Geometrical Dependence of the Highest Occupied Molecular ... · Conclusions HOMO of molecule...
Transcript of Geometrical Dependence of the Highest Occupied Molecular ... · Conclusions HOMO of molecule...
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Origin of the p Facial Stereoselectivity
of Bicyclic and Tricyclic Olefins
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Geometrical Dependence of the Highest Occupied Molecular Orbital
in Bicyclic Systems:
Motonori Tsuji Institute of Molecular Function
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M
L
S
Nu:
Cram’s Rule
OR
Less hindered side
S: small substituent M: medium substituent L: large substituent
Nu: S
L MNu
R OH
S
L MOH
R Nu
Major Product
eclipsed
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M S
L
M
L
S
O R
S
L
M
O R
M
L
S
O R
O R
Nu:
Felkin-Anh Model
s* s* p* p*
Nu: Nu:
Hyperconjugation
antiperiplanar ‡
Nu:
Nu:
s*
p*
LUMO
HOMO
Ener
gy
S: small substituent M: medium substituent L: large substituent
M
L
S
O R
M
L
S
O R
M
L
S
O R
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R
O-
H
Cieplak Model
s*‡
s
R
O-
H
s
s*‡
R
H
OH
Nu
R
H
Nu
OH
Axial Attack
Equatorial Attack
‡
‡
Major Product antiperiplanar
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X
W
W W
W
X
W
X
D
D D
D
X
D
Sterically Unbiased Systems for Facial Selectivity
X = O, CH2
W: Electron-withdrawing Group
D: Electron-donating Group
Electrophiles for Olefins Nucleophiles for Carbonyls
X
W
X
D
X
W
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X
W
W
X
D
D
X = O, CH2 W: Electron-withdrawing Group
D: Electron-donating Group
Explanation Using Cieplak Model
Steric, electrostatic, and torsional effects can ignore in these sterically unbiased systems.
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Electrophiles for Olefins Nucleophiles for Carbonyls
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A Reverse Facial Stereoselectivity with the Electron-donating Cyclopropane Ring
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Cram’s Rule, Felkin-Anh Model, Cieplak Model Steric Effect, Electrostatic Effect, Torsional Effect,
Ruled Out
anti
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Model Systems in Which the p Nature of Cyclopropane Is Maximized
Strong Electrophiles
Weak Elecrophiles
Weak Elecrophiles
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Strong Electrophiles No Selectivity
Both Electrophiles
anti
anti
anti
anti syn
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Geometrical Dependence of the HOMO in the Non-Conjugated Dienes
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A1-symmetry orbitals (circles) and B1-symmetry orbitals (squares) were obtained at the RPM3 (solid), RB3LYP/6-31G* (open), and ROVGF/6-31G*//HF/6-31G* (dotted line) methods.
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Though-space (Hyperconjugation) Though-bond Though-space
(Hyperconjugation) Though-bond
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Origin of the Facial Stereoselectivity in the Four-center Geometry
H
H
H
HHOMO
p
s
sp2 – sp2
Though-space
Hyperconjugation
Larger p lobes
Hyperconjugation p
NHOMO
p
p
s
Though-bond
Though-bond
HOMO
NHOMO
Though-space
Larger p lobes
inverse
sp2 – sp2 sp2 – sp2
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θ2
θ1
H
H
H
H
θ2 θ1 < 120º <
sp2 – sp2 s s
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Origin of the Facial Stereoselectivity in the Three-center Geometry
HOMO
p
s
sp2 – sp2
Though-space
Hyperconjugation
Larger p lobes p
NHOMO
p*
Homoconjugations (Homoaromaticity)
Larger p lobes
Strong Electrophiles
Weak Electrophiles inverse inverse
Since strong electrophiles have an activated LUMO, the olefinic molecules can not homoconjugate intramolecularly.
Weak Electrophile
LUMO
LUMO
Strong Electrophile
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θ3 120º <
θ3
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Though-bond
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W: Electron-withdrawing Group
HOMO
p
s
s sp2 – sp2
Though-bond
Hyperconjugation
HOMO
p
s
s sp2 – sp2
Though-bond
Hyperconjugation
Larger p lobes Larger p lobes
Effect of the Electron-withdrawing Group on the Facial Stereoselectivity
W
W
W
W
W
W
W
W
W
W W
W
H
H
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W
W
W
W
W
W
W
W
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D
D
D
D
D
D
D
D
D
D
D
D
H
H
HOMO
p
s
s
sp2 – sp2
Though-bond
Hyperconjugation
HOMO
p
s
s
sp2 – sp2
Though-bond
Hyperconjugation
Larger p lobes
Larger p lobes
Effect of the Electron-donating Group on the Facial Stereoselectivity
D: Electron-donating Group
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D
D
D
D
D
D
D
D
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Conclusions
HOMO of molecule depends on the geometry. This HOMO determines the facial stereoselectivity. Depending on the molecular geometry, the symmetrical p orbital is mixed with the different types of s orbitals. These s orbitals are mixed with p orbital via through-bond interactions. As a result, through-bond interactions, such as hyperconjugation, bring about the orbital rehybridization (pyramidalization) and tilting, leading to the desired transition structure. The magnitude of through-space interaction affects on the orbital sequence, thus leading to the reverse selectivity. In the three center geometry, homoconjugation (homoaromaticity) strongly affects on the orbital sequence, thus giving the ambident selectivity.
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