Stereoelectronic effects in organic chemistry, P Deslongchamps

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Content: STEREOELECTRONIC EFFECTS (S.E.) IN Organic Chemistry Pierre Deslongchamps SECTION 1 (2014) 1
Free pdf file: http://www.chm.ulaval.ca/prof_deslongchampsp.html G. Deslongchamps, P. Deslongchamps. Bent Bonds, the Antiperiplanar Hypothesis and the Theory of Resonance. A Simple Model to Understand Reactivity in Organic Chemistry. Org. Biomol. Chem. 9, 5321 (2011); ChemInform 42 (47), Nov. 22nd, 2011. G. Deslongchamps, P. Deslongchamps. Bent Bonds and the Antiperiplanar Hypothesis as a Simple Model to Predict Diels-Alder Reactivity: Retrospective or Perspective? Tetrahedron 63, 6022 (2013). 2
STEREOELECTRONIC EFFECTS IN ORGANIC CHEMISTRY Stereoelectrochemistry being an important factor in organic molecules, it means that the electronic delocalization must be taken into account in three-dimension. This leads to what is called « stereoelectronic effects ». In other words, electron delocalization can take place only when there is proper alignment of orbital. 3
Foundational Concept: Hьckel Model vs Pauling-Slater Model
J.C. Slater, Phys. Rev., 1931, 37, 481.
L. Pauling, J. Am. Chem. Soc., 1931, 53, 1367.
W.A. Bernett, J. Chem. Educ., 1967, 44, 17.
4
a-p vs t Bonds in carbonyl group and Antibonding Orbitals 5
The Antiperiplanar Hypothesis, a Traditional Stereoelectronic Concept is Based on SN2 Reaction Anomeric Effect E2 Elimination 6
Staggered vs Eclipsed Conformation
Ethane Propene (CH3-CH=CH2)
(3 kcal/mol) (2 kcal/mol) 7
Hyperconjugation Affecting Conformational Stability Propene Butene
K. Inomata, J. Synth. Org. Chem. Jpn, 2009, 67, 1172.
8
Conformational Analysis: Enol Ethers
J. D. Mersh, J. K. M. Sanders, Tetrahedron Lett., 1981, 22, 4029.
9
Enamine Conformation and Reactivity
N.B. Pyrrolidine enamines are more nucleophilic than piperidine enamines.
K. L. Brown, L. Damm, J. D. Dunitz, A. Eschenmoser, R. Hobi, C. Kratky,
Helv. Chim. Acta 1978, 61, 3108.
10
SECTION 1 Stereoelectronic Effects (S.E.) and Conformation of Acetals and Related Functions Anomeric Effect 11
Axial Isomer is More Stable than the Equatorial One in Glycosides ! Lemieux
One example: Eliel
Relative Stability (exp.)
Me
O
H+
H
OMe 0 kcal/mol (80%)
Me O
OMe
H 0.8 kcal/mol (20%)
12
Conformation of Glycosides: Exo and Endo Anomeric Effects and Relative Stability
a and Glycosides exist exclusively in A1 and E1
Evidence by NMR and Rx
13
ANOMERIC EFFECT (explanation) Electronic
REPULSION
STABILIZATION
EdWard Eliel
Altona Lemieux
HYPERCONJUGATION
BOND ­ NO BOND RESONANCE
F FCF
FF C F+
H
H
L. Brockway. J.Phys.Chem. 41, 185 (1937) L. Pauling. Nature of Chemical Bond, 314 (1960)
Altona
RO
H
nO / *C-O O
(1964)
R
Influence on bond length and oxygen basicity 14
Exo-Anomeric Effect and Steric Repulsion
longer bond
O OR H shorter bond
R O O H
- O O+ R H less steric repulsion
better than
-
R
O
O+
H more steric repulsion
15
Bond Length Changes in the Ground State of Axial and Equatorial Aryloxytetrahydropyrans (X-rays analysis)
Kirby
O n H x OAr
O n OAr x H
axial OAr x : exocyclic bond increases n: endocyclic bond decreases with a greater pKA
equatorial OAr x : exocyclic bond increases to a less extent n: endocyclic bond does not change
16
Relative Population of Two Species a and b at Equilibrium (25°C) 17
Cyclohexane Conformation 18
Steric Effects in n-Butane and CH3 CH2 CH3 OCH3
H
CH3
H
H
O CH3
H
CH3 CH3
2 x 0.85 = 1.7
2 x 0.4 = 0.8
> 4.0 kcal/mol
H
CH3
H
H
CH3
H
1.7 + 0.85 = 2.55
H
H CH3 CH3 H 0.85 H H H H
H
0
3 x 0.85 = 2.55 kcal/mol
19
Evaluation of the Anomeric Effect
57% (0)
43% (0.17 kcal/mol)
1 gauche form of n-butane =
1 OR axial to cyclohexane =
entropy factor
=
1 anomeric effect (A.E.) =
energy difference
=
+0.85 +0.8 -0.42 (favor 5) -1.4 -0.17 kcal/mol (favor 5)
Descotes
45% (0.14)
steric effect
=
1 anomeric effect (A.E.) =
energy difference
=
55% (0) +1.65 (favor 8) -1.4 (favor 7) -0.15 kcal/mol
Deslongchamps 20
1,7-Dioxaspiro[5.5]undecane (Spiroketal) Anomeric Effect and Relative Stability)
Steric Effects A.E.
9A +1.6 -2.8 -1.2 (0)
9B
9C
+2.6 -1.4 +1.2 (2.4)
+3.6 0 3.6 (4.8 kcal/mol)
Gauche form of n-propyl ether (OCH2CH2CH3) = 0.4
Gauche form of n-butane
= 0.9
Deslongchamps
21
Conformation of Mono and Dithioacetals
65%
35%
90%
10%
Eliel 22
Dithio Spiroketal
H+ thermo Deslongchamps
(98%)
(2%) 23
Monothio Spiroketal (100%) (100%) Deslongchamps 24
Conformation of 1,3-Oxazine and 1,3-Diazane
Lemieux
MAJOR
MINOR
Newton
(90%) 1 A.E. = -1.4 -1.4 0
(10%)
2 A.E. 2 gauche butane 1 gauche n-propylether
= -2.8 = +1.8 = +0.4 -0.6 +0.8 kcal/mol
Lemieux Eliel
(FAVORED)
25
Reverse Anomeric EFfect
O H N+
H + N O
HO HO HO
O H HO N+
glucose
H N+
HO HO HO H
H + N O H HO
X-rays
Lemieux
+
H
N
Deslongchamps
electrostatic attraction
26
THE ANOMERIC EFFECT by R.U. Lemieux · Unequivocal evidence for the conformational preferences of sugars and their derivatives became available in 1958 [ 1, 2 ]. · Thus, it became possible to assign a stereoelectronic contribution to the relative thermodynamic stabilities of the a- and -forms of sugar derivatives since this contribution, by favoring the axial orientation, was opposite to that expected from a consideration of non-bonded interactions [ 3 ]. [ 1 ] R.U. Lemieux, R.K. Kulling, H.J. Bernstein and W.G. Schneider. J. Am. Chem. Soc. 80, 6098 (1958). [ 2 ] R.U. Lemieux. Current Contents (Citation Classic), 26 10 (1980). [ 3 ] R.U. Lemieux and N.J. (Paul) Chь. Abstracts of Papers, Am. Chem. Soc. 133, 31N (1958). 27
THE ANOMERIC EFFECT (cont'd) by R.U. Lemieux · Riiber and Sшrensen [ 4 ], in 1933, had introduced the term "anomeric" as a class name for the a- and -forms of sugars and their glycosides. Accordingly, the term "anomeric effect" was introduced in 1959 [ 5 ] to describe this stereoelectronic feature which appeared to be a general property of acetal linkages. It was recognized [ 5 ] that the anomeric effect should have an important influence on the orientation of the aglycon of a glycoside. [ 4 ] C.N. Riiber and N.A. Sшrensen. Kgl. Norske Videnskat. Selskabs. Skrifter 7, 50 (1933). [ 5 ] R.U. Lemieux. Abstracts of Papers, Am. Chem. Soc. 135, 5E (1959). 28
THE ANOMERIC EFFECT (cont'd) by R.U. Lemieux · The term "exo-anomeric effect" was introduced in 1969 [ 6 ] to specify this component of the overall anomeric effect in the case of glycosides wherein the pyranose ring is anchored in a specific chair conformation. · The term "reverse-anomeric effect" was introduced in 1965 [ 7 ] in view of the observation that pyridinium a-glycopyranosides possessed abnormal high-energy conformations. [ 6 ] R.U. Lemieux, A.A. Pavia, J.C. Martin and K.A. Watanabe. Can. J. Chem. 47, 4427 (1969). [ 7 ] R.U. Lemieux and A.R. Morgan. Can. J. Chem. 43, 2205 (1965). 29

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