Chapter 24, Problem 24.7YT

Interpretation Introduction

Interpretation:

The generic groups (W, X, Y, and Z) on the diene in the Equation $\text{24-20}$ are to be matched to the specific groups on the diene in the given Diels-Alder reaction. Whether the cis/trans relationships among the substituents in the product in Equation $\text{24-20}$ agree with those in a given Diels-Alder reaction is to be explained.

Concept introduction:

The [$\text{4+2}$] cycloaddition reaction is generally known as Diels-Alder reaction. A Diels-Alder reaction involves a diene and a dienophile. A diene shares four π electrons from the two conjugated π bonds while a dienophile shares two π electrons from a single π bond. The Diels-Alder reaction is concerted and proceeds through a six-membered aromatic transition state. For a Diels-Alder reaction to occur, the diene must be present in the s-cis conformation because both ends of the diene must be relatively close to the dienophile in the transition state. In a standard Diels-Alder reaction, four carbon atoms are rehybridized from ${\text{sp}}^{\text{2}}$ to ${\text{ sp}}^{\text{3}}$: two from the dienophile and two from the diene. Due to this transformation, chiral centers may be introduced in the product. Thus, Diels-Alder reaction is stereospecific because the stereochemistry of the product depends on the stereochemistry of the reactants. Since Diels-Alder reaction is concerted, the stereochemistry of the diene as well as the dienophile is conserved in the product. If the dienophile is a simple ethylene molecule, the stereochemistry of the Diels-alder reaction depends only on the substituents on end carbon of the diene. If the double bonds in the diene are both trans or both cis, then the substituents at the ends of the diene become cis to each other with respect to the plane of the ring in the product. Conversely, if one of the diene’s double bonds is cis and the other is trans, the two substituents become trans to each other in the product.