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.
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.