Diels alder is the pericyclic reaction between a conjugated diene and a substituted alkene, also known as a dienophile, to form a cyclohexene compound. It is the [4n+2]𝝅 cyclo addition reaction having a concerted (single-step) mechanism. In the Diels alder reaction, two pi bonds are converted into two sigma bonds via a cyclic transition state. This results in the formation of a six-member ring with one double bond known as cycloalkenes.
This reaction was named after the German Chemists Otto Diels and Kurt Alder in 1928. Later, in 1950 they received the Noble prize in chemistry for this development.
The Diels-Alder reaction is an example of a pericyclic reaction because the reactant; conjugated diene acts as a nucleophile while the substituted dienophile acts as an electrophile and the electron from the electron-rich center travels to the electron-deficient center in a cyclic way. There are no positive or negative charge intermediates. In addition to this, this reaction is highly stereoselective, which means it preferably forms products with specific orientations of bonds.
Mechanism of Diels Alder reactions
The Diels Alder reaction highly depends upon the dienes and dienophiles used.
Diene and Dienophile
A compound having two double bonds is called diene. It is the nucleophile having conjugated double bonds. Conjugated dienes are also known as 1,3- diene. They may be open-chain or cyclic compounds. For diels alder reaction S-cis dienes are used, while S-trans diene can not be used because its end is not close enough to react with another alkene.
On the other hand, dienophiles are electron-deficient specie. They may have a double or triple bond in conjugation with electron-withdrawing groups. Normally, they are derivatives of ethene or ethyne. The reactivity of dienophiles depends upon the number and strength of electron-withdrawing groups.
The main overview of diels alder’s reaction
As diels alder reaction has a concerted mechanism, all the bond breaking and bond formation take place in a single step. In this reaction, two pi bonds are converted into two sigma bonds. The formation of a sigma bond is the driving force of Diels alder reaction as sigma bonds are thermodynamically more favorable.
The above diagram shows how the electrons move from electron-rich species to electron-deficient specie in a cycle. This results in the formation of cycloalkene derivatives.
Diels Alder reaction proceeds through a single cyclic transition state. No intermediates are formed during this reaction. The transition state involved in this reaction has six delocalized electrons. Electrons either rotate in a clockwise or anticlockwise direction resulting in the formation of the same product, such that arrows are one after another. The last arrow connects to the first arrow.
The transition state has six delocalized 𝝅 electrons. This resembles the stabilization of benzene thus transition state has an aromatic character. The transition state can be represented by using arrows or dotted lines in the figure given below:
Stereochemistry of Diels Alder reaction
Diels alder reactions are stereospecific reactions. The stereochemistry of Diels alder reaction depends upon the stereochemistry of dienophiles. Cis dienophile gives cis product whereas, trans dienophile gives trans product. There is a syn addition reaction. These reactions are also known as superficial addition reactions. New bonds are formed by using the same lobes on the same face of the reactant.
By using highly reactive species as reactants the product is a diastereomer.
Diels Alder reactions can take place at room temperature. Sometimes, they may need gentle heating. They may be warm in suitable solvents. Some of the unreactive reactants required high heating. Lewis acid catalyst may also be used in some cases. Similarly in a few cases, high pressure is also required.
Reactivity of diels alder reaction
The reactivity of Diels alder reaction depends upon a number of factors. For instance, electron-withdrawing groups in conjugation with a double or triple bond of diene increase the reactivity of the dienophile. It makes dienophiles strong electrophiles. Similarly, if the electron donating groups are present on the diene they make the diene more reactive. Diels alder reaction takes place fastly with gentle heating. So it is highly recommended to use the reactive species to get a better yield. In other cases, the reaction would be slow and requires high temperature and high pressure.
Examples of Diels Alder reaction
Synthesis of cyclohexene
This is the simplest example of Diels alder reaction. In this reaction, 1,3-butadiene react with ethene at 200ºC. This reaction is very slow. We get only 20% product. This is because both of the reactants are the least reactive. They require very high temperatures. But still, we get very little yield.
Derivative of cyclohexenes
In this example, the diene is treated with 1,3 conjugated dienophile to form the derivative of cyclohexane. This reaction gives 90% product at 140ºC.
The reaction of 1,3 butadiene with maleic anhydride
When 1,3-Butadiene is treated with melic anhydride an adduct is formed through 1.4 cyclic addition because maleic anhydride is a highly reactive dienophile owing to the fact that it has two electron-withdrawing groups. This reaction yields 100% product even at 30ºC.
- Diels alder is an important reaction for the synthesis of cycloalkene derivatives.
- It is a type of pericyclic reaction.
- These reactions are also known as the 1,4-cycloaddition reactions.
- It involves the cycloaddition of [4+2]π electrons.
- Two pi bonds are converted into two sigma bonds.
- Sigma bond formation is the driving force of the reaction.
- Sigma bonds are thermodynamically more stable.
- One new pi bond is also formed.
- It has a concerted (single-step) mechanism.
- Most of the Diels alder reactions are reversible.
- The transition state has six delocalized electrons.
- Diene act as a nucleophile whereas, dienophile acts as an electrophile.
- Dienes having electron donating groups are very reactive.
- Dienophiles having electrons withdrawing in conjugation are more reactive.
- They involve syn addition with respect to both components.
- Highly substituted reactants lead to the formation of a mixture of diastereomers.
- Endo product is kinetically more favorable in Diels alder reaction
- Exo product is thermodynamically more stable.
Why diene must have s cis conformation during diels alder reaction?
Because in trans alkene ends are not close enough to react with the dienophile to form a ring.
Can s trans diene undergo Diels alder reaction?
No, trans diene can not be used for diel alder reaction.
How we can increase the yield of Diels Alder reaction?
We can increase the yield of Diels alder reaction by using reactive diene and dienophile.
What makes diene more reactive?
When electron donating groups are in conjugation with double bonds in diene, they make diene more reactive as compared to normal dienes.
What makes dienophiles more reactive?
If electron-withdrawing groups are present on the dienophile they make it more reactive.
What is the effect of electron-withdrawing substituent on dienophiles?
When electrons withdrawing substituents are in conjugation with a double in dienophile, they increase the reactivity of the dienophile.
What factors affect the rate of Diels alder reaction?
There are different factors that affect the rate of diels alder reaction i.e,
- Electron withdrawing groups on dienophiles.
- Electron donating groups on diene.
What is meant by a concerted reaction?
The reaction in which bond breaking or bond formation takes place in a single step is called a concerted reaction. For example, diels alder reaction.
- 10th edition of Organic Chemistry by T.W Graham Solomons and Craig B. Fryhle.
- 2nd edition of Organic Chemistry by Jonathan Clayden, Nick Greeves, and Stuart Warren.
- 6th edition of Organic Chemistry Structure and Function by Peter Vollhardt and Neil Schore.
- Chap 10 by (chem.ucalgary.ca)