Sigmatropic rearrangement reactions (pericyclic reaction)

Sigmatropic Rearrangement reactions are a part of Pericyclic reaction.

1. P resented by : Sanskrita madhukailya Department of Chemistry Dibrugarh University 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 Sigma tropic 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. ψ 3 ψ 4 Thermal reaction Photochemical reaction

10. 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

11. 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

12. Stabilized by conjugation 3. 4.

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

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

15. 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

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

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

18. 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

19. [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

20. [2,3]- Sigmatropic Rearrangements

21. [2,3]- Sigmatropic Rearrangements using different hetero-atoms 1. 2.

22. =>

23. 3. => [2,3]

24. 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

25. phenylhydrazone enamine [3,3] sigmatropic rearrangement aminoacetal INDOLE 2. Fischer Indole Synthesis :

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