Photochemical microflow reactor

Photochemistry is a powerful method for generating reactive species as 

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Figure 7.22 Photochemical microflow reactor containing cooling chatmels under...

The [2 + 2] qrcloaddition reaction of maleimide and 1-hexyne in a photochemical microflow reactor has also been reported. The development of flow reactors for continuous processes can allow large-scale reactions [18]. This reaction can be achieved with 83% conversion on a large scale (0.7 kg) (Scheme 5.8). 

The photochemical chlorination of toluene-2,4-diisocyanate has also been reported (Scheme 8.3). A falling-film microflow reactor is used with irradiation of gaseous chlorine, through a quartz window, enabling the in situ generation of chlorine radicals. The optimal residence time is 9 s at 130 °C, and benzyl chloride-2,4-diisocyanate is produced in 81.0% yield. 

Addition reactions such as addition of enolates to carbonyl compounds, photochemical cydoaddition, radical addition and elimination reactions such as dehydration can be carried out in microflow reactors. Addition-elimination reactions such as Wittig reaction, Homer-Wadsworth-Emmons reaction, Baylis-Hillman reaction. 

In their pioneering work, Jensen et al. demonstrated that photochemical transformation can be carried out in a microfabricated reactor [37]. The photomicroreactor had a single serpentine-shaped microchannel (having a width of 500 pm and a depth of 250 or 500 pm, and etched on a silicon chip) covered by a transparent window (Pyrex or quartz) (Scheme 4.25). A miniature UV light source and an online UV analysis probe were integrated to the device. Jensen et al. studied the radical photopinacolization of benzophenone in isopropanol. Substantial conversion of benzophenone was observed for a 0.5 M benzophenone solution in this microflow system. Such a high concentration of benzophenone would present a challenge in macroscale reactors. This microreaction device provided an opportunity for fast process optimization by online analysis of the reaction mixture. 

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