Diels Alder Reaction
Diels Alder Reaction is the reaction of a diene with a species capable of reacting with the diene, the dienophile. A diene is a hydrocarbon that contains two carbon double bonds, while a dienophile is an electron-deficient alkene.
The Diels-Alder is also called a [4+2] cycloaddition because a ring is formed by the interaction of four pi electrons of the alkene with two pi electrons of the alkene or alkyne.
The product of the Diels-Alder reaction is usually a structure that contains a cyclohexene ring system.
The mechanism of the reaction involves a cyclic flow of electrons in a single step in which the diene, supplies 4 π electrons and the alkene or alkyne, also called the dienophile, supplies 2 π electrons. In this process, two new sigma bonds which link the former dienophile to the diene and one new pi bond between the former double bonds of the diene are formed.
The reaction involves only pi bonds in the reactants; no sigma bonds are broken. Due to the concerted mechanism, two kinds of selectivity are evident in Diels-Alder reaction products.
The stereochemistry of the dienophile is retained in the product. If the substituents of the dienophile are cis, they remain cis in the product. If they are trans, they remain trans in the product. If the diene is cyclic, and if the dienophile has cis substituents, then two stereochemical outcomes are possible.
The endo isomer is more sterically hindered than the exo. In spite of this, the endo tends to predominate in cases where the R substituents have a system that is conjugated with that of the dienophile double bond. During cyclization favorable interaction between the substituent system and that of the diene overcomes the steric hindrance and thus favors the endo isomer over the exo. Electron withdrawing groups on the dienophile and electron-donating groups on the diene can speed up this reaction. The dienophile is the part of the reaction that