For example, in the oxidation of geraniol to geranial, 4-acetamido-TEMPO is first oxidized to the oxoammonium tetrafluoroborate. In cases where secondary oxidizing agents cause side reactions, it is possible to stoichiometrically convert TEMPO to the oxoammonium salt in a separate step. The reason is when in this condition, secondary alcohols are more easily to provide an H- ion. It has been proven that secondary alcohols are more likely to be oxidized by TEMPO under an acidic environment. The oxidation of TEMPO can be highly selective. TEMPO oxidations also exhibit chemoselectivity, being inert towards secondary alcohols, but the reagent will convert aldehydes to carboxylic acids. One typical reaction example is the oxidation of ( S)-(−)-2-methyl-1-butanol to ( S)-(+)-2-methylbutanal: 4-Methoxyphenethyl alcohol is oxidized to the corresponding carboxylic acid in a system of catalytic TEMPO and sodium hypochlorite and a stoichiometric amount of sodium chlorite. With an O–H bond dissociation energy of about 70 kcal/mol, this bond is about 30% weaker than a typical O–H bond. Regardless of the reasons for the stability of the radical, the O–H bond in the hydrogenated derivative (the hydroxylamine 1-hydroxy-2,2,6,6-tetramethylpiperidine) TEMPO–H is weak. These methyl groups serve as inert substituents, whereas any CH center adjacent to the aminoxyl would be subject to abstraction by the aminoxyl.
Additional stability is attributed to the steric protection provided by the four methyl groups adjacent to the aminoxyl group. The stability is reminiscent of the stability of nitric oxide and nitrogen dioxide. The stability of this radical can be attributed to the delocalization of the radical to form a 2-center 3-electron N-O bond.
The reactive radical is well shielded by the four methyl groups. The structure has been confirmed by X-ray crystallography.
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