This is essential for UG Chemistry course

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

2. Syllabus
Pericyclic Reactions:
Mechanism, stereochemistry, regio-selectivity in case of:
Electrocyclic reactions: FMO approach involving 4π- and 6π-electrons
(thermal and photochemical) and corresponding cycloreversion reactions.
Cycloaddition reactions: FMO approach, Diels-Alder reactions,
photochemical [2+2] cycloadditions.
Sigmatropic reactions: FMO approach, sigmatropic shifts and their order;
[1,3]- and [1,5]-H shifts and [3,3]-shifts with reference to Claisen and Cope
rearrangements.
Paper – C12T (Organic)

3. Pericyclic reactions are characterized by a concerted bonding changes
taking place through the reorganization of π and σ bonding electron in a
cyclic T.S. which is associated with cyclic array of interacting orbitals.
Cyclic Transition State
Concerted Pericyclic Reaction
Concerted ionic Reaction
Transition State

4. 1. Bonding changes must be concerted i.e.; all bonds breaking and bond
forming steps are simultaneous but not necessarily synchronous. That means
that all bond breaking and making may not occur to the same extent at all
stages during the formation of transition state.
2. Pericyclic reactions are reversible in nature and follow the microscopic
reversibility path.
3. The reaction involves no distinct polar intermediates during the course of
the reaction.
4. Reaction proceeds in a single step through the formation of non polar
transition state.
5. There is little solvent effect on the rate of pericyclic reactions (unless the
reactants themselves happen to be charged species) and normally occur in the
gas phase with no solvent.
6. There is no nucleophilic or electrophilic component.

5. Characteristic Features , cont….
7. Normally no catalyst is needed to promote the reactions. However,
many transition metal complexes can catalyze pericyclic reactions by
virtue of their d-orbital participation. Lewis acids also catalyze many
pericyclic reactions, either directly, or by changing the mechanism of
the reaction so that it becomes a stepwise process.
8. Pericyclic reactions normally show very high stereo specificity and
regieselectivity.
9. No. of σ and or π electrons are important in predicting the
stereochemistry of the products.
10. Pericyclic reactions can be promoted by light as well as heat. The
stereochemistry of products under two sets of conditions is different
and it is just opposite.

6. Classifications

7. Classification , cont….

8. Classification , cont….

9. Symmetry properties of molecular orbitals
The knowledge of molecular orbitals of conjugated polyene systems and their
symmetry properties is essential in understanding the theory of pericyclic
reactions through the construction of orbital model for bonding. Before that
let me explain the symmetry properties of molecular orbitals.
A symmetry operation is a geometric operation (rotation, reflection etc)
which when performed on an object produces the indistinguishable form of
the original structure of the object. The respective symmetry elements of
different symmetry operations are i) Cn (simple axis of symmetry) ii) m
(mirror plane of symmetry) iii) i (centre of symmetry) iv) Sn (alternating
axis of symmetry). It is not necessary to consider all the symmetry elements
present in the reacting molecules or the product molecules. In order to
explain the pericyclic reactions only two-fold axis of symmetry (C2) and
mirror plane of symmetry (m) are considered.

10. Molecular orbitals and their symmetry
Pi-MO’s of Ethylene

15. 1. Frontier orbital interaction method (FOI)
(Woodward, Hoffmann, Longuet-Higgins and Abrahamson)
2. Orbital symmetry correlation method (Conservation of
orbital symmetry)
(Woodward, Hoffmann, and Fukui)
3.Hückel-Möbius transition state (aromatic and antiaromatic
TS) theory
(Dewar and Zimmerman)

16. Frontier Molecular Orbital Theory (FMO Theory)
The covalent bond formation occurs by the interactions between a filled orbital of one
molecule and a vacant orbital of the other, having appropriate symmetry. Though the
interactions are small, they are stabilizing. The energy separation between the
interacting occupied and vacant orbital is a measure of the magnitude of covalent
force. The smaller the energy difference, the greater is the stabilization due to their
interaction.

17. Therefore, Stabilization will be greater when filled and empty orbitals of equal
(degenerate orbitals) or nearly equal energy interact (Figure 2a) than the
orbitals of unequal energy (non degenerate orbitals) interact (Figure 2b). So,
molecular orbitals having similar energies are the essential criteria for a
strongest interaction between two interacting orbitals.
Frontier Molecular Orbital Theory, cont…..

18. Frontier Molecular Orbital Theory, cont…..
Since the energy difference between HOMO and LUMO is very small, the
interaction between them will be very large and hence a stronger covalent bond
will be formed by their interaction. These HOMO and LUMO are called Frontier
Molecular Orbitals (FMO). The interactions between HOMO-LUMO are called
frontier molecular orbital interactions (FMO interaction).

21. Stereochemistry of electrocyclic reactions
4np-Electron Systems:
They are highly stereospecific. The stereochemistry of the product depends upon the
following three factors:
1. The Number of electrons (4n or 4n+2) involved.
2. The stereochemistry of the reactant.
3. Mode of activation (thermal or photochemical).
SS
RR
Me
Me
Me
Me
Me
Me
Me
Me
Enantiomers
thermal
E,E-isomer
thermal
RS
Me
Me Me
Me Me
Me
Meso
Z,E-isomer

22. (4n+ 2) p-Electron Systems:
Meso
Me
Me Me
Me
Me
Me
RS
thermal
E,E-isomer
Me
Me
Me
Me
Me
Me
Me
Me
SS
RR
Enantiomers
thermal
Z,E-isomer

23. Prediction of stereochemistry of electrocyclic reactions:
Frontier molecular orbital theory

24. FMO description of electrocyclic reactions:
In FMO approach, Woodward and Hoffmann proposed that the stereochemistry and the
allowedness of electrocyclic reactions is controlled by the symmetry properties of the highest
energy occupied molecular orbital (HOMO) of the open chain form of the reactant-product
pair. If this HOMO is symmetric with respect to two-fold axis (C2) symmetry, the reaction
proceeds by conrotatory pathway and if it is symmetric with respect to mirror plane (m) of
symmetry, the direction of rotation is disrotatory.
Thermal
ring
closure
of
4np-
Electron
Systems
:

25. Photochemical ring closure of 4np-Electron Systems :

26. Thermal ring closure of (4n+2) p-Electron Systems :

27. Photochemical ring closure of (4n+2) p-Electron Systems :

28. Woodward-Hoffmann selection rules:
“all conjugated polyenes (4n or 4n+2 π electrons system) with HOMO’s
having C2 symmetry will undergo electrocyclic ring closure or ring
opening reaction by conrotatory motion and with HOMO’s having plane of
symmetry (m) will undergo electrocyclic ring closure or ring opening
reaction by disrotatory motion.”
π-System Mode of activation Symmetry of
HOMO
Allowed pathway Stereochemistry of
the reactant
Stereochemistry of
the product
4n π-System
Thermal (Δ) C2 Conrotation E,Z or Z,E Cis
Z,Z or E,E Trans
Photochemical
(hγ)
m Disrotation E,Z or Z,E Trans
Z,Z or E,E Cis
(4n+2) π-System
Thermal (Δ) m Disrotation E,Z or Z,E Trans
Z,Z or E,E Cis
Photochemical
(hγ)
C2 Conrotation E,Z or Z,E Cis
Z,Z or E,E Trans

29. Equilibrium and microscopic reversibility in electrocyclic
reactions and its deviation:

30. Deviations ………
Other examples:
Although Dewar benzene is less stable than its isomer benzene by 71 Kcal / mole, its conversion into benzene
is extremely slow with an Eact. of about 37 Kcal / mole.

31. Examples of Electrocylic reactions

32. Superimposable Mirror Images
Exactly the same compound
MESO Compound
R
S R S
DISROTATORY
y3 – HOMO
Examples of Electrocylic reactions, cont………
The following triene undergoes a thermal electrocyclic cyclisation. Using FMOs
identify all the products.

33. Examples of Electrocylic reactions, cont………
The following triene undergoes a photochemical electrocyclic cyclisation. Using
FMOs identify all the products.
Nonsuperimposable Mirror Images
Active Compound
CONROTATORY

34. The two diastereoismeric trienes 1 and 2 undergo thermal electrocyclic cyclisation reactions each
affording a pair of disubstituted conjugated cyclic dienes. Identify all four products by constructing
the transition state geometries, and state the stereochemical relationships that exist between the pairs
of stereoisimers formed from each reaction and the stereochemical relationship of the products
between the pair of reactions
y3 – HOMO
Enantiomers
R
S R S
DISROTATORY
Enantiomers
R R S S
DISROTATORY
Examples of Electrocylic reactions, cont………

35. Examples of Electrocylic reactions, cont………
The two diastereoismeric trienes 1 and 2 undergo photochemical electrocyclic cyclisation reactions
each affording a pair of disubstituted conjugated cyclic dienes. Identify all four products by
constructing the transition state geometries, and state the stereochemical relationships that exist
between the pairs of stereoisimers formed from each reaction and the stereochemical relationship of
the products between the pair of reactions
Enantiomers
CONROTATORY
Enantiomers
R
S R S
CONROTATORY

36. H
H H H
0% 100%
4p Electron Process
H H
CONROTATORY
H
H
y2
HOMO
Butadiene
6p Electron Process
H
H
H H
DISROTATORY
y3
HOMO
1,3,5-Hexatriene
Examples of Electrocylic reactions, cont………

37. H H
H H H
H
H
H
GEOMETRICALLY
IMPOSSIBLE: Hydrogen placed
inside a six-membered ring
CON
H H
H
H
H H HH
CON
H H
H H
Using FMOs we can rationalise why the two diastereoisomers have such
different reactivities.
Examples of Electrocylic reactions, cont………

38. Cascade Electrocyclic reactions
Me
Me
H
H
Me
Me
Me Me
H
H
Me
Me
y4 (3 nodes 9/4)
of 1, 3, 5, 7-octatetraene
4n - CONROTATORY
Me Me
H
H
y3 (2 nodes)
of 1, 3, 5-hexatriene
(4n + 2) - DISROTATORY
Me
Me
H
H
Me
Me
Me Me
H
H
Me Me
H
H
H
H
Me
Me

39. Cascade reactions, cont ………

40. Tandem Electrocyclic Reaction
The arrow pushing mechanism reveals that the reaction involves the ring closure of two
1,3,5-hexatriene systems. Thus, need to consider y3 HOMO of 1, 3, 5-hexatriene.
Use FMOs to predict the stereochemical
outcomes in the reaction scheme right.
In principle, there are two possible
products. Which will be formed in
highest yield. Justify your answer.
H
H
H
H
H
H
H
H
Thermodynamic
Product. Least
sterically hindered
Disrotatory
of both
triene systems
H
H
H
H
H
H
H
H

41. Electrocyclisation of cation (Nazarov cyclisation)