This is a type of pericyclic reactions

1. Pericyclic reactions-III
Dr. Satyajit Dey
Department of Chemistry
Tamralipta Mahavidyalaya
Tamluk
Midnapore (East)
Under Graduate
Organic Chemistry Course

2. A sigmatropic reaction may be defined as a rearrangement reaction in
which a σ bond with its attached groups migrates from one end of one or
more conjugated π system to the other position within the same molecule.
The sigmatropic shift of order [i, j] is defined as:
“The concerted migration of a σ bond, flanked by one or more π conjugated
systems, to a new location, whose termini are i and j atoms away from its
original point of attachment.”

3. Stereochemistry of sigmatropic rearrangements
The migration of a sigma bond (i.e.; both bond breaking and bond making)
may take place over the same face or between the opposite faces of the
conjugated pi system. If the migrating sigma bond with its attached groups
moves over the same face of the pi system, the stereochemistry of migration
is called suprafacial (above the face) shift. If the migration of a sigma bond is
associated with the opposite face of the pi system, the migration is called an
antarafacial (opposite face) shift.

4. FMO analysis of [1,n]-H shift
A. Stereochemistry of [1,3] hydrogen shift:
Photochemical [1,3]-H shifts:
Thermal [1,3]-H shifts:

5. FMO analysis of [1,n]-H shift
B. Stereochemistry of [1,5] hydrogen shift:
Thermal [1,5]-H shifts:
Photochemical [1,5]-H shifts:

6. Examples
Wolfgang Roth’s experiment-I [1964]:
Wolfgang Roth’s experiment-II [1964]:

7. Examples, contd...
Roth et al found that the reaction of (6S) –(2E, 4Z)-6-methyl-2,4-octadiene-2-d
at 250 oC provided a 1.5:1 mixture of (R)–(3E, 5Z)-3-methyl-3,5-octadiene-7-d
(X) and (S) –(3E, 5Z)-3-methyl-3,5-diene-7-d (Y), the products of symmetry
allowed suprafacial [1,5] hydrogen migration.
5-Methylcyclopentadiene, having a perfect geometry for [1,5] hydrogen
shift, undergoes a very facile thermally allowed suprafacial [1,5] H
migration at room temperature

8. FMO analysis of [1,n]-alkyl shift
A. Stereochemistry of [1,3] alkyl shift:
Thermal [1,3]-alkyl shifts:
Photochemical [1,3]-alkyl shifts:

9. FMO analysis of [1,n]-alkyl shift, contd...
A. Stereochemistry of [1,5] alkyl shift:
Thermal [1,5]-alkyl shifts:

10. Photochemical [1,5]-alkyl shifts:
FMO analysis of [1,n]-alkyl shift, contd...
A. Stereochemistry of [1,5] alkyl shift:

11. Examples
Berson and Nelson’s stereo labeling experiment[1967]:
Explanation:

12. Woodward-Hoffmann Selection Rules
Order [i,j]
Suprafacial Antarafacial
H-shift
Thermal Photochemical Thermal Photochemical
4 n
[1,3]-shift
   
4 n + 2
[1,5]-shift
   
Alkyl
shift
4 n
[1,3]-shift

(Inversion)

(Retention)
4 n + 2
[1,5]-shift

(Retention)

(Inversion)

13. Problems on [1,n] shift
1. Define sigmatropic shift of order [i,j] with an example. Show by Frontier orbital
interaction diagram a [1,5]-H suprafacial shift is allowed by the Woodward-
Hoffmann rule for thermal pericyclic reactions
3. 5-methyl 1,3-cyclopentadiene, when heated with diethyl maleate, gives a mixture
of three isomeric products – Explain mechanistically
2. Explain why concerted [1,3]-H shift is thermally forbidden while [1,3]-carbon
migration occurs with inversion of configuration at migrating center?
4. What products are obtained when each of the following compounds heated
separately? Also find out the absolute configuration of the chiral centers and E/Z
configuration of the olefinic bond of the reactant and product in each case.

14. Problems on [1,n] shift, Contd…
5. Explain the following stereo specific observations.
5. Identify the product in the following reaction showing frontier orbital interaction.

15. Analysis of [i, j] -Sigmatropic shift
[3,3] –Sigmatropic Rearrangements:
There are two types of [3,3]-Sigmatropic rearrangements:
A. Cope and Oxy-Cope Rearrangements
B. Claisen and Claisen-Cope Rearrangements
A) Cope and oxy-Cope rearrangement:

16. A) Cope and oxy-Cope rearrangement, Contd…
Cope Rearrangements:

17. A) Cope and oxy-Cope rearrangement, Contd…
Other examples:

18. A) Cope and oxy-Cope rearrangement: Examples, Contd…
Oxy-Cope Rearrangements:
The rate of the oxy-Cope rearrangement reaction is found to be accelerated by
1010 – 1017 times when the reaction is carried out in presence of metal hydride.

19. B) Claisen and Claisen-Cope rearrangement:

20. A) Claisen and Claisen-Cope rearrangement, Contd…
Proposed mechanism:
Approach to establish mechanism:
Cross-over Experiment:

21. A) Claisen and Claisen-Cope rearrangement, Contd…
Approach to establish mechanism:
Cross-over Experiment:
Isotope labeling studies:

22. A) Claisen and Claisen-Cope rearrangement, Contd…
Stereochemistry of Claisen Rearrangements (Chirality transfer):
Carroll rearrangement:

23. Problems on [3,3]-Sigmatropic Rearrangements:
Q. 1 How is Claisen rearrangement related to Cope rearrangement:
Q. 4 Suggest mechanism for the following transformation.
Q. 3 Predict the product and the mechanism involved in the following reaction.
Q. 2 What is an oxy-Cope rearrangement? Give an example.
Q. 5 What starting material would give the following compound in Claisen
rearrangement?

24. Problems on [3,3]-Sigmatropic Rearrangements, contd…
Q. 6 Predict the product of the following reaction.
Q. 7 Complete the following reaction.
Q. 8 Identify the structure of the product in the following reaction.
Q. 9 Define Sigmatropic shift of order [i,j] with suitable example.
Q. 10 Predict product of the following reaction.

25. Thanks For Your Attention
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