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Burke Group Literature Seminar
August 2, 2007
Stereoelectronic Effects
Hosted by Eric Gillis
H
H N
H
1. Introduction (ID)
2. The electronic basis of stereoelectronic effects (ID)
3. Effects on conformation (KCG)
4. Effects on reactivity (KCG, ID)
5. Substitutions at saturated centres (DSP, EMW)
6. Additions and eliminations (BMW, DMK)
7. Rearrangements and fragmentations (EPG, JSP)
8. Radical reactions (EPG)
!* C=O
Rules for Orbital Interaction
Orbitals must be close in energy to interact
Kirby, A.J. Stereoelectronic Effects. Oxford University Press, 1996.
Rules for Orbital Interaction
Orbitals must be of similar size to interact
Orbitals interact in a geometry based on their symmetry
Kirby, A.J. Stereoelectronic Effects. Oxford University Press, 1996.
Hard and Soft Bases
Ambident Nucleophiles
Kirby, A.J. Stereoelectronic Effects. Oxford University Press, 1996.
The Anomeric Effect
HO
HO
HO
O
OH
Glucose Derivative
HO
HO
HO
O
AcO
AcO
AcO
O
OH
OAc
OR
X
OH
HO
HO
HO
O
OH
OH
Axial Isomer
%
R=H
36
R = Me
67
X = OAc
86
X = Cl
94
Estimating the anomeric effect: For the equilibrium equatorial
axial,
o
Anomeric effect = A value - !G obs.
For tetrahydropyrans, the A value can be estimated as 50% greater than
with cyclohexane.
Role of the Stereoelectronic Effect
O
"
O
!
X !*
X
"
!#delocalisation
Best Donors:
nn > no > !C-C, !C-H > !C-X (X = N > O> S > Hal).
Best Acceptors: $%C=O > !%C-Hal > !%C-O > !%C-N > !%C-C, !%C-H.
"The Gauche Effect"
F
F
H
H
F
H
H
H
H
F
H
H
H
H
H
F
H
F
Effects through Bonds