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Chapter 10: Conjugation in Alkadienes and Allylic Systems |
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Diels-Alder Reaction (Nobel Prize in 1950)
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The Diels-Alder reaction is a conjugate addition reaction
of a conjugated diene to an alkene (the dienophile)
to produce a cyclohexene.
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The simplest example is the reaction of 1,3-butadiene with ethene to form
cyclohexene:
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The analogous reaction of 1,3-butadiene with ethyne to form 1,4-cyclohexadiene
is also known:
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Since the reaction forms a cyclic product, via a cyclic transition state,
it can also be described as a "cycloaddition".
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The reaction is a concerted process:
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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.
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The reaction usually thermodynamically favourable due to the conversion
of 2 p-bonds into 2 new stronger
s-bonds.
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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.
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Some common examples of the components are shown below:
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Dienes
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Dienophiles
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Stereoselectivity:
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The Diels-Alder reaction is stereospecific
with respect to both the diene and the dienophile.
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Addition is syn
on both components (bonds form from same species at the same time)
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This is shown by the examples below:
| cis-dienophile gives cis-substituents in the product. |
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| trans-dienophile gives trans-substituents in the product. |
| If both substituents on the diene are Z, then both end up on the same
face of the product |
| If substituents on the diene are E and 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)
