pericyclic reactions.pdf

7/27/2019 pericyclic reactions.pdf

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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


3/41

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


7/41

Some pericyclic reactions are induced thermally and photochemically

Me

Me Me

Me

Me

Me

hElectrocyclic reaction

Me

Me

MeMe

hMeMe


8/41

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


14/41

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


20/41

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]


22/41

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


23/41

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


24/41

Q. Suggest a mechanism

O

H3C CH3

OH

H3CCH

3

O

H3C CH3

O

H3CCH3

O

H3CCH3

H


25/41

HO [3,3]slow

HO O

KH

O O

H3O+

fast[3,3]

KH

HO

THF


26/41

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)


27/41

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


28/41

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

+



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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



30/41

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


32/41

Q. Classify the following as [m+n] cycloaddition reactions

O

CH3

CH2

CH2 OCH3

CH3

O

OCH3

+

+

+

*

*

*

General description of Diels-Alder reaction


33/41

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


36/41

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


39/41

OO O

HOMO

LUMO

primary bonding interaction

secondary bonding interaction

C l C l


40/41

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


41/41

Doubts:

Room C211, 5.00 to 6.00 PM

Chemistry Department

Inorganic classes will be started from next week onwards

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