Wittig Reaction ( Alkenes from carbonyls)

The Wittig reaction is named after its discoverer, the Nobel Prize winner Georg Wittig (1897–1987, Nobel Prize 1979).

The reaction between a carbonyl compound (aldehyde or ketone only) and a species known as a phosphonium ylid (phosphorous ylide) to form an alkene is called Wittig Reaction. An ylid (or ylide) is a molecule that has a contributing structure with positive and negative charges on adjacent atoms, each of which has an octet of electrons. Phosphonium ylide carries its positive charge on phosphorus and negative charge on carbon. Phosphonium ylides are made from Phosphonium salts by deprotonating them with a strong base. Commonly used bases are organolithium compounds such as BuLi or PhLi in tetrahydrofuran(THF). One of the best bases used is Sodium Hydride(NaH).

Phosphorous ylide are stable, but quite reactive compounds. They can be represented by two limiting resonating structures, which sometimes referred to as the ylide and ylene form.

Phosphonium salt is formed by a nucleophilic substitution reaction of trialkylphosphine with methyl halide .

Mechanism:

In Wittig reaction the mechanism starts with the attack of negative part of ylide on the carbonyl carbon.

Betaine (the dipolar ion forms a four membered ring known as oxaphosphetane, which is unstable and breaks down to form an alkene and phosphine oxide through cyclic mechanism.

The formation of P=O bond (bond energy of 575 kJ mol–1), is one of the strongest double bonds in chemistry, so the Wittig reaction is irreversible and is driven forward by the formation of this P=O bond.

The wittig reaction is stereoselective reaction. Depending upon the ylide the stereochemistry of the alkene is decided.

Generally

• Stabilized ylids are E selective in wiiting reaction.

• Unstabilized ylids are Z selective in Wittig reaction.

The question arises here, what do we mean by stabilized or unstabilized ylides?

Stabilized Ylides:

The negative charge on the carbon atom in ylides is not only stabilized by adjacent phosphorous atom but also it will be stabilized by some other functional group with which the negatively charged carbon can undergo resonance. For example

Unstabilized Ylides:

Those ylide in which the negative charge is not stabilized by resonance are called unstabilized ylides. For example,

Examples of Wittig Reaction:

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