conjugation in alkadienes and allylic systems conjugare is a latin verb meaning "to link or...
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Conjugation in Alkadienes andConjugation in Alkadienes andAllylic SystemsAllylic Systems
conjugare is a Latin verb meaning conjugare is a Latin verb meaning
"to link or yoke together" "to link or yoke together"
Isolated – p system on a single pair of adjacent atoms.
Extended – p system on a longer series of atoms. This gives extended chemical reactivity.
Classification of Allylic Systems
Conjugated:
Most stable
Requirements: Continuous π systems with adjacent “p” orbitals overlapping.
Bonding Energy: Extra bonds between p systems.
Reactivity: Reactivity differs depending on specific diene and other chemicals involved.
Types of Dienes
Continuous, overlapping p-orbitals.
Isolated:
Less stable than conjugated.
Requirements: π systems separate and are isolated by an sp3 center.
Bonding Energy: Standard bonding.
Reactivity: Same as simple alkenes.
Types of Dienes
sp3 center
Alkene p-orbital overlap.Alkene p-orbital overlap.
Cumulated:
Least stable.
Requirements: Double bonds share the sp hybridization of middle carbon.
Bonding Energy: Wrong angle, there is no overlap.
Reactivity: Same as simple alkynes.
Types of Dienes
CC C
Linear arrangement of carbons causes a nonplanar geometry.
Learning Check
Which of the following have conjugated double bonds:
A. B.
C. D.
Name Line Diagram π system Type Resonance
Propene Isolated No
1,2-propadiene Cumulated No
1,3-butadiene Conjugated Yes
1,4-pentadiene Isolated No
Dienes
CH2 C CH2
Name Line Diagram π system Type Resonance
1,3-cyclopentadiene Conjugated Yes
1,3-cyclohexadiene Conjugated Yes
1,4-cyclohexadiene Isolated No
Benzene Conjugated Yes
Dienes
sp 2sp
Bonding in Allene
sp 2
The Double Bond as a Substituent
allylic carbocation
C+C
C
allylic radical
•CC
C
conjugated diene
CCC
C
The fact that a tertiary allylic halide undergoessolvolysis (SN1) faster than a simple tertiary
alkyl halide…
Cl
CH3
CH3
CH3
relative rates: (ethanolysis, 45°C)
123 1
Allylic Carbocations Stability
CCl
CH3
CH3
CH2C CH
Provides good evidence that allylic carbocationsare more stable than other carbocations.
CH3
H2C=CH— stabilizes C+ better than does CH3—
Allylic Carbocations Stability
C C
CH3
CH3
H2C CH + +
CH3
CH3
-Must have π systems – double or triple bonds must be present.
-π electrons change positions in resonance contributors curved arrows.
-Molecular structure is composite of all the resonance contributors, with the most favorable contributing the most character.
-More resonance leads to more stability:
Resonance
C+
C+
Delocalization of electrons in the doublebond stabilizes the carbocation.
Stabilization of Allylic Carbocations
Resonance Model
CH3
CH3
H2C CH +C
CH3
CH3
H2C CH+
C
C
CH3
CH3
H2C CH ++
Allylic Free Radicals are Stabilized byElectron Delocalization
CC
C • CC
C•
Vinylic versus Allylic
CC
C
vinylic carbonsallylic
carbon
Vinylic hydrogens are attached to vinylic carbons.
Vinylic versus Allylic
CC
C
H
H
H
Allylic hydrogens are attached to allylic carbons.
Vinylic versus Allylic
CC
C
H
H
H
Vinylic versus Allylic
CC
C
X
X
X
Vinylic substituents are attached to vinylic carbons.
Vinylic versus Allylic
CC
CX
X
X
Allylic substituents are attached to allylic carbons.
Learning Check
How many allylic and vinylic hydrogens respectively are in cyclohexene?
A) 2 and 4
B) 4 and 2
C) 2 and 2
D) 6 and 2
Resonance
Molecular Orbitals
Resonance Hybrid
Allylic Carbocations
C+
C+
+ +
1/2+ 1/2+
Resonance
Molecular Orbitals
Resonance Hybrid
Allylic Radicals
Allylic Carbocations/Radicals
Either radical C’s are attacked by a radical.
Either δ+ C’s attacked by nucleophiles
Reaction Site
Delocalizing radical is stabilizing.
Delocalizing of charge is stabilizing.
Delocalized
On Terminal C’s, never on a middle C.
On Terminal C’s, never on a middle C.
Position
Resonance.Double bonds donates electron density.
Stabilization
RadicalsCarbocations
Allylic Carbocations/Radicals
Allylic bonds are often weaker and are easily
broken.
Bond Dissociation Energies
One π= 2 R groups ~ 2-propyl radical
One π= 2 R groups ~ 2-propyl cation
Stabilization
Lower energy.
Radical intermediates.
Lower energy.
Carbocation intermediates.
Intermediates
RadicalsCarbocations
+ +
A comparison of bond energies associated with radicals and allylic radicals:
Radical Bond Energies
H+ H
H
410 KJ/mol
368 KJ/mol+ H
ClCH2CHCH3
Cl
addition
500 °C
substitution
CHCH3H2C + Cl2
CHCH2ClH2C
+ HCl
Chlorination of Propene
Reaction Type: Radical Substitution.
Overall Reaction: Alkene Allyl halide and HX.
Reactivity Order: 3 > allyl > 2 > 1 > methyl
Regioselectivity: Substitution at the allylic position
due to the stability of the allylic radical
(resonance).
Stereoselectivity: None.
Requirements: Br2 or Cl2 (with light), or
N-bromosuccinimide (NBS) which can act as a source of Br2
Allylic Halogenation
N
O
O
Br
Step 1 (Initiation):
First step in radical halogenation of an allylic system is to perform homolytic cleavage of a diatomic halogen by heat or UV light.
Mechanism, Step 1
Br Br Br Br
Step 2 (Propagation):
Step 2 has two steps.
The first is the radical abstraction of H by Br
The second step adds Br to the radical and creates another Br radical.
Mechanism, Step 2
Br C
HH Br C
CBr Br BrC
Br
Step 3 (Termination):
Step 3 has three steps which ends the radical reaction. Three different products are made.
The first product forms Br2 again.
The second product forms the expected allyl bromide.
The third product is a byproduct of the two radical carbons linking together
Mechanism, Step 3
Br Br
CBr
CC
C
Br
Br Br
CC
N-Bromosuccinimide
O
O
NBr
CCl4
Br
+heat
(82-87%)
+
O
O
NH
all of the allylic hydrogens are equivalent
and
the resonance forms of allylic radicalare equivalent.
Limited Scope
Allylic halogenation is only used when:
Example
Cyclohexene satisfies both requirements.
Both resonance forms are equivalent.
H
H
H
•
H
H
H•
All allylichydrogens areequivalent.
H H
HH
Example
2-ButeneAll allylichydrogens areequivalent.
CH3CH CHCH3
Two resonance forms are not equivalent;gives mixture of isomeric allylic bromides.
But
•CH3CH CH CH2
•CH3CH CH CH2
Learning Check
Which radical would be more stable?
•CH3CH CH CH2
•CH3CH CH CH2
Example
2-ButeneAll allylichydrogens areequivalent.
CH3CH CHCH3
Two resonance forms are not equivalent;gives mixture of isomeric allylic bromides.
forms
CH3CH CH CH2 CH3CH CH CH2
Br Br
and
What is the other product formed in the reaction shown here?
A) B)
C) D)
Learning Check
Thermodynamic Factors: Corresponds to the relative stability of the products.
Kinetic Factors: Is the rate at which the product is formed.
It is possible to start off with the same material and receive two different products via different pathways.
Kinetic vs. Thermodynamic Control
Pathway 1 vs. Pathway 2
Kinetic vs. Thermodynamic Control
Reaction Coordinate
Energy
TS1
SM
P1
TS2
P2
Reaction 1 (solid) generates P1.
Faster reaction: More stable transition state.
Transition State 1 (TS1) has a lower activation barrier (ΔHact)
Product 1 (P1) is the kinetic product.
Reaction 2 (dash) generates P2.
P2 is the more stable product.
P2 has lower energy than P1
P2 is the thermodynamic product.
Increase in temperature: Average energy of the molecules increases.
Low Temperatures:
Preferred Path: Path similar to P1 on previous slide.
Reaction 1: Irreversible – it lacks the energy to go back to starting material.
Reaction 2: Is also irreversible.
Product Ratio: Is determined by the rates of formation for P1 and P2, where the rates are k1:k2
Control: Kinetic control
Control and Temperature
Intermediate Temperatures:
Preferred Path: Path similar to P1 (lower transition state), but less favored.
Reaction 1: Is reversible.
Reaction 2: Is irreversible.
Product Ratio: Dependent on time of reaction (a longer time of reaction results in more product 2 (P2).
P1 forms initially then over time goes back to starting material, then forms the more stable P2.
Major product: Depends on time of reaction
Short (time): P1
Long (time): P2
Control: Variable
Control and Temperature
High Temperatures:
Preferred Path: Path 1 is preferred, but then goes through Path 2.
Reaction 1: Is reversible.
Reaction 2: Is reversible.
Product Ratio: Dependent on equilibrium constants between P1 and P2 (K1:K2)
Major product: Depends on time of reaction, but end result is more of P2
Short (time): P1.
Long (time): P2.
Control: Variable.
Control and Temperature
Dienes can be prepared by elimination reactions of unsaturated alkyl halides and alcohols.
Elimination favors the most stable product.
Conjugated dienes major product are more stable than isolated dienes unless structure doesn’t allow.
Preparation of Conjugated Dienes
OH BrKOH
heatKHSO4
heat
1:
2:
Predict the Products
++
KOH
heat
Br2
CH2Cl2
N Br
O
O1,3-butadiene
KOH
heat
Dienes undergo electrophilic addition reactions similar to alkenes:
Isolated dienes: Double bonds react independently of one another, and therefore react like alkenes.
Cumulated dienes: React more like alkynes
Conjugated dienes: Conjugated C=C may change the reaction.
Dienes act as nucleophiles, reacting with electrophiles.
Reactions of Dienes
Nu E
Nu
EE
Nu
Three types of electrophilic addition of dienes:
Reaction with H-X:
Reaction with X2:
Reactions of Dienes
HX +
H
X
+X
H
1 42 31 42 3 1 3 42
Direct addition (1,2)
Conjugate addition (1,4)
X2 +
X
X
+X
X
1 42 31 42 3 1 3 42
Direct addition (1,2)
Conjugate addition (1,4)
Note the numbering scheme from the previous slide. The 1,2 and 1,4 addition will be discussed in detail in upcoming notes.
Third Reaction type:
Reaction with other C=C (Diels Alder):
Reactions of Dienes
+heat
Proton adds to end of diene system.
Carbocation formed is allylic.
H X
H
+
Introduction to 1,2 and 1,4 Addition
Example: H
H
H
H
H
H
HCl
Cl
HH
H
H
H
HH
?
H
H
HH
H
Cl
HH
?
HCl
Example: H
H
H
H
H
H
Cl
HH
H
H
H
HH
via: HH
H
H
H
HH
+
H
H
H
H
H
H
H X H
H
H
H
H
HH
+
Protonation of the end of
the diene unit gives an
allylic carbocation.
and: HH
H
H
H
HH
+ HH
H
H
H
HH
+
Cl–
Cl
HH
H
H
H
HH
H
HH
H
H
HH
Cl
3-Chlorocyclopentene
1,2-addition of XY
XX
YY 1,4-addition of XY
XX
YY 1,2-Addition versus 1,4-Addition
Via resonance XX
++
Two types of addition:
Direct: H-X adds directly across the ends of a C=C (1,2-addition)
Conjugate: H-X adds across the ends of a conjugated system (1,4-addition).
Distribution of product depends on conditions:
Addition of Hydrogen Halides to Dienes
81% 9%
44% 56%
BrBr+
HBr
-80 C
+ H H+
+
Br- Br-
2o 1o
20 C
Addition of Hydrogen Halides to Dienes
More substituted alkene
With secondary cationStructure
Product – more stable system
Rate of the fastest reaction and the more
stable carbocation
Control
EquilibriumRateDetermination
Reversible conditionsIrreversible conditionsReversibility
ThermodynamicKineticControl
Room TempLow TempConditions
Learning Check
Which is the product when the reaction shown is carried out under thermodynamic control?
A) B)
C) D)
Heated samples:
When heating pure samples, the major product is the direct product (1,2 addition)
PLACEHOLDER
Major product depends on conditions of the conjugate. More stable E?
Addition of Hydrogen Halides to Dienes
Reaction Type: Conjugate addition/electrophilic addition.
Overall Reaction: Diene + Dienophile (alkene) Cyclic Alkene.
Stereoselectivity: Syn and Endo or Exo addition to a cyclic compound.
Requirements: Diene + Dienophile, high temp or EDG on diene/EWG on dienophile.
Diels-Alder Reaction
W
Z
B
DC
AX
Y
W X
Y Z
A
BC
D
+ E
W X
Y Z
B
AD
C
+ E+
Mechanism
Concerted process: Happens all at once. This makes the reaction very regio- and stereoselective.
Thermodynamically favorable: 2 pi-bonds 2 new stronger sigma bonds.
heat
Aromatic like transition state.
Simple Diels Alder Examples:
1,3-butadiene + ethene cyclohexene
Diels-Alder Reaction
diene alkenediene dienophile
1,3-butadiene + ethyne 1,4-cyclohexadiene
diene dienophile diene alkyneheat
heat
Diels-Alder Reaction Forms
PLACEHOLDER SLIDE
Diels-Alder Reaction Forms
PLACEHOLDER SLIDE
Learning Check
Which diene is shown in its s-cis conformation?
A) B)
C) D)
Diels-Alder Reactivity
The most reactive dienophiles have an electron-withdrawing group (EWG) directly attached to the double bond.
Typical EWGs
C O
C N
C C
EWG
Diels-Alder Reactivity
The most reactive dienes have an electron-donating group (EDG) directly attached to nucleophilic diene.
Typical EDGs
EDG -NH2
-OH
Effect of Electron Donor/Acceptors
A molecular orbital look at the effect of electron donor/acceptors
HOMO
Orbital energy
Better Acceptor Groups
Diene Dieneophile
LUMO
Better Donor Groups
+
solvent 100°C
H2C CHCH CH2 H2C CH CH
O
(100%)
CH
O
Example
CH
Ovia:
+
solvent 100°C
O
O
O
Example
CHC CH2H2C
CH3
(100%)
H3C
O
O
O
H3C
via:O
O
O
Example Diels-Alder Questions
1. Rank the relative reactivity towards 1,3-cyclopentadiene of the following:
CH3
CO2Me OO
#1#2#3
Reason: EWG (C=O) provide resonance, thereby stabilizing the reaction.
iiiiiiiiiiii
Example Diels-Alder Questions
2. Rank the relative reactivity towards dimethyl cis-butendioate of the following:
#2#1#3
Reason: II is locked in the s-cis conformation, III can change to the s-cis, but I is locked in the s-trans conformation.
O
O
O
O
CH3
CH3
ii iiii iiiiii
Example Diels-Alder Questions
3. Rank the order of the relative reactivity towards 3-buten-2-one of the following
#2#1#3
Reason: Benzene is not a diene and would lose aromaticity (a bad thing), so is unreactive, III is sterically destabilized by the reactive s-cis conformation, making is less favorable than II.
CH3CH2
O
iii iii
Common Diels-Alder Reactants
Common Dienes:
Common Dienophiles:O
CO2Me
CO2Me
O
O
O
O
O
CN CO2Me
MeO2C
CO2Me
CO2Me
Two different conformations are possible:Endo: Dienophile is ‘under’ diene. Kinetic product.Exo: Dienophile is exposed or out. Thermodynamic product.
Endo conformations are generally the major product with exo being a minor product.
Reactions with Cyclic Dienes
O
O
OO
O
O+
O
O
O
+
Reactions with Cyclic Dienes
Secondary overlap favors the endo transition state.
H
OR
C
H
OR
C
H
O
H
O
R
R
ENDO EXO
Secondary interactions
New bonds
Diels-Alder: Both the diene and the dienophile are syn.
Cis-dienophile: cis substituted product.
Trans-dienophile: trans substituted product.
Both diene and alkene are Z both on the same side of the product.
Dienes and alkene are E and Z Are on opposite side of the product.
Diels-Alder Reaction is Stereospecific*
*A stereospecific reaction is one in which stereoisomeric starting materials yield products that are stereoisomers of each other; characterized by terms like syn addition, anti elimination, inversion of configuration, etc.
+H2C CHCH CH2
Example
C C
C6H5 COH
H H
O
only product
H
C6H5
H
COH
O
+H2C CHCH CH2
Example
C C
C6H5
COHH
H
O
only product
H
C6H5
H
COH
O
Diels-Alder Reaction is Stereospecific Examples
+
+
+
+2
CO Me
2CO Me2
CO Me
2CO Me
2CO Me
2CO Me
2CO Me
2CO Me
2CO Me
2CO Me
2CO Me
2CO Me
2CO Me
2CO Me
2CO Me
2CO Me
Cis dienophile
Trans dienophile
Both Z on diene
E and Z on diene
Diels-Alder Reaction is Stereospecific Examples
+
Product has the two ester groups TRANS – Dienophile has to be TRANS.
Predict the reactants:
Regiochemistry
Determined by the position of the electron donating group (EDG) on the diene.
Common EDG groups include ethers, amines, sulfides (the nonbonding electron pair).
Resonance structures help explain why this occurs.
CH3O -CH3O
CH3O
H
O
H
O
H
O
Regiochemistry
Determined by the position of the electron donating group (EDG) on the diene.
Common EDG groups include ethers, amines, sulfides (the nonbonding electron pair).
Resonance structures help explain why this occurs.
CH3O
HO
+CH
3O
H
O
Example Problems
1. What product might you expect when 2-amino-1,3-butadiene reacts with 3-oxo-1-butene?
Resonance:
H2N
e- rich
H2N
O
e- poor
O O
Example Problems
1. What product might you expect when 2-amino-1,3-butadiene reacts with 3-oxo-1-butene?
H2N
O
+
H2N
O
Major product
H2N
ONone
+