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16.22: Diels–Alder Reaction: Characteristics of Dienes

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Diels–Alder Reaction: Characteristics of Dienes
 
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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.

Figure1

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.

Figure2

Tags

Diels-Alder Reaction Diene Dienophile Six-membered Ring S-cis Conformation S-trans Conformation Frontier Orbital Perspective HOMO (Highest Occupied Molecular Orbital) LUMO (Lowest Unoccupied Molecular Orbital) Substituents Electron-donating Groups HOMO-LUMO Energy Gap

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