4+2

Report
Pericyclic Reactions
Cycloaddition
+
(
Electrocyclization
)n
(
)n
Ethylene -Molecular Orbitals
  2 -a n tib o n d in g
LU M O
A
171 nm
167 kcal/mol
p -o rb ita ls
S
 1 -b o n d in g
HOMO
IR
800 nm
visible
400 nm
UV
200 nm
1,3-Butadiene from Ethylene
A
LU M O
A
A
LU M O
S
S
A
HOMO
HOMO
S
S
S
1,3-Butadiene -Molecular Orbitals
214 nm
133 kcal/mol
IR
800 nm
visible
400 nm
UV
200 nm
1,3,5-Hexatriene -Molecular Orbitals
A
S
LU M O
A
HOMO
S
258 nm
109 kcal/mol
A
S
IR
800 nm
visible
400 nm
UV
200 nm
Butadiene: Orbital Coefficients
A
Frontier
Molecular
Orbitals
FMOs
S
A
S
The Allylic System: Allyl Cation
S
+
A
LU M O
S
HOMO
The Allylic System: Allyl Radical
S
LUM O
A
SOM O
S
HOMO
The Allylic System: Allyl Anion
S
A
S
LUM O
HOMO
-
Cycloaddition: Diels-Alder Reaction
An Allowed [4+2] Cycloaddition
+
d ie n e
 *4
d ie n o p h ile
A
  2 -a ntibo n d in g
LUM O
A
LUM O
S
 *3
HOMO
2
A
S
1
S
 1 -b on d in g
HOMO
e th y le n e
b u ta d ie n e
Diels-Alder Reaction:
The Effect of Electron Withdrawing Groups
+
EW G
EW G
d ie n e
d ie n o p h ile
 *4
A
  2 -a ntibo n d in g
LUM O
LUM O
S
 *3
A
EW G
HOMO
2
A
S
EW G
 1 -b on d in g
HOM O
1
S
b u ta d ie n e
d e a c tiv a te d e th y le n e
[4+2]-Cycloaddiitions
+
+
+
S
A
LU M O
S
HOMO
Diels-Alder Reaction: Mechanism
 *4
A
  2 -a ntibo n d in g
LUM O
A
LUM O
S
 *3
HOMO
2
A
S
1
S
 1 -b on d in g
HOMO
e th y le n e
b u ta d ie n e
Diels-Alder Reaction: The Endo Effect
Secondary Effect
O
What About a [2+2] Cycloaddition?
  2 -a n tib o n d in g
LU M O
  2 -a n tib o n d in g
L U M O (H O M O *)
A
A
light (hn)
S
S
 1 -b o n d in g
HOMO
 1 -b o n d in g
HOMO
Thermally Allowed Cycloadditions
4n+2 Rule
2
A
4
6
S
8
A
S
LUMOs
[4+2]=6
[6+4]=10
[8+6]=14
HOMOs
A
S
A
S
4n+2; n=1,2,3,4…..
2
Photochemically Allowed Cycloadditions
4n Rule
[2+2]=4
A
4
[4+4]=8
6
[6+6]=12
S
8
A
S
LUMOs
HOMOs
A
S
A
S
4n; n=1,2,3,4…..
and [2+6]=8; [8+4]=12
[8+8]=16
Summary of Cycloadditions
2
2
4
6
8
10
P
T
P
T
P
T
P
T
P
T
T
P
Thermal 4n+2
4
6
P
T
P
8
T
P
T
10
P
T
P
Photochemical 4n
P
T
T
P
Electrocyclizations
1,3-Butadiene-Cyclobutene
th e rm a l
hn
tra n s
con
d is
d is
con
hn
E ,Z
th e rm a l
E ,E
c is
(E,E)-1,3-Butadiene-Cyclobutene
CH3
p h o to c he m ica l
H
HOMO*
H
e x cite d sta te
CH3
d isro ta to ry
CH3
S  3 *
S
CH3
hn
CH3
CH3
th e rm a l
H
H
HOMO
g ro un d s ta te
co n ro ta to ry
CH3
CH3
A 2
A
(E,Z)-1,3-Butadiene-Cyclobutene
CH3
CH3
p h o to c he m ica l
H
HOMO*
e x cite d s ta te
CH3
CH3
d isro ta to ry
H
S  3 *
S
hn
CH3
th e rm a l
H
CH3
HOMO
g ro un d s ta te
co n ro ta to ry
CH3
H
A 2
A
CH3
(E,Z,E)-1,3,5-Hexatriene-Cyclohexadiene
th erm a l
d is
co n
(E ,Z ,E )
hn
hn
tra n s
con
d is
th erm al
(E ,Z ,Z )
(E,Z,E)-1,3,5-Hexatriene-Cyclohexadiene Orbitals
CH3
p h o to c h e m ica l
HOMO*
e xcite d sta te
CH3
co n ro ta to ry
H 3C
CH3
A  4 *
A tra n s
hn
CH3
HOMO
g ro u n d sta te
th e rm a l
d is ro ta to ry
S  3
CH3
S cis
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

similar documents