Chapter 10: Conjugation in Alkadienes and Allylic Systems

Diels-Alder Reaction (Nobel Prize in 1950)

• The Diels-Alder reaction is a conjugate addition reaction of a conjugated diene to an alkene (the dienophile) to produce a cyclohexene.
• The simplest example is the reaction of 1,3-butadiene with ethene to form cyclohexene:

• The analogous reaction of 1,3-butadiene with ethyne to form 1,4-cyclohexadiene is also known:

• Since the reaction forms a cyclic product, via a cyclic transition state, it can also be described as a "cycloaddition".
• The reaction is a concerted process:

• Due to the high degree of regio- and stereoselectivity (due to the concerted mechanism), the Diels-Alder reaction is a very powerful reaction and is widely used in synthetic organic chemistry.
• The reaction usually thermodynamically favourable due to the conversion of 2 π-bonds into 2 new stronger σ-bonds.
• The two reactions shown above require harsh reaction conditions, but the normal Diels-Alder reaction is favoured by electron withdrawing groups on the electrophilic dienophile and by electron donating groups on the nucleophilic diene.
• Some common examples of the components are shown below:

Dienes
Dienophiles

Stereoselectivity:

• The Diels-Alder reaction is stereospecific with respect to both the diene and the dienophile.
• Addition is syn on both components (bonds form from same species at the same time)
• This is illustrated by the examples below:

a cis-dienophile gives cis-substituents in the product.
a trans-dienophile gives trans-substituents in the product.
If the diene substituents have the same stereochemistry (here they are both E), then both end up on the same face of the product
If the diene substituents have opposite stereochemistry (here one is E and one Z), then they end up on opposite  faces of the product

Cyclic dienes can give stereoisomeric products depending on whether the dienophile lies under or away from the diene in the transition state.  The endo product is usually the major product (due to kinetic control)
 

Diene and dienophile aligned directly over each other gives the endo product  (dienophile under or in = endo)	Diene and dienophile staggered with respect to each other gives the exo product  (dienophile exposed or out  = exo)
Compare the relative position of the dienophile fragment in the following JMOL images

© Dr. Ian Hunt, Department of Chemistry, University of Calgary