pericyclic reactions title cycloaddition electrocyclization
TRANSCRIPT
Pericyclic Reactions
+Cycloaddition
( )n( )n
Electrocyclization
π1-bonding HOMO
π∗2-antibonding LUMO
p-orbitals
S
A
Ethylene π-Molecular Orbitals
171 nm167 kcal/mol
400 nm800 nm 200 nmvisible UVIR
1,3-Butadiene from Ethylene
HOMO
LUMO
LUMO
HOMO
S
A
A
S
SS
A
S
A
Ethylene/Butadiene
214 nm133 kcal/mol
1,3-Butadiene π-Molecular Orbitals
400 nm800 nm 200 nmvisible UVIR
Butadiene
258 nm109 kcal/mol
1,3,5-Hexatriene π-Molecular Orbitals
LUMO
HOMO
S
S
S
A
A
A
400 nm800 nm 200 nmvisible UVIR
Hexatriene
FrontierMolecularOrbitalsFMOs
Butadiene: Orbital Coefficients
S
S
A
A
HOMO
LUMO
S
S
A
The Allylic System: Allyl Cation
+
The Allylic System: Allyl Radical
SOMO
S
S
A
LUMO
HOMO
The Allylic System: Allyl Anion
-
HOMO
LUMO
S
S
A
Cycloaddition: Diels-Alder ReactionAn Allowed [4+2] Cycloaddition
+
diene dienophile
HOMO π2
LUMO π*3
π*4
π1
π1-bonding HOMO
π∗2-antibonding LUMO
butadiene
ethylene
S
A
A
S
S
A
Diels-Alder Reaction:The Effect of Electron Withdrawing Groups
+
diene dienophile
EWG EWG
HOMO π2
LUMO π*3
π*4
π1
π1-bonding HOMO
π∗2-antibonding LUMO
butadiene deactivated ethylene
EWG
EWG
S
A
A
S
S
A
[4+2]-Cycloaddiitions
++ +
HOMO
LUMO
S
S
A
Diels-Alder Reaction: Mechanism
HOMO π2
LUMO π*3
π*4
π1
π1-bonding HOMO
π∗2-antibonding LUMO
butadiene
ethylene
S
A
A
S
S
A
Diels-Alder Reaction: The Endo Effect
O
Secondary Effect
What About a [2+2] Cycloaddition?
π1-bonding HOMO
π∗2-antibonding LUMO
S
A
π1-bonding HOMO
π∗2-antibondingLUMO (HOMO*)
S
A
light (h)
Thermally Allowed Cycloadditions4n+2 Rule
HOMOs
LUMOs
A
S
2
S
A
4
S
A
6
S
A
8
[4+2]=6 [6+4]=10
[8+6]=14
4n+2; n=1,2,3,4…..
Photochemically Allowed Cycloadditions4n Rule
HOMOs
LUMOs
A
S
2
S
A
4
S
A
6
S
A
8
4n; n=1,2,3,4…..
[2+2]=4
[4+4]=8
[6+6]=12
[8+8]=16
and [2+6]=8; [8+4]=12
Summary of Cycloadditions
2 4 6 8 10
2
4
6
8
10
P P P
P
P P
P P
P
T T
P
P P
TTT
T T T
TT
P
TT
Thermal 4n+2
Photochemical 4n
Electrocyclizations
1,3-Butadiene-Cyclobutene
thermal h
h thermal
trans
cis
E,E
E,Zdis
con dis
con
(E,E)-1,3-Butadiene-Cyclobutene
CH3
CH3
CH3
CH3
HH
CH3
CH3
HH
photochemical
disrotatory
A π2
thermal
conrotatory
S π3*
HOMO*
excited state
HOMOground state
h
S
A
CH3
CH3
(E,Z)-1,3-Butadiene-Cyclobutene
CH3
CH3
CH3
CH3
H
CH3
HCH3
H
CH3
HCH3
photochemical
disrotatory
A π2
thermal
conrotatory
S π3*
HOMO*
excited state
HOMOground state
h
S
A
(E,Z,E)-1,3,5-Hexatriene-Cyclohexadiene
thermal h
h thermal
trans
(E,Z,E) (E,Z,Z)
dis
con dis
con
(E,Z,E)-1,3,5-Hexatriene-Cyclohexadiene Orbitals
photochemical
conrotatory
A π4*
thermal
disrotatory
S π3
h
HOMOground state
HOMO*excited state
S cis
A trans
CH3
CH3
CH3
H3C
CH3
CH3
CH3
CH3
Electrocyclizations
n=1,2,3,4… thermal photochemical
4n con dis
4n+2 dis con
Violations
“There are none!”
Woodward and Hoffmann,The Conservation of Orbital Symmetry