AOR No10 (2016.7.15)Thermal Pericyclic Reaction

Otto Paul Hermann Diels(1876 – 1954)

Kurt Alder(1902 – 1958)

Pericyclic Reactions

6.1 The Four Classes of Pericyclic Reactions

1. Cycloadditions (bimolecular)Cheletropic reaction

“protectionofdiene”

2. Electrocyclic reactions (Unimolecular)

3. Sigmatropic rearrangement (Unimolecular)

Claisen rearrangement[3,3]-shift

-50kJ/mol

[1,7]-shift

Mislow rearrangement

[2,3]-shift

4. Group transfer reaction (Bimolecular)

Metalla-ene reaction

“ene reaction”

M=Li,Mg,Pd

6.2 Evidence for the Concertedness of Bond Making and Breaking

No intermediates

Diels-Alder reactions: exothermichigh negative entropy of activation, -150 to -200 J K-1 mol-1, with a low enthalpy of activation

Sauer,Angew.Chem.,Int.Ed.Engl.1980,19,779

Little effect by solvent polarity

Isotope effect Substituent effect

Stereochemistry

6.3 Symmetry-allowed and Symmetry-forbidden Reaction

6.3.1 The Woodward-Hoffmann Rules� Class by Class

6.3.1.1. Cycloadditions

[2+2]

Diels-Alderreaction:[4+2]cycloaddition

“numberofelectrons”

[4 + 2]

1,3-Dipolar cycloaddition

10-electrons[8+2]

(4n + 2) electrons

X,Y,Z,A,B=(C,N,O,S)

[6+4]

A thermal pericyclic cycloaddition isallowed if the total number of electronsinvolved can be expressed in the form(4n+2), where n is an integer. If the totalnumber of electrons can be expressed inthe form (4n) it is forbidden.

Rule of cycloadditions

Suprafacial-Suprafacial Antarafacial-Antarafacial

(4n + 2)

����� ������������

Suprafacial-Antarafacial 4n (allowed)

[14 + 2]

trans

If the total number of electrons involvedcan be expressed in the form (4n+2),where n is an integer, a thermalpericyclic cycloaddition is symmetryallowed when both components react in asuprafacial manner (or both componentsreact in an antarafacial manner). If thetotal number of electrons can beexpressed in the form (4n), it is allowedif one of the components reacts in asuprafacial manner and the otherantarafacial.

Rule of cycloadditions6.3.1.2 Electrocyclic Reactions 4n,4n+2

Ringstrain,aromaticity,stabilizationofchargebysubstituent

Disrotatory (���) and Conrotatory �����

Stereochemistry

Thermal electrocyclic reactions involvinga total number of electrons that can beexpressed in the form (4n+2) aredisrotatory, and thermal electrocyclicreactions in which the total number ofelectrons can be expressed in the form(4n) are conrotatory

Woodward-Hoffman rule (1965)

Robert Burns Woodward (1917 – 1979)

Roald Hoffmann�1937- )

Examples

6.3.1.3 Sigmatropic Rearrangements

[1,n] H-shift(4n+2)

Sigmatropic rearrangements of hydrogen aresuprafacial if the total number of electrons is a(4n+2) number and antarafacial if the totalnumber of electrons is a (4n) number.

[1,5]

only

When the total number of electrons is a (4n+2)number, [1,n] sigmatropic rearrangements ofelements other than hydrogen are either suprafacialwith retention of configuration in the migratinggroup or are antarafacial with inversion ofconfiguration in the migrating group.

When the total number of electrons is a (4n) number,[1,n] sigmatropic rearrangements of elements otherthan hydrogen are either antarafacial with retentionof configuration in the migrating group or aresuprafacial with inversion of configuration in themigrating group

H => C

Wagner-Meerweinrearrangement

2e 4e

Ref)

Beckmann rearrangement

Ref)

Baeyer-Villiger

Ref)

Suprafacial [1,3]-shift Suprafacial [1,4]-shift

When the total number of electrons is a (4n+2)number, [m,n]-sigmatropic rearrangements areallowed if both migrations are suprafacial or bothantarafacial.

When the total number of electrons is a (4n)number, [m,n]-sigmatropic rearrangements areallowed if one migration is suprafacial and theother antarafacial.

(4n+2)Ireland-Claisenrearrangement

Coperearrangement

10 electrons

Stevensrearrangement

6.3.1.4 Group Transfer Reaction

When the total number of electrons is a (4n+2)number, group transfer reactions are allowedwith all-suprafacial stereochemistry.

The Generalised Woodward-Hoffmann Rule

A ground-state pericyclic change issymmetry allowed when the totalnumber of (4q+2)s and (4r)acomponents is odd.

1969

6.3.2.1 Cycloadditions

(1) which of these components is acting in a suprafacial manner andwhich in an antarafacial manner?

(1) in which of these components can the number of electrons beexpressed in the form (4q+2) and in which in the form (4r),where q and r are integers?

6.3.2.2 Electrocyclic Reactions

When both lines are drawn into the same lobe it is suprafacial, and when there is one line dawn into the top lobe and one into the bottom, it is antarafacial. Since this is neither a p nor a orbital, it is given the Greek letter w.

6.3.2.3 Sigmatropic Rearrangement

6.3.2.4 Group Transfer Reactions Report No8 (7/15)

1. Explain the following rearrangement.