Conjugated Dienes
1) Electrophilic Addition to Conjugated Dienes
a) Conjugated dienes undergo two-step electrophilic addition reactios just as do
simple alkenes. However, certain features are unique to the reactions of
conjugated dienes
b) Addition of one equivalent of HBr to 1,3-butadiene at -78°C gives a mixture of
two constitutional isomers: 3-bromo-1-butene and 1-bromo-2-butene.
CH2 CH CH CH2 + HBr
1,3-Butadiene
-78° C
Br
H
Br
H
CH2 CH CH CH2 + CH2 CH CH CH2
3-Bromo-1-butene
1-Bromo-2-butene
(90%)
(10%)
(1,2-addition)
(1,4-addition)
The designations "1,2-" and "1,4-" used here to describe additions to
conjugated dienes do not refer to IUPAC nomenclature. Rather, they refer to the
four-atom system of two conjugated double bonds and indicate that addition takes
place at either carbons 1 and 2 or 1 and 4 of the four-atom system.
The bromobutenes formed by addition of 1 mole of HBr to butadiene can
in turn undergo addition of a second mole of HBr to give a mixture of
dibromobutenes. Our concern at this point is only with the products of a single
addition of HBr.
Addition of one equivalent of Br2 at -15°C also gives a mixture of 1,2addition and 1,4-addition products.
CH2 CH CH CH2 + Br2
1,3-Butadiene
-15° C
Br
Br
Br
Br
CH2 CH CH CH2 + CH2 CH CH CH2
3,4-Dirbromo-1-butene
1,4-Dibromo-2-butene
(54%)
(46%)
(1,2-addition)
(1,2-addition)
We can account for the formation of isomeric products in the addition of
HBr in the following way. Electrophilic addition is initiated by reaction of a
terminal carbon of one of the double bonds with HBr to form an allylic
carbocation intermediate best represented as a hybrid of two contributing
resonance structures. The addition is completed by rapid reaction of the allylic
cation with bromide ion. Reaction at one carbon bearing partial positive charge
gives the 1,2-addition product; reaction at the other gives the 1,4-addition product.
i) Mechanism:
A resonance stabilized allylic carbocation
H
H
CH2 CH CH CH2 +
CH2 CH CH CH2
H Br
CH2 CH CH CH2
-
-
Br
Br
Br
H
CH2 CH CH CH2
(1,2-addition)
Br
H
CH2 CH CH CH2
(1,4-addition)
c) Kinetic versus Thermodynaic Control of Electrophilic Addition
i) Electrophilic Addition to conjugated dienes gives a mixture of 1,2-addition
and 1,4-addition products.
CH2 CH CH CH2 + HBr
1,3-Butadiene
CH2 CH CH CH2 + Br2
1,3-Butadiene
-78° C
-15° C
Br
H
Br
H
CH2 CH CH CH2 + CH2 CH CH CH2
3-Bromo-1-butene
1-Bromo-2-butene
(90%)
(10%)
(1,2-addition)
(1,4-addition)
Br
Br
Br
Br
CH2 CH CH CH2 + CH2 CH CH CH2
3,4-Dirbromo-1-butene
1,4-Dibromo-2-butene
(54%)
(46%)
(1,2-addition)
(1,2-addition)
Additional experimental observations about the products of electrophilic
additions to 1,3-butadiene.
(1) For addition of HBr at -78°C and addition of Br2 at -15°C, the 1,2-addition
product predominates over the 1,4-addition product. Generally at lower
temperatures, the 1,2-addition products predominate over 1,4-addition
products.
(2) For addition of HBr and Br2 at higher temperatures (generally 40°-60°C),
the 1,4-addition products predominate.
2
(3) If the products of low temperature addition are allowed to remain in
solution and be warmed to a higher temperature, the composition of the
products changes over time and becomes identical to the products obtained
when the reaction is carried out at higher temperatures. The same result
can be accomplished at the higher temperature in a far shorter time by
adding a Lewis acid catalyst, such as FeCl3 or ZnCl2, to the mixture of low
temperature addition products. Thus, under these higher temperature
conditions, an equilibrium is established between the 1,2- and 1,4-addition
products in which the 1,4-addition product predominates.
(4) If either the pure 1,2- or pure 1,4- addition product is dissolved in an inert
solvent at higher temperature and a Lewis acid catalyst added, an
equilibrium mixture of 1,2- and 1,4-addition product forms. The same
equilibrium is obtained regardless of which isomer is used as the starting
material.
Chemists interpret these results using the twin concepts of kinetic control and
equilibrium control of reactions.
For reactions under kinetic (rate) control, the distribution of products is
determined by the relative rates of formation of each. We see the operation of
kinetic control in the following way. At lower temperatures, the reaction is
essentially irreversible and no equilibrium is established between 1,2- and 1,4addition products. The 1,2-addition product predominates under these
conditions because the rate of 1,2-addition is greater than the rate of 1,4addition.
For reactions under thermodynamic (equilibrium) control, the distribution
of products is determined by the relative stability of each. At higher
temperatures, the reaction is reversible and an equilibrium is established
between the 1,2- and 1,4-addition products. The percentage of each product
present at equilibrium is in direct relation to the relative thermodynamic
stability of that product. The fact that the 1,4-addition product predominates at
equilibrium is because it is thermodynamically more stable than the 1,2addition product.
ii) Why is the 1,2-addition product (less stable) formed more rapidly at lower
temperatures?
First, we need to look at the resonance stabilized allylic carbocation. We must
consider the degree of substitution of both the positive carbon and the carboncarbon double bond in each contributing structure.
CH2 CH CH CH3
Less Substituted Double Bond
(Secondary carbocation)
CH2 CH CH CH3
More Substituted Double Bond
(Primary carbocation)
3
A secondary carbocation is more stable than a primary carbocation, and, if the
degree of substitution of the carbon bearing the positive charge were the more
important factor, then the Lewis structure on the left would make the greater
contribution to the hybrid. A more substituted double bond is more stable than
a less substituted one, and, if the degree of substitution of the carbon-carbon
double bond were the more important factor, then the Lewis structure on the
right would make the greater contribution to the hybrid.
We know from other experimental evidence that the location of the
positive charge in the allylic carbocation is more important than the location
of the double bond. Therefore in the hybrid, the greater fraction of the positive
charge is on the secondary carbon. Reaction with bromide ion occurs more
rapidly at this carbon, giving 1,2-addition, simply because it has a greater
density of positive charge.
iii) Is the 1,2-addition product also formed more rapidly at higher temperatures,
even though it is the 1,4-addition product that preodominates under these
conditions?
Yes. The factors affecting the structure of a resonance stabilized allylic
carbocation intermediate and the reaction of this intermediate with a
nucleophile are not greatly affected by changes in temperature.
iv) Why is the 1,4-addition product the thermodynamically more stable product?
Generally, the greater degree of substitution of a carbon-carbon double bond,
the greater the stability of the compound or ion containing it. Following are
pairs of 1,2- and 1,4- addition products. In each case, the more stable alkene is
the 1,4-addition product.
Br
CH3CHCH CH2
H3C
+
BrCH2CHCH CH2
3,4-Dibromo-1-butene
(less stable alkene)
C C
H
3-Bromo-1-butene
(less stable alkene)
Br
H
CH2Br
(E)-1-Bromo-2-butene
(more stable alkene)
BrCH2
+
H
C C
H
CH2Br
(E)-1,4-Dibromo-2-butene
(more stable alkene)
There are cases where the 1,2-addition product is more stable, and would be
the product of thermodynamic control. Addition of Bromine to 1,4-dimethyl1,3-cyclohexadiene under conditions of thermodynamic control gives 3,4-
4
dibromo-1,4-dimethylcyclohexene, because its trisubstituted double bond is
more stable than the disubstituted double bond of the 1,4-addition product.
CH3
Br
CH3
Br
Br
CH3
Br2
high
temperature
CH3
+
Br
CH3
1,4-Addition Product
(less stable)
CH3
1,2-Addition Product
(more stable)
v) What is the mechanism by which the thermodynamically less stable product is
converted to the thermodynamically more stable product?
To answer this we must look at the relationships between kinetic energy,
potential energy, and activation energy. On collision, a part of the kinetic
energy is transformed into potential energy, and, if the increase in potential
energy is equal to or greater than the activation energy for reaction, the
reaction may occur. At the higher temperatures for electrophilic addition of
HBr and Br2 to conjugated dienes, collisions are sufficiently energetic that
ionization of the 1,2-addition product occurs to re-form the resonance
stabilized allylic carbocation intermediate. It can then react again with
bromide ion to form the thermodynamically more stable 1,4-addition product.
At lower temperatures, however, the increase in potential energy on collision
is not sufficient to overcome the potential energy barrier to bring about this
ionization.
vi) Is it a general rule that where two or more products are formed from a
common intermediate, the thermodynamically less stable product is formed at
a greater rate?
No. Whether the thermodynamically more or less stable product is formed at a
greater rate from a common intermediate depends on the particular reaction
and the reaction conditions.
5
2) The Diels-Alder Reaction
a) In 1928, Otto Diels and Kurt Alder in Germany discovered another unique
reaction of conjugated dienes; they undergo cycloaddition reactions with certain
types of carbon-carbon double and triple bonds.
b) The compound with the double or triple bond that reacts with the diene is called a
dienophile, and the product of a Diels-Alder reaction is called the Diels-Alder
adduct. The designation cycloaddition refers to the fact that two reactants add
together to give a cyclic product.
c) Examples:
O
CH2
H C
+
H C
CH2
+
CH2
1,2-Butadiene
(a diene)
HC
C
CH3
CO2CH2CH3
CH2
H C
C
CH2
3-Buten-2-one
(a dienophile)
1,2-Butadiene
(a diene)
H C
O
C
C
CO2CH2CH3
Diethyl acetylenedicarboxylate
(a dienophile)
CH3
4-Cyclohexenyl methyl ketone
(a Diels-Alder adduct)
CO2CH2CH3
CO2CH2CH3
Diethyl1,4-cyclohexadiene1,2-dicarboxylate
(a Diels-Alder adduct)
Note that the four carbon atoms of the diene and two carbon atoms of the
dienophile combine to form a six-membered ring. Note further that there are two
more sigma bonds anf two fewer pi bonds in the product than in the reactants.
This exchange of two (weaker) pi bonds for two (stronger) sigma bonds is a major
driving force of the reaction.
d) Mechanism:
+
6
e) It is one of the few reactions that can be used to form a six-membered ring.
f) It is one of the few reactions that can be used to from two new carbon-carbon
bonds at the same time.
g) It is stereoselective.
h) Limitations: Stereochemistry
i) The diene must be able to assume an 2-cis conformation.
ii) We can illustrate the importance of conformation of the diene by reference to
1,3-butadiene. For maximum stability of a conjugated diene, overlap of the
four unhybridized 2p orbitals making up the pi system must be complete, a
condition that occurs only when all four carbon atoms of the diene lie in the
same plane. It follows then that if the carbon skeleton of 1,3-butadiene is
planar, the six atoms bonded to the skeleton of the diene are in the same plane.
There are two planar conformations of 1,3-butadiene, referred to as the s-trans
and th s-cis conformations where the designation s refers to the carbon-carbon
single bond of the diene. Of these, the s-trans conformation is slightly lower
in energy and, therefore, more stable.
Although s-trans-1,3-butadiene is the more stable conformation, s-cis-1,3butadiene is the reactive conformation in Diels-Alder reactions. In the s-cis
conformation, carbon atoms 1 and 4 of the conjugated system are close
enough to react with the carbon-carbon double or triple bond of the dienophile
and form a six-membered ring.
H
C
H
H
C
C
H
H
C
H
H
H
H
C
C
C H
C H
H
s-trans conformation
(slightly lower in energy)
s-cis conformation
(slightly higher in energy)
i) The Effect of Substituents on Rate.
i) The simplest example of a Diels-Alder reaction is that between 1,3-butadiene
and ethylene, both gases at room temperature. Although this reaction does
occur, it is very slow and takes place only if the reactants are heated at 200°C
under pressure.
H C
CH2
+
CH2
200°C
CH2 pressure
CH2
1,3-Butadiene Ethylene
Cyclohexene
H C
7
ii) Diels-Alders are facilitated by a combination of electron-withdrawing
substituents on one of the reactants and electron-releasing substituents on the
other. For example, placing a carbonyl group (electron-withdrawing because
of the partial positive charge on its carbon) on the dienophile facilitates the
reaction.
H C
H C
O
CH2
+
O
140°C
CCH3
CH
CCH3
CH2
CH2
1,3-Butadiene 3-Buten-2-one
Electron Releasing
Groups
Electron Withdrawing
Groups
CH3
O
CH2CH3
CH (aldehyde)
O
CH(CH3)2
CR (ketone)
C(CH3)3
O
R (other alkyl groups)
COH (carboxyl)
OR (ether)
O
O
COR (ester)
OCR (ester)
NO2 (nitro)
C N (cyano)
Placing electron releasing methyl groups on the diene further facilitates the
reaction.
H3C
+
H3C
CH2
2,3-Dimethyl1,3-butadiene
O
O
CH2
CCH3
CH
30°C
CH2
H3C
CCH3
H3C
3,2-Buten-2-one
8
j) Diels-Alder Reactions can be used to form Bicyclic Systems
i) Conjugated cyclic dienes, in which the double bonds are of necessity held in
an s-cis conformation, are highly reactive in Diels-Alder reactions. Two
particularly useful dienes for this purpose are cyclopentadiene and 1,3cyclohexadiene. In fact, cyclopentadiene is reactive both as a diene and as a
dienophile, and, on standing at room temperature, it forms a Diels-Alder
adduct known by the common name dicyclopentadiene. When
dicyclopentadiene is distilled at its normal boiling point of 170°C, a reverse
Diels-Alder reaction takes place, and cyclopentadiene is reformed.
H
room temp
+
170°C
The Diene The Dienophile
H
Dicyclopentadiene
(endo isomer)
The terms "endo" and "exo" are used for bicycli Diels-Alder products to
describe the orientation of substituents of the dienophile in relation to the twocarbon diene-derived bridge. Exo substituents are on the opposite side from
the diene-derived bridge; endo substituents are on the same side.
the double bond
derived from the
diene
exo (outside) relative to the double bond
endo (inside) relative to the double bond
For Diels-Alder reactions under kinetic control, the endo orientation of the
dienophile is favored. Treatment of cyclopentadiene with methyl propenoate
(methyl acrylate) gives the endo adduct almost exclusively. The exo adduct is
not formed.
O
+
H
OCH3
Cyclopentadiene Methyl propenoate
CO2CH3
OCH3
Methyl bicyclo[2.2.1]hept-5-en-endo-2-carboxylate
9
k) The Conformation of the Dienophile is Retained
i) If the dienophile is a cis isomer, then the substituents cis to each other in the
dienophile are cis in the Diels-Alder adduct. Conversely, if the dienophile is a
trans isomer, substituents that are trans in the dienophile are trans in the
adduct.
O
H
COCH3
H O
COCH3
H
COCH3
COCH3
H O
+
O
Dimethyl cis-2-butenedioate
O
CH3OC
H
Dimethyl cis-4-cyclohexene-1,2-dicarboxylate
H O
COCH3
+
H
COCH3
O
Dimethyl trans-2-butenedioate
COCH3
O
Dimethyl trans-4-cyclohexene-1,2-dicarboxylate
10
l) Mechanism: The Diels-Alder Reaction is a Pericyclic Reaction.
i) As chemists probed for details of the Diels-Alder reaction, they discovered
that there is no evidence for participation of either ionic or radical
intermediates. Thus, the Diels-Alder reaction is unlike any reaction studied
thus far. To account for the stereoselectivity of the Diels-Alder reaction and
the lack of evidence for any intermediates, chemists have proposed that
reaction takes place in a single step during which there is a cyclic
redistribution of electrons. During this cyclic redistribution, bond forming and
bond breaking are concerted (simultaneous). The reaction is pericyclic, that is,
a reaction that takes place in a single step, without intermediates, and involves
the cyclic redistribution of bonding electrons.
2
1
1
1
O
(endo)
3
H
(exo)
C
H3CO
3
(exo)
O
C
OCH3
(endo)
1 New Bonds Form
2 Envelope Flap Moves Up
3 H moves to exo position
CO2CH3 moves to endo position
11