Langmuir 1997, 13, 2903-2904
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Notes Discrimination of Enantiomers by Chiral Micelles Observed by NMR Spectroscopy† Gianluca Belogi, Michela Croce, and Giovanna Mancini* Centro CNR di Studio sui Meccanismi di Reazione, Dipartimento di Chimica, Universita` degli Studi di Roma “La Sapienza” P.le A. Moro 5, 00185 Roma, Italy Received November 25, 1996. In Final Form: February 24, 1997
Chiral micellar systems have been used for enantioselective kinetic recognition,1 but to the best of our knowledge they have not been used successfully as nonparamagnetic shift reagents2 with analytical purposes. It is known in fact that the interaction of the aggregate with the solute can be very specific,3 and in principle two enantiomeric solutes can interact very differently with a chiral aggregate. An example is reported4 in which enantiomeric discrimination is due to hydrogen bonding on the surface of mixed aggregates formed by a chiral surfactant in the presence of an achiral surfactant used as counterion, a system that is known5 to have quite a rigid structure. In the 1H NMR spectrum of a D2O solution 0.025 M in the axially chiral N,N ′-dibenzyl-2,2′-bipyridinium dibromide (1) and 0.10 M in sodium N-lauroyl-L-prolinate (2), the AB system of the signal relative to the benzylic methylene protons of compound 1 is split in two systems (Figure 1a), revealing two diastereomeric species. Analogously seven of the nine aromatic signals6 in the 13C NMR spectrum of the same solution are split in two signals, Figure 2a. In the spectra of the D2O solution in which the molar ratio of the two components is 2 (that means that
Figure 1. Chemical shifts of the hydrogens of the benzylic methylene of 1 in the 1H NMR spectrum of a D2O solution: (a) 0.10 M in 2 and 0.025 M in 1; (b) 0.10 M in 2 and 0.05 M in 1; (c) 0.10 M in 2 and 0.10 M in 1; (d) 0.10 M in 1.
the anionic surfactant has been neutralized by the addition of compound 1) the same signals reveal less sharply the two diastereomeric species (Figures 1b and 2b); finally when the two components are in a 1:1 molar ratio (equivalent to a cationic excess 2:1), the 1H NMR spectrum does not reveal the presence of two diastereomeric species (Figures 1c) and in the 13C NMR spectrum only two signals (151.2 and 133.4 ppm) are split (Figure 2c).
† Dedicated to Professor Alessandro Ballio on the occasion of his 75th birthday.
(1) (a) Brown, J. M.; Bunton C. A. J. Chem. Soc., Chem. Commun. 1974 969. (b) Moss, R. A.; Lee, Y.-S.; Alwis, K. W. J. Am. Chem. Soc. 1980, 102, 6648. (c) Yamada, K.; Shosenji, H.; Otsubo, Y.; Ono, S. Tetrahedron Lett. 1980, 21, 2649. (d) Ihara, H.; Ono, S.; Shosenji, H.; Yamada, K. J. Org. Chem. 1980, 45, 1623. (e) Ihara, Y.; Kunikiyo, N.; Kunimasa, T.; Nango, M.; Kuroki, N. Chem. Lett. 1981, 667. (f) Brown, J. M.; Elliott, R. L.; Griggs, C. G.; Helmchen, G.; Nill, G. Angew. Chem., Int. Ed. Engl. 1981, 890. (g) Ono, S.; Shosenji, H.; Yamada, K. Tetrahedron Lett. 1981, 22, 2391. (h) Matsumoto, Y.; Ueoka, R. Bull. Chem. Soc. Jpn. 1983, 56, 3370. (i) Ueoka, R.; Moss, R. A.; Swarup, S.; Matsumoto, Y.; Strauss, G.; Murakami, Y. J. Am. Chem. Soc. 1985, 107, 2185. (2) (a) Lanthanide Shift Reagents in Stereochemical Analysis; Morrill, T. C., Ed.; VCH Publishers: New York, 1986; pp 151-173. (b) Techniques in Pheromone Research Hummel, H. E., Miller, T. A. Eds.; SpringerVerlag: New York, 1984; pp 323-370. (3) (a) Cerichelli, G.; Grande, C.; Luchetti, L.; Mancini, G. J. Org. Chem. 1991, 56, 3025. (b) Cerichelli, G.; Coreno, M.; Mancini, G. J. Colloid Interface Sci. 1993, 158, 33. (c) Cleij, M. C.; Scrimin, P.; Tecilla, P.; Tonellato,U. Langmuir 1996, 12, 2956. (4) Jursic, B. S. Tetrahedron Lett. 1993, 34, 963. (5) (a) Ulmius, J.; Wennerstro¨m, H.; Johansson, L.-B.-A.; Lindblom, G.; Gravsholt, S. J. Phys. Chem. 1979, 83, 2232. (b) Manohar, C.; Rao, U. R. K.; Valaulikar, B. S.; Iyer, R. M. J. Chem. Soc., Chem. Commun. 1986, 379. (c) Rehage, H.; Hoffmann, H. J. Phys. Chem. 1988, 92, 4712. (d) Ambu¨hl, M.; Bangerter, F.; Luisi, P. L.; Skrabal, P.; Watzke, H. J. Langmuir 1993, 9, 36. (6) 13C NMR signals have been assigned on the basis of a 1H-1H COSY and 1H-13C HETCOR as follows: δ 131.65, 132.20, 132.55, and 133.55 (respectively C2, C3, C4, and C1 phenyl ring); 134.25, 134.95, 144.90, 150.60, 151,20 (respectively C5 and C5′, C3 and C3′, C2 and C2′, C4 and C4′, C6 and C6′ bipyridyl rings). (7) Differences in chemical shifts (up to 0.05 ppm) were relieved between neutral and basic solutions, while no differences were relieved between solution 0.039, 0.077, 0.11, 0.15, 0.21, 0.28, 0.33, and 0.60 M in NaOD.
S0743-7463(96)02041-0 CCC: $14.00
Analogous experiments were carried out on D2O solutions of (R)-2-(hexadecyldimethylammonio)butanol bromide (3) in order to discriminate the enantiomers of the sodium salt of mandelic acid (4) and of another axially chiral compound 2,2′-dicarboxy-6,6′-dinitrobiphenyl (5). In these experiments the presence of the two diastereomeric species, due to ion pairing with the chiral cationic surfactant, was not revealed by 1H and 13C NMR spectra. The discrimination of the enantiomers of mandelic acid was investigated in both a 0.050 and 0.10 M D2O solution of (R)-2-(hexadecyldimethylammonio)butanol bromide in the presence and in the absence of NaOD (NaOD has been added up to 0.60 M)7 in ratios of 1:6, 1:2, and 1:1 with respect to the cationic surfactant. 2,2′-Dicarboxy-6,6′dinitrobiphenyl was investigated in a 0.050 M solution of (R)-2-(hexadecyldimethylammonio)buthanol bromide in the presence and in the absence of NaOD (also in this case © 1997 American Chemical Society
2904 Langmuir, Vol. 13, No. 10, 1997
Notes
Figure 2. Chemical shifts of the aromatic carbons of 1 in the 13C NMR spectrum of a D2O solution: (a) 0.10 M in 2 and 0.025 M in 1; (b) 0.10 M in 2 and 0.05 M in 1; (c) 0.10 M in 2 and 0.10 M in 1.
NaOD has been added up to 0.60 M)7 in ratios 1:5, 1:12, and 1:20 with respect to the cationic surfactant. All these findings put in evidence that in order to observe recognition by chiral surfactants the binding constant of the solute, due to a combination of various factors as Coulombic forces and hydrophobic as well as specific interactions, must be very high. The observed chemical shift (δobs) for a micellized solute is expressed by eq 1
δobs ) δwxw + δmxm
(1)
where δw and δm are respectively the chemical shifts of the solute in water and in micelle and xw and xm are the molar fractions of the solute respectively in water and in micelle. In the case of two enantiomeric compounds δw is the same because in water the ions are free; on the other hand the chemical shifts of the micellized solutes (δm) are different because inside the aggregate the two species are ion paired with the surfactant forming diastereomeric domains. It is obvious that in the case of a solute with a high binding constant, the weight of the second component (δmxm) is more relevant. In Figures 1a and 2a we can assume δobs = δmfor each of the diastereomeric species. With an increase of the concentration of 1 (Figures 1b,c and 2b,c) the aggregates are saturated and the molar fraction in water becomes more and more relevant and the NMR spectra become similar to those of compound 1 in water (Figure 1d). In the case of mandelic
acid and of 2,2′-dicarboxy-6,6′-dinitrobiphenyl either the binding constant is not high enough and the molar fraction of the solute in water is relevant also in the presence of an excess of chiral cationic surfactant, or the difference in the δm term is not relevant. It must be stressed that the association of 1 with micelles of 2 did not yield the transition to larger aggregates even if it is well-known5 that generally the addition of organic hydrophobic salts with amphiphilic properties to micellar solution strongly favors the sphere to rod transition and the spontaneous formation of vesicles. In the case of a transition from spherical micelles to larger aggregates, in fact, the line width of NMR signals would have become much larger and the corresponding poor resolution of the NMR spectrum would have not allowed the observation of two separate signals. In our experiments the line widths of the NMR spectra of surfactant 2 have not been modified by addition of 1, showing no growth. In conclusion we observed an enantiomeric discrimination by a chiral surfactant thanks to the fact that to a high affinity of the solute for the aggregate did not correspond the growth of the system and the aggregates remained small sized. Methods. 1H and 13C NMR spectra were recorded at 25.0 °C on a Bruker AC 300 P spectrometer operating respectively at 300.13 and 75.47 MHz. Acknowledgment. We are indebted to Professor Giorgio Cerichelli for helpful discussions. LA9620418
