Chem 282/3
2026
DIELS-ALDER REACTION
The Diels-Alder Reaction
creates two new carbon-carbon bonds
Otto Diels and Kurt Alder shared the nobel prize in chemistry in 1950
❖ in a Diels-Alder reaction, a conjugated diene reacts with a compound
containing a carbon-carbon double bond (dienophile)
❖
❖
❖
Diels-Alder reaction is
❖ a concerted reaction: the addition of the electrophile and the nucleophile
occurs in a single step
❖ a [4+2] cycloaddition reaction which converts two  bonds into two  bonds
2
The Diels-Alder Reaction
- reactivity of dienophile ❖
the reactivity of the dienophile is increased if one or more electronwithdrawing groups are attached to its sp2 carbons
3
Dienes and Dienophiles
O
NH
O
4
The Diels-Alder Reaction
- examples -
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The Diels-Alder Reaction
- unsymmetrically substituted reactants ❖
❖
if both the diene and dienophile are unsymmetrically substituted, two
products are posibble
two products are possible because the
reactants can align in two different ways
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The Diels-Alder Reaction
- unsymmetrically substituted reactants -
❖ the partial positively charged carbon atom of the dienophile will bond
preferentially to the partial negatively charged carbon of the diene
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The Diels-Alder Reaction
- conformations of the diene ❖
a conformation results from rotation about single bonds
the s-trans conformation is little more stable than the s-cis conformation
(steric strain)
❖ for a Diels-Alder reaction, a conjugated diene must be in an s-cis
conformation
❖
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The Diels-Alder Reaction
- conformations of the diene ❖
when the diene is a cyclic compound, the product of a Diels-Alder reaction
is a bridged bicyclic compound
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The Diels-Alder Reaction
- conformations of the diene ❖
there are two possible configurations for bridged bicyclic compounds
❖ the exo configuration – the substituent points away from the double bond
❖ the endo configuration – the substituent points toward the double bond
❖
the endo product is formed faster when the dienophile has a substituent
with  electrons (due to steric and electrostatic)
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Definition of endo and exo
Exo
Endo
Exo
• Endo and exo are terms used to
identify the stereochemistry of bridged
rings. The point of reference is the
bridge that’s longer or more
unsaturated.
• An ENDO substituent is CLOSER to it
(on the same side)
• An EXO substituent is FURTHER from it
(anti to the longest bridge)
Endo
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The Diels-Alder Reaction
- stereochemistry ❖
if a Diels-Alder reaction creates a product with an asymmetric center,
identical amounts of R and S enantiomers will be formed
❖
the Diels-Alder reaction is
❖ a syn addition reaction with respect to both the diene and the dienophile
❖ a stereospecific reaction
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The Diels-Alder Reaction
- stereochemistry ❖
if the substituents in the dienophile are cis, they will be cis in the product
❖
if the substituents in the dienophile are trans, they will be trans in the
product
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The Diels-Alder Reaction
- stereochemistry ❖
the substituents in the diene will also maintain their relative configurations
in the products
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Retro Diels-Alder reaction
• The Diels-Alder cycloaddition is (in most cases) reversible.
That’s how thermodynamic control can be established to
produce the more stable exo product.
• An example is cyclopentadiene, which dimerizes slowly even
at lower temperatures. To use it as diene, it has to be distilled
fresh from a retro Diels-Alder reaction.
20 oC
2
~ 200 oC
H
H
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A Molecular Orbital Description
of the Diels Alder reaction
HOMO
LUMO
OR
LUMO
HOMO
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The Diels-Alder Reaction
- stereochemistry of bridged compoundsThe endo product, although thermodynamically less stable, is formed
preferentially!
O
Kinetic control
ENDOproduct
O
O +
20°C
O
NH
O
N
O
H
90°C
90°C
O
O
Thermodynamic control
EXOproduct
N H
O
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endo Diels-Alder Adduct
O
O
H
H
O
O
O
O
Diels-Alder reaction of
cyclopentadiene with maleic
anhydride:
Endo-type sequence for the
reaction
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Secondary FMO Interaction in the endo Transition State
HOMO
O
H
N
O
O
LUMO
NH
O
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exo Diels-Alder Adduct
Diels-Alder reaction of
cyclopentadiene with maleic
anhydride:
Exo-type sequence for the reaction
O
O
O
O
O
H
H
O
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The transition state to the exo product
O
O
• Only the primary orbital
interactions are possible
• Sterically less hindered thermodynamically more
stable
O
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Complete the following Diels-Alder reaction
schemes
CO2Me
+
CO2Me
CHO
+
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Complete the following Diels-Alder reaction
schemes
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Complete the following Diels-Alder reaction schemes and
draw the mechanism for each reaction
O
H3COOC
O
O
25 oC
O
O
O
H
COOCH3
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