Kavita maheshwari sem II,spcgca

1. “Sigmatropic Reaction”
MSc Chemistry SEMESTER 2

2. SIGMATROPIC
REACTION

3. TABLE OF CONTENT
1. Introduction
2. Pericyclic Reaction
3. Types of Reaction
4. Sigmatropic Reaction
5. Migration of H-atom
6. [3,3] Rearrangement
7. References

4. Introduction
A Sigmatropic reaction in organic chemistry is a pericyclic
reaction where in the net result is one σ-bond is changed
to another σ-bond in an uncatalyzed intramolecular
reaction.

5. Pericyclic
Reaction
 Electrons move round a circle ad there are o positive or negative charges
on any intermediates indeed ,there are no intermediates at all.
 Follows concentrated mechanism.
 Stereo-specific in nature.
 Also known as no mechanism reaction.
 No reagent is required and not affected by solvent.
 Heat/hv (Photochemical condition)
 Occurs through transition state.
https://cdn1.byjus.com/wp-content/uploads/2022/02/word-image367.png

6. Pericyclic
Reaction
Sigmatropic
Reaction
Electrocyclic
Reaction
Ene Reaction
Cyclo-addition
Reaction

7.  It involves migration of Sigma bond within a 𝜋-system.
 Migration is always from saturated to unsaturated atom..
Sigmatropic Reaction
http://www.chem.ucla.edu/harding/IGOC/S/sigmatropic_reaction.html

8. Migration of H-atom
Supra- facial Antra- facial
Migration of H-atom from the same phase of multiple bond Migration of H-atom from the opposite phase of multiple bond
+
−
+
−
+
−
+
−
+
+
+
−
+
−
+
−
+
−
+
+
+

9. Thermal Condition-
HOMO=𝜓2
[1,3] H-Shift (𝟏 + 𝟑 = 𝟒𝝅ⅇ−)
+
−
+ +
−
+
−
+
+
Supra
Antra
Symmetry=Forbidden
Geometry=Allowed
+
−
+
−
+ +
−
+
−
+
−
+
+
−
+
Supra
Antra
Symmetry=Forbidden
Geometry=Forbidden
ℎ𝛾
Symmetry=Allowed
Geometry=Forbidden
Symmetry=Allowed
Geometry=Allowed
[1,3]
H-Shift
allowed In
photo
chemical
condition
Photochemical condition – HOMO = Ψ3
_
_
+
-
+
_
+

10. Thermal Condition-
HOMO=𝜓3
[1,5] H-Shift (𝟏 + 𝟓 = 𝟔𝝅ⅇ−)
+
−
+
−
+ +
−
+
−
+
−
+
+
−
+
Supra
Antra
Symmetry=Allowed
Geometry=Allowed
+
−
+ +
−
+
−
+
+
Supra
Antra
Symmetry=Allowed
Geometry=Forbidden
Photochemical Condition-
HOMO=𝜓4
ℎ𝛾
Symmetry=Forbidden
Geometry=Forbidden
Symmetry=Forbidden
Geometry=Allowed
[1,5]
H-Shift is
possible
under
thermal
condition
_
+
_
+
_
+

11. Thermal Condition-
HOMO=𝜓4
[1,7] H-Shift (𝟏 + 𝟕 = 𝟖𝝅ⅇ−)
+
−
+ +
−
+
−
+
+
Supra
Antra
Symmetry=Forbidden
Geometry=Allowed
+
−
+
−
+ +
−
+
−
+
−
+
+
−
+
Supra
Antra
Symmetry=Forbidden
Geometry= Allowed
Photochemical Condition-
HOMO=𝜓5
ℎ𝛾
Symmetry=Allowed
Geometry=Allowed
Symmetry=Allowed
Geometry=Allowed
[1,7]
H-Shift
possible In
thermal
condition
_
+
_
+
_
+

12. [3,3] Rearrangement
Claisen cope
Rearrangement
Cope
Rearrangeme
nt
Aza-Cope
Rearrangemen
t

13. CLAISEN REARRANGEMENT
 Substrate=Allyl-Aryl ether
https://cdn1.byjus.com/wp-content/uploads/2021/07/Claisen-Rearrangement-Example.png

14. REFERENCES
Pragati Prakash
Clayden