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Diels Alder Reaction of dienes and dienophiles

Description: The Diels Alder reaction converts a diene and an alkene (usually electron-poor, called a “dienophile”) into a six-membered ring containing an alkene (cyclohexene).

Notes: X here is usually an electron withdrawing group such as a ketone, ester, or CN.

Examples: 

Notes: Example 1 shows a simple Diels-Alder.

Example 2 shows the reaction with a cyclic diene, to form a bicyclic compound.

Example 3 shows how stereochemistry on the dienophile is conserved (the groups cis to each other on the double bond will end up cis on the ring). That is to say that the reaction is stereospecific. 

Examples 4 and 5 show that the groups on the “outside” of the diene end up on the same side of the ring to each other.

Example 6 shows what happens when we have substitution on both the diene and the dienophile. A mixture of diastereomers is formed, called the “endo” and the “exo” products. Generally speaking the “endo” product is favored.

Example 7 shows an alkyne being used as a dienophile.

Examples 8 and 9 show what happens when we have electron donating groups attached to the diene. We always line up the more “electron-rich” carbon of the diene with the more “electron-poor” carbon of the dienophile.

Mechanism: The mechanism of this reaction is concerted meaning that all the bond forming and bond-breaking happens at the same time. The reaction proceeds in a 1-step cycloaddition (Step 1, arrows A, B and C).

Notes: The arrows could just as correctly been drawn proceeding in a clockwise fashion. The end result is the same.