Asymmetric reactions in continuous flow

Review on optofluidic microreactors for photocatalysis

Four interrelated issues have been arising with the development of modern industry, namely environmental pollution, the energy crisis, the greenhouse effect and the global food crisis. Photocatalysis is one of the most promising methods to solve them in the future. To promote high photocatalytic reaction efficiency and utilize solar energy to its fullest, a well-designed photoreactor is vital. Photocatalytic optofluidic microreactors, a promising technology that brings the merits of microfluidics to photocatalysis, offer the advantages of a large surface-to-volume ratio, a short molecular diffusion length and high reaction efficiency, providing a potential method for mitigating the aforementioned crises in the future. Although various photocatalytic optofluidic microreactors have been reported, a comprehensive review of microreactors applied to these four fields is still lacking. In this paper, we review the typical design and development of photocatalytic microreactors in the fields of water purification, water splitting, CO2 fixation and coenzyme regeneration in the past few years. As the most promising tool for solar energy utilization, we believe that the increasing innovation of photocatalytic optofluidic microreactors will drive rapid development of related fields in the future.

Continuous-Flow Synthesis of (R)-Tamsulosin Utilizing Sequential Heterogeneous Catalysis

We describe the continuous-flow synthesis of (R)-tamsulosin, a blockbuster therapeutic drug employed for dysuria associated with urinary stones and benign prostatic hyperplasia, by utilizing sequential heterogeneous catalysis. Two heterogeneous catalysts have been developed for the synthesis, and the key step involves reductive amination of nitriles using dimethylpolysilane-modified Pd on activated carbon/calcium phosphate. Overall, (R)-tamsulosin was obtained in 60 % yield and 64 % ee (99 % ee after recrystallization) in a flow stream through four catalytic transformations without the need for the isolation or purification of any intermediates or byproduct.

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.

Chiral Heterogeneous Scandium Lewis Acid Catalysts for Continuous-Flow Enantioselective Friedel-Crafts Carbon-Carbon Bond-Forming Reactions

While continuous-flow reactions with chiral heterogeneous catalysts provide a highly efficient method to synthesize optically active compounds, chiral heterogeneous Lewis acid catalysis has been less extensively explored. We have developed the first example of chiral heterogeneous Sc catalysts, which demonstrated excellent activity and selectivity for continuous-flow enantioselective Friedel-Crafts reactions of isatins with indoles. Noncovalent interactions between chiral Sc complexes and heteropoly acid-anchored amine-functionalized SiO₂ as support were utilized for the synthesis. The heteropoly acid was found to be crucial for the preparation, activity, and selectivity of the catalysts. The chiral ligand could be easily tuned without chemical modification and the continuous-flow synthesis of a biologically active compound was achieved.

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.

Ugi reaction-derived prolyl peptide catalysts grafted on the renewable polymer polyfurfuryl alcohol for applications in heterogeneous enamine catalysis

The multicomponent synthesis of prolyl pseudo-peptide catalysts using the Ugi reaction with furfurylamines or isocyanides is described. The incorporation of such a polymerizable furan handle enabled the subsequent polymerization of the peptide catalyst with furfuryl alcohol, thus rendering polyfurfuryl alcohol-supported catalysts for applications in heterogeneous enamine catalysis. The utilization of the polymer-supported catalysts in both batch and continuous-flow organocatalytic procedures proved moderate catalytic efficacy and enantioselectivity, but excellent diastereoselectivity in the asymmetric Michael addition of n-butanal to β-nitrostyrene that was used as a model reaction. This work supports the potential of multicomponent reactions towards the assembly of catalysts and their simultaneous functionalization for immobilization.

Biocatalytic transamination in a monolithic flow reactor: improving enzyme grafting for enhanced performance

Transaminases were immobilized onto macrocellular silica monoliths and used for carrying a continuous flow mode transamination reaction. Monoliths were prepared via an emulsion-templated sol–gel method and functionalised by amino-moieties (3-aminopropyl-triethoxysilane, APTES) in order to covalently immobilize the enzymes, using glutaraldehyde as a cross-linking agent. In order to obtain higher performance and improved reproducibility, we investigate the key parameters of APTES functionalisation and of enzyme grafting. Four functionalisation protocols were studied. We show that enhancing the homogeneity of the APTES grafting and controlling the moisture level during functionalisation led to a 3-fold increase in activity as compared to the previously reported data, and greatly improved the reproducibility. Additionally, we report a strong beneficial effect of running the enzyme immobilisation at room temperature instead of 4 °C, further enhancing the obtained activity. Finally, the popular method which consists of stabilizing the covalent attachment of the enzyme by reducing the imine bonds formed between the enzyme and the functionalised surface was investigated. We highlight a strong enzyme deactivation caused by cyanoborohydride, making this strategy irrelevant in this case. The improvements presented here led to more active macrocellular monoliths, of general interest for continuous flow mode biocatalysis.

Higher performance can be obtained in flow biocatalytic transamination reactions if the key parameters of support functionalization and of enzyme grafting are controlled.

Emerging porous materials in confined spaces: from chromatographic applications to flow chemistry

Porous materials confined within capillary columns/microfluidic devices are discussed, and progress in chromatographic and membrane separations and catalysis is reviewed.

Asymmetric fluorination of indanone-2-carboxylates using a polystyrene-supported diphenylamine-linked bis(oxazoline) complex

A highly enantioselective fluorination of indanone-2-carboxylates catalyzed by a polystyrene-supported diphenylamine-linked bis(oxazoline) (PS-box)-Cu(OTf)₂ complex has been developed in a continuous flow system. The supported complex exhibited extremely efficient catalytic performance with high activity, affording the corresponding products in excellent yields (up to 99% yield) with excellent enantioselectivities (up to 99% ee) and more than 4000 turnover number (TON).

Base-Catalyzed 1,6-Conjugate Addition of Nitroalkanes to p-Quinone Methides under Continuous Flow

A mild base-catalyzed protocol for the synthesis of substituted nitroalkane derivatives has been developed under continuous flow using a microreaction technique. This transformation basically involves the 1,6-conjugate addition of nitroalkanes to p-quinone methides, leading to the substituted nitroalkanes in good to excellent yields.

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.

1,6-Conjugate addition of zinc alkyls to para-quinone methides in a continuous-flow microreactor

An efficient protocol has been developed for the 1,6-conjugate addition of zinc alkyls to p-quinone methides under continuous-flow using a microreactor.

Enantioselective reaction monitoring utilizing two-dimensional heart-cut liquid chromatography on an integrated microfluidic chip

Chip-integrated, two-dimensional high performance liquid chromatography is introduced to monitor enantioselective continuous micro-flow synthesis. The herein described development of the first two-dimensional HPLC-chip was realized by the integration of two different columns packed with reversed-phase and chiral stationary phase material on a microfluidic glass chip, coupled to mass spectrometry. Directed steering of the micro-flows at the joining transfer cross enabled a heart-cut operation mode to transfer the chiral compound of interest from the first to the second chromatographic dimension. This allows for an interference-free determination of the enantiomeric excess by seamless hyphenation to electrospray mass spectrometry. The application for rapid reaction optimization at micro-flow conditions is exemplarily shown for the asymmetric organocatalytic continuous micro-flow synthesis of warfarin.

Tailoring the Spacer Arm for Covalent Immobilization of Candida antarctica Lipase B-Thermal Stabilization by Bisepoxide-Activated Aminoalkyl Resins in Continuous-Flow Reactors

An efficient and easy-to-perform method was developed for immobilization of CaLB on mesoporous aminoalkyl polymer supports by bisepoxide activation. Polyacrylate resins (100–300 µm; ~50 nm pores) with different aminoalkyl functional groups (ethylamine: EA and hexylamine: HA) were modified with bisepoxides differing in the length, rigidity and hydrophobicity of the units linking the two epoxy functions. After immobilization, the different CaLB preparations were evaluated using the lipase-catalyzed kinetic resolution (KR) of racemic 1-phenylethanol (rac-1) in batch mode and in a continuous-flow reactor as well. Catalytic activity, enantiomer selectivity, recyclability, and the mechanical and long-term stability of CaLB immobilized on the various supports were tested. The most active CaLB preparation (on HA-resin activated with 1,6-hexanediol diglycidyl ether—HDGE) retained 90% of its initial activity after 13 consecutive reaction cycles or after 12 month of storage at 4 °C. The specific rate (r_flow), enantiomer selectivity (E) and enantiomeric excess (ee) achievable with the best immobilized CaLB preparations were studied as a function of temperature in kinetic resolution of rac-1 performed in continuous-flow packed-bed bioreactors. The optimum temperature of the most active HA-HDGE CaLB in continuous-flow mode was 60 °C. Although CaLB immobilized on the glycerol diglycidyl ether (GDGE)-activated EA-resin was less active and less selective, a much higher optimum temperature (80 °C) was observed with this form in continuous-flow mode KR of rac-1.

Development of an Intermittent-Flow Enantioselective Aza-Henry Reaction Using an Arylnitromethane and Homogeneous Brønsted Acid-Base Catalyst with Recycle

A stereoselective aza-Henry reaction between an arylnitromethane and Boc-protected aryl aldimine using a homogeneous Brønsted acid-base catalyst was translated from batch format to an automated intermittent-flow process. This work demonstrates the advantages of a novel intermittent-flow setup with product crystallization and slow reagent addition which is not amenable to the standard continuous equipment: plug flow tube reactor (PFR) or continuous stirred tank reactor (CSTR). A significant benefit of this strategy was the integration of an organocatalytic enantioselective reaction with straightforward product separation, including recycle of the catalyst, resulting in increased intensity of the process by maintaining high catalyst concentration in the reactor. A continuous campaign confirmed that these conditions could effectively provide high throughput of material using an automated system while maintaining high selectivity, thereby addressing nitroalkane safety and minimizing catalyst usage.

A monolithic 5-(pyrrolidin-2-yl)tetrazole flow microreactor for the asymmetric aldol reaction in water–ethanol solvent

An organocatalytic polystyrene monolithic column produces chiral β-hydroxyketones under benign, continuous-flow conditions with high long-term stability.

A catalytic chiral gel microfluidic reactor assembled via dynamic covalent chemistry

A novel dynamic covalent gel strategy is reported to immobilize an asymmetric catalyst within the channels of a microfluidic flow reactor. A layer of a catalytically active Mn-salen dynamic covalent imine gel matrix was coated onto a functionalized capillary. Mn-salen active moiety was incorporated into dynamic covalent imine gel matrix via the reaction of a chiral Mn-salen dialdehyde unit with a tetraamine linker. The catalytic activity of the capillary reactor has been demonstrated in enantioselective kinetic resolution of secondary alcohols.

Aerobic flow oxidation of alcohols in water catalyzed by platinum nanoparticles dispersed in an amphiphilic polymer

Various alcohols were aerobically oxidized in an aqueous solution in a continuous-flow reactor containing a platinum nanoparticles dispersed in a polystyrene–poly(ethylene glycol) resin to give the corresponding carbonyl compounds in up to 99% yield.

Chemical reactivity and solution structure: on the way to a paradigm shift?

Reagent molecules inside solution domains {R1} and {R2} cannot contact hence react. For this reason solution structure may influence chemical reactivity.

Continuous-flow hydrogenation of olefins and nitrobenzenes catalyzed by platinum nanoparticles dispersed in an amphiphilic polymer

Continuous-flow hydrogenation of olefins and nitrobenzenes with ARP-Pt.

A practical deca-gram scale ring expansion of (R)-(−)-carvone to (R)-(+)-3-methyl-6-isopropenyl-cyclohept-3-enone-1

A route to enantiopure (R)-(+)-3-methyl-6-isopropenyl-cyclohept-3-enone-1, an intermediate for terpenoids, has been developed and includes a highly chemo- and regioselective Tiffeneau–Demjanov reaction.