Sigmatropic reactions for Undergraduate courses

A sigmatropic reaction in organic chemistry is a pericyclic reaction wherein the net result is one σ-bond is changed to another σ-bond in an uncatalyzed intramolecular reaction.

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

2. Asigmatropicreactionmaybedefinedasarearrangementreactionin
whichaσbondwithitsattachedgroupsmigratesfromoneendofoneor
moreconjugatedπsystemtotheotherpositionwithinthesamemolecule.
Thesigmatropicshiftoforder[i,j]isdefinedas:
“Theconcertedmigrationofaσbond,flankedbyoneormoreπconjugated
systems,toanewlocation,whoseterminiareiandjatomsawayfromits
originalpointofattachment.”

3. Stereochemistry of sigmatropic rearrangements
Themigrationofasigmabond(i.e.;bothbondbreakingandbondmaking)
maytakeplaceoverthesamefaceorbetweentheoppositefacesofthe
conjugatedpisystem.Ifthemigratingsigmabondwithitsattachedgroups
movesoverthesamefaceofthepisystem,thestereochemistryofmigration
iscalledsuprafacial(abovetheface)shift.Ifthemigrationofasigmabondis
associatedwiththeoppositefaceofthepisystem,themigrationiscalledan
antarafacial(oppositeface)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...
Rothetalfoundthatthereactionof(6S)–(2E,4Z)-6-methyl-2,4-octadiene-2-d
at250
o
Cprovideda1.5:1mixtureof(R)–(3E,5Z)-3-methyl-3,5-octadiene-7-d
(X)and(S)–(3E,5Z)-3-methyl-3,5-diene-7-d(Y),theproductsofsymmetry
allowedsuprafacial[1,5]hydrogenmigration.
5-Methylcyclopentadiene,havingaperfectgeometryfor[1,5]hydrogen
shift,undergoesaveryfacilethermallyallowedsuprafacial[1,5]H
migrationatroomtemperature

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
Bersonand 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.Definesigmatropicshiftoforder[i,j]withanexample.ShowbyFrontierorbital
interactiondiagrama[1,5]-HsuprafacialshiftisallowedbytheWoodward-
Hoffmannruleforthermalpericyclicreactions
3.5-methyl1,3-cyclopentadiene,whenheatedwithdiethylmaleate,givesamixture
ofthreeisomericproducts–Explainmechanistically
2.Explainwhyconcerted[1,3]-Hshiftisthermallyforbiddenwhile[1,3]-carbon
migrationoccurswithinversionofconfigurationatmigratingcenter?
4.Whatproductsareobtainedwheneachofthefollowingcompoundsheated
separately?AlsofindouttheabsoluteconfigurationofthechiralcentersandE/Z
configurationoftheolefinicbondofthereactantandproductineachcase.

14. Problems on [1,n] shift, Contd…
5.Explainthefollowingstereospecificobservations.
5.Identifytheproductinthefollowingreactionshowingfrontierorbitalinteraction.

15. Analysis of [i, j] -Sigmatropic shift
[3,3] –Sigmatropic Rearrangements:
There are two types of [3,3]-Sigmatropic rearrangements:
A.CopeandOxy-CopeRearrangements
B.ClaisenandClaisen-CopeRearrangements
A)Copeandoxy-Coperearrangement:

16. A)Copeandoxy-Coperearrangement,Contd…
Cope Rearrangements:

17. A)Copeandoxy-Coperearrangement,Contd…
Other examples:

18. A)Copeandoxy-Coperearrangement:Examples,Contd…
Oxy-Cope Rearrangements:
Therateoftheoxy-Coperearrangementreactionisfoundtobeacceleratedby
10
10
–10
17
timeswhenthereactioniscarriedoutinpresenceofmetalhydride.

19. B)ClaisenandClaisen-Coperearrangement:

20. A)ClaisenandClaisen-Coperearrangement,Contd…
Proposed mechanism:
Approach to establish mechanism:
Cross-over Experiment:

21. A)ClaisenandClaisen-Coperearrangement,Contd…
Approach to establish mechanism:
Cross-over Experiment:
Isotope labeling studies:

22. A)ClaisenandClaisen-Coperearrangement,Contd…
Stereochemistry of Claisen Rearrangements (Chirality transfer):
Carrollrearrangement:

23. Problems on [3,3]-Sigmatropic Rearrangements:
Q.1HowisClaisenrearrangementrelatedtoCoperearrangement:
Q.4Suggestmechanismforthefollowingtransformation.
Q.3Predicttheproductandthemechanisminvolvedinthefollowingreaction.
Q.2Whatisanoxy-Coperearrangement?Giveanexample.
Q.5WhatstartingmaterialwouldgivethefollowingcompoundinClaisen
rearrangement?

24. Problems on [3,3]-Sigmatropic Rearrangements, contd…
Q.6Predicttheproductofthefollowingreaction.
Q.7Completethefollowingreaction.
Q.8Identifythestructureoftheproductinthefollowingreaction.
Q.9DefineSigmatropicshiftoforder[i,j]withsuitableexample.
Q.10Predictproductofthefollowingreaction.

25. Thanks For Your Attention
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