Diels-Alder Reaction: Mechanism & Stereochemistry

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In this lesson we will learn the mechanism by which the Diels-Alder reaction creates a new six-membered ring. We will learn what reactants are necessary, and how the stereochemistry of the products turns out based on the reactants.

Diels-Alder Background

Otto Diels, a professor at the University of Kiel in Germany, was working with his student Kurt Alder in the mid 1900s to develop a reaction that could easily create new cyclohexagonal rings. Together they created the Diels-Alder reaction and later received the Nobel Prize in Chemistry for this reaction. The Diels-Alder reaction is used to make rubber and plastic as well as synthetic steroids such as cortisone and Vitamin D.

The Diels-Alder reaction is a concerted single step reaction that takes two molecules and creates a six-membered ring. In this reaction, three double bonds (pie bonds) are broken, two new single bonds (sigma bonds) are formed, and one new double bond (pie bond) is formed. Depending on the reactants, the new cyclohexagonal ring may have other rings attached on the outside or inside, as well as functional groups or substituents on the outside.

Basic Reaction

The general reaction takes a diene and a dienophile and reacts to create a six-membered ring with a double bond. The simplest Diels-Alder reaction uses 1,3-diene and ethylene.


Basic Diels Alder Reaction

The electrons from ethylene (the dienophile) attack carbon 1 on the diene, forming a single bond. The electrons from that double bond move into a double bond between carbons 2 and 3. This kicks the electrons out from between carbons 3 and 4 to form a single bond to the ethylene. All of this occurs in a single step, so really we could start with the electrons from the double bond between carbons 1 and 2 forming a new double bond between carbon 2 and 3 and go around from there. It does not matter where on the molecule we start, because it all happens at the same time.

Either the diene or the dienophile can have substituents and still react. Just keep track of where the new single bonds form and where the new double bond forms. It can look really confusing; just always draw your arrows to keep track of where the electrons are going.


reaction

This reaction can also occur with either the diene or the dienophile already in ring form (or both as rings). You just end up with two rings attached to each other.


rings

When the diene is a ring you end up with a funny looking molecule because we get a ring within a ring. Typically this is drawn so that the new double bond appears as a bridge between the two rings. In this example the new ring that was formed is numbered 1-6, while the substituents from the diene ring are labeled a and b.


Diene ring

When drawing the reaction it does not matter which side the diene and dienophile are on. But if it is easiest for you to see how the reaction will proceed in the direction as in this example simply identify which molecule is the diene and which molecule is the dienophile and reorient them as needed. Just keep track of whether the molecules are cis or trans and what substituents each molecule has.

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