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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