Study Notes On Wittig Reaction (Chemical Science) - Download PDF!

Study Notes On Wittig Reaction

WITTIG REACTION

This reaction is named after scientist Georg Wittig. It is also known as Wittig Olefination. It is a type of coupling reaction where an aldehyde or ketone reacts with triphenylphosphonium ylide to yield alkene and triphenylphosphine oxide.

Solvents used in this reaction are typically THF or diethyl ether.

Phosphonium ion generates ylide. The geometry of the resulting alkene depends upon the reactivity of ylide. If ‘R’ is an electron-withdrawing group, then the ylide is STABILISED and it is not as reactive as when ‘R’ is an alkyl group. 

The stabilized ylide predominantly gives (E)-alkene whereas, the non-stabilized ylide gives (Z)-alkene.

Reaction Mechanism- 

A four-membered cyclic intermediate (an oxaphosphetane) is formed as a result of [2 + 2] cycloaddition of ylide to the carbonyl group of substrates.

Firstly, a betaine is formed as a zwitterionic intermediate, which in turn leads to the formation of oxaphosphetane. 

Betaines may be stabilized by lithium salts which leads to side products; suitable bases in the Wittig reaction are NaOH, NaOMe, and NEt3.

Step 1: Attack of ylide Carbon on the carbonyl

Examples:

(a)

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