Application of redox system based on nitroxides to organic synthesis

Abstract

2,2,5,5-Tetramethylpiperidine-l-oxyl known as a stable radical is reduced to the hydroxylami ne and is also oxidized to the oxoaminium ion. These redox reactions are chemically reversible. In this review, some applications of nitroxyl radical to organic reactions based on its redox function were described. The hydroxylami ne was demonstrated to work as a hydration reagent of nitriles to produce amides. Further, it was found that oxygen is reduced by an anion of hydroxylamine to give superoxide ion. The oxoaminium salt could selectively oxidized hydroxide ion and alcohols to hydrogen peroxide and the corresponding carbonyl compounds, respectively. Furthermore hydroxyl groups of poly(vinyl alcohol)s were partially oxidized to carbonyl groups. The redox systems using oxoaminium salt as a mediator were investigated, and it was found that the polymeric mediator was superior to the corresponding monomeric mediator. A new kind of selectivity was found in the oxidation of ester-containing alcohols. That is, whereas 2a could little or not oxidize 2-benzoyloxyethanol and 3-benzoyloxypropanol, it did 4-benzoyloxybutanol, 5-benzoyloxypentanol, and 5-benzoyloxyhexanol to the corresponding benzoyloxyalkanals. The origin of the selectivity might be attributed to the inductive effect of the ester group. © 1990 De Gruyter