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.
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