Chapter 19: Singlet Oxygen as a Reagent in Organic Synthesis

Singlet oxygen is a perfect green reagent as it is generated from air oxygen, natural dyes and visible light in all possible solvents, in the gas phase or in restricted environments. The reactivity in organic reactions is legendary, spanning from energy- and electron-transfer steps, heteroatom oxidation, ene reactions, 2 + 2- and 4 + 2-cycloadditions. Reactivity transposition is also possible and polyoxyfunctionalization is possible either by tandem processes (multiple singlet-oxygen reactions) or by secondary transformations of the primary products. Allylic hydroperoxides from the ene process can serve as oxygen-transfer reagents and converted into epoxides, epoxy alcohols or alcohols, cyclic peroxides from 2 + 2- and 4 + 2-cycloaddition can be reduced to 1,n-diols or base-catalyzed rearranged to a plentifulness of products. The endoperoxides (from 4 + 2-cycloaddition) and 1,2-dioxetanes (from 2 + 2-cycloaddition) are also important synthetic targets due to their synthetic flexibility for further transformations, their pharmaceutical importance as well as chemoluminescence properties.