Michael Reaction Mechanism
When an α,β -unsaturated carbonyl carbon is treated with a base, the base deprotonates the α,β -unsaturated carbonyl carbon. The deprotonation of α,β -unsaturated carbonyl carbon leads to the formation of an intermediate. The intermediate formed in this reaction is carbanion. The carbanion intermediate consists of a negative charge which can be stabilized by groups that are electron-withdrawing in nature. This reaction was first worked out by an American organic chemist named Arthur Michael and after his name, this reaction is named. This reaction can also be called a nucleophilic addition reaction because the electrons of the base are being donated to a carbon center making it a nucleophile. This reaction is useful because it generates carbon-carbon bonds which is a strong covalent bond.
Michael Addition Reaction With Mechanism
The nucleophile or base which donates their electron to the proton is called Michael donor. Acyl and cyano groups act as very good nucleophiles because of their non-bonding electrons which are high in energy and are therefore ready to donate. The hydrogen attached to the substrate, methane is acidic and because of that the base or nucleophile readily abstracts it and the carbanion is formed. The substrate where the carbanion is formed is called the Michael donor whereas the other substrate which is attacked by the donor is said to be the Michael acceptor.
The reaction is thermodynamically controlled i.e. the product formed will be thermodynamically stable. The Michael donors are mostly active methylene which has electron-withdrawing groups attached to the carbon whose proton is abstracted. The abstracted proton is highly acidic because of the electron-withdrawing group attached to its adjacent carbon as they are capable of stabilizing the carbanion. The Michael acceptors are usually olefins which are electron deficient.
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Example of Compounds Showing Michael's Addition Reaction
When diethyl malonate acts as Michael donor and diethyl fumarate acts as a Michael acceptor.
Acrylonitrile acts as Michael acceptor and acetylacetone act as Michael acceptor.
Malononitrile acting as a Michael donor and ethyl vinyl ether acting as a Michael acceptor.
Michael Addition Reaction Mechanism
In the first step, the α -hydrogen is deprotonated by the base which leads to the formation of carbanion. The negative charge on the carbon is stabilized by the carbonyl carbon by resonance as the negative charge on oxygen is more stable than being on the carbon.
In the second step, the Michael acceptor being deficient in electrons acts as an acceptor which then accepts electrons from the carbanion which is rich in electrons. The reaction between them forms the carbon-carbon bond. Even though the negative charge is more stable on the oxygen during the resonance structure the carbon-carbon bond is more stable as compared to the carbon-oxygen bond. This is a 1,4- addition reaction.
In the third step, the carbonyl compound is protonated by accepting an electron from the solvent which gives us the final product. The reaction mechanism is shown below which shows which bonds were broken during the course and which were formed.
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Mukaiyama-Michael Addition Reaction
When an organosilicon group (an organic compound attached to silicon) is bonded to the oxygen of an enolate then this functional group is called silyl enol ether and when this functional group acts as a nucleophile in a Michael reaction then that reaction is called Mukaiyama-Michael reaction. The reaction requires titanium chloride to act as a catalyst in the process of forming a new carbon-carbon bond.