H2C
C
CH2
Cumulated
Isolated
Conjugated
H
H
Two separate noninteracting pi-systems.
C
C
H
H
-MO Diagrams
C
Two separate, mutually
perpendicular pi-systems.
One pi-system containing
4-electrons.
Electrophilic Addition to Dienes
... gives 1,2- and 1,4-addition products.
Br
HBr
CH3
H
CH2Br
H
CH3
CH
HO
aq. H2SO4
CH3
CH3
C
Br
H3C
CH2OH
H3C
CH3
Br
Br
Br2
Br
Can this addition of bromine involve a bromonium ion?
Br
Dienes and polyenes undergo pericyclic reactions involving a rearrangement of pi
electrons...
Electrocyclic reactions
O
What is the driving force for these reactions?
O
Cycloaddition reactions, in which two or more molecules
combine to form a cyclic product.
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.
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.
Bad Dienes
Bad Dienophiles
CO2CH3
H3CO
OCH2CH3
Cl
O
CH3
O
H
Ph
Good dienes will not react with bad dienophiles;
good dienophiles will not react with bad dienes,
but…
CH3
Other cycloaddition processes…
To understand cycloaddition reactions, we need to look at the
Frontier Molecular Orbital interactions…
What are the FMOs?
Ethylene
There are two possible combinations but the FMOs must
interact at the appropriate parts of the reactants…
But which combination is actually involved?
LUMO
LUMO
LUMO 1.2
HOMO -10.6
= 10.7 eV
= 10.9 eV
LUMO 0.3
HOMO -9.5
HOMO
HOMO
Extending conjugation (from ethylene to butadiene)
the LUMO.
the HOMO and
LUMO 1.2
HOMO -10.6
LUMO 0.3
HOMO -9.5
Electron-donating substituents (like OCH3)
Electron-withdrawing substituents (like CN) )
The Diels-Alder reaction involves the
the dienophile.
the HOMO and
the HOMO and
of the diene reacting with the
the LUMO.
the LUMO.
of
CN
OCH 3
LUMO 1.3
HOMO -9.6
LUMO -0.75
HOMO -9.9
E = 11.2 eV
E = 8.8 eV
What about the other cycloaddition reactions?
The Diels-Alder and other
cycloaddition reactions are
examples of orbital symmetry
controlled reactions.
Cycloaddition reactions are only (thermally) allowed processes
when the FMOS have…
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.
H3CO
H
H3CO2C
CO2CH3
H
H3CO
H
H
O
H3CO
H3CO
H
O
O
O
O
H
O
CO2CH3
CO2CH3
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
cisoid
CH3
CH3
CH3
CO2CH2CH3
transoid
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.
H2
C
H2
C
O
H H
C
O
C
O
endo
C
O
H
H
C O
O
exo
Generally, endo adducts are formed
faster than exo adducts in DielsAlder reactions. The reasons for
this are complex.
Diels-Alder
adducts formed
from acetylenic or
from transsubstituted
dienophiles cannot
be endo or exo.
Regioselectivity in Diels-Alder Reactions- Reactions of Unsymmetrical Dienes with
Unsymmetrical Dienophiles
CH3O
CH3O
O
C H
O
C
∆
CH3O
H
58%
These products are...
In total, how many products are possible in this reaction?
C
O
H
A Rationalization of the Ortho Rule
+
CH3O ¶
CH3O
O
¶-
C H
¶+
¶+
CH3O ¶
O
C
¶-
¶+
¶H
O
C H
Practice – Predict the product
H
CO 2CH 3
H
CN
NC
H
O
O
O
NC
H 3CO
CN
Endo/exo-Retro?
COCH 3
H
H 3CO
O
CN
OCH3
H
CHO
Symmetry allowed?
O
O
O
O
O
O
O
O
N
CH3
N
CH 3
H 2C
O
O
O
O
O
O
O
H2 C
O
SiMe 3
SiMe 3
Synthesis
HO
CO2 CH 3
CO2 CH 3
H
HO
H
O
H
O
O
CHO
O
H
Br
CN
Br
H
H
CN
H