Sigmatropic Rearrangement reactions are a part of Pericyclic reaction.

1. P r e s e n t e d b y :
S A N S K R I T A MA D H U K A I L Y A
D e p a r t m e n t o f C h e m i s t r y
D i b r u g a r h U n i v e r s i t y
SIGMATROPIC
REARRANGEMENT

2. CONTENTS
 Sigmatropic Rearrangement
 Suprafacial & Antrafacial processes
 FMO approach for allowed & forbidden reactions
 Categories of sigmatropic rearrangement
 Cope rearrangement
 Claisen rearrangement
 [2,3]- sigmatropic rearrangement
 References

3. SIGMATROPIC REARRANGEMENT
• A type of Pericyclic Reaction, involving intramolecular migration of an atom/group across
a conjugated pi-system
• σ-bond adjacent to conjugated pi-system is broken; new σ-bond formed at the end of π
system
• numbering from saturated atom;
• uncatalyzed & solvent-less reaction
• thermal as well as photochemical (rare) sigmatropic rearrangements possible
• σ-bond shift, so Sigmatropic Rearrangement

4. Formation of the new σ-bond
[1,3] Sigmatropic shift
examples:

5. [1,5] Sigmatropic shift
[3,3] Sigmatropic shift
[2,3] Sigmatropic shift
Δ
Δ
Δ

6. Suprafacial and Antrafacial processes
Migration of σ-bond across the π-bonds is through two different stereo-chemical
sources.
 migrated σ-bond gets moved across the same face of the conjugated system,
suprafacial process
 migrated σ-bond gets moved across the opposite face of the conjugated system,
antrafacial process
Diagrammatically showing the
supra & antrafacial interaction :

7. FMO approach for allowed & forbidden reactions
Considering a [1,5]-sigmatropic reaction:
Mechanism:

8. Thermal reaction Photochemical reaction

9. ψ3 ψ4
Thermal reaction Photochemical reaction

11. Categories of Sigmatropic rearrangement reactions
 Claisen Rearrangement - [3,3] Sigmatropic rearrangement
 Cope Rearrangement - [3,3] Sigmatropic rearrangement
• Oxy-Cope Rearrangement
• Claisen-Cope Rearrangement
• Aza-Cope Rearrangement
 [2,3] Sigmatropic Rearrangement reactions

12. Cope Rearrangement
•All the 6 atoms involved are Carbon atoms
•Ex. : Thermal rearrangement of 1,5-diene through [3,3] sigmatropic rearrangement
Many cope rearrangements results in a
mixture of reactants & products that are of
comparable stability
1.
2. Ring strain can make one product
predominate at equilibrium

13. Stabilized by conjugation
3.
4.

14. 5. Disadvantage:
High
Temperature
6.
Valuable synthetic method

15. 7. AZA-COPE REARRANGEMENT
Reversibility can serve as a disadvantage
Excellent synthesis of substituted pyrrolidines

16. Claisen rearrangement
1. Common example- thermal conversion of allyl aryl ethers to give o-allyl phenol via
[3,3] sigmatropic rearrangement
Aryl-allyl ether
O-allyl phenol
Δ

17. If the ortho positions are blocked:
p-allyl phenol
* If both ortho & para- positions are blocked; NO REACTION!

18. 2. Excellent stereoselectivity using allyl-vinyl ethers
Stereochemistry depends upon the geometry of the double bonds of the starting material
Δ
Δ
Δ
A

19. Δ Δ
E-
Z-
Z-
E-
Thus, rearrangement of E,E- (or Z,Z-) diene gives (>=95%) A, whereas the E,Z-
dienes give (>=95%) B
B

20. [2,3]- Sigmatropic Rearrangements
The starting material must of the type:
X can be any heteroatom
(anything except C)
Any element having lone pair
Allylic system
This carbon should always be
attached to the hetero-atom

21. [2,3]- Sigmatropic Rearrangements

22. [2,3]- Sigmatropic Rearrangements using different hetero-atoms
1.
2.

23. =>

24. 3.
=>
[2,3]

25. Some reactions involving sigmatropic rearrangement
1. Industrial synthesis of Citral (intermediate in the synthesis of Vit-A)
3-methyl-3-butenal 3-methyl-2-butenal

26. phenylhydrazone enamine
[3,3] sigmatropic
rearrangement
aminoacetalINDOLE
2. Fischer Indole Synthesis:

27. REFERENCES
1. J. Clayden, N. Greeves, S. Warren, Organic Chemistry, Second Edition, 910-918.
2. W. Carruthers, I. Coldham, Modern Methods of Organic Synthesis, Fourth Edition,
239-253
3. J. M. Hornback, Organic Chemistry, Second Edition, 956-1010.