The
Diels-Alder reaction is a cycloaddition reaction between a conjugated diene and an alkene. This reaction produces a 1,4-addition product. A typical example is the reaction of 1,3-butadiene with maleic anhydride.
The Diels-Alder reaction is favored by the presence of electron-withdrawing groups on the diene and electron-releasing groups on the dienophile, which is a group or bond that is attracted to a diene. The mechanism for the Diels-Alder reaction shows that it does not run via a carbocation intermediate. Instead, this reaction proceeds by
a pericyclic process, a mechanism of just one step, involving a cyclic redistribution of bonding electrons.
Simple alkenes and alkynes are not good dienophiles. A good dienophile generally has one or more electron-withdrawing carbonyl groups
or cyano groups
attached to it. Electron-supplying groups on the diene make the electrons of the π system more available for reaction.
Diels-Alder stereochemistry. The Diels-Alder reaction is very ste-reospecific. The original stereochemistry of the diene and the dienophile are preserved during this
syn-addition reaction. An example of this stereospecificity is the reaction of 1,3-butadiene with
cis-diethylmaleate.
Because the reaction is basically a concerted cyclization, the diene must react in the
cis conformation.
If the diene is a ring structure, the Diels-Alder reaction produces a bicyclic ring system.
A
bicyclic ring system has two carbon rings that share common sides. The previous diagram shows what appears to be a cyclohexene ring with a carbon bridge connecting the third and sixth carbons. In reality, this system is two five-membered rings, a cyclopentene ring and a cyclopentane ring, which are sharing two sides (the “carbon bridge”). The structure of this molecule is
Review of General Chemistry
Conjugated Dienes
