A field guide to flow chemistry for synthetic organic chemists

Recent advances in continuous flow synthesis of metal-organic frameworks and their composites

Metal-organic frameworks (MOFs) and their composites have garnered significant attention in recent years due to their exceptional properties and diverse applications across various fields. The conventional batch synthesis methods for MOFs and their composites often suffer from challenges such as long reaction times, poor reproducibility, and limited scalability. Continuous flow synthesis has emerged as a promising alternative for overcoming these limitations. In this short review, we discuss the recent advancements, challenges, and future perspectives of continuous flow synthesis in the context of MOFs and their composites. The review delves into a brief overview of the fundamental principles of flow synthesis, highlighting its advantages over batch methods. Key benefits, including precise control over reaction parameters, improved scalability and efficiency, rapid optimization capabilities, enhanced reaction kinetics and mass transfer, and increased safety and environmental sustainability, are addressed. Additionally, the versatility and flexibility of flow synthesis techniques are discussed. The article then explores various flow synthesis methods applicable to MOF and MOF composite production. The techniques covered include continuous flow solvothermal synthesis, mechanochemical synthesis, microwave and ultrasound-assisted flow synthesis, microfluidic droplet synthesis, and aerosol synthesis. Notably, the combination of flow chemistry and aerosol synthesis with real-time characterization is also addressed. Furthermore, the impact of flow synthesis on the properties and performance of MOFs is explored. Finally, the review discusses current challenges and future perspectives in the field of continuous flow MOF synthesis, paving the way for further development and broader application of this promising technique.

Convergent approach for direct cross-coupling enabled by flash irreversible generation of cationic and anionic species

In biosynthesis multiple kinds of reactive intermediates are generated, transported, and reacted across different parts of organisms, enabling highly sophisticated synthetic reactions. Herein we report a convergent synthetic approach, which utilizes dual intermediates of cationic and carbanionic species in a single step, hinted at by the ideal reaction conditions. By reactions of unsaturated precursors, such as enamines, with a superacid in a flow microreactor, cationic species, such as iminium ions, are generated rapidly and irreversibly, and before decomposition, they are transported to react with rapidly and independently generated carbanions, enabling direct C-C bond formation. Taking advantage of the reactivity of these double reactive intermediates, the reaction take place within a few seconds, enabling synthetic reactions which are not applicable in conventional reactions.

Copper-catalyzed asymmetric allylic substitution of racemic/meso substrates

The synthesis of enantiomerically pure compounds is a pivotal subject in the field of chemistry, with enantioselective catalysis currently standing as the primary approach for delivering specific enantiomers. Among these strategies, Cu-catalyzed asymmetric allylic substitution (AAS) is significant and irreplaceable, especially when it comes to the use of non-stabilized nucleophiles (pK a > 25). Although Cu-catalyzed AAS of prochiral substrates has also been widely developed, methodologies involving racemic/meso substrates are highly desirable, as the substrates undergo dynamic processes to give single enantiomer products. Inspired by the pioneering work of the Alexakis, Feringa and Gennari groups, Cu-catalyzed AAS has been continuously employed in deracemization and desymmetrization processes for the synthesis of enantiomerically enriched products. In this review, we mainly focus on the developments of Cu-catalyzed AAS with racemic/meso substrates over the past two decades, providing an explicit outline of the ligands employed, the scope of nucleophiles, the underlying dynamic processes and their practical applications.

In-situ Oxidation and Coupling of Anilines towards Unsymmetric Azobenzenes Using Flow Chemistry

Molecular switches, especially azobenzenes, are used in numerous applications, such as molecular solar thermal storage (MOST) systems and photopharmacology. The Baeyer-Mills reaction of anilines and nitrosobenzenes has been established as an efficient synthetic method for non-symmetric azobenzenes. However, nitrosobenzenes are not stable, depending on their substitution pattern and pose a health risk. An in-situ oxidation of anilines with Oxone® was optimized under continuous flow conditions avoiding isolation and contact. The in-situ generated nitrosobenzene derivatives were subjected to a telescoped Baeyer-Mills reaction in flow. That way azobenzenes with a broad substituent spectrum were made accessible.

Umpolung Flow Chemistry for the Synthesis of a 3-Oxo-3H-spiro[benzofuran-2,4'-piperidine] Building Block

An efficient and scalable route to tert-butyl 3-oxo-3H-spiro[benzofuran-2,4'-piperidine]-1'-carboxylate, a central prochiral intermediate in the synthesis of SHP2 inhibitor GDC-1971 (migoprotafib), was achieved. Preparation of the title compound from readily available 2-fluorobenzaldehyde included formation of a modified Katritzky benzotriazole hemiaminal, which, upon deprotonation by n-butyllithium, participated in umpolung reactivity via 1,2-addition to tert-butyl 4-oxopiperidine-1-carboxylate (N-Boc-4-piperidone). Most notably, this reaction was developed as a robust plug-flow process that could be executed on multiple kilograms without the need for pilot-scale reaction vessels operating at low cryogenic temperatures. Treatment of the resulting tetrahedral intermediate with oxalic acid resulted in collapse to the corresponding 4-(2-fluorobenzoyl)-4-hydroxypiperidine, which was isolated as a solid via crystallization. The synthesis concluded with an optimized intramolecular SNAr reaction and final crystallization to generate tert-butyl 3-oxo-3H-spiro[benzofuran-2,4'-piperidine]-1'-carboxylate as a stable, high-quality intermediate suitable for further functionalization toward GDC-1971.

Cyclopropanation of Alkenes with Halodiazirines as Halocarbene Precursors in Continuous Flow

The cyclopropanation reaction of alkenes with photolytically-generated chlorocarbenes from chlorodiazirines is reported as an effective way to prepare substituted 3-chloro-3-aryl-cyclopropanes. This practical and efficient approach allows the synthesis of various 3-chloro-3-aryl-cyclopropanes (32 examples) in continuous flow in 5-minute residence time under light-emitting diode (LED) irradiation. The conditions using 380 nm LED irradiation were successfully extended to the synthesis of substituted 3-bromo-3-aryl-cyclopropanes (3 examples).

Rapid and scalable photocatalytic C(sp2)-C(sp3) Suzuki-Miyaura cross-coupling of aryl bromides with alkyl boranes

In recent years, there has been a growing demand for drug design approaches that incorporate a higher number of sp3-hybridized carbons, necessitating the development of innovative cross-coupling strategies to reliably introduce aliphatic fragments. Here, we present a powerful approach for the light-mediated B-alkyl Suzuki-Miyaura cross-coupling between alkyl boranes and aryl bromides. Alkyl boranes were easily generated via hydroboration from readily available alkenes, exhibiting excellent regioselectivity and enabling the selective transfer of a diverse range of primary alkyl fragments onto the arene ring under photocatalytic conditions. This methodology eliminates the need for expensive catalytic systems and sensitive organometallic compounds, operating efficiently at room temperature within just 30 min. We further demonstrate the translation of the present protocol to continuous-flow conditions, enhancing scalability, safety, and overall efficiency of the method. This versatile approach offers significant potential for accelerating drug discovery efforts by enabling the introduction of complex aliphatic fragments in a straightforward and reliable manner.

Mild Strategy for the Preparation of Alkyl Sulfonyl Fluorides from Alkyl Bromides and Alcohols Using Photoredox Catalysis and Flow Chemistry

Facile access to sp3-rich scaffolds containing a sulfonyl fluoride group is still limited. Herein, we describe a mild and scalable strategy for the preparation of alkyl sulfonyl fluorides from readily available alkyl bromides and alcohols using photoredox catalysis. This approach is based on halogen atom transfer (XAT), followed by SO2 capture and fluorination. The method features mild conditions enabling fast access to high-value derivatives and has been scaled up to 5 g using a continuous stirred tank reactor cascade.

Heterogeneous Organocatalysts for Light-Driven Reactions in Continuous Flow

Within the realm of organic synthesis, photocatalysis has blossomed since the beginning of the last decade. A plethora of classical reactivities, such as selective oxidation of alcohol and amines, redox radical formation of reactive species in situ, and indirect activation of an organic substrate for cycloaddition by EnT, have been revised in a milder and more sustainable fashion via photocatalysis. However, even though the spark of creativity leads scientists to explore new reactions and reactivities, the urgency of replacing the toxic and critical metals that are involved as catalysts has encouraged chemists to find alternatives in the branch of science called organocatalysis. Unfortunately, replacing metal catalysts with organic analogues can be too expensive sometimes; however, this drawback can be solved by the reutilization of the catalyst if it is heterogeneous. The aim of this review is to present the recent works in the field of heterogeneous photocatalysis, applied to organic synthesis, enabled by continuous flow. In detail, among the heterogeneous catalysts, g-CN, polymeric photoactive materials, and supported molecular catalysts have been discussed within their specific sections, rather than focusing on the types of reactions.

Photocatalytic Functionalization of Dehydroalanine-Derived Peptides in Batch and Flow

Unnatural amino acids, and their synthesis by the late-stage functionalization (LSF) of peptides, play a crucial role in areas such as drug design and discovery. Historically, the LSF of biomolecules has predominantly utilized traditional synthetic methodologies that exploit nucleophilic residues, such as cysteine, lysine or tyrosine. Herein, we present a photocatalytic hydroarylation process targeting the electrophilic residue dehydroalanine (Dha). This residue possesses an α,β-unsaturated moiety and can be combined with various arylthianthrenium salts, both in batch and flow reactors. Notably, the flow setup proved instrumental for efficient scale-up, paving the way for the synthesis of unnatural amino acids and peptides in substantial quantities. Our photocatalytic approach, being inherently mild, permits the diversification of peptides even when they contain sensitive functional groups. The readily available arylthianthrenium salts facilitate the seamless integration of Dha-containing peptides with a wide range of arenes, drug blueprints, and natural products, culminating in the creation of unconventional phenylalanine derivatives. The synergistic effect of the high functional group tolerance and the modular characteristic of the aryl electrophile enables efficient peptide conjugation and ligation in both batch and flow conditions.

Synthesis of 3,5-disubstituted isoxazoles by domino reductive Nef reaction/cyclization of β-nitroenones

β-Nitroenones can be efficiently converted into 3,5-disubstituted isoxazoles by using tin(II)chloride dihydrate and ethyl acetate as a reducing agent and solvent, respectively. Products are obtained in good yields and several functional groups are tolerated thanks to the mild reaction conditions.

AJICAP-M: Traceless Affinity Peptide Mediated Conjugation Technology for Site-Selective Antibody-Drug Conjugate Synthesis

A traceless site-selective conjugation method, "AJICAP-M", was developed for native antibodies at sites using Fc-affinity peptides, focusing on Lys248 or Lys288. It produces antibody-drug conjugates (ADCs) with consistent drug-to-antibody ratios, enhanced stability, and simplified manufacturing. Comparative in vivo assessment demonstrated AJICAP-M's superior stability over traditional ADCs. This technology has been successfully applied to continuous-flow manufacturing, marking the first achievement in site-selective ADC production. This manuscript outlines AJICAP-M's methodology and its effectiveness in ADC production.

Fully Automated Flow Protocol for C(sp3)-C(sp3) Bond Formation from Tertiary Amides and Alkyl Halides

Herein, we present a novel C(sp3)-C(sp3) bond-forming protocol via the reductive coupling of abundant tertiary amides with organozinc reagents prepared in situ from their corresponding alkyl halides. Using a multistep fully automated flow protocol, this reaction could be used for both library synthesis and target molecule synthesis on the gram-scale starting from bench-stable reagents. Additionally, excellent chemoselectivity and functional group tolerance make it ideal for late-stage diversification of druglike molecules.

Oxidation of Alcohols in Continuous Flow with a Solid Phase Hypervalent Iodine Catalyst

One of the most useful transformations in the synthetic chemist arsenal is the oxidation of alcohols to their corresponding carbonyl congeners. Despite its seemingly straightforward nature, this transformative reaction predominantly relies on the use of metals or hazardous reagents, making these processes highly unsustainable. To address this challenge, we have developed a sustainable metal-free method for the oxidation of alcohols in continuous flow. Using a solid phase hypervalent iodine catalyst and nBu4HSO5 as a phase transfer catalyst and co-oxidant, primary and secondary alcohols were selectively oxidized to the corresponding carbonyl motifs. This operationally simple continuous-flow set-up is highly robust (15 cycles run without significant catalyst leaching or loss of reaction efficiency), uses green solvents, such as acetonitrile or acetic acid, and is readily scalable.

Advances in the Continuous Flow Synthesis of 3- and 4-Membered Ring Systems

Small carbo- and heterocyclic ring systems have experienced a significant increase in importance in recent years due to their relevance in modern pharmaceuticals, as building blocks for designer materials or as synthetic intermediates. This necessitated the development of new synthetic methods for the preparation of these strained ring systems focusing on effectiveness and scalability. The high ring strain of these entities as well as the use of high-energy reagents and intermediates has often challenged their synthesis. Continuous flow approaches have thus emerged as highly effective means to safely and reliably access these strained scaffolds. In this short review, key developments in this field are summarised showcasing the power of continuous flow approaches for accessing 3- and 4-membered ring systems via thermal, photo- and electrochemical processes.

Continuous flow reactions in the preparation of active pharmaceutical ingredients and fine chemicals

Herein, we present an overview of continuous flow chemistry, including photoflow and electroflow technologies in the preparation of active pharmaceutical ingredients (APIs) and fine chemical intermediates. Examples highlighting the benefits and challenges associated with continuous flow processes, mainly involving continuous thermal, photo- and electrochemical transformations, are drawn from the relevant literature, especially our experience and collaborations in this area, with emphasis on the synthesis and prospective scale-up.

Recent advances and applications in high-throughput continuous flow

High-throughput continuous flow technology has emerged as a revolutionary approach in chemical synthesis, offering accelerated experimentation and improved efficiency. With the aid of process analytical technology and automation, this system not only enables rapid optimisation of reaction conditions at the millimole to the picomole scale, but also facilitates automated scale-up synthesis. It can even achieve the self-planning and self-synthesis of small drug molecules with artificial intelligence incorporated in the system. The versatility of the system is highlighted by its compatibility with both electrochemistry and photochemistry, and its significant applications in organic synthesis and drug discovery. This highlight summarises its recent developments and applications, emphasising its significant impact on advancing research across multiple disciplines.

Ultrasonic or Microwave Modified Continuous Flow Chemistry for the Synthesis of Tetrahydrocannabinol: Observing Effects of Various Solvents and Acids

Synthesizing tetrahydrocannabinol is a lengthy process with minimal yields and little applicability on an industrial scale. To close the gap between bench chemistry and industry process chemistry, this paper introduces a small-scale flow chemistry method that utilizes a microwave or ultrasonic medium to produce major tetrahydrocannabinol isomers. This process produces excellent yields and minimal side products, which leads to more efficient large-scale production of the desired cannabinoids.

Preparation of Sulfonyl Chlorides by Oxidative Chlorination of Thiols and Disulfides using HNO3/HCl/O2 in a Flow Reactor

A continuous flow metal-free protocol for the synthesis of sulfonyl chlorides from thiols and disulfides in the presence of nitric acid, hydrochloric acid and oxygen was developed. The influence of the reaction parameters was investigated under batch and flow conditions. Online 19F NMR was successfully implemented to investigate different reaction conditions within a single experiment. The sulfonyl chlorides were isolated (mostly in 70-81 % yield) after performing a simple aqueous washing procedure. In particular, the protocol was successfully operated for >6 hours to convert diphenyl disulfide to its corresponding sulfonyl chloride, achieving a throughput of 3.7 g h-1. The environmental impact of the protocol was assessed and compared to an existing continuous flow protocol using 1,3-dichloro-5,5-dimethylhydantoin (DCH) as reagent. The process mass intensity (PMI) for the newly-developed flow protocol (15) compared favorably to the DCH flow process (20).

Multiphasic Continuous-Flow Reactors for Handling Gaseous Reagents in Organic Synthesis: Enhancing Efficiency and Safety in Chemical Processes

The use of reactive gaseous reagents for the production of active pharmaceutical ingredients (APIs) remains a scientific challenge due to safety and efficiency limitations. The implementation of continuous-flow reactors has resulted in rapid development of gas-handling technology because of several advantages such as increased interfacial area, improved mass- and heat transfer, and seamless scale-up. This technology enables shorter and more atom-economic synthesis routes for the production of pharmaceutical compounds. Herein, we provide an overview of literature from 2016 onwards in the development of gas-handling continuous-flow technology as well as the use of gases in functionalization of APIs.

Intensified Continuous Flow Process for the Scalable Production of Bio-Based Glycerol Carbonate

A subtle combination of fundamental and applied organic chemistry toward process intensification is demonstrated for the large-scale production of bio-based glycerol carbonate under flow conditions. The direct carbonation of bio-based glycidol with CO2 is successfully carried out under intensified flow conditions, with Barton's base as a potent homogeneous organocatalyst. Process metrics for the CO2 coupling step (for the upstream production, output: 3.6 kg day-1 , Space Time Yield (STY): 2.7 kg h-1  L-1 , Environmental factor (E-factor): 4.7) outclass previous reports. High conversion and selectivity are achieved in less than 30 s of residence time at pilot scale with a stoichiometric amount of CO2 . Supporting DFT computations reveal the unique features of the mechanism in presence of Brønsted bases.

Synthesis of Substituted Pentafluorosulfanylpyrazoles Under Flow Conditions

The development of flow conditions for the synthesis of pentafluorosulfanylpyrazoles is reported. A range of alkyl- and aryl-substituted SF5-alkynes were used in combination with different diazoacetates for this transformation. The corresponding substituted SF5-pyrazoles were obtained in up to 90% yield (average of 74% for 21 examples) as a mixture of isomers (up to 73:27 ratio). Synthetic transformations starting from an SF5-containing pyrazole were also demonstrated.

Green and sustainable approaches for the Friedel-Crafts reaction between aldehydes and indoles

The synthesis of indoles and their derivatives, more specifically bis(indolyl)methanes (BIMs), has been an area of great interest in organic chemistry, since these compounds exhibit a range of interesting biological and pharmacological properties. BIMs are naturally found in cruciferous vegetables and have been shown to be effective antifungal, antibacterial, anti-inflammatory, and even anticancer agents. Traditionally, the synthesis of BIMs has been achieved upon the acidic condensation of an aldehyde with indole, utilizing a variety of protic or Lewis acids. However, due to the increased environmental awareness of our society, the focus has shifted towards the development of greener synthetic technologies, like photocatalysis, organocatalysis, the use of nanocatalysts, microwave irradiation, ball milling, continuous flow, and many more. Thus, in this review, we summarize the medicinal properties of BIMs and the developed BIM synthetic protocols, utilizing the reaction between aldehydes with indoles, while focusing on the more environmentally friendly methods developed over the years.

Metal-free photocatalytic cross-electrophile coupling enables C1 homologation and alkylation of carboxylic acids with aldehydes

In contemporary drug discovery, enhancing the sp3-hybridized character of molecular structures is paramount, necessitating innovative synthetic methods. Herein, we introduce a deoxygenative cross-electrophile coupling technique that pairs easily accessible carboxylic acid-derived redox-active esters with aldehyde sulfonyl hydrazones, employing Eosin Y as an organophotocatalyst under visible light irradiation. This approach serves as a versatile, metal-free C(sp3)-C(sp3) cross-coupling platform. We demonstrate its synthetic value as a safer, broadly applicable C1 homologation of carboxylic acids, offering an alternative to the traditional Arndt-Eistert reaction. Additionally, our method provides direct access to cyclic and acyclic β-arylethylamines using diverse aldehyde-derived sulfonyl hydrazones. Notably, the methodology proves to be compatible with the late-stage functionalization of peptides on solid-phase, streamlining the modification of intricate peptides without the need for exhaustive de-novo synthesis.

Modular synthesis of congested β2,2-amino acids via the merger of photocatalysis and oxidative functionalisations

A two-step protocol for the modular synthesis of β2- and α-quaternary β2,2-amino acid derivatives is reported. The key steps are a photocatalytic hydroalkylation reaction, followed by an oxidative functionalisation to access N-protected β-amino acids, esters, and amides. This strategy can be effectively scaled up via continuous-flow technology.

Application of an Oscillatory Plug Flow Reactor to Enable Scalable and Fast Reactions in Water Using a Biomass-Based Polymeric Additive

The utilization of water as a sustainable reaction medium has important advantages over traditional organic solvents. Hydroxypropyl methylcellulose has emerged as a biomass-based polymeric additive that enables organic reactions in water through hydrophobic effects. However, such conditions imply slurries as reaction mixtures, where the efficacy of mass transfer and mixing decreases with increasing vessel size. In order to circumvent this limitation and establish an effectively scalable platform for performing hydroxypropyl methylcellulose-mediated aqueous transformations, we utilized oscillatory plug flow reactors that feature a smart dimensioning design principle across different scales. Using nucleophilic aromatic substitutions as valuable model reactions, rapid parameter optimization was performed first in a small-scale instrument having an internal channel volume of 5 mL. The optimal conditions were then directly transferred to a 15 mL reactor, achieving a three-fold scale-up without re-optimizing any reaction parameters. By precisely fine-tuning the oscillation parameters, the system achieved optimal homogeneous suspension of solids, preventing settling of particles and clogging of process channels. Ultimately, this resulted in a robust and scalable platform for performing multiphasic reactions under aqueous conditions.

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-Flow Enantioselective Hydroacylations under Heterogeneous Chiral Rhodium Catalysts

Transition metal-catalyzed enantioselective C-H bond functionalizations have become efficient methods for the synthesis of complex optically active molecules. Heterogeneous catalysts for this chemistry remain largely unexplored despite the advantages they offer in terms of ease of separation and reuse of catalysts. Herein, we report the development of heterogeneous chiral Rh catalysts for continuous-flow enantioselective hydroacylations. Heterogeneous catalysts could be prepared simply by mixing supports and Rh complexes. The prepared catalysts exhibited excellent activity and enantioselectivity affording optically active ketones in quantitative yields with 99 % ee's. Under the optimized reaction conditions, a turnover number >300 was achieved without the leaching of Rh species. The catalysts exhibited a wide substrate scope and in sequential-flow reactions with other heterogeneous catalysts, the syntheses of biologically active molecules and functional materials were demonstrated.

Accelerated Electrophotocatalytic C(sp3 )-H Heteroarylation Enabled by an Efficient Continuous-Flow Reactor

Electrophotocatalytic transformations are garnering attention in organic synthesis, particularly for accessing reactive intermediates under mild conditions. Moving these methodologies to continuous-flow systems, or flow ElectroPhotoCatalysis (f-EPC), showcases potential for scalable processes due to enhanced irradiation, increased electrode surface, and improved mixing of the reaction mixture. Traditional methods sequentially link photochemical and electrochemical reactions, using flow reactors connected in series, yet struggle to accommodate reactive transient species. In this study, we introduce a new flow reactor concept for electrophotocatalysis (EPC) that simultaneously utilizes photons and electrons. The reactor is designed with a transparent electrode and employs cost-effective materials. We used this technology to develop an efficient process for electrophotocatalytic heteroarylation of C(sp3 )-H bonds. Importantly, the same setup can also facilitate purely electrochemical and photochemical transformations. This reactor represents a significant advancement in electrophotocatalysis, providing a framework for its application in flow for complex synthetic transformations.

Continuous Stripping with Dense Carbon Dioxide

The integration of flow chemistry into continuous manufacturing requires efficient, controllable, and continuous methods for the concentration of diluted solutions on relatively small scales. The design and application examples of a new continuous solvent removal process are presented. The continuous stripping method employing dense carbon dioxide is based on the formation of homogeneous mixtures of dilute organic solutions of the target molecules with a large excess of carbon dioxide at temperatures as low as 35 °C and pressures around 10 MPa. Subsequent pressure reduction results in the quick release of carbon dioxide and vaporization of a significant fraction of the organic solvent. The concentration of the solute in the separated liquid phase can be up to 40 times higher than in the feed. Among the many controllable process parameters, the most significant ones are the mass-flow rate ratio of carbon dioxide to the feed and the temperature of the phase separator. By careful setting of the operational parameters, the degree of concentration enhancement may be accurately controlled. The new apparatus-despite consisting of laboratory equipment and being built in a fume hood-could easily support pilot-scale synthetic flow chemistry, being a continuous, efficient alternative to thermal concentration methods.

Highly Scalable and Inherently Safer Preparation of Di, Tri and Tetra Nitrate Esters Using Continuous Flow Chemistry

Nitrate esters are important organic compounds having wide application in energetic materials, medicines and fuel additives. They are synthesized through nitration of aliphatic polyols. But the process safety challenges associated with nitration reaction makes the production process complicated and economically unviable. Herein, we have developed a continuous flow process wherein polyol and nitric acid are reacted in a microreactor to produce nitrate ester continuously. Our developed process is inherently safer and efficient. The process was optimized for industrially important nitrate esters containing two, three and four nitro groups. Substrates include glycol dinitrates: 1,2-propylene glycol dinitrate (PGDN), ethylene glycol dinitrate (EGDN), diethylene glycol dinitrate (DEGDN), triethylene glycol dinitrate (TEGDN); trinitrates: trimethylolethane trinitrate (TMETN), 1,2,4-butanetriol trinitrate (BTTN); and tetranitrates: erythritol tetranitrate (ETN). The optimized process for each molecule provided yield >90 % in a short residence time of 1 min corresponding to a space time yield of >18 g/h/mL of reactor volume.

A Telescoped Continuous Flow Enantioselective Process for Accessing Intermediates of 1-Aryl-1,3-diols as Chiral Building Blocks

A telescoped continuous flow process is reported for the enantioselective synthesis of chiral precursors of 1-aryl-1,3-diols, intermediates in the synthesis of ezetimibe, dapoxetine, duloxetine, and atomoxetine. The two-step sequence consists of an asymmetric allylboration of readily available aldehydes using a polymer-supported chiral phosphoric acid catalyst to introduce asymmetry, followed by selective epoxidation of the resulting alkene. The process is highly stable for at least 7 h and represents a transition-metal free enantioselective approach to valuable 1-aryl-1,3-diols.

Advances in Continuous Flow Fluorination Reactions

Fluorination reactions are important in constructing organofluorine motifs, which contribute to favorable biological properties in pharmaceuticals and agrochemicals. However, fluorination reagents and reactions are associated with various problems, such as their hazardous nature, high exothermicity, and poor selectivity and scalability. Continuous flow has emerged as a transformative technology to provide many advantages relative to batch syntheses. This review article summarizes recent continuous flow techniques that address the limitations and challenges of fluorination reactions. Approaches based on different flow techniques are discussed, including gas-liquid reactions, packed-bed reactors, in-line purifications, streamlined multistep synthesis, large-scale reactions well as flow photoredox- and electrocatalysis.

Revisiting the Paradigm of Reaction Optimization in Flow with a Priori Computational Reaction Intelligence

The use of micro/meso-fluidic reactors has resulted in both new scenarios for chemistry and new requirements for chemists. Through flow chemistry, large-scale reactions can be performed in drastically reduced reactor sizes and reaction times. This obvious advantage comes with the concomitant challenge of re-designing long-established batch processes to fit these new conditions. The reliance on experimental trial-and-error to perform this translation frequently makes flow chemistry unaffordable, thwarting initial aspirations to revolutionize chemistry. By combining computational chemistry and machine learning, we have developed a model that provides predictive power tailored specifically to flow reactions. We show its applications to translate batch to flow, to provide mechanistic insight, to contribute reagent descriptors, and to synthesize a library of novel compounds in excellent yields after executing a single set of conditions.

Continuous Flow Chemistry: A Novel Technology for the Synthesis of Marine Drugs

In this perspective, we showcase the benefits of continuous flow chemistry and photochemistry and how these valuable tools have contributed to the synthesis of organic scaffolds from the marine environment. These technologies have not only facilitated previously described synthetic pathways, but also opened new opportunities in the preparation of novel organic molecules with remarkable pharmacological properties which can be used in drug discovery programs.