Chapter 24, Problem 24.9YT

Interpretation Introduction

Interpretation:

The mechanism that leads to the minor product in a given Diels–Alder reaction is to be drawn.

Concept introduction:

Diels-Alder reactions are concerted and stereospecific. When the diene and the dienophile of a Diels–Alder reaction are both unsymmetric, the major isomeric product is the one produced by the approach that exhibits the more favorable electrostatic attraction among atoms undergoing bond formation. The oppositely charged species are attracted by each other, while there is a repulsive interaction between the same charged species. Therefore, the regiochemistry in Diels-Alder reaction depends on the nature of substituents on diene as well as that on dienophile. An electron-donating substituent on the diene and an electron-withdrawing substituent on the dienophile not only enhance the rate of given Diels-Alder reaction but also decide which isomer will form predominantly. The less favorable interaction between the diene and dienophile leads to form a minor Diels-Alder product. The Diels-Alder reaction is the cycloaddition of a diene and a dienophile. The diene has a pair of conjugate double and contributes its $\text{4π}$ electrons. The dienophile contributes only one double bond and its $\text{2π}$ electrons. The movement of these six electrons is in a cyclic manner. Thus, the reaction proceeds through a cyclic transition state, which makes it a pericyclic reaction. Therefore, the Diels-Alder reaction is called a [$\text{4+2}$] cycloaddition reaction.