Diels-Alder Reaction

The series of reactions run in this lab illustrate the processes of dissociation of a dimer, the Diels-Alder reaction, and hydrolysis of an anhydride.

Dicyclopentadiene (1), the dimer of cyclopentadiene, is commercially available from coal tar. Thermolysis of 1 at its boiling point (b.p. 170°) generates cyclopentadiene (2) by a retro- (or reverse) Diels-Alder reaction. The monomeric diene is then used in a Diels-Alder reaction with maleic anhydride (3) to produce a bicyclic, unsaturated anhydride (4), which by hydrolysis affords a bicyclic dicarboxylic acid (5).

The major step in the above sequence is the Diels-Alder reaction of 2 + 3 &emdash;&emdash;> 4. This reaction is classified as a [4+2] cycloaddition in which the 4 ¹ electrons of the conjugated diene (2) interact with the 2 ¹ electrons of the dienophile (3) to form two new C-C bonds with the concomitant generation of a six-membered ring containing a C=C bond between C-2 and C-3 of the original diene. The simplest example is the reaction of butadiene with ethylene to give cyclohexene:

While two isomeric products are theoretically possible, only one (compound 4) is actually obtained in appreciable yield. The possible structures are designated exo or endo based on whether the anhydride portion is "cis" or "trans", respectively, to the methylene group.

In most Diels-Alder reactions with 2 the endo--adducts are formed more quickly and are the initially observed products (kinetic control). Although the exo-adducts are frequently more stable (thermodynamically controlled), they are seldom observed because equilibrating conditions are not in effect.

What do these structures look like in 3D? Using the Chime plugin, we can examine the starting materials, and the two possible products.

Dicyclopentadiene (1)

Maleic Anhydride (3)

Endo anhydride (4)

Exo anhydride (4')