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Pericyclic Reactions
Class test on 10/02/07 (3:30 PM to 4:30 PM)Venue will be intimated laterAll four topics of Organic
Three Classes of Organic Reactions
Polar reactions
Radical reactions
CH3CH2 Cl Cl CH3CH2Cl Cl+ +
OH
OH
OH+
OH
OH
OH
O
OH
OHH
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
Three most common types of pericyclic reactionsElcetrocyclic reactions
new σ bond
Cycloaddition reactions
+
new σ bond
new σ bondSigmatropic reactions
HH3C H3CCH2
H
new σ bond is formed
H3C H3C
σ bond is broken in the middle of the π system
Diels-Alder reaction
σ bond broken at the end of the π-system
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 in solvent
Product configuration depends on•Configuration of the reactant •The number of double bonds in the reactant•Whether the reaction is photochemical or thermal
Some pericyclic reactions are induced thermally or photochemically
[4+2]-cycloaddition (Diels-Alder reaction)
X
Y
∆hν+
X
YNo reaction
[2+2]-cycloaddition
Ph
Ph
Ph
Ph Ph
Ph
Ph
Ph+hν ∆
No reaction
Some pericyclic reactions are induced thermally and photochemically
Me
Me Me
Me
Me
Me
∆hν
Electrocyclic reaction
Me
Me
∆ Me
Me
hνMe
Me
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 conditions •What product will be formed
A molecular orbital description (ethene)
(zero probability of finding electrons)
• Overlap of in-phase p orbitals gives a bonding π molecular orbital that is lower in energy than the p atomic orbitals.
• Overlap of out-of-phase p atomic orbitals gives an antibonding πmolecular orbital that is higher in energy than the p atomic orbitals.
1,3-butadiene
Asymmetric
Nodes are more than no. of bonding interactions
Symmetric
Asymmetric
bonding interactions are more than no. of nodes
Symmetric
Four p atomic orbitals overlap to give the four π molecular orbitals
Electrocyclic Reactions
These reactions are defined as involving the cyclization of an n pi-electronsystem to an (n-2)pi + 2sigma-electron system or the reverse process.
n(π) n-2 (π) + 2 (σ )
• The reactions are reversible.
• Observance of ring opening and ring closure depends upon the thermodynamic stability of the open and closed forms.
Two distinct modes are possible….
π-system
BA A B
B B
A A
π-system
disrotatoryring closure
π-system
BA A B
B A
A B
π-system
conrotatoryring closure
HOMO(2E,4Z)-hexadiene
HOMO(2E,4E)-hexadiene
Elcetrocyclic reactions
CH3
HCH3
H
∆CH3
HCH3
Hhν H
CH3CH3
HCH3
HH
CH3
∆
CH3
HH
CH3
hνCH3
HCH3
H
Con
Dis
Con
Dis
Selection rules for electrocyclic reactions
Woodward-Hoffmann rules
No. of π-electrons
Thermally allowed, photochemically forbidden
Thermally forbidden, photochemically
allowed4n Conrotatory Disrotatory
4n+2 Disrotatory Conrotatory
n = integer
CH3
HH
CH3
CH3
CH3hν
disrotation
conrotation
CH3
CH3
CH3
HCH3
H
CH3
CH3hν
disrotation
conrotation
CH3
CH3
1,3,5-hexatriene
Six p atomic orbitals overlap to give the six π molecular orbitals
HOMO
LUMO
Q. Have the following reactions proceeded in the conrotatory or disrotatorymanner? Should they proceed under thermal or photochemical influence?
H HH H
* disrotatory
*
CH3
CH3
HH
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 equalamounts?
CH3
HH
CH3
H
CH3
H
CH3
CH3
H
CH3
H
anticlockwise clockwise
conrotatory
A
B100 C
hν-20 C
A' B'25 C° ° °
H
H
Disrotatory Conrotatory
H
H
Disrotatory
A & A’ are stereoisomersB & B’ are stereoisomers
Sigmatropic reactionsThese reactions are defined as involving migration of a σ bond that
is flanked by one or more conjugated systems to a new position within the system
etc
etc
migrating σ bond
The reaction is termed [ i, j ] sigmatropic shift when the bond migrates from position [1,1] to position [i,j].
Examples: 1
23
12
3 1
1
2
2
3
3[3,3] sigmatropic shift
H
12
3
[1,3] sigmatropic shiftH
12
3
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]*
CH3H CH3
[1,5]*
A Cope rearrangementC6H5
CH3
C6H5
CH3
A Claisen rearrangement
OCH3
O
CH3
Allyl vinyl ether
Q. Give the product of the following reaction.O
O[3,3]
O
H
OH
Q. Suggest a mechanism
O
H3C CH3
OH
H3C CH3
O
H3C CH3
O
H3C CH3
O
H3C CH3
H
HO ∆
[3,3]slow
HO O
KH
O O
H3O+fast[3,3]
KH
HO
THF
A biological reaction involving an electrocyclicreaction and a sigmatropic reaction
7-dehydrocholesterol, a steriod formed in skin, is converted intoVitamin 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 sun’s UV rays (it prevents the synthesis of too much vitamin D3)
CH3
CH3CH3
H
HO
CH3
H
H3C CH3
CH3CH3
H
HO
CH3
H3C
provitamin D3
an electrocyclic reaction
hν
7-dehydro cholesterol
CH3
CH3CH3
H
HO
CH2
H3C
vitamin D3
[1,7] sigmatropic shift
An industrial synthesis of citral
CHO
citrala key intermediate in the synthesis of vitamin A
- H2O
O [3,3]
Clasienrearrangment
O O
[3,3] Coperearrangement
CHO
OH
+∆
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
• 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
Thermal Photo chemical
HOMO Excited state HOMO
LUMOLUMO
HOMO LUMO
LUMO HOMO
Q. Classify the following as [m+n] cycloaddition reactions
O
CH3
CH2
CH2 OCH3
CH3
O
OCH3
+
+
+
*
*
*
General description of Diels-Alder reaction 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. 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
&
Q. Following compounds are synthesized by Diels-Alder reaction, find out suitable dienes and dienophiles for their synthesis.
O
CN
CN
MeCO2Me
CO2Me
OH
H MeO
O
O
Captan
(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
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)H
H
Diels-Alder reaction : endo rule
endorelationshipbetween two alkenes
Secondary bonding Interactions leading to endo product
Primary bonding interactions leading to new σ bonds
HOMO
LUMO
Reaction of cyclopentadiene with maleic anhydride
OO O
HOMO
LUMO
primary bonding interaction
secondary bonding interaction
C lC l
C l
C l C l
C l
A lderin
C lC l
C l
C l C l
C l
D ie ldrin
O
In 1950's tw o very effective pesticides
O
O
O
Cl ClCl
Cl
Cl
Cl
flame retardent
Doubts: Room C211, 5.00 to 6.00 PMChemistry Department
Inorganic classes will be started from next week onwards
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