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Synthesis

Greener, Safer Photooxidations

Singlet oxygen used in a continuous supercritical carbon dioxide-fluorous solvent reactor enables highly efficient oxidation reactions

by Stephen K. Ritter
March 5, 2012 | APPEARED IN VOLUME 90, ISSUE 10

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.

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