The document provides a comprehensive overview of pericyclic reactions, detailing their characteristics, types, and mechanisms such as electrocyclic, cycloaddition, and group transfer reactions. It explains the Woodward-Hoffmann rules that govern stereochemistry based on thermal or photochemical conditions, as well as real-world applications like the synthesis of vitamin D3 and beta-lactams. Additionally, the document covers specific reaction examples, including ene reactions and ketene cycloadditions, emphasizing their significance in organic synthesis.

1. Pallavi Kumbhar
Roll No: SMCHEM2122009
Msc Part-II (Paper-1)
Pericyclic
Reactions

2. Contents
01 What are Pericyclic Reactions?
02 Classification
03 Electrocyclic Reaction
04 Stereochemistry
05 Woodward-Hofmann Rules
06 Electrocyclic Reaction in Allylic Systems
07 Naturally Occurring Electrocyclic Reaction
08 Cycloaddition Reaction
09 Alder-Ene Reactions
10 Carbonyl-Ene Reactions
11 Ketene Cycloaddition
12 Synthesis Of Beta Lactams

3. What Are Pericyclic Reactions?
CHARACTERISTICS:
 Single step reaction.
 No intermediate is formed.
 The breaking and making of bonds occurs simultaneously.
 Independent of solvent and catalyst.
 Reactions are initiated by thermal or photochemical conditions.
 Generally stereospecific in nature.
“Pericyclic reactions are the concerted reactions involving reorganization of electrons
which occur by the way of a single cyclic transition state.”
1
PERICYCLIC REACTIONS
09/09/2021

4. Classification
Intramolecular cyclization
of conjugated polyenes.
ELECTROCYCLIC
REACTIONS
Addition of 2 unsaturated
systems leading to formation
of cyclic product.
CYCLOADDITION
REACTIONS
Involves migration of σ-bond
within π framework.
SIGMATROPIC
REARRANGEMENT
The transfer of group from one
molecule to another and formation of
new σ-bond at the expense of π-bond.
GROUP TRANFER
REACTIONS
Special class of cycloadditions in
which the two σ-bonds are either
made or broken to the same atom.
CHELETROPIC
REACTIONS
1 2 3 4 5
2
PERICYCLIC REACTIONS
09/09/2021

5. It is a reversible reaction that involves ring closure of a conjugated polyene to
a cycloalkene, or ring opening of a cycloalkene to a conjugated polyene.
RING-CLOSING REACTION:
Electrocyclic Reactions
RING-OPENING REACTION:
3
PERICYCLIC REACTIONS
09/09/2021

6.  Electrocyclic reaction is a reversible reaction that involves ring closure of a conjugated polyene to a
cycloalkene, or ring opening of a cycloalkene to a conjugated polyene.
 The ring closure of 1,3,5-hexatriene forms 1,3-cyclohexadiene product with formation of one new
sigma bond. The pi bonds are shifted which is shown with the help of arrows and one of the pi bond is
broken to form a new sigma bond.
 Similarly, ring opening of cyclobutene forms 1,3-butadiene product but here the sigma bond is broken
to form two pi bonds.
 Usually, most of the electrocyclic reactions are ring closing and only a few are ring opening reactions
such as cyclobutene as they are conformationally strained systems.
 These reactions are happening within the molecule and therefore we can say that they intramolecular.
Electrocyclic Reactions (Explaination)
4
PERICYCLIC REACTIONS
09/09/2021

7. It is a special kind of stereo selectivity observed in electrocyclic
reactions as the preference for inward or outward rotation of
substituents in disrotatory or conrotatory reactions.
Torquoselectivity
Disrotatory Conrotatory
5
PERICYCLIC REACTIONS
09/09/2021

8.  Torquoselectivity is a special kind of stereo selectivity which is mainly observed in electrocyclic reactions
as the preference for inward or outward rotation of substituents. These are termed as disrotatory and
conrotatory.
 As we know that the like phases of the orbitals overlap to form a bond and these type of reaction is
symmetry allowed. The reaction cannot occur between orbitals of opposite phase because it is symmetry
forbidden.
 Therefore, for overlapping of like phases we need to rotate the orbitals in clockwise or anticlockwise
direction depending upon the orientation of orbitals.
 When like phases of the p orbitals are on the same side of the molecule, the two orbitals must rotate
in opposite directions ie, one clockwise and one anticlockwise and this type of rotation is said to be
disrotatory.
 On the other hand, when like phases of the p orbitals are on opposite sides of the molecule, the two
orbitals must rotate in the same direction ie, both clockwise or both anticlockwise and this type of
rotation is said to be conrotatory.
Torquoselectivity (Explaination)
6
PERICYCLIC REACTIONS
09/09/2021

9. Stereoselectivity
Cis product only
Trans product only
Stereochemistry of the product of an electrocyclic reaction depends on
whether the reaction is carried out under thermal or photochemical reaction
conditions.
hv
∆
(2E,4E)-2,4- hexadiene
7
PERICYCLIC REACTIONS
09/09/2021

10.  The stereochemistry of electrocyclic reaction depends on whether the reaction is carried out
under thermal or photochemical reaction conditions.
 For example, the reaction of 2,4-Hexadiene is carried out both in thermal and photochemical
conditions.
 In thermal condition, only trans product is obtained and
 In photochemical condition, only cis product is obtained.
How do we decide if the reaction will proceed with conrotatory mode or disrotatory mode?
 Woodward and Hoffmann predicted that the number of double bonds which taking part in the
reaction and the type of reaction (whether it is thermal or photochemical), this decides whether
rotation is conrotatory or disrotatory. For this, Woodward-Hofmann rules were prepared.
Stereoselectivity (Explaination)
8
PERICYCLIC REACTIONS
09/09/2021

11. Woodward-Hoffmann Rules
(4n)π
[Even Π Bonds]
Thermal
(4n+2)π
[Odd Π Bonds]
Photochemical
Conrotatory
Reaction Conditions Allowed Mode Of Ring Closure
No. Of Π Electrons
Thermal
Photochemical
Disrotatory
Disrotatory
Conrotatory
9
PERICYCLIC REACTIONS
09/09/2021

12. Electrocyclic Ring-closure Reaction In
Conjugated Polyene Systems
4nπ conjugated system (4n+2) π conjugated system
10
PERICYCLIC REACTIONS
09/09/2021
∆
hv
Disrotatory
hv
Conrotatory
∆
Disrotatory
Conrotatory
H H CH3
H3C
+ +
- -
H
H
CH3
CH3
+ +
- -
Cis-7,8-Dimethyl-1,3,5-cyclooctatriene
H H CH3
H3C
+
+
-
-
H
H CH3
CH3
+ +
- -
Trans-7,8-Dimethyl-1,3,5-cyclooctatriene
H H CH3
H3C
+ +
- -
H H CH3
H3C
+
+
-
-
CH3
H
H
+ +
- -
CH3
Cis-5,6-Dimethyl-1,3-cyclohexadiene
Trans-5,6-Dimethyl-1,3-cyclohexadiene
CH3
H
H
+ +
- -
CH3

13. Cis Product Trans Product Trans Product Cis Product
How To Determine The
Configuration Of The Product
Substituent Pointed In The
Opposite Direction
Substituent Pointed In The
Same Direction
Disrotatory Disrotatory
Conrotatory Conrotatory
Disrotatory
Disrotatory
Conrotatory
Conrotatory
11
Pericyclic Reactions
09/09/2021

14. Electrocyclic Ring-closure In Allylic
Systems
4nπ Conjugated System (4n + 2)π Conjugated System
allyl carbanion allyl carbocation
hv
∆
CH
CH2
CH2
-
CH
CH2
CH2
+
hv
∆
hv
∆
-
12
Pericyclic Reactions
09/09/2021
hv
disrotatory
conrotatory
∆
conrotatory
disrotatory
+
+ +
- -
CH3
H3C
-
H H
CH3
CH3
-
+
+
-
-
CH3
H3C
-
H
H
CH3
CH3
-
+
+
-
-
+
+ +
- -
+
+
+
- -
+
+
+
- -
+

15. Electrocyclic Reactions In Cationic Allyl
Systems
 Cyclization of allyl cation is an intermediate step in Favorskii rearrangement also. For cyclic α-halo
ketones, the Favorskii rearrangement constitutes a ring contraction step, as shown below;
 Opening of a cyclopropyl cation to an allylic cation.
13
PERICYCLIC REACTIONS
09/09/2021

16.  The electrocyclic reactions in cationic allyl systems is known as the smallest system which is capable of
undergoing an electrocyclic reaction.
 In the reaction, since tosylate is a good leaving group, it gets removed and ring opening reaction takes
place in thermal conditions leading to formation of allylic cation.
 Here, the removal of tosylate group and opening of ring occurs simultaneously and therefore, cyclopropyl
cation is not considered an intermediate.
 Since there are 2 electrons present, the reaction proceeds via disrotatory motion. The cation formed is
then undergone hydrolysis followed by deprotonation and OH group is added in the final product.
 The stereochemistry of the cyclopropyl molecule is maintained in the product.
These type of reactions is mainly applied as an intermediate step in Favorskii rearrangement.
 Rearrangement of cyclopropanones and ketones containing halogens at alpha position leads to formation of
carboxylic acid derivatives and ring contraction takes place.
(Explaination)
14
PERICYCLIC REACTIONS
09/09/2021

17. Naturally Occurring Electrocyclic Reactions
1) Synthesis of Vitamin D3
2) Synthesis of Aranotin
15
PERICYCLIC REACTIONS
09/09/2021
..
..
..
..
..
..
..
.. ..
..
..
.. ..
..
..
..
..
.. ..
..
..
..
..
..
..
..
..
..
..
..
..
..

18.  Biosynthesis of vitamin D3.
 It is sub element of vitamin D which is essential for our bones. It is also called “sunshine vitamin” as
sunlight is a great source of vitamin D.
 The first step involves a photochemically induced conrotatory ring opening of 7-dehydrocholesterol
to form pre vitamin D3, then a [1,7]-hydride shift then forms vitamin D3.
 Biosynthesis of Aranotin.
 Aranotin is an organic heterohexacyclic compound which is known for having antiviral and antibiotic
properties.
 1st enzymatic epoxidation of the starting material takes place. In the 2nd step, disrotatory ring
opening electrocyclization reaction takes place containin 6pi e-. Then 2nd epoxidation of the ring
takes place and the nearby nucleophilic nitrogen attacks the electrophilic carbon, forming a five
membered ring. The resulting ring system is a common ring system found in aranotin and its
derivatives.
(Explaination)
16
PERICYCLIC REACTIONS
09/09/2021

19. Cycloaddition Reactions
2+2 4+2 6+2
4+4 8+2 6+4
CLASSIFICATION
A cycloaddition is a reaction between two compounds with bonds to form a cyclic
product with two new bonds.
17
PERICYCLIC REACTIONS
09/09/2021

20. Ene: Alkene with an allylic hydrogen.
Enophile: compound containing
multiple bonds.
CHARACTERISTICS
 It requires higher temperatures for breaking the allylic C-H
σ-bond.
 The enophile can also be an aldehyde, ketone or imine.
 EWGs on the enophile and EDGs on the ene favor the
reaction.
 Lewis acid catalyst such as AlCl3 increases the rate of Ene
Reactions.
Alder Ene Reactions
GENERAL REACTION:
Enophile Pentene
EXAMPLE:
+
Ene
18
PERICYCLIC REACTIONS
09/09/2021
Maleic anhydride

21.  Alder ene is a type of group transfer reactions.
 It is a special class of cycloaddition reaction taking place between an alkene having an allylic hydrogen
which is called ene and a compound containing a multiple bond which is called the enophile.
 A new σ-bond is formed with migration of the ene double bond.
Now lets study some characteristics of ene reaction,
 It requires higher temperatures because of the higher activation energy and stereoelectronic
requirement for breaking the allylic C-H σ-bond.
 The enophile can also be an aldehyde, ketone or imine.
 Electron withdrawing groups on the enophile and electron donating groups on the ene favor the reaction.
 Like the Diels-Alder, some Ene Reactions can also be catalyzed by Lewis Acids. Lewis acid catalyst such as
AlCl3 increases rate of Ene Reactions.
Alder Ene Reactions (Explainantion)
19
PERICYCLIC REACTIONS
09/09/2021

22. Production of menthol which is used to give a peppermint smell
and taste to many products. This is an intramolecular ene
reaction on another terpene derivative.
Carbonyl Ene Reactions
 Lewis-acid catalyst lowers the energy of the
LUMO increasing the rate of reaction.
 The more electrophilic carbonyl group is
favoured.
EXAMPLE
GENERAL REACTION:
20
PERICYCLIC REACTIONS
09/09/2021
..
..
..
..
..
..
..
..
..
..

23.  Carbonyl ene reactions is a type of alder ene reaction where the carbonyl group acts as enophile.
 These reactions are more affective because here carbonyl groups act as a good enophile due to
inductive effect of Oxygen.
Now what if there are more than one carbonyl group present in the enophile molecule?
 In such cases more electrophilic carbonyl group is preferred. The electrophilic Carbonyl group
then acts as enophile.
 The Ti(OR)4 acts as lewis acid to increase the rate of reaction.
 Lewis-acid catalyst lowers the energy of the LUMO increasing the rate of reaction.
 One of the major application of carbonyl ene reaction is synthesis of menthol which is used to
give a peppermint smell and taste to many products. This is an intramolecular carbonyl ene
reaction.
Carbonyl Ene Reactions (Explaination)
21
PERICYCLIC REACTIONS
09/09/2021

24. Ketene [2 + 2] Cycloaddition
 Preparation of dichloroketene follows E1cB elimination reaction.
 The most e- rich end of alkene forms bond with e- deficient central Carbon atom of
Ketene.
Cl
Cl
Dichloroketene
EtO
+ C
O
Cl
Cl
O
Cl
Cl
EtO
Most electron
deficient
Most
electron rich
22
PERICYCLIC REACTIONS
09/09/2021
..
..

25.  Ketene cycloaddition is a simple 2+2 cycloaddition involving ketene molecule.
 One of the simplest way to form ketene is dichloroketene with E1cb mechanism. The
tertiary-amine abstracts the proton due to which double bond forms and the Cl atom gets
eliminated and a carbonyl group is formed giving the product dichloroketene.
 Now, this dicloroketene formed is reacted with a molecule containing double bond to form 4
membered cyclobutene ring.
 So, the basic condition here is that the most e- rich carbon reacts with the most e- deficient
Carbon. The Carbon in ketene molecule is electrophilic due to electronegative nature of
Oxygen. So usually Electron donating substituent on the alkene is preferred.
(Explaination)
23
PERICYCLIC REACTIONS
09/09/2021

26. Regio-stereo Selectivity
Et
+ C
O O
Me Me
Me
Me
Me
Me
Et
Et
+ C
O O
Me Me
Me
Me
Me
Me
Et
Z-Alkene E-Alkene
Cis- product Trans- product
 Geometry of alkene is maintained in cyclobutanone product.
C
O
Et
+
O
Et
H
H
C
O
Ph
+
O
Ph
H
H
X
X
X =Cl, Br, Me, Et Endo
Endo
 Reaction of ketene with cyclopentadiene gives adduct with larger ketene substituent in the endo position.
Exo
24
PERICYCLIC REACTIONS
09/09/2021
..
..
..
..
..
..

27.  The stereochemistry of alkene is maintained in cyclobutanone product.
 So if Z configuration of alkene is present in the reactant, the product gives the cis product and in
case of E alkene, trans product is formed.
 Another stereochemical condition is that the substituents which are present on the ketene
molecule, the bulky group is substituted on the endo position.
 For example, in the given reaction, ethyl is bulky so it is present in the endo position and Ph
group is bulky so endo product is obtained.
 This is one of the exceptional case in ketene cycloadditions.
(Explaination)
25
PERICYCLIC REACTIONS
09/09/2021

28. Synthesis Of -Lactams
Cl
Cl
Cl
O
Et3N
C
O
Cl
Cl
NPh
Ph
Cl
N
Ph
Cl
O
O
Cl
C N
SO2Cl
N
O SO2Cl
Cl
N
O
S
Cl
O
O
O
OH
NH
O
Cl
NaHCO3
H2O
 The isocyanate with chlorosulfonyl group as substituent increases the electrophilicity of carbon. The
chlorosulfonyl group can be removed simply by hydrolysis under mild conditions via the sulfonic acid.
26
PERICYCLIC REACTIONS
09/09/2021
Ph
 The dichloroketene reacts with imine to give beta-lactam derivative.
..
..
..
..
..
..
..
..
..
..
..
..
..
..
..
..
..
..
..

29.  Now one of the major application of ketene cycloaddition is synthesis of beta-lactam ring.
 A beta lactam ring is a 4 membered ring containing Nitrogen at the beta position of the
carbonyl group.
 Here a simple ketene cycloaddition of imine with dichloroketene is carried out and beta-lactam
ring is formed.
 Since alkenes are moderately nucleophilic, to make the Carbon more electrophilic, an
alternative method is introduced where isocyanate with chlorosulfonyl group is used in place
of imine.
 The sulphonyl group is then removed by hydrolysis and the b-lactam ring is formed.
(Explaination)
27
PERICYCLIC REACTIONS
09/09/2021

30. Solvent Effect In The Electrocyclic
Reactions
PUBLISHED IN THE YEAR 1999
28
PERICYCLIC REACTIONS
09/09/2021

31. Aim: To study the solvent effects on electrocyclic ring closing and ring opening reactions’
Theory: In this, the electrocyclic ring closing and opening reaction was carried out using 18 different
solvents and there rate constant were determined and result was obtained.
The 1st reaction is ring closing reaction which involves a totally aliphatic system and the 2nd reaction
which is ring opening contains carbonyl substituents which are polar.
Procedure: So according to this paper, the kinetic determination was done using UV-vis
spectrophotometer and the product formation was detected by using H-NMR spectrum and the
observations were recorded.
Observations: So from the table, it was seen that in 1st the reaction no major deviation was
observed, but in case of 2nd reaction alcohol and acetic acid solvent showed more deviation.
Research Paper
29
PERICYCLIC REACTIONS
09/09/2021

32. Conclusion: The conclusion was made that this maybe due to the hydrogen bond donor ability of alcohol
and acetic acid.
So then, further study was done and the two reactions was then carried out in the acetonic solutions of
lithium and magnesium perchlorate. And then according to its observations, 1st reaction was ineffective
toward inorganic solvents but 2nd reaction was effective.
So finally the conclusion was made that electrocyclic reactions can be termed as pericyclic reactions with
no solvent and no cationic catalysis effect in case of totally aliphatic substrates. But substituents with
polar properties showed some significant effects with inorganic solvents acting as lewis acids.
Research Paper (Continuation)
30
PERICYCLIC REACTIONS
09/09/2021

33. BOOKS
■ Organic Chemistry; Clayden, Greeves, Warren and Wothers; Page No- 905, 943
■ Modern methods of organic synthesis; William Carruthers & Iain Coldham; 4th edition; Page No- 231, 259
■ Photochemistry & Pericyclic reactions; Jagdamba Singh, Jaya Singh; 3rd edition; Page No-17, 58
■ Pericyclic reactions by Ian Fleming; Page No- 7, 57, 84
■ Advanced Organic Chemistry Part B (Reaction and synthesis); Francis Carey and Richard Sundberg; 5th
edition; Page No- 539
■ RESEARCH PAPER: file:///C:/Users/pykum/Downloads/(sici)1099-0690(199908)1999_8_1921__aid-
ejoc1921_3.0.co%3B2-n.pdf
WEBSITES
■ https://www.asu.edu/courses/chm332/PericyclicReactions.pdf
■ http://epgp.inflibnet.ac.in/epgpdata/uploads/epgp_content/chemistry/organic_chemistry_iii/29.electrocy
clic_reactions/et/4823_et_et.pdf
■ https://www.stereoelectronics.org/webPR/PR_05.html
■ http://epgp.inflibnet.ac.in/epgpdata/uploads/epgp_content/chemistry/09.organic_chemistry-
iii/35.ene_reaction/et/5522_et_et.pdf
Bibliography
31
PERICYCLIC REACTIONS
09/09/2021

34. Does anyone have any questions?