frontier molecular orbitals and pericyclic reactions third year organic chemistry course chm3a2 -...
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Frontier Molecular Orbitals
and Pericyclic Reactions
Third Year Organic Chemistry
CourseCHM3A2
- Prof Jon A Preece -
School of ChemistryUniversity of Birmingham
Prof Preece’s Powerpoint Lecture Presentations and answers to questions can be found at…
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Part Contents
1 Pericyclic Reactions These lectures will begin with a definition of Pericyclic reactions, and will be exemplified by considering examples of cycloaddation, sigmatropic, and electrocyclic reactions. It will be highlighted how it is possible to use FMO theory (and other theories) to predict the constitution and stereochemical outcome of the products. Attention will be drawn to the cyclic transition state and the number of electrons involved (Huckel or Mobius), highlighting that when 4n+2 electrons are involved the reaction proceeds readily under thermal conditions, and the reversibility of such reactions. The concept of Linear Combination of Atomic Orbitals to form a bond(s) (and antibond(s)) will be revised, and extended to the linear combination of frontier molecular orbitals. The -molecular orbitals of ethene, butadiene and 1,3,5-hexatriene will be considered and the identities of the HOMO and LUMO will be established, as well as the FMOs of a C–H bond.
2i Electrocyclic Reactions This lecture will extend the predicative nature of FMO theory regarding the stereochemical outcomes to electrocyclic reactions for 4 and 6 -electron transition states (by defining the disrotatory or conrotatory movement of the termini of the HOMO in the Transition State).
2ii Cycloaddition Reactions These lectures will introduce cycloaddition reactions and the concepts of (i) phase relationships of the FMOs, (ii) geometry of approach of the FMOs (suprafacial and antarafacial will be defined), and (iii) minimum energy differences between the HOMO and LUMO. These concepts will be exemplified by several Diels-Alder and related reactions. Attention will be drawn to the nature (chemical and stereochemistry) of substituents and their stereochemistry in the product.
3 Photochemically Induced Pericyclic reactions These lecture will extend the predicative nature of FMO theory regarding the outcomes of electrocyclic reactions and cycloaddition reactions by considering how they can be induced photochemically, to give alternative stereochemical outcomes and allow reactions that did not go thermally.
Course Synopsis
Part 1. Frontier Molecular Orbitals
Constructing molecular orbitals and identifying the
frontier molecular orbitals
Part 2. Thermal Pericyclic Reactions
(i) Electrocyclic Reactions using FMO Theory
(ii) Cycloaddition Reactions using FMO Theory
Part 3. Photochemical Pericyclic Reactions
(i) Electrocyclic Reactions using FMO Theory
(ii) Cycloaddition Reactions using FMO Theory
Second Year Organic Chemistry CourseCHM3A2
Recommended Reading
I Fleming
Frontier Orbitals and Organic Chemical Reactions, John Wiley and
Sons, 1996.
Part 1: Ch 1 and Ch 2
Part 2 and 3: Ch 4
Second Year Organic Chemistry CourseCHM3A2
Frontier Molecular Orbitals and Pericyclic Reactions
Part 1(i):
The Questions FMO Analysis Can Answer
100% 0%
Ionic And Radical Reactions
(i) Ionic reactions
Here pairs of electrons move in one direction
e.g. SN2, SN1, E2 and E1 mechnisms
Cl
H R2
R1
R3
R4
R2R1
R4 R3
BBH
Cl
CH3 Cl Cl H3C Cl Cl
(ii) Radical reactions
Here single electrons move in a correlated manner
e.g. chlorination of alkanes
To date you have seen two broad categories of reaction:
Pericyclic Reactions
Pericyclic reactions are the third distinct class.
They involve cyclic transition states
In which all bond breaking and bond making steps take
place in commensurate manner
And there is no sense of the flow of electrons.
Pericyclic Reactions: Electrocyclic Reactions
100% 0%Clockwise
Anti-Clockwise
There is no real senses of flow for the electrons in pericyclic reactions
Stereospecific Reaction
Pericyclic Reactions: Cycloaddition Reactions
CO2Me
CO2MeCO2Me
CO2Me
CO2Me
CO2Me
100% 0%
CHOMeO CHO MeO
CHO
MeO
100%0%
Stereospecific Reaction
Regiospecific Reaction
Kinetic Product
Thermodynamic Product
Br
Br
Br
H
HH
Br H
HH
H
H
HH
Br HH
HH
1,3-syndiaxial interactions
1
23
Revision: 1,3–Syndiaxial Interactions
Br H
H
H
H
H
Br
H
H
H
axial
equitorial
Thermodynamic and Kinetic Control
MeO2C CO2Me
CO2Me
CO2Me
H
CO2Me
CO2Me
H
H
MeO2C
H
H
MeO2C
H
H
MeO2C
H
MeO2C
H
MeO2C CO2Me
Kinetic Product
Formed in Cycloaddition Reaction
Thermodynamic
Product
Not Formed in
Cycloaddition Reaction
D
MeMe
DH
Me
DH
Pericyclic Reactions: Sigmatropic Reactions
100% 0%
Stereospecific Reaction
Regiospecific Reaction
Pericyclic Reactions: Why are they so specific?
Thus, an obvious question to ask ourselves at this point is why are pericyclic reactions so selective?
Pericyclic reactions show high degrees of
(i) Stereoselectivity
(ii) Regioselectivity, and
(iii) Diastereoselectivity
To help begin to answer this question we shall briefly need to revise the SN2 reaction mechanism where YOU WILL remember that this reaction type was highly stereoselective leading to inversion of chiral centres.
Revision: SN2 Reaction Mechanism
R1
R2R3
ClNuRate = k[R-Hal][Nu]
sp3
Bimolecular Process
Rate Determinig
Step
Cl
R1
R3R2
Nu
Transition State – Energy Maxima
BondForming
2
1–2
1–
sp2
BondBreaking
R1
R2
R3Nu Cl
Inversion of Configuration
Nucleophile attacks from behind the C-Cl -bond.
This is where the *-antibonding orbital of the C-Cl bond is situated.
The concerted flow of both pairs of electrons in the SN2 reaction mechanism leads to the transition state which allows the stereochemical information to be retained,
i.e. a stereoselective reaction.
This SN2 reaction mechanism should be contrasted to the SN1 reaction mechanism where the arrow-pushing is the same but the two pairs electrons do not flow in a concerted fashion. Instead, the haloalkane C-Cl bond heterolytically cleaves to give the planar sp2 hybridised carbocation reactive intermediate. Now the nucleophile can attack from either side of the carbocation leading to racemisation,
i.e. a non-stereoselective reaction.
Revision: Transition StatesDiscussion of reaction mechanisms frequently include discussions of the nature of the transition state for each step in a reaction sequence – or at least for the slowest or rate limiting step.
A transition state is the point of highest energy in a reaction or in each step of a reaction involving more than one step.
The nature of the transition state will determine whether the reaction is a difficult one, requiring a high activation enthalpy (G‡), or an easy one.
Transition states are always energy maxima, I.e. at the top of the energy hill, and therefore, can never be isolated: there are no barriers to prevent them from immediately “rolling” downhill to form the reaction products or intermediates (or even reform the starting materials).
A transition states structure is difficult to identify accurately. It involves partial bond cleavage and partial bond formation. However, it is nigh on impossible to estimate whether the transition state is an early one (looks more like the starting materials) or a late one (looks more like the products)
A + B
Energy
Reaction Coordinate
A + B
C + DProduct
Starting Material
Revision: Transition States
Transition State
Energy Maxima
G‡
Go
Pericyclic Reactions: Transition States
Pericyclic reactions involve concerted flow of pairs of electrons going through transition states which retains stereochemical information that was present in the starting material.
Thus, now we can start to understand why pericyclic reactions are so highly stereo-, regio-, and diasteroselective.
Pericyclic Reactions Involve Cyclic Transition States
CO2Me
CO2MeCO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
Cyclic Transition State
Pericyclic reactions involve ene and polyene units.
Thus, the transition states involve the overlap of -molecular orbitals in the case of electrocyclic and cycloaddition reactions, and a -molecular orbital and -molecular orbital in the case of sigmatropic reactions.
CO2Me
CO2Me
CO2Me
CO2Me
How do the orbitals overlap?
In order to understand the selectivity of pericyclic reactions, we need to understand these molecular orbitals and how they overlap.
Frontier Molecular Orbitals
We will first revise some simple molecular orbitals of a C-H -bond and a C=C -bond and then extend this analysis to highly conjugated linear polyenes and related structures/
In particular, we need to know how the Frontier Molecular Orbitals (FMOs) interact in the starting material(s) which lead to the cyclic transition states.
Second Year Organic Chemistry CourseCHM2C3B
Frontier Molecular Orbitals and Pericyclic Reactions
Part 1(ii):
Frontier Molecular Orbitals
1
2
3
4
0 nodes
1 node
2 nodes
3 nodes
Thermal Reactions
HOMO
LUMO
Electronic Ground State
After completing PART 1 of this course you should have an understanding of, and be able to demonstrate, the
following terms, ideas and methods.
(i) Given a set of n p-orbitals you should be able to construct a molecular orbital energy level
diagram which results from their combination.
(ii) In this diagram you should be able to identify for each MO
nodes
the symmetric (S) or antisymmetric (A) nature of the MO towards a C2
axis or mirror plane
the bonding, nonbonding or antibonding nature of it
(iii) For a set of n molecular orbitals you should be able to identify the frontier molecular
orbitals.
the highest occupied molecular orbital (HOMO )
the lowest unoccupied molecular orbital (LUMO)
(iv) The HOMO (thermal reaction) interactions are important when evaluating the probability of an unimolecular reaction occurring and the stereochemical
outcome – see electrocyclic reactions.
The HOMO/LUMO (thermal reaction) interactions of the reacting species are important when evaluating the probability of (i) a bimolecular
reaction occurring and the stereochemical outcome– see cycloaddition reactions, and (ii) a unimolecular reaction occurring and the
stereochemical outcome – see sigmatropic reactions.
The geometry, phase relationship and energy of interacting HOMOs and LUMOS is important for evaluating the probability of a reaction occurring and the stereochemical outcome.
– Learning Objectives Part 1 –
Frontier Molecular Orbitals
CHM2C3B– Introduction to FMOs –
+
s ATOMIC ORBITAL
ANTI-BONDING MOLECULAR
ORBITAL
BONDING MOLECULAR
ORBITAL
Nodal Plane
ENERGY
*
Molecular Orbitals
-BondTwo s Atomic Orbitals
+
BONDING MOLECULAR
ORBITAL
sp3 ATOMIC ORBITAL
ANTI-BONDING MOLECULAR
ORBITAL
*ENERGY
Molecular Orbitals
-BondOne s Atomic Orbital and One sp3 Atomic Orbital
ENERGY
+ p-atomic orbitals
ANTI-BONDING MOLECULAR
ORBITAL
BONDING MOLECULAR
ORBITAL
*
Nodal Plane
Molecular Orbitals
-Bond:Two p Atomic Orbitals
The linear combination of
n atomic orbitals
leads to the formation of
n molecular orbitals
Cn = Coeffecient: a measure of the contribution which
the atomic orbital is making to the molecular orbital
m = Electronic distribution in the atomic orbitals
A SIMPLE Mathematical Description of a MO
= ca1 + cb2
The combination of two (or more) p-atomic orbitals (or any orbitals) to afford 2 -molecular orbitals can be described by the following simple mathematical relationship
* = cc1 + cd2
The probability of finding an electron in an occupied molecular orbital is 1.
= ca1 + cb2
* = cc1 + cd2
*
c2 = cc2 + cd
2 = 1
c2 = ca2 + cb
2 = 1
Cc = 1/√2
Ca = 1/√2
Cb = 1/√2
Cd = -1/√2 Negative
The probability of finding an electron in an occupied molecular orbital is the c2
Thus, for the ethene -molecular orbitals…
1 2
1 2
So what about the combination of 3 or 4 or 5 or 6 p-atomic orbitals.
That is to consider conjugated systems…
The Allyl Cation, Radical and Anion – 3p AOs to give 3 MOs
Cl
PolarSolvent
Cl
H
B
BH
Cl
Cl
Allyl Cation Allyl Radical Allyl Anion
Thus, allyl systems result from the combination of 3 conjugated p-orbitals.Therefore, this will result in 3 -molecular orbitals.
When we constructed the -molecular orbitals of ethene, each contributing AO was the same size, i.e. the
coeffecient c were 1/√2 or -1/√2.
When there are three or more p-atomic orbitals combining the size of each contributing p-atomic orbital will not be equal (but they will be symmetrical about the centre).
Finally, we refer to the -MOs and *-MOs as 1, 2, 3 (…n)
The Allyl -Molecular Orbitals
+_
+_
+ + + 1
2
3
+ + +
+ + +
1 2 3 4
Nodalposition4/1 = 4
Nodalposition4/2 = 2
Nodalposition4/3 = 1.33
Nodes
2
1.33
4
We can consider the molecular orbital (the electron density) being described by a SINE WAVE starting and finishing one bond length beyond the molecule…
1 = 0 Nodes
2 = 1 Nodes
3 = 2 Nodes
For our analysis of molecular orbitals we do not have to concern ourselves with the coefficients.
We can draw the p-AOs that make up the -MOs all the same size.
However, we have to always remember they are not the same size.
But it is of the utmost importance that we know how to calculate where the nodes are placed
Bonding, Non-Bonding, and Anti-bonding Levels
1
2
1
2
3
Non-bonding Level
Anti-bonding Level
Bonding Level
Energy
Anti-bonding
Non-bonding
Bonding
We can consider the molecular orbital (the electron density) being described by a sine wave starting and finishing one bond length beyond the molecule…
LUMOs and HOMOs
HOMO = Highest Occupied Molecular Orbital
LUMO = Lowest Unoccupied Molecular Orbital
1
2
3
0 nodes
1 node
2 nodes
LUMO
AllylRadical(3e)
AllylAnion(4e)
HOMO
LUMO
HOMO
LUMO
HOMO
AllylCation(2e)
Question 1: 4 p-Molecular Orbital System – Butadiene
Construct the -molecular orbitals of butadiene.
Identify the number of nodes, nodal positions, HOMO and LUMO.
Nodal Position
Number of Nodes
n
Answer 1: 4 p-Molecular Orbital System – Butadiene
Construct the -molecular orbitals of butadiene.
Identify the number of nodes, nodal positions, HOMO and LUMO.
+ + + +
+ + + +
+ + + +
+ + + +
Nodal Position
1 2 3 4 55/1 = 5
5/2 = 2.5
5/3 = 1.66
5/4 = 1.25
Number of Nodes
0
1
2
3
1
2
3
4
n
HOMO
LUMO
A Reminder: Sinusodal Wave Function
1
2
3
4
5/1
5/2
5/3
5/4
1.25
1.66
2.5
5
SIMPLE MORE COMPLEX
1 2 3 4 50 nodes
1 node
2 nodes
3 nodes
Coefficients, cn
n= ca1 + cb2 + cc3 + cnn
That is to say the probability of finding an electron in a molecular orbital is 1
Each molecular orbital is described by an equation…
c2 = 1
Where c is referred to as the coefficient
Such that the…
1.66
3
3= ca1 + cb2 + cc3 + cd4
We Keep FMO Analysis Simple!!
For the purpose of this course and the third year course
(Applied Frontier Molecular Orbitals and Stereoelectronic
Effects) you are expected
(i) to be able to place the nodal planes in the
correct place
(ii) but not to be able to assign the coefficients to
the molecular
orbitals.
That is to say you can draw the p-orbitals that
make up each
molecular orbital as the same size, whilst
remembering that in reality they are not and for
high level FMO analysis this
needs to be taken into account.
Question 2: 5 p-Molecular Orbital System – Pentadienyl
Construct the -molecular orbitals of the cyclopentenyl system.
Identify the number of nodes and nodal positions.
Nodal Position
Number of Nodes
nMolecular Orbitals
Answer 2: 5 p-Molecular Orbital System – Pentadienyl
Construct the -molecular orbitals of the cyclopentenyl system.
Identify the number of nodes and nodal positions.
Nodal Position
6/1 = 6
6/2 = 3
6/3 = 2
6/4 = 1.5
Number of Nodes
0
1
2
3
1
2
3
4
n
6/5 = 1.245
+ + + + +
+ + + + +
+ + + + +
+ + + + +
+ + + + +
1 2 3 4 65
Molecular Orbitals
1
2
3
4
5
0 nodes
1 node
2 nodes
3 nodes
4 nodes
Pentenylanion
Pentenylcation
Pentenylradical
Question 3: Pentadienyl Cation, Radical & Anion
Introduce the electrons and identify the HOMOs and LUMOs
1
2
3
4
5
HOMO
LUMO HOMO
LUMO
HOMO
LUMO
0 nodes
1 node
2 nodes
3 nodes
4 nodes
Pentenyl anion (6e)
Pentenyl cation (4e)
Pentenyl radical (5e)
Answer 3: Pentadienyl Cation, Radical & Anion
Introduce the electrons and identify the HOMOs and LUMOs
Question 4: Pentadienyl Cation & Anion
Generate the cation and anion and draw the resonance structures of the above species
Cl H
Answer 4: Pentadienyl Cation, Radical & Anion
Generate the cation and anion and draw the resonance structures of the above species
Cl H
B:
1
2
3
4
5
6
HOMO
LUMO
0 nodes
1 node
2 nodes
3 nodes
4 nodes
5 nodes
6 p-Molecular Orbital System – 1, 3, 5-Hexatriene
1
2
3
4
5
6
0 nodes
1 node
2 nodes
3 nodes
4 nodes
5 nodes
7 6 nodes
8/1 = 8
8/2 = 4
8/3 = 2.67
8/4 = 2
8/5 = 1.6
8/6 = 1.33
8/7 = 1.14
Nodal Plane Position
cation (6e)
HOMO
LUMO
radical (7e)
HOMO
LUMO
anion (8e)
HOMO
LUMO
7 p-Molecular Orbital System
Nodes m C2
1
2
3
4
5
6
m or C2Electrons
Question 5: 6p MO System
By shading the p atomic
orbitals, generate the
molecular orbitals for hexa-
1,3,5-triene .
Identify the number of nodes
characterising each molecular
orbital.
With reference to both a
mirror plane (m) and a two-
fold axis, designate the
orbitals as symmetric (S) or
antisymmetric (A).
Using arrows to represent
electrons, associate the six
p-electrons with the
appropriate molecular
orbitals of hexa-1,3,5-triene
in its ground state.
Finally, identify the HOMO
and LUMO.
Answer 5: 6p MO System
By shading the p atomic
orbitals, generate the
molecular orbitals for hexa-
1,3,5-triene .
Identify the number of nodes
characterising each molecular
orbital.
With reference to both a
mirror plane (m) and a two-
fold axis, designate the
orbitals as symmetric (S) or
antisymmetric (A).
Using arrows to represent
electrons, associate the six
p-electrons with the
appropriate molecular
orbitals of hexa-1,3,5-triene
in its ground state.
Finally, identify the HOMO
and LUMO.
Nodes m C2
1
2
3
4
5
6
m or C2
5 A S
4
3
2
1
0
A
A
A
A
A
S
S
S
S
S
HOMO
LUMO
Question 6: MO System
O OH
H3O H2O
A B
Protonation of A affords B. Draw the three
resonance structures of B in which the
positive charge has formally been shifted
from the oxygen atom onto three of the five
carbon atoms.
OH
Considering only these three resonance structures, how many
(i) carbon atoms are involved in the hybrid structure,
(ii) carbon p-orbitals are there,
(iii) -electrons are associated with the carbon atoms, and
(iv) molecular orbitals are associated with the combination of these carbon p-orbitals
In an analogous fashion to how question 1 was set out, draw out the molecular orbitals resulting from the p-orbital combination on this carbon framework, making sure you identify all of the items listed in question 1.
Answer 6: 5p MO System
Protonation of A affords B. Draw the three
resonance structures of B in which the
positive charge has formally been shifted
from the oxygen atom onto three of the five
carbon atoms.
Considering only these three resonance structures, how many
(i) carbon atoms are involved in the hybrid structure,
(ii) carbon p-orbitals are there,
(iii) -electrons are associated with the carbon atoms, and
(iv) molecular orbitals are associated with the combination of these carbon p-orbitals
In an anologous fashion to how question 5 was set out, draw out the molecular orbitals resulting from the p-orbital combination on this carbon framework, making sure you identify all of the items listed in question 5.
OH
OH
OH
OH
O OH
H3O H2O
A B
55
54
1
2
3
4
5
HOMO
LUMO
0 nodes
1 node
2 nodes
3 nodes
4 nodes
Pentenyl cation (4e)
Mirror Plane C2 axis
S A
A S
A S
S A
S A
O
Second Year Organic Chemistry CourseCHM2C3B
Frontier Molecular Orbitals and Pericyclic Reactions
Part 1(iii):
HOMO and LUMO Combination
What is the Driving Force for Controlling Pericyclic Reactions?
The driving force which controls the product
outcome in pericyclic reactions is the in
phase combination of the FMOs (the HOMO and
LUMO) of the reacting species in the
transition state.
FMO Theory is Extremely Powerful.
Pericyclic Reactions Involve Conjugated Polyene Systems
Pericyclic reactions involve conjugated polyene systems.
Enes and Polyenes are made by the linear combination of
p-AOs.
Thus, we first need to construct the molecular orbitals
of polyenes.
Then we need to identify the Frontier Molecular Orbitals.
Finally, we will need to construct the correct geometry
for orbital overlap of the FMOs in the transition states
of the reactions.
In bimolecular reactions (like the SN2 and the Diels-Alder
reaction), interaction between the two molecular components is represented by interaction between suitable molecular orbitals of each.
The extent of the interaction depends upon the geometry of approach of the components since the relative geometry affects the amount of possible overlap.
It also depends on the phase relationship of the orbitals – and also upon their energy of separation, a small energy favouring a greater interaction.
Generally, the two reactants will interact, via the highest occupied molecular orbital (HOMO) of one component and the lowest unoccupied molecular orbital (LUMO) of the other component, the so-called frontier molecular orbitals (FMOs). Consider the next five frames to appreciate this paragraph of text. Consider an
SN2 Reaction…
HOMOs and LUMOs
Highest Occupied Molecular OrbitalsLowest Unoccupied Molecular Orbitals
Revision: Transition State Geometries of Nucleophiles Attacking sp3 Tetrahedral Centres
XTETsp3 Nu XNu
= 180°
XNu
Nu
NucleophileHOMO
X
*C–X
*C–X
*C–Nu
C–Nu
Inversion of Configuration Supports this Attack Angle
NucleophileHOMOLUMO
LUMO
The orbital containing the lone pair of electrons on the Nu is the…
HOMO (Highest Occupied Molecular Orbital)
The * orbital of the C-X bond is the…
LUMO (Lowest Unoccupied Molecular Orbital)
Any bimolecular reaction can be analysed in this fashion
This analysis of FMOs (HOMOs and LUMOs) for such a simple
reactions may seem pointless for a simple SN2 reaction.
It is not!
Understand it.
Appreciate that for a bimolecular reaction the HOMO of one component interacts with the LUMO of the second component. (Additionally, for unimolecular reaction the HOMO of the molecular component dictates the reaction course).
In this course we will examine the use of FMOs to explain and predict the outcomes of a class of reactions referred to as pericyclic.
The use of FMOs is an extremely powerful tool to the synthetic organic chemist when analysing and predicting the outcome of pericyclic reactions.
Frontier Molecular Orbital Theory (FMOs)
– Summary Sheet Part 1 –
Frontier Molecular Orbitals
CHM3A2– Introduction to FMOs –
Molecular orbital theory is a powerful and versatile asset to the practice of organic chemistry. As a theory of bonding it has almost superseded the valence bond theory.
Molecular orbital theory has
proven amenable to pictorial non-mathematical expression,given the right answers to some decisive questions in organic chemistry,proven the theory of most theoretical chemists, given insight into not only to the theory of bonding, but also to the theory of making and breaking chemical bonds, andproven a theory which has been able to explain the pattern of reactivity in a class of reactions, known as pericyclic reactions.
In this course we will concentrate solely on the use of MO theory in predicting the outcome of pericyclic reactions. But it should not be forgotten that MO theory is applicable to other types of chemical reraction
To understand the importance of MO theory, we shall consider three types of pericyclic reactions and show how frontier molecular orbitals of the reactants can be used in a predicative nature to work out whether the reaction will proceed and what the stereo/regiochemical outcome will be.
The three types of pericyclic reactions we will consider are
electrcyclic reactionscycloaddition reactionssigmatropic reactions
We will see how it is possible to predict the stereoselectivity, diastereoselectivity, and regioselectivity of pericyclic reactions by the analysis of the FMOs of the transition states
The precise construction of the -molecular orbitals by the linear combination of p-atomic orbitals is extremely important if FMO theory is to yield the correct stereochemical product outcomes,
Key points to note when constructing -molecular orbitals from the combination of p-AOs are
(i) the combination of n Aos always affords n MOs(ii) The lowest -MOs (1) has no nodal planes(iii) The next highest (2) has one nodal plane, and so on(iv) The nodal planes need to be placed exactly in the Mos as described in the
lecture notes(v) Electrons fill from the lowest MO first with no more than two electrons in
each MO.