Preparation of Dienes

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Preparation of Dienes
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1,3-Butadiene
590-675C
CH3CH2CH2CH3
chromia- alumina

2H2

• More than 4 billion pounds of 1,3-butadiene
  prepared by this method in U.S. each year

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Dehydration of Alcohols
KHSO4
heat
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Dehydrohalogenation of Alkyl Halides
KOH
heat
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Reactions of Dienes

• isolated dienes double bonds react
  independently of one another
• cumulated dienes specialized topic
• conjugated dienes reactivity pattern requires
  us to think of conjugated diene system as a
  functional group of its own

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Addition of Hydrogen Halidesto Conjugated Dienes
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Electrophilic Addition to Conjugated Dienes

H
X
H

• Proton adds to end of diene system
• Carbocation formed is allylic

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Example
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Example
HCl
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via
H
X
Protonation of the end ofthe diene unit gives
anallylic carbocation.
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and
3-Chlorocyclopentene
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1,2-Addition versus 1,4-Addition
1,2-addition of XY
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HBr Addition to 1,3-Butadiene
HBr

• electrophilic addition
• 1,2 and 1,4-addition both observed
• product ratio depends on temperature

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Rationale

• 3-Bromo-1-butene (left) is formed faster
  than1-bromo-2-butene (right) because allylic
  carbocations react with nucleophiles
  preferentially at the carbon that bears the
  greater share of positive charge.

(formed faster)
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Rationale
1-Bromo-2-butene is more stable
than3-bromo-1-butene because it has amore
highly substituted double bond.

(formed faster)

• (more stable)

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Rationale
The two products equilibrate at 25C.Once
equilibrium is established, the morestable
isomer predominates.
major product at -80C
(formed faster)
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Kinetic ControlversusThermodynamic Control

• Kinetic control major product is the one formed
  at the fastest rate
• Thermodynamic control major product is the one
  that is the most stable

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HBr
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higher activation energy
formed more slowly
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Halogen Addition to Dienes

• gives mixtures of 1,2 and 1,4-addition products

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Example
Br2

(37)
(63)
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The Diels-Alder Reaction

• Synthetic method for preparing compounds
  containing a cyclohexene ring

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In general...

conjugated diene
alkene (dienophile)
cyclohexene
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via
transition state
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Mechanistic features

• concerted mechanism
• cycloaddition
• pericyclic reaction
• a concerted reaction that proceeds through a
  cyclic transition state

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Recall the general reaction...

alkene (dienophile)
conjugated diene
cyclohexene

• The equation as written is somewhat misleading
  because ethylene is a relatively unreactive
  dienophile.

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What makes a reactive dienophile?

• The most reactive dienophiles have an
  electron-withdrawing group (EWG) directly
  attached to the double bond.

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Dienes
Dienophiles
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Example

H2C
CH
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Example

benzene
100C
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Acetylenic Dienophile

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Diels-Alder Reaction is Stereospecific
A stereospecific reaction is one in which
stereoisomeric starting materials give
stereoisomeric products characterized by
terms like syn addition, anti elimination,
inversion of configuration, etc.

• Diels-Alder syn addition to alkene
• cis-trans relationship of substituents on alkene
  retained in cyclohexene product

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Example
O
C6H5
COH
H
H
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Example
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Cyclic dienes yield bridged bicyclicDiels-Alder
adducts.
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37

• is thesame as

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The ? Molecular OrbitalsofEthylene and
1,3-Butadiene
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Orbitals and Chemical Reactions

• A deeper understanding of chemical reactivity can
  be gained by focusing on the frontier orbitals of
  the reactants.
• Electrons flow from the highest occupied
  molecular orbital (HOMO) of one reactant to the
  lowest unoccupied molecular orbital (LUMO) of the
  other.

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Orbitals and Chemical Reactions

• We can illustrate HOMO-LUMO interactions by way
  of the Diels-Alder reaction between ethylene and
  1,3-butadiene.
• We need only consider only the ? electrons of
  ethylene and 1,3-butadiene. We can ignore the
  framework of ? bonds in each molecule.

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The ? MOs of Ethylene

• red and blue colors distinguish sign of wave
  function
• bonding ? MO is antisymmetric with respect to
  plane of molecule

Bonding ? orbital of ethylenetwo electrons in
this orbital
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The ? MOs of Ethylene
Antibonding ? orbital of ethyleneno electrons
in this orbital

• LUMOHOMO

Bonding ? orbital of ethylenetwo electrons in
this orbital
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The ? MOs of 1,3-Butadiene

• Four p orbitals contribute to the ? system of
  1,3-butadiene therefore, there are four ?
  molecular orbitals.
• Two of these orbitals are bonding two are
  antibonding.

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The Two Bonding ? MOs of 1,3-Butadiene
HOMO
4 ? electrons 2 ineach orbital
Lowest energy orbital
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The Two Antibonding ? MOs of 1,3-Butadiene
Highest energy orbital
LUMO
Both antibondingorbitals are vacant
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A ? Molecular Orbital Analysisof theDiels-Alder
Reaction
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MO Analysis of Diels-Alder Reaction

• Inasmuch as electron-withdrawing groups increase
  the reactivity of a dienophile, we assume
  electrons flow from the HOMO of the diene to the
  LUMO of the dienophile.

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MO Analysis of Diels-Alder Reaction
HOMO of 1,3-butadiene

• HOMO of 1,3-butadiene and LUMO of ethylene are
  in phase with one another
• allows ? bond formation between the alkene and
  the diene

LUMO of ethylene (dienophile)
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MO Analysis of Diels-Alder Reaction
HOMO of 1,3-butadiene
LUMO of ethylene (dienophile)