Pericyclic Reaction.pdf

Pericyclic reaction, stereochemistry.

1. Jagannath University



A detailed assignment on Pericyclic reactions

Course title: Reaction mechanism and stereochemistry
Course code:CHE-5121






Submitted to Submitted by
Professor MD MITHU MIA
Dr. Md Aminul Haque B150301017
MSc (Physical)
Department of
Chemistry,Jnu

2. 1. Learning Outcomes
2. Introduction
3. Classification of pericyclic reactions
3.1 Electrocyclic reaction
3.2 Cycloaddition reactions
3.3 Sigmatropic rearrangements
3.4 Ene reactions
4.The Cope Rearrangement
5.Comparism of all pericyclic reaction
6.Summary









TABLE OF CONTENTS

3. After studying this module, you shall be able to
• Know what are pericyclic reactions
• Learn about classification of pericyclic reactions
• Identify electrocyclic reaction, cycloaddition and sigmatropic shifts
• Evaluate application of Woodward-Hoffmann rules to pericyclic reactions
• Analyze which type of pericyclic mechanism is operative in a reaction




A pericyclic reaction is a reaction in which bonds are formed or
broken at the termini of one or more conjugated ??????-system.
Based on mechanism, chemical reactions are broadly classified as ionic, free radical and
pericyclic reactions. Pericyclic reactions are a unique set of reactions that takes place through a
cyclic transition state in a concerted fashion and exhibit high levels of stereospecificity.
Additionally, for pericyclic reactions no intermediates have been isolated and the reactions are
free from changes in solvent polarity, free radical generators and even catalysts (although lately
Lewis acid catalysis has been reported for some reactions). All these reactions are potentially
reversible in nature.
Unlike ionic reactions, for pericyclic reactions there is no definite sense of direction to the
movement of electrons as the electrons move in a cyclic manner. There is also a difference
between a synchronous reaction and a multi-stage concerted process as in synchronous reaction
all bond-making and bond-breaking events take place simultaneously, but in a multi-stage
concerted process some events precede others without producing an intermediate state. Most of
the pericyclic reactions are concerted and may or may not be synchronous.
Though some of the pericyclic reactions occur spontaneously, but in major of them introduction
of energy either in the form of heat or light is required. Moreover product depends on the source
of energy used.

1. Learning Outcomes
2. Introduction

4. Properties of pericyclic reactions:
 A pericyclic reaction is a concerted reaction that proceeds through a cyclic transition
state.
 Unaffected by polar reagents, solvent change, radical initiators.
 Influenced by Heat or Light
 Very stereospecfic
 No nucleophiles or electrophiles involved.
 Not generally catalysed by Lewis acids.



1. Electrocyclic reaction
2. Cycloaddition reactions
3. Sigmatropic rearrangements
4. Ene reactions




1. Classification of Pericyclic reaction

5.  Electrocyclic reaction is an intramolecular reaction in which a new
??????(sigma) bond is formed between the ends of a conjugated ??????(pi)
system or vice versa.
 The product is a cyclic compound that has one more ring and one
fewer ?????? bond than reactant.











These reactions are usually categorized by the following criteria:
 Reactions can be either photochemical or thermal.
 Reactions can be either ring-opening or ring-closing (electrocyclization).
 Depending on the type of reaction (photochemical or thermal) and the number
of pi electrons, the reaction can happen through either a conrotatory or
disrotatory mechanism.
 The type of rotation determines whether the cis or trans isomer of the product
will be formed.

1. Electrocyclic reaction
1,3,5-hexatriene 1,3-cyclohexadiene
∆
new ?????? ??????��??????
Ring-closing reaction
∆
cyclobutene
1,3-butadiene
Ring-opening reaction

6. As for example:- interconversion of cyclobutene and butadiene





According to thermodynamic considerations , the formation of the most energetically favorable
isomer, the (E,E ) diene, in which the large ester groups are as far apart as possible. The opening
of the cyclobutenes must initially give the dienes in their s-cis conformations rather than in the
lower-energy s-trans arrangements. Then the s-cis molecules will rapidly rotate to the lower-
energy s-trans conformations.


Stereospecificity of Electrocyclic reactions
∆

7.  The stereochemical outcome of the reaction dependent on
the energy source.
 Heat(∆) give one stereochemical result and light(hʋ)
another.
 In the thermalreaction, a cis 3,4- disubstituted cyclobutene
yields the (E,Z )diene.
 In the photochemical process the same cis cyclobutene
yields a pair of molecules,the (Z,Z ) diene and the (E,E )
diene.













Fig:The stereochemical outcomes of the thermal and photochemical reactions of cyclobutenes and
butadienes are different.

8. CONROTATORY PROCESSES :
In electrocyclic reaction , the rotation is called as conrotatory process if
the terminal carbons of a polyene or the saturated carbons of the
cyclized form rotate in the same direction (clockwise or counter
clockwise direction).


DISROTATORY PROCESSES:
In electrocyclic reaction , the rotation is called as disrotatory process if
the terminal carbons of a polyene or the saturated carbons of the
cyclized form rotate in different direction (clockwise or counter
clockwise direction).


Mechanism of Thermal & photochemical Reactions

9. Woodward-Hoffmann Rules:
 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










Example:
1.Interconversion of cyclobutene and butadiene
2. Interconversion 1,3,5-hexatriene and 1,3-cyclohexadiene
A summary of the type of motion to be expected from different polyenes under thermal and
photochemical conditions is shown in Table

10. 1.Interconversion of cyclobutene and butadiene:
Thermal process:





















Fig:The molecular orbitals involved in the interconversion of
cyclobutene and butadiene
The HOMO of butadiene is Φ
2
.
The thermal interconversion of cyclobutene and 1,3-
butadiene take place in a conrotatory way.

11. The cis 3,4-distributed cyclobutene can only open in conrotatory
fashion and formation of the (E,Z) diene.











Photochemical process:
Absorption of a photon promotes an electron from the HOMO to the LUMO , in this case
from Φ2 to Φ3, creating a new, “photochemical HOMO” .

∆
∆
Fig:The HOMO involved in the photochemical reaction of butadiene is Φ3.

12. Two disrotatory modes interconvert the cis 3,4-disubstituted cyclobutene and
the (Z,Z ) and (E,E ) isomers of the substituted butadiene whis is given blew:-

Now Φ
3
create a bond between the end carbonds
The photochemical interconversion of cyclobutene and
1,3-butadiene take place in a disrotatory way.
Fig:The photochemical HOMO (Φ3) demands disrotation. In
this molecular orbital, conrotation produces an antibond.
Fig: Disrotation interconverts the cis disubstituted cyclobutene and the (Z,Z ) and (E,E )
dienes

13. 2. Interconversion 1,3,5-hexatriene and 1,3-cyclohexadiene
















 In this conversion Φ3 is the HOMO for Thermal process and Φ4 is
HOMO for Photochemical reaction.
 Bond formed in thermal process by disrotation.
 Bond formed in photochemical process by conrotation



Fig:The π molecular orbitals for 1,3,5-hexatriene. The thermal HOMO is Φ
3
, and the
photochemical HOMO is Φ
4
.

14. Introduction:
A cycloaddition is a reaction in which two unsaturated molecules undergo an addition
reaction to yield a cyclic product.
 Formation of cyclic product takes place at the expense of one π (pi) bond in each of the
reacting partner and gain of two σ (sigma) bonds at the end of the both components
having π ( pi) bonds. Thus, in this reaction there is loss of two π (pi) bonds of the
reactants and gain of two σ (sigma) bonds in the product.
 Cycloaddition reactions involve the coming together of two (or, rarely, even more)
molecules to make a ring.
 The HOMOs and LUMOs of the two fragments forming the ring compound







Diels-Alder reaction is the best known [4 + 2] cycloaddition reaction. This reaction is thermally
allowed reaction. Diels-Alder reaction is photochemically forbidden. Since Diels-Alder reaction
is the most common [4 + 2] cycloaddition reaction, let us first discuss the general description of
this reaction. Diels-Alder reactions occur between a conjugated diene and an alkene usually
called the dienophile.



Two alkenes
A cyclobutane
The hypothetical combination of two alkenes to give a cyclobutane.
[4+2] Cycloadditions Reactions
Fig:The two possible HOMO–LUMO interactions in the
prototypal Diels–Alder reaction between butadiene and

15.  In the simplest Diels–Alder reaction between ethylene and 1,3-butadiene, the two
HOMO–LUMO interactions are of equal magnitude, and so we must look at the orbital
symmetry of both HOMO(diene)–LUMO(dienophile) and HOMO(dienophile)–
LUMO(diene).
 Thermal Diels–Alder reaction is allowed by orbital symmetry.

 In photochemical ,the absorption of a photon will promote an electron from the HOMO
to the LUMO. In this case, the lower-energy HOMO–LUMO gap is that in the diene
partner. Absorption of light creates a new photochemical HOMO for the diene, Φ3, and
now the HOMO–LUMO interaction with the dienophile partner involves one antibonding
overlap.
 So this photochemical Diels–Alder reaction is forbidden by orbital symmetry.



Fig: Absorption of a photon results in the formation of a new HOMO, Φ3. The
HOMO(diene)–LUMO(dienophile) interaction involves one antibond. The
photochemical Diels–Alder reaction is forbidden by orbitital symmetry.

16. I. Thermal Induced [2 + 2] Cycloaddition Reactions
 Thermal induced [2 + 2] cycloaddition reactions are symmetry forbidden reactions.
 When ethylene is heated, its π electrons are not promoted, but remain in the ground state
ψ1. If we examine the phase of the ground state HOMO of one ethylene molecule and the
LUMO of another ethylene molecule we can see why cyclisation does not occur by the
thermal induction.

For bonding to occur, the phase of the overlapping orbitals must be same. This is not the case for
the ground state HOMO and LUMO of two ethylene molecule or any other [2 + 2] system.
Because the phase of the orbitals are incorrect for bonding, a thermally induced [2 + 2]
cycloaddition is said to a symmetry forbidden reaction.

II. Photo-Induced [2 + 2] Cycloaddition Reactions.
 When ethylene is irradiated with photon of UV light, a π electron is promoted from
ψ1 to ψ2* orbital in some, but not all, of the molecules. The result is a mixture of
ground state and excited state ethylene molecules.
 Thus photo-induced cycloaddition takes place between photochemical HOMO of
one molecule and ground state LUMO of of other molecule.
[2+2] Cycloadditions Reactions

17. From the example it is clear that for cycloaddition reaction both HOMO and LUMO
should have same symmetry otherwise reaction will be symmetry forbidden.








 All are process will follow this rules for cycloaddition reactions
Table:Rules for Cycloaddition Reactions

18.  Sigmatropic rearrangements are a class of pericyclic reactions defined by the
migration of a σ bond adjacent to one or more π systems, with the π systems
becoming reorganized in the process.

 In the reaction the total number of σ or π-bonds does not change as the reactant and the
product have the same number of bonds. The σ bond that migrates may be in the middle
of the system or at the end of the system.

 These reactions are intra-molecular in nature and generally do not require a catalyst for
their completion.

 These type of rearrangement reactions are labelled by using two numbers which are set in
brackets [i, j], these numbers refer to the relative distance (in atoms) at each end of the σ-
bond which has moved.

 Sigmatropic rearrangements [1, n] are common for hydrogen atom shift with known
examples of n = 2, 3, 4, 5, 6, 7 and even for longer chains.

Sigmatropic rearrangements
1
2
3
1
2
3 [1,3]
2
1
4
3
5
[1,5]
3
1
2
4 5
6
1
[1,7]

19. Consider the following reaction:







When 1, 3-pentadiene is heated, it gives [1, 5] sigmatropic rearrangement. It is simple to
construct an arrow formalism picture of the reaction. The arrow could run in either direction,
clockwise or anticlockwise. That is not true for a polar reaction in which the convention is to run
the arrow from pair of electrons towards the electron deficient.










An arrow formalism description of the [1, 5] shift of deuterium in 1, 3-pentadiene
In the given example deuterium of sp3 hybrid carbon migrates on to the sp2 hybrid carbon
(carbon-5). The given compound has also carbon-3 as sp2 hybrid carbon. Thus this compound
can also give [1, 3] sigmatropic rearrangement on heating, but [1, 3] shift is not observed.



Mechanism of Sigmatropic Rearrrangement
∆
2
4
3
5
[1,5]
2
3
4
5
1
[1,3]
Not observed

20.  An arrow formalism can easily be written and it might be reasonably argued that the [1,
3] shift requiring a shorter path than the [1, 5] shift, should be easier than why [1, 3]
shifts not observed on heating?
 A second strange aspect of this reaction comes from photochemical experiments. When
1, 3-pentadienes are irradiated the product of the reaction include the molecules formed
through [1, 3] shift but not those of [1, 5] shifts







So any mechanism proposed must include an explanation of why thermal shifts are [1, 5]
whereas photochemically induced shifts are [1, 3]. We can use the frontier orbital approach to
analyse these reactions and see why this is so. Let us first consider the following thermally
induced sigmatropic rearrangement which is a [1, 3] shift.




For the purpose of analysing the orbitals, it is assumed that the σ (sigma) bond connecting the
migrating group to its original position undergoes homolytic cleavage to yield two free radicals.
This is not how the reaction takes place because reaction is concerted. But this assumption does
allow analysis of the molecular orbitals.





The products of the hypothetical cleavage are a hydrogen atom and an allyl free radical, which
contains three p-orbitals. The π (pi) molecular orbitals of allyl free radical are shown in Fig.
∆
Homolytic cleavage H
Allyl free radical
2
3
4
5
1
[1,3] shift
hʋ

21. The actual shift of hydrogen could take place in one of the two directions. In the first case, the
migrating group could remain on the same side of the π (pi) orbital system. Such a migration is
known as a suprafacial process. In the thermal 1, 3 sigmatropic rearrangement a suprafacial
migration is geometrically feasible but symmetry forbidden.






Let us consider the second mode of migration for a symmetry allowed [1, 3] sigmatropic shift to
occur, the migrating group must shift by an antarafacial process—that is, it must migrate to the
opposite face of the orbital system.





While symmetry-allowed a [1, 3] antarafacial sigmatropic rearrangement of hydrogen is not
geometrically favorable. Why? The problem is that the 1s orbital is smallest and cannot
effectively span the distance required for an antarafacial migration. In other words, size of 1s
orbital of hydrogen is smallest and distance between two lobes of interacting p-orbitals of carbon
Fig:Molecular orbital of allyl free radical

22. is maximum hence orbital of 1s cannot interact effectively with p-orbitals at same time in the
formation of transition state.




 [1, 3] sigmatropic shifts take place in the presence of UV light but examples are rare.
Consider again what happens when a molecule absorbs a photon. LUMO of ground state
will become HOMO of excited state known as photochemical HOMO ,















[1, 5 ] Sigmatropic Rearrangement:
The products of hypothetical cleavage in this case are hydrogen free radical and pentadienyl
radical. π MOs of pentadienyl radical is given below :
Fig:Suprafacial migration is possible in ψ*
2
photochemical HOMO of
the reaction.

23. [1, 5] Sigmatropic shift is thermally allowed and photochemically forbidden. If we again assume
a homolytic bond cleavage for purpose of analysis, we must consider the molecular orbitals of
pentadienyl radical .




Suprafacial migration:
The [1, 5] suprafacial shift is symmetry allowed and geometrically feasible. Consider
photochemical [1, 5] sigmatropic rearrangement. In this case ψ4 * will be photochemical HOMO




Antarafacial migration, symmetry allowed but geometrically difficult

24. Stereochemistry of [1, 5] sigmatropic Rearrangement:
Consider the following compound.







 In suprafacial migration of hydrogen in one conformation, the product has the (R)
configuration at C(5) and the (E ) stereochemistry at the new double bond located
between C(1) and C(2). Another possible suprafacial migration starts from the second
conformation. Here it is the opposite stereochemistries that result. The configuration of
C(5) is (S ), and the new double bond is (Z ).





 In antarafacial migrations, the hydrogen leaves from one side and is delivered from the
other. Figure shows the results of the two possible antarafacial migrations.









Rules for Allowed Sigmatropic Reactions

25.  The Cope rearrangement is an extensively studied organic reaction involving the [3,3]-
sigmatropic rearrangement of 1,5-dienes. It was developed by Arthur C. Cope. For
example 3-methyl-1,5-hexadiene heated to 300°C yields 1,5-heptadiene.
 Not all sigmatropic shift involve a hydrogen atom migrating from one location to another
.The cope rearrangement is a [3,3] sigmatropic shift in which the net result is one ??????-bond
is exchange for another without movement of Hydrogen atom









 Just like ary of sigmatropic shift , cope ararrangement is a single step ,
intramolecular reaction that is not affected by the polarity of the solvent or on
acidity of solution.

Transition state:

The Cope Rearrangement

26.  In the transition state , the sigma bond between C(1) atom is partially broken while
the single bond between C(3) atoms is partially formed. At the same time, the ??????
bonds between C(2) and C(3) is partially broken while the ??????-bond between C(1) and
C(2) is partially formed.




 In the transition state , we have partial allyl system .













Orbital diagram of Transiton state
↑↓
↑
Φ
3

Φ
2

Φ
1

HOMO
LOMO
E

27.  Under thermar condition , the Highest occupied molecular orbital for both system is Ψ2 .

Under thermal conditions , there is a perfect orbital symmetry between the two partially formed
radical allyl system . The overlap between C(3) orbital is beginning to decrease while the overlap
between C(1) orbital is being increase . Both interaction are bonding.



 There is an anti-bond interaction taking place to transition .









All types of pericyclic reactions are concerted and involve cyclic transition state without any
intermediate formed during the reaction. The characteristics which differentiate them from each
other are tabulated below
Under photochemical condition:

hʋ
3
2
1
2 3
1
COMPARISON OF DIFFERENT TYPES OF PERICYCLIC REACTIONS

28.  Pericyclic reactions are a group of reactions that takes place via a cyclic transition
state in a concerted fashion.
 Pericyclic reactions are induced either thermally or photochemically.
 Pericyclic reactions are highly stereospecific.
 Pericyclic reactions are classified into four different classes namely; electrocyclic
reactions, cycloaddition reactions, sigmatropic rearrangements and group transfer
reactions.
 Electrocyclic reactions are characterized by creation of rings from open chain
conjugated systems or opening of cyclic molecules under ring strain.
 The stereochemistry of electrocyclic reactions is decided by conrotatory or
disrotatory motion of orbitals for effective overlap.
 Cycloaddition reaction involves concerted combination of two π electron system to
form a cyclic product.
 Stereospecificity in cycloaddition reactions is maintained by suprafacial or
antarafacial interactions of orbital overlap.
S.no
Type of pericyclic
reaction
change in no. of
σ bonds
change in no. of
π bonds
comments
1)
Cycloaddition
reactions
+2 -2
A cyclic product is formed;
may be intermolecular or
intramolecular.
2)
Electrocyclic
reactions
+1 -1 Intramolecular.
3)
Sigmatropic
reactions
0 0
Intramolecular;
migration of a σ-bond;
rearrangement of π-electrons
Summary

29.  Sigmatropic reactions are defined by movement of a σ bond adjacent to one or more
π systems with reorganization of π system in the process.
 Group transfer reactions involve transfer of one or more groups or atoms from one
molecule to another in a pericyclic reaction fashion.
 Woodward-Hoffmann rules governs all the pericyclic reactions.
 A reaction forbidden by Woodward-Hoffmann rule may still take place with input
of large amount of energy.