Pericyclic Reactions!
Title
+Cycloaddition!
( )n ( )n
Electrocyclization!
π1-bonding HOMO
π∗2-antibonding LUMO
p-orbitals
S
A
Ethylene π-Molecular Orbitals!
171 nm!167 kcal/mol!
400 nm!800 nm! 200 nm!visible! UV!IR!
Ethylene
1,3-Butadiene from Ethylene !
HOMO
LUMO
LUMO
HOMO
S
A
A
S
SS
A
S
A
Ethylene/Butadiene
214 nm!133 kcal/mol!
1,3-Butadiene π-Molecular Orbitals!
400 nm!800 nm! 200 nm!visible! UV!IR!
Butadiene
258 nm!109 kcal/mol!
1,3,5-Hexatriene π-Molecular Orbitals!
LUMO
HOMO
S
S
S
A
A
A
400 nm!800 nm! 200 nm!visible! UV!IR!
Hexatriene
Butadiene Orbital Coeffients
Frontier!Molecular!Orbitals!FMOs!
!Butadiene: Orbital Coefficients!
S
S
A
A
Allyl Cation
HOMO
LUMO
S
S
A
!The Allylic System: Allyl Cation!
+
Allyl Radical !
The Allylic System: Allyl Radical!
HOMO
LUMO
S
S
A
Allyl Anion !
The Allylic System: Allyl Anion!
HOMO
LUMO
S
S
A
-
Cycloaddition: Diels-Alder Reaction!An 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
Diels-Alder 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
Allyl Cation DA !
[4+2]-Cycloadditions!
++ +
HOMO
LUMO
S
S
A
Diels-Alder Reaction: Mechanism!
Diels-Alder 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!!
Diels-Alder Endo Effect
O
Secondary Effect!
What About a [2+2] Cycloaddition?!!
2+2?
π1-bonding HOMO
π∗2-antibonding LUMO
S
A
π1-bonding HOMO
π∗2-antibondingLUMO (HOMO*)
S
A
light (hν)!
4n+2 Rule Thermally Allowed Cycloadditions!
4n+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…..!
4n Rule Photochemically Allowed Cycloadditions!
4n 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 !
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
!
Electrocyclizations!!
Butadiene-Cyclobutene 1,3-Butadiene-Cyclobutene!
!
thermal hν
hν thermal
trans
cis
E,E
E,Zdis
con dis
con
(E,E)-Butadiene-Cyclobutene (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
hν
S
A
CH3
CH3
(E,Z)-Butadiene-Cyclobutene (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
hν
S
A
(E,Z,E)-Hexatriene-Cyclohexadiene (E,Z,E)-1,3,5-Hexatriene-Cyclohexadiene!
!
thermal hν
hν thermal
trans
(E,Z,E) (E,Z,Z)
dis
con dis
con
(E,Z,E)-Hexatriene-Cyclohexadiene Orbitals
(E,Z,E)-1,3,5-Hexatriene-Cyclohexadiene Orbitals!!
photochemical
conrotatory
A π4*
thermal
disrotatory
S π3
hν
HOMOground state
HOMO*excited state
S cis
A trans
CH3
CH3
CH3
H3C
CH3
CH3
CH3
CH3
Electrocyclization Summary
Electrocyclizations!!
!n=1,2,3,4…!
!thermal!
! photochemical!
!4n!
!con!
!dis!
!4n+2!
!dis!
!con!
Violations
Violations!
“There are none!”!
Woodward and Hoffmann,The Conservation of Orbital Symmetry!