Greener, Safer Photooxidations

By integrating several synthetic strategies with a continuous-flow reactor, researchers at the University of Nottingham, in England, have created a catalytic process for carrying out highly efficient oxidation reactions using normally hard-to-handle singlet O₂ (Chem. Commun., DOI: 10.1039/c2cc17429d). Michael W. George, Richard A. Bourne, and coworkers used supercritical CO₂ in combination with a perfluorinated solvent and liquid organic substrate: This mixture is a single phase under high pressure, but it forms into liquid phases after depressurization. They also used a heavily fluorinated porphyrin as a photocatalyst: When the supercritical flow reaction stream is exposed to light, the porphyrin generates electrons to excite ground-state O₂ to singlet O₂, which in turn oxidizes the organic substrate. At the end of the flow reactor, the CO₂ is depressurized, the photocatalyst remains in the fluorinated solvent and is continuously recycled, and the pure liquid product is recovered. The researchers tested the system by oxidizing the terpene citronellol to its hydroperoxide derivative, achieving nearly 100% yield while using 95% less catalyst than is required in a nonfluorous system.