Post on 22-Dec-2015
Chemistry 125: Lecture 56February 25, 2011
Generalized AromaticityCycloaddition – Diels-Alder
Electrocyclic StereochemistryDewar Benzene
This
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Generalization of Aromaticity:
4n+2 Stability
Transition State “Aromaticity”Cycloadditions &
Electrocyclic Reactionse.g. J&F Sec. 13.6 pp. 582-595
Generalized Aromaticity
pKa 15vs. 16 for H2O
H H
HH
H H
e.g. J&F Sec. 13.6pp. 587, 592
cyclo-C7H8 cyclo-C7H7- pKa 39 (despite more resonance structures)
6 electrons (4n+2)
8 electrons (4n, antiaromatic)
R H
R R
+ Ph3C+
2 electrons (4n+2)
H
HH
H H
OH-
unusually stable cation (triply benzylic)
+ Ph3CH
R
R R
+
even more stable
e.g. J&F Sec. 13.6 p. 591
Same for cyclo-C7H8 + Ph3C+ cyclo-C7H7
+
(cycloheptatrienyl or “tropylium”) 6 electrons (4n+2)
Electrocyclic Reactions
Pericyclic Reactions(in which transition states are “aromatic”)
Cycloadditions: Diels-Alder (e.g. J&F Sec. 12.12, 14.3)
H
H
Cycloadditions: Diels-Alder
4 + 2 electrons
Ring
4 + 2 electrons
enediene
LUMO
HOMO
How does become ?
Approach parallel to p-orbital axes.
foldedtransition
state
flattened product
H
H
H
H
H
H
cis
Z
H
H
H
H
Z
E tran
s
Cycloadditions: Diels-AlderRegiochemistry
H
H
CO2CH3
H
H
H
H
H
CH3
H
H
CO2CH3
H
CO2CH3H3C
CO2CH3
H3C
H
H
H
HCH3
9% yield
45% yield
20°C Perhaps an
allylic+ / enolate- intermediate
stabilized by terminal CH3
or unsymmetrical Transition State?
?
Perhaps StericHindrance?Note: Diene is
over C=O as well as C=C
CH3O2C
H
H
CO2CH3
trans alkene trans cyclohexene
cis alkene cis cyclohexene
H
H
CO2CH3
CO2CH3
Cycloadditions: Diels-Alder
Stereochemistry (Ene)Diene just “sits down” on Ene
68% yield
84% yield
e.g. J&F Sec. 12.12, p. 549
150-160°C
150-160°C
forming two -bonds simultaneously from the same face.
CO2CH3
CO2CH3
H
H
No rotatable intermediate with only one new bond
H
CO2CH3
H
CO2CH3
O
O
O
H
H
O
O
O
H
H
Cycloadditions: Diels-AlderStereochemistry (Diene)
H
H
H
H
O
O
O
CH2OH
CH3
5 min
120°C
(2E,4E)-2,4-hexadien-1-ol
maleic anhydride
all cis81% yield
15 hr
150°Cone trans
H
H
H
H
O
O
O
H
CH3
CH3Prefers s-trans
conformation, which is not
reactive.
CH3
CH2OH
H
H
CH3
H
H3C
H
CH3
H
(2E,4Z)-2,4-hexadiene
CNNC
Diels-Alder Variety
O H
propenal (“acrolein”)
OO
H
H
OHO
He.g. J&F Sec. 14.3, pp. 628-630
CN
CNCNNC
C
C
CO2CH3
CO2CH3
CH3 CH3
CO2CH3
CO2CH3
160°C
150°C
20°C
k ~1 M-1s-1
LUMO
HOMO
Diels-Alder Reactioncyclic electrontransition state
HOMO
LUMO
TransitionState
Motion
front view side view
TransitionState
HOMO-1
TransitionState
HOMO
p. 1351
A-T-T-G
DNA Double Helix
T-A-A-C
T-T
T-T
h (UVB)
Thymine photodimerization causes a chain kink that inhibits DNA replication & transcription and is believed to be the main source of mutation / melanomas.
N
N N
N
O
O
O
O
chainchain
CH3CH3
H H
HH
Pericyclic Reactions(in which transition states are “aromatic”)
Cycloadditions: Diels-Alder
Electrocyclic Reactions
conrotation disrotation
requires twistin 1 of 2 ways
Hückel
Transition StateMotion
top
touches
top(even #
of nodes)
top touches
bottom
(odd # of nodes)
Dav
id B
enb
en
nick Möbius
Preserves Mirror
Preserves Axis
node
13
conrotation disrotationMöbius
2
Aromatic Analogue (Hückel Connectivity)
Hückel
2
1
3
4
5
6
1
2
3
4
Hückel
21
3
4
5
6
2
1
3
4
5
6
1
2
3
4
Möbius
!
Track the MOs of hexatriene as they transform into those
of cyclohexadiene:
Preserves Mirror
Preserves Axis
H
-16 kcal/mole
H
+11 kcal/mole
How to study whether Conrotationis preferred for 4n-electron shift?
Disrotation preferred for 6-electron shift
CH3 CH3 CH3 CH3
CH3
CH3
CH3 CH3
(forms the less stable isomer)
(less stable isomer)• •• •
(4n+2)
The transition state favored in going from A to B, must also be favored in going from B to A. (“Microscopic Reversibility”)
CON4e
CON for 4nCON
8e
DIS6e DIS for 4n+2
e.g. J&F Sec. 27.2 pp. 1343-1346
H3C CH3
~0.005%CH3 CH3
CH3
CH3
99.9%
280°C
CH3 CH3 CH3 CH3
(forms the less stable isomer)
(less stable isomer)
Bias >11 kcal/mole
-10°C
CH3
CH3CH3 CH3
2
Transition State HOMO-1
1
disrotation6e Hückel
bottom touches top(odd # of nodes)
top touches top(even # of nodes)
conrotation4e Möbius
If you could run it forwards!
Calculated Isomers of Benzene
(2004)
84 are calculated to be < 100 kcal above benzene.
6 > 100 kcal above benzene have been prepared.(single bond breaking gives even less stable species)
Dewar Benzene (1963) is 74 kcal above benzene but lasts 2 days at room temperature!
CCC angles require disrotatory
motion
HOMO* LUMO
t1/2 = 2 days (room temp)
-11 kcal
-75 kcal
25
33
conrotatory
more strain
aromatic
HOMO
66 kcal/mole more exothermic,but only 8 kcal/mole “faster”?
good for 4n electrons
* LUMO
But shouldn’t “aromatic” 6--electron transition state be good for disrotation?
It is more fundamental that LUMO doesn’t overlap HOMOs (& vice versa).
Spectroscopy forStructure and Dynamics
“Sunbeams..passing through a Glass Prism to the opposite Wall, exhibited there a Spectrum of divers colours”
Newton (1674)
“Specters or straunge Sights, Visions and Apparitions” (1605)
O.E.D.
Electronic (Visible/UV) e.g. F&J sec. 12.7-12.8 pp. 533
Vibrational (Infrared) e.g. F&J sec. 15.4, pp. 707-713
NMR (Radio) e.g. F&J sec. 15.5-15.9, pp. 713-749
End of Lecture 56February 25, 2011
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