Conjugation in Alkadienes and Allylic Systems
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Transcript of Conjugation in Alkadienes and Allylic Systems
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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"
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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
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Conjugated:
Requirements: Continuous _____ systems with adjacent ___ orbitals overlapping.
Bonding Energy: Extra bonds between
Reactivity: Reactivity differs depending on specific diene and other chemicals involved.
Types of Dienes
Continuous, overlapping p-orbitals.
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Isolated:
_________ stable than conjugated.
Requirements: ____ systems separate and are isolated by an ________ center.
Bonding Energy: _______ bonding.
Reactivity: __________simple alkenes.
Types of Dienes
sp3 center
Alkene p-orbital overlap.Alkene p-orbital overlap.
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Cumulated:
_________stable.
Requirements: Double bonds _____________hybridization of middle carbon.
Bonding Energy:
Reactivity: Same as simple alkynes.
Types of Dienes
CC C
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Name Line Diagram π system Type Resonance
Propene
1,2-propadiene
1,3-butadiene
1,4-pentadiene
Dienes
CH2 C CH2
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Name Line Diagram π system Type Resonance
1,3-cyclopentadiene
1,3-cyclohexadiene
1,4-cyclohexadiene
Benzene
Dienes
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sp 2sp
Bonding in Allene
sp 2
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The Double Bond as a Substituent
carbocation
C+C
C
radical
•CC
C
diene
CCC
C
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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)
Allylic Carbocations Stability
CCl
CH3
CH3
CH2C CH
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Provides good evidence that allylic carbocationsare __________________________________.
CH3
stabilizes C+ better than does
Allylic Carbocations Stability
C C
CH3
CH3
H2C CH + +
CH3
CH3
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-Must have π systems –
-__________change positions in resonance contributors shown by ______________.
-Molecular structure is composite of all the resonance contributors, with the most favorable contributing the most character.
-More resonance leads to __________ stability:
Resonance
C+
C+
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Delocalization of electrons in the doublebond stabilizes the carbocation.
Stabilization of Allylic Carbocations
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Resonance Model
CH3
CH3
H2C CH +C
C
CH3
CH3
H2C CH
C
CH3
CH3
H2C CH
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Allylic Free Radicals are Stabilized byElectron Delocalization
CC
C • CC
C
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Vinylic versus Allylic
CC
C
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Vinylic versus Allylic
CC
C
H
H
H
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Allylic hydrogens are attached to allylic carbons.
Vinylic versus Allylic
CC
C
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Vinylic versus Allylic
CC
C
Vinylic substituents are attached to vinylic carbons.
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Vinylic versus Allylic
CC
C
Allylic substituents are attached to allylic carbons.
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Resonance
Molecular Orbitals
Resonance Hybrid
Allylic Carbocations
C+
C+
+ +
1/2+ 1/2+
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Resonance
Molecular Orbitals
Resonance Hybrid
Allylic Radicals
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Allylic Carbocations/Radicals
Either _____________ by a radical.
Either ____________ by nucleophiles
Reaction Site
_______ radical is stabilizing.
________ of charge is stabilizing.
Delocalized
On _______l C’s,
never on a ______ C.
On __________l C’s,
never on a ______ C.
Position
Double bonds ______ electron density.
Stabilization
RadicalsCarbocations
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Allylic Carbocations/Radicals
Allylic bonds are often ________ and are
_______ broken.
Bond Dissociation Energies
One π= ____ R groups
~ ____-propyl radical
One π= _____ R groups ~ _____-propyl cation
Stabilization
_________ energy than alkyl
Radical intermediates.
_________ energy then alkyl
Carbocation intermediates.
Intermediates
RadicalsCarbocations
+ +
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A comparison of bond energies associated with radicals and allylic radicals:
Radical Bond Energies
H+ H
H
KJ/mol
KJ/mol+ H
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ClCH2CHCH3
Cl
500 °C
CHCH3H2C + Cl2
CHCH2ClH2C
+ HCl
Chlorination of Propene
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Reaction Type:
Overall Reaction: Alkene
Reactivity Order:
Regioselectivity: Substitution at the
______position due to the stability of the ______
radical (resonance).
Stereoselectivity:
Requirements: Br2 or Cl2 with ________, or
N-bromosuccinimide (NBS) which can act as a source of Br2
Allylic Halogenation
N
O
O
Br
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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
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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
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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
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N-Bromosuccinimide
O
O
NBr
CCl4+
heat+
O
O
NH
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all of the allylic hydrogens are ____________
and
the resonance forms of allylic radicalare ________________.
Limited Scope
Allylic halogenation is only used when:
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Example
Cyclohexene satisfies both requirements.
H
H
H
•
H
H
H•
H H
HH
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Example
2-Butene CH3CH CHCH3
But
•CH3CH CH CH2
•CH3CH CH CH2
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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
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Thermodynamic Factors: Corresponds to the relative
____________of the products.
Kinetic Factors: Is the ______ 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
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Pathway 1 vs. Pathway 2
Kinetic vs. Thermodynamic Control
Reaction Coordinate
Energy
SM Reaction 2 (dash) generates ______.
P2 is the ________ stable product.
P2 has ________ energy than P1
P2 is the _______________ product.
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Increase in temperature: Average energy of the molecules increases.
Low Temperatures:
Preferred Path: Path similar to ______
(on previous slide.)
Reaction 1:
Reaction 2:
Product Ratio: Is determined by the
Control:
Control and Temperature
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Intermediate Temperatures:
Preferred Path: Path similar to __________
Reaction 1:
Reaction 2:
Product Ratio: Dependent on ______________
(a ________ of reaction results in more product ____
______ forms initially then over time goes back
to starting material, then forms the ____________.
Major product: Depends on time of reaction
Short (time):
Long (time):
Control: Variable
Control and Temperature
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High Temperatures:
Preferred Path: ___________ is preferred, but then goes through ___________.
Reaction 1:
Reaction 2:
Product Ratio: Dependent on _______________
between P1 and P2
Major product: Depends on time of reaction, but end result is more _______
Short (time):
Long (time):
Control:
Control and Temperature
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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
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Dienes undergo electrophilic addition reactions similar to alkenes:
Isolated dienes: Double bonds react ___________ one another, and therefore react like ___________.
Cumulated dienes: React more ___________
Conjugated dienes: Conjugated C=C changes the reactivity.
Dienes act as ____________, reacting with _______________.
Reactions of Dienes
Nu E
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Three types of electrophilic addition of dienes:
Reaction with H-X:
Reaction with X2:
Reactions of Dienes
HX ++
X2 ++
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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
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Proton adds to ________ of diene system.
Carbocation formed is __________.
H X
H
+
Introduction to 1,2 and 1,4 Addition
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Example: H
H
H
H
H
H
HCl HH
H
H
HH
?
H
HH
H
HH
?
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via: HH
H
H
HH
H
H
H
H
H
H
H X HH
H
H
HH
Protonation of the end of
the diene unit gives an
________________.
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and: HH
H
H
H
H HH
H
H
H
H
HH
H
H
H
H
H
HH
H
H
H
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1,2-addition of XY 1,4-addition of XY 1,2-Addition versus 1,4-Addition
Via resonance
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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
+
HBr
-80 C
+
20 C
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Addition of Hydrogen Halides to Dienes
Structure
Control
Determination
Reversibility
Control
Room TempLow TempConditions
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Reaction Type:
Overall Reaction: Diene + Dienophile (alkene)
Stereoselectivity: Syn and Endo or Exo
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+
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Mechanism
____________ process:
This makes the reaction very __________ and ____________selective.
Thermodynamically favorable:
Aromatic like transition state.
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Simple Diels Alder Examples:
1,3-butadiene + ethene
Diels-Alder Reaction
1,3-butadiene + ethyne
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Diels-Alder Reactivity
The most reactive dienes have an electron-___________ group (E__G) directly attached to nucleophilic diene.
Typical E___Gs
E___G
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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
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+
solvent 100°C
H2C CHCH CH2 H2C CH CH
O
Example
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+
solvent 100°C
O
O
O
Example
CHC CH2H2C
CH3
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Example Diels-Alder Questions
1. Rank the relative reactivity towards 1,3-cyclopentadiene of the following:
CH3
CO2Me OO
iiiiiiiiiiii
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Example Diels-Alder Questions
2. Rank the relative reactivity towards dimethyl cis-butendioate of the following:
O
O
O
O
CH3
CH3
ii iiii iiiiii
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Example Diels-Alder Questions
3. Rank the order of the relative reactivity towards 3-buten-2-one of the following
CH3CH2
O
iii iii
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Common Diels-Alder Reactants
Common Dienes:
Common Dienophiles:O
CO2Me
CO2Me
O
O
O
O
O
CN CO2Me
MeO2C
CO2Me
CO2Me
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Two different conformations are possible:Endo: Dienophile is ‘_________’ diene.
___________________ product.Exo: Dienophile is ________.
__________________ product.
______ conformations are generally the major product with _______ being a minor product.
Reactions with Cyclic Dienes
O
O
OO
O
O+
O
O
O
+
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Reactions with Cyclic Dienes
____________________________favors the endo transition state.
H
OR
C
H
OR
C
H
O
H
O
R
R
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Diels-Alder: Both the diene and the dienophile are _______
Cis-dienophile: __________ substituted product.
Trans-dienophile: __________ substituted product.
Both diene and alkene are Z (or E) both on the _________side of the product.
Dienes and alkene are E and Z Are on ________ 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.
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+H2C CHCH CH2
Example
C C
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+H2C CHCH CH2
Example
C C
H
H
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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
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Diels-Alder Reaction is Stereospecific Examples
Product has the two ___________groups ___________
– Dienophile has to be _______________
Predict the reactants:
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Regiochemistry
Determined by the position of the electron donating group (EDG) on the diene.
Common EDG groups include ethers, amines, sulfides
(Using the the nonbonding electron pair).
CH3OCH3O
CH3O
H
O
H
O
H
O
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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).
CH3O
HO
+
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Example Problems
1. What product might you expect when 2-amino-1,3-butadiene reacts with 3-oxo-1-butene?
H2N H2N
O O O
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Example Problems
1. What product might you expect when 2-amino-1,3-butadiene reacts with 3-oxo-1-butene?
H2N
O
+