Journal of Flow Chemistry最新文献

The recent advances in the area of electrophotocatalysis (EPC) show that it is a highly suitable technique to yield greener and more sustainable organic synthesis. The overall productivity of EPC however is constrained by a multitude of practical limitations, which impose difficulties in effectively harmonizing the photochemical and electrochemical steps, let alone in accelerating both steps simultaneously. In this contribution, we have tackled these limitations by developing a parallel plate flow cell that permits the execution of EPC in continuous flow. By using a transparent electrode, such as fluorine-doped tin oxide (FTO) or indium tin oxide (ITO) coated glass, the interelectrode distance could be reduced while improving photon absorption. By enhancing both the photochemical and electrochemical steps simultaneously, a notable increase in productivity and space–time-yield (a ten-fold and 300-fold improvement, respectively) of the N-arylation of different azoles was observed. In addition, this was achieved in a single-pass process under electrolyte-free conditions.

An explorative study on the continuous flow generation of the N-F reagent 2,6-dichloro-1-fluoro-pyridinium tetrafluoroborate from 2-6-dichloropyridine and 10% F₂/N₂ and its telescoped downstream electrophilic fluorination reaction with an enamine is reported. The 2-step procedure was performed in a modular lab-scale silicon carbide flow reactor, which safely allowed processing corrosive F₂ and precise temperature control. Both reaction sequences turned out to be very fast when carried out in flow at − 10 °C: the N-F generation step could be done within 7.9 s and only 6.6 s were necessary for the fluorination of the enamine.

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We are currently experiencing a resurgence in the realm of electrochemical organic synthesis, driven by the transformative potential of conducting redox chemistry under mild conditions through the simple use of electrons, thereby circumventing the use of harmful reductants and oxidants. This renaissance is further bolstered by the fusion of electrochemistry with flow chemistry, which not only grants precise control over reaction parameters but also promotes sustainability and heightened reproducibility. Despite these promising advancements, the application of flow electrochemistry to steer asymmetric processes remains in its nascent stage. This perspective delves into the limited contributions to date, shedding light on critical challenges and presenting prospective solutions that are essential for fully unleashing the untapped potential of this field.

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Reactive organometallic intermediates present a distinct opportunity for the creation of novel carbon-carbon and carbon-heteroatom bonds. Whereas their utility in synthesis is well-established, the thermal sensitivity of these species often imposes the requirement for stringent reaction conditions, including strict control of reaction temperatures, concentrations, and use of additives. Moreover, their strong reactivity can pose challenges in achieving the desired selectivity. Since pioneering works in the 2000s, the advent of flow microreactor technology has revolutionized this field, expanding the possibilities of reactive organometallic intermediates within synthetic chemistry. In this review, we provide an overview of the recent advancements in this dynamic area, focusing on breakthroughs that have emerged within the past four years.

In a continuous flow microreactor system, a continuous nitration process of 2-(2,4-dichloro-5-nitrophenyl)-4-(difluoromethyl)-5-methyl-1,2,4-triazol-3-one which is the key intermediate for the synthesis of important triazolinone herbicide Sulfentrazone was developed. The effects of molar ratio of mixed acids, molar ratio of nitric acid to substrate, reaction temperature, total flow rate and residence time in the microreactor on nitration reaction were studied. The results showed that when the flow rate of the material was 60 mL/min, the molar ratio of nitrate to sulfur mixed acid was 1:6, the molar ratio of nitric acid to raw material was 1.1:1, the reaction temperature was 60 ℃, and the residence time was 30 s, the product can be obtained in 97% yield. Compared with the results of nitration process using traditional batch reactors, the use of continuous flow microreactors improved reaction efficiency and achieved higher yields. A characterization kinetics study was conducted on this reaction, and the pre-exponential-factor and activation energy for 2-(2,4-dichlorophenyl)-4-(difluoromethyl)-5-methyl-1,2,4-triazol-3-one nitration were obtained. The activation energy of the reaction is 40.204 kJ/mol. The continuous flow microreactor system greatly increased liquid-liquid two phases mass transfer efficiency, while accurately controlling the reaction temperature and residence time in the reactor.

The synthesis of trisubstituted isoxazoles generally requires multiple individual chemical steps, making them amenable to improvements in efficiency by telescoping as a multistep flow process. Three steps (oximation, chlorination and cycloaddition) were developed in continuous flow mode, aiming to function as an high-yielding and efficient sequence. We demonstrate this sequence using two aldehyde starting materials of interest: one carbocyclic and one heterocyclic. Between these two substrates, significant differences in solubility and reactivity necessitated modifications to the route. Most notably, the chlorination step could be carried out using either an organic N-Cl source (applicable for the carbocyclic aldehyde) or Cl₂ generated on-demand in a flow setup (applicable for the heterocyclic aldehyde). By selecting the most effective method for each substrate, good yields could be achieved over the telescoped sequence.

In order to improve process safety, product purity, and production efficiency in the synthesis of N-n-butyl-N-(2-nitroxy-ethyl)nitramine (BuNENA), a two-stage continuous flow microreactor system was constructed by sequentially connecting the self-designed heart-shaped channel microreactor and the caterpillar microreactor. n-Butylethanolamine was used as the raw material, nitric acid and acetic anhydride were used as the nitrating agents. The results showed that when the flow rate of n-butylethanolamine was 1.00 mL.min^− 1, the temperature of the heart-shaped channel microreactor was 10 ℃, the temperature of the caterpillar microreactor was 35 ℃, the molar ratio of ZnCl₂ to n-butylethanolamine was 2%, the molar ratio of nitric acid to n-butylethanolamine was 2.4, and the molar ratio of ZnCl₂ to n-butylethanolamine was 2.4, the result was best. Under the conditions, the reaction time was shortened to 300 s, the purity of BuNENA was up to 98.1%, and the yield was 87.1%.

This study examined the photochemical transformation of an oxazolone derivative in a continuous microreactor irradiated by a UVC LED array (273 nm). The aim of this study was to transfer the reaction protocol originally developed under batch conditions to continuous flow and to further evaluate the scope of this application. A custom-built UVC-LED panel was combined with a microchip, and this microflow system allowed to work under perfectly controlled operating conditions. NMR and LC-MS were used to identify and quantify the main products obtained during the reaction. From this, an HPLC method was developed for imine separation, allowing for an easy and fast monitoring of the reaction progress. Subsequently, the influence of the operating conditions (residence time, photon flux density, temperature) on the selectivity and conversion was investigated to identify the most favorable conditions for a specific product. Temperature did not affect conversion but had an impact on the reaction’s selectivity. The developed UVC-LED-driven continuous-flow microreactor was found to be very efficient since a quantum photon balance ratio of 0.7 was enough to convert all the reactant, while at the same time achieving the maximal yield of the target product. Exhaustive irradiation did not change the molar ratio of each compound present in the reaction medium, thus excluding follow-up photoreactions of the products. This work opens promising perspectives for boosting flow photochemistry in the UV-C domain.

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The number of biocatalyzed reactions at industrial level is growing rapidly together with our understanding on how we can maximize the enzyme efficiency, stability and productivity. While biocatalysis is nowadays recognized as a greener way to operate in chemistry, its combination with continuous processes has lately come up as a powerful tool to enhance process selectivity, productivity and sustainability. This perspective aims at describing the recent advances of this technology and future developments leading to smart, efficient and greener strategies for process optimization and large-scale production.

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