Difference between revisions of "Wolff–Kishner reduction"

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The Wolff–Kishner reduction is a reaction used in organic chemistry to reduce carbonyl groups into methylene groups.

Description

The reaction mechanism first involves the in situ generation of a hydrazone by condensation of hydrazine with the ketone or aldehyde substrate. The hydrazone is deprotonated by alkoxide base followed by a concerted, rate-determining step in which a diimide anion is formed. Collapse of this alkyldiimide with loss of nitrogen leads to formation of an alkylanion which can be protonated by solvent to give the desired product.^[1]

The Wolff-Kishner reduction doesn't work on esters or ethers.

Procedure

As this reaction involves the use of hydrazine hydrate, work must be performed under a fumehood, as pure hydrazine is very toxic.

Modifications

Huang Minlon modification

The modified procedure consists of refluxing the carbonyl compound in 85% hydrazine hydrate with three equivalents of sodium hydroxide followed by distillation of water and excess hydrazine and elevation of the temperature to 200 °C. Significantly reduced reaction times and improved yields can be obtained using this modification.

Barton modification

This method features vigorous exclusion of water, higher temperatures, and longer reaction times as well as sodium in diethylene glycol instead of alkoxide base.

Cram modification

Slow addition of preformed hydrazones to potassium tert-butoxide in DMSO as reaction medium instead of glycols allows hydrocarbon formation to be conducted successfully at temperatures as low as 23 °C.

Henbest modification

Henbest extended Cram’s procedure by refluxing carbonyl hydrazones and potassium tert-butoxide in dry toluene. Slow addition of the hydrazone is not necessary and it was found that this procedure is better suited for carbonyl compounds prone to base-induced side reactions than Cram's modification.

Caglioti reaction

Involves treatment of tosylhydrazones with hydride donors, like sodium cyanoborohydride, sodium triacetoxyborohydride, or catecholborane to obtain the corresponding alkanes.

Uses

• Reduce ketones and aldehydes

References

1. ↑ https://en.wikipedia.org/wiki/Wolff%E2%80%93Kishner_reduction

Relevant Scencemadness threads

• ethylbenzene by Wolff-Kishner (Huang-Minlon)
• alpha-keto acids and hydrazone formation/wolff-kishner stuffs
• Modified Wolff-Kishner