The Michael Reaction

The Michael reaction is a 1,4-addition of a nucleophilic enolate to an alpha-beta unsaturated carbonyl compound.

Michael Reaction
Scheme 1: An example of the Michael reaction, addition of ethyl acetate to 3-buten-2-one using sodium ethoxide.

This reaction involves the addition of the nucleophilic enolate donor to the beta carbon of the alpha beta unsaturated carbonyl electrophile.

The enolates formed for the reaction, usually from beta-ketoesters and beta-diesters, tend to be stable and must be fairly acidic for an appreciable concentration to form. The alpha beta unsaturated carbonyl compounds used tend to be unhindered.

This reaction is similar to the alkylation of carbanions with alkyl halides, though in the Michael reaction base is used as a catalyst not in stochiometric amounts. This makes the reaction reversible. The driving force for this reaction is the formation of new carbon - carbon bonds.

Mechanism for the Michael Reaction
Scheme 2: The mechanism for the Michael Reaction.

A large number of alpha-beta unsaturated carbonyl compounds can be used as the electrophile in the Michael reaction. These compounds include:

There is also a large number of enolates that can be used as the nucleophile in the Michael reaction. These compounds include:

More nucleophiles and electrophiles that can be used for the Michael reaction.

Cyclic alpha-beta unsaturated carbonyl compounds can also react with nucleophilic enolates to yield Michael reaction products

Michael reaction of a cyclic alpha-beta unsaturated ketone
Scheme 3: A Michael reaction of a cyclic alpha-beta unsaturated ketone.

If an excess of the alpha-beta unsaturated carbonyl compound is used the Michael product can be dialkylated.

Dialkylation in the Michael reaction
Scheme 4: Dialkylation of the Michael reaction product.

References

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Author: Allyson Doyle (document modification date: 5th December 2001)