The Diels-Alder Reaction

Diels Alder Reaction: Dienes and Dienophiles

In this post, we will discuss the reactivity and specifics of the diene and the dienophile in the Diels-Alder reaction. First, a reminder that the Diels-Alder reaction is a type of pericyclic reaction between a conjugated diene (two double bonds) and a dienophile (an alkene with an electron-withdrawing group).

 

 

 

Reactivity of the Diene

Let’ start with the diene. The first thing to remember here is that it can only react in the cis conformation. We are not talking about the traditional cis and trans isomerism of the alkenes. Conjugated dienes can also be in cis and trans geometry. However, these are conformations since they interconvert through a rotation about the sigma bond between the two double bonds, and we call them s-cis and s-trans:

 

 

Acyclic dienes usually prefer the s-trans conformation. Cyclic dienes, on the other hand, are locked whether they are cis or trans since there is no free rotation about the sigma bond:

 

 

These conformations are two extremes, as the locked trans dienes do not react in the Diels-Alder, while the cyclic cis dienes are so reactive that they may react with themselves.

For example, cyclopentadiene dimerizes because one molecule acts as the diene and the other as the dienophile. Cyclic dienes produce bridged bicyclic compounds:

 

 

 

Electronic effects

The second factor is the electronic effect. Remember that electron-donating groups increase the reactivity of the diene:

 

 

This is a result of the HOMO-LUMO interaction in the Diels-Alder reaction. Even though it is a concerted mechanism, one can imagine the electron flow being initiated from the highest occupied molecular orbitals (HOMO) of the diene and going to the lowest occupied molecular orbitals (LUMO) of the dienophile. Therefore, electron-donating groups on the diene increase its reactivity, while electron-withdrawing groups on the dienophile lower the LUMO energy level, thus supporting this electron flow as well.

 

 

Reactivity of the Dienophile

As we just mentioned above, electron-withdrawing groups increase the reactivity of the dienophile.

 

 

Some of the commonly used dienophiles are shown below:

 

 

 

Stereochemistry

A few more points about the stereochemistry of the Diels-Alder reaction depending on the structure of the diene and the dienophile.

First, remember that if there are two groups on the dienophile, the product will have them cis or trans exactly as they initially appear in the dienophile. This indicates that Diels-Alder is a stereospecific reaction:

 

 

If the diene is cyclic as well, then bicyclic compounds are formed. The main concept here is the endo-exo isomerism. In the endo product, the substituents of the dienophile are pointing towards the larger bridge, while in the exo isomer, they are pointing away from the larger bridge:

 

 

In general, endo is the major product because it is formed when the electron-withdrawing groups of the dienophile are pointing towards the π electrons of the diene. You can read more details, such as the transition state and the endo-exo definition, when acyclic dienes are reacted here.

 

 

If the cyclic diene is reacted with a mono-substituted dienophile then the product contains one stereogenic center and the endo and exo products are formed as two enantiomers:

 

 

And any combination of an endo and exo product represents a pair of diastereomers:

 

 

If the diene is also unsymmetrical, then you need to consider the regiochemistry of the Diels-Alder as well. It would be too much for today, so you can check this post.

 

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