16.22:
Diels–Alder Reaction: Characteristics of Dienes
The starting materials for a Diels–Alder reaction include a conjugated π system called the diene and a compound with at least one π bond called the dienophile.
Dienes are electron-rich systems, and dienophiles are electron-deficient. A flow of electrons from the HOMO of the diene to the LUMO of the dienophile forms a cyclic product with new σ bonds.
Let's examine the two essential characteristics of dienes: conformation and reactivity.
Only dienes that can adopt an s-cis conformation undergo Diels–Alder reactions.
Between competing isomers, the s-cis form with the least steric hindrance is more reactive. Additionally, an increase in substituents further destabilizes the s-cis conformer, rendering it unreactive.
Next, the reactivity of dienes is significantly enhanced by electron-donating groups. This can be rationalized by examining the HOMO–LUMO energy gap.
The energy of the diene's HOMO increases with an increase in the electron-donating ability of the substituent. Consequently, the HOMO–LUMO energy gap decreases, favoring the interaction between the frontier orbitals and increasing the rate of the Diels–Alder reaction.
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.
