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CHAPTER 7. Continuous-flow Photooxygenations: An Advantageous and Sustainable Oxidation Methodology with a Bright Future

Photooxygenations allow for the incorporation of molecular oxygen into substrates under mild and sustainable conditions. They only use light, oxygen and catalytic amounts of an organic dye to generate singlet oxygen (1O2), a short-lived reactive species, which can subsequently react with a multitude of functional groups within a substrate. Photooxygenations have found applications in fine chemical and pharmaceutical industries, for example in the manufacture of fragrances, building block chemicals and pharmaceuticals. Unlike batch processes, photooxygenations under flow conditions avoid accumulation of potentially explosive intermediates or products by continuously producing small amounts of materials, which can be subsequently quenched or converted safely. Continuous-flow reactors also allow for precise temperature control and the utilization of low-power light sources such as LEDs or OLEDs. The combination of flow operation, miniaturized dimensions and photooxygenations enables an environmentally friendly and safe adaptation to green organic synthesis. This chapter highlights various flow photoreactor technologies and successful examples of photooxygenations in flow.