The document discusses electrocyclic reactions, which involve the conversion of a conjugated polyene to an unsaturated cyclic compound with one less carbon-carbon double bond. It notes that these reactions can occur thermally or photochemically, and with high stereoselectivity. It provides examples of electrocyclic reactions involving butadiene and hexatriene, and discusses the correlation between molecular orbital symmetry and the conrotatory or disrotatory nature of the reaction. It also addresses electrocyclic reactions involving reactants with an odd number of atoms, such as cations and anions, as well as photochemical cyclizations.

1. Dr. Harish Chopra
Professor
Department of Chemistry
SLIET, LONGOWAL

2. Electrocyclic reactions are a class of pericyclic
reactions in which a conjugated polyene inter-
converts with an unsaturated cyclic compound
containing one less carbon–carbon double bond than
the polyene. The reactions can be promoted
thermally or photo-chemically and take place with a
very high degree of stereo-selectivity.

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

6.  In a Thermal Reaction, an open-chain system
containing 4n π electrons, the orbital symmetry of the
highest occupied molecule orbital (HOMO) is such that
a bonding interaction between the ends must involve
overlap between orbital envelopes on opposite faces of
the system and this can only be achieved in a
CONROTATORY process.
 In open systems containing (4n + 2) π electrons,
terminal bonding interaction within ground-state
molecules requires overlap of orbital envelopes on the
same face of the system, attainable only by
DISROTATORY displacements.
 In a PHOTOCHEMICAL REACTION an electron in the
HOMO of the reactant is promoted to an excited state
leading to a reversal of terminal symmetry relationships
and stereo-specificity.

7. Buta-1,3-diene has 4 -electrons in the ground
state and thus proceeds through a
conrotatory ring-closing mechanism.

8. In the electrocyclic ring-closure of the
substituted hexa-1,3,5-triene, the reaction
proceeds through a disrotatory mechanism.

9. In the case of a photochemically driven
electrocyclic ring-closure of buta-1,3-diene,
electronic promotion causes to become the
HOMO and the reaction mechanism must be
disrotatory..

10. A molecular orbital correlation diagram
correlates molecular orbitals of the starting
materials and the product based upon
conservation of symmetry.
If a symmetry element is present throughout
the reaction mechanism (reactant, transition
state, and product), it is called a conserved
symmetry element.
The transition state of a conrotatory
closure has C2 symmetry, whereas
the transition state of a disrotatory
opening has mirror symmetry.

11. The correlation diagram of CONROTATORY
4 electron electrocyclic ring closure of 1,3-
butadiene (the symmetry element is the C2
axis ) is shown below:

12. The correlation diagram of DISROTATORY 4
electron electrocyclic ring closure of 1,3-
butadiene (the symmetry element is the σ
mirror plane) is shown below :

13. Thermal and Photochemical Cyclizations of
1,3,5-Hexatrienes.

14. Based on these rules, following generalization is seen
in electrocyclic reactions:
"It should be emphasized that our hypothesis specifies
in any case which of two types of geometrical
displacements will represent a favored process, but
does not exclude the operation of the other under very
energetic conditions."

15. Electrocyclic reactions in Reactants with
Odd number of Atoms
DISROTATORY If
reactant is a cation
(two electrons), the
reaction is
ALLOWED because
y1 and s orbitals
have the same
symmetry.
However, if the
chain is an anion
(four electrons), an
excited state would
be produced and the
reaction is
FORBIDDEN.
CONTOTATORY PROCESS:
Allowed: Anion
Forbidden: Cation

16. Electrocyclic reactions in Reactants with
Odd number of Atoms (Cation & Anion)

17. Electrocyclic reactions in Reactants with
Odd number of Atoms (Cation & Anion)
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.

18. Electrocyclic reactions in Reactants with
Odd number of Atoms (Cation & Anion)

19. Electrocyclic reactions in Reactants with
Odd number of Atoms (Cation & Anion)

20. Electrocyclic reactions in Reactants with
Odd number of Atoms (Cation & Anion)

21. Electrocyclic reactions in Reactants with
Odd number of Atoms (Cation & Anion)
[Formation and Cyclization of Dipolar Molecules]

22. Electrocyclic reactions in Reactants with
Odd number of Atoms (Cation & Anion)
[Formation and Cyclization of Dipolar Molecules]
The reaction
proceeds through
CONROTATORY
PROCESS

23. Electrocyclic reactions in Reactants with
Odd number of Atoms (Cation & Anion)
[Formation and Cyclization of Dipolar Molecules]

24. Electrocyclic reactions
[Photochemical Cyclizations]

25. Electrocyclic reactions
[Photochemical Cyclizations]

26. Electrocyclic reactions
[Photochemical Cyclizations]
STEREOSPECIFIC REACTIONS

27. Electrocyclic reactions
[Photochemical Cyclizations]
NON-STEREOSPECIFIC REACTIONS
1,3-Butadiene cyclize on photo-irradiation with UV light
with wavelengths above 220 nm to form the predicted
DISROTATORY ring closure products

28. Electrocyclic reactions
[Photochemical Cyclizations]
NON-STEREOSPECIFIC REACTIONS

29. Examples of Electrocyclic reactions
Ring-opening of the aldehyde, formed by oxidation of
the alcohol, occurs to give the diene with>97% isomeric
purity, in which the aldehyde rather than the alkyl
group has rotated ‘inward’

30. Ring-opening of Benzocyclobutenes
to o-Quinodimethanes.
Synthesis of the steroid estrone, the
benzocyclobutane, prepared by a cobaltmediated
cyclotrimerization, was converted on heating to
the o-quinodimethane, which undergoes
cycloaddition to the tetracycle.

31. E,Z,E-triene 340 is converted to the cis product
341 with >99.5% diastereomeric purity,
indicating a completely stereospecific
THERMAL DISROTATORY electrocyclization. The
isomeric E,Z,Z-triene 342 is converted to the trans
product 343. In this latter case the product 344
(derived from the diene 343 by a 1,5-hydrogen
shift) is also produced and the triene 342 has
been found to interconvert readily with the
Z,Z,Z-isomer by consecutive 1,7-hydrogen shifts.
Electrocyclic Cyclization of 1,3,5-Hexatrienes

32. In the synthesis of the carbazole hyellazole 346,
the divinyl-indole 345 was heated to promote
electrocyclization, followed by dehydrogenation
with palladium on charcoal to give hyellazole.
The parent substrate, stilbene can be converted to
phenanthrene, a process that involves conrotatory
electrocyclization under photochemical conditions
Examples of Electrocyclic reactions

33. In a synthesis of cervinomycin A,
photochemical electrocyclization of the
mixture of E- and Z-diaryl alkenes 347 gave
the polycyclic aromatic compound 348 after
oxidation with iodine.
Examples of Electrocyclic reactions

34. The cyclization of a pentadienyl cation to a
cyclopentenyl cation offers a useful entry to
five-membered carbocyclic compounds. One
such reaction is the Nazarov cyclization of
divinyl ketones. The Nazarov cyclization is a 4-
electron cyclization and occurs thermally by a
conrotatory process. The stereochemical outcome
across the new carbon–carbon bond is often
obscured by the loss of a proton at one of these
centres during the cyclopentenone formation
Examples of Electrocyclic reactions

35. Thank You