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Diels–Alder Reaction Forming Cyclic Products: Stereochemistry
The simplest example of a Diels–Alder reaction is between 1,3-butadiene and ethene, forming cyclohexene.
This reaction is syn-stereospecific, proceeding via a transition state in which the diene's HOMO interacts with the dienophile's LUMO in a suprafacial manner.
Due to the concerted nature of the reaction, both components remain locked in their original configuration, giving rise to stereospecific outcomes.
For example, cis and trans dienophiles form products with syn and anti stereochemistry. The same applies to substituted dienes.
Interestingly, for a diene to undergo a Diels–Alder reaction, it must adopt an s-cis conformation.
Recall that 1,3-butadiene exits as an equilibrium mixture of s-cis and s-trans conformers.
Although the s-trans is more stable, the p orbitals on the terminal carbons are too far apart to simultaneously overlap with the dienophile. This is not the case in the s-cis conformation, making it more reactive.
Lastly, dienes where the s-cis form exhibits severe steric strain or dienes locked in an s-trans form will not undergo Diels–Alder reactions.
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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.
Conformational Aspects
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.
