16.23:
Diels–Alder Reaction: Characteristics of Dienophiles
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.
16.23:
Diels–Alder Reaction: Characteristics of Dienophiles
In a Diels–Alder reaction, the diene is usually an electron-rich system and acts as a nucleophile, whereas the dienophile is electron-deficient and functions as an electrophile. Much like the diene, the nature of the dienophile significantly impacts the outcome of the reaction.
Characteristics of Dienophiles
Generally, the best dienophiles are alkenes containing electron-withdrawing substituents such as carbonyl, nitrile, and nitro groups. The feasibility of a Diels–Alder reaction depends on the energy difference between the HOMO of the diene and the LUMO of the dienophile. The smaller the HOMO–LUMO gap, the faster the reaction. Electron-withdrawing substituents on the dienophile lower the energy of the LUMO, reducing the HOMO–LUMO gap and speeding up the reaction.
With 1,2-disubstituted dienophiles, the stereochemistry of the double bond is preserved, leading to stereospecific outcomes.
Cyclic dienophiles give bicyclic products.
