A Microflow Electrolysis Cell for Laboratory Synthesis on the Multigram Scale

Csp2-H functionalization of phenols: an effective access route to valuable materials via Csp2-C bond formation

In the vast majority of top-selling pharmaceutical and industrial products, phenolic structural motifs are highly prevalent. Non-functionalized simple phenols serve as building blocks in the synthesis of value-added chemicals. It is worth mentioning that lignin, being the largest renewable biomass source of aromatic building blocks in nature, mainly consists of phenolic units, which enable the production of structurally diverse phenols. Given their remarkable applicability in the chemical value chain, many efforts have been devoted to increasing the molecular complexity of the phenolic scaffold. Among the key techniques, direct functionalization of Csp2-H is a powerful tool, enabling the construction of new Csp2-C bonds in an economical and atomic manner. Herein we present and summarize the large plethora of direct Csp2-H functionalization methods that enables scaffold diversification of simple, unprotected phenols, leading to the formation of new Csp2-C bonds. In this review article, we intend to summarize the contributions that appeared in the literature mainly in the last 5 years dealing with the functionalization of unprotected phenols, both catalytic and non-catalytic. Our goal is to highlight the key findings and the ongoing challenges in the stimulating and growing research dedicated to the development of new protocols for the valorization of phenols.

Deconstructive Functionalization of Unstrained Cycloalkanols via Electrochemically Generated Aromatic Radical Cations

Herein we report an electrochemical approach for the deconstructive functionalization of cycloalkanols, where various alcohols, carboxylic acids, and N-heterocycles are employed as nucleophiles. The method has been demonstrated across a broad range of cycloalkanol substrates, including various ring sizes and substituents, to access useful remotely functionalized ketone products (36 examples). The method was demonstrated on a gram scale via single-pass continuous flow, which exhibited increased productivity in relation to the batch process.

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.

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.

Electrochemical Deconstructive Functionalization of Cycloalkanols via Alkoxy Radicals Enabled by Proton-Coupled Electron Transfer

Herein, we report a new electrochemical method for alkoxy radical generation from alcohols using a proton-coupled electron transfer (PCET) approach, showcased via the deconstructive functionalization of cycloalkanols. The electrochemical method is applicable across a diverse array of substituted cycloalkanols, accessing a broad range of synthetically useful distally functionalized ketones. The orthogonal derivatization of the products has been demonstrated through chemoselective transformations, and the electrochemical process has been performed on a gram scale in continuous single-pass flow.

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.

Development of a multistep, electrochemical flow platform for automated catalyst screening

An integrated flow platform enables the electrochemical synthesis of base-metal catalysts with high-throughput screening and rapid data generation.

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.

Flash Electrochemical Approach to Carbocations

A novel flow electrochemical reactor that accomplishes electrolysis within a few seconds in a single passage was developed. By using the flow reactor system, the flash electrochemical generation of short-lived carbocations, including oxocarbenium ions, N -acyliminium ions, glycosyl cations, and Ferrier cations was achieved within a few seconds, enabling the subsequent reaction with nucleophiles before their decomposition. Moreover, continuous operation based on the present system enabled the rapid synthesis of pharmaceutical precursors on demand.

Electrochemical oxidative Z-selective C(sp2)-H chlorination of acrylamides

An electrochemical method for the oxidative Z-selective C(sp²)-H chlorination of acrylamides has been developed. This catalyst and organic oxidant free method is applicable across various substituted tertiary acrylamides, and provides access to a broad range of synthetically useful Z-β-chloroacrylamides in good yields (22 examples, 73% average yield). The orthogonal derivatization of the products was demonstrated through chemoselective transformations and the electrochemical process was performed on gram scale in flow.

Progress in the Synthesis of N-Acyl-N,O-acetals

N-Acyl-N,O-acetals are key components in a variety of bioactive natural products. Furthermore, they are synthetic equivalents of unstable N-acylimines and building blocks in organic synthesis. Tremendous efforts have been made in the synthesis of such acetals, these methods can be broadly classified into two categories: electrochemical oxidation and chemical methods. Herein, we will summarize progress in the preparation of these subunits, which may aid the development of new synthetic methods for N-acyl-N,O-acetals.

1 Introduction

2 Synthetic Methods for Preparing N-Acyl-N,O-acetals

2.1 Electrochemical Oxidation

2.2 Chemical Methods

2.3 Other Methods

3 Summary and Outlook

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.

Application of reactor engineering concepts in continuous flow chemistry: a review

The adoption of flow technology for the manufacture of chemical entities, and in particular pharmaceuticals, has seen rapid growth over the past two decades with the technology now blurring the lines between chemistry and chemical engineering.

Electrosynthesis of 3,3′,5,5’-Tetramethyl-2,2′-biphenol in Flow

3,3′,5,5’-Tetramethyl-2,2′-biphenol is well known as an outstanding building block for ligands in transition-metal catalysis and is therefore of particular industrial interest. The electro-organic method is a powerful, sustainable, and efficient alternative to conventional synthetic approaches to obtain symmetric and non-symmetric biphenols. Here, we report the successive scale-up of the dehydrogenative anodic homocoupling of 2,4-dimethylphenol (4) from laboratory scale to the technically relevant scale in highly modular narrow gap flow electrolysis cells. The electrosynthesis was optimized in a manner that allows it to be easily adopted to different scales such as laboratory, semitechnical and technical scale. This includes not only the synthesis itself and its optimization but also a work-up strategy of the desired biphenols for larger scale. Furthermore, the challenges such as side reactions, heat development and gas evolution that arose during optimization are also discussed in detail. We have succeeded in obtaining yields of up to 62% of the desired biphenol.

Synthetic Photoelectrochemistry

Photoredox catalysis (PRC) and synthetic organic electrochemistry (SOE) are often considered competing technologies in organic synthesis. Their fusion has been largely overlooked. We review state-of-the-art synthetic organic photoelectrochemistry, grouping examples into three categories: 1) electrochemically mediated photoredox catalysis (e-PRC), 2) decoupled photoelectrochemistry (dPEC), and 3) interfacial photoelectrochemistry (iPEC). Such synergies prove beneficial not only for synthetic "greenness" and chemical selectivity, but also in the accumulation of energy for accessing super-oxidizing or -reducing single electron transfer (SET) agents. Opportunities and challenges in this emerging and exciting field are discussed.

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.

Electroorganic Synthesis under Flow Conditions

Despite the long history of electroorganic synthesis, it did not participate in the mainstream of chemical research for a long time. This is probably due to the lack of equipment and standardized protocols. However, nowadays organic electrochemistry is witnessing a renaissance, and a wide range of interesting electrochemical transformations and methodologies have been developed, not only for academic purposes but also for large scale industrial production. Depending on the source of electricity, electrochemical methods can be inherently green and environmentally benign and can be easily controlled to achieve high levels of selectivity. In addition, the generation and consumption of reactive or unstable intermediates and hazardous reagents can be achieved in a safe way. Limitations of traditional batch-type electrochemical methods such as the restricted electrode surface, the necessity of supporting electrolytes, and the difficulties in scaling up can be alleviated using electrochemical flow cells. Microreactors offer high surface-to-volume ratios and enable precise control over temperature, residence time, flow rate, and pressure. In addition, efficient mixing, enhanced mass and heat transfer, and handling of small volumes lead to simpler scaling-up protocols and minimize safety concerns. Electrolysis under flow conditions reduces the possibility of overoxidation as the reaction mixture is flown continuously out of the reactor in contrast to traditional batch-type electrolysis cells. In this Account, we highlight our contributions in the area of electroorganic synthesis under flow conditions over the past decade. We have designed and manufactured different generations of electrochemical flow cells. The first-generation reactor was effectively used in developing a simple one-step synthesis of diaryliodonium salts and used in proof-of-concept reactions such as the trifluoromethylation of electron-deficient alkenes via Kolbe electrolysis of trifluoroacetic acid in addition to the selective deprotection of the isonicotinyloxycarbonyl (iNoc) group from carbonates and thiocarbonates. The improved second-generation flow cell enabled the development of efficient synthesis of isoindolinones, benzothiazoles, and thiazolopyridines, achieving gram-scale for some of the products easily without changing the reactor design or reoptimizing the reaction parameters. In addition, the same reactor was used in the development of an efficient continuous flow electrochemical synthesis of hypervalent iodine reagents. The generated unstable hypervalent iodine reagents were easily used without isolation in various oxidative transformations in a coupled flow/flow manner and could be easily transformed into bench-stable reagents via quantitative ligand exchange with the appropriate acids. Our second-generation reactor was further improved and commercialized by Vapourtec Ltd. We have demonstrated the power of online analysis in accelerating optimizations and methodology development. Online mass spectrometry enabled fast screening of the charge needed for the cyclization of amides to isoindolinones. The power of online 2D-HPLC combined with a Design of Experiments approach empowered the rapid optimization of stereoselective electrochemical alkoxylations of amino acid derivatives.

Manganese-Catalyzed Electrochemical Deconstructive Chlorination of Cycloalkanols via Alkoxy Radicals

A manganese-catalyzed electrochemical deconstructive chlorination of cycloalkanols has been developed. This electrochemical method provides access to alkoxy radicals from alcohols and exhibits a broad substrate scope, with various cyclopropanols and cyclobutanols converted into synthetically useful β- and γ-chlorinated ketones (40 examples). Furthermore, the combination of recirculating flow electrochemistry and continuous inline purification was employed to access products on a gram scale.

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.