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1.1 INTRODUCTION
Pericyclic reactions are defined as the reactions that occur by a concerted cyclic shift of electrons.This definition states two key points that characterise a pericyclic reaction. First point is that
reaction is concerted. In concerted reaction, reactant bonds are broken and product bonds areformed at the same time, without intermediates. Second key point in pericyclic reactions
involves a cyclic shift of electrons. The word pericyclic means around the circle. Pericyclicword comes from cyclic shift of electrons. Pericyclic reactions thus are characterised by a
cyclic transition state involving the π bonds.
or by UV light (Photo Induction). Pericyclic reactions are stereospecific and it is not uncommonThe energy of activation of pericyclic reactions is supplied by heat (Thermal Induction),
that the two modes of induction yield products of opposite stereochemistry.
is the electrocyclic reaction We shall concern with four major types of pericyclic reactions. The first type of reaction: A reaction in which a ring is closed (or opened) at the expense of a
conjugated double (or triple bond) bond.
New σ bond closes a ring Loss of onegain of one π σbond and bond
more πThe second type of reaction is the electron systems react to form a ring at the expense of one^ cycloaddition reaction : A reaction in which two or π bond in each of the
reacting partners.
+ →^ h ←ν
+ D
New sbond
New sbond
Overall there is loss of two^ In this reaction formation of two new π ( pi ) bonds in reactants and gain of two^ σ^ ( sigma ) bonds takes place which close a ring. σ ( sigma ) bonds in a product.
which aThe third type of reaction is the σ ( sigma ) bond formally migrates from one end to the other end of^ sigmatropic rearrangement^ (or reaction): A reaction in π ( pi ) electron
system and the net number of π bonds remains the same.
CHAPTER
Pericyclic Reactions
2 Photochemistry and Pericyclic Reactions
C
D H H
D
C
D D
D H
D
C
H
σ Bonded group moved from one end to theother end of the π electron system
more groups or atoms transfer from one molecule to another molecule. In this reaction bothThe fourth type of reaction is the^ group transfer reaction : A reaction in which one or
molecules are joined together by σ ( sigma ) bond.
H
H
M M
The three features of any pericyclic reaction are intimately interrelated. These are:
1. Activation: However, many reactions that require heat are not initiated by light and Pericyclic reactions are activated either by thermal energy or by UV light. vice-versa.
2. The number of π ( pi ) bonds involved in the reaction.
3. The stereochemistry of the reaction.
Consider the following three reactions:
CH 3 CH 3
H H
D H 3 C CH^3
H 3 C H H CH 3 H 3 C CH
3
D ...(2)
H 3 C H H CH 3 H
3 C^ CH^3
h n ...(3)
photochemical reaction activated by light. The relationship between the mode of activationFirst two reactions are thermal reactions activated by heat and third reaction is
4 Photochemistry and Pericyclic Reactions
four p^ In 1, 3-butadiene, we have a system of fourorbitals will overlap to produce four π molecular orbitals. We can get four new MOs in a^ p^ orbitals on four adjacent carbons. These
number of equivalent ways. One of the ways to obtain four new molecular orbitals is by linearcombination of two molecular orbitals of ethylene. Linear combination of orbitals is also known
asalways takes place between two orbitals (two atomic orbitals, two molecular orbitals or one perturbation theory or perturbation molecular orbital ( PMO ) theory. Linear combination
atomic and one molecular orbitals) having minimum energy difference. This means that weneed to look only at the results of the π ± π and π* ± π* interactions and do not have to consider
π ± π* (Fig. 1.2).
y 1 = p+p
Bonding interaction
Antibonding interaction
y 2 = p–p
y 3 * = p* +p*
Bonding interaction
Antibonding interaction
y 4 * = p* – p*
p* p*
p p
Fig. 1.2 The schematic formation of thefrom the π molecular orbitals of ethylene π molecular orbitals of 1, 3-butadiene
CHAPTER 1
Pericyclic Reactions 5
There are three bonding interactions, and the electrons are delocalised over four nuclei (Fig. 1.3).^ The lowest energy orbital (ψ^1 ) of 1, 3-butadiene is exceptionally stable for two reasons:
Bonding Bonding Bonding
Fig. 1.3 (ψ 1 = π + π) Bonding interaction between two ethylene bonding MOs
interaction between two bonding molecular orbitals of ethylene. The^ The second molecular orbital^ ψ^2 of 1, 3-butadiene is obtained from the antibonding ψ
and one antibonding interaction, so we would expect it to be a bonding orbital (two bonding –^2 orbital has two bonding
one antibonding = one bonding). Thus, energy oforbital has one node between C ψ 2 is more than that of ψ 1. ψ 2 molecular
(Fig. 1.4).^2 –C^3. A node is a plane where the wave function drops to zero
Bonding Antibonding Bonding
Fig. 1.4 ψ 2 = π π (Antibonding interaction between two ethylene π molecular orbitals)
bonding interaction between^ The third butadiene MO, π* and^ ψ^3 * has two nodes. This molecular orbital is obtained from the π* of two ethylene molecules. There is a bonding interaction
of the Cother at the C 2 —C 3 bond and there are two antibonding interactions: One at C 1 –C 2 bond and the
antibonding) having two nodes. Thus energy of this^3 —C^4 bond. This is an antibonding orbital (one bonding – two antibonding = one ψ 3 * orbital is more than the energy of ψ 2
MO (Fig. 1.5).
Antibonding Bonding Antibonding
Fig. 1.5 (ψ 3 * = π* + π*) Bonding interaction between two antibonding MOs of ethylene
π* and^ The fourth molecular orbital ( π* of two ethylene molecules. This molecular orbital has three nodes and is totallyψ^4 *) is obtained from the antibonding interaction between
antibonding. This MO has the highest energy (Fig. 1.6).
Antibonding Antibonding Antibonding
Fig. 1.6 (ψ 4 * = π* π) Antibonding interaction between π and π* of ethylene
CHAPTER 1
Pericyclic Reactions 7
m Fig. 1.8 (a)
is mirror plane asymmetry, abbreviated as^ In Fig. 1.8 ( a ) both orbitals are not mirror images to each other. Thus in this MO there m (A).
lines are drawn on one side and extended an equal distance on the other side, will meet the^ C^2 - Symmetry : The centre of symmetry is a point in the molecular axis from which if
same phases of the orbitals [Fig.1.8 ( b )].
Centre of the molecular axis Fig. 1.8 (b)
axis. Thus in this MO there is centre of symmetry, abbreviated as^ Both orbitals in Fig. 1.8 ( b ) are symmetrical with respect to centre of the molecular C
2 (S).
m (S) means plane of symmetry.
C 2 (S) means centre of symmetry.
1, 3-butadiene (Fig. 1.10).Let us take examples for the purpose of symmetry properties in ethylene (Fig. 1.9) and
E
Y 1
Y 2 *
Numberof nodes 1 0
Orbital Y 2 * Y 1
m A S
C 2
S
A
Fig. 1.9 Symmetries in the π molecular orbitals of ethylene
8 Photochemistry and Pericyclic Reactions
Numberof nodes 3
Orbital Y 4 *
m A
C 2
S
–^ Y^1
–^ Y^2
–^ Y^4 *
–^ Y^3 *
– 2 Y^3 *^ S^ A
1 Y 2 A S
0 Y 1 S A
Fig. 1.10 Symmetries in the π molecular orbitals of 1, 3-butadiene
points^ On the basis of the above two examples we can conclude the followingfor linear conjugated π systems:^ very important
1. The wave function ψ n will have ( n – 1) nodes.
2. When n is odd, ψ n will be symmetric with m and asymmetric with C 2.
3. When n is even, ψ n will be symmetric with C 2 and asymmetric with m. Table 1.1.
Table 1.1 Symmetry elements in the orbital ψ n of a linear conjugated polyene
Wave function nodes (n – 1) m C 2
ψodd : ψ 1 , ψ 3 , ψ 5 , ... 0 or even S A
ψeven : ψ 2 , ψ 4 , ψ 6 , ... odd A S
1.4 FILLING OF ELECTRONS IN π MOLECULAR ORBITALS IN CONJUGATED
POLYENES
Conjugated polyenes always contain even number of carbon atoms. These polyenes containeither (4 n )π or (4 n + 2)π conjugated electrons. The filling of electrons in the π molecular orbitals
of a conjugated polyene is summarised below:
1. Number of bonding π MOs and antibonding π MOs are same.
2. Number of electrons in any molecular orbital is maximum two.
10 Photochemistry and Pericyclic Reactions
( i ) The ethylene bonding orbital,ing way and moving down in energy to give π gives linear combination with the ψ p orbital in a bond-
( ii ) The ethylene antibonding orbital (π*) gives linear combination with^1. p orbital in an
antibonding way and moving up in energy to give ψ 3 *.
( iii ) Thethere is double mixing for the p orbital mixes with both the bonding and antibonding orbitals of ethylene. Thus, p orbital. The lower energy ethylene bonding orbital (π)
mixing in an antibonding way to push thethe ethylene antibonding orbital mixing in a bonding way to push to p orbital up in the energy ( ψ p 2 ′orbital down = π – p ) but
in energy (situation a nodal plane always passes through the central carbon of the chain. Thisψ 2 ″ = π* + p ). Thus, the net result in the energy change is zero. Under this
means an electron in i.e. , central molecular orbital must be non-bonding molecular orbital. Thus, the ψ 2 has no electron density on the central carbon. Thus, the ψ 2 ,
molecular orbitals of the allylic system is represented as follows (Fig. 1.12).
y 3 ; antibonding MO; m s ( )
- (^) +
- y 2 ; non-bonding MO;^ C 2^ ( ) s
- y 1 ; bonding MO; m s ( )
Fig. 1.12 π molecular orbitals of allyl system
molecular orbitals of the conjugated open chain system (cation, anion and radical) containing^ From the example of allyl system following generalisation can be made to construct the
odd number of carbon atoms:
1. Number of conjugated atoms (orhas three orbitals and 2, 4-pentadienyl system has five orbitals. p orbitals) are always odd. For example, allyl system
2. These systems have always one non-bonding molecular orbital whose energy is alwaysequal to the unhybrid p orbital. Non-bonding molecular orbital is always central
molecular orbital of the system (Fig. 1.12).
3. If system has m (which is always odd) atomic p orbitals then it has:
( i ) m^ 2 − 1 bonding molecular orbitals,
( ii ) m^ 2 − 1 antibonding molecular orbitals, and
( iii ) one non-bonding molecular orbital.
4. In non-bonding molecular orbital all the nodal planes ( wave function) pass through the carbon nucleus (or nuclei) (Fig. 1.13). i.e. , n – 1 nodal planes for ψ n
CHAPTER 1
Pericyclic Reactions 11
5. For odd ψ n (ψ 1 , ψ 3 , ψ 5 , ......), all nodal planes pass between two carbon nuclei (Fig. 1.13).
6. For evenand remaining nodal planes pass between two carbon atoms (Fig. 1.13). ψ n (ψ 2 , ψ 4 , ψ 6 , ......), one nodal plane passes through the central carbon atom
to seven carbon atoms in length with symmetries and nodal planes.Figure 1.13 illustrates schematically the forms of the molecular orbitals for chains up
-^ y^5 *;^ m s ( )
-^ y^4 *;^ C^2^ ( ) s
y 3 ; m s ( )
y 2 ; C 2 ( ) s
-^ y^1 ;^ m s ( )
y 3 *; m s ( )
y 2 ; C 2 ( ) s
-^ y^1 ;^ m s ( )
-^ y^7 *;^ m s ( )
y 6 *; C 2 ( ) s
y 5 *; m s ( )
y 4 ; C 2 ( ) s
y 3 ; m s ( )
-^ y^1 ;^ m s ( )
E
3C system
5C system
7C system Fig. 1.13 π molecular orbitals of allyl 2, 4-pentadienyl and 2, 4, 6-heptatrienyl system
CHAPTER 1
Pericyclic Reactions 13
Notice that cations, radicals and anions involving the same π system have the same
molecular orbitals. For example, the MOs of the allyl system apply equally well to the allylcation, allyl radical and allyl carbanion because all the three species contain the same p orbitals.
These species differ only in thecolumn of Fig. 1.14. number of π electrons, as shown in the “ electron occupancy ”
1.6 FRONTIER MOLECULAR ORBITALS
Two π molecular orbitals are of particular importance in understanding pericyclic reactions.
One is the occupied molecular orbital of highest energy, known as orbital (HOMO). The other is the unoccupied molecular orbital of lowest energy known as highest occupied molecular
lowest unoccupied molecular orbital opposite symmetries (Fig 1.16). HOMO and LUMO are referred to as (LUMO). HOMO and LUMO of any given compound have frontier molecular orbitals.
y 4 * (^) C (^) 2 ( ) s y 3 * (^) m s ( ) y 2 C (^) 2 ( ) s y 1 m s ( )
LUMO or ground state LUMO HOMO or ground state HOMO
Fig. 1.16 Electronic state of ground state of 1, 3-butadiene
LUMO of the ground state of the species is known as ground state LUMO.^ HOMO of the ground state species is also known as ground state HOMO. Similarly,
reaction? The electrons in the HOMO of a molecule are like the outer shell electrons of anWhy are the LUMO and HOMO so important in determining the course of a concerted
atom. They can be removed with the least expenditure of energy because they are already in ahighest energy level than any of the other electrons in the molecule. The LUMO of a molecule
is the orbital to which electrons can be transformed with the least expenditure of energy.
from it. The lower is the energy of the LUMO of the molecule, the more easily electrons can beThe higher is the energy of HOMO of a molecule, the more easily electrons can be removed
transferred into it. Therefore, the interaction between a molecule with a high HOMO and alow LUMO is particularly strong.
the LUMO of another with which it is reacting, the stronger is the interaction between the twoIn general, the smallest the difference in energy between HOMO of one molecule and
molecules.
1.7 EXCITED STATES
The molecules and ions we have been discussing can absorb energy from electromagnetic
radiation of certain wavelengths. This process is shown schematically in Fig. 1.17 for1, 3-butadiene. Let us refer to the normal electronic configuration of 1, 3-butadiene as the
ground state from the HOMO (. When 1, 3-butadiene absorbs a photon of proper wavelengthψ an electron is promoted
2 ) to the LUMO (ψ 3 *). The species with the promoted electron is an^ excited
14 Photochemistry and Pericyclic Reactions
state of 1, 3-butadiene. The orbital ψ 3 * becomes HOMO and ψ 4 * becomes LUMO of the excited
state. HOMO of excited state is termed as excited state HOMO or photochemical HOMO.Similarly, LUMO of the excited state is termed as excited state LUMO or photochemical LUMO
(Fig. 1.17).
y 4 * y 3 * y 2 y 1
LUMO
HOMO
ground state
E h n
¯
y 4 * y 3 * y 2 y 1 excited state
excited state or photochemical LUMO excited state or photochemical HOMO
Fig. 1.
Similarly, LUMO of the ground state and excited state also have opposite symmetries.^ Notice that the HOMO of ground state and excited state have opposite symmetries.
1.8SYMMETRIES IN CARBON-CARBON SIGMA BOND
The sigma orbital of a carbon-carbon covalent bond has a mirror plane symmetry, and since arotation of 180° through its midpoint regenerates the same sigma orbital, it also has a C
symmetry. A sigma antibonding molecular orbital is asymmetric with respect to both m and^2
C 2 (Table 1.2).
Table 1.2 Symmetric properties of σ and σ* molecular orbitals
Orbital m C 2 σ* A A σ S S
1.9 THEORY OF PERICYCLIC REACTIONS
Pericyclic reactions are defined as reactions that occur by concerted cyclic shift of electrons.According to the Woodward and Hoffmann symmetry of the molecular orbitals that participate
in the chemical reaction determines the course of the reaction. They proposed what they calledthe principle of the conservation of orbital symmetry ; in the concerted reactions. In the most
general terms, the principle means that in concerted pericyclic reactions, the molecular orbitalsof the starting materials must be transformed into the molecular orbitals of the product in
16 Photochemistry and Pericyclic Reactions
- T.L. Gilchrist and R.C. Storr,University Press, Cambridge, U.K., 1979. Organic Reactions and Orbital Symmetry , 2nd ed. Cambridge
- J.M. Coxon and B.Halton,U.K., 1974. Organic Photochemistry , Cambridge University Press, Cambridge,
PROBLEMS
1. (^) (Sketch the a ) 1, 3-butadiene pi molecular orbitals of ( b ) 1, 3, 5-hexatriene 2. Sketch thetion, allyl free radical and allyl carbanion. pi molecular orbitals of the allyl system. Give electron occupancy in allyl carbona- 3. Sketch thelar orbitals. Show electron occupancy in its carbocation, free radical and carbanion. pi molecular orbitals of 2, 4-pentadienyl system. Also draw nodal points in molecu- 4. Give symmetric properties of HOMO and LUMO of the following systems: ( i ) Butadiene ( ii ) Excited state butadiene ( iii ) 1, 3, 5-hexatriene ( iv ) Excited state hexatriene. 5. Show the electronic configuration of the ground state of 1, 3, 5-hexatriene. Give symmetricproperties of all the molecular orbitals of 1, 3, 5-hexatriene. 6. Show the electronic configuration of the excited state of 2, 4-pentadienyl cation. Give symmet-ric properties of HOMO and LUMO of the system. 7. Draw molecular orbitals of 1, 3-butadiene from the linear combination of molecular orbitals oftwo ethylene molecules. 8. Draw molecular orbitals of allyl system by the use of the linear combination of molecularorbital of one ethylene molecule and one p -atomic orbital. 9. Explain why ψ 3 of butadiene has higher energy than the ψ 2? 10. Explain why energy of ψ 5 of 1, 3, 5-hexatriene is less than the ψ 6? 11. What are HOMO and LUMO? Why these two orbitals are so important in pericyclic reactions? 12. 13. Give classification of pericyclic reactions with one example each.( i ) Draw the p -orbital array in the π molecular orbitals of the following ions ( ii ) Draw diagrams showing the occupied orbitals of the ground states and indicate the HOMOs. ( i ) CH 3 —CH CH— CH^ ⊕ —CH 3 ( ii ) CH 2 CH— CH^ Θ —CH (^3) 14. ( iii How many approaches have been made to explain the results of pericyclic reactions?) CH^3 —CH^ CH—CH^ CH—^ CH^ Θ^ —CH^3. 15. What do you understand from frontier orbital and orbital symmetry?
