An extended channel length microflow electrolysis cell for convenient laboratory synthesis

Spatio-temporal detachment of homogeneous electron transfer in controlling selectivity in mediated organic electrosynthesis

In electrosynthesis, electron transfer (ET) mediators are normally chosen such that they are more easily reduced (or oxidised) than the substrate for cathodic (or anodic) processes; setting the electrode potential to the mediator therefore ensures selective heterogeneous ET with the mediator at the electrode, rather than the substrate. The current work investigates the opposite, and counter intuitive, situation for a successful mediated electroreductive process where the mediator (phenanthrene) has a reduction potential that is negative to that of the substrate, and the cathode potential is set negative to both (Eele < EM < Es). Simulations reveal a complex interplay between mass transport, the relative concentrations of the mediator and substrate as well as the heterogeneous and homogeneous rate constants for multiple steps, which under suitable conditions, leads to separation of the homogeneous chemistry in a reaction layer detached from the electrode. Reaction layer detachment is a spatio-temporal effect arising due to opposing fluxes of the mediator radical anion M˙- and the substrate 1, which ultimately prevents 1 from reaching the electrode, thereby affording a different reaction pathway. Simulations representative of unstirred batch (1D) and flow (2D) electrolysis are presented, which qualitatively reproduce the experimental selectivity outcomes for mediated and unmediated electroreductive cyclisation of aryl iodide 1. The potential to use highly reducing homogeneous ET agents, possessing reduction potentials beyond those of the substrates, offers exciting opportunities in mediated electrosynthesis.

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.

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.

C-H Bond Functionalization under Electrochemical Flow Conditions

Electrochemical C-H functionalization is a rapidly growing area of interest in organic synthesis. To achieve maximum atom economy, the flow electrolysis process is more sustainable. This allows shorter reaction times, safer working environments, and better selectivities. Using this technology, the problem of overoxidation can be reduced and less emergence of side products or no side products are possible. Flow electro-reactors provide high surface-to-volume ratios and contain electrodes that are closely spaced where the diffusion layers overlap to give the desired product, electrochemical processes can now be managed without the need for a deliberately added supporting electrolyte. Considering the importance of flow electrochemical C-H functionalization, a comprehensive review is presented. Herein, we summarize flow electrolysis for the construction of C-C and C-X (X=O, N, S, and I) bonds formation. Also, benzylic oxidation and access to biologically active molecules are discussed.

Self-Optimization of Continuous Flow Electrochemical Synthesis Using Fourier Transform Infrared Spectroscopy and Gas Chromatography

A continuous-flow electrochemical synthesis platform has been developed to enable self-optimization of reaction conditions of organic electrochemical reactions using attenuated total reflection Fourier transform infrared spectroscopy (ATR FT-IR) and gas chromatography (GC) as online real-time monitoring techniques. We have overcome the challenges in using ATR FT-IR as the downstream analytical methods imposed when a large amount of hydrogen gas is produced from the counter electrode by designing two types of gas-liquid separators (GLS) for analysis of the product mixture flowing from the electrochemical reactor. In particular, we report an integrated GLS with an ATR FT-IR probe at the reactor outlet to give a facile and low-cost solution to determining the concentrations of products in gas-liquid two-phase flow. This approach provides a reliable method for quantifying low-volatile analytes, which can be problematic to be monitored by GC. Two electrochemical reactions the methoxylation of 1-formylpyrrolidine and the oxidation of 3-bromobenzyl alcohol were investigated to demonstrate that the optimal conditions can be located within the pre-defined multi-dimensional reaction parameter spaces without intervention of the operator by using the stable noisy optimization by branch and FIT (SNOBFIT) algorithm.

Development of an electrochemical flow-through cell for the fast and easy generation of isotopically labeled metabolite standards

In drug development, metabolite standards of new chemical entities are required for a comprehensive safety evaluation. Stable isotope-labeled internal metabolite standards at the milligram scale, which are difficult and expensive to synthesize in common bottom-up approaches, are necessary for metabolite quantification using liquid chromatography/mass spectrometry. A preparative electrochemical flow-through cell is presented here as a powerful tool for the cheap and straightforward synthesis of milligram amounts of isotopically labeled metabolite standards. The developed cell scales up established, so-called "coulometric" electrochemical cells. Problems like electrode fouling and cross contamination between syntheses are addressed by the use of exchangeable working electrodes. The applicability of the developed cell for the synthesis of metabolite standards is demonstrated using isotopically labeled acetaminophen as a model system for the generation of a biologically relevant phase II metabolite.

The Longer Route can be Better: Electrosynthesis in Extended Path Flow Cells

This personal account provides an overview of work conducted in my research group, and through collaborations with other chemists and engineers, to develop flow electrolysis cells and apply these cells in organic electrosynthesis. First, a brief summary of my training and background in organic synthesis is provided, leading in to the start of flow electrosynthesis in my lab in collaboration with Derek Pletcher. Our work on the development of extended path electrolysis flow reactors is described from a synthetic organic chemist's perspective, including laboratory scale-up to give several moles of an anodic methoxylation product in one day. The importance of cell design is emphasised with regards to achieving good performance in laboratory electrosynthesis with productivities from hundreds of mg h^-1 to many g h^-1 , at high conversion in a selective fashion. A simple design of recycle flow cell that can be readily constructed in a small University workshop is also discussed, including simple modifications to improve cell performance. Some examples of flow electrosyntheses are provided, including Shono-type oxidation, anodic cleavage of protecting groups, Hofer-Moest reaction of cubane carboxylic acids, oxidative esterification and amidation of aldehydes, and reduction of aryl halides.

Electrochemical Radiofluorination of Small Molecules: New Advances

The development of new ¹⁸ F-based radiopharmaceuticals constantly demands innovations in the search for new radiofluorination methods. [¹⁸ F]fluoride is the simplest and most convenient chemical form of the isotope for the synthesis of ¹⁸ F-based radiopharmaceuticals. The ease of production and handling, as well as the possibility of obtaining high molar activities, makes it the preferred choice for radiofluorination. However, the use of [¹⁸ F]fluoride in late-stage radiofluorination comes with challenges, especially for the radiolabeling of electron-rich molecules where S_N 2 and S_N Ar reactions are not suitable. New developments in fluorination chemistry have been extensively studied to overcome these difficulties. Selective electrochemical oxidation of precursors, using a controlled potential, is one method to create reactive intermediates and overcome the activation energy required for nucleophilic fluorination of electron-rich moieties. This method has been used for years in cold fluorination of organic molecules and more recently has been adapted as an alternative to traditional radiofluorination methods. Although relatively young, this field stands out as a promising route for the synthesis of new PET probes as well as fluorinated pharmaceuticals. This review focuses on recent advances in electrochemical radiofluorination as an alternative for the late-stage radiolabeling of organic molecules.

Bridging Lab and Industry with Flow Electrochemistry

A revitalization of organic electrosynthesis has incited the organic chemistry community to adopt electrochemistry as a green and cost-efficient method for activating small molecules to replace highly toxic and expensive redox chemicals. However, many of the critical challenges of batch electrosynthesis, especially for organic synthesis, still remain. The combination of continuous flow technology and electrochemistry is a potent means to enable industry to implement large scale electrosynthesis. Indeed, flow electrosynthesis helps overcome problems that mainly arise from macro batch electro-organic systems, such as mass transfer, ohmic drop, and selectivity, but this is still far from being a flawless and generic applicable process. As a result, a notable increase in research on methodology and hardware sophistication has emerged, and many hitherto uncharted chemistries have been achieved. To better help the commercialization of wide-scale electrification of organic synthesis, we highlight in this perspective the advances made in large-scale flow electrosynthesis and its future trajectory while pointing out the main challenges and key improvements of current methodologies.

Translating batch electrochemistry to single-pass continuous flow conditions: an organic chemist’s guide

The recent renaissance of electrochemical methods for organic synthesis has also attracted increased interest towards flow electrochemistry as the most suitable scale-up strategy. Many electrochemical methods using flow cells are based on recirculation of the electrolyte solution. However, single-pass processing is very attractive as it permits integration of the electrochemical reaction with other synthetic or purification steps in a continuous stream. Translation of batch electrochemical procedures to single-pass continuous flow cells can be challenging to beginners in the field. Using the electrochemical methoxylation of 4-methylanisole as model, this paper provides newcomers to the field with an overview of the factors that need to be considered to develop a flow electrochemical process, including advantages and disadvantages of operating in galvanostatic and potentiostatic mode in small scale reactions, and the effect of the interelectrode gap, supporting electrolyte concentration and pressure on the reaction performance. A comparison of the reaction efficiency in batch and flow is also presented.

Cubane Electrochemistry: Direct Conversion of Cubane Carboxylic Acids to Alkoxy Cubanes Using the Hofer-Moest Reaction under Flow Conditions

The highly strained cubane system is of great interest as a scaffold and rigid linker in both pharmaceutical and materials chemistry. The first electrochemical functionalisation of cubane by oxidative decarboxylative ether formation (Hofer-Moest reaction) was demonstrated. The mild conditions are compatible with the presence of other oxidisable functional groups, and the use of flow electrochemical conditions allows straightforward upscaling.

A design of flow electrolysis cell for ‘Home’ fabrication

Optimising the performance of a simple electrolysis flow cell design in recycle mode; application to selective anodic and cathodic electrosyntheses.

Metal Nanowire Felt as a Flow-Through Electrode for High-Productivity Electrochemistry

Flow-through electrodes such as carbon paper are used in redox flow batteries, water purification, and electroorganic syntheses. This work examines the extent to which reducing the size of the fibers to the nanoscale in a flow-through electrode can increase the productivity of electrochemical processes. A Cu nanowire felt, made from nanowires 45 times smaller than the 10 μm wide fibers in carbon paper, can achieve a productivity 278 times higher than carbon paper for mass-transport-limited reduction of Cu ions. Higher increases in productivity were predicted for the Cu nanowire felt based on the mass-transport-limited current, but Cu ion reduction became charge transfer-limited on Cu nanowire felt at high concentrations and flow rates when the mass-transport-limited current became comparable to the charge transfer-limited current. In comparison, the reaction rate on carbon paper was mass-transport-limited under all concentrations and flow rates because its mass-transport-limited current was much lower than its charge transfer-limited current. Higher volumetric productivities were obtained for the Cu nanowire felt by switching from Cu ion reduction to Alizarin Red S (ARS) reduction, which has a higher reaction rate constant. An electroorganic intramolecular cyclization reaction with Cu nanowire felt achieved a productivity 4.2 times higher than that of carbon paper, although this reaction was also affected by charge transfer kinetics. This work demonstrates that large gains in productivity can be achieved with nanostructured flow-through electrodes, but the potential gains can be limited by the charge transfer kinetics of a reaction.

Continuous-Flow Electrosynthesis of Benzofused S-Heterocycles by Dehydrogenative C-S Cross-Coupling

Reported herein is the synthesis of benzofused six-membered S-heterocycles by intramolecular dehydrogenative C-S coupling using a modular flow electrolysis cell. The continuous-flow electrosynthesis not only ensures efficient product formation, but also obviates the need for transition-metal catalysts, oxidizing reagents, and supporting electrolytes. Reaction scale-up is conveniently achieved through extended electrolysis without changing the reaction conditions and equipment.

An Easy-to-Machine Electrochemical Flow Microreactor: Efficient Synthesis of Isoindolinone and Flow Functionalization

Flow electrochemistry is an efficient methodology to generate radical intermediates. An electrochemical flow microreactor has been designed and manufactured to improve the efficiency of electrochemical flow reactions. With this device only little or no supporting electrolytes are needed, making processes less costly and enabling easier purification. This is demonstrated by the facile synthesis of amidyl radicals used in intramolecular hydroaminations to produce isoindolinones. The combination with inline mass spectrometry facilitates a much easier combination of chemical steps in a single flow process.

The Hitchhiker's Guide to Flow Chemistry ∥

Flow chemistry involves the use of channels or tubing to conduct a reaction in a continuous stream rather than in a flask. Flow equipment provides chemists with unique control over reaction parameters enhancing reactivity or in some cases enabling new reactions. This relatively young technology has received a remarkable amount of attention in the past decade with many reports on what can be done in flow. Until recently, however, the question, "Should we do this in flow?" has merely been an afterthought. This review introduces readers to the basic principles and fundamentals of flow chemistry and critically discusses recent flow chemistry accounts.

Continuous direct anodic flow oxidation of aromatic hydrocarbons to benzyl amides

No oxidant needed – just electric current! Continuous synthesis of benzyl amides directly from aromatic hydrocarbons.