Diels Alder Reaction

Diels-Alder reaction mechanism proceeds through the suprafacial (same-face involvement of the 𝝅 system or isolated orbital in the process) interaction between a 4𝝅 electron system with a 2𝝅 electron system. Diels-Alder reaction involves cycloaddition reactions resulting in the formation of a new ring from two reactants.

In Diels-Alder reaction, the 4𝝅 electron system refers to the diene structure whereas the 2𝝅 electron system refers to the dienophile structure. This interaction now leads to a transition state without any additional energy barrier from the imposition of orbital symmetry.

What is Diels-Alder Reaction?

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 (i.e. all bond breakage and bond formation occurs in a single step).

This reaction was discovered by the German chemists 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.

An illustration of the reaction is provided below.

From the illustration given above, it can be observed that two pi bonds were converted into two sigma bonds. This occurs due to the concerted bonding of two independent pi-electron systems. The Diels-Alder reaction involves the shift of four pi electrons of the diene and two pi electrons of the dienophile.

This reaction is used in the production of vitamin B6. The reverse reaction (also called the retro-Diels-Alder reaction) is used in the production of cyclopentadiene on an industrial scale.

Mechanism of Diels-Alder Reaction 

Since pi bonds are converted into stronger sigma bonds, the reaction is thermodynamically favourable. The Diels-Alder reaction is favoured by electrophilic dienophiles with electron-withdrawing groups attached to them. It is also favoured by nucleophilic dienes with electron-donating groups in them. Given below are a few examples for good dienes and dienophiles for the Diels-Alder reaction.

Since the Diels-Alder reaction mechanism is concerted, the reaction proceeds in a single step cycloaddition reaction. Here, two unsaturated molecules combine to form a cyclic adduct. There is a net reduction in bond multiplicity. All the bond formations and bond breakages happen simultaneously. Given below is an illustration of the simple reaction mechanism.

Thus, the diene and the dienophile react to give a cyclohexene derivative. It can be observed from the illustration of the mechanism that three carbon-carbon pi bonds break but only 1 pi bond forms whereas two sigma bonds are formed.

Stereoselectivity & Variations

The Diels-Alder reaction has several modifications. Some of the variations of this reaction are listed below.

1. The Hetero Diels-Alder Variation

These reactions involve one or more heteroatoms (any atom other than carbon or hydrogen).

When carbonyl groups are reacted with dienes, dihydropyran products are formed.

The aza Diels-Alder reaction involves the use of imines as the dienophile (or diene substituents). The product formed in this reaction is an N-heterocyclic compound.

If a nitroso compound is used as the dienophile, the resulting reaction with the diene yields oxazines.

2. Usage of Lewis Acids

In this variation, a Lewis acid is used as a catalyst.

Examples of the Lewis acids that can be used in these reactions include aluminium chloride, boron trifluoride, tin tetrachloride, and zinc chloride.

In these reactions, the electrophilicity of the dienophile complex is increased by the Lewis acid.

The advantages of this variation include increased reaction rates and improved stereoselectivity and regioselectivity. These Diels-Alder reactions can proceed at relatively low temperatures.

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. Organocatalysts which have relatively small molecules can also be used to modify the stereoselectivity of this reaction.

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.

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