview of the progress regarding ligand-to-substrate single-electron transfer as a relatively new mode of operation to exploit ligand-centered reactivity and catalysis based thereon.

Electrooxidative Amination of sp2 C-H Bonds: Coupling of Amines with Aryl Amides via Copper Catalysis

Metal-catalyzed cross-coupling reactions are among the most important transformations in organic synthesis. However, the use of C-H activation for sp² C-N bond formation remains one of the major challenges in the field of cross-coupling chemistry. Described herein is the first example of the synergistic combination of copper catalysis and electrocatalysis for aryl C-H amination under mild reaction conditions in an atom-and step-economical manner with the liberation of H₂ as the sole and benign byproduct.

Electrochemical study of fast blue BB. A green strategy for sulfination of fast blue BB

This research has led to the development of an environmentally friendly method for the synthesis of some new sulfone derivatives with high yields and without using any toxic reagents and solvents.

Electrochemical strategies for C-H functionalization and C-N bond formation

Conventional methods for carrying out carbon-hydrogen functionalization and carbon-nitrogen bond formation are typically conducted at elevated temperatures, and rely on expensive catalysts as well as the use of stoichiometric, and perhaps toxic, oxidants. In this regard, electrochemical synthesis has recently been recognized as a sustainable and scalable strategy for the construction of challenging carbon-carbon and carbon-heteroatom bonds. Here, electrosynthesis has proven to be an environmentally benign, highly effective and versatile platform for achieving a wide range of nonclassical bond disconnections via generation of radical intermediates under mild reaction conditions. This review provides an overview on the use of anodic electrochemical methods for expediting the development of carbon-hydrogen functionalization and carbon-nitrogen bond formation strategies. Emphasis is placed on methodology development and mechanistic insight and aims to provide inspiration for future synthetic applications in the field of electrosynthesis.

Modern Electrochemical Aspects for the Synthesis of Value-Added Organic Products

The use of electricity instead of stoichiometric amounts of oxidizers or reducing agents in synthesis is very appealing for economic and ecological reasons, and represents a major driving force for research efforts in this area. To use electron transfer at the electrode for a successful transformation in organic synthesis, the intermediate radical (cation/anion) has to be stabilized. Its combination with other approaches in organic chemistry or concepts of contemporary synthesis allows the establishment of powerful synthetic methods. The aim in the 21st Century will be to use as little fossil carbon as possible and, for this reason, the use of renewable sources is becoming increasingly important. The direct conversion of renewables, which have previously mainly been incinerated, is of increasing interest. This Review surveys many of the recent seminal important developments which will determine the future of this dynamic emerging field.

Electrochemically modified Corey-Fuchs reaction for the synthesis of arylalkynes. The case of 2-(2,2-dibromovinyl)naphthalene

The electrochemical reduction of 2-(2,2-dibromovinyl)naphthalene in a DMF solution (Pt cathode) yields selectively 2-ethynylnaphthalene or 2-(bromoethynyl)naphthalene in high yields, depending on the electrolysis conditions. In particular, by simply changing the working potential and the supporting electrolyte, the reaction can be directed towards the synthesis of the terminal alkyne (Et₄NBF₄) or the bromoalkyne (NaClO₄). This study allowed to establish that 2-(bromoethynyl)naphthalene can be converted into 2-ethynylnaphthalene by cathodic reduction.