16.22 : 狄尔斯-阿尔德反应：二烯的特征

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.