The Witting
Reaction:
- Witting reaction is an important method of preparation of alkene from aldehyde or ketone.
- This reaction was discovered by the scientist Georg Witting in 1954, for which he was awarded the Nobel Prize in Chemistry in 1979.
In this reaction, when an aldehyde or ketone
reacts with a triphenyl phosphonium ylide, it gives out substituted alkene and
triphenyl phosphine oxide.
Example of
witting reaction:
There are few examples of witting reaction.
Mechanism of
witting reaction:
Witting reaction is carried out under mild
condition. It is basically a three steps process.
Step I: In step I
when a carbonyl compound reacts with ylide, a dipolar charge-seperated species
betaine is formed.
Step II: With
rearrangement, betaine is converted into four membered heterocyclic structure
which is called as an oxaphosphatane.
Step III: Cleaveage
of oxaphosphatane takes place which leads to alkene and phosphine oxide as
products.
What is
ylide?
Triphenyl phosphonium ylide is also known as
witting reagent. It is a neutral molecule but have positive and negative
centres on the adjacent atoms that are connected by a sigma ( – ) bond.
The ylides is prepared in two steps:
1. SN2 reaction between triphenyl phosphine and an alkyl halide
2. Abstraction of a proton by a strong base
The mechanism of the reaction is given follow:
1.
2.
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