Article pubs.acs.org/JPCB
Raman Optical Activity of Enantiopure Cryptophanes Nicolas Daugey,† Thierry Brotin,*,‡ Nicolas Vanthuyne,§ Dominique Cavagnat,† and Thierry Buffeteau*,† †
Institut des Sciences Moléculaires (UMR 5255 - CNRS), Université de Bordeaux, 351 Cours de la Libération, 33405 Talence, France Laboratoire de Chimie de l’ENS LYON (UMR 5182 - CNRS), École Normale Supérieure de Lyon, 46 Allée d’Italie, 69364 Lyon, France § Aix Marseille Université, Centrale Marseille, CNRS, iSm2 UMR 7313, 13397 Marseille, France ‡
S Supporting Information *
ABSTRACT: Raman optical activity (ROA) and density functional theory (DFT) calculations were used to determine the absolute configuration of enantiopure cryptophane molecules and to obtain conformational information about their three ethylenedioxy linkers. ROA spectra recorded in chloroform solution for the two resolved enantiomers of cryptophanes derivatives bearing five (2), six (1), nine (3 and 4), and 12 (5) methoxy substituents are presented for the first time. The number of methoxy substituents (cryptophanes 1, 3, and 5) and the arrangement of the three linkers (anti for 3 and syn for 4) are two important parameters that significantly affect the ROA spectra. DFT calculations, at the B3PW91/6-31G** level, for cryptophane bearing six methoxy substituents establish, besides the absolute configuration, the preferential all-trans conformation of the ethylenedioxy linkers of the chloroform−cryptophane complex. This study shows that the ROA/DFT approach exhibits a higher selectivity for the conformation of the linkers than vibrational circular dichroism (VCD) associated with theoretical calculations.
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INTRODUCTION Cryptophanes are nearly spherical cage molecules composed of two cyclotriveratrylene (CTV) bowls connected by three aliphatic linkers. The rigid bowl-shaped structure of the cryptophane cavity generates a lipophilic cavity suitable for the encapsulation of small neutral molecules.1 The specific molecular recognition is mainly determined by the internal volume of the cavity, which in turn is controlled by the length of the aliphatic linkers (81, 95, and 121 Å3 for methylenedioxy, ethylenedioxy, and propylenedioxy linkers, respectively). Thus, cryptophane derivatives can encapsulate a variety of guest molecules, such as halogenomethanes and ammonium salts or even a xenon atom in organic or aqueous solutions.2 Recently, we have shown that water-soluble cryptophanes bearing hydroxyl functions could also efficiently encapsulate cesium and thallium cations under basic conditions.3 Besides their interesting binding properties, cryptophane derivatives are inherently chiral molecules. The chiroptical properties of enantiopure cryptophanes and the molecular recognition of chiral or achiral guest molecules have been investigated by chiroptical techniques such as polarimetry, electronic circular dichroism (ECD), and vibrational circular dichroism (VCD).4 The results obtained by these techniques have revealed that the chiroptical properties of cryptophane-A congeners (bearing three ethylenedioxy linkers) are strongly © 2014 American Chemical Society
dependent on some external parameters such as the nature of the solvent (organic or aqueous) and the ability of a guest molecule to enter their cavity. For instance, significant and specific ECD responses upon complexation have been observed for various cryptophanes that depend on the size of the guest. Indeed, spectral modifications are clearly observed in the 200− 320 nm region (corresponding to the allowed 1Bb and the two forbidden 1La and 1Lb transitions of the benzene rings) for guest molecules having different molecular volumes ranging from V vdw = 42 Å3 (chloromethane) to V vdw = 72 Å3 (chloroform).4f,g Similarly, spectral modifications have been also observed in the VCD spectra and, in particular, in the increase in the VCD intensity with the size of the guest molecules. 4e−g These spectral modifications have been interpreted from molecular dynamics (MD) and ab initio calculations at the density functional theory (DFT) level by subtle conformational changes of the three linkers upon complexation. Thus, the encapsulation of large guest molecules, such as chloroform, favors the trans conformation of the linkers. Generally, cryptophane-A derivatives adopt a gauche− trans−trans (GTT) conformation of the linkers. This Received: March 17, 2014 Revised: April 24, 2014 Published: April 25, 2014 5211
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conformations of cryptophanes 1 and have been compared to the corresponding experimental ROA spectrum. Second, we wish to investigate the effect of the symmetry of cryptophanes and of the number of methoxy substituents on the experimental ROA spectra. Cryptophanes 1 and 2, having D3 and C1 symmetry, respectively, are the most appropriate systems for addressing this point. A comparison of the ROA spectra of cryptophanes 1, 3, 5, bearing 6, 9, and 12 methoxy substituents, respectively, may also give complementary information. Finally, since the syn diasteromer of cryptophane bearing nine methoxy substituents (4) is also chiral, it has also been interesting to compare cryptophanes 3 and 4 to show how the arrangement of the three linkers (anti or syn) affects the overall ROA spectra of cryptophanes.
conformation has been observed, for instance, in the X-ray crystallographic structure of the CHCl3@cryptophane-A complex.5 In addition, VCD spectroscopy, associated with DFT calculations, has allowed the determination of the absolute configuration (AC) of numerous cryptophanes derivatives.4c,e Raman optical activity (ROA) is another chiroptical spectroscopy technique which can establish the absolute configuration and/or conformation of chiral molecules in solution. It measures a small difference in the intensity of vibrational Raman scattering from chiral molecules in right and left circularly polarized incident light (incident circular polarization, ICP) or, equivalently, the difference in the intensities of the right and left circularly polarized components of the scattered light using incident light of fixed polarization (scattered circular polarization, SCP).6 The advantage of ROA relative to the VCD is the possibility of accurately analyzing aqueous solutions and the possibility of obtaining vibrational information at lower wavenumbers (up to 100 cm−1). Thus, ROA is well suited to the study of the structure of biomolecules such as proteins, peptides, and carbohydrates in aqueous solutions.7 Nevertheless, ROA experiments can also be performed in organic solutions (chloroform, for example), allowing investigations of a broad variety of types of molecules.8 Valuable conformational information has been obtained on these systems, since reliable DFT calculations can be performed to predict ROA spectra. In addition, large systems are now accessible due to the development of computational techniques.9 In this article, we complete the chiroptical analysis of cryptophanes derivatives by reporting for the first time the ROA spectra of molecules presented in Scheme 1. There are
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EXPERIMENTAL SECTION Synthesis of Cryptophanes 1−5. The synthesis route to obtaining enantiopure cryptophanes 1 and 2 was previously reported.4b−e Cryptophanes 3−5 were synthesized as racemic compounds according to a known procedure.10a The separation of the two enantiomers of compounds 3−5 was achieved by HPLC using chiral stationary phases as described in ref 10b. ROA Measurements. Raman and ROA spectra were recorded at ambient temperature on a chiral Raman spectrometer (BioTools, Inc.), equipped with a Millennia PRO 2sJ (Spectra-Physics) diode-pumped solid-state laser (Nd:YV04) operating at 532 nm. This spectrometer employs backscattering geometry and a scattered circular polarization (SCP) setup as designed by Hug.6a,11 The ROA spectra are presented as intensity differences (IR − IL), and the parent Raman spectra, as intensity sums (IR + IL), with IR and IL denoting the Raman scattering intensities with right and left circular polarization states, respectively. The two enantiomers of cryptophanes 1−5 were dissolved in CDCl3 to a concentration of 0.1 M and used to fill an ROA fused silica microcell (4 × 3 × 10 mm3, BioTools, Inc.). Additional experiments were performed for cryptophane 1 in C2D2Cl4 at a concentration of 0.1 M. The power of the laser was 200 mW (∼80 mW at the sample). Each presented spectrum is an average over about 50 h of spectra of 15 min acquisition time, corresponding to 1536 scans. The exposure time was 0.3 s to prevent the saturation of the CCD detector. Raman and ROA spectra were collected in the range of 2500−100 cm−1 with a spectral resolution of about 7 cm−1. All measured spectra were exported into the Origin 7.5 software package for data treatment and figure preparation. After the artifact spikes were removed (false CCD detector signal, coming from cosmic rays), the ROA spectra were averaged and slightly smoothed (Stavisky-Golay algorithm, nine points). An ROA spectrum of the solvent, recorded under the same experimental conditions, was subtracted from the bare ROA spectra of the samples to remove the artifact signal of the solvent. DFT Calculations. The geometry optimizations, vibrational frequencies, and ROA intensities were calculated with the Gaussian 09 program12 on the DELL cluster of the MCIA computing center of the University Bordeaux I. Calculations of the optimized geometry of PP-1 for the T1T1T1, T2T2T2, G−G−G−, and G+G+G+ conformations13 of the three ethylenedioxy linkers were performed at the density functional theory level using a B3PW91 functional14 and a 6-31G** basis set.15 Additional calculations were performed for the CDCl3@ PP-1 complex, considering the T1T1T1, G−T1T1, and G−G−G− conformers. Vibrational frequencies and ROA intensity tensors
Scheme 1. Chemical Structures of Cryptophanes 1−5a
a
Only a single enantiomer (MM) is shown for compounds 1−3 and 5. Only the MP enantiomer is shown for compound 4.
several motivations for the present investigation. First, we wish to learn whether ROA may give complementary information about the conformation of the linkers. In other words, are the ROA spectra more sensitive to the linker’s conformation than VCD spectra? To answer this question, DFT calculations at the B3PW91/6-31G** level have been performed for various 5212
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C2D2Cl4 solutions (0.1 M) are quasi-identical to those recorded in CDCl3 solution (Supporting Information, S1), as previously observed from VCD experiments.4c The assignment of the most important bands of cryptophanes 1 and 2 observed in the ROA spectra has been performed with respect to the literature16 and on the basis of the visual observation with the Agui graphical interface of Ampac software17 of the vibrational modes calculated at the B3PW91/6-31G** level (vide infra). The band located at 1607 cm−1 is assigned to the ν8bCC stretching vibration of the rings.18 The ν8aCC and ν19bCC stretching vibrations observed in the Raman spectrum with weak intensities at 1580 and 1515 cm−1, respectively, do not give ROA contributions. It is noteworthy that the ν19bCC stretching vibration was the most important band in the infrared spectrum of cryptophanes, with a significant contribution in the VCD spectrum.4c The bisignate shape observed at 1440 and 1405 cm−1 comes from the δaCH3 and δsCH3 bending modes of the methyl groups, respectively, coupled with the δCH2 bending mode of the methylene groups of the two bowls. The wagging and twisting of the CH2 groups (linkers and bowls) give rise to the three bands observed with the same sign in the 1350−1250 cm−1 spectral range. The region below 1200 cm−1 is more complex because the observed bands correspond to coupled modes involving several vibrations. Nevertheless, the most intense ROA bands can be associated with specific vibrations. The band at 1211 cm−1 can be assigned to the νaC−O−C asymmetric stretching vibration of methoxy substituents and ethylenedioxy linkers coupled with the twisting mode of the CH2 groups of the linkers. The band at 1140 cm−1 comes from the coupling of the parallel rocking mode of the methyl groups and the twisting mode of the CH2 groups (linkers and bowls). The νsC−O−C symmetric stretching vibration of methoxy substituents and ethylenedioxy linkers is involved in the two bands observed at 1002 and 982 cm−1. The C−H in-plane and out-of-plane bending vibrations of the rings give rise to very weak ROA contributions at 1180 and 878 cm−1, respectively. The normal mode ν1 of rings (breathing mode in benzene derivatives) is observed at 746 cm−1 with a significant contribution in the ROA spectra of compounds 1 and 2. The intensity and the frequency of this mode are strongly dependent on the substitution of the rings.15 Finally, the most intense band in the ROA spectra of compounds 1 and 2 is observed at 140 cm−1 and can be ascribed to the rotation of the methyl groups. This band exhibits a very weak intensity in the parent Raman spectra. Effect of the Number of Methoxy Substituents. Experimental Raman and ROA spectra obtained for 0.1 M chloroform solutions of cryptophane derivatives bearing 9 (3) or 12 (5) methoxy substituents are shown in Figure 2. The small drift of the baseline is due to the slight increase of the fluorescence background observed in the parent Raman spectra. Adding a methoxy group to the six rings (compound 5) modifies their substitution, allowing a drastic change in the intensities, and sometimes in the signs, of the ROA bands. The intensities of the ν8b and ν1 modes of the rings at 1607 and 746 cm−1, respectively, decrease strongly in the ROA spectra of compound 5 in Figure 2b, whereas the intensity of the band at 1002 cm −1, associated with the ν sC−O−C symmetric stretching vibration of methoxy substituents, increases significantly. On the other hand, in the 1400−1050 and 650−450 cm−1 spectral ranges, the signs of several bands are changed for cryptophanes 1 and 5 (Supporting Information, S2b and S2c).
(excitation at 532 nm) were calculated at the same level of theory for the isolated molecule in vacuo. For comparison to experiment, the calculated frequencies were scaled by 0.968 and the calculated intensities were converted to Lorentzian bands with a half-width of 7 cm−1.
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RESULTS AND DISCUSSION Effect of the Symmetry of Cryptophanes. Experimental Raman optical activity (ROA) and Raman spectra obtained for 0.1 M chloroform solutions of cryptophanes 1 and 2 are shown in Figure 1. The Raman spectra of compounds 1 and 2 reveal
Figure 1. (A) Experimental Raman and ROA spectra recorded at 295 K for (+)-1 and (−)-1 in CDCl3 solution (0.1 M). (B) Experimental Raman and ROA spectra recorded at 295 K for (+)-2 and (−)-2 in CDCl3 solution (0.1 M).
the difficulty of performing experiments at moderate concentration (0.1 M) since the intensities of the cryptophane peaks, visible essentially in the 1700−1000 cm−1 spectral range, are very low with respect to those of the deuterated chloroform solvent at 908, 737, 650, 364, and 260 cm−1. The Raman spectra of the two enantiomers of 1 and 2 overlay nearly exactly and exhibit very low fluorescence backgrounds. Moreover, the ROA spectra of the two enantiomers are nearly perfect mirror images for the two compounds, as expected for enantiopure materials. The ROA spectra presented in Figure 1 were obtained with a very good signal-to-noise ratio, allowing a direct comparison between cryptophane derivatives. It is noteworthy that the ROA spectra of the (+)-1 and (+)-2 compounds are similar in sign and intensity, revealing that the substitution of H by OCH3 does not affect the overall shape of the ROA spectra. Thus, the symmetry of cryptophanes 1 and 2 has not been modified enough to observe significant changes in the ROA spectra. ROA spectra of (+)-1 and (−)-1 enantiomers in 5213
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Figure 3. Comparison of ROA spectra of (+)-4 and (+)-3 recorded at 295 K in CDCl3 solution (0.1 M).
spectra are very sensitive to the linkage of the two CTV units, whereas VCD spectra are less perturbed.10b The ROA spectra seem to be very dependent on the backbone of the cryptophane derivatives and should also be dependent on the conformation of the three linkers. This last point will be discussed in the following paragraphs. Conformational Analysis of Empty PP-1 and CDCl3@ PP-1 Complex. The analysis of the ROA spectra of empty PP1 and CDCl3@PP-1 complex begins with the prediction of their optimized geometries. As mentioned previously,4c,e 20 nonequivalent conformations of the linkers are possible for the cryptophane 1 molecule because each O−CH2−CH2−O bridge gives rise to four possible conformations (T1, T2, G+, G−).13 It is not reasonable to investigate all conformers for this large molecule because calculations of the optimized geometry, vibrational frequencies, and ROA intensities are very timedemanding for each conformer. We have built and optimized the structures of the four conformers of highest symmetry (T1T1T1, T2T2T2, G−G−G−, and G+G+G+ conformations of the three ethylenedioxy linkers). Using the starting O−C−C−O dihedral angles close to 180° for T1 and T2 conformations and ±60° for G+ and G− conformations, the geometries of the empty cavity of PP-1 were optimized at the B3PW91/6-31G** level. Harmonic vibrational frequencies have been calculated at the same level in order to confirm that all structures are stable conformations and to enable free energies to be calculated. The electronic and the Gibbs energies of the four conformers are reported in Table 1. On the basis of the ab initio-predicted Gibbs free energies, it can be concluded that the T1T1T1 and
Figure 2. (A) Experimental Raman and ROA spectra recorded at 295 K for (+)-3 and (−)-3 in CDCl3 solution (0.1 M). (B) Experimental Raman and ROA spectra recorded at 295 K for (+)-5 and (−)-5 in CDCl3 solution (0.1 M).
Finally, the strong band at 140 cm−1, due to the rotation of the methyl groups and observed in Figure 1a for compound 1, decreases significantly and splits in two components for compound 5. This result indicates that the two methoxy groups linked to each ring do not contribute similarly to the ROA spectra, certainly explaining the more structured ROA spectra for compound 5. It is noteworthy that the ROA spectra of compound 3, bearing nine methoxy substituents, are almost half the sum of the ROA spectra of compounds 1 and 5 (Supporting Information, S2a). This feature shows that the spectral modifications observed in Figure 2 come from the difference in the CTV units rather than a conformational modification of the linkers. Very specific ROA spectra have been recorded for cryptophane derivatives bearing different substituents, revealing the high potential of this spectroscopic technique to investigate the chiroptical properties of large molecules, as has been shown in recent years. Effect of the Arrangement of the Linkers (anti or syn) of Cryptophane Derivatives. If the two CTV units are different, then the anti and syn cryptophanes are chiral molecules. To study the effect of the linkage (anti or syn) of the two CTV units on the chiroptical properties of cryptophane derivatives, we have recorded the experimental Raman and ROA spectra obtained for 0.1 M chloroform solutions of compound 4 (Supporting Information, S3). The two ROA spectra of the (+) enantiomers of compounds 3 and 4 are reported in Figure 3. As shown in this figure, the ROA spectra of the two isomers are totally different. Most of the bands exhibit opposite signs, whereas optical rotation measurements give the same positive sign for the two compounds. The ROA
Table 1. Conformations and Energies of PP-1 and CDCl3@ PP-1 Complex Calculated at the B3PW91/6-31G** Level energy (hartrees) conformers T1T1T1 T2T2T2 G−G−G− G+G+G+ T1T1T1 G−T1T1 G−G−G− 5214
electronic
Gibbs
Empty PP-1 −2991.74 332 093 −2090.848 055 −2991.73 746 172 −2990.841 291 −2991.74 852 316 −2990.846 709 −2991.74 297 946 −2990.840 444 CDCl3@PP-1 −4410.84 246 010 −4409.936 530 −4410.84 282 064 −4409.933 195 −4410.83 971 133 −4409.928 516
ΔG (kcal/mol) 0 4.25 0.84 4.78 0 2.09 5.03
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for VCD experiments.4c Therefore, the ROA technique seems to be more sensitive to the conformational effects induced by the three linkers than does VCD.
G−G−G− conformers are the most favorable for empty PP-1. Indeed, the T2T2T2 and G+G+G+ conformers are more than 4 kcal/mol higher in free energy. In order to have a more realistic system, similar calculations were performed for the CDCl3@ PP-1 complex. The lower symmetry of the complex (with respect to the empty molecule) significantly increases the computational times, and only the most stable forms of trans and gauche conformations (T1T1T1 and G−G−G− conformers) have been considered. Under this condition, the T1T1T1 conformers yield a final optimized structure of the complex that is 5 kcal/mol lower in energy than the G −G − G − conformers. The addition of chloroform molecule to the PP1 cage stabilizes the T1T1T1 conformer. This result is not surprising due to the better size matching between the chloroform (ca. 72 Å3) and the cryptophane cavity in its alltrans conformation (ca. 95 Å3). We have also calculated the optimized geometry for the G−T1T1 conformer since this conformation of the aliphatic bridges has been determined from the X-ray structure of the crystalline CHCl3@PP-1 complex.5 The G−T1T1 conformer leads to an intermediate Gibbs free energy that is 2 kcal/mol higher than for the T 1T 1 T1 conformers. Calculated ROA Spectra of Empty PP-1 and CDCl3@ PP-1 Complex. The ROA spectra calculated at the B3PW91/ 6-31G** level for T1T1T1, G−T1T1, and G−G−G− conformers of the CDCl3@PP-1 complex are compared in Figure 4 to the
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CONCLUSIONS In this article, we show that ROA associated with DFT calculations provides an accurate description of the absolute configuration and conformation of enantiopure cryptophane derivatives. The (+)-PP-1 configuration of cryptophane bearing six methoxy substituents (cryptophane-A), determined previously by VCD experiments, is confirmed in this study. We confirm also that the all-trans conformation of the ethylenedioxy linkers is the most favorable for the CDCl3@PP-1 complex. The ROA spectra calculated for various conformations of the linkers are more specific to these conformations and, more generally, to the backbone of the cryptophane than those calculated for VCD experiments. This study also reveals that the change in the symmetry of the cryptophanes (D3 for 1 and C1 for 2) does not affect the overall shape of the ROA spectra. In contrast, the ROA spectra are strongly modified when a second methoxy substituent is added to the rings of CTV units (cryptophanes 3 and 5).
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ASSOCIATED CONTENT
S Supporting Information *
Experimental Raman and ROA spectra recorded at 295 K for (+)-1 and (−)-1 in C2D2Cl4 solution (0.1 M). Experimental Raman and ROA spectra recorded at 295 K for (+)-4 and (−)-4 in CDCl3 solution (0.1 M). Comparison of the experimental ROA spectrum of (+)-1 in CDCl3 solution with calculated spectra at the B3PW91/6-31G** level for T1T1T1, T2T2T2, G−G−G−, and G+G+G+ conformers of PP-1. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: t.buff[email protected]. Author Contributions
The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript.
Figure 4. Comparison of experimental ROA spectrum of (+)-1 in CDCl3 solution with calculated spectra at the B3PW91/6-31G** level for T1T1T1, G−T1T1, and G−G−G− conformers of CDCl3@PP-1.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are indebted to the CNRS (Chemistry Department) and to Région Aquitaine for financial support with respect to ROA equipment. We also acknowledge the computational facilities provided by the MCIA (Mésocentre de Calcul Intensif Aquitain) of the Université de Bordeaux and of the Université de Pau et des Pays de l’Adour, financed by the Conseil Régional d’Aquitaine and the French Ministry of Research and Technology.
experimental spectrum of (+)-1 in CDCl3 solution. The spectra calculated for T1T1T1 and G−T1T1 conformers reproduce fairly well the sign of the bands observed on the experimental ROA spectrum in the 1800−700 cm−1 spectral range, allowing the determination of the absolute configuration (+)-PP-1 of the molecule. For wavenumbers lower than 700 cm−1, the agreement is not good and the calculated intensities are very weak with respect to those observed in the experimental spectrum. The spectrum calculated for the T1T1T1 conformer of the CDCl3@PP-1 complex is similar to that calculated for the empty molecule (Supporting Information, S4). The presence of chloroform in the cryptophane cavity does not affect the conformation of the host molecule, suggesting a weak interaction between guest and host molecules. Finally, the spectrum calculated for the G−G−G− conformer is totally different from those calculated for T1T1T1 and G−T1T1 conformers, whereas this spectral difference was less marked
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Chem. Soc., Chem. Commun. 1987, 219−221. (b) Garel, L.; Lozach, B.; Dutasta, J.-P.; Collet, A. Remarkable Effect of Receptor Size in the Binding of Acetylcholine and Related Amonium Ions to Water-Soluble Cryptophanes. J. Am. Chem. Soc. 1993, 115, 11652−11653. (c) Spence, M. M.; Ruiz, E. J.; Rubin, S. M.; Lowery, T. J.; Winssinger, N.; Schultz, P. G.; Wemmer, D. E.; Pines, A. Development of a Functionalized Xenon Biosensor. J. Am. Chem. Soc. 2004, 126, 15287−15294. (d) Huber, G.; Brotin, T.; Dubois, L.; Desvaux, H.; Dutasta, J.-P.; Berthault, P. Water Soluble Cryptophanes Showing Unprecedented Affinity for Xenon: Candidates as NMR-Based Biosensors. J. Am. Chem. Soc. 2006, 128, 6239−6246. (e) Fogarthy, H. A.; Berthault, P.; Brotin, T.; Huber, G.; Desvaux, H.; Dutasta, J.-P. A Cryptophane Core Optimized for Xenon Encapsulation. J. Am. Chem. Soc. 2007, 129, 10332−10333. (f) Chaffee, K. E.; Fogarty, H. A.; Brotin, T.; Goodson, B. M.; Dutasta, J.-P. Encapsulation of Small Gas Molecules by Cryptophane-111 in Organic Solution. 1. Size- and Shape-Selective Complexation of Simple Hydrocarbons. J. Phys. Chem. A 2009, 113, 13675−13684. (g) Hill, P. A.; Wei, Q.; Troxler, T.; Dmochowski, I. J. Substituent Effects on Xenon Binding Affinity and Solution Behavior of Water-Soluble Cryptophanes. J. Am. Chem. Soc. 2009, 131, 3069− 3077. (h) Berthault, P.; Desvaux, H.; Wendlinger, T.; Gyejacquot, M.; Stopin, A.; Brotin, T.; Dutasta, J.-P.; Boulard, Y. Effect of pH and Counterions on the encapsulation Properties of Xenon in WaterSoluble Cryptophanes. Chem.Eur. J. 2010, 16, 12941−12946. (i) Fairchild, R. M.; Joseph, A. I.; Holman, K. T.; Fogarty, H. A.; Brotin, T.; Dutasta, J.-P.; Boutin, C.; Huber, G.; Berthault, P. A WaterSoluble Xe@cryptophane-111 Complex Exhibits Very High Thermodynamic Stability and a Peculiar 129Xe NMR Chemical Shift. J. Am. Chem. Soc. 2010, 132, 15505−15507. (j) Seward, G. K.; Bai, Y.; Khan, N. S.; Dmochowski, I. J. Cell-Compatible, Integrin-Targeted Cryptophane 129Xe NMR Biosensors. Chem. Sci. 2011, 2, 1103− 1110. (k) Givelet, C.; Sun, J.; Xu, D.; Emge, T. J.; Dhokte, A.; Warmuth, R. Templated Dynamic Cryptophane Formation in Water. Chem. Commun. 2011, 47, 4511−4513. (l) Traoré, T.; Clavé, G.; Delacour, L.; Kotera, N.; Renard, P.-Y.; Romieu, A.; Berthault, P.; Boutin, C.; Tassali, N.; Rousseau, B. The First Metal-Free WaterSoluble Cryptophane-111. Chem. Commun. 2011, 47, 9702−9704. (m) Kotera, N.; Tassali, N.; Léonce, E.; Boutin, C.; Berthault, P.; Brotin, T.; Dutasta, J.-P.; Delacour, L.; Traoré, T.; Buisson, D. A.; et al. Sensitive Zinc-Activated 129Xe MRI Probe. Angew. Chem., Int. Ed. 2012, 51, 4100−4103. (n) Takacs, Z.; Brotin, T.; Dutasta, J.-P.; Lang, J.; Todde, G.; Kowalewski, J. Inclusion of Chloromethane Guests Affects Conformation and Internal Dynamics of Cryptophane-D Host. J. Phys. Chem. B 2012, 116, 7898−7913. (o) Takacs, Z.; Soltesova, M.; Kowalewski, J.; Lang, J.; Brotin, T.; Dutasta, J.-P. Host-Guest Complexes Between Cryptophane-C and Chloromethanes Revisited. Magn. Reson. Chem. 2013, 51, 19−31. (3) (a) Brotin, T.; Montserret, R.; Bouchet, A.; Cavagnat, D.; Linares, M.; Buffeteau, T. High Affinity of Water-Soluble Cryptophanes for Cesium Cations. J. Org. Chem. 2012, 77, 1198−1201. (b) Brotin, T.; Cavagnat, D.; Berthault, P.; Montserret, R.; Buffeteau, T. WaterSoluble Molecular Capsule for the Complexation of Cesium and Thallium Cations. J. Phys. Chem. B 2012, 116, 10905−10914. (c) Brotin, T.; Goncalves, S.; Berthault, P.; Cavagnat, D.; Buffeteau, T. Influence of the Cavity Size of Water-Soluble Cryptophanes on Their Binding Properties for Cesium and Thallium Cations. J. Phys. Chem. B 2013, 117, 12593−12601. (4) (a) Canceill, J.; Collet, A.; Gottarelli, G.; Palmieri, P. Synthesis and Exciton Optical Activity of D3-Cryptophanes. J. Am. Chem. Soc. 1987, 109, 6454−6464. (b) Brotin, T.; Barbe, R.; Darzac, M.; Dutasta, J.-P. Novel Synthetic Approach for Optical Resolution of Cryptophanol-A: A Direct Access to Chiral Cryptophanes and Their Chiroptical Properties. Chem.Eur. J. 2003, 9, 5784−8792. (c) Brotin, T.; Cavagnat, D.; Dutasta, J.-P.; Buffeteau, T. Vibrational Circular Dichroism Study of Optically Pure Cryptophane-A. J. Am. Chem. Soc. 2006, 128, 5533−5540. (d) Cavagnat, D.; Buffeteau, T.; Brotin, T. Synthesis and Chiroptical Properties of Cryptophanes Having C1Symmetry. J. Org. Chem. 2008, 73, 66−75. (e) Brotin, T.; Cavagnat, D.; Buffeteau, T. Conformational Changes in Cryptophane Having C15216
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(12) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al. Gaussian 09, revision A.1; Gaussian Inc.: Wallingford, CT, 2009. (13) The two trans T1 and T2 conformations have a O−C−C−O dihedral angle close to 180° but differ in the position of the CH2 groups with respect to the O−O direction. The two gauche G+ and G− conformations correspond to O−C−C−O dihedral angles of +60 and −60°, respectively. (14) Perdew, J. P.; Wang, Y. Accurate and Simple Analytic Representation of the Electron-Gas Correlation Energy. Phys. Rev. B 1992, 45, 13244−13249. (15) Ditchfield, R.; Hehre, W. J.; Pople, J. A. Self-Consistent Molecular-Orbital Methods. IX. An Extended Gaussian-Type Basis for Molecular-Orbital Studies of Organic Molecules. J. Chem. Phys. 1971, 54, 724−728. (16) (a) Socrates, G. In Infrared Characteristic Group Frequencies; Wiley-Interscience: New York, 1980. (b) Varsanyi, G. Vibrational Spectra of Benzene Derivatives; Academic Press: New York, 1969. (17) AMPAC-9, Semichem, 12456W, 62nd Terrace, Suite D, Shawnee, KS 66216. (18) The ν8bCC and ν19bCC vibrations are found at higher frequencies than ν8aCC and ν19aCC vibrations in the ptetrasubstitution of benzene groups. See ref 15b, pp 156 and 167.
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Raman Optical Activity of Enantiopure Cryptophanes Nicolas Daugey,† Thierry Brotin,*,‡ Nicolas Vanthuyne,§ Dominique Cavagnat,† and Thierry Buffeteau*,† †
Institut des Sciences Moléculaires (UMR 5255 - CNRS), Université de Bordeaux, 351 Cours de la Libération, 33405 Talence, France Laboratoire de Chimie de l’ENS LYON (UMR 5182 - CNRS), École Normale Supérieure de Lyon, 46 Allée d’Italie, 69364 Lyon, France § Aix Marseille Université, Centrale Marseille, CNRS, iSm2 UMR 7313, 13397 Marseille, France ‡
S Supporting Information *
ABSTRACT: Raman optical activity (ROA) and density functional theory (DFT) calculations were used to determine the absolute configuration of enantiopure cryptophane molecules and to obtain conformational information about their three ethylenedioxy linkers. ROA spectra recorded in chloroform solution for the two resolved enantiomers of cryptophanes derivatives bearing five (2), six (1), nine (3 and 4), and 12 (5) methoxy substituents are presented for the first time. The number of methoxy substituents (cryptophanes 1, 3, and 5) and the arrangement of the three linkers (anti for 3 and syn for 4) are two important parameters that significantly affect the ROA spectra. DFT calculations, at the B3PW91/6-31G** level, for cryptophane bearing six methoxy substituents establish, besides the absolute configuration, the preferential all-trans conformation of the ethylenedioxy linkers of the chloroform−cryptophane complex. This study shows that the ROA/DFT approach exhibits a higher selectivity for the conformation of the linkers than vibrational circular dichroism (VCD) associated with theoretical calculations.
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INTRODUCTION Cryptophanes are nearly spherical cage molecules composed of two cyclotriveratrylene (CTV) bowls connected by three aliphatic linkers. The rigid bowl-shaped structure of the cryptophane cavity generates a lipophilic cavity suitable for the encapsulation of small neutral molecules.1 The specific molecular recognition is mainly determined by the internal volume of the cavity, which in turn is controlled by the length of the aliphatic linkers (81, 95, and 121 Å3 for methylenedioxy, ethylenedioxy, and propylenedioxy linkers, respectively). Thus, cryptophane derivatives can encapsulate a variety of guest molecules, such as halogenomethanes and ammonium salts or even a xenon atom in organic or aqueous solutions.2 Recently, we have shown that water-soluble cryptophanes bearing hydroxyl functions could also efficiently encapsulate cesium and thallium cations under basic conditions.3 Besides their interesting binding properties, cryptophane derivatives are inherently chiral molecules. The chiroptical properties of enantiopure cryptophanes and the molecular recognition of chiral or achiral guest molecules have been investigated by chiroptical techniques such as polarimetry, electronic circular dichroism (ECD), and vibrational circular dichroism (VCD).4 The results obtained by these techniques have revealed that the chiroptical properties of cryptophane-A congeners (bearing three ethylenedioxy linkers) are strongly © 2014 American Chemical Society
dependent on some external parameters such as the nature of the solvent (organic or aqueous) and the ability of a guest molecule to enter their cavity. For instance, significant and specific ECD responses upon complexation have been observed for various cryptophanes that depend on the size of the guest. Indeed, spectral modifications are clearly observed in the 200− 320 nm region (corresponding to the allowed 1Bb and the two forbidden 1La and 1Lb transitions of the benzene rings) for guest molecules having different molecular volumes ranging from V vdw = 42 Å3 (chloromethane) to V vdw = 72 Å3 (chloroform).4f,g Similarly, spectral modifications have been also observed in the VCD spectra and, in particular, in the increase in the VCD intensity with the size of the guest molecules. 4e−g These spectral modifications have been interpreted from molecular dynamics (MD) and ab initio calculations at the density functional theory (DFT) level by subtle conformational changes of the three linkers upon complexation. Thus, the encapsulation of large guest molecules, such as chloroform, favors the trans conformation of the linkers. Generally, cryptophane-A derivatives adopt a gauche− trans−trans (GTT) conformation of the linkers. This Received: March 17, 2014 Revised: April 24, 2014 Published: April 25, 2014 5211
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conformations of cryptophanes 1 and have been compared to the corresponding experimental ROA spectrum. Second, we wish to investigate the effect of the symmetry of cryptophanes and of the number of methoxy substituents on the experimental ROA spectra. Cryptophanes 1 and 2, having D3 and C1 symmetry, respectively, are the most appropriate systems for addressing this point. A comparison of the ROA spectra of cryptophanes 1, 3, 5, bearing 6, 9, and 12 methoxy substituents, respectively, may also give complementary information. Finally, since the syn diasteromer of cryptophane bearing nine methoxy substituents (4) is also chiral, it has also been interesting to compare cryptophanes 3 and 4 to show how the arrangement of the three linkers (anti or syn) affects the overall ROA spectra of cryptophanes.
conformation has been observed, for instance, in the X-ray crystallographic structure of the CHCl3@cryptophane-A complex.5 In addition, VCD spectroscopy, associated with DFT calculations, has allowed the determination of the absolute configuration (AC) of numerous cryptophanes derivatives.4c,e Raman optical activity (ROA) is another chiroptical spectroscopy technique which can establish the absolute configuration and/or conformation of chiral molecules in solution. It measures a small difference in the intensity of vibrational Raman scattering from chiral molecules in right and left circularly polarized incident light (incident circular polarization, ICP) or, equivalently, the difference in the intensities of the right and left circularly polarized components of the scattered light using incident light of fixed polarization (scattered circular polarization, SCP).6 The advantage of ROA relative to the VCD is the possibility of accurately analyzing aqueous solutions and the possibility of obtaining vibrational information at lower wavenumbers (up to 100 cm−1). Thus, ROA is well suited to the study of the structure of biomolecules such as proteins, peptides, and carbohydrates in aqueous solutions.7 Nevertheless, ROA experiments can also be performed in organic solutions (chloroform, for example), allowing investigations of a broad variety of types of molecules.8 Valuable conformational information has been obtained on these systems, since reliable DFT calculations can be performed to predict ROA spectra. In addition, large systems are now accessible due to the development of computational techniques.9 In this article, we complete the chiroptical analysis of cryptophanes derivatives by reporting for the first time the ROA spectra of molecules presented in Scheme 1. There are
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EXPERIMENTAL SECTION Synthesis of Cryptophanes 1−5. The synthesis route to obtaining enantiopure cryptophanes 1 and 2 was previously reported.4b−e Cryptophanes 3−5 were synthesized as racemic compounds according to a known procedure.10a The separation of the two enantiomers of compounds 3−5 was achieved by HPLC using chiral stationary phases as described in ref 10b. ROA Measurements. Raman and ROA spectra were recorded at ambient temperature on a chiral Raman spectrometer (BioTools, Inc.), equipped with a Millennia PRO 2sJ (Spectra-Physics) diode-pumped solid-state laser (Nd:YV04) operating at 532 nm. This spectrometer employs backscattering geometry and a scattered circular polarization (SCP) setup as designed by Hug.6a,11 The ROA spectra are presented as intensity differences (IR − IL), and the parent Raman spectra, as intensity sums (IR + IL), with IR and IL denoting the Raman scattering intensities with right and left circular polarization states, respectively. The two enantiomers of cryptophanes 1−5 were dissolved in CDCl3 to a concentration of 0.1 M and used to fill an ROA fused silica microcell (4 × 3 × 10 mm3, BioTools, Inc.). Additional experiments were performed for cryptophane 1 in C2D2Cl4 at a concentration of 0.1 M. The power of the laser was 200 mW (∼80 mW at the sample). Each presented spectrum is an average over about 50 h of spectra of 15 min acquisition time, corresponding to 1536 scans. The exposure time was 0.3 s to prevent the saturation of the CCD detector. Raman and ROA spectra were collected in the range of 2500−100 cm−1 with a spectral resolution of about 7 cm−1. All measured spectra were exported into the Origin 7.5 software package for data treatment and figure preparation. After the artifact spikes were removed (false CCD detector signal, coming from cosmic rays), the ROA spectra were averaged and slightly smoothed (Stavisky-Golay algorithm, nine points). An ROA spectrum of the solvent, recorded under the same experimental conditions, was subtracted from the bare ROA spectra of the samples to remove the artifact signal of the solvent. DFT Calculations. The geometry optimizations, vibrational frequencies, and ROA intensities were calculated with the Gaussian 09 program12 on the DELL cluster of the MCIA computing center of the University Bordeaux I. Calculations of the optimized geometry of PP-1 for the T1T1T1, T2T2T2, G−G−G−, and G+G+G+ conformations13 of the three ethylenedioxy linkers were performed at the density functional theory level using a B3PW91 functional14 and a 6-31G** basis set.15 Additional calculations were performed for the CDCl3@ PP-1 complex, considering the T1T1T1, G−T1T1, and G−G−G− conformers. Vibrational frequencies and ROA intensity tensors
Scheme 1. Chemical Structures of Cryptophanes 1−5a
a
Only a single enantiomer (MM) is shown for compounds 1−3 and 5. Only the MP enantiomer is shown for compound 4.
several motivations for the present investigation. First, we wish to learn whether ROA may give complementary information about the conformation of the linkers. In other words, are the ROA spectra more sensitive to the linker’s conformation than VCD spectra? To answer this question, DFT calculations at the B3PW91/6-31G** level have been performed for various 5212
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C2D2Cl4 solutions (0.1 M) are quasi-identical to those recorded in CDCl3 solution (Supporting Information, S1), as previously observed from VCD experiments.4c The assignment of the most important bands of cryptophanes 1 and 2 observed in the ROA spectra has been performed with respect to the literature16 and on the basis of the visual observation with the Agui graphical interface of Ampac software17 of the vibrational modes calculated at the B3PW91/6-31G** level (vide infra). The band located at 1607 cm−1 is assigned to the ν8bCC stretching vibration of the rings.18 The ν8aCC and ν19bCC stretching vibrations observed in the Raman spectrum with weak intensities at 1580 and 1515 cm−1, respectively, do not give ROA contributions. It is noteworthy that the ν19bCC stretching vibration was the most important band in the infrared spectrum of cryptophanes, with a significant contribution in the VCD spectrum.4c The bisignate shape observed at 1440 and 1405 cm−1 comes from the δaCH3 and δsCH3 bending modes of the methyl groups, respectively, coupled with the δCH2 bending mode of the methylene groups of the two bowls. The wagging and twisting of the CH2 groups (linkers and bowls) give rise to the three bands observed with the same sign in the 1350−1250 cm−1 spectral range. The region below 1200 cm−1 is more complex because the observed bands correspond to coupled modes involving several vibrations. Nevertheless, the most intense ROA bands can be associated with specific vibrations. The band at 1211 cm−1 can be assigned to the νaC−O−C asymmetric stretching vibration of methoxy substituents and ethylenedioxy linkers coupled with the twisting mode of the CH2 groups of the linkers. The band at 1140 cm−1 comes from the coupling of the parallel rocking mode of the methyl groups and the twisting mode of the CH2 groups (linkers and bowls). The νsC−O−C symmetric stretching vibration of methoxy substituents and ethylenedioxy linkers is involved in the two bands observed at 1002 and 982 cm−1. The C−H in-plane and out-of-plane bending vibrations of the rings give rise to very weak ROA contributions at 1180 and 878 cm−1, respectively. The normal mode ν1 of rings (breathing mode in benzene derivatives) is observed at 746 cm−1 with a significant contribution in the ROA spectra of compounds 1 and 2. The intensity and the frequency of this mode are strongly dependent on the substitution of the rings.15 Finally, the most intense band in the ROA spectra of compounds 1 and 2 is observed at 140 cm−1 and can be ascribed to the rotation of the methyl groups. This band exhibits a very weak intensity in the parent Raman spectra. Effect of the Number of Methoxy Substituents. Experimental Raman and ROA spectra obtained for 0.1 M chloroform solutions of cryptophane derivatives bearing 9 (3) or 12 (5) methoxy substituents are shown in Figure 2. The small drift of the baseline is due to the slight increase of the fluorescence background observed in the parent Raman spectra. Adding a methoxy group to the six rings (compound 5) modifies their substitution, allowing a drastic change in the intensities, and sometimes in the signs, of the ROA bands. The intensities of the ν8b and ν1 modes of the rings at 1607 and 746 cm−1, respectively, decrease strongly in the ROA spectra of compound 5 in Figure 2b, whereas the intensity of the band at 1002 cm −1, associated with the ν sC−O−C symmetric stretching vibration of methoxy substituents, increases significantly. On the other hand, in the 1400−1050 and 650−450 cm−1 spectral ranges, the signs of several bands are changed for cryptophanes 1 and 5 (Supporting Information, S2b and S2c).
(excitation at 532 nm) were calculated at the same level of theory for the isolated molecule in vacuo. For comparison to experiment, the calculated frequencies were scaled by 0.968 and the calculated intensities were converted to Lorentzian bands with a half-width of 7 cm−1.
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RESULTS AND DISCUSSION Effect of the Symmetry of Cryptophanes. Experimental Raman optical activity (ROA) and Raman spectra obtained for 0.1 M chloroform solutions of cryptophanes 1 and 2 are shown in Figure 1. The Raman spectra of compounds 1 and 2 reveal
Figure 1. (A) Experimental Raman and ROA spectra recorded at 295 K for (+)-1 and (−)-1 in CDCl3 solution (0.1 M). (B) Experimental Raman and ROA spectra recorded at 295 K for (+)-2 and (−)-2 in CDCl3 solution (0.1 M).
the difficulty of performing experiments at moderate concentration (0.1 M) since the intensities of the cryptophane peaks, visible essentially in the 1700−1000 cm−1 spectral range, are very low with respect to those of the deuterated chloroform solvent at 908, 737, 650, 364, and 260 cm−1. The Raman spectra of the two enantiomers of 1 and 2 overlay nearly exactly and exhibit very low fluorescence backgrounds. Moreover, the ROA spectra of the two enantiomers are nearly perfect mirror images for the two compounds, as expected for enantiopure materials. The ROA spectra presented in Figure 1 were obtained with a very good signal-to-noise ratio, allowing a direct comparison between cryptophane derivatives. It is noteworthy that the ROA spectra of the (+)-1 and (+)-2 compounds are similar in sign and intensity, revealing that the substitution of H by OCH3 does not affect the overall shape of the ROA spectra. Thus, the symmetry of cryptophanes 1 and 2 has not been modified enough to observe significant changes in the ROA spectra. ROA spectra of (+)-1 and (−)-1 enantiomers in 5213
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Figure 3. Comparison of ROA spectra of (+)-4 and (+)-3 recorded at 295 K in CDCl3 solution (0.1 M).
spectra are very sensitive to the linkage of the two CTV units, whereas VCD spectra are less perturbed.10b The ROA spectra seem to be very dependent on the backbone of the cryptophane derivatives and should also be dependent on the conformation of the three linkers. This last point will be discussed in the following paragraphs. Conformational Analysis of Empty PP-1 and CDCl3@ PP-1 Complex. The analysis of the ROA spectra of empty PP1 and CDCl3@PP-1 complex begins with the prediction of their optimized geometries. As mentioned previously,4c,e 20 nonequivalent conformations of the linkers are possible for the cryptophane 1 molecule because each O−CH2−CH2−O bridge gives rise to four possible conformations (T1, T2, G+, G−).13 It is not reasonable to investigate all conformers for this large molecule because calculations of the optimized geometry, vibrational frequencies, and ROA intensities are very timedemanding for each conformer. We have built and optimized the structures of the four conformers of highest symmetry (T1T1T1, T2T2T2, G−G−G−, and G+G+G+ conformations of the three ethylenedioxy linkers). Using the starting O−C−C−O dihedral angles close to 180° for T1 and T2 conformations and ±60° for G+ and G− conformations, the geometries of the empty cavity of PP-1 were optimized at the B3PW91/6-31G** level. Harmonic vibrational frequencies have been calculated at the same level in order to confirm that all structures are stable conformations and to enable free energies to be calculated. The electronic and the Gibbs energies of the four conformers are reported in Table 1. On the basis of the ab initio-predicted Gibbs free energies, it can be concluded that the T1T1T1 and
Figure 2. (A) Experimental Raman and ROA spectra recorded at 295 K for (+)-3 and (−)-3 in CDCl3 solution (0.1 M). (B) Experimental Raman and ROA spectra recorded at 295 K for (+)-5 and (−)-5 in CDCl3 solution (0.1 M).
Finally, the strong band at 140 cm−1, due to the rotation of the methyl groups and observed in Figure 1a for compound 1, decreases significantly and splits in two components for compound 5. This result indicates that the two methoxy groups linked to each ring do not contribute similarly to the ROA spectra, certainly explaining the more structured ROA spectra for compound 5. It is noteworthy that the ROA spectra of compound 3, bearing nine methoxy substituents, are almost half the sum of the ROA spectra of compounds 1 and 5 (Supporting Information, S2a). This feature shows that the spectral modifications observed in Figure 2 come from the difference in the CTV units rather than a conformational modification of the linkers. Very specific ROA spectra have been recorded for cryptophane derivatives bearing different substituents, revealing the high potential of this spectroscopic technique to investigate the chiroptical properties of large molecules, as has been shown in recent years. Effect of the Arrangement of the Linkers (anti or syn) of Cryptophane Derivatives. If the two CTV units are different, then the anti and syn cryptophanes are chiral molecules. To study the effect of the linkage (anti or syn) of the two CTV units on the chiroptical properties of cryptophane derivatives, we have recorded the experimental Raman and ROA spectra obtained for 0.1 M chloroform solutions of compound 4 (Supporting Information, S3). The two ROA spectra of the (+) enantiomers of compounds 3 and 4 are reported in Figure 3. As shown in this figure, the ROA spectra of the two isomers are totally different. Most of the bands exhibit opposite signs, whereas optical rotation measurements give the same positive sign for the two compounds. The ROA
Table 1. Conformations and Energies of PP-1 and CDCl3@ PP-1 Complex Calculated at the B3PW91/6-31G** Level energy (hartrees) conformers T1T1T1 T2T2T2 G−G−G− G+G+G+ T1T1T1 G−T1T1 G−G−G− 5214
electronic
Gibbs
Empty PP-1 −2991.74 332 093 −2090.848 055 −2991.73 746 172 −2990.841 291 −2991.74 852 316 −2990.846 709 −2991.74 297 946 −2990.840 444 CDCl3@PP-1 −4410.84 246 010 −4409.936 530 −4410.84 282 064 −4409.933 195 −4410.83 971 133 −4409.928 516
ΔG (kcal/mol) 0 4.25 0.84 4.78 0 2.09 5.03
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for VCD experiments.4c Therefore, the ROA technique seems to be more sensitive to the conformational effects induced by the three linkers than does VCD.
G−G−G− conformers are the most favorable for empty PP-1. Indeed, the T2T2T2 and G+G+G+ conformers are more than 4 kcal/mol higher in free energy. In order to have a more realistic system, similar calculations were performed for the CDCl3@ PP-1 complex. The lower symmetry of the complex (with respect to the empty molecule) significantly increases the computational times, and only the most stable forms of trans and gauche conformations (T1T1T1 and G−G−G− conformers) have been considered. Under this condition, the T1T1T1 conformers yield a final optimized structure of the complex that is 5 kcal/mol lower in energy than the G −G − G − conformers. The addition of chloroform molecule to the PP1 cage stabilizes the T1T1T1 conformer. This result is not surprising due to the better size matching between the chloroform (ca. 72 Å3) and the cryptophane cavity in its alltrans conformation (ca. 95 Å3). We have also calculated the optimized geometry for the G−T1T1 conformer since this conformation of the aliphatic bridges has been determined from the X-ray structure of the crystalline CHCl3@PP-1 complex.5 The G−T1T1 conformer leads to an intermediate Gibbs free energy that is 2 kcal/mol higher than for the T 1T 1 T1 conformers. Calculated ROA Spectra of Empty PP-1 and CDCl3@ PP-1 Complex. The ROA spectra calculated at the B3PW91/ 6-31G** level for T1T1T1, G−T1T1, and G−G−G− conformers of the CDCl3@PP-1 complex are compared in Figure 4 to the
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CONCLUSIONS In this article, we show that ROA associated with DFT calculations provides an accurate description of the absolute configuration and conformation of enantiopure cryptophane derivatives. The (+)-PP-1 configuration of cryptophane bearing six methoxy substituents (cryptophane-A), determined previously by VCD experiments, is confirmed in this study. We confirm also that the all-trans conformation of the ethylenedioxy linkers is the most favorable for the CDCl3@PP-1 complex. The ROA spectra calculated for various conformations of the linkers are more specific to these conformations and, more generally, to the backbone of the cryptophane than those calculated for VCD experiments. This study also reveals that the change in the symmetry of the cryptophanes (D3 for 1 and C1 for 2) does not affect the overall shape of the ROA spectra. In contrast, the ROA spectra are strongly modified when a second methoxy substituent is added to the rings of CTV units (cryptophanes 3 and 5).
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ASSOCIATED CONTENT
S Supporting Information *
Experimental Raman and ROA spectra recorded at 295 K for (+)-1 and (−)-1 in C2D2Cl4 solution (0.1 M). Experimental Raman and ROA spectra recorded at 295 K for (+)-4 and (−)-4 in CDCl3 solution (0.1 M). Comparison of the experimental ROA spectrum of (+)-1 in CDCl3 solution with calculated spectra at the B3PW91/6-31G** level for T1T1T1, T2T2T2, G−G−G−, and G+G+G+ conformers of PP-1. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: t.buff[email protected]. Author Contributions
The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript.
Figure 4. Comparison of experimental ROA spectrum of (+)-1 in CDCl3 solution with calculated spectra at the B3PW91/6-31G** level for T1T1T1, G−T1T1, and G−G−G− conformers of CDCl3@PP-1.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are indebted to the CNRS (Chemistry Department) and to Région Aquitaine for financial support with respect to ROA equipment. We also acknowledge the computational facilities provided by the MCIA (Mésocentre de Calcul Intensif Aquitain) of the Université de Bordeaux and of the Université de Pau et des Pays de l’Adour, financed by the Conseil Régional d’Aquitaine and the French Ministry of Research and Technology.
experimental spectrum of (+)-1 in CDCl3 solution. The spectra calculated for T1T1T1 and G−T1T1 conformers reproduce fairly well the sign of the bands observed on the experimental ROA spectrum in the 1800−700 cm−1 spectral range, allowing the determination of the absolute configuration (+)-PP-1 of the molecule. For wavenumbers lower than 700 cm−1, the agreement is not good and the calculated intensities are very weak with respect to those observed in the experimental spectrum. The spectrum calculated for the T1T1T1 conformer of the CDCl3@PP-1 complex is similar to that calculated for the empty molecule (Supporting Information, S4). The presence of chloroform in the cryptophane cavity does not affect the conformation of the host molecule, suggesting a weak interaction between guest and host molecules. Finally, the spectrum calculated for the G−G−G− conformer is totally different from those calculated for T1T1T1 and G−T1T1 conformers, whereas this spectral difference was less marked
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REFERENCES
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