Mechanism for Diels-Alder reaction- examples, diene, dienophile-Chemarticle

cycloaddition reactions between 1,3-diene and dienophile, are butadiene and maleic anhydride. They are involved in the 1,4-Cycloaddition..

Introduction

Welcome to Chem Article. Today we will discuss the Diels-Alder reaction. This reaction is a cycloaddition reaction. We know what a cycloaddition reaction is, which is already discussed in the pericyclic reaction part-3. This is pericyclic reaction part-4, now today we will discuss cycloaddition reaction example- 1. Diels-Alder Reaction 2. 1,3-cycloaddition Reaction. 

Mechanism for Diels-Alder reaction- examples, diene, dienophile-Chemarticle.com
created by: Chem Article


In the previous class we were discussing- how to form Diels-Alder reaction?, what is the formation of products in Diels-Alder Reaction? 

More Read: Previous class- cycloaddition Reaction in Organic Synthesis-Mechanism of cycloaddition Reaction in Thermal and Photochemical condition.

Now, In this article, we will briefly discuss- what is Diels-Alder's reaction? What is Diels alder reaction used for?

What is Diels-Alder Reaction?

The most popular pericyclic reaction, the Diels-Alder cycloaddition reaction, is a 1,3-diene reaction with dienophile formation of six-membered transition state rings. And finally formed a six-member ring. In the Diels-Alder cycloaddition reaction formation of two σ-bonds and one new π-bond, form three π-bonds of starting materials. 

What is Diels-Alder Reaction? Diels-Alder reaction mechanism
Diels-Alder reaction mechanism: [4+2] cycloaddition reaction- chem Article; the reaction between electron-rich of diene (Cyclopentadiene) and electron-deficient of dienophile (which is attached with electron-withdrawing substituents).


The Diels-Alder reaction takes place in room temperature or Heating conditions. This cycloaddition reaction is reversible in nature. Mainly all Pericyclic reaction is reversible in nature we already see in 1st class in pericyclic reaction. 

Diels-Alder reaction catalysts

The Diels-Alder reaction is a reversible process we have to use quietly at low temperatures. In case the Highly heating procedure is not favorable and in the case forward reaction is facilitated. Most of the cases in the Diels-Alder reaction use Lewis acid-catalyzed. In a few cases, it uses high pressure, which is a facility to better production yield.

Diels-Alder reaction mechanism

The Diels-Alder cycloaddition reaction, the example of cycloaddition reactions between 1,3-diene and dienophile, are butadiene and maleic anhydride. They are involved in the 1,4-Cycloaddition of alkyne and a conjugated diene or 1,3-diene. Any formation of carbon-carbon single bonds. In case conjugated dienes must occur in cisoid.  The transoid does not react with alkyne or we can say this transoid does not participate in the Diels-Alder cycloaddition reaction. Due to the transoid molecule, Conjugated double bonds have different planes. That's why the transoid conjugated double bonds rotate and form a cisoid configuration. Then participate in the Diels-Alder cycloaddition reaction

The mechanism of the Diels-Alder reaction is much debated, the rate-determining step is the formation of biradical. Where two components carbon p-orbitals overlap parallel planes. And the formation of two new σ-bonds. Where the starting materials π-bonds are endwise overlap of carbon atom p-orbitals, therefore the formation of a new σ-bond.  

Diels-Alder reaction molecular orbitals symmetry concept.

The Diels-Alder reaction is a versatile part of the pericyclic reaction, which is a concerned process. Which pericyclic reaction has no intermediate? The mechanism of such a process occurs in the molecular orbital symmetry concept. 

Diels-Alder reaction: Molecular orbitals symmetry concept.Chem Article; the interaction between Highest Occupied Molecular Orbital (HOMO) of diene and Lowest Unoccupied Molecular Orbital (LUMO) of dienophile. Which is symmetrically allowed cycloaddition reaction.
Diels-Alder reaction mechanism: Molecular orbitals symmetry concept- Chem Article; the interaction between Highest Occupied Molecular Orbital (HOMO) of diene and Lowest Unoccupied Molecular Orbital (LUMO) of dienophile. Which is an asymmetrically allowed cycloaddition reaction


Generally, the Diels-Alder reaction involves an electron-rich diene and electron-deficient dienophile. The whole reaction occurs in presence of electron-donating substituents in the diene and electron-withdrawing substituents in alkene which is dienophile. In the concept of Orbital symmetry, we can say the interaction between the Highest Occupied Molecular Orbital (HOMO) of diene and the Lowest Unoccupied Molecular Orbital (LUMO) of dienophile. Due to the small energy difference in these two frontier orbitals, the interaction between these orbital. This makes for a better reaction.

Why is the Diels-Alder reaction stereospecific?

The example of the Diels-Alder cycloaddition reaction is highly stereoselective. Cycloaddition reaction of trans and cis 1,2-dichloroethane to the formation of Cyclopentadiene. This whole reaction mechanism shows two steps. This reaction proceeds at a diradical intermediate. Due to the interconversion of lower energy to higher-level energy, they have small types of differences. In this case, the trans and cis isomers are the different types of products. The rotation of bonding the intermediate show is ring-closure. In this case use of cis 1,2-dichloroethane, more ring-closure products are formed compared to trans 1,2-dichloroethane.

Diels-Alder reaction dienophile

Different types of dienophiles participated in the Diels-Alder reaction. The dienophile must have one double or triple bond presence, which is better to attach with electron-deficient substituents. Different types of dienophile reactivity have different types of probability, which depends on the structure. Generally, more reactive dienophiles have double or triple bonds, which are attached with more electron-attracting substituents. That causes the lowering of the energy level LUMO of dienophiles. In case of better interaction with LUMO of dienophile and HOMO of diene, the great part of the substituent is attached with a dienophile. That is the most common dienophile.

However, the Diels-Alder reaction takes place between an electron-rich dienophile and electron-deficient diene. These types of Diels-Alder reactions are called inverse electron demand.

We can see the maleic anhydride, Which is the most common and popular dienophile. In maleic anhydride, two electron-deficient carbonyl groups are present. Therefore double bond of the dienophile goes to the more electron-deficient character. That's why the maleic anhydride is a great dienophile.

Reactivity rate of dienophile in Diels-Alder reaction

The reactivity order of dienophile in the Diels-Alder reaction: I>II>III>IV. We can see in the picture of the dienophile, increasing the numbers of cyano group( substituent)  corresponds to increasing the reactivity rate of the Diels-Alder Reaction. Due to increases, the electrons withdrawing group corresponds to a decrease in the electrons density on a double bond. That’s why the electron-deficient character, the increasing electronegativity of substituents.

Reactivity rate of dienophile in Diels-Alder reaction
Reactivity rate of dienophile in Diels-Alder reaction-Chem Article; the order of the reactivity rate Tetracyanoethene (I), Tricyanoethene (II), 1,1-dicyanoethene(III) and Acrylonitrile(IIV). The order is I>II>III>IV.


Diels-Alder Reaction with Hetero-dienophile

In case use of hetero-dienophile like without one or more carbon atom. In case carbonyl groups in aldehyde and ketone are reacted with 1,3-diene formation cycloaddition reaction product. The hetero-dienophile reactivity depends on attaching electron-withdrawing groups. If the electron-withdrawing group is higher then the reactivity of the Diels-Alder Reaction will be increased.

Diels-Alder Reaction Diene

The variety of 1,3-diene, which participated in Diels-Alder Reaction. They include open-chain or Acyclic diene, cyclic chain. When One or more atoms are heteroatoms like Nitrogen, Oxygen, boron atom, etc. Then these types of diene are called Hetero-dienes. 

  • Acyclic diene or Open Chain diene.
  • Cyclic diene

Acyclic diene in Diels-Alder Reaction

In the conjugated Acyclic diene, there is a possibility of configuration- one is cisoid and another is transoid. The trans configuration does not participate in the Diels-Alder reaction. Due to two double bonds being present in the different planes. If possible by rotation of transoid to cisoid then diene participates in this reaction, otherwise not.

Acyclic diene reactivity rate 

  • In acyclic diene, reactivity rates are dependent on their structure.
  • In this case, without substituents, the cisoid dienes are more reactive than transoid diene.
  • The reactivity of diene in the Diels-Alder reaction, which is dependent on substituted groups. If the substituted groups of diene are more electron-donating then we will call grate reactive diene in the Diels-Alder reaction. That means the attaching electron-donating groups their character will be increased then the diene reactivity rate corresponds to increase. 
  • Attached with more electron-withdrawing (-I Effect) group- the diene is less reactive.
  • Attached with a more electron-donating group (+M, +I Effect)- Diene is more reactive in the Diels-Alder reaction. 
  • Generally, In diene have more electron density clouds.

The substituents in diene participate in cycloaddition sterically. In case cisoid configurations are unstable due to bulky substituents are present. That causes the steric hindrance of two bulker groups. 

The example of Acyclic diene in Diels-Alder reaction.


Cyclic diene in Diels-Alder reaction

In case the double bonds are attached with cyclic ring-like cyclopentadiene, pyrrole, furan, etc. the cyclic diene two conjugated double bonds are cisoid configuration in the same plane.

Cyclic diene in Diels-Alder reaction: the reaction between cyclopentadiene and dienophile, the Diels-Alder reaction between Furan and Maleic anhydride- Chem Article
Cyclic diene in Diels-Alder reaction: the reaction between cyclopentadiene and dienophile, the Diels-Alder reaction between Furan and Maleic anhydride- Chem Article


The use of cyclo-diene formation of products is Exo form or endo form.

In case the Diels-Alder reaction between furan and maleic anhydride formation of major products is exo configuration. The Exo isomer is a thermodynamically stable product, which occurs at 20℃ temperature.

In case the Diels-Alder reaction between cyclopentadiene and maleic anhydride formation of major products is endo configuration. Where the Endo product is a kinetically stable product and the Exo product is thermodynamically stable. 

The kinetically stable products are fast produced due to the formation of a more stable cyclic transition state(T.S) compared to the thermodynamic stable product cyclic transition state.

Conclusion

In this class were to see briefly- what is the mechanism of the Diels-Alder reaction? The concept of Reactivity rate of diene and dienophile. The reactivity of diene and dienophile is changed during Various structures and Reactivity Changes with changing their substituent. This is the Diels-alder cycloaddition reaction mechanism. Next class, we will see the stereochemistry of the Diels-Alder reaction, Regio-chemistry of the Diels-Alder reaction. What is the Endo addition rule? Intermolecular Diels-Alder cycloaddition reaction, all are seen in the next class. 

Frequently asked questions

Q.1. Why is the Diels-Alder reaction important?

Ans: For a particular product yield, In organic synthesis is easily found by Diels-Alder reaction. Diels alder reaction is the most popular cycloaddition reaction Which is most commonly used in organic synthesis. Using Diels-Alder Reaction we have to easily find out the stereochemical product, stereospecific product, and Regio-selective product except with great yield. This type of major product is easily Distinguished by another minor product.


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