[Development of versatile oxidation systems based on the design of oxoammonium salts]

Organic nitroxyl radical catalysts have recently attracted great attention because they realize efficient alcohol oxidation under mild and environmentally benign conditions. A representative of this class is 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO). We have also developed 2-azaadamantane N-oxyls (AZADOs) as highly efficient oxidation catalysts. These nitroxyl radicals are generally oxidized by a cooxidant to generate oxoammonium salts, which are active species for alcohol oxidation. In the oxidation systems presented in this paper, we focus on the differences between these two species in terms of oxidation state and counter anion. Herein, the effects of a counter anion of an oxoammonium species on its reaction selectivity are shown. On the basis of the control of the counter anion, we have developed catalytic oxidative rearrangement of tertiary allylic alcohols to β-substituted α,β-unsaturated carbonyl compounds. Moreover, we have developed novel useful oxidation systems utilizing a catalytic oxoammonium salt; namely, a one-pot oxidation of primary alcohols to carboxylic acids and an aerobic alcohol oxidation.