Selective Photooxygenation of Dihydroartemisinic Acid in a Reusable Microreactor with Physically Immobilized Photocatalysts

Abstract

Photocatalytic production of organic materials including Artemisinin has been hampered in the pharmaceutical industry because of low reusability and selectivity of photocatalysts. In this work, reusable photocatalysts were synthesized through novel physical and electrostatic immobilizations. After four reaction cycles of dihydroartemisinic acid (DHAA) photooxygenation using physically supported photosensitizers, the conversion was slightly reduced from 81 to 78%, indicating their high reusability. This occurred while maintaining the selectivity of the desired product above 85%, which was higher than that of the homogeneous photosensitizers. Then, a continuous-flow microreactor functionalized with these reusable polystyrene-supported photocatalysts was applied toward the photooxygenation of DHAA at ambient temperature and pressure to produce Artemisinin with a high yield confirmed by the 1H-NMR analysis. Moreover, theoretical evaluations using finite element method simulation showed a 2-µm high local singlet oxygen (1O2) concentration around the reusable photocatalysts which agreed very satisfyingly with the data available in the literature.