16.23 : 第尔斯-阿尔德反应：亲双烯体的特征

Alkenes, substituted alkenes, and alkynes are some of the commonly used dienophiles in Diels–Alder reactions.

Let's examine a few typical characteristics of dienophiles.

Ethylene is the simplest dienophile that reacts with 1,3-butadiene to form cyclohexene. However, this reaction is slow and requires high temperatures.

One way to speed up the process is by introducing electron-withdrawing groups to the dienophile.

Recall that a normal Diels–Alder reaction is driven by the flow of electrons from the HOMO of the diene to the LUMO of the dienophile. So, the reaction rate is influenced by the HOMO–LUMO energy gap.

Electron-withdrawing groups make the dienophile more electrophilic and lower the energy of the LUMO. A decrease in the HOMO–LUMO energy gap facilitates the transfer of electrons and increases the reaction rate.

Next, dienophiles with substituents on both carbons yield stereospecific products, implying that the stereochemistry of the substituents is retained during the reaction.

Lastly, cyclic dienophiles like maleic anhydride undergo Diels–Alder reactions to form bicyclic products.