Elektrifizierung der organischen Synthese

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.

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.

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

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

Electrochemical synthesis of biaryls by reductive extrusion from N,N'-diarylureas

The synthesis of biaryl compounds by the transition-metal free coupling of arenes is an important contemporary challenge, aiming to avoid the toxicity and cost profiles associated with the metal catalysts commonly used in the synthesis of these pharmaceutically relevant motifs. In this paper, we describe an electrochemical approach to the synthesis of biaryls in which aniline derivatives are coupled through the formation and reduction of a temporary urea linkage. The conformational alignment of the arenes in the N,N'-diaryl urea intermediates promotes C-C bond formation following single-electron reduction. Our optimized conditions are suitable for the synthesis of a variety of biaryls, including sterically hindered examples carrying ortho-substituents, representing complementary reactivity to most metal catalysed methods.

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.

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.

Peroxodicarbonate as a Green Oxidizer for the Selective Degradation of Kraft Lignin into Vanillin

Lignin, the world's largest resource of renewable aromatics, with annually roughly 50 million tons of accruing technical lignin, mainly Kraft lignin, is highly underdeveloped regarding the production of monoaromatics. We demonstrate the oxidative depolymerization of Kraft lignin at 180 °C to produce vanillin 1 in yields up to 6.2 wt % and 92 % referred to the maximum yield gained from the quantification reaction utilizing nitrobenzene. Using peroxodicarbonate (C₂ O₆ ^2- ) as "green" oxidizer for the degradation, toxic and/or harmful reagents are prevented. Also, the formed waste can serve as makeup chemical in the pulping process. Na₂ C₂ O₆ is synthesized in an ex-cell electrolysis of aqueous Na₂ CO₃ at BDD anodes, achieving a yield of Na₂ C₂ O₆ with 41 %. At least, the oxidation and degradation of Kraft lignin is analysis via UV/Vis and NMR spectroscopy.

Preferential Adsorption of Ethylene Oxide on Fe and Chlorine on Ni Enabled Scalable Electrosynthesis of Ethylene Chlorohydrin

Industrial manufacturing of ethylene chlorohydrin (ECH) critically requires excess corrosive hydrochloric acid or hypochlorous acid with dealing with massive by-products and wastes. Here we report a green and efficient electrosynthesis of ECH from ethylene oxide (EO) with NaCl over a NiFe₂ O₄ nanosheet anode. Theoretical results suggest that EO and Cl preferentially adsorb on Fe and Ni sites, respectively, collaboratively promoting the ECH synthesis. A Cl radical-mediated ring-opening process is proposed and confirmed, and the key Cl and carbon radical species are identified by high-resolution mass spectrometry. This strategy can enable scalable electrosynthesis of 185.1 mmol of ECH in 1 h with 92.5 % yield at a 55 mA cm^-2 current density. Furthermore, a series of other chloro- and bromoethanols with good to high yields and paired synthesis of ECH and 4-amino-3,6-dichloropyridine-2-carboxylicacid via respectively loading and unloading Cl are achieved, showing the promising potential of this strategy.

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.

Tunable Electrochemical Peptide Modifications: Unlocking New Levels of Orthogonality for Side-Chain Functionalization

Electrochemical transformations provide enticing opportunities for programmable, residue-specific peptide modifications. Herein, we harness the potential of amidic side-chains as underutilized handles for late-stage modification through the development of an electroauxiliary-assisted oxidation of glutamine residues within unprotected peptides. Glutamine building blocks bearing electroactive side-chain N,S-acetals are incorporated into peptides using standard Fmoc-SPPS. Anodic oxidation of the electroauxiliary in the presence of diverse alcohol nucleophiles enables the installation of high-value N,O-acetal functionalities. Proof-of-principle for an electrochemical peptide stapling protocol, as well as the functionalization of dynorphin B, an endogenous opioid peptide, demonstrates the applicability of the method to intricate peptide systems. Finally, the site-selective and tunable electrochemical modification of a peptide bearing two discretely oxidizable sites is achieved.

Electrophotochemical Metal-Catalyzed Enantioselective Decarboxylative Cyanation

In contrast to the rapid growth of electrophotocatalysis in recent years, enantioselective catalytic reactions powered by this unique methodology remain rare. In this work, we report an electrophotochemical metal-catalyzed protocol for direct asymmetric decarboxylative cyanation of aliphatic carboxylic acids. The synergistic merging of electrophotochemical cerium catalysis and asymmetric electrochemical copper catalysis permits mild reaction conditions for the formation and utilization of the key carbon centered radicals by combining the power of light and electrical energy. Electrophotochemical cerium catalysis enables radical decarboxylation to produce alkyl radicals, which could be effectively intercepted by asymmetric electrochemical copper catalysis for the construction of C-CN bonds in a highly stereoselective fashion. This environmentally benign method smoothly converts a diverse array of arylacetic acids into the corresponding alkyl nitriles in good yields and enantioselectivities without using chemical oxidants or pre-functionalization of the acid substrates and can be readily scaled up.

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.

Electrochemical Synthesis of gem-Difluoro- and γ-Fluoro-Allyl Boronates and Silanes

The electrochemical synthesis of fluorinated allyl silanes and boronates was disclosed. The addition of electrogenerated boryl or silyl radicals onto many α-trifluoromethyl or α-difluoromethylstyrenes in an undivided cell allowed the formation of a large panel of synthetically useful gem-difluoro and γ-fluoroallyl boronates and silanes (64 examples, from 31 % to 95 % yield). In addition, a scale up of the reactions under continuous flow was showcased using an electrochemical reactor with promising volumetric productivity (688 g.L-1 .h-1 and 496 g.L-1 .h-1 ). Moreover, the synthetic utility of these building blocks was highlighted through versatile transformations. Finally, plausible reaction mechanisms were suggested to explain the formation of the products.

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.

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

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

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.

Visible-Light-Induced, Single-Metal-Catalyzed, Directed C-H Functionalization: Metal-Substrate-Bound Complexes as Light-Harvesting Agents

C-H functionalization represents one of the most rapidly advancing areas in organic synthesis and is regarded as one of the key concepts to minimize the ecological and economic footprint of organic synthesis. The ubiquity and low reactivity of C-H bonds in organic molecules, however, poses several challenges, and often necessitates harsh reaction conditions to achieve this goal, although it is highly desirable to achieve C-H functionalization reactions under mild conditions. Recently, several reports uncovered a conceptually new approach towards C-H functionalization, where a single transition-metal complex can be used as both the photosensitizer and catalyst to promote C-H bond functionalization in the absence of an exogeneous photosensitizer. In this Minireview, we will provide an overview on recent achievements in C-H functionalization reactions, with an emphasis on the photochemical modulation of the reaction mechanism using such catalysts.

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.

Electrochemical Migratory Cyclization of N-Acylsulfonamides

Benzoxathiazine dioxide, as a bioisostere of the clinically widely used diazoxide, exhibits interesting biological activity. However, limited success has been achieved in terms of its concise and direct synthesis. We report herein a facile electrochemical migratory cyclization of N-acylsulfonamides to access a diverse array of benzoxathiazine dioxides. The inclusion of electrochemistry is crucial for realizing such a novel transformation, which is substantiated both by the experiments and density-functional-theory calculations.

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.

Direct Dehydrogenative Access to Unsymmetrical Phenones

The first non-directed dehydrogenative phenone coupling method of methylarenes with aromatic C-H bonds, displaying a large substrate scope, is herein reported. This reaction represents a far more direct atom- and step-efficient alternative to the classical Friedel-Crafts or Suzuki-Miyaura derived acylation reactions. The method can be carried out on a gram scale and was successfully applied to the synthesis of several Ketoprofen drug analogues.

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

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

Synthetic Molecular Photoelectrochemistry: New Frontiers in Synthetic Applications, Mechanistic Insights and Scalability

Synthetic photoelectrochemistry (PEC) is receiving increasing attention as a new frontier for the generation and handling of reactive intermediates. PEC permits selective single-electron transfer (SET) reactions in a much greener way and broadens the redox window of possible transformations. Herein, the most recent contributions are reviewed, demonstrating exciting new opportunities, namely, the combination of PEC with other reactivity paradigms (hydrogen-atom transfer, radical polar crossover, energy transfer sensitization), scalability up to multigram scale, novel selectivities in SET super-oxidations/reductions and the importance of precomplexation to temporally enable excited radical ion catalysis.