Wolff-Kishner Reduction
The Wolff-Kishner reduction is the deoxygenation of aldehydes and ketones to hydrocarbons via the corresponding hydrazones under basic conditions and heat.
General features:
1. The reaction is commonly employed to remove a carbonyl group after it has served its synthetic purpose in preceding steps. 2. The reaction is usually carried out in high boiling solvents. 3. Sterically hindered carbonyl compounds are deoxygenated more slowly. 4. The rate-determining step is the proton capture at the carbon terminal. 5. Since the reaction requires highly basic conditions, it is unsuitable for base-sensitive substrates.
Wolff-Kishner Reduction
Reaction Mechanism
1. In situ generation of a hydrazone by condensation of hydrazine with the ketone. 2. De-protonation of the hydrazone by an alkoxide base. 3. Formation of the hydrocarbon via loss of dinitrogen to afford an alkyl ion. 4. The carbanionic carbon undergoes an acid-base reaction with solvent to give the corresponding alkane.
Experimental Procedure
To a stirred solution of the aldehyde (1.1 mmol, 1.0 eq) and hydrazine monohydrate (20.0 eq) in diethylene glycol monomethyl ether (DGME, 22 mL) was added KOH (6.0 eq) at room temperature. The resulting mixture was heated successively at 110 °C for 1 h, at 194 °C for 4 h, and was then cooled to room temperature. The mixture was quenched by addition of 1M aqueous HCl, and extracted with Et2O. The combined organic layers were dried over Na2SO4, filtered, and evaporated to dryness. The resulting residue was purified by column chromatography to furnish the desired product (61% yield).
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