16.20: Diels–Alder Reaction Forming Cyclic Products: Stereochemistry
The Diels–Alder reaction is one of the robust methods for synthesizing unsaturated six-membered rings. The reaction involves a concerted cyclic movement of six π electrons: four π electrons from the diene and two π electrons from the dienophile.
For the electrons to flow seamlessly between the two π systems, specific stereochemical and conformational requirements must be met.
Stereochemical Orbital Symmetry
The frontier molecular orbitals that satisfy the symmetry requirements are the HOMO of the diene and the LUMO of the dienophile. The two molecular orbitals interact suprafacially, meaning that the addition occurs at the same face of the π system, like a syn addition. The reaction is stereospecific, and the stereochemistry of the diene and the dienophile are retained in the product.
Acyclic conjugated dienes such as 1,3-butadiene exist as a mixture of s-cis and s-trans conformers, with the latter being more stable. However, the s-cis conformer is preferred, as the geometry allows for a better overlap between the terminal carbons of the diene and the dienophile. Therefore, to undergo a Diels–Alder reaction, a diene must adopt the s-cis conformation.