H2C
C
CH2
Cumulated
Isolated
Two separate noninteracting pi-systems.
Conjugated
One pi-system containing
4-electrons.
H
C
H
C
C
H
H
Two separate, mutually
perpendicular pi-systems.
H
H2C
H2C
=
=
CH2
C
C
H
CH2
Electrophilic Addition to Dienes
Br
HBr
CH
CH3
H
CH2Br
H
CH3
The mechanism is the that of monoalkenes but involves the formation of an allylic
carbocation.
H
Br
Br
CH3
H
CH3
Br
Br
CH
CH3
HO
aq. H2SO4
CH3
CH3
C
Br
H
CH2
H
CH3
H
CH2Br
H3C
CH2OH
H3C
CH3
Br
Br
Br2
Br
Dienes and polyenes undergo pericyclic reactions involving a rearrangement of pi
electrons...
Electrocyclic reactions
Sigmatropic Rearrangements
O
Cycloaddition Reactions
CH2
CH2
CH2
CH2
hν
The most important cycloaddition reaction is the Diels-Alder reaction:
This is the prototypical Diels-Alder
reaction. The products are usually referred
to as Diels-Alder adducts. These adducts
always have a cyclohexene core.
Cycloaddition Reactions: The Diels-Alder reaction
The Diels-Alder reaction has two components: the diene and the dienophile.
C
N
CN
CO2CH3
CO2CH3
O
O
CO2CH3
CO2CH3
CH2
CH2
C
H2
O
O
O
Acetylenic dienophiles give 1,4cyclohexadiene
products.
Cyclic dienes give bicyclic
adducts.
In some cases, the diene can also
act as a dienophile such as in the
dimerization of cyclopentadiene.
Diels-Alder
reactions
are
equilibrium reactions and so are
reversible.
Substituent Effects on the Reaction Rate
Diels-Alder reactions work best for electron-rich dienes, though unactivated
dienes (like butadiene) react easily.
Electron poor dienophiles (that have a C=O, C≡N, directly attached) are usually
necessary for successful reaction.
Good Dienes
Good Dienophiles
H3C
H3CO
O
CO2CH3
H
H3CO2C
Me3SiO
CN
CO2CH2CH3
CO2CH3
O
OCH3
O
Danishefsky's
Diene
CO2CH2CH3
O
O
Bad Dienes
Bad Dienophiles
CO2CH3
H3CO
OCH2CH3
Cl
O
CH3
O
H
Ph
CH3
The Diels-Alder reaction is an orbital symmetry controlled process.
It involves the π-orbitals...
CH2
π-MOs of butadiene
These show the phases of
the 2p atomic orbitals
reflected in the π-MOs
Ψ4
CH2
Ψ3
4x2p
Ψ2
Ψ1
CH2
π-MOs of ethylene
CH2
2x2p
Because the Diels-Alder reaction involves the reaction of a 4π electron system and a 2π
electron system, it is referred to as a 4 + 2 cycloaddition.
The Diels-Alder reaction involves the in-phase overlap of the HOMO of the diene with
the LUMO of the dienophile. The molecules stack in the transition state.
Stereochemistry of the Diels-Alder Reaction
If the dienophiles substituents are trans before the reaction, they are trans after the
reaction. The converse is also true, of course - cis substituted dienes give cis products.
H
H3CO
H3CO2C
CO2CH3
H
H3CO
H
H
O
H3CO
CO2CH3
O
H
H3CO
CO2CH3
O
O
H
O
O
The Diene must be in a cisoid conformation in order to react. Note how the
stereochemistry of the two terminal substituents determines their stereochemistry in the
final product.
CH3
CH3
The Diene must be in a cisoid
conformation in order to react.
Note how the stereochemistry of
the two terminal substituents
determines their stereochemistry in
the final product.
CH3
CO2CH2CH3
CH3
CH3
CH3
CO2CH2CH3
C
C
CH3
CO2CH2CH3
CO2CH2CH3
CH3
Note that when acetylenic dienophiles are used in the Diels-Alder reaction, a
cyclohexadiene is the product.
Endo and Exo Products in Diels-Alder Adducts - Bicyclic products
O
C
O
CH2
C
H
O
C
0°C
short time
O
H
C
O
O
C
C
H
O
H O
trace
O
98% Yield
H2
C
What is the stereochemical
relationship between these to
isomers?
How many chiral centers are
formed in this reacton?
H2
C
O
H H
C
O
C
C
O
O
H
H
C O
O
These adducts are distinguished by the terms endo and
exo.
If the dienophile substituent sticks into the
cavity (i.e. the boat of the six-membered cyclohexene ring)
then the product is an endo adduct. If the substituent is out
of the cavity, then it is an exo adduct.
O
H
O
O
O
H
H
C
H
C
H
H2
C
H
H
CO2CH3
H
H
H
CO2CH3
Regioselectivity in Diels-Alder Reactions- Reactions of Unsymmetrical Dienes with
Unsymmetrical Dienophiles
CH3O
CH3O
O
C H
O
C
∆
CH3O
H
C
58%
H
O
These products are...
A Rationalization of the Ortho Rule
+
CH3O ∂
CH3O
O
∂−
O
C H
C H
∂+
∂−
+
CH3O ∂
O
C
∂−
H
∂+
∂−
Similar arguments predict (correctly) the outcome of this reaction:
H3CO
CH3O
C H
O
CH3O
C
O
H
In total, how many products are possible in this reaction?
O
C
H
H
H
O
O
C
C
H
CH2
O
O
C
H
H
CH3
C
H
H
C
H
CH3
CH2
O
C
H
H
CH2
CH3
H
OHC
C
H C
O
C
C
C H
CH3
C
C
H
O
C H
H C
CH2
H
O
H
H
C
C
O
CHO
CH3
H
O
C
H
H