High-Throughput Electrochemistry: State of the Art, Challenges, and Perspective

An Electrochemical Screening Reactor Kit for Rapid Optimization of Electrosynthesis Applications

Electrosynthetic processes powered by renewable energy present a viable solution to decarbonize the chemical industry, while producing essential chemical products for modern society. However, replacing well-established thermocatalytic methods with renewable-powered electrosynthesis requires cost-efficient and highly optimized systems. Current optimization of electrolysis conditions towards industrial applications involving scalable electrodes is time-consuming, highlighting the necessity for the development of electrochemical setups aimed at rapid and material efficient testing. To address this challenge, we introduce a 3D-printed electrochemical screening reactor designed for rapid optimization of relevant electrochemical parameters, utilizing electrode and membrane materials comparable to those in scalable electrolyzers. The reactor comprises eight individual two-compartment cells that can be operated simultaneously and independently. To evaluate the reactor's ability to provide meaningful insights on scalable cell designs, trends were compared with data from conventional scalable systems for electrochemical hydrogenations (EChH), demonstrating fast and accurate parameter optimization with the screening reactor. A detailed description of the reactor design and construction data files are provided using open-source tools, enabling easy modification for anyone. We believe this screening reactor will be a valuable tool for the scientific community, for facilitating the discovery of reactions with customized electrode designs and rapidly improving conditions in established large-scale electrolyzers.

Harnessing the Reactivity of Nitroarene Radical Anions to Create Quinoline N-Oxides by Electrochemical Reductive Cyclization

Electrochemical reduction of 2-allyl-substituted nitroarenes using a simple, undivided electrochemical cell with non-precious electrodes to generate nitroarene radical anions was developed. The nitroarene radical anion intermediates participate in 1,5-hydrogen atom transfer reactions to construct quinoline N-oxides bearing aryl-, heteroaryl-, alkenyl-, benzyl-, sulfonyl-, or carboxyl groups.

Light-harvesting microelectronic devices for wireless electrosynthesis

High-throughput experimentation (HTE) has accelerated academic and industrial chemical research in reaction development and drug discovery and has been broadly applied in many domains of organic chemistry1,2. However, application of HTE in electrosynthesis-an enabling tool for chemical synthesis-has been limited by a dearth of suitable standardized reactors3-7. Here we report the development of microelectronic devices, which are produced using standard nanofabrication techniques, to enable wireless electrosynthesis on the microlitre scale. These robust and inexpensive devices are powered by visible light and convert any traditional 96-well or 384-well plate into an electrochemical reactor. We validate the devices in oxidative, reductive and paired electrolysis and further apply them to achieve the library synthesis of biologically active compounds and accelerate the development of two electrosynthetic methodologies. We anticipate that, by simplifying the way electrochemical reactions are set up, this user-friendly solution will not only enhance the experience and efficiency of current practitioners but also substantially reduce the barrier for nonspecialists to enter the field of electrosynthesis, thus allowing the broader community of synthetic chemists to explore and benefit from new reactivities and synthetic strategies enabled by electrochemistry8-12.

An Automated Electrochemical Flow Platform to Accelerate Library Synthesis and Reaction Optimization

Automated batch and flow setups are well-established for high throughput experimentation in both thermal chemistry and photochemistry. However, the development of automated electrochemical platforms is hindered by cell miniaturization challenges in batch and difficulties in designing effective single-pass flow systems. In order to address these issues, we have designed and implemented a new, slug-based automated electrochemical flow platform. This platform was successfully demonstrated for electrochemical C-N cross-couplings of E3 ligase binders with diverse amines (44 examples), which were subsequently transferred to a continuous-flow mode for confirmation and isolation, showing its applicability for medicinal chemistry purposes. To further validate the versatility of the platform, Design of Experiments (DoE) optimization was performed for an unsuccessful library target. This optimization process, fully automated by the platform, resulted in a remarkable 6-fold increase in reaction yield.

Enhancing electrochemical reactions in organic synthesis: the impact of flow chemistry

Utilizing electrons directly offers significant potential for advancing organic synthesis by facilitating novel reactivity and enhancing selectivity under mild conditions. As a result, an increasing number of organic chemists are exploring electrosynthesis. However, the efficacy of electrochemical transformations depends critically on the design of the electrochemical cell. Batch cells often suffer from limitations such as large inter-electrode distances and poor mass transfer, making flow cells a promising alternative. Implementing flow cells, however, requires a foundational understanding of microreactor technology. In this review, we briefly outline the applications of flow electrosynthesis before providing a comprehensive examination of existing flow reactor technologies. Our goal is to equip organic chemists with the insights needed to tailor their electrochemical flow cells to meet specific reactivity requirements effectively. We also highlight the application of reactor designs in scaling up electrochemical processes and integrating high-throughput experimentation and automation. These advancements not only enhance the potential of flow electrosynthesis for the synthetic community but also hold promise for both academia and industry.

Proving the automatic benchtop electrochemical station for the development of dopamine and paracetamol sensors

The introduced work represents an implementation of the automatic benchtop electrochemical station (BES) as an effective tool for the possibilities of high-throughput preparation of modified sensor/biosensors, speeding up the development of the analytical method, and automation of the analytical procedure for the determination of paracetamol (PAR) and dopamine (DOP) as target analytes. Within the preparation of gold nanoparticles modified screen-printed carbon electrode (AuNPs-SPCE) by electrodeposition, the deposition potential EDEP, the deposition time tDEP, and the concentration of HAuCl4 were optimized and their influence was monitored on 1 mM [Ru(NH3)6]3+/2+ redox probe and 50 μM DOP. The morphology of the AuNPs-SPCE prepared at various modification conditions was observed by SEM. The analytical performance of the AuNPs-SPCE prepared at different modification conditions was evaluated by a construction of the calibration curves of DOP and PAR. SPCE and AuNPs-SPCE at modification condition providing the best sensitivity to PAR and DOP, were successfully used to determine PAR and DOP in tap water by "spike-recovery" approach. The BES yields better reproducibility of the preparation of AuNPs-SPCE (RSD = 3.0%) in comparison with the case when AuNPs-SPCE was prepared manually by highly skilled laboratory operator (RSD = 7.0%).

Interfacing High-Throughput Electrosynthesis and Mass Spectrometric Analysis of Azines

Combinatorial electrochemistry has great promise for accelerated reaction screening, organic synthesis, and catalysis. Recently, we described a new high-throughput electrochemistry platform, colloquially named "Legion". Legion fits the footprint of a 96-well microtiter plate with simultaneous individual control over all 96 electrochemical cells. Here, we demonstrate the versatility of Legion when coupled with high-throughput mass spectrometry (MS) for electrosynthetic product screening and quantitation. Electrosynthesis of benzophenone azine was selected as a model reaction and was arrayed and optimized using a combination of Legion and nanoelectrospray ionization MS. The combination of high-throughput synthesis with Legion and analysis via MS proves a compelling strategy for accelerating reaction discovery and optimization in electro-organic synthesis.

Autonomous closed-loop mechanistic investigation of molecular electrochemistry via automation

Electrochemical research often requires stringent combinations of experimental parameters that are demanding to manually locate. Recent advances in automated instrumentation and machine-learning algorithms unlock the possibility for accelerated studies of electrochemical fundamentals via high-throughput, online decision-making. Here we report an autonomous electrochemical platform that implements an adaptive, closed-loop workflow for mechanistic investigation of molecular electrochemistry. As a proof-of-concept, this platform autonomously identifies and investigates an EC mechanism, an interfacial electron transfer (E step) followed by a solution reaction (C step), for cobalt tetraphenylporphyrin exposed to a library of organohalide electrophiles. The generally applicable workflow accurately discerns the EC mechanism's presence amid negative controls and outliers, adaptively designs desired experimental conditions, and quantitatively extracts kinetic information of the C step spanning over 7 orders of magnitude, from which mechanistic insights into oxidative addition pathways are gained. This work opens opportunities for autonomous mechanistic discoveries in self-driving electrochemistry laboratories without manual intervention.

Nanomaterials Synthesis Discovery via Parallel Electrochemical Deposition

Electrodeposition of nanoparticles is investigated with a multichannel potentiostat in electrochemical and chemical arrays. De novo deposition and shape control of palladium nanoparticles are explored in arrays with a two-stage strategy. Initial conditions for electrodeposition of materials are discovered in a first stage and then used in a second stage to logically expand chemical and electrochemical parameters. Shape control is analyzed primarily with scanning electron microscopy. Using this approach, optimized conditions for the electrodeposition of cubic palladium nanoparticles were identified from a set of previously untested electrodeposition conditions. The parameters discovered through the array format were then successfully extrapolated to a traditional bulk three-electrode electrochemical cell. Electrochemical arrays were also used to explore electrodeposition parameters reported in previous bulk studies, further demonstrating the correspondence between the array and bulk systems. These results broadly highlight opportunities for electrochemical arrays, both for discovery and for further investigations of electrodeposition in nanomaterials synthesis.

Identifying opportunities for late-stage C-H alkylation with high-throughput experimentation and in silico reaction screening

Enhancing the properties of advanced drug candidates is aided by the direct incorporation of specific chemical groups, avoiding the need to construct the entire compound from the ground up. Nevertheless, their chemical intricacy often poses challenges in predicting reactivity for C-H activation reactions and planning their synthesis. We adopted a reaction screening approach that combines high-throughput experimentation (HTE) at a nanomolar scale with computational graph neural networks (GNNs). This approach aims to identify suitable substrates for late-stage C-H alkylation using Minisci-type chemistry. GNNs were trained using experimentally generated reactions derived from in-house HTE and literature data. These trained models were then used to predict, in a forward-looking manner, the coupling of 3180 advanced heterocyclic building blocks with a diverse set of sp3-rich carboxylic acids. This predictive approach aimed to explore the substrate landscape for Minisci-type alkylations. Promising candidates were chosen, their production was scaled up, and they were subsequently isolated and characterized. This process led to the creation of 30 novel, functionally modified molecules that hold potential for further refinement. These results positively advocate the application of HTE-based machine learning to virtual reaction screening.

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.

Legion: An Instrument for High-Throughput Electrochemistry

Electrochemical arrays promise utility for accelerated hypothesis testing and breakthrough discoveries. Herein, we report a new high-throughput electrochemistry platform, colloquially called "Legion," for applications in electroanalysis and electrosynthesis. Legion consists of 96 electrochemical cells dimensioned to match common 96-well plates that are independently controlled with a field-programmable gate array. We demonstrate the utility of Legion by measuring model electrochemical probes, pH-dependent electron transfers, and electrocatalytic dehalogenation reactions. We consider advantages and disadvantages of this new instrumentation, with the hope of expanding the electrochemical toolbox.

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.

Ni- and Ni/Pd-Catalyzed Reductive Coupling of Lignin-Derived Aromatics to Access Biobased Plasticizers

Lignin-derived aromatic chemicals offer a compelling alternative to petrochemical feedstocks, and new applications are the focus of extensive interest. 4-Hydroxybenzoic acid (H), vanillic acid (G), and syringic acid (S) are readily obtained via oxidative depolymerization of hardwood lignin substrates. Here, we explore the use of these compounds to access biaryl dicarboxylate esters that represent biobased, less toxic alternatives to phthalate plasticizers. Chemical and electrochemical methods are developed for catalytic reductive coupling of sulfonate derivatives of H, G, and S to access all possible homo- and cross-coupling products. A conventional NiCl₂/bipyridine catalyst is able to access the H-H and G-G products, but new catalysts are identified to afford the more challenging coupling products, including a NiCl₂/bisphosphine catalyst for S-S and a NiCl₂/phenanthroline/PdCl₂/phosphine cocatalyst system for H-G, H-S, and G-S. High-throughput experimentation methods with a chemical reductant (Zn powder) are shown to provide an efficient screening platform for identification of new catalysts, while electrochemical methods can access improved yields and/or facilitate implementation on larger scale. Plasticizer tests are performed with poly(vinyl chloride), using esters of the 4,4'-biaryl dicarboxylate products. The H-G and G-G derivatives, in particular, exhibit performance advantages relative to an established petroleum-based phthalate ester plasticizer.

Stirring-Free Scalable Electrosynthesis Enabled by Alternating Current

Alternating current (AC) electrolysis is receiving increased interest as a versatile tool for mild and selective electrochemical transformations. This work demonstrates that AC can enable the concept of a stirring-free electrochemical reactor where the periodic switch of electrode polarity, inherent to AC, provides uniform electrolysis across the whole volume of the reactor. Such design implies a straightforward approach for scaling up electrosynthesis. This was demonstrated on the range of electrochemical transformations performed in three different RVC-packed reactors on up to a 50-mmol scale. Redox-neutral, oxidative, and reductive processes were successfully implemented using the suggested design and the applicable frequency ranges were further investigated for different types of reactions. The advantages of the AC-enabled design - such as the absence of stirring and a maximized surface area of the electrodes - provide the possibility for its universal application both for small-scale screening experimentation and large-scale preparative electrosynthesis without significant optimization needed in between.

Flash Synthesis and Continuous Production of C-Arylglycosides in a Flow Electrochemical Reactor

Electrochemistry provides a green and atom-efficient route to synthesize pharmaceutical and useful functional molecules, as it eliminates the need for the harsh chemical oxidants and reductants commonly used in traditional chemical reactions. To promote the implementation of electrochemical processes in the industry, there is a strong demand for the development of technologies that would allow for scale-up and a shortened reaction process time. Herein, we report that electrolysis was successfully accomplished using a flow-divided-electrochemical reactor within a few seconds, enabling the desired chemical conversion in a short period of time. Moreover, the narrow electrode gap of the flow reactor, which offers greener conditions than the conventional batch reactor, resulted in the continuous flash synthesis of C-arylglycosides.

Working at the interfaces of data science and synthetic electrochemistry

Electrochemistry is quickly entering the mainstream of synthetic organic chemistry. The diversity of new transformations enabled by electrochemistry is to a large extent a consequence of the unique features and reaction parameters in electrochemical systems including redox mediators, applied potential, electrode material, and cell construction. While offering chemists new means to control reactivity and selectivity, these additional features also increase the dimensionalities of a reaction system and complicate its optimization. This challenge, however, has spawned increasing adoption of data science tools to aid reaction discovery as well as development of high-throughput screening platforms that facilitate the generation of high quality datasets. In this Perspective, we provide an overview of recent advances in data-science driven electrochemistry with an emphasis on the opportunities and challenges facing this growing subdiscipline.

Photons or Electrons? A Critical Comparison of Electrochemistry and Photoredox Catalysis for Organic Synthesis

Redox processes are at the heart of synthetic methods that rely on either electrochemistry or photoredox catalysis, but how do electrochemistry and photoredox catalysis compare? Both approaches provide access to high energy intermediates (e.g., radicals) that enable bond formations not constrained by the rules of ionic or 2 electron (e) mechanisms. Instead, they enable 1e mechanisms capable of bypassing electronic or steric limitations and protecting group requirements, thus enabling synthetic chemists to disconnect molecules in new and different ways. However, while providing access to similar intermediates, electrochemistry and photoredox catalysis differ in several physical chemistry principles. Understanding those differences can be key to designing new transformations and forging new bond disconnections. This review aims to highlight these differences and similarities between electrochemistry and photoredox catalysis by comparing their underlying physical chemistry principles and describing their impact on electrochemical and photochemical methods.

Catalytic Furfural/5-Hydroxymethyl Furfural Oxidation to Furoic Acid/Furan-2,5-dicarboxylic Acid with H2 Production Using Alkaline Water as the Formal Oxidant

Furfural and 5-hydroxymethyl furfural (HMF) are abundantly available biomass-derived renewable chemical feedstocks, and their oxidation to furoic acid and furan-2,5-dicarboxylic acid (FDCA), respectively, is a research area with huge prospective applications in food, cosmetics, optics, and renewable polymer industries. Water-based oxidation of furfural/HMF is a lucrative approach for simultaneous generation of H2 and furoic acid/FDCA. However, this process is currently limited to (photo)electrochemical methods that can be challenging to control, improve, and scale up. Herein, we report well-defined ruthenium pincer catalysts for direct homogeneous oxidation of furfural/HMF to furoic acid/FDCA, using alkaline water as the formal oxidant while producing pure H2 as the reaction byproduct. Mechanistic studies indicate that the ruthenium complex not only catalyzes the aqueous oxidation but also actively suppresses background decomposition by facilitating initial Tishchenko coupling of substrates, which is crucial for reaction selectivity. With further improvement, this process can be used in scaled-up facilities for a simultaneous renewable building block and fuel production.

Picomole-Scale Transition Metal Electrocatalysis Screening Platform for Discovery of Mild C-C Coupling and C-H Arylation through in Situ Anodically Generated Cationic Pd

Development of new transition-metal-catalyzed electrochemistry promises to improve overall synthetic efficiency. Here, we describe the first integrated platform for online screening of electrochemical transition-metal catalysis. It utilizes the intrinsic electrochemical capabilities of nanoelectrospray ionization mass spectrometry (nano-ESI-MS) and picomole-scale anodic corrosion of a Pd electrode to generate and evaluate highly efficient cationic catalysts for mild electrocatalysis. We demonstrate the power of the novel electrocatalysis platform by (1) identifying electrolytic Pd-catalyzed Suzuki coupling at room temperature, (2) discovering Pd-catalyzed electrochemical C-H arylation in the absence of external oxidant or additive, (3) developing electrolyzed Suzuki coupling/C-H arylation cascades, and (4) achieving late-stage functionalization of two drug molecules by the newly developed mild electrocatalytic C-H arylation. More importantly, the scale-up reactions confirm that new electrochemical pathways discovered by nano-ESI can be implemented under the conventional electrolytic reaction conditions. This approach enables in situ mechanistic studies by capturing various intermediates including transient transition metal species by MS, and thus uncovering the critical role of anodically generated cationic Pd catalyst in promoting otherwise sluggish transmetalation in C-H arylation. The anodically generated cationic Pd with superior catalytic efficiency and novel online electrochemical screening platform hold great potential for discovering mild transition-metal-catalyzed reactions.