Reproduzierbarkeit in der elektroorganischen Synthese – Mythen und Missverständnisse

Electrochemical Dehydration of Dicarboxylic Acids to Their Cyclic Anhydrides

An intramolecular electrochemical dehydration reaction of dicarboxylic acids to their cyclic anhydrides is presented. This electrolysis allows dicarboxylic acids as naturally abundant, inexpensive, safe, and readily available starting materials to be transformed into carboxylic anhydrides under mild reaction conditions. No conventional dehydration reagent is required. The obtained cyclic anhydrides are highly valuable reagents in organic synthesis, and in this report, we use them in-situ for acylation reactions of amines to synthesize amides. This work is part of the recent progress in electrochemical dehydration, which - in contrast to electrochemical dehydrogenative reactions for example - is an underexplored field of research. The reaction mechanism was investigated by 18O isotope labeling, revealing the formation of sulfate by electrochemical oxidation and hydrolysis of the thiocyanate-supporting electrolyte. This transformation is not a classical Kolbe electrolysis, because it is non-decarboxylative, and all carbon atoms of the carboxylic acid starting material are contained in the carboxylic anhydride. In total, 20 examples are shown with NMR yields up to 71 %.

Electrochemical Multicomponent Synthesis of Alkyl Alkenesulfonates using Styrenes, SO2 and Alcohols

A novel electrochemical approach to access alkyl alkenesulfonates via a multicomponent reaction was developed. The metal-free method features easy-to-use SO2 stock solution forming monoalkylsulfites from alcohols with an auxiliary base in-situ. These intermediates serve a dual role as starting materials and as supporting electrolyte enabling conductivity. Anodic oxidation of the substrate styrene, radical addition of these monoalkylsulfites and consecutive second oxidation and deprotonation preserve the double bond and form alkyl β-styrenesulfonates in a highly regio- and stereoselective fashion. The feasibility of this electrosynthetic method is demonstrated in 44 examples with yields up to 81 %, employing various styrenes and related substrates as well as a diverse set of alcohols. A gram-scale experiment underlines the applicability of this process, which uses inexpensive and readily available electrode materials.

Selective Electrochemical Degradation of Lignosulfonate to Bio-Based Aldehydes

A sustainable electrochemical pathway for degradation and thermal treatment of technical lignosulfonate is presented. This approach is an opportunity to produce remarkable quantities of low molecular weight compounds, such as vanillin and acetovanillone. For the electrochemical degradation, a simple two-electrode arrangement in aqueous media is used, which is also easily scalable. The oxidation of the biopolymer occurs at the anode whereas hydrogen is evolved at the cathode. The subsequent thermal treatment supports the degradation of the robust chemical structure of lignosulfonates. With optimized electrolytic conditions, vanillin could be obtained in 9.7 wt% relative to the dry mass of lignosulfonate used. Aside from vanillin, by-products such as acetovanillone or vanillic acid were observed in lower yields. A new and reliable one-pot, two-step degradation of different technically relevant lignosulfonates is established with the advantages of using electrons as an oxidizing agent, which results in low quantities of reagent waste.

Electrooxidative Activation of B-B Bond in B2 cat2 : Access to gem-Diborylalkanes via Paired Electrolysis

This report describes the unprecedented electrooxidation of a solvent (e.g., DMF)-ligated B₂ cat₂ complex, whereby a solvent-stabilized boryl radical is formed via quasi-homolytic cleavage of the B-B bond in a DMF-ligated B₂ cat₂ radical cation. Cyclic voltammetry and density functional theory provide evidence to support this novel B-B bond activation strategy. Furthermore, a strategy for the electrochemical gem-diborylation of gem-bromides via paired electrolysis is developed for the first time, affording a range of versatile gem-diborylalkanes, which are widely used in synthetic society. Notably, this reaction approach is scalable, transition-metal-free, and requires no external activator.

Electroreduction Enables Regioselective 1,2-Diarylation of Alkenes with Two Electrophiles

Precisely introducing two similar functional groups into bulk chemical alkenes represents a formidable route to complex molecules. Especially, the selective activation of two electrophiles is in crucial demand, yet challenging for cross-electrophile-coupling. Herein, we demonstrate a redox-mediated electrolysis, in which aryl nitriles are both aryl radical precursors and redox-mediators, enables an intermolecular alkene 1,2-diarylation with a remarkable regioselectivity, thereby avoiding the involvement of transition-metal catalysts. This transformation utilizes cyanoarene radical anions for activating various aryl halides (including iodides, bromides, and even chlorides) and affords 1,2-diarylation adducts in up to 83 % yield and >20 : 1 regioselectivity with more than 80 examples, providing a feasible approach to complex bibenzyl derivatives.

Direct Electrochemical Synthesis of 2,3-Disubstituted Quinoline N-oxides by Cathodic Reduction of Nitro Arenes

The use of electric current in synthetic organic chemistry offers a sustainable tool for the selective reductive synthesis of quinoline N-oxides starting from easily accessible nitro compounds. The reported method employs mild and reagent-free conditions, a simple undivided cell, and constant current electrolysis set-up which provides conversion with a high atom economy. The synthesis of 30 differently substituted quinoline N-oxides was successfully performed in up to 90 % yield. Using CV studies, the mechanism of the selective formation of the quinoline N-oxides was elucidated. The technical relevance of the described reaction could be shown in a 50-fold scale-up reaction.

Electrochemical Synthesis of Pyrazolines and Pyrazoles via [3+2] Dipolar Cycloaddition

Pyrazolines and pyrazoles are common and important motifs of pharmaceutical agents and agrochemicals. Herein, the first electrochemical approach for their direct synthesis from easily accessible hydrazones and dipolarophiles up to decagram scale is presented. The application of a biphasic system (aqueous/organic) even allows for the conversion of highly sensitive alkenes, wherein inexpensive sodium iodide is employed in a dual role as supporting electrolyte and mediator. In addition, mechanistic insight into the reaction is given by the isolation of key step intermediates. The relevance of the presented reaction is underlined by the synthesis of commercial herbicide safener mefenpyr-diethyl in good yields.

A Liquid-Liquid-Solid System to Manipulate the Cascade Reaction for Highly Selective Electrosynthesis of Aldehyde

Electrocatalytic synthesis of aldehydes from alcohols exhibits unique superiorities as a promising technology, in which cascade reactions are involved. However, the cascade reactions are severely limited by the low selectivity resulting from the peroxidation of aldehydes in a traditional liquid-solid system. Herein, we report a novel liquid-liquid-solid system to regulate the selectivity of benzyl alcohol electrooxidation. The selectivity of benzaldehyde increases 200-fold from 0.4 % to 80.4 % compared with the liquid-solid system at a high current density of 136 mA cm^-2 , which is the highest one up to date. In the tri-phase system, the benzaldehyde peroxidation is suppressed efficiently, with the conversion of benzaldehyde being decreased from 87.6 % to 3.8 %. The as-produced benzaldehyde can be in situ extracted to toluene phase and separated from the electrolyte to get purified benzaldehyde. This strategy provides an efficient way to efficiently enhance the selectivity of electrocatalytic cascade reactions.

Controlling Selectivity in Shuttle Hetero-difunctionalization Reactions: Electrochemical Transfer Halo-thiolation of Alkynes

Shuttle hetero-difunctionalization reaction, in which two chemically distinct functional groups are transferred between two molecules, has long been an unmet goal due to the daunting challenges in controlling the chemo-, regio-, and stereoselectivity. Herein, we disclose an electrochemistry enabled shuttle reaction (e-shuttle) to selectively transfer one RS^- and one X^- group between β-halosulfides and unsaturated hydrocarbons via a consecutive paired electrolysis mechanism. The preferential anodic oxidation of one anion over the other, which is controlled by their distinct redox potentials, plays a pivotal role in controlling the high chemoselectivity of the process. This easily scalable methodology enables the construction of a myriad of densely functionalized β-halo alkenyl sulfides in unprecedented chemo-, regio-, and stereoselectivity using benign surrogates, e.g., 2-bromoethyl sulfide, avoiding the handling of corrosive and oxidative RS-Br reagents. In a broader context, these results open up new strategies for selective shuttle difunctionalization reactions.

Counter Electrode Reactions-Important Stumbling Blocks on the Way to a Working Electro-organic Synthesis

Over the past two decades, electro-organic synthesis has gained significant interest, both in technical and academic research as well as in terms of applications. The omission of stoichiometric oxidizers or reducing agents enables a more sustainable route for redox reactions in organic chemistry. Even if it is well-known that every electrochemical oxidation is only viable with an associated reduction reaction and vice versa, the relevance of the counter reaction is often less addressed. In this Review, the importance of the corresponding counter reaction in electro-organic synthesis is highlighted and how it can affect the performance and selectivity of the electrolytic conversion. A selection of common strategies and unique concepts to tackle this issue are surveyed to provide a guide to select appropriate counter reactions for electro-organic synthesis.

A General Electro‐Synthesis Approach to Amaryllidaceae Alkaloids

Amaryllidaceae alkaloids appeal to organic chemists with their attractive structures and their impressive antitumor and acetylcholinesterase inhibitory properties. We demonstrate a highly versatile access to this family of natural products. A general protocol with high yields in a sustainable electro‐organic key transformation on a metal‐free anode to spirodienones facilitates functionalization to the alkaloids. The biomimetic syntheses start with the readily available, inexpensive biogenic starting materials methyl gallate, O‐methyl tyramine, and vanillin derivatives. Through known dynamic resolutions, this technology provides access to both enantiomeric series of (epi‐)martidine, (epi‐)crinine, siculine, and galantamine, clinically prescribed for the treatment of Alzheimer's disease.

Robust and Self-Cleaning Electrochemical Production of Periodate

Periodate, a platform oxidizer, can be electrochemically recycled in a self-cleaning process. Electrosynthesis of periodate is well established at boron-doped diamond (BDD) anodes. However, recovered iodate and other iodo species for recycling can contain traces of organic impurities from previous applications. For the first time, it was shown that the organic impurities do not hamper the electrochemical re-oxidation of used periodate. In a hydroxyl-mediated environment, the organic compounds form CO₂ and H₂ O during the degradation process. This process is often referred to as "cold combustion" and provides orthogonal conditions to periodate synthesis. To demonstrate the strategy, different dyes, pharmaceutically active ingredients, and iodine compounds were added as model contaminations into the process of electrochemical periodate production. UV/Vis spectroscopy, NMR spectroscopy, and mass spectrometry (MS) were used to monitor the degradation of organic molecules, and liquid chromatography-MS was used to control the purity of periodate. As a representative example, dimethyl 5-iodoisophthalate (2 mm), was degraded in 90, 95, and 99 % while generating 0.042, 0.054, and 0.082 kilo equiv. of periodate, respectively. In addition, various organic iodo compounds could be fed into the periodate generation for upcycling such iodo-containing waste, for example, contrast media.

Oxidation Potential Gap (ΔEox ): The Hidden Parameter in Redox Chemistry

Oxidative biaryl coupling of aryls with different electronic features generally fails. However, this has not been systematically studied via theoretical analysis, and thus, the crucial factor governing coupling efficiency remains unclear. Herein, we propose that the "oxidation potential gap (ΔE_ox )" is a key parameter in predicting the efficiency of an intramolecular oxidative coupling reaction, with ΔE_ox defined as a difference in the oxidation potentials of the relevant aromatic rings. Our experimental and computational analyses revealed that the efficiency of an aromatic intramolecular coupling reaction correlates with the activation energy (ΔE^≠ ) of C-C bond formation of the radical cation intermediates. Furthermore, ΔE^≠ correlates with ΔE_ox . Therefore, we demonstrate the tuning of ΔE_ox by attaching cleavable extra electron-donating/-withdrawing groups, enabling the rational synthesis of a phenanthridone skeleton using aromatic rings with an electronic gap.

Direct and Selective Electrocarboxylation of Styrene Oxides with CO2 for Accessing β-Hydroxy Acids

Highly selective and direct electroreductive ring-opening carboxylation of epoxides with CO2 in an undivided cell is reported. This reaction shows broad substrate scopes within styrene oxides under mild conditions, providing practical and scalable access to important synthetic intermediate β-hydroxy acids. Mechanistic studies show that CO2 functions not only as a carboxylative reagent in this reaction but also as a promoter to enable efficient and chemoselective transformation of epoxides under additive-free electrochemical conditions. Cathodically generated α-radical and α-carbanion intermediates lead to the regioselective formation of α-carboxylation products.

Beyond Kolbe and Hofer-Moest: Electrochemical Synthesis of Carboxylic Anhydrides from Carboxylic Acids

Herein we report a conceptually new non-decarboxylative electrolysis of carboxylic acids to obtain their corresponding anhydrides as highly valuable reagents in organic synthesis. All carbon atoms of the starting material are preserved in the product in an overall redox-neutral reaction. In a broad substrate scope of carboxylic acids the anhydrides are generated with high selectivity, which demonstrates the versatility of the developed method. Beneficially, no dehydrating reagents are required in comparison to conventional methods and the synthesis is based on uncritical starting materials using graphite and stainless steel as very inexpensive and eco-friendly electrode materials.

Lessons from an Array: Using an Electrode Surface to Control the Selectivity of a Solution-Phase Chemical Reaction

Electrochemistry offers a variety of novel means by which selectivity can be introduced into synthetic organic transformations. In the work reported, it is shown how methods used to confine chemical reactions to specific sites on a microelectrode array can also be used to confine a preparative reaction to the surface of an electrode inserted into a bulk reaction solution. In so doing, the surface of a modified electrode can be used to introduce new selectivity into a preparative reaction that is not observed in the absence of either the modified electrode surface or the effort to confine the reaction to that surface. The observed selectivity can be optimized in the same way that confinement is optimized on an array and is dependent on the nature of the functionalized surface.

Trifluoroethanol as a Unique Additive for the Chemoselective Electrooxidation of Enamines to Access Unsymmetrically Substituted NH-Pyrroles

An electrochemical method for the synthesis of unsymmetrically substituted NH-pyrroles is described. The synthetic strategy comprises a challenging heterocoupling between two structurally diverse enamines via sequential chemoselective oxidation, addition, and cyclization processes. A series of aryl- and alkyl-substituted enamines were effectively cross-coupled from an equimolar mixture to synthesize various unsymmetrical pyrrole derivatives up to 84 % yield. The desired cross-coupling was achieved by tuning the oxidation potential of the enamines by utilizing a "magic effect" of the additive trifluoroethanol (TFE). Additionally, extensive computational studies reveal the unique role of TFE in promoting the heterocoupling process by regulating the activation energies of the reaction steps through H-bonding and C-H⋅⋅⋅π interactions. Importantly, the developed electrochemical protocol was found to be equally efficient for the homocoupling of enamines to form symmetric pyrroles up to 92 % yield.

Anodic Dehydrogenative Cyanamidation of Thioethers: Simple and Sustainable Synthesis of N-Cyanosulfilimines

A novel and very simple to perform electrochemical approach for the synthesis of several N-cyanosulfilimines in good to excellent yields was established. This method provides access to biologically relevant sulfoximines by consecutive oxidation using electro-generated periodate. This route can be easily scaled-up to gram quantities. The S,N coupling is carried out at an inexpensive carbon anode by direct oxidation of sulfide. Therefore, the designed process is atom economic and represents a new "green route" for the synthesis of sulfilimines and sulfoximines.