Multi-step continuous-flow synthesis

Visible-Light-Induced Divergent C-H Esterification/Alkylation of Quinoxalin-2(1H)-ones with Aldehydes under Mild Conditions

Herein, we introduce an efficient and straightforward strategy for the selective C-H esterification and alkylation of quinoxalin-2(1H)-ones with aldehydes. A key feature of our study is the ability to perform both C-H esterification and alkylation using different types of aldehydes. The reaction system is highly compatible with a range of quinoxalin-2(1H)-ones and aldehydes, yielding C3-esterified and C3-alkylated products in moderate-to-good yields. The applicability of this approach is further enhanced by its scalability through continuous-flow synthesis, late-stage modification of significant molecules, and product derivatization. Our mechanistic investigations reveal a radical relay mechanism, triggered by a hydrogen atom transfer process.

Photo- and Cerium-Mediated C─C Bond Cleavage for the Deconstructive Diversification of Cyclic Acids

The selective cleavage of inert carbon-carbon bonds in unstrained rings continues to pose a formidable challenge in chemical synthesis. Current methods for C(sp3) ─C(sp3) bond cleavage are highly limited, typically relying on transition-metal catalysis to facilitate ring-opening via small-ring strain or inducing β-fragmentation after generating radicals from oxygen or nitrogen atoms pre-installed in the substrate. Herein, we introduce an effective strategy for the decarboxylative ring-opening functionalization of α-trisubstituted carboxylic acids, mediated by both light and cerium. This method enables the ring-opening of carboxylic acids with ring sizes ranging from 3 to 12 members, allowing the construction of C─CN, C-halide, C─C, C─Se, and C─oxime bonds. Notably, this reaction does not require the pre-installation of an oxygen atom in the substrate, as the carbonyl group is derived from atmospheric oxygen. Furthermore, late-stage modification establishes distally functionalized carbonyl compounds, which serve as versatile synthons for accessing valuable building blocks.

Photocatalytic E → Z isomerization of gem-bromofluoroalkenes: stereoselective synthesis of β-fluorostyrene derivatives

Stereoselective synthesis of β-fluorostyrene derivatives has been achieved. Selective isomerization of gem-bromofluoroalkenyl benzenes bearing various ortho-substituents is enabled by using Ir photocatalysts with high triplet energy. Subsequent one-pot transition-metal (TM)-catalyzed reactions enable pot-economical synthesis of monofluoroalkenes in a stereoselective manner.

Bimetallic Nickel-Palladium Nanoparticles Supported on Multiwalled Carbon Nanotubes for Suzuki Cross-Coupling Reactions in Continuous Flow

An efficient Suzuki cross-coupling reaction under continuous flow conditions was developed utilizing an immobilized solid supported catalyst consisting of bimetallic nickel-palladium nanoparticles (Ni-Pd/MWCNTs). In this process, the reactants can be continuously pumped into a catalyst bed at a high flow rate of 0.6 mL/min and the temperature of 130 °C while the Suzuki products are recovered in high steady-state yields for prolonged continuous processing. The catalyst was prepared by mechanical shaking of the appropriate nickel and palladium salts using ball-mill energy without the requirement of any solvent or reducing agent. This straightforward, facile, and simple method allows for bulk production of Ni-Pd/MWCNTs nanoparticles with a small particle size ideal for application in continuous flow cross-coupling catalysis. The as-prepared catalyst mostly contains nickel (7.9%) with a very small amount of palladium (0.81%) according to ICP-OES analysis. This remarkable immobilized catalyst can be used several times for different Suzuki reactions with a minimum loss of reactivity and no detectable leaching of the metal nanoparticles. Notably, by modifying the groups on both aryl halides and phenylboronic acids, the method provides access to a diverse array of the Suzuki products in flow with high steady-state yield, making it suitable for applications in industrial and pharmaceutical scales. Moreover, several spectroscopic techniques were employed to identify the structure and composition of the as-prepared Ni-Pd/MWCNTs nanoparticles before and after the reaction in flow such as transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), BET surface area (physisorption), and FTIR spectroscopy.

Room-Temperature Vortex Fluidic Flow Chemistry Synthesis of Full Color Carbon Dots

Carbon dots (CDs) as a new class of photoluminescent zero-dimension carbon nanoparticles have attracted significant research interests owing to their extraordinary opto-electro-properties and biocompatibility. So far, almost all syntheses of CDs require either heat treatment or exertion of high energy fields. Herein, a scalable room-temperature vortex fluidic method is introduced to the CDs synthesis using the angled vortex fluidic device (VFD). By judicious selection of the solvent, typical CDs precursor of phenylenediamine has been converted into high crystalline CDs through VFD processing. The VFD-synthesized CDs cover the full color spectrum from blue to red with the highest quantum yield of 45.6 %. The synthesis shows that the dynamic thin liquid film generated by VFD spun at high rotational speed (7-9 k RPM) is able to induce cycloaddition reactions. The new method for CD synthesis is facile, occurring under ambient conditions in the VFD, potentially offering industrial scaling up of production of full color carbon dots.

Auto-RapTAC: A Versatile and Sustainable Platform for the Automated Rapid Synthesis and Evaluation of PROTAC

The tedious synthesis and limited throughput biological evaluation remain a great challenge for discovering new proteolysis targeting chimera (PROTAC). To rapidly identify potential PROTAC lead compounds, we report a platform named Auto-RapTAC. Based on the modular characteristic of the PROTAC molecule, a streamlined workflow that integrates lab automation with "click chemistry" joint building-block libraries was constructed. This facilitates the autonomous generation of a variety of PROTACs, each with distinct linkers and E3 ligase ligands, all stored in biocompatible solutions. The ready-for-screening (R4S) approach, when paired with fluorescence-based assays, enables the efficient assessment of the PROTAC degradation activity in a high-throughput manner. To further test the capability of the platform, we identify six new PROTACs that target CDK2, CDK12, and BCL6 within a mere 8-day time frame for each target. In all, this platform could find broad application not only in discovering new PROTACs but also in the rapid development of novel heterobifunctional modalities.

Green Synthesis of Alzheimer's Disease Probes Aftobetin and Analogues Enabled by Flow Technology and Heterogeneous Photocatalysis

Aftobetin is a non-invasive diagnosis of Alzheimer's disease, that can bind with aggregated β-amyloid peptide in eye's lenses, used for early diagnosis of Alzheimer's disease in a rapid and painless mode. The reported synthesis of this probe fell short in the aspects of greenness and economy due to the involvement of toxic Chromium(IV) oxidant, noble palladium catalyst, elevated reaction temperature, the long reaction time as well as the cumbersome workup. Herein, a holistic optimization of the synthetic process was achieved via the employment of flow technology and heterogenous photocatalysis. Firstly, the integration of heterogenous carbon nitrides photocatalysis and circulation flow technology furnished the air oxidation of alcohol and nickel catalyzed C-N coupling at 20 g scale, thus avoiding the use of toxic Chromium and precious palladium species respectively. Flow-intensified esterification between acyl chloride and alcohol, just taking 30 seconds replaced the Steglich esterification of 6 hours, also avoiding the generation of difficult-to-remove dicyclohexylurea. Finally, C-N coupling, esterification and Knoevenagel condensation were telescoped together, thus simplifying the reaction workup. This fully-flow protocol led to the on-demand synthesis of eight probes.

Pyrazine derivative synthesis in a continuous-flow system: a green synthesis of pyrazinamide from pyrazine esters and amines catalyzed by Lipozyme® TL IM from Thermomyces lanuginosus

Pyrazinamide derivatives have been extensively studied for their biological activities, such as anti-tuberculosis activity and antiviral activities. In this work, a continuous-flow system was developed for the synthesis of pyrazinamide derivatives from pyrazine esters and amines (aliphatic amine, benzylamines and morpholine) catalyzed by Lipozyme® TL IM from Thermomyces lanuginosus, which was used for the first time. The reaction parameters including solvent, substrate ratio, reaction temperature and reaction time/flow rate were also studied in detail. A total of 23 pyrazinamide derivatives can be obtained through this method in parallel. Compared with other works, this method can be conducted at 45 °C for 20 min in a greener tert-amyl alcohol solvent and maximum yield (91.6%) was obtained as well. In brief, a more efficient and greener method for the synthesis of pyrazinamide derivatives was developed with good scalability, various substrates including aliphatic amines, benzylamines and morpholines can be applied to this method and achieve a desirable yield. Through the construction and research of amide bonds, this method provides a greener and more efficient biocatalytic continuous technology for the development of pyrazine-derived drugs, and provides a basis for the rapid synthesis of pyrazine-derived drugs in the future.

Sequential-Flow Synthesis of Donepezil: A Green and Sustainable Strategy Featuring Heterogeneous Catalysis and Hydrogenation

An atom-economical sequential-flow synthesis of donepezil, a widely prescribed drug for Alzheimer's disease, was accomplished using inexpensive, commercially available precursors. This achievement was made possible by reconfiguring the synthetic route to include only heterogeneous catalytic addition and condensation reactions, with a particular emphasis on skeletal transformation and bond formation through hydrogenation processes. Notably, water was the sole byproduct in this synthesis. A crucial aspect of this work was the development of appropriate continuous-flow processes to achieve a one-flow synthesis. This was accomplished by implementing in-line treatments of the main reaction stream to eliminate inhibitory factors that could affect catalyst performance in the hydrogenation steps.

Continuous flow synthesis of the antiviral drug tecovirimat and related sp3-rich scaffolds

Herein we report a 2-step continuous flow synthesis of the antiviral drug tecovirimat, which is used for the treatment of monkeypox and smallpox. This work exploits a high-temperature pericyclic cascade process between cycloheptatriene and maleic anhydride generating a key sp3-rich scaffold, which affords the desired API after further condensation with an acyl hydrazide. Additional investigations of the key intermediate in reactions with different hydrazines revealed the accessibility of different heterocyclic chemotypes, depending on the substitution pattern of the hydrazine used. Ultimately, the streamlined and scalable access to these sp3-rich scaffolds enables improved access to tecovirimat and structurally related entities with high drug-like character.

Structure-Directed Quaternary Ammonium Hydroxide Resins: High-Performance Heterogeneous Base Catalysts for Continuous-Flow Carbon-Carbon Bond-Forming Reactions

Quaternary ammonium hydroxide resins are widely recognized for their effectiveness as immobilized bases and are frequently utilized as commodity chemicals for anion-exchange applications. However, despite their accessibility, chemically controlling their properties has proven challenging, hindering research and development efforts toward their use as solid base catalysts. This study investigates the synthetic factors that are crucial for achieving high functional performance in polystyrene resins. Using commercially available resin as a benchmark and the continuous-flow Henry reaction as a model system, we explore and evaluate newly synthesized resin catalysts to maximize their basic catalytic activity.

Continuous-Flow Catalysis Using Phosphine-Metal Complexes on Porous Polymers: Designing Ligands, Pores, and Reactors

Continuous-flow syntheses using immobilized catalysts can offer efficient chemical processes with easy separation and purification. Porous polymers have gained significant interests for their applications to catalytic systems in the field of organic chemistry. The porous polymers are recognized for their large surface area, high chemical stability, facile modulation of surface chemistry, and cost-effectiveness. It is crucial to immobilize transition-metal catalysts due to their difficult separation and high toxicity. Supported phosphine ligands represent a noteworthy system for the effective immobilization of metal catalysts and modulation of catalytic properties. Researchers have been actively pursuing strategies involving phosphine-metal complexes supported on porous polymers, aiming for high activities, durabilities, selectivities, and applicability to continuous-flow systems. This review provides a concise overview of phosphine-metal complexes supported on porous polymers for continuous-flow catalytic reactions. Polymer catalysts are categorized based on pore sizes, including micro-, meso-, and macroporous polymers. The characteristics of these porous polymers are explored concerning their efficiency in immobilized catalysis and continuous-flow systems.

Stable and Versatile Pd Precursors for the Preparation of Robust Pd Catalysts Under Continuous-Flow

To achieve global sustainability, the chemical and engineering communities require the development of versatile precursors that can be used to synthesize robust catalysts. To meet this demand, we developed a new Pd precursor for highly efficiently incorporating fine Pd-metal into supports. An atmospherically stable Pd precursor (Pd-80) was prepared by thermally promoting the aerobic oxidation of tetrakis(triphenylphosphine)palladium. The physical properties of Pd-80 were investigated by NMR spectroscopy, SEM, XPS, solvent-relaxation NMR spectroscopy, and dynamic light scattering (DLS) experiments. We also prepared a cordierite-supported Pd catalyst (Pd/cordierite) by stirring Pd-80 and cordierite powder in chloroform at room temperature. Pd/cordierite selectively catalyzed the hydrogenation of various reducible functional groups, including alkynes, azides, nitro groups, olefins, CO2Bn, N-Cbz, O-Bn, aromatic ketones, and styrene oxide, in continuous-flow hydrogenation reactions. The Pd/cordierite-catalyzed continuous-flow hydrogenation of nitrobenzene derivatives afforded the corresponding anilines, with catalytic activities maintained for over 250 h of continuous operation with a turnover number (TON) of 61,090.

Modularization of Immobilized Multienzyme Cascades for Continuous-Flow Enantioselective C-H Amination

Multienzyme cascades (MECs) have gained much attention in synthetic chemistry but remain far from being a reliable synthetic tool. Here we report a four-enzyme cascade comprising a cofactor-independent and a cofactor self-sustaining bienzymatic modules for the enantioselective benzylic C-H amination of arylalkanes, a challenging transformation from bulk chemicals to high value-added chiral amines. The two modules were subsequently optimized by enzyme co-immobilization with microenvironmental tuning, and finally integrated in a gas-liquid segmented flow system, resulting in simultaneous improvements in enzyme performance, mass transfer, system compatibility, and productivity. The flow system enabled continuous C-H amination of arylalkanes (up to 100 mM) utilizing the sole cofactor NADH (0.5 mM) in >90 % conversion, achieving a high space-time yield (STY) of 3.6 g ⋅ L-1 ⋅ h-1, which is a 90-fold increase over the highest value previously reported.

Self-Driving Laboratories for Chemistry and Materials Science

Self-driving laboratories (SDLs) promise an accelerated application of the scientific method. Through the automation of experimental workflows, along with autonomous experimental planning, SDLs hold the potential to greatly accelerate research in chemistry and materials discovery. This review provides an in-depth analysis of the state-of-the-art in SDL technology, its applications across various scientific disciplines, and the potential implications for research and industry. This review additionally provides an overview of the enabling technologies for SDLs, including their hardware, software, and integration with laboratory infrastructure. Most importantly, this review explores the diverse range of scientific domains where SDLs have made significant contributions, from drug discovery and materials science to genomics and chemistry. We provide a comprehensive review of existing real-world examples of SDLs, their different levels of automation, and the challenges and limitations associated with each domain.

In-Flow Generation of Thionyl Fluoride (SOF2) Enables the Rapid and Efficient Synthesis of Acyl Fluorides from Carboxylic Acids

Herein, we report an approach for generating thionyl fluoride (SOF2) from the commodity chemicals thionyl chloride (SOCl2) and potassium fluoride (KF). The methodology relies on a microfluidic device that can efficiently produce and dose this toxic gaseous reagent under extremely mild and safe conditions. Subsequently, the in situ-generated thionyl fluoride is reacted with an array of structurally and electronically differing carboxylic acids, leading to the direct and efficient synthesis of highly sought-after acyl fluorides. Importantly, our investigation also highlights the inherent modularity of this flow-based platform. We demonstrate the adaptability of this approach by not only synthesizing acyl fluorides but also directly converting carboxylic acids into a diverse array of valuable compounds such as esters, thioesters, amides, and ketones. This versatility showcases the potential of this approach for a wide range of synthetic applications, underscoring its significance in the realm of chemical synthesis.

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.

Simultaneous reaction- and analytical model building using dynamic flow experiments to accelerate process development

In modern pharmaceutical research, the demand for expeditious development of synthetic routes to active pharmaceutical ingredients (APIs) has led to a paradigm shift towards data-rich process development. Conventional methodologies encompass prolonged timelines for the development of both a reaction model and analytical models. The development of both methods are often separated into different departments and can require an iterative optimization process. Addressing this issue, we introduce an innovative dual modeling approach, combining the development of a Process Analytical Technology (PAT) strategy with reaction optimization. This integrated approach is exemplified in diverse amidation reactions and the synthesis of the API benznidazole. The platform, characterized by a high degree of automation and minimal operator involvement, achieves PAT calibration through a "standard addition" approach. Dynamic experiments are executed to screen a broad process space and gather data for fitting kinetic parameters. Employing an open-source software program facilitates rapid kinetic parameter fitting and additional in silico optimization within minutes. This highly automated workflow not only expedites the understanding and optimization of chemical processes, but also holds significant promise for time and resource savings within the pharmaceutical industry.

Telescoped Flow Synthesis of Azacyclic Scaffolds Exploiting the Chromoselective Photolysis of Vinyl Azides and Azirines

An efficient chromoselective photochemical process is presented for the synthesis of 2H-azirines and 1,3-diazabicylo[3.1.0]hex-3-enes from readily available vinyl azides. The method exploits continuous flow photochemistry to enable the safe consumption of the hazardous azide group and provides uniform irradiation using high-power LEDs at 365-450 nm. Additionally, a scaled telescoped process has been developed providing access to drug-like 1,6-dihydropyrimidines and pyrimidines via integrated ring-expansion and oxidation reactions. Given the prevalence of various azacyclic targets in pharmaceutical, agrochemical and materials applications it is anticipated that this methodology will enable further exploitations of these important scaffolds.

Effect of Brønsted Acids on the Activation of Mixed Anhydride/Mixed Carbonic Anhydride and C-Terminal-Free N-Methylated Peptide Synthesis in a Micro-Flow Reactor

Amidations employing mixed (carbonic) anhydrides have long been favoured in peptide synthesis because of their cost-effectiveness and less waste generation. Despite their long history, no study has compared the effects of additives on the activation of mixed anhydrides and carbonic anhydrides. In this study, we investigated the amidation of mixed (carbonic) anhydride in the presence of a base and/or Brønsted acids. The use of NMI⋅HCl significantly improved the conversion of the mixed carbonic anhydride, while expediting nucleophilic attacks on the desired carbonyl group. In contrast, in the case of mixed anhydrides, neither the conversion nor the desired nucleophilic attack improved significantly. We developed a C-terminus-free N-methylated peptide synthesis method using mixed carbonic anhydrides in a micro-flow reactor. Fourteen N-alkylated peptides were synthesized in moderate to high yields (55-99 %) without severe racemization (<1 %). Additionally, a significant enhancement in the amidation between mixed carbonic anhydrides and bis-TMS-protected N-methyl amino acids with the inclusion of NMI⋅HCl was observed for the first time. In addition, we observed unexpected C-terminal epimerization of the C-terminus-free N-methyl peptides.

Continuous and autonomous-flow separation of laccase enzyme utilizing functionalized aqueous two-phase system with computer vision control

Downstream processing of biomolecules, particularly therapeutic proteins and enzymes, presents a formidable challenge due to intricate unit operations and high costs. This study introduces a novel cysteine (cys) functionalized aqueous two-phase system (ATPS) utilizing polyethylene glycol (PEG) and potassium phosphate, referred as PEG-K3PO4/cys, for selective extraction of laccase from complex protein mixtures. A 3D-baffle micro-mixer and phase separator was meticulously designed and equipped with computer vision controller, to enable precise mixing and continuous phase separation under automated-flow. Microfluidic-assisted ATPS exhibits substantial increase in partition coefficient (Kflow = 16.3) and extraction efficiency (EEflow = 88 %) for laccase compared to conventional batch process. Integrated and continuous-flow process efficiently partitioned laccase, even in low concentrations and complex crude extracts. Circular dichroism spectra of laccase confirm structural stability of enzyme throughout the purification process. Eventually, continuous-flow microfluidic bioseparation is highly useful for seamless downstream processing of target biopharmaceuticals in integrated and autonomous manner.

C(sp3)-H sulfinylation of light hydrocarbons with sulfur dioxide via hydrogen atom transfer photocatalysis in flow

Sulfur-containing scaffolds originating from small alkyl fragments play a crucial role in various pharmaceuticals, agrochemicals, and materials. Nonetheless, their synthesis using conventional methods presents significant challenges. In this study, we introduce a practical and efficient approach that harnesses hydrogen atom transfer photocatalysis to activate volatile alkanes, such as isobutane, butane, propane, ethane, and methane. Subsequently, these nucleophilic radicals react with SO2 to yield the corresponding sulfinates. These sulfinates then serve as versatile building blocks for the synthesis of diverse sulfur-containing organic compounds, including sulfones, sulfonamides, and sulfonate esters. Our use of flow technology offers a robust, safe and scalable platform for effectively activating these challenging gaseous alkanes, facilitating their transformation into valuable sulfinates.

Cascade Sequence of Photooxygenation-Epoxidation for the Flow Synthesis of Epoxy Alcohols

A photooxygenation-epoxidation cascade sequence converting alkenes to epoxy alcohols was developed and evaluated in batch and continuous-flow systems. In the batch system, the undesired interactions between the photooxygenation and epoxidation catalysts resulted in suboptimal yields, whereas the fine control of reaction parameters in the flow system allowed the allyl hydroperoxides produced through photooxygenation of alkenes to be rapidly converted to epoxy alcohols in yields of up to 93%. The developed procedure allows one to avoid an important synthetic bottleneck, works well where traditional batch synthesis fails, and can be scaled up to meet the needs of industrial production, thus presenting a valuable addition to the toolbox of practicing organic chemists.

A Coalescing Filter for Liquid-Liquid Separation and Multistage Extraction in Continuous-Flow Chemistry

Presented here is the design and performance of a coalescing liquid-liquid filter, based on low-cost and readily available meltblown nonwoven substrates for separation of immiscible phases. The performance of the coalescer was determined across three broad classes of fluid mixtures: (i) immiscible organic/aqueous systems, (ii) a surfactant laden organic/aqueous system with modification of the type of emulsion and interfacial surface tension through the addition of sodium chloride, and (iii) a water-acetone/toluene system. The first two classes demonstrated good performance of the equipment in effecting separation, including the separation of a complex emulsion system for which a membrane separator, operating through transport of a preferentially wetting fluid through the membrane, failed entirely. The third system was used to demonstrate the performance of the separator within a multistage liquid-liquid counterflow extraction system. The performance, robust nature, and scalability of coalescing filters should mean that this approach is routinely considered for liquid-liquid separations and extractions within the fine chemical and pharmaceutical industry.

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.

Rapid in situ generation of 2-(halomethyl)-5-phenylfuran and nucleophilic addition in a microflow reactor

2,5-Disubstituted furans are frequently found in pharmaceuticals and bioactive natural products. Nucleophilic substitution reactions on the carbon atom adjacent to the furan ring are useful for producing various furan derivatives. However, the formation of 5-substituted 2-halomethylfuran and the subsequent nucleophilic substitution reactions are often limited by severe undesired reactions caused by the highly reactive halomethylfurans. This paper reports the successful rapid synthesis of various 2,5-disubstituted furans using microflow technology, which suppresses undesired reactions including dimerization and ring opening of the furans. We observed that Brønsted acids had a significant effect on the nucleophilic substitution reaction and the use of HBr and HI gave the best results. A plausible mechanism of the Brønsted acid-mediated nucleophilic substitutions in the developed approach was proposed.

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.

Microflow, Sequential Coupling and Cyclization Approach for Synthesis of Cyclic Phosphotriesters from PCl3

An approach for the synthesis of cyclic phosphotriesters with various ring sizes (5- to 8-membered rings) from phosphorus trichloride and diols was developed. The major challenge in developing this approach is the suppression of the undesired reactions caused by substrates containing multiple highly reactive sites. These undesired reactions were successfully suppressed by microflow technology, which can precisely control the reaction time and temperature. Two optimal conditions were developed, depending on the speed of cyclization. Fifteen cyclic phosphotriesters and their analogs were synthesized. A plausible mechanism for suppressing undesired reactions is proposed.

Enhancing continuous-flow reactions via compression-molding of solid catalysts and dilutants in packed-bed systems

In this study, we present an improved packed-bed system designed for continuous-flow reactions using platinum (Pt)-black powder and silica gel (SiO2). The Pt-leaching from the reaction column is suppressed via compression-molding of the Pt and SiO2. Scanning electron microscopy results and particle-size distribution analysis demonstrate that crushed and downsized SiO2 is effective in suppressing outflow. Furthermore, we successfully conducted a scaled-up experiment of the flow reaction using a large column, achieving excellent productivity.

Amberlyst-A26-Mediated Corey-Chaykovsky Cyclopropanation of 9-Alkylidene-9H-fluorene under Continuous Process

Herein, we have developed a continuous-process for the direct cyclopropanation of various alkenes nonconjugated with carbonyl using trimethylsulfoxonium iodide as a methylene source via the Corey-Chaykovsky cyclopropanation reaction in the presence of Amberlyst-A26 as a heterogeneous base. Several 9-alkylidene-9H-fluorene derivatives successfully undergo Corey-Chaykovsky cyclopropanation to afford spiro[cyclopropane-1,9'-fluorene] in excellent yields under the continuous-process module. Furthermore, continuous process for the cyclopropanation of 3-benzylideneindolin-2-one derivatives using Amberlyst-A26 as a heterogeneous base has been described, which afford spiro[cyclopropane-1,3'-indolin]-2'-one derivatives.

Vortex fluidic regulated phospholipid equilibria involving liposomes down to sub-micelle size assemblies

Conventional channel-based microfluidic platforms have gained prominence in controlling the bottom-up formation of phospholipid based nanostructures including liposomes. However, there are challenges in the production of liposomes from rapidly scalable processes. These have been overcome using a vortex fluidic device (VFD), which is a thin film microfluidic platform rather than channel-based, affording ∼110 nm diameter liposomes. The high yielding and high throughput continuous flow process has a 45° tilted rapidly rotating glass tube with an inner hydrophobic surface. Processing is also possible in the confined mode of operation which is effective for labelling pre-VFD-prepared liposomes with fluorophore tags for subsequent mechanistic studies on the fate of liposomes under shear stress in the VFD. In situ small-angle neutron scattering (SANS) established the co-existence of liposomes ∼110 nm with small rafts, micelles, distorted micelles, or sub-micelle size assemblies of phospholipid, for increasing rotation speeds. The equilibria between these smaller entities and ∼110 nm liposomes for a specific rotational speed of the tube is consistent with the spatial arrangement and dimensionality of topological fluid flow regimes in the VFD. The prevalence for the formation of ∼110 nm diameter liposomes establishes that this is typically the most stable structure from the bottom-up self-assembly of the phospholipid and is in accord with dimensions of exosomes.

Fluoride-triggered phase transition of metallogels for on-demand in situ containment of fluids

Sol-gel transition regulates mass transport in fluidic systems. We designed pre-gelators that react with fluoride anions to form a metallogel barrier. A combination of spectroscopic, rheological, and X-ray spectroscopic studies elucidated the mechanism of gelation involving desilylation followed by metal coordination-driven self-assembly, the kinetics of which can be finely controlled by the chemical structure of the silyl substituents. Protonation-induced degelation restores flow, allowing the metallogel to function as a reversible chemical valve.

Accelerated Diffusion of a Copper(I)-Functionalized COF Packed Bed Reactor for Efficient Continuous Flow Catalysis

Flow chemistry provides a neo-orientation for the research and development of chemical technology, in which heterogeneous continuous catalysis based on packed beds can realize rapid separation and recycling. However, options for heterogeneous catalysts are still limited. In this work, we gradually grow covalent organic frameworks (COFs, TpBpy) on the surface of a silica gel (SiO2)-supported substrate to obtain a stable copper(I)-chelated high-loading heterogeneous catalyst (SiO2@CuI-TpBpy). SiO2@CuI-TpBpy shows high catalytic activity in three-component Huisgen 1,3-dipolar cycloaddition, giving the corresponding triazoles with excellent yields and reposeful recyclability under batch conditions. The structures of the catalysts remain steady, and the copper contents are basically unchanged after five cycles. Then, the catalysts are successfully applied for three-component heterogeneous catalysis in a one-pot continuous flow to prepare rufinamide in 89% yield for 24 h stably and efficiently with mere traces of copper ions remaining. More importantly, the catalytic system reveals a minuscule effect of catalyst particle size on internal diffusion. This COF encapsulation strategy presents a new possibility for the design of industrial heterogeneous catalysts with high metal loading and low internal diffusion resistance.

Iodobenzene-Catalyzed Synthesis of Fully Functionalized NH-Pyrazoles and Isoxazoles from α,β-Unsaturated Hydrazones and Oximes via 1,2-Aryl Shift

An iodine(III)-catalyzed general method for the synthesis of fully functionalized NH-pyrazoles and isoxazoles from α,β-unsaturated hydrazones and oximes, respectively, via cyclization/1,2-aryl shift/aromatization/detosylation, has been developed. The reaction progresses through an anti-Baldwin 5-endo-trig cyclization. It gives direct access to an advanced intermediate for the preparation of valdecoxib and parecoxib, drugs used for COX-inhibition. In addition, a method for N-alkynylation of pyrazoles has also been developed in the presence of TIPS-EBX.

One-Flow Synthesis of Substituted Indoles via Sequential 1,2-Addition/Nucleophilic Substitution of Indolyl-3-Carbaldehydes

Substituted indoles are important as drugs. A number of valuable indoles have been synthesized via nucleophilic substitution at the 3'-position of indoles. However, the preparation of an indolylmethyl electrophile containing a tertiary carbon at the 3'-position and its subsequent nucleophilic substitution are challenging owing to the instability of the electrophile. Herein, we demonstrated the rapid one-flow synthesis of indoles via sequential 1,2-addition/nucleophilic substitution of indolyl-3-carbaldehydes. The use of a microflow technology helped in suppressing the undesired reactions caused by the unstable intermediates, resulting in significantly higher yields and reproducibility compared to those under batch conditions. A crown ether was effective when 1-alkylindole-3-carboxaldehyde was used as a substrate. However, the crown ether exerted a detrimental effect when 1H-indole-3-carboxaldehyde was used. A total of 15 structurally diverse indole derivatives were obtained in generally acceptable to good yields.

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.

Developing Amphetamine Certified Reference Materials: From Batch and Continuous-Flow Synthesis to Certification Protocol

Certified reference materials (CRM) of amphetamine derivatives were produced through a simple, rapid and efficient synthesis in both batch and continuous-flow conditions, accompanied by the development of a comprehensive certification protocol for this class of substances. Our chemistry enabled the synthesis of MDA, MDMA, PMA and PMMA in two steps from safrole and estragole with overall yields of 38-61 % in 48 hours under batch conditions and 61-65 % in 65 minutes under continuous-flow conditions, followed by the development of a certification protocol for these materials through identity checking, homogeneity, stability, and characterization studies. Furthermore, as result of this work, a very pure CRM of MDA.HCl with 99.1±1.4 g/100 g of certified characterization value was produced. Considering the importance of supplying amphetamine calibrants for public security efforts in Forensic Chemistry, the potential therapeutical applications, and responding to the rising demand for the synthesis of CRM, this work presents a pioneering approach for the production of amphetamine and related compounds.

Flow chemistry based catalytic hydrogenation for improving the synthesis of 1-deoxynojirimycin (DNJ) from an l-sorbose derived precursor

1-Deoxynojirimycin (1-DNJ) is a glycoprocessing inhibitor, and it serves as a synthetic precursor to two of three currently marketed iminosugar drugs, miglustat (N-butyl DNJ/Zavesca®) and miglitol (Glyset®). Herein a continuous flow procedure is presented that shortens a synthesis of 1-DNJ from an intermediate prepared from l-sorbose. Batch reactions involving an azide reduction, subsequent reductive amination-based cyclisation, and O-benzyl deprotection in a previous report required two steps and the use of an acid. Here, this sequence is achieved in one step using the H-Cube® MiniPlus continuous flow reactor. Subsequent reductive amination of 1-DNJ with butanal using the H-Cube® gave NB-DNJ.

Process Intensification of a Napabucasin Manufacturing Method Utilizing Microflow Chemistry

Microflow chemistry is one of the newest and most efficient technologies used today for the safe and effective production of medicines. In this paper, we show the use of this technology in the development of a manufacturing method for napabucasin, which has potential in the treatment of colorectal and pancreatic cancers. In conventional "batch-type" reactor systems, the generation of side products can be controlled with traditional techniques such as reagent reverse-addition and temperature control. However, there is a limitation to which the yield and purity can be improved by these methods, as both are constrained by the efficiency of heat/mass transfer. Applying microflow chemistry technology alters the parameters of the constraint through the use of precise mixing in a microchannel, which offers increased possibility for improving yields and process intensification of the napabucasin process. Reported herein is a proof-of-concept study for the scale-up production of napabucasin using microflow chemistry techniques for manufacturing at the kilogram scale.

Quid Pro Flow

How do you get into flow? We trained in flow chemistry during postdoctoral research and are now applying it in new areas: materials chemistry, crystallization, and supramolecular synthesis. Typically, when researchers think of "flow", they are considering predominantly liquid-based organic synthesis; application to other disciplines comes with its own challenges. In this Perspective, we highlight why we use and champion flow technologies in our fields, summarize some of the questions we encounter when discussing entry into flow research, and suggest steps to make the transition into the field, emphasizing that communication and collaboration between disciplines is key.

Will the next generation of chemical plants be in miniaturized flow reactors?

For decades, a production paradigm based on centralized, stepwise, large scale processes has dominated the chemical industry horizon. While effective to meet an ever increasing demand for high value-added chemicals, the so-called macroscopic batch reactors are also associated with inherent weaknesses and threats; some of the most obvious ones were tragically illustrated over the past decades with major industrial disasters and impactful disruptions of advanced chemical supplies. The COVID pandemic has further emphasized that a change in paradigm was necessary to sustain chemical production with an increased safety, reliable supply chains and adaptable productivities. More than a decade of research and technology development has led to alternative and effective chemical processes relying on miniaturised flow reactors (a.k.a. micro and mesofluidic reactors). Such miniaturised reactors bear the potential to solve safety concerns and to improve the reliability of chemical supply chains. Will they initiate a new paradigm for a more localized, safe and reliable chemical production?

Machine learning for nanoplasmonics

Plasmonic nanomaterials have outstanding optoelectronic properties potentially enabling the next generation of catalysts, sensors, lasers and photothermal devices. Owing to optical and electron techniques, modern nanoplasmonics research generates large datasets characterizing features across length scales. Furthermore, optimizing syntheses leading to specific nanostructures requires time-consuming multiparametric approaches. These complex datasets and trial-and-error practices make nanoplasmonics research ripe for the application of machine learning (ML) and advanced data processing methods. ML algorithms capture relationships between synthesis, structure and performance in a way that far exceeds conventional simulation and theory approaches, enabling effective performance optimization. For example, neural networks can tailor the nanostructure morphology to target desired properties, identify synthetic conditions and extract quantitative information from complex data. Here we discuss the nascent field of ML for nanoplasmonics, describe the opportunities and limitations of ML in nanoplasmonic research, and conclude that ML is potentially transformative, especially if the community curates and shares its big data.

In-line Multidimensional NMR Monitoring of Photochemical Flow Reactions

This work demonstrates the in-line monitoring of a flow photochemical reaction using 1D and ultrafast 2D NMR methods at high magnetic field. The reaction mixture exiting the flow reactor is flown through the NMR spectrometer and directly analyzed. In the case of simple substrates, suitable information can be obtained through 1D ¹ H spectra, but for molecules of higher complexity the use of 2D experiments is key to address signal overlaps and assignment issues. Here we show the usefulness of ultrafast 2D COSY experiments acquired in 70 s or less, for the in-line monitoring of photochemical reactions, and the possibility to obtain reliable quantitative information. This is a powerful framework to, for example, efficiently screen reaction conditions.

Bayesian Self-Optimization for Telescoped Continuous Flow Synthesis

The optimization of multistep chemical syntheses is critical for the rapid development of new pharmaceuticals. However, concatenating individually optimized reactions can lead to inefficient multistep syntheses, owing to chemical interdependencies between the steps. Herein, we develop an automated continuous flow platform for the simultaneous optimization of telescoped reactions. Our approach is applied to a Heck cyclization-deprotection reaction sequence, used in the synthesis of a precursor for 1-methyltetrahydroisoquinoline C5 functionalization. A simple method for multipoint sampling with a single online HPLC instrument was designed, enabling accurate quantification of each reaction, and an in-depth understanding of the reaction pathways. Notably, integration of Bayesian optimization techniques identified an 81 % overall yield in just 14 h, and revealed a favorable competing pathway for formation of the desired product.

Fischer indole synthesis in DMSO/AcOH/H2O under continuous flow conditions

A new continuous flow synthetic method for preparing indole and its derivatives are successfully developed to overcome the disadvantages of traditional batch methods, such as low conversion rates, long reaction times, and amplification effects. The method represents a sustainable and efficient preparation of indole and its derivatives without the need for additional catalysts. By investigating the effects of the reaction temperature, the solvent, the equivalence ratio, and the residence time, high conversion rates and excellent yields were simultaneously achieved within 20 min under optimized conditions. For the template reaction, DMSO/H2O/AcOH = 2:1:1 is used as the solvent, the reaction temperature is 110 °C, and the ratio of phenylhydrazine hydrochloride to cyclopentanone is 1:1.05. Indole and a wide array of its derivatives are synthesized to verify the universality of the method, and most of the reactions exhibit satisfactory conversion rates and high yields are obtained. This new continuous flow method is more suitable for industrial scale-up relative to traditional batch methods.