Diels alder is the pericyclic reaction between a conjugated diene and a substituted dienophile to form cycloalkenes. It is the [4n+2]𝝅 cyclo addition reaction having a concerted mechanism. In the Diels alder reaction, two pi bonds are converted into two sigma bonds via a cyclic transition state in a single step. This results in the formation of a six-member ring with one double bond(cyclo alkenes).
This reaction was named after the German Chemists Otto Diels and Kurt Alder. They received the Noble prize in 1950 for this development. It is a type of pericyclic reaction. It proceeds on heating. This reaction involves cyclo addition of 4𝝅 electron of conjugated diene and 2𝝅 electron of substituted dienophile. This results in the formation of cyclo alkenes derivatives.
Diels alder is a reaction between Nucleophiles and electrophiles. It has a concerted mechanism. dIn this reaction conjugated diene acts as a nucleophile whereas substituted dienophile acts as an electrophile. In this reaction, electrons move from electron-rich nucleophiles to electron-deficient electrophiles in a cyclic way. There is no positive or negative charge intermediate.
Mechanism of Diels Alder reactions
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. Because sigma bonds are thermodynamically more favorable.
Here electron moves from electron-rich species to electron-deficient specie in a cyclic way. It results in the formation of cycloalkene derivatives. Products are formed via a cyclic transition state.
The transition state has six delocalized electrons. Electrons either rotate in a clockwise or anticlockwise direction resulting in the formation of the same product. Arrows are one after another. The last arrow connects to the first arrow.
Transition state of diels alder reaction
Diels Alder reaction proceeds through a single cyclic transition state. No intermediates are formed during this reaction. The transition state has six delocalized 𝝅 electrons. This resembles to stabilization of benzene thus transition state has an aromatic character. The transition state can be represented by using arrows or dotted lines.
Overlapping of molecular orbitals
During the Diels alder reaction highest occupied molecular orbitals (HOMO) of the diene interact with the lowest unoccupied molecular orbitals (LUMO) of the dienophile. Here diene is the electron-rich species whereas, dienophile is electron deficient specie. HOMO electron will always be in 𝝅 orbitals whereas LUMO will always be in 𝝅* orbital.
These reactions can easily take place by mixing components at room temperature. Sometimes these reactions need gentle heating. They may be warm in suitable solvents. Some of the unreactive reactants required high heating. Lewis acid catalyst may be also used in some cases. Similarly in a few cases, high pressure is also required for diels alder reaction.
What is diene?
A compound having two double bonds is called diene. Di means two and ene is for double bond. In diels alder reaction dienes are 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. S-trans diene can not be used because their end is not close enough to react with another alkene.
What is a dienophile?
Dienophile is the electron-deficient specie. It is in search of nucleophiles. Dienophiles 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.
Reactivity of Diels alder reaction depends upon a number of factors:
If electron-withdrawing groups are present in conjugation with a double or triple bond of diene it increases the reactivity of the dienophile. It makes dienophile strong electrophile.
Similarly, if the electron donating groups are present on diene they make diene more reactive. With the reactive reactants, Diels alder reaction takes place fastly with gentle heating. Otherwise, the reaction would be slow and require high heating and high pressure. These substituent does not affect the reaction mechanism.
Examples of Diels Alder reaction
This is the simplest example of Diels alder reaction. In this reaction, 1,3-Butadiene react with ethene at 2000C. 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.
In this example, we use a reactive dienophile. Because it has an electron-withdrawing group in conjugation with a double bond. That makes it more electron deficient. We get 90% product at 1400C.
In this reaction, we treated 1,3-Butadiene with melic anhydride. A very reactive dienophile. Because it has two electron-withdrawing groups. We get 100% product even at 1000C.
In this example, we have used diene which is substituted with alkyl groups. We know that alkyl groups are electron donating. They increase the reactivity of diene. We get 100% yield even at 300C.
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 in 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.
When we use very reactive reactants are highly substituted reactants we get mixtures of diastereomers. We get endo as well as Exo product. In a kinetically controlled reaction Edno product is in excess whereas, in a thermodynamically controlled reaction, Exo product is in excess. Endo products are stabilized by secondary orbital interactions.
- Diels alder is an important reaction for the synthesis of cyclo alkene derivatives.
- It is the type of pericyclic reaction.
- These reactions are also known as the 1,4-Cycloaddition reactions.
- It involves the cycloaddition of [4+2]pi 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 mechanism. (single step)
- Most of the Diels alder reactions are reversible.
- Similarly, it has a cyclic transition state.
- 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.
- Similarly, dienophiles having electrons withdrawing in conjugation are more reactive.
- These are stereospecific reactions.
- 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.
- Exo product is thermodynamically more stable.
Why diene must have s cis conformation during diel 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 diels 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.
Which of the dienophile is more reactive?
What is the effect of electron-withdrawing substituent on dienophiles?
When electrons withdrawing substituent are in conjugation with a double in dienophile it increases the reactivity of 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.
- Diels alder reaction by (chem.ucalgary.ca)