Pericyclic Reactions PDF [PDF]

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Pericyclic Reactions

Class test on 10/02/07 (3:30 PM to 4:30 PM) Venue will be intimated later All four topics of Organic

Three Classes of Organic Reactions Polar reactions

+

Y

R

X

R

+

Y

X

R = aliphatic as well as aromatic O

OH +

H

OH

OH

..

..

OH

O H

OH

O

OH2

OH OH

O

OH

H

O

O

O

Radical reactions

CH3CH2 +

Cl

Cl

CH3CH2Cl + Cl

Pericyclic reactions This reaction occurs as a result of a cyclic reorganization of electrons

Three most common types of pericyclic reactions Elcetrocyclic reactions new σ bond

Cycloaddition reactions new σ bond

Diels-Alder reaction

+

new σ bond

Sigmatropic reactions H3 C

H3 C

new σ bond is formed

σ bond is broken in the middle of the π system

σ bond broken at the end of

CH 2 H3C

H

H 3C

H

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 highly stereoselective • 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

hν

+

No reaction

X

∆

Y

Y [2+2]-cycloaddition Ph Ph

hν Ph Ph

Ph

Ph

+ Ph

∆ No reaction

Ph

Some pericyclic reactions are induced thermally and photochemically Electrocyclic reaction Me

hν

Me

Me

Me

∆

Me

Me

hν

Me

Me

∆

Me Me

Me

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 π molecular 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-electron system 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

π-system

disrotatory ring closure B

A

A

B

B

B

A

A

π-system

π-system

conrotatory ring closure B

A

A

B

B

A

A

B

HOMO (2E,4Z)-hexadiene

HOMO (2E,4E)-hexadiene

Elcetrocyclic reactions CH3

∆

H CH3 H

CH3

Con

H CH3

hν

H

Dis

H

CH3 CH3 CH3

H

∆ H H CH3

CH3

H

Con

hν

H

CH3 CH3 H CH3

Dis

H

Selection rules for electrocyclic reactions Woodward-Hoffmann rules

No. of πelectrons

Thermally allowed, photochemically forbidden

Thermally forbidden, photochemically allowed

4n

Conrotatory

Disrotatory

4n+2

Disrotatory

Conrotatory

n = integer

CH3

tion a t o disr

CH3

hν

CH3

H H CH3

conrot ation

CH3

CH3

CH3

on i t a ot r s i d

CH3

hν

CH3

H CH3 H

c on

r ot atio

CH3

n

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 disrotatory manner? Should they proceed under thermal or photochemical influence?

*

disrotatory H

H

H CH3

*

H

CH3 H H

H H

conrotatory

CH3

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? H CH3

CH3 H H CH3

CH3

H

CH3

H

clockwise

anticlockwise H

CH3

H

H

H 100 °C

25 °C B'

A'

B Disrotatory

H

hν -20 °C Conrotatory

A

A & A’ are stereoisomers B & B’ are stereoisomers

Disrotatory

Sigmatropic reactions These 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

migrating σ bond etc

The reaction is termed [ i, j ] sigmatropic shift when the bond migrates from position [1,1] to position [i,j]. Examples:

2

2

1

1

3 3

1

3 3

1 2

2

H

H 3

1 2

[3,3] sigmatropic shift

1

3 2

[1,3] sigmatropic shift

Q. Classify the following sigmatropic reactions of order [i,j] H

H H

H H

*

H3C

H

H D

D

H

[1,5]

D2C H

[2,3]

S

*

S R1

R2 R1

H

*

CH3

R2

CH3

[1,5]

A Cope rearrangement C6H5

C6H5

CH3

CH3

A Claisen rearrangement CH3

CH3

O

O

Allyl vinyl ether

Q. Give the product of the following reaction. O

O

O [3,3]

H

OH

Q.

Suggest a mechanism OH

O CH3

H3C

CH3

H3C

O O

CH3

H3C

H3C

CH3

H O H3C

CH3

∆

HO

HO

O

[3,3] slow

KH

O

fast [3,3]

H3O+ O

HO KH THF

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 sun’s UV rays (it prevents the synthesis of too much vitamin D3)

H 3C

CH3

H3C

CH3

CH3

CH3

an electrocyclic reaction

CH3 H

H

CH3

CH3

hν

HO

CH3

H HO

provitamin D3

7-dehydro cholesterol

H3C CH3

CH2

[1,7] sigmatropic shift

H HO

vitamin D3

CH3 CH3

An industrial synthesis of citral

CHO

∆

CHO + OH

citral a key intermediate in the synthesis of vitamin A

- H2O [3,3]

O

Cope rearrangement

[3,3] O Clasien rearrangment

O

Cycloaddition reactions

H3C

+

H3C

CH3

O O O

CH3

O

a [4+2] cycloaddition reaction (Diels-Alder reaction)

O

O

O

O

+

heat O

O

UV light

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

LUMO

LUMO

LUMO

HOMO

HOMO

LUMO

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

*

CH3

+

CH3 O

*

O

+

CH2

*

OCH3 +

CH2

OCH3

General description of Diels-Alder reaction CHO

CHO

diene

dienophile

adduct

+ NO2

diene

dienophile

NO2

adduct

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

Me

O CN CN

CO2Me CO2Me

H

H

O

MeO

O

Captan (an industrial application of Diels-Alder reaction) O

H

O

H

O

NH3

O Cl

NH

O

O

+

H

H

O

CCl3 S

N

H

S

O

Captan (agricultural fungicide)

CCl3

O

H

O

The endo rule for Diels-Alder reaction O

O

H

O

O

+

O

H

O O

H

O

H O

H

O

O

H

'exo' adduct (not formed)

O

'endo' adduct (formed) H +

H

'endo' adduct

Diels-Alder reaction : endo rule

Primary bonding interactions leading to new σ bonds

HOMO

endo relationship between two alkenes

LUMO

Secondary bonding Interactions leading to endo product

Reaction of cyclopentadiene with maleic anhydride

HOMO

O O

O

LUMO

primary bonding interaction secondary bonding interaction

Cl

Cl

Cl

Cl

Cl

Cl

O Cl

Cl

Cl

A ld erin

Cl D ie ldrin

Cl

Cl

In 1 950's tw o very effective p es ticide s

Cl Cl

Cl Cl

O Cl

Cl

O O

flame retardent

Doubts: Room C211, 5.00 to 6.00 PM Chemistry Department

Inorganic classes will be started from next week onwards