This is the second Thematic Series on chemistry in flow systems. Flow chemistry is an enabling technology that was introduced to the laboratories of synthetic organic chemists around ten years ago and has flourished now for about half a decade. This series contains representative examples of fields in organic synthesis where continuous flow conditions have already made a significant impact. First of all, there is the application to photochemistry, which has the chance of experiencing a renaissance particularly in an industrial environment. Second, flow chemistry lends itself naturally to the synthesis and direct application of reactive intermediates or reactive reagents, which are difficult to handle in a batch reactor. And third, new heating concepts, including inductive heating, are rendered possible, allowing accelerated synthesis under pressure, up to supercritical conditions, but whereby only a small volume of the reaction mixture inside the flow reactor is exposed to these extreme conditions.
See also the Thematic Series:
Chemistry in flow systems III
Chemistry in flow systems
See videos about flow chemistry at Beilstein TV.
Graphical Abstract
Scheme 1: Azide–nitrile cycloaddition under batch microwave conditions.
Figure 1: HPLC-UV chromatograms (215 nm) of crude reaction mixtures from the cycloaddition of diphenylacetoni...
Figure 2: HPLC-UV chromatogram (215 nm) showing the decomposition of tetrazole 2 in NMP/AcOH/H2O 5:3:2 (0.125...
Scheme 2: Possible decomposition mechanisms for tetrazole 2 in NMP/AcOH/H2O.
Scheme 3: Reaction steps for the degradation of tetrazole 2 and the corresponding rate equations.
Figure 3: Decomposition of tetrazole 2 at 240 °C in a NMP/AcOH/H2O 5:3:2 mixture (0.125 M) (points: experimen...
Figure 4: Decomposition of tetrazole 2 in a 4 mL resistance heated stainless steel coil at a nominal temperat...
Scheme 4: Mizoroki–Heck coupling under continuous flow conditions.
Scheme 5: Nucleophilic aromatic substitution of 4-fluoro-1-nitrobenzene (15) with pyrrolidine (16) under cont...
Figure 5: Nucleophilic aromatic substitution reaction of 1-fluoro-4-nitrobenzene (15) with pyrrolidine (16) i...
Graphical Abstract
Figure 1: Schematic diagram of the continuous flow reactor (left) and the column top (right).
Scheme 1: Hydrogenation of ethyl cinnamate.
Scheme 2: Hydrogenation of trans-stilbene and trans-chalcone.
Scheme 3: Hydrogenation of nitrobenzene and deprotection of the Cbz group.
Scheme 4: Hydrogenation in water.
Graphical Abstract
Figure 1: Sequential gas/liquid–liquid/liquid flow reactor for the synthesis of 4-fluoropyrazole derivatives.
Figure 2: H-bonded cycles in structures 4a (a) and 4f (b) (only one disordered pyrazole hydrogen atom is show...
Graphical Abstract
Scheme 1: General photodecarboxylation involving phthalimides (the broken line indicates intra- as well as in...
Figure 1: Microreactor (dwell device, mikroglas chemtech) under a UV exposure panel (Luzchem) and connected t...
Figure 2: UV-spectrum of DMBP (in MeCN) versus emission spectrum of the UVA lamp. The vertical dotted line re...
Figure 3: Light-penetration profile for a 1.5 mM solution of DMBP at 350 nm. The vertical lines represent the...
Scheme 2: DMBP mediated α-photodecarboxylation of N-phthaloylglycine (1).
Scheme 3: Photodecarboxylation of potassium phthaloyl-γ-aminobutyrate (3).
Scheme 4: Photodecarboxylative cyclization of potassium phthalimidomethylsulfanylacetate (6).
Scheme 5: Photodecarboxylative benzylation of 2.
Scheme 6: Photodecarboxylative addition of 11 to 2.
Scheme 7: Photodecarboxylative addition of 11 to DMBP.
Scheme 8: Mechanistic scenario (the broken line indicates intra- and intermolecular reactions).
Graphical Abstract
Scheme 1: Halogen–lithium exchange of p-bromoanisole followed by reaction with methanol.
Figure 1: Flow microreactor system for halogen–lithium exchange of aryl halide followed by reaction with meth...
Figure 2: Effects of the temperature (T) and the residence time in R1 (tR1) on the yield of anisole in the Br...
Scheme 2: Halogen-lithium exchange of p-bromoanisole followed by oxidative homocoupling with FeCl3.
Figure 3: Integrated flow microreactor system for oxidative homocoupling reaction of aryllithium with FeCl3. ...
Figure 4: Effects of the temperature (T) and the residence time in R2 (tR2) on the yield of 4,4'-dimethoxybip...
Graphical Abstract
Scheme 1: Electrochemically generated N-acyliminium ions 1 and subsequent reactions.
Figure 1: Electrochemical microreactor.
Scheme 2: Electrolysis of furan.
Scheme 3: Kolbe electrolysis of phenylacetic acids 6 in flow.
Scheme 4: Synthesis of diaryliodonium salts 11 in flow.
Graphical Abstract
Scheme 1: Products of aryl azide photolysis.
Scheme 2: Optimisation of the photolysis of aryl azide 8a.
Figure 1: Relationship between residence time and relative composition of the crude reaction mixture.
Scheme 3: Preparation of side product 10.
Scheme 4: General conditions for the photolysis of aryl azides in continuous flow.
Graphical Abstract
Scheme 1: Reduction of 4-(4-azidopiperidin-1-yl)benzonitrile on the H-Cube®.
Figure 1: SAH3 schematic with injector port and UV detector. Components: (A) pump; (B) six-position manual in...
Figure 2: Variation in UV absorbance during the reduction of styrene to ethylbenzene, in the controlled mode ...
Figure 3: Box plot of data showing the percent conversion of styrene to ethylbenzene as a function of pressur...
Figure 4: Conversion of styrene to ethylbenzene over 20 reactions in sequence through a single 30 mm 10% Pd/C...
Figure 5: Cartridge-to-cartridge variability of the substrate conversion based on the reduction of styrene to...
Figure 6: Conversion of styrene to ethylbenzene in a sequence of reactions alternating between 10% Pd/C and q...
Figure 7: Leached catalyst from 30 mm 10% Pd/C CatCart®. First 10 mL wash aliquot (A) compared to second 10 m...
Graphical Abstract
Figure 1: SEM image of silica monolith.
Scheme 1: Suzuki–Miyaura reaction of bromobenzene with phenylboronic acid.
Figure 2: Reactivity of the Pd-monolith-3.2 and Pd-monolith-6.4 for the Suzuki–Miyaura reaction between bromo...
Figure 3: Reactivity of the Pd-monolith-3.2 and Pd-monolith-6.4 for the Suzuki–Miyaura reaction between bromo...
Figure 4: TEM image of Pd-monolith catalyst (scale bar: 100 nm).
Graphical Abstract
Figure 1: Schematic the for contacting modes of biphasic gas–liquid in (a) batch reactor, (b) dual-channel, a...
Figure 2: Optical image of the triple-channel microreactor (for demonstration purposes, the inner channel for...
Figure 3: Photosensitized oxygenation in the triple-channel microreactor.
Scheme 1: Photosensitized oxygenation of citronellol (a key step in the synthesis of rose oxide).
Graphical Abstract
Scheme 1: Eschenmoser coupling reaction with secondary S-alkylated thioamide derivatives of type 3.
Scheme 2: Eschenmoser coupling sequence of S-alkylated ternary thioamides of type 7.
Figure 1: Conversion of 3aa to 4aa under different flow conditions.
Figure 2: Reaction kinetics analysis. Left: Rate constants with 0.1 M reaction solution. Right: Arrhenius-plo...
Scheme 3: Exclusive formation of thiazol 13 with dihydropyrimidine derivatives 11 take place in the case of a...
Figure 3: Flow chemistry setup scheme.
Figure 4: Capillary reactor with jacketed cover removed, and the process controller.
Graphical Abstract
Figure 1: Hastelloy-made micromixer (MiChS β-150H).
Figure 2: Hastelloy-made microextraction unit.
Figure 3: Acid-tolerant microflow system used for the Koch–Haaf reaction.
Scheme 1: Synthesis of 1-adamantanecarboxylic acid (2a) in a microflow system.
Scheme 2: Koch–Haaf reaction of 1b and 1c in a microflow system.
Scheme 3: Multigram scale flow synthesis of 1-adamantanecarboxylic acid (2a).
Graphical Abstract
Scheme 1: Hydrogenation of ethyl pyruvate.
Scheme 2: Hydrogenation of dimethyl itaconate.
Scheme 3: a) Enantioselective hydrogenation of N-(1-phenylethylidene)aniline in IL–CO2; b) Enantioselective h...
Scheme 4: Selective hydroformylation with a silica supported Rh catalyst.
Scheme 5: Enantioselective hydroformylation of styrene.
Scheme 6: Enantioselective hydrovinylation of styrene.
Scheme 7: Enantioselective cyclopropanation of styrene catalyzed by supported Cu–BOX, Cu–PyOX and Rh–PyBOX ca...
Scheme 8: Continuous hydrogenation of acetophenone coupled with the kinetic resolution of the product.
Scheme 9: Kinetic resolution of phenylethanol using CALB immobilized in ILs and supported ILs.
Graphical Abstract
Figure 1: Illustration of synthetically interesting diaryl ethers.
Scheme 1: Illustration of the model reaction used to compare the enabling technologies of microwave and micro...
Scheme 2: Illustration of the model reaction used to benchmark Labtrix S1 against batch and stopped-flow micr...
Figure 2: Photograph illustrating Labtrix® S1, the automated microreactor development apparatus from Chemtrix...
Figure 3: Schematic illustrating the 10 µL reactor manifold used for the SNAr reactions described herein (322...
Figure 4: Schematic illustration of the reactor manifold used to evaluate the continuous-flow synthesis of 2-...
Figure 5: Comparison of the results obtained in Labtrix® S1 with reported data generated in a microwave synth...
Figure 6: Screen shot from the Labtrix® S1 control software illustrating the system file that enables the use...
Figure 7: Summary of the results obtained for the organic base screen towards the SNAr reaction between DCNB (...
Figure 8: Illustration of the substituent effect on the synthesis of diaryl ethers under continuous flow (res...
Figure 9: Schematic illustrating the mixing of immiscible reagent streams in a microfluidic channel, whereby ...
Figure 10: Comparison of base effect on the synthesis of 2-chloro-1-(4-methoxyphenoxy)-4-nitrobenzene (7) (res...
Figure 11: Graphical representation of an automated flow reaction for equilibration and screening of reactor t...
Graphical Abstract
Figure 1: Experimental setup for catalyst synthesis in the tubular flow reactor; 1: Reaction mixture reservoi...
Figure 2: Measured temperature profile along the tubular reactor.
Figure 3: TGA weight loss curves for pristine CNT, HNO3 oxidized CNT, Pt/CNT-oil bath and Pt/CNT-tubular reac...
Figure 4: TEM micrographs of catalyst samples: a) Pt/CNT tubular reactor and b) Pt/CNT oil bath.
Figure 5: X-ray diffraction patterns for the as-received CNT and the three Pt/CNT samples taken at intervals ...
Figure 6: Comparison of performance in DMFC with Pt/CNT oil bath and Pt/CNT tubular reactor samples as cathod...
Graphical Abstract
Scheme 1: Hassner's synthesis of vinyl azides and a stable, nonexplosive analogue 5 of iodine azide (1).
Scheme 2: Preparation of polymer-bound bisazido iodate(I) 5 and polymer-bound 1,8-diaza-[5.4.0]bicyclo-7-unde...
Scheme 3: Two-step protocol for the preparation of vinyl azides 4a–e and 4g–i under flow conditions.
Scheme 4: Regeneration of functionalized polymers 5 and 8.
Scheme 5: Preparation of triazoles 12a–l by using inductively heated copper turnings as a packed-bed material...
Graphical Abstract
Figure 1: Metabolic pathways in a living cell as an example of efficient coupled-reaction processes. A: Subst...
Figure 2: Four generations of biotransformations. I: Single-reaction processes; II: Single-reaction processes...
Scheme 1: Production of L-leucine (3) in a continuously operating enzyme membrane reactor (EMR). E1: L-Leucin...
Scheme 2: Production of D-mandelic acid (5) in a continuously operating enzyme membrane reactor. E1: D-(−)-Ma...
Scheme 3: Simultaneous synthesis of gluconic acid (9) and glutamic acid (8) in a continuously operated membra...
Scheme 4: Production of L-tert-leucine (11) in a continuously operated enzyme membrane reactor equipped with ...
Scheme 5: Continuous oxidation of lactose (12) to lactobionic acid (13) in a dynamic membrane-aerated reactor...
Scheme 6: Production of N-acetylneuraminic acid (17) in a continuously operated enzyme membrane reactor. E1: ...
Scheme 7: Chemo-enzymatic epoxidation of 1-methylcyclohexene (18) in a packed-bed reactor (PBR) containing No...
Scheme 8: Continuous production of (R)-1-phenylethyl propionate (24) by dynamic kinetic resolution of (rac)-1...
Scheme 9: Synthesis of D-xylulose (28) from D,L-serine (26) and D,L-glyceraldehyde (25) in a continuously ope...
Scheme 10: Continuous production of L-alanine (31) from fumarate (29) in a two-stage enzyme membrane reactor. ...
Scheme 11: Continuous synthesis of 1-phenyl-(1S,2S)-propanediol (35) in a cascade of two enzyme membrane react...
Scheme 12: Production of a dipeptide 39 in a cascade of two continuously operated membrane reactors. E1: Carbo...
Scheme 13: Continuous production of GDP-mannose (43) from mannose 1-phosphate (40) in a cascade of two enzyme ...
Scheme 14: Continuous solvent-free chemo-enzymatic synthesis of ethyl (S)-3-(benzylamino)butanoate (48) in a s...
Scheme 15: Continuous chemo-enzymatic synthesis of grossamide (52) in a cascade of packed-bed reactors. E: Per...
Scheme 16: Chemo-enzymatic synthesis of 2-aminophenoxazin-3-one (56) in a cascade of continuously operating pa...
Scheme 17: Continuous conversion of 3-phospho-D-glycerate (57) into D-ribulose 1,5-bisphosphate (58) in a casc...
Scheme 18: Continuous hydrolysis of 4-cyanopyridine (59) to isonicotinic acid (61) in a cascade of two packed-...
Scheme 19: Continuous fermentative production of ethanol (64) from hardwood lignocellulose (62) in a stirred-t...
Scheme 20: Production of hydrogen by anaerobic fermentation of glucose (7) using Clostridium acetobutylicum ce...
Scheme 21: Continuous production of (2R,5R)-hexanediol (67) in an enzyme membrane reactor containing whole cel...
Scheme 22: Synthesis of L-phenylalanine (69) in a continuously stirred tank reactor equipped with a hollow-fib...
Scheme 23: Continuous epoxidation of 1,7-octadiene (70) to (R)-7-epoxyoctene (72) by a strain of Pseudomonas o...
Scheme 24: Oxidation of styrene (73) to (S)-styrene oxide (74) in a continuously operated biofilm tube reactor...
Scheme 25: Reduction of estrone (75) to β-estradiol (76) by Saccharomyces cerevisiae in a cascade of two stirr...
Graphical Abstract
Scheme 1: Proline-catalyzed direct enantioselective α-aminoxylation of aldehydes.
Figure 1: Polystyrene-immobilized hydroxyproline 1a.
Scheme 2: Preparation of the immobilized catalysts 1a and 1b.
Scheme 3: Direct enantioselective α-aminoxylation of propanal catalyzed by resins 1a and 1b.
Figure 2: Experimental setup for the continuous-flow α-aminoxylation of aldehydes.
Graphical Abstract
Scheme 1: The two proposed mechanistic pathways for the Appel reaction.
Scheme 2: Functionalisation of the triphenylphosphine monolith by using carbon tetrabromide in a recycling pr...
Figure 1: a. Unfunctionalised triphenylphosphine monolith; b. Monolith after fuctionalisation with carbon tet...
Scheme 3: Flow synthesis of bromides from alcohols by using the functionalised triphenylphosphine monolith.
Figure 2: Linear decrease of the brown decolourisation.
Graphical Abstract
Figure 1: Methods for catalyst use in flow.
Scheme 1: Prior results for batch α-aminoxylation reaction.
Figure 2: General reactor setup. A) A glass Omnifit column is packed with 1 g of proline. B) The column is th...
Figure 3: Schematic of the reactor setup. As the starting aldehyde and thiourea 3b (A) enter the proline pack...
Figure 4: The long-term stability of a proline packed bed in the α-aminoxylation reaction of hexanal. A solut...
Scheme 2:
Reaction with 3-phenylpropionaldehyde through reactor setup.
aIsolated yield, due to the instability...
Scheme 3:
Reaction with isovaleraldehyde through reactor setup.
aIsolated yield, due to the instability of the...
Graphical Abstract
Scheme 1: The schematic arrangement of the continuous-flow system.
Scheme 2: Preparation of chalcone 3b on larger scale.