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| free radicals | |
| nitrogen heterocycles | |
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2,2,6,6-Tetramethylpiperidine 1-oxyl or '''TEMPO''' is a remarkably stable Radical with many applications in Organic Synthesis . TEMPO was discovered by chemists Lebelev and Kazarnowskii in 1960. It is prepared by oxidation of 2,2,6,6-tetramethylpiperidine . TEMPO is widely used in Electron Spin Resonance spectroscopy as a probe for biological systems and also as a radical trap . TEMPO is called a '''hindered amine'''. 4 bulky Methyl groups surround the nitroxyl amine group and drive selectivity in reactions. TEMPO is employed in organic synthesis as a very efficient Oxidizing Agent for Alcohol groups to Carbonyl groups.The actual oxidant is the N-oxoammonium Salt . In a Catalytic Cycle with Sodium Hypochlorite , Hypochlorous Acid makes sure that the N-oxoammonium salt is generated from the nitroxyl radical. In the oxidation step the oxoammonium salt is regenerated. In this reaction TEMPO is present is catalytic amounts and it is the sodium hypochlorite that is consumed. 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 show great Chemoselectivity being able to react exclusively with a primary alcohol group and being inert towards a secondary alcohol group in the same molecule. When secondary oxidizing agents cause side reactions it is also possible to stoichiometric convert TEMPO to the oxoammonium salt in a separate step. For example in the oxidation of Geraniol to Geranial 4-acetamido-TEMPO is first oxidized to the oxoammonium tetrafluoroborate . EXTERNAL LINKS REFERENCES |
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