Transcript Document

Chemistry 125: Lecture 55
February 24, 2010
This
(4n+2) Aromaticity
Cycloaddition
Electrocyclic Reactions
For copyright
notice see final
page of this file
Furan
4 ABNs
2 ABNs
0 anti-bonding nodes
SHMo2
(Simple Hückel Molecular Orbital Program)
Crude  calculation shows heterocycle analogy.
identical shape
energy
N
lower energy N
node
highonNN
density
larger on N
N
lower energy
Benzene
Pyridine
Cyclodecapentaene  Naphthalene
(SHMo2)
same as
ethylene
same as
butadiene
same as
cyclodecapentaene
& butadiene
The Cyclodecapentaene
SystemAngewandte Chemie
International Edition in
EnglishVolumeﾊ3, Issueﾊ3,
Date:ﾊMarch 1964, Pages:ﾊ
228-229E. Vogel, H. D. Roth
Another Criterion of Aromaticity
is the PMR Chemical Shift
(coming soon, Chapter 15)
Generalized Aromaticity
H
H
H
H
H
OH-
H
H
pKa 15
vs. 16 for H2O
H
cyclo-C7H8
R
H
H
H
6  electrons (4n+2)
cyclo-C7H7- pKa 39 (despite more resonance structures)
Sec. 13.6 p. 591
8  electrons (4n, antiaromatic)
R
H
Ph3C+
+
R
R
unusually
stable cation
(triply benzylic)
Same for cyclo-C7H8
(cycloheptatrienyl
+
+
R
Ph3CH
R
even more stable
2  electrons (4n+2)
cyclo-C7H7+
6  electrons (4n+2)
“tropylium”)
Sec. 13.6
pp. 587, 592
Pericyclic Reactions
(in which transition states are “aromatic”)
Cycloadditions: Diels-Alder
(Sec. 12.12, 14.3)
Electrocyclic Reactions
Cycloadditions: Diels-Alder (Sec. 15.3)
ene
diene
4 + 2  electrons
ene
diene
Ring
4  + 2  electrons
Cycloadditions: Diels-Alder
Stereochemistry (ene) (Sec. 12.12, p. 549)
Ene just “sits down” on Diene
Cycloadditions: Diels-Alder
Stereochemistry (diene)
Diene just “sits down” on Ene
Cycloadditions: Diels-Alder (Sec. 14.3, pp. 628-630)
p. 1351

Transition
State
Motion
HOMO
LUMO
Transition
State
HOMO-1
HOMO

HOMO
LUMO
front view
Diels-Alder Reaction
cyclic electron
transition state
side view
Transition
State
Motion
front view
Diels-Alder Reaction
cyclic electron
transition state
side view
?
HOMO () orthogonal to LUMO (*)
h
Shift electron from HOMO to LUMO
p. 1046
Pericyclic Reactions
(in which transition states are “aromatic”)
Cycloadditions: Diels-Alder
Electrocyclic Reactions
231
6
5
4
3
2
1

Hückel
connectivity
4
3 !
6
5
4
3
2
1
6
5
4
3
2
1

2
requires twist
in 1 of 2 ways
1

top
touches
bottom
(odd # of nodes)
Transition State
Motion
top
touches
top
Möbius
Hückel
conrotation disrotation
(even #
of nodes)
4
3
2
1

Sec. 27.2
pp. 1343-1346
CON
4e
(shown backwards in our text;
same favored “aromatic”
conrotatory transition state)
DIS
6e
CON
8e
CON for 4n
DIS for 4n+2
4e Möbius
conrotation
2
1
Transition State HOMO -1


bottom touches top
(odd # of nodes)
6e Hückel
disrotation
top touches top
(even # of nodes)
End of Lecture 55
Feb. 24, 2010
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