On being green: can flow chemistry help?

Flow-Integrated Preparation of Norbornadiene Precursors for Solar Thermal Energy Storage

Molecular solar thermal (MOST) energy storage systems are getting increased attention related to renewable energy storage applications. Particularly, 2,3-difunctionalized norbornadiene-quadricyclane (NBD-QC) switches bearing a nitrile (CN) group as one of the two substituents are investigated as promising MOST candidates thanks to their high energy storage densities and their red-shifted absorbance. Moreover, such NBD systems can be prepared in large quantities (a requirement for MOST-device applications) in flow through Diels-Alder reaction between cyclopentadiene and appropriately functionalized propynenitriles. However, these acetylene precursors are traditionally prepared in batch from their corresponding acetophenones using reactive chemicals potentially leading to health and physical hazards, especially when working on a several-grams scale. Here, we develop a multistep flow-chemistry route to enhance the production of these crucial precursors. Furthermore, we assess the atom economy (AE) and the E-factor showing improved green metrics compared to classical batch methods. Our results pave the way for a complete flow synthesis of NBDs with a positive impact on green chemistry aspects.

Continuous Ligand-Free Catalysis Using a Hybrid Polymer Network Support

Although the pharmaceutical and fine chemical industries primarily utilize batch homogeneous reactions to carry out chemical transformations, emerging platforms seek to improve existing shortcomings by designing effective heterogeneous catalysis systems in continuous flow reactors. In this work, we present a versatile network-supported palladium (Pd) catalyst using a hybrid polymer of poly(methylvinylether-alt-maleic anhydride) and branched polyethyleneimine for intensified continuous flow synthesis of complex organic compounds via heterogeneous Suzuki-Miyaura cross-coupling and nitroarene hydrogenation reactions. The hydrophilicity of the hybrid polymer network facilitates the reagent mass transfer throughout the bulk of the catalyst particles. Through rapid automated exploration of the continuous and discrete parameters, as well as substrate scope screening, we identified optimal hybrid network-supported Pd catalyst composition and process parameters for Suzuki-Miyaura cross-coupling reactions of aryl bromides with steady-state yields up to 92% with a nominal residence time of 20 min. The developed heterogeneous catalytic system exhibits high activity and mechanical stability with no detectable Pd leaching at reaction temperatures up to 95 °C. Additionally, the versatility of the hybrid network-supported Pd catalyst is demonstrated by successfully performing continuous nitroarene hydrogenation with short residence times (<5 min) at room temperature. Room temperature hydrogenation yields of >99% were achieved in under 2 min nominal residence times with no leaching and catalyst deactivation for more than 20 h continuous time on stream. This catalytic system shows its industrial utility with significantly improved reaction yields of challenging substrates and its utility of environmentally-friendly solvent mixtures, high reusability, scalable and cost-effective synthesis, and multi-reaction successes.

Synthesis of Dolutegravir Exploiting Continuous Flow Chemistry

An efficient continuous flow process for the synthesis of dolutegravir, an active pharmaceutical ingredient (API) for HIV treatment, was investigated. The synthetic procedure starts from a readily available benzyl-protected pyran via six chemical transformations using continuous flow reactors. The significant advantage of this flow process includes the reduction of the overall reaction time from 34.5 h in batch to 14.5 min. The overall yield of each reaction step improved dramatically upon flow optimization. Another key feature of this synthesis is telescoping multiple steps.

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.

An Automated Computer-Vision "Bubble-Counting" Technique to Characterise CO2 Dissolution into an Acetonitrile Flow Stream in a Teflon AF-2400 Tube-in-Tube Flow Device

A Teflon AF-2400 based tube-in-tube device was used to generate flow streams of CO₂ in acetonitrile and a computer-vision based 'bubble counting' technique was used to estimate the amount of CO₂ that had passed into solution whilst in the tube-in-tube device by quantifying the amount of CO₂ that left solution (forming separate gas-phase segments) downstream of the back-pressure regulator. For both CO₂ pressures used, there appeared to be a minimum residence time below which no CO₂ was observed to leave solution. This was assumed to be due to residual CO₂ below (or close to) the saturation concentration at atmospheric pressure and, by taking this into account, we were able to fit curves corresponding to simple gradient-driven diffusion and which closely matched previously obtained colorimetric titration data for the same system. The estimated value for the residual concentration of CO₂ (0.37 M) is higher than, but in reasonable general correspondence with, saturation concentrations previously reported for CO₂ in acetonitrile (0.27 M).

Amide Bonds Meet Flow Chemistry: A Journey into Methodologies and Sustainable Evolution

Formation of amide bonds is of immanent importance in organic and synthetic medicinal chemistry. Its presence in "traditional" small-molecule active pharmaceutical ingredients, in linear or cyclic oligo- and polypeptidic actives, including pseudopeptides, has led to the development of dedicated synthetic approaches for the formation of amide bonds starting from, if necessary, suitably protected amino acids. While the use of solid supported reagents is common in traditional peptide synthesis, similar approaches targeting amide bond formation in continuous-flow mode took off more significantly, after a first publication in 2006, only a couple of years ago. Most efforts rely upon the transition of traditional approaches in flow mode, or the combination of solid-phase peptide synthesis principles with flow chemistry, and advantages are mainly seen in improving space-time yields. This Review summarizes and compares the various approaches in terms of basic amide formation, peptide synthesis, and pseudopeptide generation, describing the technological approaches and the advantages that were generated by the specific flow approaches. A final discussion highlights potential future needs and perspectives in terms of greener and more sustainable syntheses.

Understanding flow chemistry for the production of active pharmaceutical ingredients

Multi-step organic syntheses of various drugs, active pharmaceutical ingredients, and other pharmaceutically and agriculturally important compounds have already been reported using flow synthesis. Compared to batch, hazardous and reactive reagents can be handled safely in flow. This review discusses the pros and cons of flow chemistry in today’s scenario and recent developments in flow devices. The review majorly emphasizes on the recent developments in the flow synthesis of pharmaceutically important products in last five years including flibanserin, imatinib, buclizine, cinnarizine, cyclizine, meclizine, ribociclib, celecoxib, SC-560 and mavacoxib, efavirenz, fluconazole, melitracen HCl, rasagiline, tamsulosin, valsartan, and hydroxychloroquine. Critical steps and new development in the flow synthesis of selected compounds are also discussed.

Rapid mild macrocyclization of depsipeptides under continuous flow: total syntheses of five cyclodepsipeptides

A systematic investigation of the flow macrocyclization approaches for five destruxin analogues natural products at three different cyclization point has been reported.

Development of a modular photoreactor for the upscaling of continuous flow photochemistry

The upscaling of biphasic photochemical reactions is challenging because of the inherent constraints of liquid–gas mixing and light penetration. Using semi-permeable coaxial flow chemistry within a modular photoreactor, the photooxidation of the platform chemical furfural was scaled up to produce routinely 29 gram per day of biobased building block hydroxybutenolide, a precursor to acrylate alternatives.

An easily-built, user-friendly modular photoreactor enables optimization and upscaling of flow photooxidation.

Synthesis of novel chiral spiro-β-lactams from nitrile oxides and 6-(Z)-(benzoylmethylene)penicillanate: batch, microwave-induced and continuous flow methodologies

The first examples of the diastereoselective 1,3-dipolar cycloaddition reaction of nitrile oxides and 6-alkylidene penicillanates leading to chiral spiroisoxazoline-penicillanates are reported. The synthesis of this new type of penicillanate involved the selective generation of two consecutive stereogenic centers, including a quaternary chiral center. Furthermore, the present work also describes the outcomes of these 1,3-dipolar cycloaddition reactions under three distinct reaction conditions (conventional heating, microwave irradiation and continuous flow). The successful use of the continuous flow technique as well as the proper selection of the reaction media allowed the development of a sustainable route to chiral spiroisoxazoline-penicillanates.

The first examples of the diastereoselective 1,3-dipolar cycloaddition reaction of nitrile oxides and 6-alkylidene penicillanates leading to chiral spiroisoxazoline-penicillanates are reported.

Continuous Flow Synthesis of Anticancer Drugs

Continuous flow chemistry is by now an established and valued synthesis technology regularly exploited in academic and industrial laboratories to bring about the improved preparation of a variety of molecular structures. Benefits such as better heat and mass transfer, improved process control and safety, a small equipment footprint, as well as the ability to integrate in-line analysis and purification tools into telescoped sequences are often cited when comparing flow to analogous batch processes. In this short review, the latest developments regarding the exploitation of continuous flow protocols towards the synthesis of anticancer drugs are evaluated. Our efforts focus predominately on the period of 2016-2021 and highlight key case studies where either the final active pharmaceutical ingredient (API) or its building blocks were produced continuously. It is hoped that this manuscript will serve as a useful synopsis showcasing the impact of continuous flow chemistry towards the generation of important anticancer drugs.

Evaluating the Green Credentials of Flow Chemistry towards Industrial Applications

Continuous flow chemistry is becoming an established technology platform that finds frequent application in industrial chemical manufacture with support and endorsements by the FDA for pharmaceuticals. Amongst the various advantages that are commonly cited for flow chemistry over batch processing, sustainability continues to require further advances and joint efforts by chemists and chemical engineers in both academia and industry. This short review highlights developments between 2015 and early 2021 that positively impact on the green credentials associated with flow chemistry, specifically when applied to the preparation of pharmaceuticals. An industrial perspective on current challenges is provided to whet discussion and stimulate further investment towards achieving greener modern synthetic technologies.

1 Introduction

2 Subject Areas and Relevant Case Studies

3 Industrial Outlook on Future Sustainability Driven through Continuous Manufacturing Approaches

4 Conclusions and Outlook

One-flow upscaling neutralization of an organophosphonate-derived pesticide/nerve agent simulant to value-added chemicals in a novel Teflon microreactor platform

Synthesizing value-added products from chemical warfare agents is a concept well beyond the usual notion of simply neutralizing the agents.

From circular synthesis to material manufacturing: advances, challenges, and future steps for using flow chemistry in novel application area

We review the emerging use of flow technologies for circular chemistry and material manufacturing, highlighting advances, challenges, and future directions.

Automated synthesis of prexasertib and derivatives enabled by continuous-flow solid-phase synthesis

Recent advances in end-to-end continuous-flow synthesis are rapidly expanding the capabilities of automated customized syntheses of small-molecule pharmacophores, resulting in considerable industrial and societal impacts; however, many hurdles persist that limit the number of sequential steps that can be achieved in such systems, including solvent and reagent incompatibility between individual steps, cumulated by-product formation, risk of clogging and mismatch of timescales between steps in a processing chain. To address these limitations, herein we report a strategy that merges solid-phase synthesis and continuous-flow operation, enabling push-button automated multistep syntheses of active pharmaceutical ingredients. We demonstrate our platform with a six-step synthesis of prexasertib in 65% isolated yield after 32 h of continuous execution. As there are no interactions between individual synthetic steps in the sequence, the established chemical recipe file was directly adopted or slightly modified for the synthesis of twenty-three prexasertib derivatives, enabling both automated early and late-stage diversification.

Optimization of a Catalytic Chemoenzymatic Tandem Reaction for the Synthesis of Natural Stilbenes in Continuous Flow

In view of the development of efficient processes for the synthesis of high-value compounds, the combination of bio- and chemocatalysis is highly promising. In addition, implementation of immobilized catalysts into continuous setups allows a straightforward separation of the target compound from the reaction mixture and ensures uniform product quality. In this work, we report the optimization of a chemoenzymatic tandem reaction in continuous flow and its extended application for the synthesis of pharmacologically active resveratrol and pterostilbene. The tandem reaction involves enzymatic decarboxylation of coumaric acid employing encapsulated phenolic acid decarboxylase from B. subtilis and a Heck coupling of the obtained vinylphenol with an aryl iodide using heterogeneous Pd-Ce-Sn oxides implemented in a packed bed reactor. By optimization of the reaction conditions for the limiting cross-coupling step, the yield of (E)-4-hydroxystilbene using the fully continuous setup could be more than doubled compared to previous work. Furthermore, the improved chemoenzymatic cascade could also be applied to the synthesis of resveratrol and pterostilbene in a continuous fashion. Leaching of the metal catalyst at high temperatures limited the process in many perspectives. Therefore, the feasibility of a reactor setup with reversed flow was experimentally evaluated and approved.

A practical experiment to teach students continuous flow and physico-chemical methods: acetylation of ethylene diamine in liquid bi-phase

Despite growing applications being reported both in academia and industry, continuous flow chemistry remains a relatively untaught field across most chemistry undergraduate courses. This is particularly true in laboratory practical classes, where it is often deemed simpler to carry out synthetic reactions in traditional batch mode using round-bottomed flasks. Herein, we report the development of an undergraduate project that utilises cheap and readily available materials to construct continuous flow reactors. The students compare the performance of different types of reactors and conditions in a biphasic selective acetylation of a symmetrical diamine. Throughout the investigation, the students can vary multiple parameters as they optimise the reaction, thus actively learning and readjusting them based on their improved understanding. The experiments give the students an appreciation of continuous flow techniques in comparison to batch.

Compartmentalized cross-linked enzymatic nano-aggregates (c-CLEnA) for efficient in-flow biocatalysis

Nano-sized enzyme aggregates, which preserve their catalytic activity are of great interest for flow processes, as these catalytic species show minimal diffusional issues, and are still sizeable enough to be effectively separated from the formed product. The realization of such catalysts is however far from trivial. The stable formation of a micro-to millimeter-sized enzyme aggregate is feasible via the formation of a cross-linked enzyme aggregate (CLEA); however, such a process leads to a rather broad size distribution, which is not always compatible with microflow conditions. Here, we present the design of a compartmentalized templated CLEA (c-CLEnA), inside the nano-cavity of bowl-shaped polymer vesicles, coined stomatocytes. Due to the enzyme preorganization and concentration in the cavity, cross-linking could be performed with substantially lower amount of cross-linking agents, which was highly beneficial for the residual enzyme activity. Our methodology is generally applicable, as demonstrated by using two different cross-linkers (glutaraldehyde and genipin). Moreover, c-CLEnA nanoreactors were designed with Candida antarctica Lipase B (CalB) and Porcine Liver Esterase (PLE), as well as a mixture of glucose oxidase (GOx) and horseradish peroxidase (HRP). Interestingly, when genipin was used as cross-linker, all enzymes preserved their initial activity. Furthermore, as proof of principle, we demonstrated the successful implementation of different c-CLEnAs in a flow reactor in which the c-CLEnA nanoreactors retained their full catalytic function even after ten runs. Such a c-CLEnA nanoreactor represents a significant step forward in the area of in-flow biocatalysis.

c-CLEnA are obtained via cross-linking enzymes in the nanocavity of supramolecular stomatocytes. Such c-CLEnA can be recycled while retaining its activity – an excellent nanoreactors platform for in-flow bio-catalysis.

Continuous Flow Photochemistry for the Preparation of Bioactive Molecules

The last decade has witnessed a remarkable development towards improved and new photochemical transformations in response to greener and more sustainable chemical synthesis needs. Additionally, the availability of modern continuous flow reactors has enabled widespread applications in view of more streamlined and custom designed flow processes. In this focused review article, we wish to evaluate the standing of the field of continuous flow photochemistry with a specific emphasis on the generation of bioactive entities, including natural products, drugs and their precursors. To this end we highlight key developments in this field that have contributed to the progress achieved to date. Dedicated sections present the variety of suitable reactor designs and set-ups available; a short discussion on the relevance of greener and more sustainable approaches; and selected key applications in the area of bioactive structures. A final section outlines remaining challenges and areas that will benefit from further developments in this fast-moving area. It is hoped that this report provides a valuable update on this important field of synthetic chemistry which may fuel developments in the future.

Recent advances in continuous-flow organocatalysis for process intensification

The progresses on continuous-flow organocatalysis from 2016 to early 2020 are reviewed with focus on transition from batch to flow.

The rise of continuous flow biocatalysis – fundamentals, very recent developments and future perspectives

Very recent developments in the field of biocatalysis in continuously operated systems. Special attention on the future perspectives in this key emerging technological area ranging from process analytical technologies to digitalization.

Rapid production of block copolymer nano-objects via continuous-flow ultrafast RAFT dispersion polymerisation

Continuous-flow reactors are exploited for conducting ultrafast RAFT dispersion polymerisation for the preparation of diblock copolymer nanoparticles.

Development of a multistep reaction cascade for the synthesis of a sacubitril precursor in continuous flow

The active pharmaceutical ingredient sacubitril acts as a neprilysin inhibitor in the body and is administered to patients suffering from high blood pressure and chronic heart failure. In this paper, we report the development of a three-step setup for the synthesis of an advanced sacubitril precursor in continuous flow. The key transformation of our cascade is a Suzuki-Miyaura coupling facilitated by a heterogeneous palladium catalyst. Its implementation in a packed-bed reactor and the application of continuous flow methodologies allow intensification of the cross-coupling reaction compared to batch processing. The subsequent steps for the synthesis of the target molecule involve Boc-deprotection as well as N-succinylation, which have been optimized using the statistical “Design of Experiments” (DoE) approach. In this way, the individual as well as interactive effects of selected parameters on the output of the reactions could be investigated very efficiently. The consecutive performance of the three reaction steps using an integrated setup enabled the synthesis of a late-stage sacubitril precursor in continuous flow with 81% overall yield.

Reconfigurable Flow Platform for Automated Reagent Screening and Autonomous Optimization for Bioinspired Lignans Synthesis

Naturally occurring benzoxanthenones, which belong to the vast family of lignans, are promising biologically relevant targets. They are biosynthetically produced by the oxidative dimerization of 2-propenyl phenols. In this manuscript, we disclose a powerful automated flow-based strategy for identifying and optimizing a cobalt-catalyzed oxidizing system for the bioinspired dimerization of 2-propenyl phenols. We designed a reconfigurable flow reactor associating online monitoring and process control instrumentation. Our machine was first configured as an automated screening platform to evaluate a matrix of 4 catalysts (plus the blank) and 5 oxidants (plus the blank) at two different temperatures, resulting in an array of 50 reactions. The automated screening was conducted on micromole scale at a rate of one fully characterized reaction every 26 min. After having identified the most promising cobalt-catalyzed oxidizing system, the automated screening platform was straightforwardly reconfigured to an autonomous self-optimizing flow reactor by implementation of an optimization algorithm in the closed-loop system. The optimization campaign allowed the determination of very effective experimental conditions in a limited number of experiments, which allowed us to prepare the natural products carpanone and polemannone B as well as synthetic analogues.

Integrating continuous flow synthesis with in-line analysis and data generation

Continuous flow synthesis of fine chemicals has successfully advanced from an academic niche area to a rapidly growing field of its own that directly impacts developments and applications in industrial settings. Whilst the numerous advantages of flow over batch processing are widely recognised and have led to a wider uptake of continuous flow synthesis within the community, we have reached a point where continuous flow synthesis has to transition from a stand-alone enabling technology to a readily integrated synthesis concept. Thus it is paramount to embrace a multitude of in-line analysis and purification techniques to not only allow for efficiently telescoped multi-step sequences but ultimately generate bioactivity data concomitantly on newly synthesised entities. This short review summarises the state of the art in this field and presents both challenges and opportunities that arise from this ambitious endeavour.

Peroxidation of 2-oxindole and barbituric acid derivatives under batch and continuous flow using an eco-friendly ethyl acetate solvent

We have demonstrated the magnetically retrievable Fe(OH)₃Fe₃O₄catalyzed C–H peroxidation of 2-oxindole and barbituric acid derivatives under batch and continuous flow process for the first time.

A general and atom-efficient continuous-flow approach to prepare amines, amides and imines via reactive N-chloramines

Chloramines are an important class of reagents, providing a convenient source of chlorine or electrophilic nitrogen. However, the instability of these compounds is a problem which makes their isolation and handling difficult. To overcome these hazards, a continuous-flow approach is reported which generates and immediately reacts N-chloramines directly, avoiding purification and isolation steps. 2-Chloramines were produced from the reaction of styrenes with N-alkyl-N-sulfonyl-N-chloramines, whilst N-alkyl or N,N'-dialkyl-N-chloramines reacted with anisaldehyde in the presence of t-BuO₂H oxidant to afford amides. Primary and secondary imines were produced under continuous conditions from the reaction of N-chloramines with base, with one example subsequently reduced under asymmetric conditions to produce a chiral amine in 94% ee.

Continuous Platform to Generate Nitroalkanes On-Demand (in situ) using Peracetic Acid-Mediated Oxidation in a PFA Pipes-in-Series Reactor

The synthetic utility of the aza-Henry reaction can be diminished on scale by potential hazards associated with the use of peracid to prepare nitroalkane substrates, and the nitroalkanes themselves. In response, a continuous and scalable chemistry platform to prepare aliphatic nitroalkanes on-demand is reported, using the oxidation of oximes with peracetic acid and direct reaction of the nitroalkane intermediate in an aza-Henry reaction. A uniquely designed pipes-in-series plug flow tube reactor addresses a range of process challenges including stability and safe handling of peroxides and nitroalkanes. The subsequent continuous extraction generates a solution of purified nitroalkane which can be directly used in the following enantioselective aza-Henry chemistry to furnish valuable chiral diamine precursors in high selectivity, thus, completely avoiding isolation of potentially unsafe low molecular weight nitroalkane intermediate. A continuous campaign (16 h) established that these conditions were effective in processing 100 g of the oxime and furnishing 1.4 L of nitroalkane solution.

Chiral auxiliary recycling in continuous flow: automated recovery and reuse of Oppolzer's sultam

The telescoping of a three-stage, chiral auxiliary-mediated transformation in flow is described, including continuous separation of the product and auxiliary, enabling automated auxiliary reuse.

The telescoping of a three-stage, chiral auxiliary-mediated transformation in flow is described, including continuous separation of the product and auxiliary. The auxiliary can either be collected for later reuse, or directly fed back to the beginning of the process for recycling in real time, enabling each molecule of auxiliary to make multiple equivalents of chiral product and thus minimizing the step- and atom-economy issues associated with auxiliary-mediated synthesis. This concept is demonstrated for the asymmetric hydrogenation of olefins using Oppolzer's sultam, shortening the total reaction time >100 fold compared to batch, and demonstrating formal sub-stoichiometric auxiliary loading with respect to the process by automating auxiliary recycling within a closed loop.