Mechanism of the Diels-Alder reaction in which a conjugated diene and a substituted alkene the dienophile react to form a cyclohexene. There is little evidence to support simple radical or polar mechanism of the Diels-Alder reaction. As the result of many studies the reaction seems best formulated as a process in which the bonds between the diene and the dienophile are formed essentially simultaneously:
In other words, the reaction is concerted, there being no evidence for any discrete intermediate. Referring to Figure 13-1, we may say that the reaction follows curve A and not curve B on the plots of energy versus reaction coordinate. Although it is difficult to prove experimentally that the reaction is concerted, we shall see in Chapter 21 that there are theoretical reasons to expect it to be so, despite the high degree of ordering (unfavorable entropy, Section 4-4B) that the transition state must have in order that all of the participating bonds can be made and broken at once.
We already have discussed a few addition reactions that appear to occur in a concerted manner. These include the addition of diimide, ozone, and boron hydrides to alkenes. Concerted reactions that have cyclic transition states often are called pericyclic reactions. Other examples will be considered in later chapters.
Figure 13-1 Schematic representation of energy versus reaction coordinate for (A) a concerted reaction and (B) a stepwise reaction involving formation of an unstable intermediate. Curve B has been drawn to have the highest energy point (the transition state) come before the intermediate is formed. For many processes the highest energy point comes after the intermediate is formed. If the highest energy point comes after the intermediate is formed, then the intermediate will be more or less in equilibrium with the reactants.
FMO Analysis and Mechanism of the Diels-Alder Reaction video
This video shows how molecular orbital theory explain nearly every experimental observation on the mechanism of the Diels-Alder reaction.
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