14.5: The Diels-Alder Cycloaddition Reaction
-
- Last updated
- Save as PDF
Objectives
After completing this section, you should be able to
- write an equation to represent a typical Diels-Alder reaction.
- draw the structure of the product formed when a given conjugated diene reacts with a given dienophile in a Diels-Alder reaction.
- identify the diene and dienophile that must be used to prepare a given compound by a Diels-Alder reaction.
- explain the general mechanism of the Diels-Alder reaction, without necessarily being able to describe it in detail.
Key Terms
Make certain that you can define, and use in context, the key terms below.
- Diels-Alder cycloaddition
- pericyclic reaction
Study Notes
The Diels-Alder reaction is an example of an organic chemical reaction which does not proceed by either a polar or a free radical pathway, but rather a pericyclic reaction.
Although we do not expect you to be able to provide a detailed account of the mechanism of this reaction, you should learn enough about the Diels-Alder reaction to fulfillthe objectives stated above. You will find it useful to contrast the mechanism of the Diels-Alder reaction with the polar and radical mechanisms studied earlier.
The unique character of conjugated dienes manifests itself dramatically in the Diels-Alder Cycloaddition Reaction . The Diels-Alder reaction is an important and widely used synthetic method for making six-membered rings. In the Diels-Alder reaction, a conjugated diene, simply referred to as the diene , reacts with a double or triple bond co-reactant called the dienophile , because it combines with (has an affinity for) the diene. During the reaction, two pi-bonds are converted to two sigma-bonds. The Diels-Alder cycloaddition is classified as a pericyclic process. Pericyclic reactions involve the redistribution of bonding electrons in a single step mechanism and will be discussed in greater detail in Chapter 30 . In particular, the Diels-alder reaction is called a [4+2] process because the diene has four pi-electrons that shift position in the reaction and the dienophile has two.
General Reaction
An example of the Diels-Alder reaction is the cycloaddition of 1,3-butadiene to cyanoethene (acrylonitrile) to form 4-cyanocyclohexene.
Example \(\PageIndex{1}\)
Mechanism
All of the electron rearrangements of the Diels-Alder reaction take place once in a single mechanistic step. During this step carbons 1 and 4 of the diene and both alkene carbons of the dienophile, rehybridize from sp 2 to sp 3 and electrons rearrange to create two new sigma bonds in the cyclic product. Carbons 2 and 3 of the diene remain sp 2 hybridized and form a new pi bond in the product.
The mechanism occurs through a cyclic transition state in which there is head-on overlap of two p orbitals on carbons 1 and 4 of the diene with the two p orbitals from the alkene of the dienophile to form two new sigma bonds in the cyclohexene product. The remaining two p orbitals from the diene overlap to form the new pi bond.
Problems
Answers