Continuous‐flow technology with channel dimension in the millimeter region found widespread applications in numerous applications. The epoxidation of methyl oleate with molecular oxygen using an aldehyde as sacrificial reductant (Mukaiyama reaction) was implemented in a segmented flow microreactor. The effect of aldehyde structure and aldehyde/methyl oleate molar ratio on the conversion to epoxides was studied. Total conversion of methyl oleate (0.67 M in heptane) with a selectivity higher than 99% and a cis/trans ratio ∼75/25 was reach in less than 4 min, at room temperature, using three equivalents of 2‐ethylhexanal, 100 ppm of Mn(II) as catalyst and five bar of oxygen.
Practical applications: As the result of the small reactor volumes, very high surface‐to‐volume ratio and high heat exchange efficiency compared to classical batch reactors, the overall safety of oxidation processes is significantly improved using continuous‐flow technology. Microreactors also provide better mass transfer, and thus, improve the productivity of the aerobic epoxidation of methyl oleate using aldehyde as sacrificial reductant while maintaining the high selectivity of the process.
(i) Epoxidation of methyl oleate with molecular oxygen is performed safely using continuous‐flow technology. (ii) Total conversion is obtained in 4 min at r.t. using 100 ppm of Mn(II) and a sacrificial reductant. (iii) The selectivity and cis/trans ratio are comparable to batch.
An efficient and metal-free method for the oxidation of aldehydes to the corresponding carboxylic acids has been developed. In a simple continuous-flow photochemical reactor, the use of camphorquinone (CQ) irradiated with a white light-emitting diode (LED) source enhanced the autoxidation of aldehydes. Under 5 bar of oxygen, visible light, and 0.3 mol% of CQ, the rate of oxidation was increased from 6 times with 2-ethylhexanal to 30 times for n-nonanal. The large interfacial area generated by a segmented flow apparatus associated with radicals formed by photooxidation of CQ ensures metal-free high throughput of carboxylic acids under safe conditions.
A simple continuous flow photochemical reactor was optimized for the safe photo‐oxidation of citronellol into precursors of rose oxide, a flagrance of commercial value. The reactor productivity was increased by a factor of 4 compared to the current literature for a conversion above 99 % and in safe conditions (up to 0.72 mM min−1 mLreactor−1). The reactor was based on a classical design described by Booker‐Milburn et al., i. e. multiple loops of perfluoroalkoxy (PFA) tubing wrapped tightly around a visible LED lamp under slug flow conditions. The influence of the type of visible white LED light (6500 K vs. 3000 K), of photosensitizers (tetraphenylporphyrin vs. methylene blue), of solvent (CH2Cl2 vs. MeOH), concentration of citronellol (0.1 M vs. 2 M) and of mass efficiency of the reactors (PFA tubing of 0.75 mm to 0.25 mm) was paramount to increase the productivity of the reactor.
The front cover artwork is provided by Jaqueline Leib, the Laboratoire de Génie des Procédés Catalytiques (UMR CNRS 5285, Université de Lyon/CPE Lyon) and the Conservatoire National des Arts et Métiers. The image shows the transformation of citronellol (main constituent of the essential oil from geranium, Pelargonium graveolens) by photo‐oxygenation to give rose oxide, a floral green note, with a strong, diffusive rose scent. The productivity as well as the safety of the process was improved using a photo‐microreactor. Read the full text of the Communication at 10.1002/cptc.201800201.
The Front Cover illustrates the transformation of essential oil from geranium (i.e. citronellol) into precursors of rose oxide (a fragrance of commercial value) using singlet oxygen. A simple continuous‐flow photochemical reactor was optimized for the safe photo‐oxidation of citronellol using a visible‐light LED. Reactor productivity was increased by a factor of four compared to the current literature. More information can be found in the Communication by Z. E. Hamami et al. on page 122 in Issue 3, 2019 (DOI: 10.1002/cptc.201800201).
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