DAMIETTA
UNIVERSITY
CHEM-405:
PERICYCLIC REACTIONS
LECTURE 2
Dr Ali El-Agamey
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• Three approaches have been employed to explain the
pericyclic reactions and these are:
1- Frontier orbitals
2- Correlation diagrams
3- Aromatic transition states
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A Molecular Orbital Description of
Ethene
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Four p atomic orbitals interact to give the four  MOs of
1,3-butadiene
LUMO
LUMO
HOMO
HOMO
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Drawing HOMO and LUMO of various
(neutral) polyenes1
 Draw the HOMO and LUMO of 1,3,5-hexatriene??
 Can we draw the whole molecular orbital diagram of
1,3,5-hexatriene??
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A Molecular Orbital Description of 1,3,5-hexatriene
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The Allyl cation
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The Allyl radical
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The Allyl anion
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The pentadienyl system
• The pentadienyl system has five MOs.
• In pentadienyl cation, 2 (asymmetric) is the HOMO and
3 (symmetric) is the LUMO.
• In pentadienyl free radical and anion, 3 (symmetric)
is the HOMO and 4 (asymmetric) is the LUMO.
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Electronic Structure of the Atom
• An atom has a dense,
positively charged nucleus
surrounded by a cloud of
electrons.
• The electron density is
highest at the nucleus and
drops off exponentially with
increasing distance from the
nucleus in any direction.
Chapter 1
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Rule
• For systems containing 4n + 2 electrons, the HOMO is
symmetric.
• For systems containing 4n electrons, the HOMO is
asymmetric.
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Questions
 Draw the HOMO and LUMO of 1,3,5,7-octatetraene?
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An electrocyclic reaction is completely stereoselective
and completely stereospecific1
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1
Photochemical means it is driven by light energy١٧
To form the new  bond in the electrocyclic reaction, the  orbitals
at the end of the conjugated system must overlap head-to-head
١٨
Only the symmetry of the HOMO is important in determining
the course of the reaction
Bonding interaction
Antibonding interaction
Bonding interaction
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Only the symmetry of the HOMO is important in determining
the course of the reaction
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(2)
Heat
Heat
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(3)
Heat
The symmetry of the
HOMO of the
compound undergoing
ring closure controls
the stereochemical
outcome of an
electrocyclic reaction
Heat
٢٢
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• In reactions under photochemical conditions every thing is reversed.1
• The ground state and excited state HOMO’s have opposite symmetries. If
1
the ground state HOMO is symmetric, the excited state HOMO is asymmetric.
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The configuration of the product formed depends on:
• the configuration of the reactant
• the number of conjugated double bonds or pairs of
electrons in the reacting system
• whether the reaction is a thermal or a photochemical
reaction
A photochemical reaction takes place when a reactant absorbs light
A thermal reaction takes place without the absorption of light
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 Robert Woodward (1917-1979) received the Nobel
Prize in 1965 for his work on the synthesis of natural
products. Had he lived, he would undoubtedly have
shared a second Nobel Prize in 1981. Roald Hoffmann
shared the Nobel Prize in chemistry in 1981 with
K. Fukui of Japan for their work on molecular orbital
theory and pericyclic reactions.
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Woodward-Hoffmann rules for electrocyclic reactions
No. of
electrons
Mode of
activation
Motion
4n
thermal
conrotatory
4n
photochemical
disrotatory
4n + 2
thermal
disrotatory
4n + 2
photochemical
conrotatory
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According to the principle of microscopic reversibility, the reverse
process of thermal ring opening takes exactly the same path.1
The sigma bond will open (via a conrotatory motion) so as to give
the resulting p orbitals which will have the symmetry of
٢٨
2 (HOMO).1
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The direction taken by an electrocyclic reaction is dependent on
the relative stabilities of the ring and open-chain reactants.1
The strained ring of the cyclobutene makes this reaction take place
in the ring-opening sense, while hexatriene and octatetraene
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reactions are ring closures.2
The diene must assume a s-cis-conformation in order to make
the terminal carbons p orbitals overlap.1
The “s” in the terms “s-cis” and “s-trans” refers to a sigma bond
and indicates that these are conformations about a single
bond and not configurations about a double bond.2
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Now we can easily depict conrotations and disrotations directly
Without drawing molecular orbitals.1
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Questions
 Write the structures of the products for the thermal electrocyclic reactions
for the following compounds? For (a) and (c), show terminal orbitals during
the course of reaction and omit it for (b).
 In addition, specify the type of rotation and write the names of reactants
and products and specify which of them is thermodynamically favored?
(a)
(b)
(c)
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Questions
 Write the structures of the products for the photochemical electrocyclic
reactions for the following compounds? For (a) and (c), show terminal orbitals
during the course of reaction and omit it for (b).
 In addition, specify the type of rotation and write the names of reactants
and products and specify which of them is thermodynamically favored?
trans-3,4-Dimethylcyclobutene
(a)
(b)
(c)
٣٤
There are always two conrotatory modes clockwise and
anticlockwise and both are equally probable. Similarly, there are
two disrotatory modes.1
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The ring opening of 3-susbtituted cyclobutene can result in a
mixture of isomers (cis and trans-butadiene) due to the motion of
the substituent either inward or outward during the breaking of
the sigma bond.1
Thermolysis of 3-methylcyclobutene yields exclusively
trans-penta-1,3-diene. This would involve the motion of the
methyl group outward, which is preferred because of the
minimum steric effect involved in the T.S.1
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However, the ring openeing of 3-alkyl-3-methylcyclobutenes
gave a mixture of isomers.1
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Torquoselectivity: the stereoselectivity due to the inward/ outward
motion of the substituents in the electrocyclic ring opening
reactions.1
Torquoselectivity is controlled by
(1) Steric effect
(2) Electronic effect.
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(1) Steric effect
In the elctrocyclic ring opening of trans-3,4-dimethylcyclobutene,
two products might be obtained from conrotatory ring openings but
in fact only the trans, trans product is obtained because there are
severe steric interactions in the TS leading to the cis, cis product.2
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(2) Electronic effect
The preference for the outward rotation of the substituent
increases with the increasing of pi-donar nature of the substituent,
whereas with increasing the pi-acceptor nature of the substituent,
the inward motion is preferred.1
Methoxy is better pi-donar than methyl.
cis-Penta-2,4-dienal is formed exclusively
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Questions
 (a) Why Dewar-benzene (extraordinarily high-energy molecule) is not
easily converted to benzene or compound 2 at room temperature, although
isomerization of Dewar-benzene to benzene has been estimated to be
exothermic by about 79 kcal/mol. (b) In your opinion, what are the reasons
responsible for the high exothermicity of this reaction?
2
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