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Conformation Elucidation of Tethered Donor-Acceptor Binaphthyls from the Anisotropy Factor of a Charge-Transfer Band Masaki Nishizaka, Tadashi Mori,* and Yoshihisa Inoue* Department of Applied Chemistry, Osaka University, 2-1 Yamada-oka, Suita 565-0871, Japan
ABSTRACT Biaryls, in particular 1,10 -binaphthyls, are widely employed as effective chiral auxiliaries and ligands in asymmetric synthesis and chirality sensing, where the amplitude and splitting energy of the main-band couplet are often used as conventional tools for elucidating the conformational details of biaryls in conjunction with relatively simple theoretical models. In this study, the chiroptical properties of a series of tethered donor-acceptor binaphthyls DAn were investigated. The theoretical calculations at the RI-CC2 level successfully reproduced the experimental spectra of all the examined DAn and revealed that the above parameters are quantitatively less informative in the conformational investigations. Alternatively, the anisotropy factor at the charge transfer (CT) band was found to be a more sensitive and robust parameter that can be exploited as a tool for analyzing the solution-phase conformation of donor-acceptor binaphthyls. A substantial contribution from the linker atoms on the anisotropy factor of the CT band was also highlighted. SECTION Molecular Structure, Quantum Chemistry, General Theory
T
he unique axial chirality and the frequent use in asymmetric synthesis and chirality sensing render substituted biaryls and binaphthyls as one of the most intriguing and explored chiral compounds.1,2 The excitoncoupled circular dichroism (CD) spectra of biaryls are discussed in terms of the couplet amplitude (A) and the splitting energy (Δλobs max), both of which are critical functions of the dihedral angle (θ) between the two aryl rings. A recent conformational study on C2-symmetric 2,20 -disubstituted 1,10 binaphthyls in solution disclosed a simple but useful qualitative relationship of these parameters with θ.3,4 The A value at the binaphthyl's main band (∼220 nm) is very sensitive to various factors (such as substituent, solvent, and temperature), which makes the conformational analysis rather complicated. The Davydov splitting (ΔλDav), which is also explained by the exciton model, is a relatively robust parameter,3,4 although not very predictive. Our recent chiroptical study on a donor-acceptor binaphthyl derivative revealed that the conformer distribution over a wide range of θ on a fairly flat potential surface is essential for interpreting the CD spectra of such a flexible system.5 In this study, we synthesized a series of donoracceptor-type 1-naphthylisoquinolinium derivatives (DAn, n = the number of atoms in the ring) (Chart 1), in which the 2,20 -positions are linked with a tether of varying lengths to restrict the θ in a certain range, and investigated their chiroptical properties. Our results revealed that only the anisotropy factor of the charge transfer (CT) band (gCT) is nicely correlated with the dihedral angle θ and theoretically
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well-reproduced, and thus can be used as a tool for analyzing the solution-phase conformation of binaphthyls, while the amplitude A and splitting ΔλDav at the main band are not. A series of cyclic DAn (n = 7-9) with different tether lengths were prepared as tetrafluoroborate salts by the reaction of 1-(2-hydroxy-1-naphthyl)isoquinoline with appropriate dibromoalkanes and the subsequent anion exchange. The CT band appears at around 380 nm, showing donor-acceptor interaction in DAn (Figure S4 in the Supporting Information). The Marcus-Hash analysis of the CT band deconvoluted by Gaussian peak fitting gave the electronic coupling element (Hab),6,7 which steadily increases with decreasing tether length (Table 1). Enantiomers of DAn were separated by reverse-phase chiral high-performance liquid chromatography (HPLC; Figure S5), and their experimental CD spectra were measured in acetonitrile and dichloromethane to reveal that the effects of solvent and counterions are essentially negligible (Figures S6 and S7). The experimental anisotropy (g) factors were compared with the theoretical ones calculated at the most reliable RI-CC2/TZVPP level for DFT-D-B97-D/TZV2P optimized geometries (Figure 1).8,9 Due to the limited rotational freedoms, the experimental g factors of tethered DAn are much larger than that of untethered DA in most of the
Received Date: July 2, 2010 Accepted Date: July 15, 2010 Published on Web Date: July 22, 2010
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transitions (Figure 1), and less sensitive to the temperature (Figure S8). As shown in Table 1, the splitting energy Δλmaxobs
gradually decreased with increasing tether length or θ, while the amplitude A exhibited an apparently parabolic dependence on θ, maximizing at ∼70°. The theoretical CD spectra calculated for (R)-DAs showed excellent agreement in most of the spectral regions with the experimental ones shown in Figures 1 and S9, allowing us to definitely assign the absolute configurations of these (-)-DAs as R.10 With a minor deviation in the 1La region, the simulation nicely reproduced the experimental CD spectra in both shape and intensity. For example, the amplitude of the negative couplet at the main band (∼220 nm) is well reproduced by the theory for a series
Chart 1. Chiral Tethered Donor-Acceptor 1,10 -Binaphthyl Derivatives DAn and Freely Rotating DA
Table 1. Comparison of Structural/Spectral Parameters for DAn and DA a θ, r (s-cis) DA7 DA8
θ, r (s-trans)
58.9, 1.474 66.9, 1.476
λCT
εCT
Hab
ΔεCT
103gCT
Δλmaxobs
A
389 369
5070 3870
2140 2060
þ10.2 þ5.3
þ1.9 þ1.3
14.0 13.6
330 450
DA9
71.7, 1.479
95.1, 1.480
368
2140
1650
þ6.1
þ2.9
12.4
720
DA
76.1, 1.479
98.1, 1.480
379
1180
1260
þ1.0
þ0.81
11.8
560
a Dihedral angle (θ/°), C1-C10 bond length (r/Å), absorption maximum and molar extinction coefficient at the CT band (λCT/nm and εCT/M-1 cm-1), electronic coupling element (Hab/cm-1), anisotropy factor at the CT band (103 gCT), apparent energy splitting and the coupling amplitude at the main band (Δλmaxobs/nm and A/M-1 cm-1). The CT bands were deconvoluted from the observed spectra in dichloromethane at 25 °C. The Hab values were obtained by the equation Hab = (0.0206/rDA) (νCT Δν0.5 εCT)0.5. Distance between the aromatic rings (rDA)was calculated from the values obtained by the DFT-D-B97-D/TZV2P optimized geometry (4.6 Å for DA7/DA8 and 4.7 Å for DA9/DA).
Figure 1. Experimental and theoretical anisotropy (g) factors of DA7 (blue), DA8 (green), DA9 (red), and DA (dotted black). Top: Experimental spectra at 25 °C. (left: high energy region in acetonitrile; right: low energy region in dichloromethane.) Bottom: Theoretical spectra at the RI-CC2/TZVPP level. Only the s-cis conformer was considered in the case of DAn, while the Boltzmann population of the s-cis and s-trans conformers, based on the relative energies calculated by the SCS-MP2/TZVPP, was taken into account in the case of DA; see ref 5.
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of DAs (strength of the g factors at 230-240 nm were found in the order |DA7| > |DA9| g |DA8| > |DA| for both the experiment and the theory), and the magnitude of the Cotton effect at the 1Lb band is also well reproduced (g factors at 300-320 nm were in the order |DA7| > |DA8| > | DA9| > |DA| for both the experiment and the theory). Additionally, the positive g factors at the CT band were also well reproduced (g factors at >360 nm were in the order DA9 > DA7 > DA8 > DA for both the experiment and the theory). It is also important to note that the computationally less demanding time-dependent density functional theoroy (TD-DFT) methods using conventional hybrid functionals also afforded qualitatively similar but quantitatively poorer results, even after appropriate scaling and/or wavelength shift were applied (Figure S10).11 The geometrical parameters of DAs were obtained from their optimized structures at the DFT-D-B97-D/TZV2P level (Figure 2).12,13 The dispersion correction was applied, as the donor-acceptor interaction is effectively incorporated without any additional computational cost. As is the case with acyclic DA, DA9 can take s-cis and s-trans conformations. However, the Boltzmann distribution to the s-trans in DA9 was calculated as ∼0.1% at 298 K due to the much higher energy compared to that of the s-cis (ΔESCS-MP2/TZVPP14 = 4.5 kcal mol-1), and hence its contribution was ignored throughout the work. Figure 3 shows the theoretical (RI-CC2/TZVPP, open circle) and experimental (closed circle) A, ΔλDav, and gCT
values as functions of θ. The solid lines indicate the theoretical values calculated at the TD-DFT-BH-LYP/TZV2P level for DA (with no tether atom) of varying θ as reference structures. The observed energy splitting Δλobs max gradually decreased with increasing dihedral angle θ at least in the examined range (50° < θ < 90°), in accordance with the previous report.3,4 The theoretically estimated Δλobs max showed a similar trend, but were always overestimated. More seriously, the parameter was not very sensitive to θ, varying only in a narrow range of 12-14 nm, and hence seems somewhat unsafe for the use in conformational analysis. The main-band couplet amplitude A was much more sensitive to θ, varying from 330 to 720 M-1cm-1 with a maximum at 70°. Although such a trend was qualitatively reproduced by the theoretical calculations, there were appreciable deviations from the experimental ones. As the couplet is more susceptible to various factors such as overlapping transition, solvent-induced change in splitting energy, and the dynamic rotation around the central axis,5 the parameter A is again not conclusive in the conformational analysis. It was revealed that, as the dihedral angle θ decreases, the molar extinction coefficient and thus the electronic coupling element at the CT band (an indicator of the strength of donor-acceptor interaction) progressively increase as a consequence of the increased through-bond donor-acceptor interaction (vide supra). Accordingly, we propose the use of the anisotropy factor gCT for the conformational analysis of axially chiral donor-acceptor molecules. The g factor of the CT band is primarily a direct function of the dihedral angle θ, but is also affected significantly by the linker conformation. Nevertheless, an excellent agreement was found between the observed and RI-CC2-calculated g factors of all DAn and DA (see Figure 3, right), justifying its use in the conformational analysis. Moreover, the estimated values obtained by the less expensive TD-DFT method were found to provide more or less similar results, which seems favorable as the coupled cluster calculations are not always applicable to medium to
Figure 2. The optimized geometries of tethered donor-acceptor binaphthyls DAn at the DFT-D-B97-D/TZV2P level.
Figure 3. Left: Comparison of the experimental (closed circle) and RI-CC2 calculated (open circle) coupling amplitude (A, green) and the observed energy splitting (Δλobs max, blue) at the main band for geometry-optimized DAn and DA. Solid lines: theoretically estimated values calculated at the TD-DFT-BH-LYP/TZV2P level for DA of varying θ as reference structures. Right: Comparison of the experimental (closed circle) and theoretical (colored circle) anisotropy factors at the CT band (gCT) for geometry-optimized DAn and DA. Solid lines: theoretically estimated values calculated at the TD-DFT-BH-LYP/TZV2P level for DA of varying θ as reference structures. The theoretical values were obtained either at the RI-CC2/TZVPP level (colored open circle) or at the TD-DFT-BH-LYP/TZV2P level (open circle in black).
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large sized molecules in question. The deviations of the experimental (and calculated) g factors from the theoretical lines calculated for DA of varying θ, show an odd-even symptom (which was properly reproduced by the theory). Thus, the gCT value is also susceptive to the linker conformation because the rotatory strength of the CT transition critically depends on the flip angle (or the exo/endo position) of the methylene group adjacent to the ether oxygen. In summary, in addition to the classical parameters such as energy splitting and coupling amplitude at the main band, the anisotropy factor at the CT band, which can be predicted by the conventional quantum chemical calculations, was found to be a useful parameter for the conformational study of chiral donor-acceptor binaphthyls. Further studies on the chiroptical behavior of axially chiral donor-acceptor molecules are in progress.
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SUPPORTING INFORMATION AVAILABLE Details of the experimental procedure, theoretical calculation, and spectroscopic data for DAn. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: tmori@ chem.eng.osaka-u.ac.jp.
ACKNOWLEDGMENT Financial support by a Grant-in-Aid for Scientific Research, JSPS, the Mitsubishi Chemical Corporation Fund, and the Sumitomo Foundation is greatly acknowledged.
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