Flow gas/liquid reactions are often carried out using a tubular reactor and mixer under slug flow conditions. This is due to the extended residence time from 15 to 30 s. Indeed, when the reaction was carried out with a shallow reactor angle such as 5°, the conversion of 1 increased from 49 to 61% ( Table 1, entries 8 and 10). The irradiation at 600 W gave an almost similar result ( Table 1, entries 8 and 9), which suggested that 240 W sufficed the reaction. When a lower feeding of 1 (29.6 mmol/min) and an excess amount of Cl 2 (3.09 equiv) were used, higher conversion of 1 (76%) was attained with the selectivity of 84:16 ( Table 1, entry 7). The reaction of 1 with 1.97 equiv of Cl 2 resulted in 61% conversion of 1 and an 86:14 ratio of 2 and 2’ ( Table 1, entry 6). Then, we limited the feeding of 1 (flow rate: 46.4 mmol/min) in order to increase conversion, which worked well. When the reaction mixture was circulated twice, we observed a higher conversion of 1 (87%) and obtained a 74:26 mixture of 2 and 2’ ( Table 1, entry 5).
The reaction of 1 with one equivalent of Cl 2 gave 2 and 2’ in a ratio of 89:11 with 39% conversion of 1 ( Table 1, entry 4). When 0.45 equiv of Cl 2 was used, the conversion of 1 increased to 21% and the selectivity of 2 became 91% ( Table 1, entry 3). When 0.23 equiv of Cl 2 was used, the selectivity became 96% with 12% conversion of 1, in which a small amount of undesired 1,2-dichloroethylene carbonate ( 2’) was detected by GC ( Table 1, entry 2). When the reaction of ethylene carbonate ( 1, flow rate: 74.9 mmol/min, containing 0.03% of H 2O) with 0.17 equiv of Cl 2 gas (flow rate: 12.5 mmol/min) was carried out under irradiation by UV-LED (240 W) with a 15° reactor angle, the desired chloroethylene carbonate ( 2) was formed selectively with a 9% conversion of 1 ( Table 1, entry 1). Ethylene carbonate ( 1) contains 0.03% of H 2O.
Photoirradiation was carried out by using LEDs (365 ± 5 nm at the power of 240 or 600 W). The design concepts including angle settings to ensure a thin liquid layer are summarized in Figure 1.ĪReactions were conducted by using LX-1 with a reactor angle of 15° or 5° (entry 10). The flow photoreactor is embedded into an aluminum frame equipped with a heat carrier channel. Each channel track has a 2 mm depth and 557 mm length, while the width varies from 6 or 13 mm depending on the number of channels 7 or 5, respectively. In this study, we tested a novel photoflow setup consisting of quartz-made straight-line reactors, which are provided from MiChS (LX-1, Figure 1a) and a high-power LED (MiChS LED-s, 365 ± 5 nm, Figure 1b). We thought that if rationally designed scalable photoflow setups were available, flow C–H chlorination reactions using chlorine gas would be able to focus on production. More recent studies on flow C–H chlorination reactions focused on the use of Cl 2 gas in situ generated by photolysis of sulfuryl chloride or by acid treatment of NaOCl. While the flow rate employed was quite low (0.12 mL/min of toluene), the residence time was less than 14 seconds. In 2002, Jähnisch and co-workers reported the first microflow chlorination of 2,4-diisocyano-1-methylbenzene, which used a falling-film reactor developed by IMM. Radical chain mechanism for a photo-induced C–H chlorination reaction.įlow C–H chlorination using a compact flow reactor is highly desirable in terms of efficiency and safety in handling highly toxic gases such as chlorine. We found that the substrate contamination with water negatively influenced the performance of the C–H chlorination. At a higher conversion of ethylene carbonate such as 61%, the selectivity for monochlorinated ethylene carbonate over dichlorinated ethylene carbonate was 86%. Near-complete selectivity for single chlorination required the low conversion of ethylene carbonate such as 9%, which was controlled by limited introduction of chlorine gas. The partial irradiation of the flow channels also sufficed for the C–H chlorination, which is consistent with the requirement of photoirradiation for the purpose of radical initiation. Such short time of exposition sufficed the photo C–H chlorination. When ethylene carbonate was introduced to the reactor, the residence time was measured to be 15 or 30 s, depending on the slope of the reactor set at 15 or 5°, respectively. The setup employed sloped channels so as to make the liquid phase thinner, ensuring a high surface-to-volume ratio. A novel photoflow setup designed for a gas–liquid biphasic reaction turned out to be useful for the direct use of chlorine gas. We report the high-speed C–H chlorination of ethylene carbonate, which gives chloroethylene carbonate, a precursor to vinylene carbonate.
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