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.
What is Michael Addition?
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.
Examples of Michael Addition
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.
To learn more about the mechanism of the Michael addition reaction and those of other important name reactions, register with BYJU’S and download the mobile application on your smartphone.
Frequently Asked Questions – FAQs
What is the Michael reaction?
The Michael reaction is a nucleophilic addition reaction involving the addition of a carbanion (or any other suitable nucleophile) to an 𝛼,𝛽-unsaturated carbonyl compound that contains a functional group which is electron-withdrawing in nature. This named reaction is extremely useful for facilitating the formation of new carbon-carbon bonds.
What are Michael donors and Michael acceptors?
In Michael addition reactions, the nucleophile is commonly referred to as the Michael donor. The substituent groups present on Michael donors are usually electron-withdrawing. Michael acceptors are the substituent groups on the activated unsaturated compound. Common examples of Michael acceptors are ketone groups and nitro groups.
List some examples of Michael addition.
The reaction between diethyl malonate with diethyl fumarate proceeds via Michael addition. Another important example of this named reaction is the synthesis of the anticoagulant warfarin from the chemical reaction between benzylideneacetone with 4-hydroxycoumarin.
What is Mukaiyama-Michael addition?
The Mukaiyama-Michael addition is a variation of the Michael reaction in which a silyl enol ether is employed as the nucleophile. Silyl enol ethers are a class of organic compounds which share a common functional group that consists of an enolate bonded to an organosilicon group via an oxygen atom. The catalyst usually used in Mukaiyama-Michael addition reactions is titanium tetrachloride.
What are asymmetric Michael reactions?
Asymmetric Michael additions involve the enantioselective synthesis of the products. Such reactions can be achieved via chiral phase transfer catalysis. An example of an asymmetric Michael reaction is the production of asymmetric quaternary salts of the ammonium ion from Cinchona alkaloids.