NMR and chirality 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 IdenticalIsomeric Constitutional isomersStereoisomers DiastereoisomersEnantiomers Classification of homomorphic nuclei Homomorphic nuclei HomotopicHeterotopic Constitutionally heterotopicStereoheterotopic DiastereotopicEnantiotopic 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 havethe same !G? Are NMR spectra of racemates identical with those of the individual enantiomers?R + R RR S + S SS R + S RS Dihydroquinine NNOHHOCH3pure () 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. !" increases with enantiomer ratio (reflecting increased proportion of heterochiral aggregates vs. homochiral). ! K = R" " " S[ ]R[ ] S[ ]! K = R" " " R[ ]R[ ]2= S" " " S[ ]S[ ]2R + R RR S + S SS R + S RS Chemical shifts reflect time averaged and concentration-weighedenvironments of nuclei in (R # RR # RS) compared to (S # SS # RS)]. Self-induced anisochrony Dihydroquinine NNOHHOCH3pure () 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 COOHOCH3HCOOHOCH3F3CCOOHOCH3H3CFFFFFCH3CH3OPOOClOO PClOOH F3CCOOHCOOHCN FFFFFFNCOOCH3F3C OSi ClCOOCH3HSi CH3COOHR + R $ RR S + R $ SR Chiral derivatization agents NMR methods for determination of enantiomer ratios based on diastereotopicity COOHOCH3HCOOHOCH3F3CR + 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 reactionforced to completion to avoid kinetic resolution (control withracemate). Chiral solvating agents NMR methods for determination of enantiomer ratios based on diastereotopicity OHH F3COHH F3COHH F3CNH2H H3CNH2H H3CHNH H3CONO2NO2COOHOH HOHOHquininecinchonine, other alkaloids NMR methods for determination of enantiomer ratios based on diastereotopicity NOH3C CH3ONCH3CH3NOH3C CH3ONCH3CH3COOHOH HOCH2 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 ! "dip= K 3cos2# $1r3Pseudocontacs shift (positive or neg.) ORHLr ! 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 athigher fields!). Apolar solvents required. Chiral shift reagent Much larger !" (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 CSRNNCH32'3'OCF3OYb/3CH3H3CH3CNMR methods for determination of enantiomer ratios based on diastereotopicity Determination of ratios between nicotine enantiomers using CSRNNCH32'3'OCF3OYb/3CH3H3CH3CMethods 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. COClOCH3F3CCOClCF3H3CO1-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. CHOMTPA!" > 0 !" < 0!" = "S "R MTPA plane (R)-MTPA ester(S)-MTPA ester OCF3H OMeO PhL1L2OCF3H OPh OMeL1L2L1 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.!" defined differently for different reagents [e.g., "S "R 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 !" behavior (1H 2D NMR better than 19F NMR) = advanced Moshers method. 8.Computational results show that the Moshers model is simplified and the conformational behavior is complex (explains some anomalies) 9.MPA and analogs better than MTPA. Methods for determination of absolute configuration Alternatives to MTPA COOHOCH3HCOOHOCH3HCOOHOCH3HMPA