1. 4
Sigmatropic
Reactions

2. Theory of Sigmatropic Shifts
Pericyclic Rearrangements
Rearrangement processes proceed directly by concerted
pericyclic mechanisms. (change in position of one s bond
as well as of several p bonds)
(example 1) Cope rearrangement (1,5-hexadiene)
(example 2) Migration of hydrogen atoms in conjugated
dienes

3. Theory of Sigmatropic Shifts
Pericyclic Rearrangements
Identification of the order of a particular sigmatropic shift
(1) The two atom forming the bond being broken are both
numbered 1
(2) The atoms in each direction from the bond being broken
are numbered consecutively as atoms 2, 3, and so on

4. Theory of Sigmatropic Shifts
Suprafacial and Antarafacial Shifts
Sigmatropic shift might result in retention or inversion
of geometry of the migrating group

5. Theory of Sigmatropic Shifts
Suprafacial and Antarafacial Shifts
Suprafacial rearrangement
The migrating group remains on the original face of the
p system

6. Theory of Sigmatropic Shifts
Suprafacial and Antarafacial Shifts
Antarafacial rearrangement
The migrating group ends up on the opposite face of the
p system (retention of the geometry)

7. Theory of Sigmatropic Shifts
Suprafacial and Antarafacial Shifts
Antarafacial rearrangement
The migrating group ends up on the opposite face of the
p system (inversion of the geometry)

8. Theory of Sigmatropic Shifts
Suprafacial and Antarafacial Shifts
Antarafacial rearrangement
The migrating group ends up on the opposite face of the
p system (inversion of the geometry)
[1,5] antarafacial migration of the methyl group

9. Theory of Sigmatropic Shifts
Application of Symmetry Conservation Rules
Aromatic transition state approach
Prediction of the stereochemistry of a sigmatropic
rearrangement is by counting the number of electrons
involved in the reaction and applying the aromatic
transition state approach
Sigmatropic shift involving 4n electrons proceed via
an odd number of antarafacial interactions which will
result in an odd number of inversions.
Sigmatropic shift involving 4n+2 electrons proceed via
an even number of antarafacial interactions which will
result in an even number of inversions

10. Theory of Sigmatropic Shifts
Application of Symmetry Conservation Rules
[1,5] shift of a hydrogen is a six-electron process
Three other stereoisomers, resulting from rearrangements
with two inversions, could be formed by allowed processes
However, their formation would require strained TS and is
extremely unlikely.

11. Theory of Sigmatropic Shifts
Frontier Orbital Approach
Regard all the reacting bonds (p bonds and s bonds)
and then to consider the symmetry of its HOMO.
In a Cope rearrangement, consider two allylic radicals.
HOMOs of the two radicals would have the same
symmetry and would recombine in a suprafacial manner

12. Theory of Sigmatropic Shifts
Frontier Orbital Approach
In a [1,5] hydrogen shift, the TS would resemble a
hydrogen atom associated with a pentadienyl radical
Since HOMO (f3) is symmetric, suprafacial migration
is possible. Suprafacial [1,5] shift would be allowed.

13. Theory of Sigmatropic Shifts
Frontier Orbital Approach
In a [1,3] hydrogen shift, the TS would resemble a
hydrogen atom associated with a allyl radical
Since HOMO (f2) is antisymmetric, suprafacial migration
would be forbidden. Antarafacial [1,3] shift would be
allowed.

14. Theory of Sigmatropic Shifts
Frontier Orbital Approach
In a [1,3] methyl group shift, suprafacial migration with
Inversion of the configuration of the migrating group
would be allowed. An antarafacial migration with
retention would be unlikely to occur.

15. Experimental Observation
[1,3] Sigmatropic Shifts

16. Experimental Observation
[1,3] Sigmatropic Shifts

17. Experimental Observation
[1,3] Sigmatropic Shifts

18. Experimental Observation
Effects of Polar Substituents

19. Experimental Observation
Effects of Polar Substituents
Less forbidden in highly polar system than in other nonpolar
molecules. The presence of strong EWG on one radical unit
in the TS would lower the energy levels of its MO. Similarly,
the presence of EDG on one radical would raise the energy
of its HOMO closer th that of the LUMO of the other radical
unit and would facilitate their interaction in the TS.

20. Experimental Observation
Photochemical [1,3] Shifts
- Photochemical [1,3] shifts, if cencerted, should yield
products resulting from suprafacial migrations.
- However, many of these reactions proceed via excited
triplet states (diradicals) which then recombine to form
the rearrangement products. (No stereospecific reaction)
- [1,3] Shifts resulting from excited singlet states of
alkene can also proceed via recombination of free
radical intermediates.
- In some cases rearrangement from singlet states do
appear to proceed by concerted mechanisms.

21. Experimental Observation
Photochemical [1,3] Shifts
Above reactions proceed stereospecifically with
retention of the configuration

22. Experimental Observation
[1,5] Shifts
Hydrogen migrations
- Suprafacial pathway in small cyclic system is common
at 200 oC and above.

23. Experimental Observation
[1,5] Shifts
Hydrogen migrations
- Suprafacial pathway in open-chain system is also
common at 200 oC and above.

24. Experimental Observation
[1,5] Shifts
Hydrogen migrations
- When indenes are heated to 100 oC, they undergo
entirely [1,5] shifts in stead of [1,3] hydrogen migrations.

25. Experimental Observation
[1,5] Shifts
Hydrogen migrations.

26. Experimental Observation
[1,5] Shifts
[1,5] migrations of alkyl, aryl and acyl groups
- Alkyl and aryl groups have never been observed to
undergo thermal [1,5] shifts in acyclic systems.
- Thermal [1,5] shifts in cyclic systems are common.

27. Experimental Observation
[1,5] Shifts
Migratory aptitudes in [1,5] shifts
- Carbonyl and carboxyl groups are the very best migrators,
with hydrogen atoms second best. Aromatic rings and
vinyl groups are also good migrators, but not as good as
carbonyl groups or hydrogens
Alkyl groups are the poorest migrators.
benzyl>cyclopropylmethyl>isopropyl>ethyl>methyl

28. Experimental Observation
[1,5] Shifts
Migrations in blocked aromatic systems
- Migrations can take place at relatively low temperature.
Why?
The blocked aromatic rings attain an appreciable degree
of aromatic stabililty in the TS for the rearrangements.

29. Experimental Observation
[1,5] Shifts
Migrations in blocked aromatic systems
(example 1)

30. Experimental Observation
[1,5] Shifts
Migrations in blocked aromatic systems
(example 2)

31. Experimental Observation
[1,5] Shifts
Migrations in blocked aromatic systems
(example 3)
Exceptionally rapid [1,5] benzyl group migration take
place when compound 17 is treated with base at 0 oC
because of the fact that a new aromatic ring is formed.

32. Experimental Observation
[1,5] Shifts
Migrations in blocked aromatic systems
(example 4)
Although [1,5] migration of saturated alkyl groups in
cyclohexadienes are very rare, gentle heating of 19
results in [1,5] alkyl shifts to form 20 because of the
fact that a new aromatic ring is formed.

33. Experimental Observation
[1,7] Shifts
Thermal [1,7] shift of a hydrogen atom proceed entirely
by antarafacial paths.

34. Experimental Observation
[1,7] Shifts
Thermal [1,7] shift of carbon atoms are extremely rare but
do occur in the interconversion of the bicyclic nonatrienes.

35. Experimental Observation
[1,7] Shifts
Photochemical [1,7] shifts should proceed suprafacially.
Hydrogen atom and alkyl groups have been observed in
cycloheptatrienes.