NMR Spectroscopy Using a Chiral Lanthanide Shift Reagent To ~ssessthe optical purity of l-Phenylethylamine Tito Viswanathan and Alan Toland University of Arkansas at Little Rock, Little Rock. AR 72204 To obtain the optical purity of a sample using polarimetry requires the value of the absolute rotation of the pure enantiomer. However, this is a condition that cannot always be met. The separation of a racemic mixture into its constituent enantiomers (i.e., resolution or optical resolution) and the determination of the optical purity of the resolved material has r - - - , ' ' ' ' , -5.0 25.0 20.0 15.0 10.0 5.0 0.0 PPH therefore provided a challenge for chemists. The problem is acute because the only differences between enantiomeric Figure 1. 200 MHz 'H-NMR spectrum of the chiral LSR-Yb(TFC)3in CDCI,. pairs can be subtle. In contrast, diastereomers Le., stereoisomers that are not enantiomers) are distinguishable even in achiral surroundings and exhibit different physical properties, for example, melting point, solubility, and IR and NMR spectra. I t thus seems logical that most methods of resolution involve the conversion of enantiomers into I I I \ I / 1 I diastereomers (1). I .!. .lo 115 ,!. .!5 610 #!l5 . 0 4 . 5 4.0 1.8 3.0 i., ,.I ,.a 0.0 ..I W" The resolution of a racemic form involves reaction with a chiral substance Figure 2. 'H-NMR spectrum of (S)-(-)-1-phenylaminein CDCI,. (ppeferably optically pure), called a resolving agent, to produce a mixture of diinteraction with chiral molecules. The diastereomeric astereomers. Although the constituents of a racemic mixproducts have the same structure but different configurature have the same solubility in achiral solvents, the diastions; they are not mirror images. tereomers produced usually do not. They can usually be The (Rj-(+j-amine (R),(R)-(+)-tartrateis soluble in separated by crystallization from an achiral solvent. methanol, whereas its diastereomeric companion (S)-(-1An experiment to exemplify the use of NMR in deteramine (R),(Rj-(+)-tartrateis relatively insoluble in methamining the extent of resolution is being routinely carnol and hence is easily separated. The amine salt is conried out in our organic laboratory. An enantiomeric mixverted to the free base by addition of concentrated NaOH t u r e of l-phenylethylamine (a-phenylethylamine, and then extracted with ether. The experimental protocol a-methylbenzylaminej, is reacted with the chiral refor accomplishing this may be found in standard laboraagent (R),(Rj-(+)-tartaric acid to give diastereomeric tory texts (e.g., see ref2). salts that differ greatly in their solubility in methanol. NMR Spectroscopy The enantiomeric reactants have the same structure but different configurations, being mirror images; they have Enantiomeric forms of l-phenylethylamine cannot be identical physical and chemical properties, except for distinguished by 'H NMR or 13CNMR because the groups the direction of rotation of plane-polarized light and the attached to the stereocenter are in an enantiotopic environment. However, the chemical shifts of the proton and carbons attached to the stereocenter can be differentially altered so that they become distinguishable in the 'H and 13CNMR snectra. This is accom~lishedbv usine one of several chiral'lanthanidr shift res&nts I ~ S k ' sf:;'., An exrensive rejiew of chin11 LSR's in oreanlc chernistrv has been (+(+)-atnine (*(-Famine (RJ,(R)
