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The Diels-Alder reaction is an organic chemical reaction (specifically, a cycloaddition) between a conjugated diene and a substituted alkene (commonly termed the dienophile) to form a substituted cyclohexene system. The reaction can proceed even if some of the atoms in the newly-formed ring are not carbon. Some of the Diels-Alder reactions are reversible; the decomposition reaction of the cyclic system is then called the Retro-Diels-Alder.
This reaction belongs to a larger class known as 2 + 4 cycloadditions, which are characterized by the formation of a ring by a process involving two pi electrons of one reactant and four pi electrons of the other. Such reactions tend to be especially rapid because the transition state (see activation energy) involves six pi electrons delocalized around a ring, much like benzene. It therefore has some aromatic character and is particularly stable. By comparison, 2 + 2 and 4 + 4 cycloadditions tend to be much slower, as the transition state is anti-aromatic. These reactions can be performed photochemically.
The diene component in the Diels-Alder reaction can be open-chain or cyclic and it can have many different kinds of substituents. There is only one limitation: it must be able to take up the cis-conformation. Butadiene normally prefers the s-trans conformation with the two double bonds as far away from each other as possible for steric reasons. The barrier to rotation about the central bond is small and rotation to the less favourable but reactive s-cis conformation is rapid. Cyclic dienes that are permanently in the s-cis conformation are exceptionally good at Diels-Alder reactions (cyclopentadiene is a classic example), but cyclic dienes that are permanently in the s-trans conformation and cannot adopt the s-cis conformation will not do the reaction at all.
In a typical Diels-Alder reaction, the dienophile has an electron-withdrawing group conjugated to the alkene. Though common, this feature is not exclusive of Diels-Alder dienophiles. There must be some extra conjugation, at least a phenyl group or chlorine atom.
The Diels-Alder reaction is generally considered the "Mona Lisa" of reactions in organic chemistry since it requires very little energy to create the very useful six-membered ring .
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