Oxoammonium-catalyzed oxidation reactions involve the conversion of organic substrates to more highly oxidized materials through the action of an N-oxoammonium species. Nitroxides may also be used in catalytic amounts in the presence of a stoichiometric amount of a terminal oxidant. Nitroxide radical species used are either 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) or derivatives thereof. (1) Mechanism and stereochemistry One-electron oxidation of the nitroxide produces a highly electrophilic oxoammonium species, which serves as the active oxidizing agent. The nitroxide can be used as a catalyst in conjunction with cheaper stoichiometric oxidants such as sodium hypochlorite or bis(acetoxy)iodobenzene (BAIB). Under neutral or slightly acidic conditions (in the presence of silica gel, for instance), oxidation occurs by an initial hydrogen bond between the hydroxyl group and the oxoammonium nitrogen, followed by concerted proton transfer and hydride abstraction. The need for hydrogen bonding is supported by the low reactivity of β-alkoxy and β-amino alcohols, which exhibit competitive intramolecular hydrogen bonding. The mechanism of oxidation under weakly basic (pyridine) conditions is similar, except that pyridine neutralizes the hydroxyammonium species, and this intermediate "comproportionates" with oxoammonium salt to give nitroxide radicals and pyridinium salts (see equation (3) below). Because this reaction consumes base and active oxidant, two equivalents of base and oxidant are necessary under weakly basic conditions. A unified mechanism under neutral and basic conditions in presented in a recent article. The authors present a comprehensive analysis of a number of oxoammonium salt mediated oxidations. (2) Under strongly basic conditions, the deprotonated substrate reacts with the N-oxyammonium species. Attack of the substrate alkoxide on either nitrogen or oxygen may occur, although the former is believed to operate on the basis of on observations of oxidations of N-alkoxy amines (which, presumably, proceed via intermediate 1). Comproportionation of the reduced product (a hydroxylamine) with the oxoammonium ion competes with oxidation; thus, an excess of the oxidizing agent is often required. (3) Nitroxide-catalyzed oxidations involve N-oxoammonium intermediates as the active oxidizing agent. The mechanism of oxidation of the nitroxide radical depends on the terminal oxidant employed. Two-electron oxidants, such as NaOCl, are able to directly convert nitroxides into oxoammoniums. (4) One-electron oxidants, such as copper(II), operate via a more complex mechanism involving dioxygen as the terminal oxidant. Copper(II) oxidizes four equivalents of nitroxide to oxoammonium, two equivalents of which (blue) react with alcohols to form carbonyl compounds. The other two equivalents of oxoammonium (red) undergo comproportionation to re-form nitroxy radicals (pink).