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Pericyclic Reactions
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Class test on 10/02/07 (3:30 PM to 4:30 PM)
Venue will be intimated later
All four topics of Organic
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Three Classes of Organic Reactions
Polar reactions
Radical reactions
CH3CH2 Cl Cl CH3CH2Cl Cl+ +
OH
OH
O
H+
OH
OH
OH
O
OH
OH
H
O
OH2
OH
O
O H
O O
.. ..
Y R X R Y X+ +
R = aliphatic as well as aromatic
Pericyclic reactions
This reaction occurs as a result of a cyclic reorganization of electrons
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Three most common types of pericyclic reactions
Elcetrocyclic reactions
new bond
Cycloaddition reactions
+
newbond
newbondSigmatropic reactions
HH3C H3C
CH2
H
new bond is formed
H3C H3C
bond is broken in themiddle of the system
Diels-Alder reaction
bond broken at the end ofthe -system
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Certain common features of pericyclic reactions
They are all concerted reactions, electron
reorganization takes place in single step.
Therefore, there is one TS and no intermediates
As the reactions are concerted, they are highlystereoselective
The reactions are generally not effected by
catalysts or by a change insolvent
Product configuration depends onConfiguration of the reactant
The number of double bonds in the reactant
Whether the reaction is photochemical or thermal
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Some pericyclic reactions are induced thermally or photochemically
[4+2]-cycloaddition (Diels-Alder reaction)
X
Y
h+
X
YNo reaction
[2+2]-cycloaddition
Ph
Ph
Ph
Ph Ph
Ph
Ph
Ph+h
No reaction
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Some pericyclic reactions are induced thermally and photochemically
Me
Me Me
Me
Me
Me
hElectrocyclic reaction
Me
Me
MeMe
hMeMe
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Conservation of Orbital Symmetry Theory
R. B. Woodward and Roald Hoffmann (1965)
In phase orbitals overlap during the course of a pericyclic reaction
Based on Frontier orbital theory (1954, Fukui)
(HOMO, LUMO)
Molecular orbital symmetry controls
Weather or not compound undergoes a reaction under particular condit ions
What product will be formed
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A molecular orbital description (ethene)
(zero probability of finding electrons)
Overlap of in-phasep orbitals gives a bonding molecular orbitalthat is lower in energy than thep atomic orbitals.
Overlap of out-of-phasep atomic orbitals gives an antibonding molecular orbital that is higher in energy than thep atomic orbitals.
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1,3-butadiene
Asymmetric
Nodes are more than no.
of bonding interactions
Symmetric
Asymmetric
bonding interactions are
more than no. of nodes
Symmetric
Fourp atomic orbitals overlap to give the four molecular orbitals
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Electrocyclic Reactions
These reactions are defined as involving the cyclization of an npi-electron
system to an (n-2)pi + 2sigma-electron system or the reverse process.
n() n-2 () + 2 ()
The reactions are reversible.
Observance of ring opening and ring closure depends
upon the thermodynamic stability of the open and closed forms.
T di ti t d ibl
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Two distinct modes are possible.
-system
BA A B
B B
A A
-system
disrotatory
ring closure
-system
B A A B
B A
A B
-system
conrotatory
ring closure
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HOMO
(2E,4Z)-hexadiene
HOMO
(2E,4E)-hexadiene
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Elcetrocyclic reactions
CH3
HCH3
H
CH3
H
CH3
Hh H
CH3CH3
HCH3
H
H
CH3
CH3
HH
CH3
h
CH3
HCH3
Con
Dis
Con
Dis
Selection rules for electrocyclic reactions
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Selection rules for electrocyclic reactions
Woodward-Hoffmann rules
No. of-electrons
Thermally allowed,
photochemically forbidden
Thermally forbidden,
photochemically
allowed
4n Conrotatory Disrotatory
4n+2 Disrotatory Conrotatory
n = integer
CH
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CH3
HH
CH3
CH3
CH3h
disrot
ation
conrotation
CH3
CH3
CH3
HCH3
H
CH3
CH3h
disrot
ation
conrotation
CH3
CH3
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1,3,5-hexatriene
Six p atomic orbitals overlap to give the six molecular orbitals
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HOMO
LUMO
Q. Have the following reactions proceeded in the conrotatory or disrotatory
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Q g p y y
manner? Should they proceed under thermal or photochemical influence?
H HH H
* disrotatory
*
CH3
CH3
H
H
CH3
HH
CH3
Q. Show that the cyclobutane below open by two alternative conrotarory procsses.
What is the product in each instance? Do you expect them to be formed in equal
amounts?
CH3
H
H
CH3
CH3
H
CH3
CH
H
CH3
H
anticlockwise clockwise
conrotatory
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A
B
100 C
h
-20 C
A ' B '25 C
H
H
Disrotatory Conrotatory
H
H
Disrotatory
A & A are stereoisomersB & B are stereoisomers
Sigmatropic reactions
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S g op c e c o s
These reactions are defined as involving migration of a bond thatis flanked by one or more conjugated systems to a new position
within the system
etc
etc
migrating bond
The reaction is termed [ i, j ] sigmatropic shift when the bond migrates
from position [1,1] to position [i,j].
Examples:
1
2
3
12
3 1
1
2
2
3
3[3,3] sigmatropic shift
H
1
2
3
[1,3] sigmatropic shiftH
1 2 3
Q Classify the following sigmatropic reactions of order [i j]
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Q. Classify the following sigmatropic reactions of order [i,j]
H
D D
H
H3C
H
H D2C
H H
HH
H
[1,5]*
S
R1 R2
S
R1 R2
[2,3]
*
CH3HCH3
[1,5]
*
A Cope rearrangement
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p g
C6H5
CH3
C6H5
CH3
A Claisen rearrangement
O CH3 O
CH3
Allyl vinyl ether
Q. Give the product of the following reaction.O
O[3,3]
O
H
OH
Q Suggest a mechanism
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Q. Suggest a mechanism
O
H3C CH3
OH
H3CCH
3
O
H3C CH3
O
H3CCH3
O
H3CCH3
H
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HO [3,3]slow
HO O
KH
O O
H3O+
fast[3,3]
KH
HO
THF
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A biological reaction involving an electrocyclic
reaction and a sigmatropic reaction
7-dehydrocholesterol, a steriod formed in skin, is converted into
Vitamin D3
by two pericyclic reactions
Deficiency in vitamin D causes rickets. Deficiency in vitamin D can be prevented by getting
enough sun.
Too much vitamin D is also harmful causes calcification of soft tissues
skin pigmentation protects the skin from the suns UV rays (it prevents
the synthesis of too much vitamin D3)
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CH3
CH3CH3
H
HO
CH3
H
H3C CH3
CH3CH3
H
HO
CH3
H3C
provitamin D3
an electrocyclic reaction
h
7-dehydro cholesterol
CH3
CH3CH3
H
HO
CH2
H3C
vitamin D3
[1,7] sigmatropic shift
An industrial synthesis of citral
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An industrial synthesis of citral
CHO
citral
a key intermediate in the synthesis ofvitamin A
- H2O
O [3,3]
Clasienrearrangment
O O
[3,3] Coperearrangement
CHO
OH
+
Cycloaddition reactions
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Cycloaddition reactions
H3C CH3H3C CH3
OO O
OO
O
a [4+2]
cycloaddition reaction
(Diels-Alder reaction)
+
+ O
O
O
heat
UV light
O
O
O
a [2+2]
cycloaddition reaction
Cycloaddition reactions
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y
Why does maleic anhydride react easily with butadiene,
but not at all with ethylene?
O
O
O
+ O
O
O
O
O
O
+O
O
O
HOMO of one reactant should react with LUMO of the other reactant
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Thermal Photo chemical
HOMO Excited state HOMO
LUMOLUMO
HOMO LUMO
LUMO
HOMO
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Q. Classify the following as [m+n] cycloaddition reactions
O
CH3
CH2
CH2 OCH3
CH3
O
OCH3
+
+
+
*
*
*
General description of Diels-Alder reaction
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p
CHO
NO2
NO2
CHO
+
diene dienophile adduct
adductdiene dienophile
Dienes permanently in s-cis conformation undergo Diels-Alder reactions with ease
O
Dienes permanently in s-trans conformation cannot undergo Diels-Alder reaction
Q Whi h f th f l l i d t d t th Di l Ald ti di ?
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Q. Which of the following do not respond to the Diels-Alder reaction as a diene?
Q. Arrange the following dienes according to their reactivity
towards Diels-Alder reaction
But But
But
&
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Q. Following compounds are synthesized by Diels-Alder reaction, find
out suitable dienes and dienophiles for their synthesis.
O
CN
CN
Me
CO2Me
CO2Me
OH
H MeO
O
O
Captan
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p
(an industrial application of Diels-Alder reaction)
ClS
CCl3
O
O
O
N
H
H
O
O
S CCl3
O
H
H
O
O
NH3NH
H
H
O
O
+
Captan
(agricultural fungicide)
The endo rule for Diels-Alder reaction
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O
H
H
O
O
'endo' adduct (formed)
H
H
'endo' adduct
+
O+ O
H
H
O
O
O
O
O
O
O
'exo' adduct (not formed)
HH
Diels-Alder reaction : endo rule
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endorelationship
between two alkenes
Secondary bonding
Interactions leading to
endo product
Primary bonding
interactions leading
to new bonds
HOMO
LUMO
Reaction of cyclopentadiene with maleic anhydride
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OO O
HOMO
LUMO
primary bonding interaction
secondary bonding interaction
C l C l
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C lC l
C lC l
C l
A lderin
C lC l
C lC l
C l
Dieldrin
O
In 1950's tw o ve ry e ffec t i v e p es t i c id es
O
O
O
Cl ClCl
Cl
Cl
Cl
flame retardent
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Doubts:
Room C211, 5.00 to 6.00 PM
Chemistry Department
Inorganic classes will be started from next week onwards