Continuous-flow synthesis of (E)-2-Hexenal intermediates using a two-stage microreactor system
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Continuous-flow synthesis of (E)-2-Hexenal intermediates using a two-stage microreactor system Jian Deng 1 & Pengcheng Zou 1 & Kai Wang 1
&
Guangsheng Luo 1
Received: 4 May 2020 / Accepted: 18 August 2020 # Akadémiai Kiadó 2020
Abstract A flow synthesis method of (E)-2-hexenal intermediates by employing a two-stage microreactor system was reported as an example of autothermal Prins-type reaction. The continuous flow technology not only overcame the low efficiency of batch reactions, it also met the demand for high selectivity and required less consumption of reactants. Temperature monitors of a micro-mixer and a reaction tube covered by thick thermal insulation materials provided an apparent reaction enthalpy of −112 kJ· mol−1 for the generations of hexenal intermediates and by-products. A two-stage reaction platform was therefore developed to enable efficient control of the reaction temperature, which was from 20 °C and 110 °C. Under optimized operating conditions, the yield of (E)-2-hexenal intermediates reached 72%, with a space time yield of 273 kg·h−1 L−1, which was over 1200 times that of a traditional batch reactor with dripping fed reactants. The final yield of (E)-2-hexenal was maintained at 72% after hydrolysis of the intermediates. Keywords (E)-2-hexenal . Autothermal reaction . Prins-type reaction . Flow synthesis . Microreactor
Introduction (E)-2-hexenal exists in fruits and vegetables such as tomatoes, apples, and strawberries. It has a pleasant, fresh, and fruity odor at concentrations below 0.1 wt.% and is commonly used as a fragrance ingredient in foods and beverages [1]. Due to its low natural abundance in plants, commercial (E)-2-hexenal is obtained mainly via organic synthesis at present. As a straightchain α-β unsaturated aldehyde, (E)-2-hexenal is difficult to synthesize via classic methods, such as aldolization of saturated aldehydes followed by dehydration of the aldol [2], due to the generation of undesired branched-chain aldehydes. Currently, (E)-2-hexenal is manufactured via a two-step batch Highlights 1. An autothermal Prins-type synthesis was realized for (E)-2-hexenal intermediates. 2. A two-stage microreactor system with high reaction yield of (E)-2hexenal intermediates was developed. 3. Space time yield of (E)-2-hexenal intermediates was over 1000 times that of a traditional batch reactor. * Kai Wang [email protected] 1
The State Key Lab of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China
method involving a Prins-type reaction followed by hydrolysis, as shown in Scheme 1 [3, 4]. The Prins-type reaction involves the acid catalyzed addition of aldehydes and olefinic compounds to produce 1,3-dioxane derivative, which are useful intermediates in the preparation of fragrance ingredients, medicines, and natural compounds [5–10]. As shown in Scheme 1, the synthesis of (E)-2hexenal employs butyraldehyde (BA) and vinyl ethyl ether (VE) as starting materials to create both 1,3-dioxane intermediate (IM1) and an open-ri
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