16.22 : Diels–Alder Reaction: Characteristics of Dienes

The Diels–Alder reaction brings together a diene and a dienophile to form a six-membered ring. Both components have unique characteristics that influence the rate of the reaction.

Characteristics of the diene

Conformation

The simplest example of a diene is 1,3-butadiene, an acyclic conjugated π system. At room temperature, the molecule exists as a mixture of s-cis and s-trans conformers by virtue of rotation around the carbon–carbon single bond. Although the s-trans isomer is more stable, the terminal carbons are too far apart to overlap with the carbons of the dienophile. However, in an s-cis configuration, the carbons are close enough to interact with the dienophile. As a result, for a diene to undergo a Diels–Alder reaction, it must adopt an s-cis conformation.

Reactivity

From a frontier orbital perspective, the dominant interaction is between the HOMO of the diene and the LUMO of the dienophile. The rate of a Diels–Alder reaction depends on the HOMO–LUMO energy gap, which can be altered by adding substituents to the diene. Electron-donating groups push the HOMO of the diene closer to the LUMO of the dienophile. This decreases the HOMO–LUMO energy gap and increases the rate of the Diels–Alder reaction.