Difference Between Michael Addition and Robinson Annulation

The key difference between Michael addition and Robinson annulation is that Michael addition forms an aliphatic compound, whereas Robinson annulation forms a ring structure.

Overall, Michael addition and Robinson annulation are organic synthesis reactions. Both these reactions come under the category of addition reaction because both these reactions involve the addition of two compounds together, giving a different compound as the final product.

CONTENTS

1. Overview and Key Difference
2. What is Michael Addition
3. What is Robinson Annulation
4. Side by Side Comparison – Michael Addition vs Robinson Annulation in Tabular Form
5. Summary

What is Michael Addition?

Michael reaction is the nucleophilic addition of a nucleophile to an α,β-unsaturated carbonyl compound. Furthermore, this is the most suitable method for the mild formation of carbon-carbon bonds. Originally, this reaction was defined by the scientist Arthur Michael. The reaction is as follows:

R and R’ of the nucleophile are electron-withdrawing groups, i.e. acyl and cyano groups. B is a base which provides the medium for the reaction while involving in the reaction. Moreover, R’’ substituent on the α,β-unsaturated compound is called “Michael acceptor”, and usually, it is a ketone group. But sometimes it is a nitro group. Furthermore, the reaction mechanism for Michael addition is as follows:

What is Robinson Annulation?

Robinson annulation is an organic reaction in which a ring structure forms by the formation of three new C-C bonds. Moreover, the reactants of this reaction are a ketone and a methyl vinyl ketone. Further, this reaction includes Michael addition followed by aldol condensation. Furthermore, it is very useful in the formation of fused ring structures. The reaction is as follows:

Further, this reaction was first published by William Rapson and Robert Robinson.

The above figure shows the mechanism of Robinson annulation. Here, the reaction begins with the nucleophilic attack of ketone on vinyl ketone, which produces an intermediate Michael adduct. Subsequently, aldol type ring closure occurs, leading to the formation of keto alcohol, which is then dehydrated, producing the annulation product.

What is the Difference Between Michael Addition and Robinson Annulation?

Michael reaction is the nucleophilic addition of a nucleophile to an α,β-unsaturated carbonyl compound while Robinson annulation is an organic reaction in which a ring structure forms by the formation of three new C-C bonds. Therefore, the key difference between Michael addition and Robinson annulation is that Michael addition forms an aliphatic compound, whereas Robinson annulation forms a ring structure.

Moreover, Michael addition is important for the formation of mild C-C bonds while Robinson reaction is important in forming fused ring structures.

Below infographic shows more details on the difference between Michael addition and Robinson annulation.

Summary – Michael Addition vs Robinson Annulation

Michael reaction is the nucleophilic addition of a nucleophile to an α,β-unsaturated carbonyl compound whereas Robinson annulation is an organic reaction in which a ring structure forms by the formation of three new C-C bonds. The key difference between Michael addition and Robinson annulation is that Michael addition forms an aliphatic compound, whereas Robinson annulation forms a ring structure.

Reference:

1. “24.8: The Michael Reaction.” Chemistry LibreTexts, Libretexts, 5 June 2019, Available here.

Image Courtesy:

1. “Michael Reaction general” By Nothing serious – Own work (Public Domain) via Commons Wikimedia
2. “Michael Reaction Mechanism” (CC BY-SA 3.0) via Commons Wikimedia
3. “Reaction Scheme for Robinson Annulation” By Ehart4 – Own work (CC BY-SA 3.0) via Commons Wikimedia
4. “Robinson annulation” By Alsosaid1987 – Own work (CC BY-SA 4.0) via Commons Wikimedia