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Langmuir 1998, 14, 1960-1962
Deracemization of an Axially Chiral Biphenylic Structure in Chiral Micellar Aggregates Stefano Borocci, Maura Erba, and Giovanna Mancini* Centro CNR di Studio sui Meccanismi di Reazione, c/o Dipartimento di Chimica, Universita` degli Studi di Roma “La Sapienza”, P.le A. Moro 5, 00185 Roma, Italy
Anita Scipioni Dipartimento di Chimica, Universita` degli Studi di Roma “La Sapienza”, P.le A. Moro 5, 00185 Roma, Italy Received September 29, 1997. In Final Form: December 17, 1997 An example of deracemization1 of an axially chiral biphenylic system in micellar type aggregates is reported. Two independent techniques (1H NMR and CD) give evidence of the phenomenon, which is due to a combination of chiral induction and organization.
Introduction Information of chirality at a molecular level can induce, under aggregating conditions, formation of organized systems such as enantiomerically pure domains. After discovery of the existence of enantiomers by Pasteur,2 numerous examples of organization into enantiomeric domains were reported.3 Both spontaneous3a-e and induced3b-c,f resolution of enantiomers are examples of organization in self-assemblies on the basis of a stereochemical code. All the reported examples concern systems with a high degree of organization like crystal lattices, Langmuir films, or liquid crystals, while, to the best of our knowledge, no evidence was reported concerning less organized structures such as micellar aggregates. We recently reported4 the organization in domains following E/Z stereochemical information in an amidic surfactant, under micellar aggregating conditions. We report here on the 1H NMR and CD observation of deracemization of axially chiral 2-carboxy-6-nitro-2′dodecyloxybiphenyl (1)5 in an aqueous solution of N-
(1) The term deracemization is used according to March, J. Advanced Organic Chemistry, 4th ed.; John Wiley & Sons: New York, 1992; p 124, because we describe a process in which one enantiomer is converted to the other, thanks to the interaction with an outside optically active substance, so that a racemic mixture is converted to a mixture enriched in one enantiomer. (2) Pasteur, L. C. R. Hebd. Se´ ances Acad. Sci. Paris 1848, 26, 535. (3) (a) Paquette, L. A.; Lau, C. J. J. Org. Chem. 1987, 52, 1634. (b) March, J. Advanced Organic Chemistry, 4th ed.; John Wiley & Sons: New York, 1992; p 123. (c) Eliel, E. L.; Wilen, S. H. Stereochemistry of Organic Compounds; John Wiley & Sons: New York, 1994; Chapter 7. (d) Steven, F.; Dyer, D. J.; Walba, D. M. Angew. Chem., Int. Ed. Engl. 1996, 35, 900. (e) Steven, F.; Dyer, D. J.; Walba, D. M. Langmuir 1996, 12, 436. (f) Arnett, E. M.; Harvey, N. G.; Rose, P. L. Acc. Chem. Res. 1989, 22, 131. (4) Cerichelli, G.; Luchetti, L.; Mancini, G. Langmuir 1997, 13, 4767. (5) Synthesis and characterization of 2-carboxy-6-nitro-2′-dodecyloxybiphenyl (1) will be described elsewhere.
hexadecyl-N-methyl-L-prolinolinium bromide (2). As far as the chiral surfactant 2 is concerned, we used the diastereomeric mixture 1:1 obtained by quaternization of N-methyl-L-prolinol with 1-bromohexadecane in acetonitrile.6 The ratio between the two diastereomers was determined by integrating the signals of methyl on nitrogen relative to the two diastereomers in the 1H NMR spectrum (Figure 1). For 2-carboxy-6-nitro-2′-alkyloxybiphenyl the rotational barrier is about 20 kcal/mol,7 which allows equilibration of enantiomers at ambient temperature. Results and Discussion 1H
NMR and CD experiments were performed on The an aqueous solution 25 mM in N-hexadecyl-N-methyl-Lprolinolinium bromide (2) and 4.7 mM in 2-carboxy-6nitro-2′-dodecyloxybiphenyl (1)8 between 25.0 and 60.0 °C, the relative spectra being reported in Figure 2 and Figure 3. At 25.0 °C (Figure 2a) the 1H NMR signals are broad showing the existence of large aggregates;8 moreover, the nonsymmetric shape of the signals suggests the presence of less intense signals which could be diagnostic of a partial deracemization. By increasing the temperature (T ) 40.0 °C, Figure 2b, and T ) 50.0 °C, Figure 2c), we obtained better resolved spectra which show minor signals (indicated by the arrows) close to the seven signals expected for the aromatic protons, the two being approximately in a ratio 5:1. These minor signals could be the evidence of a deracemization; in fact, the enantiomeric biphenyls, thanks to diastereomeric interactions with the chiral surfactant, should yield diastereomeric signals. At 60.0 °C (Figure 2d) the rate of interconversion of the diasteromeric biphenyls is fast compared to the NMR time scale and the spectrum shows only signals corresponding to a single species. It must be pointed out that when the temperature is increased, the size of the aggregates de(6) Up to now attempts to obtain a mixture enriched in one diastereomer by using different reaction conditions were not successful. (7) Li, C. C.; Adams, R. J. Am. Chem. Soc. 1935, 57, 1565. (8) The signal line width in the 1H NMR spectra show that these concentration conditions yield micellar type aggregates; below 30 °C there is evidence of slightly larger aggregates that could be rods more than classical micelles. Different concentration conditions yield different phases which are under investigation.
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Figure 1. Integration of the signals of methyl on nitrogen relative to the two diastereomers of N-hexadecyl-N-methyl-L-prolinolinium bromide (2) in a CD3OD solution 38 mM in 2.
Figure 2. Aromatic region of the 1H NMR spectra of an aqueous solution 4.7 mM in 2-carboxy-6-nitro-2′-dodecyloxybiphenyl (1) and 25 mM in N-hexadecyl-N-methyl-L-prolinolinium bromide (2) at 25.0 °C (a), 40.0 °C (b), 50.0 °C (c), and 60.0 °C (d).
creases (as shown by line width) with respect to the sizes at lower temperatures and they become looser and more hydrated.9 The CD spectra of the same solution show a dichroic band at 330 nm (a spectral region in which the aqueous solution of the chiral surfactant 2 is transparent) in the range of temperature between 25.0 and 60.0 °C (Figure 3). In fact in the presence of an enantiomeric excess a dichroic band is expected independently of the rate of interconversion between the two biphenylic species, since the observation by CD depends only on their equilibrium constant. Of course, the CD band is expected unless the temperature affects dramatically the equilibrium constant and/or the size and the structure of the aggregates.9 Actually the CD band observed at 60.0 °C (Figure 3b) is less intense than that at 25.0 °C (Figure 3a); this result can be attributed to the presence of smaller and looser aggregates evidenced by the NMR spectrum and/or to a different equilibrium constant. (9) Cerichelli, G.; Luchetti, L.; Mancini, G.; Savelli, G.; Bunton, C. A. J. Colloid Interface Sci. 1993, 160, 85.
Figure 3. CD spectra of an aqueous solution 4.7 mM in 2-carboxy-6-nitro-2′-dodecyloxybiphenyl (1) and 25 mM in N-hexadecyl-N-methyl-L-prolinolinium bromide (2) at 25.0 °C (a) and 60.0 °C (b).
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Figure 4. Aromatic region of the 1H NMR spectrum of a CDCl3 solution 4.7 mM in 2-carboxy-6-nitro-2′-dodecyloxybiphenyl, (1) (a) and of a CDCl3 solution 4.7 mM in 2-carboxy-6-nitro-2′-dodecyloxybiphenyl (1) and 25 mM in N-hexadecyl-N-methyl-L-prolinolinium bromide (2) (b).
CD experiments were also performed on aqueous solutions 25 mM in N-hexadecyl-N-methyl-L-prolinolinium bromide (2) in the presence of two different achiral chromophores (4.7 mM), 4,4′-bipyridyl and N-dodecylaniline, respectively, to assess if the CD band observed in the presence of compound 1 could be an induced effect due to the chiral environment. In the cases of the two achiral chromophores no CD signal was observed; this evidence is in favor of a phenomenon of deracemization as far as 1 was concerned. The 1H NMR and CD experiments performed on a CDCl3 solution 25 mM in N-hexadecyl-N-methyl-L-prolinolinium bromide (2) and 4.7 mM in 2-carboxy-6-nitro-2′-dodecyloxybiphenyl (1) at 25.0 °C show no evidence of deracemization: in fact neither a CD band in the area of absorbance of the biphenylic system nor less intense 1H NMR signals were observed. At the same time the 1H NMR spectrum shows an interaction between the biphenylic structure 1 and the chiral surfactant 2; in fact there is a slight variation in the chemical shift of the aromatic signals with respect to the spectrum of 4.7 mM 1 in CDCl3 (Figure 4). These experiments demonstrate that the organization due to aggregation is essential for observing the deracemization process.
In conclusion, by using two techniques characterized by different time scales, we observed a high extent of deracemization of 2-carboxy-6-nitro-2′-dodecyloxybiphenyl (1). The combination of chiral induction and organization has driven the biphenylic system to deracemization. In other words, there is recognition and induction by the chiral auxiliary and recognition of the chiral code of the biphenylic structure and consequently organization. The novelty of this result consists not only in the induction of a high degree of deracemization but also in the fact that such an organization process takes place in micellar type aggregates which are generally considered to be disorganized.10 Experimental Procedure 1H
NMR were performed on a Bruker AC 300 P spectrometer operating at 300.13 MHz. The signals of HDO (δ, 4.750) CHCl3 (δ, 7.25), and CD3OD (δ, 3.3) were used as standards in D2O, CDCl3, and CD3OD solutions, respectively, for the 1H NMR spectra. CD spectra were recorded on a Jasco spectropolarimeter J500 A using quartz cells of 0.1- and 0.05-cm path length.
LA971072F (10) Menger, F. M.; Ding, J. Angew. Chem., Int. Ed. Engl. 1996, 35, 2137.
