Chapter 10: Conjugation in Alkadienes and Allylic Systems

Diels-Alder Reaction (discovered 1928, Nobel Prize in 1950)

• The Diels-Alder reaction is a conjugate addition reaction of a conjugated diene to an alkene or alkyne (the dienophile) to produce a cyclohexene.
• The simplest example is the reaction of 1,3-butadiene with ethene to form cyclohexene (not a very efficient example):

• 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 both regio- and stereoselectivity (due to the concerted mechanism), the Diels-Alder reaction is a very powerful reaction and is therefore widely used in synthetic organic chemistry.
• The reaction is usually thermodynamically favourable due to the conversion of 2 π-bonds into 2 new stronger σ-bonds.
• In order for the diene to react, it must be able to adopt the s-cis conformation. For 1,3-butadiene, the s-trans conformation is about 12 kJ/mol (2.8 kcal/mol) more stable than the s-cis but the activation barrier to interconversion is low and the conformations readily interconvert by rotation about the C2-C3 σ-bond at room temperatue:



• The two reactions shown above require harsh reaction conditions, but the normal Diels-Alder reaction is favoured by electron withdrawing groups (e.g. carbonyl containing functional groups such as aldehydes, ketones and esters or nitriles) on the electrophilic dienophile and by electron donating groups (e.g. alkyl and alkoxy groups) on the nucleophilic diene.

• Some common examples of the components are shown below:

Dienes
Dienophiles

 

Study Tips: Use the simplest Diels-Alder reaction as a "template" for deriving the products of more complex examples. It helps to visualise the reaction and helps with locating the C=C and the substituents.

 

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 where you should compare the relative positions substituents in the JSMOL images:

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

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)
Highlight dienophile fragment		Highlight dienophile fragment
Compare the relative position of the dienophile fragment in the JSMOL images above.

© Dr. Ian Hunt, Department of Chemistry