Applications of Flow Microreactors in Electrosynthetic Processes

Electrochemical Synthesis of β-Keto Sulfones from Enol Acetates and Sulfonyl Hydrazides

A novel and environmentally friendly strategy has been developed for the efficient electrochemical synthesis of β-keto sulfones. This method enables the synthesis of β-keto sulfones by reacting easily available sulfonylhydrazide with enol acetate under mild conditions, especially without the need for transition metal catalysts or oxidants, which can achieve high yields. The scope of this reaction was systematically explored with various sulfonyl hydrazides and enol acetates. The scale-up synthesis of β-keto sulfones was successfully accomplished using an electrochemical flow cell, demonstrating the industrial applicability of this approach. Moreover, the underlying reaction mechanism was further investigated through free radical scavenging experiments and cyclic voltammetry studies.

Microfluidics for Electrochemical Energy Conversion and Storage: Prospects Toward Sustainable Ammonia Production

Ammonia is a key chemical in the production of fertilizers, refrigeration and an emerging hydrogen-carrying fuel. However, the Haber-Bosch process, the industrial standard for centralized ammonia production, is energy-intensive and indirectly generates significant carbon dioxide emissions. Electrochemical nitrogen reduction offers a promising alternative for green ammonia production. Yet, current reaction rates remain well below economically feasible targets. This work examines the application of electrochemical microfluidics for the enhancement of the rates of electrochemical ammonia synthesis. The review is built on the introduction to electrochemical microfluidics, corresponding cell designs, and the main applications of microfluidics in electrochemical energy conversion/storage. Based on recent advances in electrochemical ammonia synthesis, with an emphasis on the critical role of robust experimental controls, electrochemical microfluidics represents a promising route to environmentally friendly, on-site and on-demand ammonia production. This review aims to bridge the knowledge gap between the disciplines of electrochemistry and microfluidics and promote interdisciplinary understanding and innovation in this transformative field.

Navigating electrochemical oxidative functionalization of olefins: selected mechanistic and synthetic examples

The functionalization of olefins to form added-value compounds is a cornerstone of modern organic chemistry, promoting the synthesis of complex molecules from simple feedstock materials. In parallel, electrochemistry has emerged as a powerful and sustainable technique for enabling challenging transformations under mild conditions by generating reactive intermediates in a controlled manner. This review highlights recent advances in oxidative electrochemical methods for olefin functionalization, showcasing key developments that underscore the versatility of this approach. Using selected representative examples, we explore diverse mechanistic pathways, bond-forming strategies, and the integration of electrochemical techniques with catalytic systems. By providing a concise overview of this rapidly evolving field, we aim to inspire further innovation in electrochemical methodologies to expand the frontiers of olefin chemistry.

Paired Electrosynthesis at Interdigitated Microband Array Electrodes without Intentionally Added Electrolyte: C-C Coupling of Dicyanobenzenes with Methanol

The formation of substituted benzyl alcohols from dicyanobenzenes and methanol (C-C coupling) is demonstrated as a model system for paired electrosynthesis and investigated at interdigitated microband array electrodes in a microreactor with/without intentionally added supporting electrolyte. A Pt-Pt microband array with 5 μm bands separated by 5 μm gaps is employed in a dimethyl sulfoxide (DMSO) solvent. Yields are optimized to approximately 50% at the point of 100% conversion. The mechanism is investigated by employing isotope labeling (CD3OD, CH3OD, d 6-DMSO, 13CH3OH). The methylene group (12C or 13C) is obtained with H2, D2, and DH substitution patterns, and a hypothesis for a corresponding mechanism is discussed aided with density functional theory (DFT) calculations. Implications for sustainable electrosynthesis at paired microband electrodes are discussed.

Electrochemical Dehydrogenative sp2-Coupling Reaction of Naphthols Accessing a Polycyclic Naphthalenone Motif

A novel polycyclic naphthalenone motif was obtained by electrochemical synthesis starting from naphthols. The process is solvent controlled, and the highly diastereoselective cyclization is due to a solvent cage. The direct, anodic dehydrogenative sp2-coupling was carried out by flow electrolysis. Ten derivatives containing this motif were synthesized in yields up to 88%, resulting in novel polycycles structurally similar to bioactive compounds like Daldionin, potentially exploring the bioactive profile.

Electrochemical Trifluoromethylation of Enamides under Microflow Conditions

The development of sustainable trifluoromethylations of enamides is of great interest to the pharmaceutical industry. Herein, we demonstrate a sustainable direct electrochemical trifluoromethylation method in a microflow cell, using Langlois reagent, without the need for a supporting electrolyte, oxidants, or any additive under mild conditions. This method can be applied to various substrates with a yield of up to 84%. Additionally, the batch process yielded significantly less (22%), highlighting the microflow cell's efficiency.

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.

The Strategies Towards Electrochemical Generation of Aryl Radicals

The advancement in electrochemical techniques has unlocked a new path for achieving unprecedented oxidations and reductions of aryl radical precursors in a controlled and selective manner. This approach facilitates the construction of aromatic carbon-carbon and carbon-heteroatom bonds. In light of the green merits and the growing importance of this technique in aryl radical chemistry, this review aims to provide an overview of the recent advance in the electrochemical generation of aryl radicals organized by the aryl radical precursor type, with a focus on the substrate scope, limitation, and underlying mechanism, thereby inspiring future work on electrochemical aryl radical generation.

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.

Nickel-Catalyzed Electrochemical Cross-Electrophile C(sp2)-C(sp3) Coupling via a NiII Aryl Amido Intermediate

Cross-electrophile coupling (XEC) between aryl halides and alkyl halides is a streamlined approach for C(sp2)-C(sp3) bond construction, which is highly valuable in medicinal chemistry. Based on a key NiII aryl amido intermediate, we developed a highly selective and scalable Ni-catalyzed electrochemical XEC reaction between (hetero)aryl halides and primary and secondary alkyl halides. Experimental and computational mechanistic studies indicate that an amine secondary ligand slows down the oxidative addition process of the Ni-polypyridine catalyst to the aryl bromide and a NiII aryl amido intermediate is formed in situ during the reaction process. The relatively slow oxidative addition is beneficial for enhancing the selectivity of the XEC reaction. The NiII aryl amido intermediate stabilizes the NiII-aryl species to prevent the aryl-aryl homo-coupling side reactions and acts as a catalyst to activate the alkyl bromide substrates. This electrosynthesis system provides a facile, practical, and scalable platform for the formation of (hetero)aryl-alkyl bonds using standard Ni catalysts under mild conditions. The mechanistic insights from this work could serve as a great foundation for future studies on Ni-catalyzed cross-couplings.

Solvent-dependent reaction mechanisms in the electrooxidative coupling of phenols: insights by operando Raman spectroelectrochemistry

The electrochemical oxidative phenol coupling reaction is a sustainable method for accessing biphenolic compounds. Using the dimerization of sesamol as a model reaction, insights into the reaction mechanism were gained via operando Raman spectroscopy. By varying the solvent and electrodes, different reaction mechanisms were identified and correlated with the respective product yields.

Redox-neutral electrochemical decontamination of hypersaline wastewater with high technology readiness level

Industrial hypersaline wastewaters contain diverse pollutants that harm the environment. Recovering clean water, alkali and acid from these wastewaters can promote circular economy and environmental protection. However, current electrochemical and advanced oxidation processes, which rely on hydroxyl radicals to degrade organic compounds, are inefficient and energy intensive. Here we report a flow-through redox-neutral electrochemical reactor (FRER) that effectively removes organic contaminants from hypersaline wastewaters via the chlorination-dehalogenation-hydroxylation route involving radical-radical cross-coupling. Bench-scale experiments demonstrate that the FRER achieves over 75% removal of total organic carbon across various compounds, and it maintains decontamination performance for over 360 h and continuously treats real hypersaline wastewaters for two months without corrosion. Integrating the FRER with electrodialysis reduces operating costs by 63.3% and CO2 emissions by 82.6% when compared with traditional multi-effect evaporation-crystallization techniques, placing our system at technology readiness levels of 7-8. The desalinated water, high-purity NaOH (>95%) and acid produced offset industrial production activities and thus support global sustainable development objectives.

Paired Electrosynthesis at Interdigitated Microband Electrodes: Exploring Diffusion and Reaction Zones in the Absence of a Supporting Electrolyte

Electrosynthesis traditionally requires dedicated reactor systems and an added electrolyte, although some paired electrosynthesis processes are possible at interdigitated microband electrodes simply immersed in solution and without an intentionally added electrolyte. Here, 1,1'-ferrocenedimethanol oxidation and activated olefin electro-hydrogenation reactions are investigated as model processes at a Pt-Pt interdigitated microband array electrode with 5 μm width and with 5 μm interelectrode gap. Voltammetric responses for electro-hydrogenation are discussed, and product yields are determined in methanol (MeOH) in the presence/absence of an added electrolyte (LiClO4). An isotope effect is observed in CH3OD solvent, leading to olefin monodeuteration linked to a fast EC-type process close to the cathode surface (in the cathode reaction zone) rather than to charge annihilation in the interelectrode zone. A finite element simulation is employed to visualize/discuss reaction zones and to contrast the rate of charge annihilation processes with/without a supporting electrolyte.

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.

Closing the Gap: Towards a Fully Continuous and Self-Regulated Kolbe Electrosynthesis

In this article, we address the transition of the Kolbe electrolysis of valeric acid (VA) to n-octane as an exemplary electrosynthesis process from a batch reaction to a continuous, self-regulated process. Based on a systematic assessment of chemical boundary conditions and sustainability aspects, we propose a continuous electrosynthesis including a simple product separation and electrolyte recirculation, as well as an online-pH-controlled VA feeding. We demonstrate how essential performance parameters such as product selectivity (S) and coulombic efficiency (CE) are significantly improved by the transition from batch to a continuous process. Thus, the continuous and pH-controlled electrolysis of a 1 M valeric acid, starting pH 6.0, allowed a constantly high selectivity of around 47 % and an average Coulomb efficiency about 52 % throughout the entire experimental duration. Under otherwise identical conditions, the conventional batch operation suffered from lower and strongly decreasing performance values (Sn-octane, 60min =10.4 %, Sn-octane, 240min =1.3 %; CEn-octane, 60min =7.1 %, CEn-octane, 240min =0.5 %). At the same time, electrolyte recirculation significantly reduces wastes and limits the use of electrolyte components.

Electrifying Friedel-Crafts Intramolecular Alkylation toward 1,1-Disubstituted Tetrahydronaphthalenes

In this work, we successfully employed electrochemical conditions to promote a Hofer-Moest, intramolecular Friedel-Crafts alkylation sequence. The reaction proceeds under mild conditions, employing carboxylic acids as starting materials. Notably, the electrochemical process performed in batch was adapted to a continuous flow electrolysis apparatus to provide a significant improvement. This catalyst-free, electrochemical approach produces an array of tetrahydronaphthalenes that could be used for API 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.

Developing electrochemical hydrogenation towards industrial application

Electrochemical hydrogenation reactions gained significant attention as a sustainable and efficient alternative to conventional thermocatalytic hydrogenations. This tutorial review provides a comprehensive overview of the basic principles, the practical application, and recent advances of electrochemical hydrogenation reactions, with a particular emphasis on the translation of these reactions from lab-scale to industrial applications. Giving an overview on the vast amount of conceivable organic substrates and tested catalysts, we highlight the challenges associated with upscaling electrochemical hydrogenations, such as mass transfer limitations and reactor design. Strategies and techniques for addressing these challenges are discussed, including the development of novel catalysts and the implementation of scalable and innovative cell concepts. We furthermore present an outlook on current challenges, future prospects, and research directions for achieving widespread industrial implementation of electrochemical hydrogenation reactions. This work aims to provide beginners as well as experienced electrochemists with a starting point into the potential future transformation of electrochemical hydrogenations from a laboratory curiosity to a viable technology for sustainable chemical synthesis on an industrial scale.

Electrosynthesis and Microanalysis in Thin Layer: An Electrochemical Pipette for Rapid Electrolysis and Mechanistic Study of Electrochemical Reactions

Electrochemistry represents unique approaches for the promotion and mechanistic study of chemical reactions and has garnered increasing attention in different areas of chemistry. This expansion necessitates the enhancement of the traditional electrochemical cells that are intrinsically constrained by mass transport limitations. Herein, we present an approach for designing an electrochemical cell by limiting the reaction chamber to a thin layer of solution, comparable to the thickness of the diffusion layer. This thin layer electrode (TLE) provides a modular platform to bypass the constraints of traditional electrolysis cells and perform electrolysis reactions in the timescale of electroanalytical techniques. The utility of the TLE for electrosynthetic applications benchmarked using NHPI-mediated electrochemical C-H functionalization. The application of microscale electrolysis for the study of drug metabolites was showcased by elucidating the oxidation pathways of the paracetamol drug. Moreover, hosting a microelectrode in the TLE, was shown to enable real-time probing of the profiles of redox-active components of these rapid electrosynthesis reactions.

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).

Scalable electrosynthesis of commodity chemicals from biomass by suppressing non-Faradaic transformations

Electrooxidation of biomass platforms provides a sustainable route to produce valuable oxygenates, but the practical implementation is hampered by the severe carbon loss stemming from inherent instability of substrates and/or intermediates in alkaline electrolyte, especially under high concentration. Herein, based on the understanding of non-Faradaic degradation, we develop a single-pass continuous flow reactor (SPCFR) system with high ratio of electrode-area/electrolyte-volume, short duration time of substrates in the reactor, and separate feeding of substrate and alkaline solution, thus largely suppressing non-Faradaic degradation. By constructing a nine-stacked-modules SPCFR system, we achieve electrooxidation of glucose-to-formate and 5-hydroxymethylfurfural (HMF)-to-2,5-furandicarboxylic acid (FDCA) with high single-pass conversion efficiency (SPCE; 81.8% and 95.8%, respectively) and high selectivity (formate: 76.5%, FDCA: 96.9%) at high concentrations (formate: 562.8 mM, FDCA: 556.9 mM). Furthermore, we demonstrate continuous and kilogram-scale electrosynthesis of potassium diformate (0.7 kg) from wood and soybean oil, and FDCA (1.17 kg) from HMF. This work highlights the importance of understanding and suppressing non-Faradaic degradation, providing opportunities for scalable biomass upgrading using electrochemical technology.

Current State of Microflow Trifluoromethylation Reactions

The trifluoromethyl group is a powerful structural motif in drugs and polymers; thus, developing trifluoromethylation reactions is an important area of research in organic chemistry. Over the past few decades, significant progress has been made in developing new methods for the trifluoromethylation of organic molecules, ranging from nucleophilic and electrophilic approaches to transition-metal catalysis, photocatalysis, and electrolytic reactions. While these reactions were initially developed in batch systems, more recent microflow versions are highly attractive for industrial applications owing to their scalability, safety, and time efficiency. In this review, we discuss the current state of microflow trifluoromethylation. Approaches for microflow trifluoromethylation based on different trifluoromethylation reagents are described, including continuous flow, flow photochemical, microfluidic electrochemical reactions, and large-scale microflow reactions.

Electrochemical oxo-functionalization of cyclic alkanes and alkenes using nitrate and oxygen

Direct functionalization of C(sp³)-H bonds allows rapid access to valuable products, starting from simple petrochemicals. However, the chemical transformation of non-activated methylene groups remains challenging for organic synthesis. Here, we report a general electrochemical method for the oxidation of C(sp³)-H and C(sp²)-H bonds, in which cyclic alkanes and (cyclic) olefins are converted into cycloaliphatic ketones as well as aliphatic (di)carboxylic acids. This resource-friendly method is based on nitrate salts in a dual role as anodic mediator and supporting electrolyte, which can be recovered and recycled. Reducing molecular oxygen as a cathodic counter reaction leads to efficient convergent use of both electrode reactions. By avoiding transition metals and chemical oxidizers, this protocol represents a sustainable oxo-functionalization method, leading to a valuable contribution for the sustainable conversion of petrochemical feedstocks into synthetically usable fine chemicals and commodities.

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.

Cell Voltage-Dependent Structural Dichotomy: Electrochemical C-H Acyloxylation and N-Acylation of 2H-Indazoles

A simple and efficient electrochemical method that utilizes modulation of the cell voltage to cause structural alterations in 2H-indazole is introduced. This method enables the C-3 acyloxylation of 2H-indazole and promotes the transfer of the acyl group from C-3 to N-1, allowing the N-acylation of 2H-indazoles. Additionally, the application of the μ-electro flow reactor was demonstrated, showcasing its effectiveness in achieving gram-scale production of 3x within a short residence time.

Microfluidic Biosensor Decorated with an Indium Phosphate Nanointerface for Attomolar Dopamine Detection

Developing functional materials that directly integrate into miniaturized devices for sensing applications is essential for constructing the next-generation point-of-care system. Although crystalline structure materials such as metal organic frameworks are attractive materials exhibiting promising potential for biosensing, their integration into miniaturized devices is limited. Dopamine (DA) is a major neurotransmitter released by dopaminergic neurons and has huge implications in neurodegenerative diseases. Integrated microfluidic biosensors capable of sensitive monitoring of DA from mass-limited samples is thus of significant importance. In this study, we developed and systematically characterized a microfluidic biosensor functionalized with the hybrid material composed of indium phosphate and polyaniline nanointerfaces for DA detection. Under the flowing operation, this biosensor displays a linear dynamic sensing range going from 10-18 to 10-11 M and a limit of detection (LOD) value of 1.83 × 10-19 M. In addition to the high sensitivity, this microfluidic sensor showed good selectivity toward DA and high stability (>1000 cycles). Further, the reliability and practical utility of the microfluidic biosensor were demonstrated using the neuro-2A cells treated with the activator, promoter, and inhibiter. These promising results underscore the importance and potential of microfluidic biosensors integrated with hybrid materials as advanced biosensors systems.

Copper Nanowire Networks: An Effective Electrochemical Peroxymonosulfate Activator toward Nitrogenous Pollutant Abatement

Herein, we developed an electrochemical filtration system for effective and selective abatement of nitrogenous organic pollutants via peroxymonosulfate (PMS) activation. Highly conductive and porous copper nanowire (CuNW) networks were constructed to serve simultaneously as catalyst, electrode, and filtration media. In one demonstration of the CuNW network's capability, a single pass through a CuNW filter (τ < 2 s) degraded 94.8% of sulfamethoxazole (SMX) at an applied potential of -0.4 V vs SHE. The exposed {111} crystal plane of CuNW triggered atomic hydrogen (H*) generation on sites, which contributed to effective PMS reduction. Meanwhile, with the involvement of SMX, a Cu-N bond was formed by the interactions between the -NH₂ group of SMX and the Cu sites of CuNW, accompanied by the redox cycling of Cu²⁺/Cu⁺, which was facilitated by the applied potential. The different charges of the active Cu sites made it easier to withdraw electrons and promote PMS oxidation. Theoretical calculations and experimental results were combined to suggest a mechanism for pollution abatement with CuNW networks. The results showed that system efficacy for the degradation of a wide array of nitrogenous pollutants was robust across a broad range of solution pH and complex aqueous matrices. The flow-through operation of the CuNW filter outperformed conventional batch electrochemistry due to convection-enhanced mass transport. This study provides a new strategy for environmental remediation by integrating state-of-the-art material science, advanced oxidation processes, and microfiltration technology.

A field guide to flow chemistry for synthetic organic chemists

Flow chemistry has unlocked a world of possibilities for the synthetic community, but the idea that it is a mysterious "black box" needs to go. In this review, we show that several of the benefits of microreactor technology can be exploited to push the boundaries in organic synthesis and to unleash unique reactivity and selectivity. By "lifting the veil" on some of the governing principles behind the observed trends, we hope that this review will serve as a useful field guide for those interested in diving into flow chemistry.

Transition Metal-Catalyzed C-H Functionalization Through Electrocatalysis

Electrochemically promoted transition metal-catalyzed C-H functionalization has emerged as a promising area of research over the last few decades. However, development in this field is still at an early stage compared to traditional functionalization reactions using chemical-based oxidizing agents. Recent reports have shown increased attention on electrochemically promoted metal-catalyzed C-H functionalization. From the standpoint of sustainability, environmental friendliness, and cost effectiveness, electrochemically promoted oxidation of a metal catalyst offers a mild, efficient, and atom-economical alternative to traditional chemical oxidants. This Review discusses advances in the field of transition metal-electrocatalyzed C-H functionalization over the past decade and describes how the unique features of electricity enable metal-catalyzed C-H functionalization in an economic and sustainable way.

Will the next generation of chemical plants be in miniaturized flow reactors?

For decades, a production paradigm based on centralized, stepwise, large scale processes has dominated the chemical industry horizon. While effective to meet an ever increasing demand for high value-added chemicals, the so-called macroscopic batch reactors are also associated with inherent weaknesses and threats; some of the most obvious ones were tragically illustrated over the past decades with major industrial disasters and impactful disruptions of advanced chemical supplies. The COVID pandemic has further emphasized that a change in paradigm was necessary to sustain chemical production with an increased safety, reliable supply chains and adaptable productivities. More than a decade of research and technology development has led to alternative and effective chemical processes relying on miniaturised flow reactors (a.k.a. micro and mesofluidic reactors). Such miniaturised reactors bear the potential to solve safety concerns and to improve the reliability of chemical supply chains. Will they initiate a new paradigm for a more localized, safe and reliable chemical production?

Electrochemically driven [4+2] benzannulation: synthesis of polycyclic (hetero)aromatic compounds

A green and economical electrochemical protocol has been developed to synthesize polycyclic (hetero)aromatic compounds by the [4+2] benzannulation of biaryldiazonium salts with alkynes. This protocol features a broad substrate scope. Instead of requiring diazonium reagents, these reactions can begin from anilines and can be carried out in one pot. Moreover, the readily accessible scale-up synthesis achieved by using an electrochemical flow cell demonstrates the synthetic potential of this protocol.

Towards Antibiotic Synthesis in Continuous-Flow Processes

Continuous-flow chemistry has become a mainstream process and a notable trend among emerging technologies for drug synthesis. It is routinely used in academic and industrial laboratories to generate a wide variety of molecules and building blocks. The advantages it provides, in terms of safety, speed, cost efficiency and small-equipment footprint compared to analog batch processes, have been known for some time. What has become even more important in recent years is its compliance with the quality objectives that are required by drug-development protocols that integrate inline analysis and purification tools. There can be no doubt that worldwide government agencies have strongly encouraged the study and implementation of this innovative, sustainable and environmentally friendly technology. In this brief review, we list and evaluate the development and applications of continuous-flow processes for antibiotic synthesis. This work spans the period of 2012-2022 and highlights the main cases in which either active ingredients or their intermediates were produced under continuous flow. We hope that this manuscript will provide an overview of the field and a starting point for a deeper understanding of the impact of flow chemistry on the broad panorama of antibiotic synthesis.

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.

Machine-Learning-Guided Discovery of Electrochemical Reactions

The molecular structures synthesizable by organic chemists dictate the molecular functions they can create. The invention and development of chemical reactions are thus critical for chemists to access new and desirable functional molecules in all disciplines of organic chemistry. This work seeks to expedite the exploration of emerging areas of organic chemistry by devising a machine-learning-guided workflow for reaction discovery. Specifically, this study uses machine learning to predict competent electrochemical reactions. To this end, we first develop a molecular representation that enables the production of general models with limited training data. Next, we employ automated experimentation to test a large number of electrochemical reactions. These reactions are categorized as competent or incompetent mixtures, and a classification model was trained to predict reaction competency. This model is used to screen 38,865 potential reactions in silico, and the predictions are used to identify a number of reactions of synthetic or mechanistic interest, 80% of which are found to be competent. Additionally, we provide the predictions for the 38,865-member set in the hope of accelerating the development of this field. We envision that adopting a workflow such as this could enable the rapid development of many fields of chemistry.

Fuel Cell Reactors for the Clean Cogeneration of Electrical Energy and Value-Added Chemicals

Fuel cell reactors can be tailored to simultaneously cogenerate value-added chemicals and electrical energy while releasing negligible CO2 emissions or other pollution; moreover, some of these reactors can even “breathe in” poisonous gas as feedstock. Such clean cogeneration favorably offsets the fast depletion of fossil fuel resources and eases growing environmental concerns. These unique reactors inherit advantages from fuel cells: a high energy conversion efficiency and high selectivity. Compared with similar energy conversion devices with sandwich structures, fuel cell reactors have successfully “hit three birds with one stone” by generating power, producing chemicals, and maintaining eco-friendliness. In this review, we provide a systematic summary on the state of the art regarding fuel cell reactors and key components, as well as the typical cogeneration reactions accomplished in these reactors. Most strategies fall short in reaching a win–win situation that meets production demand while concurrently addressing environmental issues. The use of fuel cells (FCs) as reactors to simultaneously produce value-added chemicals and electrical power without environmental pollution has emerged as a promising direction. The FC reactor has been well recognized due to its “one stone hitting three birds” merit, namely, efficient chemical production, electrical power generation, and environmental friendliness. Fuel cell reactors for cogeneration provide multidisciplinary perspectives on clean chemical production, effective energy utilization, and even pollutant treatment, with far-reaching implications for the wider scientific community and society. The scope of this review focuses on unique reactors that can convert low-value reactants and/or industrial wastes to value-added chemicals while simultaneously cogenerating electrical power in an environmentally friendly manner.

Graphical Abstract

A schematic diagram for the concept of fuel cell reactors for cogeneration of electrical energy and value-added chemicals

Ultrastable Super-Hydrophobic Surface with an Ordered Scaly Structure for Decompression and Guiding Liquid Manipulation

Directional droplet manipulation is very crucial in microfluidics, intelligent liquid management, etc. However, excessive liquid pressure tends to destroy the solid-gas-liquid (SAL) composite interface, creating a highly adhesive surface, which is not conducive to liquid transport. Herein, we propose a strategy to enhance the surface durability, in which the surface cannot withstand liquid pressure only by "blocking" but must instead guide liquid transport for "decompression". Learning from the water resistance of water strider legs and the drag reduction of shark skin, we present a continuous integrated system to obtain an ultrastable super-hydrophobic surface with a highly ordered scaly structure via a liquid flow-induced alignment method for lossless unidirectional liquid transport. The nonwetting scaly structure can both buffer liquid pressure and drive droplet motion to further reduce the vertical pressure of the liquid. Moreover, droplets can be manipulated unidirectionally using a voice. This work could aid in manufacturing scalable anisotropic micro-nanostructure surfaces, which inspires efforts in realizing lossless continuous liquid control on demand and related microfluidic applications.

Scalable Microreactor Concept for the Continuous Kolbe Electrolysis of Carboxylic Acids Using Aqueous Electrolyte

The Kolbe electrolysis is a promising reaction to combine the usage of electrons as reagents and the application of renewable generated carboxylic acids as raw materials producing value added chemicals. Within this study, the electrolysis was conducted in a specially developed concept electrochemical microreactor and draws the particular attention to continuous operation and reuse of the aqueous electrolyte as well as of the dissolved unreacted feedstock. The electrolysis was conducted in alkaline aqueous solution using n-octanoic acid as model substance. Platinized titanium as anode material in an undivided cell setup was shown to give Kolbe selectivity above 90 %. During the technically relevant conditions of current densities up to 0.6 A cm-2 and overall electrolysis times of up to 3 h, a high electrode stability was observed. Finally, a proof-of-concept continuous operation and the numbering up potential of the ECMR could be demonstrated.

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.

Cathodic Radical Cyclisation of Aryl Halides Using a Strongly-Reducing Catalytic Mediator in Flow

Electro-reductive radical cyclisation of aryl halides affords the corresponding hetero- and carbo-cycles in an undivided flow reactor equipped with steel and carbon electrodes using an organic mediator. A dissolving metal anode is not needed, and the mediator can be employed in a sub-stoichiometric amount (0.05 equiv), increasing the practical utility of cathodic radical cyclisation. The methodology is applied to O-, N-, and C-tethers, yielding tricyclic fused and spiro systems. In the absence of mediator, the major pathway is hydrogenolysis of the C-X bond, a 2 e- process occurring at the cathode. Predominance of the radical pathway in presence of a strongly reducing mediator (M) is consistent with homogeneous electron-transfer in a reaction layer detached from the cathode surface, where the flux of M.- leaving the electrode is such that little aryl halide reaches the cathode.

First example of organocatalysis by cathodic N-heterocyclic carbene generation and accumulation using a divided electrochemical flow cell

In this paper we present the first electrochemical generation of NHC carried out in a divided flow cell. The flow cell operated in the recycle mode. The need for a divided cell derived from the anodic electroactivity of the electrogenerated carbene. In order to have NHC accumulation in the catholyte, the Nafion membrane (cell separator) was pretreated with an alkaline solution. The formation of NHC was quantified as its reaction product with elemental sulfur. The NHC was successfully used as organocatalyst in two classical umpolung reactions of cinnamaldehyde: its cyclodimerization and its oxidative esterification.

Rapid access to organic triflates based on flash generation of unstable sulfonium triflates in flow

Flash (extremely fast) electrochemical generation of unstable arylbis(arylthio)sulfonium triflates [ArS(ArSSAr)]⁺ [OTf]^- that are unsuitable for accumulation in batch processes was achieved within 10 s in a divided-type flow electrochemcial reactor, enabling one-flow access to vinyl triflates, short-lived oxocarbenium triflates and glycosyl triflates.