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TEMPO
(2,2,6,6-Tetramethylpiperidin-1-yl)oxyl or (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl, commonly known as TEMPO, is a chemical compound with the formula (CH2)3(CMe2)2NO. This heterocyclic compound is a red-orange, sublimable solid. As a stable aminoxyl radical, it has applications in chemistry and biochemistry. TEMPO is used as a radical marker, as a structural probe for biological systems in conjunction with electron spin resonance spectroscopy, as a reagent in organic synthesis, and as a mediator in controlled radical polymerization.
TEMPO was discovered by Lebedev and Kazarnowskii in 1960. It is prepared by oxidation of 2,2,6,6-tetramethylpiperidine.
The structure has been confirmed by X-ray crystallography. The reactive radical is well shielded by the four methyl groups.
The stability of this radical can be attributed to the delocalization of the radical to form a two-center three-electron N–O bond. The stability is reminiscent of the stability of nitric oxide and nitrogen dioxide. Additional stability is attributed to the steric protection provided by the four methyl groups adjacent to the aminoxyl group. These methyl groups serve as inert substituents, whereas any CH center adjacent to the aminoxyl would be subject to abstraction by the aminoxyl.
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. With an O–H bond dissociation energy of about 70 kcal/mol (290 kJ/mol), this bond is about 30% weaker than a typical O–H bond.
TEMPO is employed in organic synthesis as a catalyst for the oxidation of primary alcohols to aldehydes. The actual oxidant is the N-oxoammonium salt. In a catalytic cycle with sodium hypochlorite as the stoichiometric oxidant, hypochlorous acid generates the N-oxoammonium salt from TEMPO.
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. TEMPO oxidations also exhibit chemoselectivity, being inert towards secondary alcohols, but the reagent will convert aldehydes to carboxylic acids.
The oxidation of TEMPO can be highly selective. In basic conditions, TEMPO oxidizes primary alcohols before secondary alcohols. But in acid, secondary alcohols provide an H− ion more easily, and oxidize first instead.
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TEMPO
(2,2,6,6-Tetramethylpiperidin-1-yl)oxyl or (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl, commonly known as TEMPO, is a chemical compound with the formula (CH2)3(CMe2)2NO. This heterocyclic compound is a red-orange, sublimable solid. As a stable aminoxyl radical, it has applications in chemistry and biochemistry. TEMPO is used as a radical marker, as a structural probe for biological systems in conjunction with electron spin resonance spectroscopy, as a reagent in organic synthesis, and as a mediator in controlled radical polymerization.
TEMPO was discovered by Lebedev and Kazarnowskii in 1960. It is prepared by oxidation of 2,2,6,6-tetramethylpiperidine.
The structure has been confirmed by X-ray crystallography. The reactive radical is well shielded by the four methyl groups.
The stability of this radical can be attributed to the delocalization of the radical to form a two-center three-electron N–O bond. The stability is reminiscent of the stability of nitric oxide and nitrogen dioxide. Additional stability is attributed to the steric protection provided by the four methyl groups adjacent to the aminoxyl group. These methyl groups serve as inert substituents, whereas any CH center adjacent to the aminoxyl would be subject to abstraction by the aminoxyl.
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. With an O–H bond dissociation energy of about 70 kcal/mol (290 kJ/mol), this bond is about 30% weaker than a typical O–H bond.
TEMPO is employed in organic synthesis as a catalyst for the oxidation of primary alcohols to aldehydes. The actual oxidant is the N-oxoammonium salt. In a catalytic cycle with sodium hypochlorite as the stoichiometric oxidant, hypochlorous acid generates the N-oxoammonium salt from TEMPO.
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. TEMPO oxidations also exhibit chemoselectivity, being inert towards secondary alcohols, but the reagent will convert aldehydes to carboxylic acids.
The oxidation of TEMPO can be highly selective. In basic conditions, TEMPO oxidizes primary alcohols before secondary alcohols. But in acid, secondary alcohols provide an H− ion more easily, and oxidize first instead.