Retro-Diels-Alder Reaction: Mechanism & Overview

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 about the Retro-Diels-Alder reaction and how it works as the reverse of the Diels-Alder reaction. We will learn how to determine what products are formed and the stereochemistry of those products.

Diels-Alder and Retro-Diels-Alder

Let's say that you were asked to create cholesterol in the laboratory. What product would you use? Where would you start?

When we look at the structure of cholesterol we notice that there is a six membered ring with a double bond. We remember that the Diels-Alder reaction can create a six membered ring with a double bond, but what reactants do we need to use to create this ring? We can determine what reactants to use with the retro Diels-Alder reaction.

Cholesterol has a six membered ring with a double bond suggesting that this could be synthesized using the Diels-Alder Reaction
Cholesterol structure

The retro-Diels-Alder Reaction occurs with intensive heat in one concerted step. It really is just the Diels-Alder reaction backwards.

Remember, the Diels-Alder reaction takes a diene (two double bonds separated by one single bond in the cis conformation) and a dienophile (any compound with an alkene) to create a six membered ring with one double bond.

It breaks 3 double bonds, and creates 1 new double bond and 2 single bonds. So if the retro-Diels-Alder reaction is backwards, then is breaks 2 single bonds and 1 double bond to make 3 double bonds.

General Mechanism

Since this reaction all occurs in one step it doesn't really matter where you begin, although it is usually easiest to begin at the double bond.


Simple reaction


First, draw an arrow coming from the double bond (bond 1-2) to the next bond (bond 2-3). It does not matter if you go clockwise or counter-clockwise, just keep going in the direction that you started.

Then draw an arrow coming from bond 3-4 to bond 4-5, then from bond 5-6 to bond 6-1. Where arrows begin, a bond is broken, and where arrows end, a bond is formed.

So this is what we end up with:

  • One of the double bonds at bond 1-2 is broken. But since there is still another bond, those two carbon atoms remain connected.
  • A new bond is formed at bond 2-3. Since there was already a bond there, this becomes a double bond. Same with 4-5.
  • The bond is broken at bond 3-4. Since this was a single bond, carbon atoms 3 and 4 are no longer connected.
  • The bond is broken at carbons 5-6, disconnecting carbons 5 and 6.
  • A new bond is formed at bond 6-1, forming a double bond.

So we end up with two compounds, the diene and the dienophile.

Stereochemistry

The stereochemistry of the reactant (the three dimensional structure) is important in determining the stereochemistry of the products.

In the reactant the stereochemistry is going to be drawn using wedges (to indicate it is pointing up) and dashes (to indicate it is pointing down).

In the products this stereochemistry is around a double bond so it won't be pointing up or down. Instead, substituents will either be on the same side of the double bond (cis) or on opposite sides of the double bond (trans).


Trans configuration product


If the reactants had both substituents pointing up or both pointing down, then the product will be in the cis conformation. If the reactants had the substituents pointing in opposite directions (one up and one down) then the product will be trans.

Many Options

Sometimes when drawing the arrows you have several different options where the electrons can move. It isn't always straight forward which bonds will be broken and formed. When this occurs, draw out each option and then determine which products are the most likely.

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