Experimental and Theoretical Studies on the Chiroptical Properties of

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Experimental and Theoretical Studies on the Chiroptical Properties of Donor-Acceptor Binaphthyls. Effects of Dynamic Conformer Population on Circular Dichroism Masaki Nishizaka, Tadashi Mori,* and Yoshihisa Inoue* Department of Applied Chemistry, Osaka University, 2-1 Yamada-oka, Suita 565-0871, Japan

ABSTRACT The axial chirality of biaryls has attracted much attention as effective chiral auxiliaries and ligands in asymmetric synthesis and chirality sensing. In this study, the chiroptical properties of donor-acceptor binaphthyl DA and symmetrical DD were investigated. The amplitude of the main-band couplet was enhanced in the circular dichroism (CD) spectra of DA, due to the effective cation-π interactions altering the potential energy profile against the rotational angle about the binaphthyl's C1-C10 bond. A shallow unsymmetrical potential curve was obtained for DA, which is in contrast to the almost symmetrical double-well potential for DD. The theoretical calculations of the CD spectra at the RI-CC2 level using the two-state model composed of the s-cis and s-trans conformations successfully reproduce the experimental CD. Dynamic conformer distribution over a wide range of dihedral angle in the chiral binaphthyls was shown to be critical in discussing the performance of such axially chiral molecules. SECTION Molecular Structure, Quantum Chemistry, General Theory

iaryls, in particular 1,10 -binaphthyls, have been utilized as effective chiral auxiliaries and ligands in asymmetric synthesis and chirality sensing.1,2 Their unique axial chirality has also been a subject of the theoretical studies at various levels, which have often been combined with the synthetic as well as spectroscopic investigations.3,4 As the two connected chromophores strongly couple to each other, the major absorption band splits to show a strong bisignate Cotton effect in circular dichroism (CD) spectrum. The CD amplitude (A) and the split energy (ΔλDav) are critical functions of the dihedral angle (θ) between the two chromophores. Some recent investigations on chiral binaphthyls successfully utilized these variants as parameters for assessing the solutionphase conformation.5,6 X-ray crystallographic studies on the structure of 1,10 -binaphthyl derivatives provide important information on the solid-state conformation. Thus, unsubstituted 1,10 -binaphthyl exists in the s-cis conformation (θ = 69°) in racemate crystal, while enantiopure 1,10 -binaphthyl is s-trans (θ=103°) in conglomerate crystal.7,8 Besides such morphological interest, the comprehensive understanding of the conformational variation and population is indispensable for precisely predicting the chiroptical properties in solution, as the s-cis and s-trans conformers were claimed to show almost mirrorimaged CD spectra, albeit having the same absolute configuration.9 More importantly, the effective dihedral angle θ is believed to decisively control the stereochemical outcomes of the asymmetric reactions conducted in (organic) solvents.10 A Raman spectroscopic study revealed that the 1,10 -binaphthyl adopts almost perpendicular orientation in solution (θ = 84-90°).11 A recent vibrational CD spectral investigation also

supported this conclusion that θ is close to 90° for most of the binaphthyl derivatives in solution.12 Although it was shown that the s-cis conformation is favored in general for 2,20 disubstituted 1,10 -binaphthyl derivatives (unless the substituents are too large),9 the theoretical investigation predicted a shallow double-well potential along θ, which, however, allows nearly free rotation of the two naphthyl rings at ambient temperatures.13 In this study to elucidate the effects of dynamic conformational equilibrium on the chiroptical properties of binaphthyls, we synthesized donor-acceptor and donor-donor type 1, 10 -binaphthyl derivatives (Chart 1, DA and DD), and experimentally and theoretically investigated the CD spectral behavior in solution by using state-of-the-art theoretical calculations. The cation-π interaction, introduced to DA as a tool for manipulating the conformation, is expected to affect the conformational preference (vide infra) and alter the CD spectrum. Besides the exciton coupling at the main band, the Cotton effect observed for the charge-transfer (CT) band in DA will also be useful for assessing the difference in conformation between DA and DD. It is of our particular interest to theoretically reproduce the experimental CD spectra in solution by taking into account the dynamic conformer population, which enables us not only to interpret the chiral recognition behavior and stereochemical outcomes but also to design more effective chiral binaphthyl auxiliaries.

B

r 2010 American Chemical Society

Received Date: May 5, 2010 Accepted Date: May 21, 2010 Published on Web Date: May 26, 2010

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DOI: 10.1021/jz100574e |J. Phys. Chem. Lett. 2010, 1, 1809–1812

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Chart 1. Chiral Donor-Acceptor and Donor-Donor Type 1,10 -Binaphthyl Derivatives

Information). The approximation by the two-state model, in which the spectra were estimated by weighted-averaging for the s-cis and s-trans conformers, nicely reproduced the experimental spectra (Figure 2, colored lines), which enabled us to assign the absolute configurations of both DA and DD as (R)-(-). It is to be noted that the computationally less expensive TD-DFT method afforded qualitatively similar results, except for the 1La transition (Figure S4, right). This defect in calculating the excitation energy of the 1La band has already been pointed out for the TD-DFT calculations with common density functionals.19 In the condensed phase, the two naphthyl rings are freely rotating within a certain angular range around the C1-C10 bond; in other words, the conformers are dynamically equilibrating in solution.13 To better simulate the dynamics of the axial chirality of binaphthyl derivatives, we calculated the relative energies of the conformers with different θ at the SCSM2/TZVPP and DFT-D-B97-D/TZV2P level. As illustrated in Figure 3 (right), DD gave shallow double-well potentials. Similar behavior was reported for unsubstituted 1,10 -binaphthyl calculated at the BPW91/6-31G** level.12 In contrast, DA gave significantly different, less symmetrical potential curves shown in Figure 3 (left). At an ambient temperature, where the Boltzmann quantum kT ∼ 0.6 kcal mol-1 (horizontal dotted lines in Figure 3), the estimated range of θ for DA (70-105°) is narrower and more s-cis-oriented than that for DD (60-120°). Since the potential energy profile is flat in this range, the twostate model is, in principle, less accurate approximation to simulate the observed CD spectrum, than the ensemble of all

DA was prepared by the N-methylation of 1-(2-methoxy-1naphthyl)isoquinoline and optically resolved by reversephase chiral HPLC (see the Supporting Information). The X-ray crystallographic study of racemic DA revealed that the s-trans conformation is preferred with θ = 98.3° and the C1-C10 bond length (r) = 1.486 Å (Figure 1, right and Table 1), both of which are smaller than those (θ = 112.1°, r = 1.503 Å) of DD that lacks the cation-π interaction. We also obtained the optimized structures of both s-cis and s-trans conformers of DA and DD at the DFT-D-B97-D/TZV2P level (Figure 1, left).14,15 The dispersion correction was applied as the donor-acceptor or cation-π interaction can be effectively incorporated with reasonable computational costs. The optimized s-trans structures sensibly agreed with the X-ray structures (Table 1). The SCS-MP2 relative energy,16 which is comparable in accuracy to the CCSD(T) method, was employed to calculate the relative Boltzmann population at 25 °C as s-cis/s-trans = 66:34 for DA and 55:45 for DD, most probably due to the cation-π interaction. As can be seen from Figure 2, the CD spectrum of DA (left, black line), measured in acetonitrile at 25 °C, was very similar in pattern to that of DD (right, black line), except for the additional positive Cotton effect at the absorption edge (ca. 380 nm), which is most probably ascribed to the CT transition. The relative intensities, however, differ significantly, as the couplet at the main band (ca. 230 nm) was much stronger in amplitude (A) for DA (A = 560) than for DD (A = 360); the Davydov splitting (ΔλDav) was also appreciably enhanced for DA (ΔλDav = 11.8 nm; 0.30 eV) than for DD (ΔλDav = 10.2 nm; 0.24 eV). The theoretical CD spectra of the s-cis and s-trans conformers of DA and DD were calculated at the most reliable RI-CC2/TZVPP level,17,18 and the other individual DA or DD conformers were also simulated by the time-dependent density functional theory (TD-DFT) methods; they differ significantly from each other (Figures S4 and S5, in the Supporting

Table 1. Comparison of Structural Parameters (Dihedral Angle θ, and Bond Length, r) of DA and DD and Relative Energies between the Conformers (in kcal mol-1) a θ/°

r/Å

ΔEDFT-D

ΔESCS-MP2

% population b

DA (s-cis)

76.1

1.479

0

0

65.6

DA (s-trans) DA (X-ray)

98.1 98.3

1.480 1.486

0.36

0.38

34.4

79.0

1.490

0

0

55.1

DD (s-trans)

DD (s-cis)

101.9

1.491

0.10

0.12

44.9

DD (X-ray) c

112.1

1.503

a The geometry optimizations for two isomeric donor-acceptor and donor-donor binaphthyls (DA and DD) were performed at the DFT-DB97-D/TZV2P level. b Relative Boltzmann population at 25 °C based on the SCS-MP2 energy. c From CCDC No. 258234.

Figure 1. Left: The optimized geometries of two isomeric donor-acceptor binaphthyl DA at the DFT-D-B97-D/TZV2P level. Right: ORTEP drawing of crystal DA. The BF4- anion is omitted for clarity.


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Figure 2. Comparison of the theoretical and experimental CD spectra of DA and DD. Color: Spectra at the RI-CC2/TZVPP level (shift: 0.1 eV). Black: Experimental (acetonitrile at 25 °C).

Figure 3. Potential curves against the dihedral angle θ for DA and DD. Solid lines: SCS-MP2/TZVPP level. Dotted lines: DFT-D-B97-D/ZTV2P level.

interaction to the binaphthyl system (as in DA), the s-cis conformation is more favored, and the potential curve becomes less flat than that of symmetrical binaphthyl (such as DD). Such conformational control can be achieved via other attractive/repulsive interactions, and the very recent example is found for the binaphthyl surrogate molecules with intramolecular hydrogen bonding.21 As the dihedral angle θ is one of the most important factors in sensing the molecular chirality and controlling the stereochemical outcomes in asymmetric synthesis, the dynamic distribution of the conformers of different θ should be taken into account in designing the chiral materials based on axial chirality. Nevertheless, the two-state approximation with appropriate theoretical models can be still valid for reproducing the observed CD spectra of binaphthyl derivatives in solution.

possible conformers. The success of the two-state model may be explained by an effective cancellation between the nearby conformers, as judged from the calculated CD spectra for conformers with different θ values (Figure S5, Supporting Information) and also from the small temperature effects on the anisotropy factor observed for the main-band region (Figure S6). Interestingly, the ratio (0.67) of the possible θ range for DA versus DD is reasonably close to the (reversed) ratio of the coupling amplitude (ADD/ADA = 0.65), indicating that the coupling amplitude can be critically controlled by changing the possible θ range. It should be noted that the standard DFT method failed to give the optimized s-trans geometries of DA as the local minima, in opposition to the X-ray crystallographic structure obtained (vide supra). This is most probably due to the neglect of solvent effect in our calculations, which is certainly important (but not easily incorporated) for the cation-π systems.20 In summary, we have shown the dynamic conformational population differences between the donor-acceptor and the neutral binaphthyls (DA vs DD). By introducing the cation-π


SUPPORTING INFORMATION AVAILABLE Details of the experimental procedure, theoretical calculation, and spectroscopic data. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: tmori@ chem.eng.osaka-u.ac.jp.

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ACKNOWLEDGMENT Financial support by a Grant-in-Aid for Scientific Research, JSPS, the Mitsubishi Chemical Corporation Fund, and the Sumitomo Foundation are greatly acknowledged.

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