Conjugated Dienes &
Cycloadditions
© 2000, Paul R. Young
University of Illinois at Chicago, All Rights Reserved
Organic Chemistry OnLine ©2000
Vitamin A: Retinol
CH3 CH3
OH
CH3
A conjugated system consists of a series of
adjacent sp or sp2 centers such that there can
be overlap of -electrons.
Organic Chemistry OnLine ©2000
Vitamin A: Retinol
CH3 CH3
OH
CH3
A conjugated system consists of a series of
adjacent sp or sp2 centers such that there can
be overlap of -electrons.
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Simple Conjugated Systems
Conjugated systems poses a series of
adjacent sp2 or sp centers
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Simple Conjugated Systems
Alkene p-orbitals overlap
to form a system
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Simple Conjugated Systems
Alkene p-orbitals overlap
to form a system
Conjugated double bonds
overlap to form a
continuous system
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Simple Conjugated Systems
The overlap presents an
enhanced barrier to
rotation around single
bonds in conjugated
systems.
Conjugated double bonds
overlap to form a
continuous system
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Synthesis of Conjugated Dienes
Br
-
NBS
(CH3 )3 CO
CCl4
(CH3 )3 COH
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Synthesis of Conjugated Dienes
Br
H
Br
-
NBS
(CH3 )3 CO
CCl4
(CH3 )3 COH
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Synthesis of Conjugated Dienes
Br
H
Br
-
NBS
(CH3 )3 CO
CCl4
(CH3 )3 COH
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Synthesis of Conjugated Dienes
Br
-
NBS
(CH3 )3 CO
CCl4
(CH3 )3 COH
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Synthesis of Conjugated Dienes
Br
-
NBS
(CH3 )3 CO
CCl4
(CH3 )3 COH
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Synthesis of Conjugated Dienes
Br
Br
δ
δ
-
NBS
(CH3 )3 CO
CCl4
(CH3 )3 COH
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Ionic Addition Reactions of Conjugated
Dienes
Br
1,2 addition
3
+ HBr
1
2
4
Br
1,4 addition
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Ionic Addition Reactions of Conjugated
Dienes
Br
1,2 addition
3
+ HBr
1
2
4
Br
1,4 addition
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CH2
H
+ HBr
CH2
H
allylic carbocation
δ
δ
Br
CH2
-
Br
1,4 addition
H
Protonation on the terminal carbon generates the allylic carbocation
with cationic character on both carbons #1 and 3
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1,2 & 1,4 Ionic Addition Reactions of
Conjugated Dienes
The 1,2- addition product forms rapidly at low
temperatures.
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1,2 & 1,4 Ionic Addition Reactions of
Conjugated Dienes
The 1,2- addition product forms rapidly at low
temperatures.
The 1,4-addition product is predominant at higher
temperatures.
Organic Chemistry OnLine ©2000
1,2 & 1,4 Ionic Addition Reactions of
Conjugated Dienes
The 1,2- addition product forms rapidly at low
temperatures.
The 1,4-addition product is predominant at higher
temperatures.
Even at low temperatures, 1,4-addition products will
predominate if given enough time.
Organic Chemistry OnLine ©2000
1,2 & 1,4 Ionic Addition Reactions of
Conjugated Dienes
The 1,2- addition product forms rapidly at low
temperatures.
The 1,4-addition product is predominant at higher
temperatures.
Even at low temperatures, 1,4-addition products will
predominate if given enough time.
The addition of HBr to butadiene is reversible and isolated
1,2-addition product will convert to the 1,4-product at
higher temperatures or at longer times.
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Effect of Temperature and Time on Product
Distribution
Energy
high activation energy,
slower reaction
lower activation energy’
faster reaction
Br
Br
Organic Chemistry OnLine ©2000
Effect of Temperature and Time on Product
Distribution
Energy
high activation energy,
slower reaction
lower activation energy’
faster reaction
Br
Br
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Effect of Temperature and Time on Product
Distribution
Energy
Therefore, 1,2 addition is
faster, but forms a less stable
product, while 1,4 addition is
slower, but gives a more
stable product.
∆G˚
Br
∆G˚
Br
smaller ∆G˚, less
favored at equilibrium,
but formed faster
larger ∆G˚, more
favored at equilibrium,
but formed more slowly
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Effect of Temperature and Time on Product
Distribution
...at equilibrium (Thermodynamic
Energy
Control) the more stable 1,4
addition product will be favored,
but if you examine the initial
product distribution, the more
rapidly formed 1,2 product will
predominate (Kinetic Control).
∆G˚
Br
∆G˚
Br
smaller ∆G˚, less
favored at equilibrium,
but formed faster
larger ∆G˚, more
favored at equilibrium,
but formed more slowly
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kinetic product
+ HBr
CH3
thermodynamic product
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Br
kinetic product
+ HBr
CH3
Br
thermodynamic product
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Br
Br
H
H
3˚ carbocation initially
formed; product also has
less steric hindrance
2˚ carbocation initially
formed
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Cycloaddition Reactions
The Diels-Alder Reaction
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4 + 2 Cycloaddition Reactions
heat
+
...and a
dienophile
a diene...
heat
+
O
O
C
heat
+
CH3
CH3
Organic Chemistry OnLine ©2000
4 + 2 Cycloaddition Reactions
heat
+
...and a
dienophile
a diene...
heat
+
O
O
C
heat
+
CH3
CH3
Organic Chemistry OnLine ©2000
O
O
C
heat
CH3
a diene
CH3
a dieneophile
Cycloaddition reactions work best with dienes containing
electron donating substituents and dienophiles containing
electron withdrawing substituents.
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free rotation
s-trans
s-cis
s-cis stereochemistry is required for a
4+2 cycloaddition reaction
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concerted
transition
state
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a diene
CN
NC
a dieneophile
NC
CN
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©2000
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Drawing the cyclohexene as a “chair” allows the
stereochemistry of attached groups to be clearly described;
the cyclohexene ring, however, is actually distorted due to
the 120˚ bond angles of the sp2 centers.
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Common Dienophiles
O
H
H
H
C
H
H
poor
H
C
C
C
C
H
C C
C
O
O
H
C
C
C
O
CH3
H
H
H
O
H
C
H
H
H
O
H
O
H
C
C
C
C
O
C
C
H
H
H
H
C
C
C
N
C
C
C
OCH3
H
COOCH3
C
C
H
H
Organic Chemistry OnLine ©2000
...benzene can be formed from a 2+2+2 cycloaddition reaction
H
H
H
H
H
H
H
H
H
H
H
H
Organic Chemistry OnLine ©2000
CN
+
+
COOCH3
CHO
+
O
+
C
CH3
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CN
CN
+
+
COOCH3
CHO
+
O
+
C
CH3
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CN
CN
COOCH3
COOCH3
+
+
CHO
+
O
+
C
CH3
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CN
CN
COOCH3
COOCH3
CHO
CHO
+
+
+
O
+
C
CH3
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CN
CN
COOCH3
COOCH3
CHO
CHO
O
O
C
C
+
+
+
+
CH3
CH3
Organic Chemistry OnLine ©2000
CN
+
O
+
O
O
+
C
CH3
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H
CN
+
CN
O
+
O
O
+
C
CH3
Organic Chemistry OnLine ©2000
H
H
+
CN
CN
O
+
O
O
+
C
CH3
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H
H
+
CN
CN
O
+
O
O
+
C
CH3
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H
CN
+
CN
endo isomer
O
+
O
O
+
C
CH3
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CN
CN
exo
H
CN
not observed H
+
CN
H
H
CN
CN
endo
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H
Consider the reaction of
cyclopentadiene with
H butanedial...
CHO
CHO
H
O
H
O
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H
Consider the reaction of
cyclopentadiene with
H butanedial...
CHO
CHO
H
O
H
O
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Consider the reaction of
cyclopentadiene with
butanedial...
H
O
H
O
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Consider the reaction of
cyclopentadiene with
butanedial...
H
O
H
O
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H
With the carbonyl groups
lined up underneath the
double bonds of the diene,
there can be interactions between the
diene and the electron
withdrawing groups on
the dienophile.
O
H
O
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H
H
CHO
CHO
H
O
H
O
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H
CN
+
CN
endo isomer
O
+
O
O
+
C
CH3
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H
CN
+
CN
O
O
O
O
+
O
+
C
CH3
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H
CN
+
CN
O
O
+
O
O
O
+
C
H3 C
C
O
H
CH3
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O
H3 C
C
O
H
H3 C
C
H
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O
H3 C
C
O
H
H3 C
C
H
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H
CN
+
CN
O
O
+
O
O
O
+
C
H3 C
C
O
H
CH3
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CH3
H
CH3 H
+
H
CHO
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CH3
H
CH3 H
+
H
CHO
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CH3
H
CH3 H
+
H
CHO
H
CH3
CH3 H
H
CHO
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CH3
H
CH3 H
+
H
CHO
H
CH3
CH3 H
H
CHO
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H
CH3
H
CH3
CH3 H
CH3 H
+
H
H
CHO
rotate up
CHO
trans stereochemistry
CH3
CH3
H
H
H
CH3
H
H
H
H
CHO
CH3
CH3 H
H
OHC
CH3
CHO
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CH3
H3 C
H
H
+
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CH3
H3 C
H
H
CH3
H3 C
H
H
+
H
rotate up
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CH3
H3 C
H
H
CH3
H3 C
H
H
+
H
rotate up
H3 C
H
CH3
H
H
H3 C
H3 C
H
CH3
cis stereochemistry
H
H3 C
Organic Chemistry OnLine ©2000
cis
1,4-trans
trans
CH3
CH3
H
CH3 H
+
H
H
1
4
H
CHO
CH3
CHO
trans
1,4-cis
trans
CH3
H3 C
H
H
+
H
H
H3 C
1
4
H
CH3
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trans
1,4-trans
cis
1,2- cis
cis
trans
1,4- cis
trans
1,2-trans
trans
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+
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+
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H
CH3
+
H3 C
trans dienophile
H
trans adduct
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CH3
H
+
H
CH3
H3 C
H3 C
H
trans-trans diene
H
cis adduct
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Suggest a synthesis for
each of the following
utilizing a 4 + 2
cycloaddition reaction:
NC
H
O
H
NH 2
O
H
CN
NC
H
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NC
H
O
H
NH 2
O
H
CN
NC
H
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NC
H
CN
+
O
H
NH 2
O
H
CN
NC
H
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NC
H
CN
+
O
H
NH 2
O
H
CN
NC
H
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NC
H
CN
+
O
C
O
+
O
H
NH 2
NH 2
O
H
CN
NC
H
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NC
H
CN
+
O
C
O
+
O
H
NH 2
NH 2
O
H
CN
NC
H
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NC
H
CN
+
O
C
O
+
O
H
NH 2
NH 2
O
H
CN
+
CN
NC
NC
H
Organic Chemistry OnLine ©2000
Suggest a synthesis for
each of the following
utilizing a 4 + 2
cycloaddition reaction:
H
CH3
O
H
H O
O
O
O
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H
CH3
O
H
H O
O
O
O
Organic Chemistry OnLine ©2000
+
heat
H
CH3
O
H
H O
O
O
O
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+
heat
H
CH3
O
H
H O
O
O
O
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+
heat
O
C
+
H
CH3
CH3
O
H
H O
O
O
O
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+
heat
O
C
+
H
CH3
CH3
O
H
H O
O
O
O
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+
heat
O
C
+
H
CH3
O
CH3
O
H
H O
O
O
+
O
O
O
O
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O
O
CH3
H3 C
H3 C
CH3
Cl
H
H
H
Suggest a synthesis for the
molecule shown above utilizing
a 4 + 2 cycloaddition reaction:
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O
O
CH3
H3 C
H3 C
CH3
Cl
H
H
H
The molecule is a diketone, we can make ketones by:
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O
O
CH3
H3 C
H3 C
CH3
Cl
H
H
H
The molecule is a diketone, we can make ketones by:
a) ozonolysis or oxidation of an alkene
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O
O
CH3
H3 C
H3 C
CH3
Cl
H
H
H
The molecule is a diketone, we can make ketones by:
a) ozonolysis or oxidation of an alkene
b) hydration of an alkyne
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O
O
CH3
H3 C
H3 C
CH3
Cl
H
H
H
The molecule is a diketone, we can make ketones by:
a) ozonolysis or oxidation of an alkene
b) hydration of an alkyne
Because ozonolysis will give us both carbonyls at once,
that is our first choice.
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O
O
2
H3 C 1
CH3
Cl
3
6
H3 C
CH3
H
5
H
4
H
The molecule is a diketone, we can make ketones by:
a) ozonolysis or oxidation of an alkene
b) hydration of an alkyne
Because ozonolysis will give us both carbonyls at once,
that is our first choice.
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O
O
CH3
H3 C
H3 C
Cl
H
CH3
H
H
+
O3 ; Zn/H
H3 C
6
1
5
3
H3 C 2
H
4
Cl
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O
O
CH3
H3 C
H3 C
Cl
H
CH3
H
H
+
4+2
O3 ; Zn/H
Cl
H3 C
H
H3 C
Cl
Organic Chemistry OnLine ©2000
H3 C
H3 C
H
H
H
H3 C
H3 C
Cl
Suggest a synthesis for the
molecule shown above utilizing a
4 + 2 cycloaddition reaction:
Organic Chemistry OnLine ©2000
H3 C
H3 C
H
H
H
H3 C
H3 C
Cl
The molecule is a saturated alkyl halide, we must use a
cycloaddition reaction which will yield an alkene. We could
therefore make this molecule by either:
Organic Chemistry OnLine ©2000
H3 C
H3 C
H
H
H
H3 C
H3 C
Cl
The molecule is a saturated alkyl halide, we must use a
cycloaddition reaction which will yield an alkene. We could
therefore make this molecule by either:
a) reduction of an alkene
Organic Chemistry OnLine ©2000
H3 C
H3 C
H
H
H
H3 C
H3 C
Cl
The molecule is a saturated alkyl halide, we must use a
cycloaddition reaction which will yield an alkene. We could
therefore make this molecule by either:
a) reduction of an alkene
b) addition of HCl to an alkene
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H3 C
H3 C
H
H
H
H3 C
H3 C
Cl
The molecule is a saturated alkyl halide, we must use a
cycloaddition reaction which will yield an alkene. We could
therefore make this molecule by either:
a) reduction of an alkene
b) addition of HCl to an alkene
Because the double bond will form between the carbons of
the diene, reduction of that double bond is our first
choice.
Organic Chemistry OnLine ©2000
H
H3 C
H
H
H3 C
H3 C
Cl
H3 C
H2 /Pt
H3 C
H
H3 C
Cl
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H
H3 C
H
H
H3 C
H3 C
Cl
H3 C
4+2
Cl
H2 /Pt
H3 C
H
H3 C
Cl
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HOOC
COOH
Br
Suggest a synthesis for the
molecule shown above utilizing a
4 + 2 cycloaddition reaction:
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HOOC
COOH
Br
The molecule is a dicarboxylic acid. The only way we know
to make carboxylic acids is by:
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HOOC
COOH
Br
The molecule is a dicarboxylic acid. The only way we know
to make carboxylic acids is by:
oxidation of an alkene
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HOOC
COOH
Br
The molecule is a dicarboxylic acid. The only way we know
to make carboxylic acids is by:
oxidation of an alkene
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H
H
C
C
Br
The molecule is a dicarboxylic acid. The only way we know
to make carboxylic acids is by:
oxidation of an alkene
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HOOC
COOH
Br
+
MnO4 /H
H
Br
Organic Chemistry OnLine ©2000
HOOC
COOH
Br
4+2
+
MnO4 /H
Br
H
Br
Organic Chemistry OnLine ©2000
Suggest a synthesis for the molecule shown on the right,
using a cycloaddition reaction.
H
CN
H3 C
H
Organic Chemistry OnLine ©2000
Suggest a synthesis for the molecule shown on the right,
using a cycloaddition reaction.
H
CN
+
CN
H3 C
H3 C
H
Organic Chemistry OnLine ©2000
Suggest a synthesis for the molecule shown on the right,
using a cycloaddition reaction.
CN
CN
Organic Chemistry OnLine ©2000
Suggest a synthesis for the molecule shown on the right,
using a cycloaddition reaction.
CN
CN
CN
CN
+
Organic Chemistry OnLine ©2000
Suggest a synthesis for the molecule shown on the right,
using a cycloaddition reaction.
H
CH3
CH3
CN
H3 C
H
H
H
Organic Chemistry OnLine ©2000
trans
1,4-trans
cis
1,2- cis
cis
trans
1,4- cis
trans
1,2-trans
trans
Organic Chemistry OnLine ©2000
Suggest a synthesis for the molecule shown on the right,
using a cycloaddition reaction.
H
CH3
CH3
CN
H3 C
H
H
H
Organic Chemistry OnLine ©2000
Suggest a synthesis for the molecule shown on the right,
using a cycloaddition reaction.
CH3
CN
+
CH3
H
CH3
CH3
CN
H3 C
CH3
H
H
H
Organic Chemistry OnLine ©2000
Suggest a synthesis for the molecule shown on the right,
using a cycloaddition reaction.
CH3
CH3
H
NC
H
H
Organic Chemistry OnLine ©2000
Suggest a synthesis for the molecule shown on the right,
using a cycloaddition reaction.
CH3
CH3
H
NC
H
H
Organic Chemistry OnLine ©2000
Suggest a synthesis for the molecule shown on the right,
using a cycloaddition reaction.
CH3
CH3
CN
CH3
H
+
NC
CH3
H
H
Organic Chemistry OnLine ©2000
Suggest a synthesis for the molecule shown on the right,
using a cycloaddition reaction.
H3 C
CN
CN
Organic Chemistry OnLine ©2000
Suggest a synthesis for the molecule shown on the right,
using a cycloaddition reaction.
H3 C
CN
+
CN
H3 C
CN
CN
Organic Chemistry OnLine ©2000
Suggest a synthesis for the molecule shown on the right,
using a cycloaddition reaction.
H
CH3
CH3
CN
H
H
CH3
H
Organic Chemistry OnLine ©2000
Suggest a synthesis for the molecule shown on the right,
using a cycloaddition reaction.
CH3
H
CH3
CN
CH3
CN
+
H
CH3
CH3
H
CH3
H
Organic Chemistry OnLine ©2000
Cycloadditions & Isomerism
Draw all of the potential isomers from the cycloaddition
reaction shown below.
CH3
Cl
Organic Chemistry OnLine ©2000
CH3
H
Cl
H
Cl
First, you must consider
that attack can occur
from either the top, or the
bottom face.
H
CH3
H
Organic Chemistry OnLine ©2000
CH3
H
Cl
H
Cl
First, you must consider
that attack can occur
from either the top, or the
bottom face.
H
CH3
H
Organic Chemistry OnLine ©2000
CH3
Cl
First, you must consider
that attack can occur
from either the top, or the
bottom face.
H
Cl
H
H
CH3
H
H
H
CH3
Cl
Organic Chemistry OnLine ©2000
CH3
Cl
First, you must consider
that attack can occur
from either the top, or the
bottom face.
H
Cl
H
CH3
H
H
H
H
Cl
H
CH3
H3 C
H
Cl
Organic Chemistry OnLine ©2000
CH3
Cl
First, you must consider
that attack can occur
from either the top, or the
bottom face.
H
Cl
H
H
H
Cl
The compounds are
enantiomers.
H3 C
CH3
H
CH3
H
H
H
Cl
Organic Chemistry OnLine ©2000
Therefore, all additions will lead to the formation of a pair
of enantiomers.
Cl
CH3
CH3
H
H
H
H
CH3
H
H
H
Cl
H
H
H
Cl
A pair of enantiomers;
a racemic mixture.
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Next, the dienophile can approach the diene from either the
endo (favored) or exo face.
CH3
H
endo
Cl
H
CH3
H
exo
H
Cl
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Next, the dienophile can approach the diene from either the
endo (favored) or exo face.
endo
Cl
CH3
CH3
H
H
H
Cl
H
CH3
H
exo
H
Cl
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Next, the dienophile can approach the diene from either the
endo (favored) or exo face.
endo
Cl
CH3
CH3
H
H
H
Cl
H
H
H
CH3
H
H
H
H
Cl
CH3
H
exo
H
Cl
Organic Chemistry OnLine ©2000
Next, the dienophile can approach the diene from either the
endo (favored) or exo face.
endo
Cl
CH3
CH3
H
H
H
Cl
H
H
CH3
H
H
H
H
exo
H
CH3
CH3
H
H
Cl
H
H
Cl
Cl
Organic Chemistry OnLine ©2000
Next, the dienophile can approach the diene from either the
endo (favored) or exo face.
endo
Cl
CH3
CH3
H
H
H
Cl
H
H
CH3
H
H
H
H
exo
H
CH3
CH3
H
H
Cl
H
H
H
Cl
H
H
CH3
Cl
H
Cl
H
H
Organic Chemistry OnLine ©2000
Next, the dienophile can approach the diene from either the
endo (favored) or exo face.
endo
Cl
CH3
CH3
H
H
H
Cl
H
H
CH3
H
H
H
H
exo
H
CH3
CH3
H
H
Cl
H
H
Cl
(Each as a pair of
enantiomers.)
H
H
H
CH3
Cl
H
Cl
H
H
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Further, the dienophile can approach syn or anti to the methyl
group on the diene.
exo
endo
H
CH3
H
H
syn
Cl
H
H
CH3 H
H
H
H
Cl
CH3
Cl
H
H
H
CH3
anti
Cl
Organic Chemistry OnLine ©2000
Further, the dienophile can approach syn or anti to the methyl
group on the diene.
exo
endo
H
CH3
H
CH3
H
H
H
H
H
Cl
H
Cl
H
anti
H
CH3 H
Cl
CH3
H
H
H
syn
Cl
H
H
H
CH3 H
H
H
H
Cl
endo
CH3
H
Cl
H
H
exo
Organic Chemistry OnLine ©2000
Further, the dienophile can approach syn or anti to the methyl
group on the diene.
exo
endo
H
CH3
H
Cl
H
e
H
CH3
i
H
CH3 H
CH3
Cl
H
Cl
H
H
H
H
anti
H
H
H
syn
Cl
H
H
H
CH3 H
H
H
H
Cl
endo
CH3
H
Cl
H
H
exo
Organic Chemistry OnLine ©2000
Further, the dienophile can approach syn or anti to the methyl
group on the diene.
exo
endo
H
CH3
H
H
Cl
H
e
H
CH3 H
H
H
syn
i
H
CH3
Cl
H
Cl
H
H
These two are diasteromers.
H
CH3
H
anti
Cl
H
H
H
CH3 H
H
H
H
Cl
endo
CH3
H
Cl
H
H
exo
Organic Chemistry OnLine ©2000
Further, the dienophile can approach syn or anti to the methyl
group on the diene.
exo
endo
H
CH3
H
H
Cl
e
H
H
CH3 H
H
H
syn
i
H
CH3
Cl
H
Cl
H
H
These two are diasteromers.
H
CH3
H
anti
Cl
H
i
H
H
e
H
Cl
endo
H
H
CH3 H
CH3
H
Cl
H
H
exo
Organic Chemistry OnLine ©2000
Further, the dienophile can approach syn or anti to the methyl
group on the diene.
exo
endo
H
CH3
H
H
Cl
e
H
H
CH3 H
H
H
syn
i
H
CH3
Cl
H
Cl
H
H
These two are diasteromers.
These two are also diasteromers.
H
CH3
H
anti
Cl
H
i
H
H
e
H
Cl
endo
H
H
CH3 H
CH3
H
Cl
H
H
exo
Organic Chemistry OnLine ©2000
The two sets are isomers, but are different chemical compounds.
exo
endo
H
CH3
H
H
Cl
e
H
H
CH3 H
H
H
syn
i
H
CH3
Cl
H
Cl
H
H
cis- and trans-3-chloro-2-methylcyclohexene
cis- and trans-4-chloro-2-methylcyclohexene
CH3
H
anti
Cl
H
H
i
H
H
e
H
Cl
endo
H
H
CH3 H
CH3
H
Cl
H
H
exo
Organic Chemistry OnLine ©2000
Cycloadditions & Isomerism
Conclusion: the reaction yields two sets of diastereomers
and their mirror images.
H
H
H
H
H
H
CH3
Cl
H
CH3 H
H
Cl
H
H
H
CH3
Cl
H
H
H
H
H
CH3 H
H
H
Cl
H
CH3
H
Cl
H
H
(Each compound is formed as a
pair of enantiomers.)
Organic Chemistry OnLine ©2000