Pinnick Oxidation
The Pinnick oxidation (Pinnick–Kraus oxidation) is a reaction in organic chemistry in which aldehydes are oxidized into their corresponding carboxylic acids using sodium chlorite, mild acidic conditions, and a scavenger.
General features:
1. The typical solvent is a combination of tert-butanol with another solvent (THF, water…) 2. The use of an excess of a scavenger (2-methyl-2-butene) is needed. The HClO is formed as a by-product of the oxidation process, and it can cause side reactions such as the consumption of the oxidizing agent NaClO2 to form chlorine oxide (ClO2). To avoid this sabotaging side reaction, scavengers provide a double bond that consumes the generated by-product HClO. 3. The reaction tolerates a wide range of functional groups.
Note: Despite the reaction is commonly referred to as the Pinnick oxidation, Tetrahedron 1981, 37, 2091., the reaction conditions were first developed by Kraus. See seminal publications: J. Org. Chem. 1980, 45, 4825. and J. Org. Chem. 1980, 45, 1175.
Pinnick Oxidation
Reaction Mechanism
1. Formation of chlorous acid. 2. The chlorous acid adds to the aldehyde. 3. The resulting intermediate undergoes pericyclic reaction. 4. Formation of the carboxylic acid and hypochlorous acid. 5. The scavenger consumes the hypochlorous acid to avoid side reactions with the starting material.
Experimental Procedure
To a stirred solution of the aldehyde (0.636 mmol, 1.0 eq) in t-BuOH (3.0 mL) and H2O (3.0 mL) were added 2-methyl-2-butene (20 eq), NaH2PO4 (10 eq), and NaClO2 (10 eq) at room temperature. After stirring for 14 hours, the mixture was quenched with aqueous NaHSO3 at room temperature and extracted with EtOAc three times. The combined organic layer was dried over Na2SO4. After filtration, removal of the solvent under reduced pressure gave a crude material, which was purified by silica gel chromatography to afford the corresponding carboxylic acid (71% yield).
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