3. Methods of determination of enatiomeric ratios based on diastereotopicity NMR of diastereomers Chiral derivatizing agents (CDAs) Chiral solvating agents (CSAs) Chiral shift and relaxation reagents (CSRs, CRRs)
Lecture outline
1. Classification of compounds and ligands
4. Methods for determination of absolute stereochemistry
2. NMR properties of stereoisomers
Classification of compounds
Compounds with identical molecular formula
Identical Isomeric
Constitutional isomers Stereoisomers
Diastereoisomers Enantiomers
Classification of homomorphic nuclei
Homomorphic nuclei
Homotopic Heterotopic
Constitutionally heterotopic Stereoheterotopic
Diastereotopic Enantiotopic
Isochrony (chemical shift equivalence) and anisochrony in enantiomers and racemates
Do enantiomers have identical NMR spectra (all respective pairs of nuclei are isochronous)?
Do NMR spectra of racemates show one set of signals?
Do homochiral and heterochiral nonbonded interactions have the same ΔG?
Are NMR spectra of racemates identical with those of the individual enantiomers?
R + R R……R
S + S S……S
R + S R……S
Dihydroquinine
N
N
OH
H
OCH3pure (–)
racemate
1:1 mixture of (–) and racemate
NMR spectra of enantiomers and racemates
Solid state 13C NMR spectra of enantiomers and racemates are normally different.
Enantiomer discrimination: measurable differences between physical properties of enantiomers vs. racemates due to energetic differences between homochiral and heterochioral nonbonded intramolecular interactions.
Solid state 13C NMR can be used to determine enantiomer purity of a sample.
Solid state:
Isochrony (chemical shift equivalence) and anisochrony in enantiomers and racemates
Solutions (in achiral media):
Isochrony (chemical shift equivalence) and anisochrony in enantiomers and racemates
Enantiopure and racemic compounds generally give identical but sometimes different NMR spectra.
The anisochrony occurs under conditions of fast exchange.
Δδ increases with enantiomer ratio (reflecting increased proportion of heterochiral aggregates vs. homochiral).
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K =R ⋅ ⋅ ⋅S[ ]R[ ] S[ ]
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K =R ⋅ ⋅ ⋅R[ ]R[ ]2
=S ⋅ ⋅ ⋅S[ ]S[ ]2
R + R R……R
S + S S……S
R + S R……S
Chemical shifts reflect time averaged and concentration-weighed environments of nuclei in (R ⇔ RR ⇔ RS) compared to (S ⇔ SS ⇔ RS)].
Self-induced anisochrony
Dihydroquinine
N
N
OH
H
OCH3pure (–)
racemate
1:1 mixture of (–) and racemate
Lessons:
Do not try to compare NMR spectra of samples with different or unknown enantiomeric composition.
Isochrony (chemical shift equivalence) and anisochrony in enantiomers and racemates
….These extra peaks may not be impurities….
Direct determination of enantiomeric excess!
Chiral derivatization agents
NMR methods for determination of enantiomer ratios based on diastereotopicity
COOHOCH3H
COOHOCH3F3C
COOHOCH3H3C
F
FF
F
FCH3
CH3
OPOO
Cl OOPCl
O
OHF3CCOOH
COOHCNF
F
FF
F
F
NCOOCH3F3C O
Si Cl
COOCH3H
Si CH3
COOH
R + R → RR
S + R → SR
Chiral derivatization agents
NMR methods for determination of enantiomer ratios based on diastereotopicity
COOHOCH3H
COOHOCH3F3C
R + R → RR S + R → SR
R + R → RR R + S → RS
Sharp, well resolved resonances should be present
The CDA must be enantiomerically pure and stable
Reagent should be added in large excess and the reaction forced to completion to avoid kinetic resolution (control with racemate).
Chiral solvating agents
NMR methods for determination of enantiomer ratios based on diastereotopicity
OHHF3C
OHHF3C
OHHF3C
NH2HH3C
NH2HH3C
HNHH3C
ONO2
NO2
COOHOHH OH
OH
quinine"cinchonine, "other alkaloids
NMR methods for determination of enantiomer ratios based on diastereotopicity
N
O
H3C CH3O
NCH3
CH3
N
O
H3C CH3O
NCH3
CH3COOH
OHH
–OCH2– group, 400 MHz, CDCl3
1.5%
98.5%
Chiral solvating agents
NMR methods for determination of enantiomer ratios based on diastereotopicity
Sharp, well resolved resonances should be present.
The CSA do not need to be enantiomerically pure and stable (as always when transient, dynamic species are involved; absence of enantiomeric purity diminishes anisochrony).
Anisochrony strongly CSA-concentration dependent
Apolar solvents preferred.
NMR methods for determination of enantiomer ratios based on diastereotopicity
Chiral shift reagent
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Δ dip = K 3cos2ϑ −1r3
Pseudocontacs shift (positive or neg.)
OR
H
L r ϑ
NMR methods for determination of enantiomer ratios based on diastereotopicity
Enhance anisochrony (LIS); externally enantiotopic groups become diastereotopic.
The CSR do not need to be enantiomerically pure and stable (as always when transient, dynamic species are involved; absence of enantiomeric purity diminishes anisochrony).
Resonance broadening by chemical exchange (especially at higher fields!).
Apolar solvents required.
Chiral shift reagent
Much larger Δδ (10-50 times larger) compared to CSAs.
CSRs are decomposed by strongly coordinating compounds.
NMR methods for determination of enantiomer ratios based on diastereotopicity
Determination of ratios between nicotine enantiomers using CSR
N
NCH3
2'3'
O
CF3
OYb/3
CH3H3C
H3C
NMR methods for determination of enantiomer ratios based on diastereotopicity
Determination of ratios between nicotine enantiomers using CSR
N
NCH3
2'3'
O
CF3
OYb/3
CH3H3C
H3C
Methods for determination of absolute configuration
Methods based on chiral derivatization agents (CDAs).
Transformation of a chiral compound with two enantiomeric CDAs to TWO diastereomeric species followed by comparison of spectra of the latter.
COClOCH3F3C
COClCF3H3CO
1-Methoxy-1-trifluoromethylphenylacetic
acid MTPA
MOSHER METHOD
Methods for determination of absolute configuration
Methods based on chiral derivatization agents (CDAs).
Transformation of a chiral compound with two enantiomeric CDAs to TWO diastereomeric species followed by comparison of spectra of the latter.
CH
OMTPAΔδ > 0Δδ < 0
Δδ = δS – δR MTPA plane
(R)-MTPA ester (S)-MTPA ester
OCF3
H O
MeO Ph
L1
L2O
CF3H O
Ph OMe
L1
L2
L1 and L2 are ligands connected to the chiral carbon of the secondary alcohol
Methods for determination of absolute configuration
Methods based on chiral derivatization agents (CDAs).
Transformation of a chiral compound with two enantiomeric CDAs to TWO diastereomeric species followed by comparison of spectra of the latter.
(R)-MTPA acid gives (S)-MTPA chloride – avoid confusion!!!!
Methods for determination of absolute configuration
MOSHER METHOD (AND RELATED METHODS)
1. CDA must have a bulky polar group to fix a well-defined conformation. 2. Carboxylic acid generally used for covalent derivatization 3. Aromatic group to induce anisotropic effect. 4. Δδ defined differently for different reagents [e.g., δS – δR for MPA,
(methoxyphenylacetic esters)]. 5. Originally described as an empirical rule, but is founded on conformational
preferences (similarly as, e.g., asymmetric induction rules). 6. Success depends on the presence of the expected, well-defined
conformation. 7. One should use as many resonances as possible, not just one resonance,
and they should exhibit consistent Δδ behavior (1H 2D NMR better than 19F NMR) = “advanced Mosher’s method”.
8. Computational results show that the Mosher’s model is simplified and the conformational behavior is complex (explains some anomalies)
9. MPA and analogs better than MTPA.