Finally, dioxygen re-oxidizes four equivalents of copper(I) back to copper(II). Overall, a single molecule of dioxygen mediates the oxidation of two equivalents of alcohol, with the formation of two equivalents of water. (5) Stereoselective variants Enantioselective oxidations are typically either kinetic resolutions of chiral alcohols or desymmetrization reactions. These oxidations may be facilitated through the use of chiral nitroxide radicals in the catalytic mode. A good example is provided by the kinetic resolution of racemic 1-phenylethanol. Oxidative desymmetrization processes employing oxoammonium oxidants, on the other hand, are relatively rare. (6) Scope Oxidations using oxoammonium salts may be carried out either in the stoichiometric or catalytic mode under acidic or basic conditions. This section describes the most commonly used conditions for the stoichiometric and catalytic oxidation of alcohols to carbonyl compounds with oxoammonium salts. Although a wide variety of alcohols may be oxidized using TEMPO, competitive oxidation of more electron-rich functionality sometimes takes place. In addition, the site selectivity of oxidation of polyols may differ depending on the conditions used. Stoichiometric oxidations Under mildly acidic or neutral conditions, oxoammonium salts such as Bobbitt's salt oxidize allylic, benzylic, propargylic, or aliphatic alcohols to the corresponding aldehydes or ketones. Secondary alcohols react faster than primary ones, although selectivity is low. A convenient experimental protocol allows for recycling of the oxoammonium salt. (7) Amines, benzylic ethers, and alkenes are oxidized more rapidly than unactivated alcohols; thus, selective stoichiometric oxidation of unactivated alcohols in the presence of these functional groups is not possible. Alcohols with β-nitrogen or β-oxygen substituents react sluggishly under acidic conditions. Allylic and benzylic alcohols can be selectively oxidized under these conditions (8) Under basic conditions, two equivalents of oxidant are needed because of competitive comproportionation between reduced nitroxide and unreacted oxoammonium (see equation (3) above). Pyridine is usually employed as the base. These are the most common conditions for nitroxide oxidations in the stoichiometric mode. (9) Tertiary allylic alcohols can also be stoichiometrically oxidized by oxoammonium salts to enones in a variation of the Babler-Dauben reaction. Catalytic oxidations Catalytic oxoammonium oxidation may be facilitated using sodium hypochlorite as the terminal oxidant. The pH must be maintained below 10 using a buffer for the reaction to proceed. The active oxidizing agent of nitroxide is hypobromite anion; hence, potassium bromide is used as an additive. No epimerization of α-stereogenic centers in carbonyl-containing products takes place. (10) The use of chlorites as terminal oxidants