Advanced Electroanalysis for Electrosynthesis

Electrosynthesis is a popular, environmentally friendly substitute for conventional organic methods. It involves using charge transfer to stimulate chemical reactions through the application of a potential or current between two electrodes. In addition to electrode materials and the type of reactor employed, the strategies for controlling potential and current have an impact on the yields, product distribution, and reaction mechanism. In this Review, recent advances related to electroanalysis applied in electrosynthesis were discussed. The first part of this study acts as a guide that emphasizes the foundations of electrosynthesis. These essentials include instrumentation, electrode selection, cell design, and electrosynthesis methodologies. Then, advances in electroanalytical techniques applied in organic, enzymatic, and microbial electrosynthesis are illustrated with specific cases studied in recent literature. To conclude, a discussion of future possibilities that intend to advance the academic and industrial areas is presented.

Self-Standing Metal Foam Catalysts for Cathodic Electro-Organic Synthesis

Although electro-organic synthesis is currently receiving renewed interest because of its potential to enable sustainability in chemical processes to value-added products, challenges in process development persist: For reductive transformations performed in protic media, an inherent issue is the limited choice of metallic cathode materials that can effectively suppress the parasitic hydrogen evolution reaction (HER) while maintaining a high activity toward the targeted electro-organic reaction. Current development trends are aimed at avoiding the previously used HER-suppressing elements (Cd, Hg, and Pb) because of their toxicity. Here, this work reports the rational design of highly porous foam-type binary and ternary electrocatalysts with reduced Pb content. Optimized cathodes are tested in electro-organic reductions using an oxime to nitrile transformation as a model reaction relevant for the synthesis of fine chemicals. Their electrocatalytic performance is compared with that of the model CuSn7Pb15 bronze alloy that has recently been endorsed as the best cathode replacement for bare Pb electrodes. All developed metal foam catalysts outperform both bare Pb and the CuSn7Pb15 benchmark in terms of chemical yield and energetic efficiency. Moreover, post-electrolysis analysis of the crude electrolyte mixture and the cathode's surfaces through inductively coupled plasma mass spectrometry (ICP-MS) and scanning electron microscopy (SEM), respectively, reveal the foam catalysts' elevated resistance to cathodic corrosion.

Electrochemical deoxygenative reduction of ketones

Electrochemical reduction via paired electrolysis has been used to achieve deoxygenative reduction of ketones. As a result of the complexing of ketones with the triphenylphosphine radical cation generated by anodic oxidation, the reduction of carbonyl groups occurs readily. Through spontaneous β-scission of phosphoranyl radicals, C-O bonds are cleaved to form benzylic radical intermediates. These radical species are either able to abstract hydrogen from MeCN or undergo reduction at the cathode to give carbanions, upon workup forming reductive hydrogenation of ketones.

On-line Electrode Dissolution Monitoring during Organic Electrosynthesis: Direct Evidence of Electrode Dissolution during Kolbe Electrolysis

Electrode dissolution was monitored in real-time during Kolbe electrolysis along with the characteristic products. The fast determination of appropriate reaction conditions in electro-organic chemistry enables the minimization of electrode degradation while keeping an eye on the optimal formation rate and distribution of products. Herein, essential parameters influencing the dissolution of the electrode material platinum in a Kolbe electrolysis were pinpointed. The formation of reaction products and soluble platinum species were monitored during potentiodynamic and potentiostatic experiments using an electroanalytical flow cell coupled to two different mass spectrometers. The approach opens new vistas in the field of electro-organic chemistry because it enables precise and quick quantification of dissolved metals during electrosynthesis, also involving electrode materials other than platinum. Furthermore, it draws attention to the vital topic of electrode stability in electro-organic synthesis, which becomes increasingly important for the implementation of green chemical processes utilizing renewable energy.

Synthesis of active pharmaceutical ingredients using electrochemical methods: keys to improve sustainability

Organic electrochemistry is receiving renewed attention as a green and cost-efficient synthetic technology. Electrochemical methods promote redox transformations by electron exchange between electrodes and species in solution, thus avoiding the use of stoichiometric amounts of oxidizing or reducing agents. The rapid development of electroorganic synthesis over the past decades has enabled the preparation of molecules of increasing complexity. Redox steps that involve hazardous or waste-generating reagents during the synthesis of active pharmaceutical ingredients or their intermediates can be substituted by electrochemical procedures. In addition to enhance sustainability, increased selectivity toward the target compound has been achieved in some cases. Electroorganic synthesis can be safely and readily scaled up to production quantities. For this pupose, utilization of flow electrolysis cells is fundamental. Despite these advantages, the application of electrochemical methods does not guarantee superior sustainability when compared with conventional protocols. The utilization of large amounts of supporting electrolytes, enviromentally unfriendly solvents or sacrificial electrodes may turn electrochemistry unfavorable in some cases. It is therefore crucial to carefully select and optimize the electrolysis conditions and carry out green metrics analysis of the process to ensure that turning a process electrochemical is advantageous.

Versatile Tools for Understanding Electrosynthetic Mechanisms

Electrosynthesis is a popular, green alternative to traditional organic methods. Understanding the mechanisms is not trivial yet is necessary to optimize reaction processes. To this end, a multitude of analytical tools is available to identify and quantitate reaction products and intermediates. The first portion of this review serves as a guide that underscores electrosynthesis fundamentals, including instrumentation, electrode selection, impacts of electrolyte and solvent, cell configuration, and methods of electrosynthesis. Next, the broad base of analytical techniques that aid in mechanism elucidation are covered in detail. These methods are divided into electrochemical, spectroscopic, chromatographic, microscopic, and computational. Technique selection is dependent on predicted reaction pathways and electrogenerated intermediates. Often, a combination of techniques must be utilized to ensure accuracy of the proposed model. To conclude, future prospects that aim to enhance the field are discussed.

Cathodic Corrosion of Metal Electrodes-How to Prevent It in Electroorganic Synthesis

The critical aspects of the corrosion of metal electrodes in cathodic reductions are covered. We discuss the involved mechanisms including alloying with alkali metals, cathodic etching in aqueous and aprotic media, and formation of metal hydrides and organometallics. Successful approaches that have been implemented to suppress cathodic corrosion are reviewed. We present several examples from electroorganic synthesis where the clever use of alloys instead of soft neat heavy metals and the application of protective cationic additives have allowed to successfully exploit these materials as cathodes. Because of the high overpotential for the hydrogen evolution reaction, such cathodes can contribute toward more sustainable green synthetic processes. The reported strategies expand the applications of organic electrosynthesis because a more negative regime is accessible within protic media and common metal poisons, e.g., sulfur-containing substrates, are compatible with these cathodes. The strongly diminished hydrogen evolution side reaction paves the way for more efficient reductive electroorganic conversions.

An Electrochemical Approach to Designer Peptide α-Amides Inspired by α-Amidating Monooxygenase Enzymes

Designer C-terminal peptide amides are accessed in an efficient and epimerization-free approach by pairing an electrochemical oxidative decarboxylation with a tandem hydrolysis/reduction pathway. Resembling Nature's dual enzymatic approach to bioactive primary α-amides, this method delivers secondary and tertiary amides bearing high-value functional motifs, including isotope labels and handles for bioconjugation. The protocol leverages the inherent reactivity of C-terminal carboxylates, is compatible with the vast majority of proteinogenic functional groups, and proceeds in the absence of epimerization, thus addressing major limitations associated with conventional coupling-based approaches. The utility of the method is exemplified through the synthesis of natural product acidiphilamide A via a key diastereoselective reduction, as well as bioactive peptides and associated analogues, including an anti-HIV lead peptide and blockbuster cancer therapeutic leuprolide.

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.

Nickel Nanoparticle Catalyzed Mono- and Di-Reductions of gem-Dibromocyclopropanes Under Mild, Aqueous Micellar Conditions

Mild mono- and di-hydrodehalogenative reductions of gem-dibromocyclopropanes are described, providing an easy and green approach towards the synthesis of cyclopropanes. The methodology utilizes 0.5-5 mol % TMPhen-nickel as the catalyst, which, when activated with a hydride source such as sodium borohydride, cleanly and selectively dehalogenates dibromocyclopropanes. Double reduction proceeds in a single operation at temperatures between 20-45 °C and at atmospheric pressure in an aqueous designer surfactant medium. At lower loading and either in the absence of ligand or in the presence of 2,2'-bipyridine, this new technology can also be used to gain access to not only monobrominated cyclopropanes, interesting building blocks for further use in synthesis, but also mono- or di-deuterated analogues. Taken together, this base-metal-catalyzed process provides access to cyclopropyl-containing products and is achieved under environmentally responsible conditions.

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.

Selective and Scalable Electrosynthesis of 2H-2-(Aryl)-benzo[d]-1,2,3-triazoles and Their N-Oxides by Using Leaded Bronze Cathodes

Electrosynthesis of 2H-2-(aryl)benzo[d]-1,2,3-triazoles and their N-oxides from 2-nitroazobenzene derivatives is reported. The electrolysis is conducted in a very simple undivided cell under constant current conditions with a leaded bronze cathode and a glassy carbon anode. The product distribution between 2H-2-(aryl)benzo[d]-1,2,3-triazoles and their N-oxides can be guided by simply controlling the current density and the amount of the charge applied. The reaction tolerates several sensitive functional groups in reductive electrochemistry. The usefulness and the applicability of the synthetic method is demonstrated by a formal synthesis of an antiviral compound.

Electrochemical-Induced Ring Transformation of Cyclic α-(ortho-Iodophenyl)-β-oxoesters

Cyclic α-(ortho-iodophenyl)-β-oxoesters were converted in a ring-expanding transformation to furnish benzannulated cycloalkanone carboxylic esters. The reaction sequence started by electrochemical reduction of the iodoarene moiety. In a mechanistic rationale, the resulting carbanionic species was adding to the carbonyl group under formation of a strained, tricyclic benzocyclobutene intermediate, which underwent carbon-carbon bond cleavage and rearrangement of the carbon skeleton by retro-aldol reaction. The scope of the reaction sequence was investigated by converting cyclic oxoesters with different ring sizes yielding benzocycloheptanone, -nonanone and -decanone derivatives in moderate to good yields. Furthermore, acyclic starting materials and cyclic compounds carrying additional substituents on the iodophenyl ring were submitted to this reaction sequence. The starting materials for this transformation are straightforwardly obtained by conversion of β-oxoesters with phenyliodobis(trifluoroacetate).

About the selectivity and reactivity of active nickel electrodes in C–C coupling reactions

Active anodes which are operating in highly stable protic media such as 1,1,1,3,3,3-hexafluoroisopropanol are rare. Nickel forms, within this unique solvent, a non-sacrificial active anode at constant current conditions, which is superior to the reported powerful molybdenum system. The reactivity for dehydrogenative coupling reactions of this novel active anode increases when the electrolyte is not stirred during electrolysis. Besides the aryl–aryl coupling, a dehydrogenative arylation reaction of benzylic nitriles was found while stirring the mixture providing quick access to synthetically useful building blocks.

Active anodes which are operating in highly stable protic media such as 1,1,1,3,3,3-hexafluoroisopropanol are rare.

Hydrodehalogenation of Aryl Halides through Direct Electrolysis

A catalyst- and metal-free electrochemical hydrodehalogenation of aryl halides is disclosed. Our reaction by a flexible protocol is operated in an undivided cell equipped with an inexpensive graphite rod anode and cathode. Trialkylamines nBu₃ N/Et₃ N behave as effective reductants and hydrogen atom donors for this electrochemical reductive reaction. Various aryl and heteroaryl bromides worked effectively. The typically less reactive aryl chlorides and fluorides can also be smoothly converted. The utility of our method is demonstrated by detoxification of harmful pesticides and hydrodebromination of a dibrominated biphenyl (analogues of flame-retardants) in gram scale.

Selective Functionalization of Styrenes with Oxygen Using Different Electrode Materials: Olefin Cleavage and Synthesis of Tetrahydrofuran Derivatives

Electrode materials can have a significant impact on the course of an electrolysis reaction. Of particular interest is that different electrodes can generate different products from the same substrate. The electrode-material-selective transformations of styrene derivatives with molecular oxygen are reported. Platinum electrodes afford carbonyl products via cleavage of olefins, whereas tetrahydrofuran formation is achieved with carbon electrodes. A variety of different styrenes are available for both reactions. Electrolysis allows straightforward and mild chemical conversions that are metal- and oxidant-free. Electrochemical measurements illuminate the different effects of platinum and carbon electrodes on styrenes. The key to the differing reactions is probably that the oxidation potentials of the substrates are lower (higher HOMO energy) on carbon electrodes than on platinum electrodes. The adsorption of the substrates on carbon electrodes can also promote tetrahydrofuran formation.

Switchable Interfaces: Redox Monolayers on Si(100) by Electrochemical Trapping of Alcohol Nucleophiles

Organic electrosynthesis is going through its renaissance but its scope in surface science as a tool to introduce specific molecular signatures at an electrode/electrolyte interface is under explored. Here, we have investigated an electrochemical approach to generate in situ surface-tethered and highly-reactive carbocations. We have covalently attached an alkoxyamine derivative on an Si(100) electrode and used an anodic bias stimulus to trigger its fragmentation into a diffusive nitroxide (TEMPO) and a surface-confined carbocation. As a proof-of-principle we have used this reactive intermediate to trap a nucleophile dissolved in the electrolyte. The nucleophile was ferrocenemethanol and its presence and surface concentration after its reaction with the carbocation were assessed by cyclic voltammetry. The work expands the repertoire of available electrosynthetic methods and could in principle lay the foundation for a new form of electrochemical lithography.

Synthetic Organic Electrochemistry: Calling All Engineers

Unmet potential: Electrochemistry is the most simple and basic way of altering the redox-states of organic molecules. Despite extensive studies and its demonstrated promise, it has yet to take off in mainstream synthesis. The reason is due to engineering challenges in instrument design.

A mild electroassisted synthesis of (hetero)arylphosphonates

The first example of a nickel-catalyzed electrochemical coupling between dimethyl phosphite and (hetero)aryl halides to furnish (hetero)arylphosphonates is described.

Synthetic Organic Electrochemical Methods Since 2000: On the Verge of a Renaissance

Electrochemistry represents one of the most intimate ways of interacting with molecules. This review discusses advances in synthetic organic electrochemistry since 2000. Enabling methods and synthetic applications are analyzed alongside innate advantages as well as future challenges of electroorganic chemistry.

Anti-addition of Dimethylsulfoxonium Methylide to Acyclic α,β-Unsaturated Ketones and Its Application in Formal Synthesis of an Eicosanoid

Cyclopropanation using dimethylsulfoxonium methylide (Corey-Chaykovsky reaction) was examined with a series of linear α,β-unsaturated ketones, and the results showed that the major trajectory for the addition of the sulfur ylide to the enones is anti, related to the γ-substituent. The stereochemical assignment for the generated cyclopropanes was achieved by X-ray crystallography or comparing with the reported spectroscopic data. We found that the diastereoselectivity was influenced by several factors, including the protecting groups, solvents, and temperatures, and good anti/syn ratios (>10:1) were often obtained using the tert-butyldimethylsilyl and tert-butyldiphenylsilyl-protected substrates. The method was applied to a formal synthesis of a natural eicosanoid with good efficiency.

Insights into the Mechanism of Anodic N-N Bond Formation by Dehydrogenative Coupling

The electrochemical synthesis of pyrazolidine-3,5-diones and benzoxazoles by N-N bond formation and C,O linkage, respectively, represents an easy access to medicinally relevant structures. Electrochemistry as a key technology ensures a safe and sustainable approach. We gained insights in the mechanism of these reactions by combining cyclovoltammetric and synthetic studies. The electron-transfer behavior of anilides and dianilides was studied and led to the following conclusion: The N-N bond formation involves a diradical as intermediate, whereas the benzoxazole formation is based on a cationic mechanism. Besides these studies, we developed a synthetic route to mixed dianilides as starting materials for the N-N coupling. The compatibility with valuable functionalities like triflates and mesylates for follow-up reactions as well as the comparison of different electrochemical set-ups also enhanced the applicability of this method.

The Hitchhiker's Guide to Flow Chemistry ∥

Flow chemistry involves the use of channels or tubing to conduct a reaction in a continuous stream rather than in a flask. Flow equipment provides chemists with unique control over reaction parameters enhancing reactivity or in some cases enabling new reactions. This relatively young technology has received a remarkable amount of attention in the past decade with many reports on what can be done in flow. Until recently, however, the question, "Should we do this in flow?" has merely been an afterthought. This review introduces readers to the basic principles and fundamentals of flow chemistry and critically discusses recent flow chemistry accounts.

Electrochemical synthesis of benzoxazoles from anilides – a new approach to employ amidyl radical intermediates

The electroorganic synthesis of a variety of benzoxazoles starting from easy accessible anilides is reported.

Electrochemical Allylic Oxidation of Olefins: Sustainable and Safe

The power you're supplying: With the application of an optimized electrochemical approach, the allylic oxidation of olefins, which is an important C-H activation process that provides access to enones, becomes a sustainable, versatile, and potent key reaction for organic synthesis.