Electroorganic Synthesis under Flow Conditions

Electrochemical Selenized Reaction of N-Arylbicyclo[1.1.0]butane-1-carboxamides: Access to 3-(Arylselanyl)spiro[cyclobutane-1,3'-indolin]-2'-one Derivatives

A novel selenized reaction of N-arylbicyclo [1.1.0]butane-1-carboxamides with diselenide for the synthesis of polycyclic indoline derivatives is developed under electrochemical conditions. The synthesis is achieved by the bicyclo[1.1.0]butane strain-release reaction and intramolecular cyclization process. In addition, this approach features a wide range of substrates, good group tolerance, shorter reaction time, and mild conditions.

Electrochemical Ferrier Rearrangement of Glycals

The Ferrier rearrangement (FR) is a well-documented reaction that relies on strong acids or oxidants to convert glycals into unsaturated glycosyl derivatives. In this work, we introduce an electrochemical variant of the FR, offering a broad substrate compatibility. Various nucleophiles and glycal derivatives afford 2,3-unsaturated glycosyl derivatives in high yields with excellent diastereoselectivities. This sustainable method promises to expand the electrochemistry applications in sugar chemistry.

Synthesis of N-heterocyclic amides from imidazoheterocycles through convergent paired electrolysis

An efficient ring opening of imidazoheterocycles induced by a direct C-H azidation resulting in an unusual formation of N-heterocyclic amides has been successfully developed through convergent paired electrolysis. A broad scope of pyridylbenzamides could be obtained in moderate to excellent yields under exogenous-oxidant, electrolyte- and metal-free electrochemical conditions. The methodology was transferred to continuous flow conditions resulting in notable improvements particularly in terms of cost-efficiency over traditional batch versions.

Low-Cost, Safe, and Anion-Flexible Method for the Electrosynthesis of Diaryliodonium Salts

An electrochemical approach toward the synthesis of diaryliodonium salts based on anodic C-I coupling between aryl iodides and arenes is presented. In contrast to previous protocols, our method requires no chemical oxidants, strong acids, or fluorinated solvents. A further advantage is that by use of the appropriate supporting electrolyte, the counterion of choice can be introduced, which is time- and cost-saving as compared to postsynthesis ion exchange. This "anion-flexibility" is particularly interesting when considering the pronounced effect of the counterion on the reactivity of diaryliodonium species in aryl transfer reactions. The scope of our method comprises 24 examples with isolated yields of up to 99%. Scalability was demonstrated by the synthesis on a gram scale. Furthermore, it was shown that the diaryliodonium-containing post-electrolysis solution can be used without further workup as a reactive medium for O-arylation reactions. Finally, a series of para-substituted diaryliodonium compounds was studied using linear sweep voltammetry on a microelectrode and analyzed with respect to the influence of the electronic structure on the redox behavior.

Isolation, biological activity, and synthesis of isoquinoline alkaloids

Covering: 2019 to 2023Isoquinoline alkaloids, an important class of N-based heterocyclic compounds, have attracted considerable attention from researchers worldwide. To follow up on our prior review (covering 2014-2018) and present the progress of this class of compounds, this review summarizes and provides updated literature on novel isoquinoline alkaloids isolated during the period of 2019-2023, together with their biological activity and underlying mechanisms of action. Moreover, with the rapid development of synthetic modification strategies, the synthesis strategies of isoquinoline alkaloids have been continuously optimized, and the total synthesis of these classes of natural products is reviewed critically herein. Over 250 molecules with a broad range of bioactivities, including antitumor, antibacterial, cardioprotective, anti-inflammatory, neuroprotective and other activities, are isolated and discussed. The total synthesis of more than nine classes of isoquinoline alkaloids is presented, and thirteen compounds constitute the first total synthesis. This survey provides new indications or possibilities for the discovery of new drugs from the original naturally occurring isoquinoline alkaloids.

Recent Advances in Electrochemical Carboxylation with CO2

ConspectusCarbon dioxide (CO2) is recognized as a greenhouse gas and a common waste product. Simultaneously, it serves as an advantageous and commercially available C1 building block to generate valuable chemicals. Particularly, carboxylation with CO2 is considered a significant method for the direct and sustainable production of important carboxylic acids. However, the utilization of CO2 is challenging owing to its thermodynamic stability and kinetic inertness. Recently, organic electrosynthesis has emerged as a promising approach that utilizes electrons or holes as environmentally friendly redox reagents to produce reactive intermediates in a controlled and selective manner. This technique holds great potential for the CO2 utilization.Since 2015, our group has been dedicated to exploring the utilization of CO2 in organic synthesis with a particular focus on electrochemical carboxylation. Despite the significant advancements made in this area, there are still many challenges, including the activation of inert substrates, regulation of selectivity, diversity in electrolysis modes, and activation strategies. Over the past 7 years, our team, with many great experts, has presented findings on electrochemical carboxylation with CO2 under mild conditions. In this context, we primarily highlight our contributions to selective electrocarboxylations, encompassing new reaction systems, selectivity control methods, and activation approaches.We commenced our research by establishing a Ni-catalyzed electrochemical carboxylation of unactivated aryl halides and alkyl bromides in conjunction with a useful paired anodic reaction. This approach eliminates the need for sacrificial anodes, rendering the carboxylation process sustainable. To further utilize the widely existing yet cost-effective alkyl chlorides, we have developed a deep electroreductive system to achieve carboxylation of unactivated alkyl chlorides and poly(vinyl chloride), allowing the direct modification and upgrading of waste polymers.Through precise adjustment of the electroreductive conditions, we successfully demonstrated the dicarboxylation of both strained carbocycles and acyclic polyarylethanes with CO2 via C-C bond cleavage. Furthermore, we have realized the dicarboxylative cyclization of unactivated skipped dienes to produce the valuable ring-tethered adipic acids through single-electron reduction of CO2 to the CO2 radical anion (CO2•-). In terms of the asymmetric carboxylation, Guo's and our groups have recently achieved the nickel-catalyzed enantioselective electroreductive carboxylation reaction using racemic propargylic carbonates and CO2, paving the way for the synthesis of enantioenriched propargylic carboxylic acids.In addition to the aforementioned advancements, Lin's and our groups have also developed new electrolysis modes to achieve regiodivergent C-H carboxylation of N-heteroarenes dictated by electrochemical reactors. The choice of reactors plays a crucial role in determining whether the hydrogen atom transfer (HAT) reagents are formed anodically, consequently influencing the carboxylation pathways of N-heteroarene radical anions in the distinct electrolyzed environments.

Auto-Optimized Electro-Flow Reactor Platform for the in-situ Reduction of P(V) Oxide to P(III) and Their Application

Trivalent phosphine catalysis is mostly utilized to activate the carbon-carbon multiple bonds to form carbanion intermediate species and is highly sensitive to certain variables. Random manual multi-variables are critical for understanding the batch disabled regeneration of trivalent phosphine chemistry. We need the artificial intelligence-based system which can change the variable based on previously conducted failed experiment. Herein, we report an auto-optimized electro-micro-flow reactor platform for the in-situ reduction of stable P(V) oxide to sensitive P(III) and further utilized the method for Corey-Fuchs reaction.

Electrocatalytic continuous flow chlorinations with iodine(I/III) mediators

Electrochemistry offers tunable, cost effective and environmentally friendly alternatives to carry out redox reactions with electrons as traceless reagents. The use of organoiodine compounds as electrocatalysts is largely underdeveloped, despite their widespread application as powerful and versatile reagents. Mechanistic data reveal that the hexafluoroisopropanol assisted iodoarene oxidation is followed by a stepwise chloride ligand exchange for the catalytic generation of the dichloroiodoarene mediator. Here, we report an environmentally benign iodine(I/III) electrocatalytic platform for the in situ generation of dichloroiodoarenes for different reactions such as mono- and dichlorinations as well as chlorocyclisations within a continuous flow setup.

Electrochemically-driven difunctionalization of the isocyanide and Mumm rearrangement cascade: expeditious synthesis of N-acyl-N-alkyl S-thiocarbamates

We describe the novel electrochemical multicomponent reaction (e-MCR) of readily available isocyanides, thiols and carboxylic acids to form N-substituted S-thiocarbamate derivatives that are found in several biologically active compounds. The effectiveness of the μ-electro flow reactor (μ-EFR) was showcased through significant reduction in electrolyte volume during the reaction, achieving gram-scale production of 4a within a short 12 min residence time using a Pt/Pt flow cell.

Continuous Flow Electroselenocyclization of Allylamides and Unsaturated Oximes to Selenofunctionalized Oxazolines and Isoxazolines

The synthesis of selenofunctionalized oxazolines and isoxazolines from N-allyl benzamides and unsaturated oximes with diselenides was studied by utilizing a continuous flow electrochemical approach. At mild reaction conditions and short reaction times of 10 min product yields of up to 90% were achieved including a scale-up reaction. A broad substrate scope was studied and the reaction was shown to have a wide functional group tolerance.

Electrochemical Continuous-Flow Scholl Reaction toward Polycyclic Aromatic Hydrocarbons

The preparation of polycyclic aromatic hydrocarbons (PAHs) by the Scholl reaction is typically performed by using superstoichiometric oxidants. Herein, we develop an electrochemical continuous-flow Scholl reaction to access PAHs that features a reduction in the use of supporting electrolytes and easy scale-up without changing the reaction conditions and setups. This reaction allows the synthesis of distorted PAHs containing three [5]helicene units that possess intriguing electronic and optical properties.

Unlocking the Potential of Oxidative Asymmetric Catalysis with Continuous Flow Electrochemistry

In the field of catalytic asymmetric synthesis, the less-treated path lies in oxidative catalytic asymmetric transformations. The hurdles of pinpointing the appropriate chemical oxidants and addressing their compatibility issues with catalysts and functionalities present significant challenges. Organic electrochemistry, employing traceless electrons for redox reactions, is underscored as a promising solution. However, the commonly used electrolysis in batch cells introduces its own set of challenges, hindering the advancement of electrochemical asymmetric catalysis. Here we introduce a microfluidic electrochemistry platform with single-pass continuous flow reactors that exhibits a wide-ranging applicability to various oxidative asymmetric catalytic transformations. This is exemplified through the sulfenylation of 1,3-dicarbonyls, dehydrogenative C-C coupling, and dehydrogenative alkene annulation processes. The unique properties of microfluidic electrochemical reactors not only eliminate the need for chemical oxidants but also enhance reaction efficiency and reduce the use of additives and electrolytes. These salient features of microfluidic electrochemistry expedite the discovery and development of oxidative asymmetric transformations. In addition, the continuous production facilitated by parallel single-pass reactors ensures straightforward reaction upscaling, removing the necessity for reoptimization across various scales, as evidenced by direct translation from milligram screening to hectogram asymmetric synthesis.

Anion-π catalysis on carbon allotropes

Anion-π catalysis, introduced in 2013, stands for the stabilization of anionic transition states on π-acidic aromatic surfaces. Anion-π catalysis on carbon allotropes is particularly attractive because high polarizability promises access to really strong anion-π interactions. With these expectations, anion-π catalysis on fullerenes has been introduced in 2017, followed by carbon nanotubes in 2019. Consistent with expectations from theory, anion-π catalysis on carbon allotropes generally increases with polarizability. Realized examples reach from enolate addition chemistry to asymmetric Diels-Alder reactions and autocatalytic ether cyclizations. Currently, anion-π catalysis on carbon allotropes gains momentum because the combination with electric-field-assisted catalysis promises transformative impact on organic synthesis.

Spiers Memorial Lecture: Old but new organic electrosynthesis: history and recent remarkable developments

Organic electrosynthesis has a long history. However, this chemistry is still new. Recently, we have seen its second renaissance with organic electrosynthesis being considered a typical green chemistry process. Therefore, a number of novel electrosynthetic methodologies have recently been developed. However, there are still many problems to be solved from a green and sustainable viewpoint. After an explanation of the historical survey of organic electrosynthesis, this paper focuses on recent remarkable developments in new electrosynthetic methodologies, such as novel electrodes, recyclable nonvolatile electrolytic solvents and recyclable supporting electrolytes, as well as new types of electrolytic flow cells. Furthermore, novel types of organic electrosynthetic reactions will be mentioned.

Alkene reactions with superoxide radical anions in flow electrochemistry

Alkenes were cleaved to ketones by using dioxygen in an electrochemical flow set-up. The pressurised system allowed efficient gas-liquid mixing with a stabilised flow. This mild and straightforward approach avoids the use of transition metals and harsh oxidants.

Concluding remarks: A summary of the Faraday Discussion on electrosynthesis

A summary of the Faraday Discussion presented in this issue and a perspective on that discussion is presented. The work highlights the specific science contributions made and the key conclusions associated with those findings so that readers can identify papers that they would like to explore in more detail.

Photoelectrocatalyzed undirected C-H trifluoromethylation of arenes: catalyst evaluation and scope

During the last few years, photoelectrocatalysis has evolved as an increasingly viable tool for molecular synthesis. Despite several recent reports on the undirected C-H functionalization of arenes, thus far, a detailed comparison of different catalysts is still missing. To address this, more than a dozen different mediators were employed in the trifluoromethylation of (hetero-)arenes to compare them in their efficacies.

Electric field-assisted anion-π catalysis on carbon nanotubes in electrochemical microfluidic devices

The vision to control the charges migrating during reactions with external electric fields is attractive because of the promise of general catalysis, emergent properties, and programmable devices. Here, we explore this idea with anion-π catalysis, that is the stabilization of anionic transition states on aromatic surfaces. Catalyst activation by polarization of the aromatic system is most effective. This polarization is induced by electric fields. The use of electrochemical microfluidic reactors to polarize multiwalled carbon nanotubes as anion-π catalysts emerges as essential. These reactors provide access to high fields at low enough voltage to prevent electron transfer, afford meaningful effective catalyst/substrate ratios, and avoid interference from additional electrolytes. Under these conditions, the rate of pyrene-interfaced epoxide-opening ether cyclizations is linearly voltage-dependent at positive voltages and negligible at negative voltages. While electromicrofluidics have been conceived for redox chemistry, our results indicate that their use for supramolecular organocatalysis has the potential to noncovalently electrify organic synthesis in the broadest sense.

Electrochemical Late-Stage Functionalization

Late-stage functionalization (LSF) constitutes a powerful strategy for the assembly or diversification of novel molecular entities with improved physicochemical or biological activities. LSF can thus greatly accelerate the development of medicinally relevant compounds, crop protecting agents, and functional materials. Electrochemical molecular synthesis has emerged as an environmentally friendly platform for the transformation of organic compounds. Over the past decade, electrochemical late-stage functionalization (eLSF) has gained major momentum, which is summarized herein up to February 2023.

Continuous Flow Electrochemistry Enables Practical and Site-Selective C-H Oxidation

The selective oxygenation of ubiquitous C(sp3 )-H bonds remains a highly sought-after method in both academia and the chemical industry for constructing functionalized organic molecules. However, it is extremely challenging to selectively oxidize a certain C(sp3 )-H bond to afford alcohols due to the presence of multiple C(sp3 )-H bonds with similar strength and steric environment in organic molecules, and the alcohol products being prone to further oxidation. Herein, we present a practical and cost-efficient electrochemical method for the highly selective monooxygenation of benzylic C(sp3 )-H bonds using continuous flow reactors. The electrochemical reactions produce trifluoroacetate esters that are resistant to further oxidation but undergo facile hydrolysis during aqueous workup to form benzylic alcohols. The method exhibits a broad scope and exceptional site selectivity and requires no catalysts or chemical oxidants. Furthermore, the electrochemical method demonstrates excellent scalability by producing 115 g of one of the alcohol products. The high site selectivity of the electrochemical method originates from its unique mechanism to cleave benzylic C(sp3 )-H bonds through sequential electron/proton transfer, rather than the commonly employed hydrogen atom transfer (HAT).

Electrochemical flow aziridination of unactivated alkenes

Aziridines derived from bioactive molecules may have unique pharmacological activities, making them useful in pharmacology (e.g. mitomycin C). Furthermore, the substitution of the epoxide moiety in epothilone B with aziridine, an analog of epoxides, yielded a pronounced enhancement in its anticancer efficacy. Thus, there is interest in developing novel synthetic technologies to produce aziridines from bioactive molecules. However, known methods usually require metal catalysts, stoichiometric oxidants and/or pre-functionalized amination reagents, causing difficulty in application. A practical approach without a metal catalyst and extra-oxidant for the aziridination of bioactive molecules is in demand, yet challenging. Herein, we report an electro-oxidative flow protocol that accomplishes an oxidant-free aziridination of natural products. This process is achieved by an oxidative sulfonamide/alkene cross-coupling, in which sulfonamide and alkene undergo simultaneous oxidation or alkene is oxidized preferentially. Further anticancer treatments in cell lines have demonstrated the pharmacological activities of these aziridines, supporting the potential of this method for drug discovery.

Wireless Electrochemical Reactor for Accelerated Exploratory Study of Electroorganic Synthesis

Electrosynthesis is an emerging tool to construct value-added fine chemicals under mild and sustainable conditions. However, the complex apparatus required impedes the facile development of new electrochemistry in the laboratory. Herein, we proposed and demonstrated the concept of wireless electrochemistry (Wi-eChem) based on wireless power transfer technology. The core of this concept is the dual-function wireless electrochemical magnetic stirrer that provides an electrolysis driving force and mixing simultaneously in a miniaturized form factor. This Wi-eChem system allowed electrochemists to execute electrochemical reactions in a manner similar to traditional organic chemistry without handling wire connections. The controllability, reusability, and versatility were validated with a series of modern electrosynthesis reactions, including electrodecarboxylative etherification, electroreductive olefin-ketone coupling, and electrochemical nickel-catalyzed oxygen atom transfer reaction. Its remarkably simplified operation enabled its facile integration into a fully automated robotic synthesis platform to achieve autonomous parallel electrosynthesis screening.

Cobaltaelectro-Catalyzed C-H Annulation with Allenes for Atropochiral and P-Stereogenic Compounds: Late-Stage Diversification and Continuous Flow Scale-Up

The 3d metallaelectro-catalyzed C-H activation has been identified as an increasingly viable strategy to access valuable organic molecules in a resource-economic fashion under exceedingly mild reaction conditions. However, the development of enantioselective 3d metallaelectro-catalyzed C-H activation is very challenging and in its infancy. Here, we disclose the merger of cobaltaelectro-catalyzed C-H activation with asymmetric catalysis for the highly enantioselective annulation of allenes. A broad range of C-N axially chiral and P-stereogenic compounds were thereby obtained in good yields of up to 98% with high enantioselectivities of up to >99% ee. The practicality of this approach was demonstrated by the diversification of complex bioactive compounds and drug molecules as well as decagram scale enantioselective electrocatalysis in continuous flow.

Grafting and Solubilization of Redox-Active Organic Materials for Aqueous Redox Flow Batteries

This study concerns the development of sustainable design strategies of aqueous electrolytes for redox flow batteries using redox-active organic materials. A green spontaneous grafting reaction occurs between a redox-active organic radical and an electrochemically activated structural modifier at room temperature through a simple mixing step. Then, a physical mixing method is used to formulate a structured aqueous electrolyte and enables aqueous solubilization of the organic solute from below 0.5 to 1.5 m beyond the conventional dissolution limit. The as-obtained concentrated mixture can be readily used as catholyte for a redox flow battery. A record high discharge cell voltage (1.6 V onset output voltage) in aqueous non-hybrid flow cell is attained by using the studied electrolytes.

Three-Component Electrochemical Aminoselenation of 1,3-Dienes

Azoles and organoselenium compounds are pharmacologically important scaffolds in medicinal chemistry and natural products. We developed an efficient regioselective electrochemical aminoselenation reaction of 1,3-dienes, azoles, and diselenide derivatives to access selenium-containing allylazoles skeletons. This protocol is more economical and environmentally friendly and features a broad substrate scope; pyrazole, triazole, and tetrazolium were all tolerated under the standard conditions, which could be applied to the expedient synthesis of bioactive molecules and in the pharmaceutical industry.

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.

Synthesis of 3-Thiocyanobenzothiophene via Difunctionalization of Active Alkyne Promoted by Electrochemical-Oxidation

A novel and green method for the synthesis of 3-thiocyanatobenzothiophenes via electrochemical-oxidation promoted difunctionalization of active alkyne has been developed. In this protocol, inexpensive and easily available potassium thiocyanate was chosen as the thiocyanation reagent, 2-alkynylthioanisoles as the substrates, a variety of 3-thiocyanatobenzothiophenes were obtained in moderate to good yields under oxidant- and catalyst-free conditions. Moreover, the continuous flow system has good applicability for this transformation, the use of continuous flow system has overcome the disadvantage of low efficiency in traditional electrochemical amplification, and realized the stable and excellent yields of target products in the scale-up reactions.

Electrochemical rhodium catalysed alkyne annulation with pyrazoles through anodic oxidation - a metal oxidant/additive free methodology

Pyrazole and its derivatives are important azole heteroarenes prevalent in pharmaceutical compounds and have been used as ligands for protein binding, making them valuable targets for synthetic applications. Herein we disclose an electrochemical intermolecular C-H/N-H oxidative annulation of 2-phenylpyrazoles with alkynes using a rhodium(III) redox regime without any external metal oxidants in a water compatible solvent system. Both symmetrical and unsymmetrical alkynes were shown to be compatible with the optimized conditions.

Dehydrogenative Imination of Low-Valent Sulfur Compounds-Fast and Scalable Synthesis of Sulfilimines, Sulfinamidines, and Sulfinimidate Esters

Herein, we describe an electrochemical pathway for the synthesis of sulfilimines, sulfoximines, sulfinamidines, and sulfinimidate esters from readily available low-valent sulfur compounds and primary amides or their analogues. The combination of solvents and supporting electrolytes together act both as an electrolyte as well as a mediator, leading to efficient use of reactants. Both can be easily recovered, enabling an atom-efficient and sustainable process. A broad scope of sulfilimines, sulfinamidines, and sulfinimidate esters with N-EWGs is accessed in up to excellent yields with broad functional group tolerance. This fast synthesis can be easily scaled up to multigram quantities with high robustness for fluctuation of current densities of up to 3 orders of magnitude. The sulfilimines are converted into the corresponding sulfoximines in an ex-cell process in high to excellent yields using electro-generated peroxodicarbonate as a green oxidizer. Thereby, preparatively valuable NH sulfoximines are accessible.

A High Voltage Aqueous Zinc-Vanadium Redox Flow Battery with Bimodal Tin and Copper Clusters by a Continuous-Flow Electrometallic Synthesis

Aqueous zinc-based redox flow batteries are promising large-scale energy storage applications due to their low cost, high safety, and environmental friendliness. However, the zinc dendritic growth has depressed the cycle performance, stability, and efficiency, hindering the commercialization of the zinc-based redox flow batteries. We fabricate the carbon felt modified with bimodal sized tin and copper clusters (SCCF) with the electrometallic synthesis in a continuous-flow cell. The SCCF electrode provides a larger zinc nucleation area and lower overpotential than pristine carbon felt, which is ascribed to the well-controlled interfacial interaction of bimodal tin and copper particle clusters by suppressing unwanted alloy formation. The zinc symmetric flow battery and the zinc-based hybrid redox flow battery show the improved zinc plating and stripping efficiency. The SCCF electrode exhibits 75% improved cycling stability compared to the pristine carbon felt electrode in the zinc symmetric flow battery. Notably, the high-voltage aqueous zinc-vanadium redox flow battery demonstrates a high average cell voltage of 2.31 V at 40 mA cm^-2, showing a Coulombic efficiency of 99.9% and an energy efficiency of 87.6% for 100 cycles. We introduce a facile strategy to suppress the zinc dendritic growth, enhancing the performance of the zinc-based redox flow batteries.

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.

Two-step continuous-flow synthesis of 6-membered cyclic iodonium salts via anodic oxidation

We describe a multi-step continuous-flow procedure for the generation of six-membered diaryliodonium salts. The accompanying scalability and atom economy are significant improvements to existing batch methods. Benzyl acetates are submitted to this two-step procedure as highly available and cheap starting materials. An acid-catalyzed Friedel-Crafts alkylation followed by an anodic oxidative cyclization yielded a defined set of cyclic iodonium salts in a highly substrate-dependent yield.

Inline purification in continuous flow synthesis – opportunities and challenges

Continuous flow technology has become the method of choice for many academic and industrial researchers when developing new routes to chemical compounds of interest. With this technology maturing over the last decades, robust and oftentimes automated processes are now commonly exploited to generate fine chemical building blocks. The integration of effective inline analysis and purification tools is thereby frequently exploited to achieve effective and reliable flow processes. This perspective article summarizes recent applications of different inline purification techniques such as chromatography, extractions, and crystallization from academic and industrial laboratories. A discussion of the advantages and drawbacks of these tools is provided as a guide to aid researchers in selecting the most appropriate approach for future applications. It is hoped that this perspective contributes to new developments in this field in the context of process and cost efficiency, sustainability and industrial uptake of new flow chemistry tools developed in academia.

Divergent C-H Amidations and Imidations by Tuning Electrochemical Reaction Potentials

Electrochemical C-H functionalizations are attractive transformations, as they are capable of avoiding the use of transition metals, pre-oxidized precursors, or suprastoichiometric amounts of terminal oxidants. Herein an electrochemically tunable method was developed that enabled the divergent formation of cyclic amines or imines by applying different reaction potentials. Detailed cyclic voltammetry analyses, coupled with chronopotentiometry experiments, were carried out to provide insight into the mechanism, while atom economy was assessed through a paired electrolysis. Selective C-H amidations and imidations were achieved to afford five- to seven-membered sulfonamide motifs that could be employed for late-stage modifications.

A flow electrochemistry-enabled synthesis of 2-substituted N-(methyl-d)piperidines

A synthesis of N-monodeuteriomethyl-2-substituted piperidines is described. An efficient and readily scalable anodic methoxylation of N-formylpiperidine in an undivided microfluidic electrolysis cell delivers methoxylated piperidine 3, which is a precursor to a N-formyliminium ion and enables C-nucleophiles to be introduced at the 2-position. The isotopically labelled N-deuteriomethyl group is installed using the Eschweiler-Clarke reaction with formic acid-d₂ and unlabelled formaldehyde. Monodeuterated N-methyl groups in these molecular systems possess small isotropic proton chemical shift differences important in the investigation of molecules that are able to support long-lived nuclear spin states in solution nuclear magnetic resonance.

The Application of Sulfonyl Hydrazides in Electrosynthesis: A Review of Recent Studies

Sulfonyl hydrazides are viewed as alternatives to sulfinic acids and their salts or sulfonyl halides, which are broadly used in organic synthesis or work as active pharmaceutical substances. Generally, sulfonyl hydrazides are considered good building blocks and show powerful value in a diverse range of reactions to construct C-S bonds or C-C bonds, and even C-N bonds as sulfur, carbon, or nitrogen sources, respectively. As a profound synthetic tool, the electrosynthesis method was recently used to achieve efficient and green applications of sulfonyl hydrazides. Interestingly, many unique and novel electrochemical syntheses using sulfonyl hydrazides as radical precursors have been developed, including cascade reactions, functionalization of heterocycles, as well as a continuous flow method combining with electrochemical synthesis since 2017. Accordingly, it is necessary to specifically summarize the recent developments of electrosynthesis with only sulfonyl hydrazides as radical precursors to more deeply understand and better design novel electrochemical synthesis reactions. Herein, electrosynthesis research using sulfonyl hydrazides as radical precursors since 2017 is reviewed in detail based on the chemical structures of products and reaction mechanisms.

High-Productivity Single-Pass Electrochemical Birch Reduction of Naphthalenes in a Continuous Flow Electrochemical Taylor Vortex Reactor

We report the development of a single-pass electrochemical Birch reduction carried out in a small footprint electrochemical Taylor vortex reactor with projected productivities of >80 g day-1 (based on 32.2 mmol h-1), using a modified version of our previously reported reactor [Org. Process Res. Dev. 2021, 25, 7, 1619-1627], consisting of a static outer electrode and a rapidly rotating cylindrical inner electrode. In this study, we used an aluminum tube as the sacrificial outer electrode and stainless steel as the rotating inner electrode. We have established the viability of using a sacrificial aluminum anode for the electrochemical reduction of naphthalene, and by varying the current, we can switch between high selectivity (>90%) for either the single ring reduction or double ring reduction with >80 g day-1 projected productivity for either product. The concentration of LiBr in solution changes the fluid dynamics of the reaction mixture investigated by computational fluid dynamics, and this affects equilibration time, monitored using Fourier transform infrared spectroscopy. We show that the concentrations of electrolyte (LiBr) and proton source (dimethylurea) can be reduced while maintaining high reaction efficiency. We also report the reduction of 1-aminonaphthalene, which has been used as a precursor to the API Ropinirole. We find that our methodology produces the corresponding dihydronaphthalene with excellent selectivity and 88% isolated yield in an uninterrupted run of >8 h with a projected productivity of >100 g day-1.

Process intensification in continuous flow organic synthesis with enabling and hybrid technologies

Industrial organic synthesis is time and energy consuming, and generates substantial waste. Traditional conductive heating and mixing in batch reactors is no longer competitive with continuous-flow synthetic methods and enabling technologies that can strongly promote reaction kinetics. These advances lead to faster and simplified downstream processes with easier workup, purification and process scale-up. In the current Industry 4.0 revolution, new advances that are based on cyber-physical systems and artificial intelligence will be able to optimize and invigorate synthetic processes by connecting cascade reactors with continuous in-line monitoring and even predict solutions in case of unforeseen events. Alternative energy sources, such as dielectric and ohmic heating, ultrasound, hydrodynamic cavitation, reactive extruders and plasma have revolutionized standard procedures. So-called hybrid or hyphenated techniques, where the combination of two different energy sources often generates synergistic effects, are also worthy of mention. Herein, we report our consolidated experience of all of these alternative techniques.

Electrochemical aromatic C-H hydroxylation in continuous flow

The direct hydroxylation of arene C-H bonds is a highly sought-after transformation but remains an unsolved challenge due to the difficulty in efficient and regioselective C-H oxygenation and high reactivity of the phenolic products leading to overoxidation. Herein we report electrochemical C-H hydroxylation of arenes in continuous flow for the synthesis of phenols. The method is characterized by broad scope (compatible with arenes of diverse electronic properties), mild conditions without any catalysts or chemical oxidants, and excellent scalability as demonstrated by the continuous production of 1 mol (204 grams) of one of the phenol products.

Cu-Electrocatalytic Diazidation of Alkenes at ppm Catalyst Loading

The 1,2-diamine motif is prevalent in natural products, small-molecule pharmaceuticals, and catalysts for asymmetric synthesis. Transition metal catalyzed alkene diazidation has evolved to be an attractive strategy to access vicinal primary diamines but remains challenging, especially for practical applications, due to the restriction to a certain type of olefins, the frequent use of chemical oxidants, and the requirement for high loadings of metal catalysts (1 mol % or above). Herein we report a scalable Cu-electrocatalytic alkene diazidation reaction with 0.02 mol % (200 ppm) of copper(II) acetylacetonate as the precatalyst without exogenous ligands. In addition to its use of low catalyst loading, the electrocatalytic method is scalable, compatible with a broad range of functional groups, and applicable to the diazidation of α,β-unsaturated carbonyl compounds and mono-, di-, tri-, and tetrasubstituted unactivated alkenes.

Electrochemical selenium-π-acid promoted hydration of alkynyl phosphonates

An unprecedented electrochemical selenium-π-acid promoted hydration of internal alkynes bearing a phosphonate auxiliary was described. Thus, valuable (hetero)aryl and alkyl ketones could be accessed under mild, metal- and external oxidant-free conditions. This protocol features high atom-economy, good chemo- and regio-selectivity, excellent functional group tolerance and easily transformable products. Control experiments demonstrate that phosphonate assistance is essential for this transformation.

Electropolymerization without an electric power supply

Electrifying synthesis is now a common slogan among synthetic chemists. In addition to the conventional two- or three-electrode systems that use batch-type cells, recent progress in organic electrochemical processes has been significant, including microflow electrochemical reactors, Li-ion battery-like technology, and bipolar electrochemistry. Herein we demonstrate an advanced electrosynthesis method without the application of electric power based on the concept of streaming potential-driven bipolar electrochemistry. As a proof-of-concept study, the electrochemical oxidative polymerization of aromatic monomers successfully yielded the corresponding polymer films on an electrode surface, which acted as an anode under the flow of electrolyte in a microchannel without an electric power supply.

Biomimetic total synthesis of (-)-galanthamine via intramolecular anodic aryl-phenol coupling

(-)-Galanthamine as a drug for the treatment of Alzheimer's disease has attracted synthetic chemists for decades. However, previous total synthetic and biomimetic approaches often use stoichiometric oxidants (metal oxidants or hypervalent iodine) to prepare the target product. Anodic oxidative coupling offers a sustainable alternative method which is, for the first time, successfully applied to the total synthesis of (-)-galanthamine. We report a new asymmetric total synthesis of (-)-galanthamine by using an anodic aryl-phenol coupling as the key synthetic step.

Electrochemical Cage Activation of Carboranes

Carboranes are boron-carbon molecular clusters that possess unique properties, such as their icosahedron geometry, high boron content, and delocalized three-dimensional aromaticity. These features render carboranes valuable building blocks for applications in supramolecular design, nanomaterials, optoelectronics, organometallic coordination chemistry, and as boron neutron capture therapy (BNCT) agents. Despite tremendous progress in this field, stoichiometric chemical redox reagents are largely required for the oxidative activation of carborane cages. In this context, electrosyntheses represent an alternative strategy for more sustainable molecular syntheses. It is only in recent few years that considerable progress has been made in electrochemical cage functionalization of carboranes, which are summarized in this Minireview. We anticipate that electrocatalysis will serve as an increasingly powerful stimulus within the current renaissance of carborane electrochemistry.

Synthesis of 1,3-diselenyl-dihydroisobenzofurans via electrochemical radical selenylation with substituted o-divinylbenzenes and diselenides

An efficient electrochemical method for the direct synthesis of complicated 1,3-diselenyl-dihydroisobenzofurans was developed under external oxidant free conditions at room temperature from substituted o-divinylbenzenes and diselenides. A radical mechanism is proposed for this novel and practical transformation.