Wittig Reaction
Wittig Reaction
It is the synthesis
of an alkene (olefin) from the reaction between acarbonyl compound specifically
aldehyde and ketone with a phosphonium ylide (wittig reagent), via either a
betaine and/or oxaphosphetane intermediate. Therefore, this reaction is generally
known as the Wittig reaction, Wittig olefination, or Wittig olefin synthesis.
This Reaction is
named after its discoverer, the German chemist Georg Wittig. He was also
awarded the 1979 Nobel Prize in Chemistry for this discovery
General Formula of Wittig reaction:
The Wittig reaction consists of the following consecutive steps:
- Preparation of phosphonium salt by reacting
triphenylphosphine (or
trialkylphosphine) with alkyl halide.
- Deprotonation of the corresponding phosphonium
salt to form the phosphorus ylide (i.e., phosphorane
- Condensation of phosphorane and carbonyl
compound.
- Rapid decomposition of the condensation
intermediate (either betaine or oxaphosphetane) to give olefin and phosphine
oxide.
Effect of
temperature on the Wittig reaction:
- It may increase the reaction rate and yield of the Wittig reaction, because of the enhanced dissociation of base at higher temperature.
- Due to the higher reaction rate, the stereoselectivity may be decreased or enhanced according to the structure of ylides.
Preparation of Wittig reagent:
- Sn2 reaction of alkyl halides with phosphines
to give a phosphonium salt.
- Deprotonation of C-H with strong base gives the
ylide.
Mechanism of Wittig reaction:
Examples of Wittig reaction:
Example1
Example2
Example3
Example4
Example5
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