Diels Alder reaction mechanism

The Diels-Alder reaction is an important organic chemical reaction where the reactants include a substituted alkene and a conjugated diene. This substituted alkene is commonly referred to as a dienophile. This reaction gives rise to a substituted derivative of cyclohexene. The Diels Alder reaction is a very good example of pericyclic reactions which proceed via concerted mechanisms that all bond breakage and bond formation occur in a single step.

This reaction was discovered by the German chemist Otto Diels and Kurt Alder in the year 1928, for which they received the Nobel Prize in chemistry in the year 1950. The Diels Alder reaction can be used to form six-membered rings since there is a simultaneous construction of two new carbon-carbon bonds.

The retro Diels Alder reaction is the microscopic reverse of the Diels Alder reaction the formation of a diene and dienophile from cyclohexene.It can be accomplished spontaneously with heat, or with acid or base.

What is Diels Alder’s reaction?

The Diels-Alder reaction is an organic reaction that is used to convert a conjugated diene (a molecule with two alternating double bonds) and a dienophile (an alkene) to a cyclic olefin. This process is concerted, where bonds form and break at the same time, and the entire reaction takes place in one step in the presence of heat. The class of reactions to which Diels-Alder belong is termed cycloaddition. The electrons are transferred cyclically between the diene and the alkene to form a cyclic adduct. Diels-Alder reactions are stereospecific. The substituents attached to both the diene and the dienophile retain their stereochemistry throughout the reaction. Electron withdrawing groups on the dienophile and electron-donating groups on the diene facilitate a reaction.

Reaction: 

The electrons are transferred cyclically between the diene and the alkene to form a cyclic adduct. Diels-Alder reactions are stereospecific.

Examples of Diels Alder reaction:

• Diels Alder’s reaction to cyclohexene

• Diels Alder reaction for cyclopentadiene

Mechanism of Diels Alder reaction:

Diels Alder reaction mechanism proceeds through the suprafacial same face involvement of the pi system or isolated orbital in the process. Interaction between a 4 pi-electron system with a 2 pi-electron system. Diels Alder reaction involves cycloaddition reactions that result in the formation of a new ring from two reactants. The Diels-Alder reaction is a concerted, cycloaddition reaction between a conjugated diene and an alkene. The reaction occurs in two steps:

      1. The diene reacts with the alkene to form a new cyclohexene ring.

      2. The new cyclohexene ring rearranges to form the final product.

Stereoselectivity of Diels Alder Reaction:

The diels-alder reaction is stereoselective. This means that the two molecules that are reacting will form a new molecule with a specific stereochemistry. In the diels-alder reaction, the two molecules that are reacting are a diene and an alkene. The diene will have two carbon atoms that are double-bonded to each other. The alkene will have two carbon atoms that are single-bonded to each other. When the two molecules are reacting, the diene will bond to the alkene to form a new molecule. This new molecule will have four carbon atoms that are double-bonded to each other.

Some variations of this reaction are listed below:

1. The Hetero Diels alder reaction: The hetero Diels-Alder variation is a reaction that can be used to form six-membered heterocycles. The hetero Diels-Alder variation is a reaction between a diene and a heteroalkene. The heteroalkene can be substituted or unsubstituted alkene. The hetero Diels-Alder variation can be used to form six-membered heterocycles that contain one or more heteroatoms. The hetero Diels-Alder variation is a useful reaction for forming heterocycles that contain nitrogen, oxygen, or Sulphur atoms.

2. Usage of Lewis acids: A Lewis acid is an electron-pair acceptor. It is a molecule that can accept an electron pair from another molecule. This electron pair is then used to form a covalent bond between the two molecules.

3. The Asymmetric variation: There exist many variations of this reaction that influence its stereoselectivity. One such example is the use of a chiral auxiliary. Organic catalysts that have relatively small molecules can also be used to modify the stereoselectivity of this reaction.

Retro Diels Alder reaction:

The retro Diels Alder reaction is the microscopic reverse of the Diels Alder reaction, thereaction the formation of a diene and dienophile from cyclohexene. It can be accomplished spontaneously with heat, or with acid or base. 

The retro Diels-Alder reaction is the exact reverse of the Diels-Alder. It passes through the same transition state when the heat is applied. For example, cyclohexene breaks down into butadiene and ethylene at a temperature of 800c.

Applications of Diels Alder reaction:

• The Diels-Alder reaction is used extensively in synthetic organic chemistry as a means of creating carbon-carbon bonds. It is particularly useful for creating six-membered rings.

• Some important applications of the Diels-Alder reaction include its role in the production of vitamin B6 and the role of its reverse reaction in the production of cyclopentadiene on an industrial scale.

• The Diels alder reaction is one of the most popular transformations for organic chemists to generate molecular complexity efficiently.it is used in industrial applications for the synthesis of pharmacologically active ingredients, agrochemicals, and flavors and fragrances.

• The Diels-Alder reaction is used in the synthesis of natural products like rubber and plastic. It also finds its application in pharmaceuticals and biomedical engineering. It is used to make synthetic steroids, such as cortisone and Vitamin D.

Conclusion:

The Diels Alder reaction is a chemical reaction between a conjugated diene and a substituted alkene, commonly termed the dienophile to form a substituted cyclohexene derivative. It is the prototypical example of a pericyclic reaction with a concerted mechanism. Thethe Diels-Alder reaction includes its role in the production of vitamin B6 and the role of its reverse reaction in the production of cyclopentadiene on an industrial scale.