The Michael addition mechanism starts off with the deprotonation of the 𝛼,𝛽-unsaturated carbonyl compound by the base. This generates a carbanion, which finds stability due to its electron withdrawing groups. The Michael reaction was first defined by Arthur Michael, an American organic chemist.
Michael Addition can be defined as the nucleophilic addition of a nucleophile (or a carbanion) to an 𝛼,𝛽-unsaturated carbonyl compound, and it belongs to a group of reactions that are considered very useful in the mild formation of carbon-carbon bonds.
The Michael Addition Reaction
Acyl groups and cyano groups are very good substituents on the nucleophile for it to act as a good Michael donor, as they make the methylene hydrogen acidic which leads to the formation of a carbanion when it reacts with the base (written as B in the illustration below).
The substituent group of the activated alkenes can be a ketone or a nitro group. This substituent is known as a Michael acceptor. The general format of the Michael addition reaction is provided below.
A few examples of Michael addition reactions are the reactions between:
- Diethyl malonate with methyl crotonate.
- Diethyl malonate with diethyl fumarate.
- Mesityl oxide with diethyl malonate.
- Nitropropane with methyl vinyl ketone.
- 2-Nitropropane with methyl acrylate.
Michael Addition Mechanism
The carbonyl-containing compound is attacked by the base in the first step of the Michael addition mechanism. Subsequently, the alpha hydrogen of the carbonyl compound is deprotonated by the base, leading to the formation of the carbanion (which has enolate ions in two out of three resonance structures).
The electron withdrawing groups of the carbanion give it stability. The enolate ion and the Michael acceptor now react together, leading to the formation of a new carbon-carbon bond. Thus, the enolate ion completes a 1,4 addition on the 𝛼,𝛽-unsaturated carbonyl compound.
The compound formed from the 1,4 addition of the enolate on the 𝛼,𝛽-unsaturated carbonyl compound is now protonated, giving the required product. The Michael addition mechanism is illustrated below along with the list of bonds formed and broken during the Michael reaction.
Thus, the 1,5 dicarbonyl compound is formed from the addition of the enolate of a carbonyl compound to an 𝛼,𝛽-unsaturated carbonyl compound. This product is commonly referred to as a Michael Adduct.
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