哈佛大学高等有机

Chemistry 206 Advanced Organic Chemistry
Lecture Number 2
Stereoelectronic Effects-2
! "Positive" and "Negative" Hyperconjugation ! Anomeric and Related Effects ! Peracid & Dioxirane Epoxidation (Stereoelectronics)
D. A. Evans
Hyperconjugation, The Anomeric Effect, and More
Useful LIterature Reviews
Chem 206
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Kirby, A. J. (1982). The Anomeric Effect and Related Stereoelectronic Effects at Oxygen. New York, Springer Verlag. Box, V. G. S. (1990). “The role of lone pair interactions in the chemistry of the monosaccharides. The anomeric effect.” Heterocycles 31: 1157. Box, V. G. S. (1998). “The anomeric effect of monosaccharides and their derivatives. Insights from the new QVBMM molecular mechanics force field.” Heterocycles 48(11): 2389-2417. Graczyk, P. P. and M. Mikolajczyk (1994). “Anomeric effect: origin and consequences.” Top. Stereochem. 21: 159-349. Juaristi, E. and G. Cuevas (1992). “Recent studies on the anomeric effect.” Tetrahedron 48: 5019 (PDF)
R H H C
Physical Evidence for Hyperconjugation
■ Bonds participating in the hyperconjugative interaction, e.g. C–R, will be lengthened while the C(+)–C bond will be shortened. First X-ray Structure of an Aliphatic Carbocation
! Less e-density at X
D. A. Evans
Lone Pair Delocalization: N2F2
The trans Isomer
Chem 206
Now carry out the same analysis with the same 2 orbitals present in the trans isomer.
+
C
R H H H H C C H H
The graphic illustrates the fact that the C-R bonding electrons can "delocalize" to stabilize the electron deficient carbocationic center. Note that the general rules of drawing resonance structures still hold: the positions of all atoms must not be changed.
Syn Orientation
R
""
Chem 206
antibonding !" C–R R:– H H C X+ H H antibonding !" C–R R:– R X+ H H C filled X hybrid orbital
""
! Delocalization of nonbonding electron pairs into vicinal antibonding orbitals is also possible R H H C
Carey & Sundberg: Part A; Chapter 3 pp 151-156
Question: First hour Exam 2000 (Database Problem 34)
Question 4. (15 points). The useful epoxidation reagent dimethyldioxirane (1) may be prepared from "oxone" (KO3SOOH) and acetone (eq 1). In an extension of this epoxidation concept, Shi has described a family of chiral fructose-derived ketones such as 2 that, in the presence of "oxone", mediate the asymmetric epoxidation of di- and tri-substituted olefins with excellent enantioselectivities (>90% ee) (JACS 1997, 119, 11224). Me O Me R2 R1 R2
D. A. Evans
Hyperconjugation: Carbocation Stabilization
Chem 206
! The interaction of a vicinal bonding orbital with a p-orbital is referred to as hyperconjugation. This is a traditional vehicle for using valence bond to denote charge delocalization.
The Expected Structural Perturbations
Change in Structure
X
!!
Spectroscopic Probe
X-ray crystallography X-ray crystallography Infrared Spectroscopy Infrared Spectroscopy NMR Spectroscopy NMR Spectroscopy
H
The Molecular Orbital Description
!" C–R
! Overlap between two orbitals is better in the anti orientation as stated in "Bonding Generalizations" handout.
!" C–R !" C–R
T. Laube, Angew. Chem. Int. Ed. 1986, 25, 349
+
C
H H
+
C
H H
The Adamantane Reference (MM-2)
H
1.528 Å
110 °
! C–R
! C–R
Me Me
Me
1.530 Å
! Take a linear combination of ! C–R and CSP2 p-orbital:
The interaction of filled orbitals with adjacent antibonding orbitals can have an ordering effect on the structure which will stabilize a particular geometry. Here are several examples: Case 1: N2F2 F N N This molecule can exist as either cis or trans isomers F F N N F There are two logical reasons why the trans isomer should be more stable than the cis isomer. ! The nonbonding lone pair orbitals in the cis isomer will be destabilizing due to electron-electron repulsion. ! The individual C–F dipoles are mutually repulsive (pointing in same direction) in the cis isomer.
+
1.431 Å
[F5Sb–F–SbF5]–
+
C
Stereoelectronic Requirement for Hyperconjugation: Syn-planar orientation between interacting orbitals
100.6 °
1.608 Å
Me
Me Me
The Molecular Orbital Description