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TEMPO oxidizes alcohols to aldehydes and ketones
In 1987, Anelli reported the oxidation of alcohols to aldehydes and ketones using TEMPO(CA.1% mol), NaOCl, KBr and NaHCO3 in dichloromethane - water two-phase solvent.
TEMPO also called2,2,6,6-tetramethylpiperidine-1-oxide, or 2,2,6,6-tetramethylpiperidinyloxy. 2,2,6, 6-tetramylperidol free radical is a commercially available and stable nitroxyl radical compound that acts as a catalyst in this reaction.
This oxidation can be highly selective between primary alcohol and secondary alcohol according to the condition of reaction.
The addition of sodium bicarbonate in the reaction is to keep the pH in the system at 8.6 -- 9.5, because the pH of commercially available sodium hypochlorite is 12.7, and the reaction is too alkaline to affect the reaction. When the reaction substrate is sensitive to alkali, 0.1 M HCl can be used to adjust the pH to 6.5 -- 7.5.
The effect of adding KBr to the reaction might be to produce some HOBr, to speed up the reaction. Reaction generally at 0℃ to room temperature reaction, the reaction rate is very fast (oxidation to aldehyde generally 3min, oxidation to ketone 7-10min).
The oxidation of secondary alcohols to ketones will stop, but the oxidation of aldehydes may continue to form carboxylic acids, but the reaction is very slow, and excessive sodium hypochlorite must be added to continue the oxidation. For specific content, refer to the previous release: TEMPO oxidation.
For some alcohols with large steric hindrance, the reaction can be accelerated by adding quaternary ammonium salt as phase transfer catalyst. But it also speeds up the peroxide process to produce carboxylic acids.
Sodium hypochlorite produces HOCl, which can react with some electron-rich substrates such as alkenes. This process can be resolved by tempo-Phi (OAc)2(Protocol of Piancatelli and Margarita).
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