1. 2
Electrocyclic
Reactions

2. Introduction Electrocyclic reactions
Reactions in which conjugated polyenes close to form
rings or in which rings open to form polyenes simply by
having electrons chase to each other’s tails.
favor to ring formation
favor to ring opening
Pericyclic reactions
Electrocyclic reactions belong to a broader class of
reactions. These reactions proceed via transition state.
Usually favor the ring formation because p bond concert
to s bond, but strain in cyclobutene causes to open.

3. Introduction The difference in energy between the open-chain and
closed-ring isomers in electrocyclic reactions are usually
not great. Thus, partial conversion of the more stable isomers in this reactions to the less stable forms may
easily occur as steps.
(example) cis,cis-2,4-hexadiene converted to
trans,trans-2,4-hexadiene via two electrocyclic steps

4. Conrotatory and Disrotatory Processes
Conrotatory Process
In electrocyclic reaction, the terminal carbons of a polyene
or the saturated carbons of the cyclized form rotate in the
same direction (clockwise or counterclockwise direction)

5. Conrotatory and Disrotatory Processes
In electrocyclic reaction, the terminal carbons of a polyene
or the saturated carbons of the cyclized form rotate in the
different direction.
(one clockwise and other counterclockwise direction)

6. Conrotatory and Disrotatory Processes
Woodward-Hoffmann Rules
Stereochemistry of pericyclic reactions are governed by
selection rules. (allowed or forbidden)
Forbidden reactions are expected to occur with much
higher activation energy than allowed reaction.
“thermal electrocyclic reactions involving 4n electrons
are allowed if they proceed by conrotatory paths:
thermal electrocyclic reactions involving 4n+2 electrons
are allowed if they proceed by disrotatory paths”
Example: Ring closure of 1,3-butadiene and ring opening
of cyclobutene (4n electrons) – conrotatory path
Ring closure of 1,3,5-hexatriene and ring opening
of cyclohexadiene (4n+2 electrons) – disrotatory
path

7. Conrotatory and Disrotatory Processes
Conrotatory path (allowed)
No detection
Disrotatory path
(forbidden)

8. Conrotatory and Disrotatory Processes
Woodward-Hoffmann rules should apply to thermal
Electrocyclic reactions involving any number of electrons
4n electrons : conrotatory path allowed

9. Conrotatory and Disrotatory Processes
There are two modes of rotation in electrocyclic reaction
- clockwise and counterclockwise
Electron donating groups (EDG) and weakly electron
withdrawing groups (EWG) tend to end up “outside”
position, while strongly electron accepting substituents
rotate toward to “inside” position
Weakly EWG
outside position
Stongly EWG
Inside position

10. Conrotatory and Disrotatory Processes
Woodward-Hoffmann rules can account for the stabilities
of some molecules that might be quite reactive.
ring strain effect
Compound 2 is stable at up to 260 oC even though cyclobutenes normally open to butadienes on heating at steam-bath

11. Conrotatory and Disrotatory Processes
cis-Fused isomer requires a temperature nearly 200 oC
than trans-fused isomer to have the central ring open
Conrotatory
ring opening
allowed
- Formation of
Strained rings
from the cis
isomer

12. Conrotatory and Disrotatory Processes
Woodward-Hoffmann rules can explain the existence of
some extraordinarily high-energy molecules.
Aromatic ring (stable)
exothermic
Strained isomer
Compound 3 is stable at room temperature, although it
could isomerize to benzene simply by changing the bond
angle or by only slightly higher temperature.

13. Explanations for The Woodward-Hoffmann Rules
Symmetries of Molecular Orbitals (MO)

14. Explanations for The Woodward-Hoffmann Rules
Symmetries of Molecular Orbitals (MO)

15. Explanations for The Woodward-Hoffmann Rules
Symmetries of Molecular Orbitals (MO)

16. Explanations for The Woodward-Hoffmann Rules
Symmetries of Molecular Orbitals (MO)

17. Explanations for The Woodward-Hoffmann Rules
Symmetries of Molecular Orbitals (MO)

18. Explanations for The Woodward-Hoffmann Rules
Symmetries of Molecular Orbitals (MO)
A : Antisymmtric
S : Symmetric

19. Explanations for The Woodward-Hoffmann Rules
Symmetries of Molecular Orbitals (MO)
A : Antisymmtric
S : Symmetric

20. Explanations for The Woodward-Hoffmann Rules
Symmetries of Molecular Orbitals (MO)
HOMO (highest occupied molecular orbital)
LUMO (lowest unoccupied molecular orbital)

21. Explanations for The Woodward-Hoffmann Rules
The Frontier Orbital Approach
It is common for the chemical properties of atoms to be
approximated by considering only the valence orbitals
(highest occupied orbital). A similar approach can be
employed with molecules (HOMO : the frontier orbitals)
Disrotatory reaction

22. Explanations for The Woodward-Hoffmann Rules
The Frontier Orbital Approach
It is common for the chemical properties of atoms to be
approximated by considering only the valence orbitals
(highest occupied orbital). A similar approach can be
employed with molecules (HOMO : the frontier orbitals)
Conrotatory reaction

23. Explanations for The Woodward-Hoffmann Rules
Correlation Diagrams
A chart that follows the molecular orbitals of the starting
materials in a reaction and shows how they are converted
to the molecular orbitals of the products.

24. Explanations for The Woodward-Hoffmann Rules
Correlation Diagrams
The lowest-energy orbital
of triene j1 can converted
to lowest-energy orbital of
cyclohexadiene s.
The second lowest-energy
orbital of triene j2 can
converted to third lowest-
energy orbital of cyclo-
hexadiene j2 (HOMO).
The third lowest-energy
orbital of triene j3 can
converted to second lowest-
hexadiene j1.

25. Explanations for The Woodward-Hoffmann Rules
Correlation Diagrams
The lowest-energy orbital
of diene j1 can converted
to lowest-energy orbital of
cyclobutene s.
The second lowest-energy
orbital of diene j2 can
converted to third lowest-
energy orbital of cyclo-
butene p*.
The third lowest-energy
orbital of triene j3 can
converted to second lowest-
butene p.
This process would lead to
a very high-energy excited
state of the product.

26. Explanations for The Woodward-Hoffmann Rules
Correlation Diagrams

27. Explanations for The Woodward-Hoffmann Rules
Correlation Diagrams
The lowest-energy orbital
of diene j1 can converted
to lowest-energy orbital of
cyclobutene p.
The second lowest-energy
orbital of diene j2 can
converted to the lowest-
energy orbital of cyclo-
butene s.
The two filled orbitals of 1,3-butadienes can be converted
to the two lower-energy orbitals of cyclobutene, so that
The conrotatory ring closure is a symmetry-allowed process

28. Electrocyclic Reactions with Odd Numbers of Atoms
Theoretical Predictions
If SM is a cation
(two electrons),
the reaction is
allowed because
j1 and s orbitals
have the same
symmtry.
However, if the
chain is an anion
(four electrons),
an excited state
would be produced
and the reaction is
forbidden.
In contrast, a contotatory process is allowed for the
anion but is forbidden for the cation

29. Electrocyclic Reactions with Odd Numbers of Atoms
Reactions of Cations and Anions

30. Electrocyclic Reactions with Odd Numbers of Atoms
Reactions of Cations and Anions
The ring opening of cyclopropyl
derivatives to allylic cations do
proceed in disrotatory fashion.
At higher temperature, cation
6a isomerize to 6b, which in
turn, as the temperature is
raised, isomerizes to the least
crowded cation, 6c.

31. Electrocyclic Reactions with Odd Numbers of Atoms
Reactions of Cations and Anions 7

32. Electrocyclic Reactions with Odd Numbers of Atoms
Reactions of Cations and Anions

33. Electrocyclic Reactions with Odd Numbers of Atoms
Reactions of Cations and Anions

34. Electrocyclic Reactions with Odd Numbers of Atoms
Formation and Cyclization of Dipolar Molecules

35. Electrocyclic Reactions with Odd Numbers of Atoms
Formation and Cyclization of Dipolar Molecules
Conrotatory
path allowed

36. Electrocyclic Reactions with Odd Numbers of Atoms
Formation and Cyclization of Dipolar Molecules

37. Photochemical Cyclizations
Photochemical Reactions

38. Photochemical Cyclizations
Photochemical Reactions
Woodward-Hoffmann rules to photochemical cyclizations
The stereochemistry of photochemical cyclization should be opposite to that of thermal cyclization : 4n electrons (disrotatory) and 4n+2
electrons (conrotatory)

39. Photochemical Cyclizations
Stereochemistry of Photochemical Electrocyclic Reactions

40. Photochemical Cyclizations
Stereochemistry of Photochemical Electrocyclic Reactions

41. Photochemical Cyclizations
Nonstereospecific Ring Openings
1,3-Butadiene cyclize on photoirradiation with UV light with
Wavelengths above 220 nm to form the predicted disrotatory
Ring closure products

42. Photochemical Cyclizations
Nonstereospecific Ring Openings