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Molecular Orbitals
Pericyclic Reactions
Electrocyclic Reactions
Pi Bonding MO (bond along x-axis; orbitals in xy plane)
x
y
z
mirror in yz plane
Symmetrical mirror plane (yz)
Pi Bonding MO(bond along x-axis; orbitals in xy plane) x
y
z
rotate 180o
about z-axis in xy-plane
Pi Bonding MO(bond along x-axis; orbitals in xy plane) x
y
z
Antisymmetric about the C2 axis
Pi Antibonding MO (bond along x-axis; orbitals in xy plane)
x
y
z
mirror in yz plane
Antisymmetric mirror plane (yz)
Pi Bonding MO(bond along x-axis; orbitals in xy plane) x
y
z
rotate 180o
about z-axis in xy-plane
Symmetric about C2 axis
Pi Bond MO
Mirror C2 - axis
AS
Bonding
SA
Antibonding
node
Allylic Resonance
Energy of anisolated p-orbital
allyl cationallyl radicalallyl anion
bonding -
0 nodes
SA
anti-bonding -
2 nodesSA
non-bonding -
1 nodeAS
M.O.s of 1,3-Butadiene
HighestOccupiedMolecularOrbital
LowestUnoccupiedMolecularOrbital
Energy of isolatedp-atomic orbital (A.O.)
bonding M.O. - 2
HOMOAS
bonding M.O. - 1SA
anti-bonding M.O. - 3*
LUMOSA
anti-bonding M.O. - 4* AS
AS
SA
SA
AS
AS
SAHOMO
LUMO
Orbital Symmetry in the Diels-Alder Reaction
Equal HOMO-LUMO Energy Difference
AS
SA
SA
AS
HOMO
LUMO AS
SA
EWG
EWG
Diels-Alder Reaction with a Deactivated Dienophile
EWG Lowers LUMO Energy
Dienophile LUMO (3*)
DieneHOMO (2)
The Diels-Alder Reaction Transition State
EWG EWG
HOMOthermal rxn.
hE
HOMOphotochemical rxn.
Photochemical Excitation in 1,3-Butadiene
1,3-Butadiene Cyclobutene
Conrotatory
Disrotatory
h
2
3*
but one cyclobutene is just like another cyclobutene
enter stereochemistry
1,3-Butadiene Cyclobutene
Conrotatory
Disrotatory
h
2
MeMe HH
H
HMe
Me
3*
MeMe
H
MeMe
H
Thermal Con
Thermal Con
hDis
hDis
Stereocontrol in the 1,3-Diene Cyclization
E,E E,Z
trans
cis