Synthesis, Structures, and Properties of Hexapole Helicenes

Article pubs.acs.org/JACS

Synthesis, Structures, and Properties of Hexapole Helicenes: Assembling Six [5]Helicene Substructures into Highly Twisted Aromatic Systems Tomoka Hosokawa,† Yusuke Takahashi,† Tomoya Matsushima,‡ Soichiro Watanabe,‡ Shoko Kikkawa,§ Isao Azumaya,§ Akihiro Tsurusaki,*,† and Ken Kamikawa*,† †

Department of Chemistry, Graduate School of Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan Department of Biomolecular Science, Faculty of Science, and Research Center for Materials with Integrated Properties, Toho University, Miyama 2-2-1, Funabashi, Chiba 274-8510, Japan § Faculty of Pharmaceutical Sciences, Toho University, Miyama 2-2-1, Funabashi, Chiba 274-8510, Japan ‡

S Supporting Information *

ABSTRACT: Hexapole helicenes 1, which contain six [5]helicene substructures, were synthesized by Pd-catalyzed [2+2+2]cycloadditions of aryne precursor 6. Among the possible 20 stereoisomers, which include ten pairs of enantiomers, HH-1 was obtained selectively. Density functional theory (DFT) calculations identified HH-1 as the second most stable isomer that quantitatively isomerizes under thermal conditions into the most stable isomer (HH-2). Both enantiomers of HH-2 can be separated by chiral HPLC. Single-crystal X-ray diffraction analyses revealed a saddle-like structure for (P,M,P,P,M,P) HH-1 and a propeller-like structure for (P,M,P,M,P,M) HH-2. Because of the helical assembly and the resulting steric repulsion, the structure of HH-1 is significantly distorted and exhibits the largest twisting angle reported so far (up to 35.7° per benzene unit). Electrochemical studies and DFT calculations indicated a narrow HOMO−LUMO gap on account of the extended π-system. Kinetic studies of the isomerization from HH-1 to HH-2 and the racemization of enantiomerically pure HH-2 were conducted based on 1H NMR spectroscopy, HPLC analysis, and DFT calculations.

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INTRODUCTION Helicenes, i.e., helically ortho-fused polyaromatic hydrocarbons, have received much attention due to their unique physical properties.1 The helical structure of helicenes has been attributed to the steric repulsion of the terminal rings, inducing chirality despite the absence of asymmetric carbon or other chiral centers. Additionally, the helical structure induces high levels of optical rotation, high circular dichroism values, and intensifies some other physico-organic properties.1 The characteristic features of helicenes render them promising prospects for applications in asymmetric catalysis, supramolecular chemistry, liquid crystals, and organic devices.1a−d Further interest in helicenes arose from theoretical investigations that were concerned with the limits of aromaticity in order to examine helical chirality in polyaromatic compounds with distorted π-systems. In addition to the chemistry of ordinary helicenes, that of multiple helicenes with plural helicities has grown rapidly in recent years, and developed into an attractive research field (Figure 1).2−6 Multiple helicenes exhibit highly unusual conformations and three-dimensional intermolecular interactions. In particular, they often afford highly distorted structures based on multiple repulsive interactions, thereby allowing insight into the distortion of π© 2017 American Chemical Society

systems. Many research groups have devoted efforts toward the discovery of multiple helicenes with unusual structures and properties. In 2015, we reported the synthesis, structure, and physical properties of double [5]helicene (c), which was obtained from a Pd-catalyzed double Suzuki−Miyaura crosscoupling reaction.2e During our ongoing quest toward the development of higher multiple helicenes, we then subsequently focused on hexabenzotriphenylene (e)3g,h as a platform, given that it exhibits an inherent 3-fold helicity (Figure 2). A compound that contains an additional six fused aromatic rings at the exterior region of e should generate three outer helicene substructures in a single polyaromatic framework that comprises three inner helical structures. Consequently, the thus obtained hexapole helicene should afford 20 stereoisomers, including ten pairs of enantiomers. A related second-generation dendrimer-like multiple helicene was postulated in 1999 by Pascal Jr. et al. as a hypothetical molecule, but such helicenes have not been generated in the almost two decades since.3g Meanwhile, few studies on compounds such as g,5c which is composed of six-[4]helicenes, and h,5a which consists of fourReceived: July 8, 2017 Published: September 6, 2017 18512

DOI: 10.1021/jacs.7b07113 J. Am. Chem. Soc. 2017, 139, 18512−18521

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Journal of the American Chemical Society

[4]helicene/two-[5]helicene subunits in a molecule framework, have been reported. These compounds can be categorized as hexapole helicenes. Nevertheless, the development of molecules consisting of higher helicene substructures inducing all six[5]helicene subunits and a high molecular symmetry still remains a formidable challenge. Moreover, the multirepulsive interactions exert a strong deformation on the benzene ring.7,8 For instance, Hatakeyama reported in 2014 a highly distorted benzene ring in a phosphine-fused double helicene (twisting angle: 25.0°).2o We have also disclosed double carbohelicene c with 4-fold helicity (twisting angle per benzene unit on the central naphthalene core: 29.9°).2e A further deformation of the aromatic ring was achieved by Segawa and Itami in quadruple helicene f, which exhibits the largest twisting angle reported so far (35.3°).4 Thus, the development of multiple helicenes that enable the dense assembly of several repulsive interactions should provide a unique carbon nanostructure with significant undulations, and afford an insight into the limits of deformability of aromatic rings. It should also be feasible to assume that the level of deformation in hexapole helicenes may surpass that in double, triple, and quadrapole helicenes. In this article, we report the first synthesis of hexapole helicene 1, which is composed of six-[5]helicene substructures, via a Pd-catalyzed [2+2+2]cycloaddition. As expected, 1 exhibits a highly distorted benzene ring (twisting angle: 35.7°), which is, to the best our knowledge, the largest value reported to date. We also disclose the dynamic behavior of 1, theoretical studies regarding its isomerization, and its physical properties.

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Figure 1. Some representative examples for multiple helicenes.

RESULTS AND DISCUSSION Synthesis of Hexapole Helicenes 1 and Their Thermal Isomerization. Hexapole helicene 1 was synthesized by a Pdcatalyzed [2+2+2]cycloaddition of 6 as illustrated in Scheme 1.3h,9 Initially, 7,8-bibromo[5]helicene (2) was prepared from 2,2′-dimethyl-1,1′-binaphthyl in two steps following a previously reported procedure.10 A nucleophilic substitution in 2 using sodium methoxide afforded 7-methoxy[5]helicene (3) in 51%. The ortho-bromination of 3 with NBS in the presence of diisopropylamine furnished 4 in 94%. Demethylation of 4 with BBr3 furnished 7-bromo-8-hydroxy[5]helicene (5) in 98%. [5]Helicene 5 was subsequently treated with hexamethyldisilazane (HMDS), and the thus obtained trimethylsilyl ether was ortho-lithiated with n-butyllithium to generate crude 7-oxy-8trimethylsilyl[5]helicene via a retro-Brook rearrangement. The resulting crude product was quenched with Tf2O to afford 6 in 88% from 5.11 A Pd-catalyzed [2+2+2]cycloaddition of 6 in the presence of 10 mol % of Pd2(dba)3 and CsF at room temperature furnished hexapole helicene 1 (HH-1) in 54%. 1 exhibits 6-fold helicity and affords 20 stereoisomers, including ten pairs of enantiomers (Figure 3). Ten of these stereoisomers (not regarding the enantiomers) are categorized as follows: C1 symmetry (C1-I and C1-II), C2 symmetry (C2-I to C2-VI), and D3 symmetry (D3-I and D3-II). 1 was formed stereoselectively and the stereochemistry of the obtained product was confirmed by a single-crystal X-ray diffraction analysis (Figure 5), which revealed a (P,M,P,P,M,P) configuration (C2-IV; HH-1), characterized by a clockwise rotation (Scheme 1). Moreover, HH-1 (C2-IV) could be quantitatively converted into the thermodynamically more stable HH-2 (D3-II) isomer by heating in toluene (100 °C, 3 h) (Figure 4). The 1H NMR spectra clearly indicate that the proton signals of HH-1 with C2 symmetry converge to a simpler spectrum, demonstrating the

Figure 2. Concept for the design of hexapole helicenes composed of six-[5]helicene subunits.

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Journal of the American Chemical Society Scheme 1. Synthesis of Hexapole Helicenes 1

Figure 3. Stereoisomers of hexapole helicene 1 and their relative free energies calculated at the B3LYP/6-31G(d) level of theory. The labeling of each benzene ring in hexapole helicene 1 is shown for C1-I.

formation of an isomer (HH-2) with a higher symmetry. The stereochemistry of HH-2 (P,M,P,M,P,M) with D3 symmetry was confirmed by single-crystal X-ray diffraction analysis (Figure 5); all outer and inner helicities adopt helical P and M substructures, respectively. This result is also consistent with an NMR spectrum for HH-2 that indicates higher symmetry than C2. Both inside protons in the inner and outer helical substructures such as H(19) and H(25) in HH-2 overlap at 8.57 ppm, which is downfield shifted due to the anisotropic effect of the aromatic rings. DFT calculations revealed that HH-

2 is the most stable, whereas HH-1 is the second most stable isomer (Figure 3). However, the details of the stereoselective formation of HH-1 still remain unclear. As HHs that are composed of six [5]helicene substructures inherently contain multiple helicities, they provide an interesting motif for chiral polyaromatic hydrocarbons. Therefore, we examined the optical resolution of HHs. Although we were unable to completely separate both enantiomers of HH-1 using a representative series of chiral columns, the enantiomers of propeller-shaped HH-2 could be separated by HPLC using a CHIRALPAK IB 18514


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[5]helicene moieties in HH-1 and HH-2 adopt a relative stereochemistry of P,M,P,P,M,P and P,M,P,M,P,M, respectively, which includes the enantiomers in the unit cell. It should be noted that the two [5]helicene moieties with M chirality in HH-1 are more distorted than the other four with P chirality, i.e., the dihedral angles of the two [5]helicene moieties with M chirality (55.5° and 56.0°) are higher than those with the P chirality (48.1°−51.4°).13 The dihedral angle described above is defined as the angle formed by the two benzene rings located at the terminal edges of each [5]helicene subunit. The most notable feature of HHs is the degree of deformation of the benzene rings, especially for the central triphenylene core. The twisting angles (Figure 5) of the three benzene rings surrounding the central benzene ring A in HH-1 are 15.3° (B), 14.5° (C), and 35.7° (D).13 These values are higher than those of the corresponding triple helicene e with C 2 symmetry,3f i.e., 7.4°, 5.1°, and 28.8° for rings B−D, respectively,14 due to the additional repulsion arising from the outer [5]helicene moiety in HH-1. Furthermore, the highest deformation of ring D (35.7°) is also higher than that of c (28.7−29.9°),2e and slightly exceeds the maximum twisting angle of the benzene ring in quadruple helicene f (35.3°),4 which has been reported as the highest deformation value in a benzene ring so far. HH-2 with D3 symmetry also exhibits a deformed geometry. Interestingly, the dihedral angles of the inner helicity of HH-2 are notably increased (70.3−71.4°) by the conformational change from HH-1 to HH-2, whereas the values of the outer helicities (48.5−50.2°) are slightly smaller than those in HH-1 (51.0−55.5°). The twisting angles of the benzene rings B−D in HH-2 (29.9−30.1°) are comparable to each other, reflecting the 3-fold geometry, and higher than that

Figure 4. 1H NMR spectra of HH-1 (green) and HH-2 (brown) in toluene-d8.

column (>99% ee; Figure S13). The specific optical rotation of the first eluting peak ([α]D22 = −340°; c = 0.13; CHCl3) was, to our surprise, smaller than those of single helicenes, which generally exhibit relatively large [α]D values.12 The kinetic studies for the racemization of HH-2 were also examined at elevated temperatures in 1,2,4-trichlorobenzene (vide inf ra). Structures. Single crystals suitable for X-ray diffraction analyses were obtained by room-temperature vapor diffusion of petroleum ether or hexane into dichloromethane solutions of HH-1 or HH-2, respectively. The X-ray diffraction analysis of HH-2 revealed that its unit cell contained two crystallographically independent molecules (HH-2-A and HH-2-B), and that HH-2-B exhibited a 3-fold rotational axis through the centroid of the central benzene ring. HH-1 adopts a saddle structure, whereas HH-2 exhibits a propeller-type geometry (Figure 5). The distances between the least-squares plane of the central benzene ring and the farthest carbon atoms of the terminal [5]helicene are 3.32 and 3.61 Å (HH-1), 2.90 and 2.77 Å (HH-2-A), and 2.90 and 2.60 Å (HH-2-B). The six

Figure 5. X-ray structures of (a) HH-1 (C2-IV) and (b) HH-2-A (D3-II), their twisting angles relative to the central triphenylene core, and their HOMA and NICS values (in parentheses; red numbers: HOMA; black numbers: NICS). ORTEP drawings are shown at 30% probability levels, and all hydrogen atoms are omitted for clarity. The twisting angle is defined by the two carbon atoms and the two bond midpoints (black circles) and these dihedral angles are illustrated by red lines [top right figures for panels a and b]. NICS values for HH-1 and HH-2-A were calculated at the B3LYP/6-311+G(2d,p) level of theory. 18515


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Journal of the American Chemical Society of triple helicene e with D3 symmetry (e:3g 22.6−24.0°; t-butyl substituted derivative of e:3e 18.5−22.6°).14 It is noteworthy that the fusion of six benzene rings at the periphery of hexabenzotriphenylene e induces a higher twisting angle of benzene rings B−D than the introduction of the sterically bulky tBu groups in e, which confirms the introduction of profound deformation in nonplanar π-electron conjugated molecules upon accumulation of multiple repulsive interactions between helical substructures. Moreover, the D ring in HH-1 and the B−D rings in HH-2 exhibit a unique twist-boat conformation in cyclohexane. The deformation of the D ring in HH-1 is also reflected in the C−C bond lengths: the nonfused C−C bond lengths [C(5)−C(47): 1.456(4) Å; C(6)−C(66): 1.464(4) Å; C(56)− C(57): 1.465(4) Å] are almost 4% longer than those of the other C−C bonds [1.396(4)−1.406(4) Å]. These longer C−C bonds are comparable to C−C single bonds between typical sp2-hybridized carbon atoms (1.46−1.48 Å).15 Similarly, the nonfused C−C bonds lengths in the B−D rings in HH-2 [1.458(4)−1.467(4) Å] are longer than those of the other C− C bonds [1.395(4)−1.414(4) Å]. In addition, the nonplanarity (NP)15 values of ring D (0.152) in HH-1 and of rings B−D (0.123−0.126) in HH-2 are higher than those of the other rings (0.010−0.114; Table S2) and of hexabenzotriphenylene e (C2: 0.005−0.106; D3: 0.005−0.104). Subsequently, we evaluated the local aromaticity of the individual rings using the harmonic oscillator model of aromaticity (HOMA)16 and the nucleus-independent chemical shift (NICS).17 Though the HOMA values of the HHs decreased from the periphery to the inner benzene rings, the NICS values increased in the same direction (Table S2), indicating a reduction of the aromatic character of the inner rings. Ring D in HH-1 exhibits the lowest HOMA value (0.270), which is lower than that of the central naphthalene core of Itami’s quadruple helicene f (0.345 and 0.392),4 whereas the A−C rings in the triphenylene core exhibit values of 0.511, 0.455, and 0.437, respectively. In general, the local aromaticity of [n]helicenes and [n]phenacenes varies from the external to central rings in an alternating fashion.18 According to this trend, the B−D rings, corresponding to the third ring of both edges of the [7]phenacene (e.g., F-E-B-A-C-K-L ring) or [5]helicene subunits (e.g. H-G-B-E-F ring) in the HHs, were expected to exhibit higher HOMA values compared to the adjacent rings. However, in contrast, small HOMA values were observed experimentally. This result should probably be rationalized in terms of the highly twisted geometry of B−D in HH-1, which would reduce the HOMA values. The B−D rings in HH-2 also show small HOMA (0.247−0.312) and high NICS values (0.02−0.10), which is indicative of low aromaticity. The HOMA value of the central ring A in HH-2 (0.687) is higher than that in HH-1 (0.511). The 3-foldsymmetrical orientation of the outer [5]helicene moieties should increase the planarity of the central ring A in HH-2. Photophysical Properties. HH-1 (C2-IV) and HH-2 (D3-II) show yellow in solution, whereas orange under UV-irradiation in solution. The UV−vis spectra of HH-1 and HH-2 in CHCl3 showed comparable absorption spectra, irrespective of the different arrangement of the six helicities per molecule (Figure 6, Table S3). This result stands in sharp contrast to the previously reported multiple helicenes a,2g b,2f d,2d f,4 and h5a (Figure 1), whose diastereomers show different absorption maxima. The absorption onset of both HH-1 and HH-2 (520 nm) corresponds to an optical band gap of 2.38 eV. This

Figure 6. UV−vis absorption spectra (solid lines) and photoluminescence (dashed lines) of HH-1 (C2-IV; red) and HH-2 (D3II; blue) in CHCl3. Inset: 450−550 nm.

absorption is significantly red-shifted compared to those of [5]helicene (350 nm; 3.54 eV)13 and double helicene c (437 nm; 2.84 eV),2c and should be ascribed to the extended πconjugation. The longest-wavelength absorption maxima were observed at 393 nm (HH-1, ε = 57 800; HH-2, ε = 48 200). Compared to HH-1, HH-2 showed a more vibronic fine structure, probably due to its higher symmetry. On the other hand, for HH-1, a more intense absorption was observed around 450 nm (shoulder) relative to HH-2. In order to understand the nature of these absorptions, time-dependent (TD) DFT calculations were carried out at the B3LYP-D3/6311+G(2d,p)/B3LYP/6-31G(d) level of theory. The resulting energy diagrams and selected molecular orbitals are shown in Figure 7. In HH-2, both the HOMO and the LUMO are almost doubly degenerated. The HOMO and LUMO exhibit dominant contributions from the π- and π*-orbitals, which are delocalized over the [7]phenacene moiety (F-E-B-A-C-K-L in Figure 7), together with additional contributions from a [5]helicene moiety (P-O-D-M-N). The HOMO−1 and the LUMO+1 exhibit contributions from the π- and π*-orbitals, which are mainly delocalized over two [5]helicene moieties (IC-A-B-G and N-M-D-O-P). The partial inversion of the helicities from HH-2 (D3-II) to HH-1 (C2-IV) leads to a remarkable change in the overlap of the π- and π*-orbitals. Initially, the HOMO and LUMO in HH-2 stabilize and destabilize to afford the HOMO−1 and LUMO+1 in HH-1, respectively, which should be rationalized in terms of an effective π-conjugation derived from the more coplanar geometry of the [7]phenacene moiety (Figure S23). Conversely, the more distorted naphthalene core (A and D) from 31.1° (HH-2) to 67.2° (HH-1) suppresses the conjugation between the two [5]helicene moieties, leading to the destabilization and stabilization of the HOMO−1 and the LUMO+1 in HH-2 (corresponding to the HOMO and LUMO in HH-1), respectively. Thus, the HOMO−LUMO gap in HH1 (3.14 eV) is smaller than that in HH-2 (3.30 eV). The longer and somewhat more intense absorption at ∼450 nm in HH-1 should thus be attributed to the HOMO−LUMO transition (λcalc = 457.2 nm, f = 0.158), which is bathochromically shifted compared to that in HH-2 (λcalc = 444.8 nm, f = 0.048). A mixture of the two transitions, i.e., HOMO−1 → LUMO/ HOMO → LUMO+1 and HOMO → LUMO/HOMO−1 → LUMO+1, was calculated to be the most intense absorption observed at 393 nm (HH-1: λcalc = 419.5 nm, f = 0.855 and λcalc 18516


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transition in the longest-wavelength region (HH-1, λcalc = 469.8 nm, f = 0.0006; HH-2, λcalc = 468.2 nm, f = 0.0000). Electrochemical Properties. The electrochemical properties of HH-1 (C2-IV) and HH-2 (D3-II) were examined by cyclic voltammetry (CV) measurements, which were carried out at room temperature in CH2Cl2 using [nBu4N][ClO4] as the supporting electrolyte (Figure 8, Table S6). In contrast to the

Figure 8. Cyclic voltammograms of HH-1 (C2-IV; red) and HH-2 (D3-II; blue) in CH2Cl2 at room temperature using [nBu4N][ClO4] as the supporting electrolyte. E1/2 (Fc/Fc+) = +0.50 V vs Ag/Ag+.

theoretically predicted energy difference of 0.07 eV, clear differences between HH-1 and HH-2 were not observed. One reversible oxidation couple was observed for HH-1 (0.71 V vs Fc/Fc+) and HH-2 (0.72 V), which allowed estimating the HOMO level of HH-1 (−5.81 eV) and HH-2 (−5.82 eV) relative to the formal potential of the redox couple of Fc/Fc+ (−5.10 eV on the Fermi scale).20 Circular Dichromism. Whereas several reports on the CD of double helicenes have been reported in recent years,2b,d,g,j,l−n,q,3a those on multiple helicenes with opposing chirality in the same molecule are limited to quadruple helicene f (two P helicities in the dithia[6]helicenes and two M helicities in the [5]helicenes), whose CD spectrum shows a complicated pattern with a Δε value in the range of ±100 M−1 cm−1.4 The CD spectrum of (−)-HH-2, which is the first eluting peak in the HPLC analysis, exhibits a positive Cotton effect in the range of 380−460 nm (Δε433 nm = +26 M−1 cm−1, Δε393 nm = +26 M−1 cm−1) and below 305 nm (Δε282 nm = +133 M−1 cm−1), whereas it shows a negative Cotton effect (Δε324 nm = −131 M−1 cm−1) in the range of 305−380 nm (Figure 9). The

Figure 7. Energy diagram for the frontier orbitals of HH-1 (C2-IV; left) and HH-2 (D3-II; right) calculated at the B3LYP/6-31G(d) level of theory, together with the labeling scheme for the benzene rings in HH (top).

= 414.0 nm, f = 0.733; HH-2: λcalc = 418.1 nm, f = 0.966 and λcalc = 418.1 nm, f = 0.966; Table S10). Similar to the UV−vis spectra, the fluorescence spectra of HH-1 and HH-2 in CHCl3 were virtually comparable (Table S3), although the maximum of HH-1 (λem = 517 nm) was slightly red-shifted compared to that of HH-2 (λem = 513 nm). Furthermore, HH-2 exhibited a more vibronic fine structure than that of HH-1, suggesting that HH-2 should be more rigid than HH-1. The fluorescence quantum yield (Φf) and lifetime (τs) values between HH-1 and HH-2 were also similar. The fluorescence quantum yield values of HH-1 (Φf = 0.041) and HH-2 (Φf = 0.039) were comparable to that of [5]helicene (Φf = 0.04),19 whereas the fluorescence lifetime values of HH-1 (τs = 5.0 ns) and HH-2 (τs = 5.5 ns) were smaller than that of [5]helicene (τσ = 25.5 ns).19 According to kr = Φf/τs and knr = (1−Φf)/τs, the radiative (kr) and nonradiative (knr) decay rate constants from the singlet excited state were determined as kr = 8.2 × 106 s−1 and knr = 1.9 × 108 s−1 (HH-1), as well as kr = 7.0 × 106 s−1 and knr = 1.7 × 108 s−1 (HH-2). As in the case of [5]helicene, the small kr value resulting in a low emission quantum yield should be attributed to the symmetry-forbidden

Figure 9. CD spectra of (−)-HH-2 [red; (P,M,P,M,P,M)] and (+)-HH-2 [blue; (M,P,M,P,M,P)] in CHCl3. The gray bars show the TDDFT-derived rotatory strength values for the (P,M,P,M,P,M) isomer calculated at the B3LYP-D3/6-311+G(2d,p)//B3LYP/631G(d) level of theory. 18517


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Figure 10. Energy diagram for the isomerization from HH-1 (C2-IV) to HH-2 (D3-II).

corresponding anisotropy factors (g-values: g = Δε/ε) are +2.3 × 10−3 (433 nm) and −4.8 × 10−3 (324 nm) (Table S5) These g-values are comparable to that of (P)-(+)-[5]helicene (Δε310 nm = +161 M−1 cm−1; g = +4.2 × 10−3).12 (+)-HH-2, which elutes later in the HPLC analysis, showed a mirror-imaged CD spectrum. On the basis of TDDFT calculations, (−)-HH-2 should be assigned to (P,M,P,M,P,M)HH-2, which revealed a large negative rotatory strength at ∼350 nm and a positive strength at ∼450 nm (Figure S25). The shape of the CD spectra of HH-2 in a the longerwavelength region (>330 nm) should predominantly depend on the inner M helicities, but not the outer P helicities. TDDFT calculations on hexabenzotriphenylene (M,M,M)-e, which exhibits only three inner M helicities, predicted a positive Cotton effect at >360 nm and a negative Cotton effect in the range 290−360 nm (Figure S26), which results in a pattern similar to that of (−)-HH-2.21 Kinetics of the Interconversion Pathway between Hexapole Helicenes. Thermal Isomerization from HH-1 (C2-IV) to HH-2 (D3-II). The kinetics of the isomerization from HH-1 to HH-2 were studied experimentally by monitoring the decrease of the integrals of HH-1 in the 1H NMR spectra in toluene-d8. Because of the energetic difference between HH-1 and HH-2, a potential retro-conversion could be ruled out. The first-order rate constants kc (s−1) of the forward conversion at various temperatures were estimated according to the following equation: ln([HH‐1]t /[HH‐1]0 ) = −kct

ln k /T = −ΔH ‡/RT + ΔS‡/R + ln kB/h

where R is the gas constant, T the measured temperature, Ea the activation energy, A the frequency factor, ΔH‡ the activation enthalpy, kB the Boltzmann constant, h the Planck constant, and ΔS‡ the activation entropy. These plots provided the activation parameters ΔH‡ [29.5(2) kcal·mol−1], ΔS‡ [3.8(4) cal·mol−1·K−1], Ea [30.2(2) kcal·mol−1], and ΔG‡ [30.6(3) kcal·mol−1] at 298 K (Figure S17). The conversion energy from HH-1 to HH-2 [Ea = 30.2(2) kcal·mol−1] is thus higher than that of the inversion of [5]helicene [Ea = 23.5 kcal·mol−1; ΔG‡ = 24.1 kcal·mol−1],22 as well as the thermal isomerizations of double helicene b (ΔG‡ = 28.7 kcal·mol−1),2f quadruple helicene f (Ea = 29.3 kcal·mol−1),4 and hexapole helicene h (ΔG‡ = 27.7 kcal mol−1).5a The larger activation energy for the conversion of HH-1 to HH-2 should be due to additional repulsion from the neighboring [5]helicene substructures in the transition state. Furthermore, the individual steps of the conversion from HH-1 to HH-2 were revealed by theoretical calculations. All possible 20 transition states with face-to-face oriented outer or inner aromatic rings of the helical substructures associated with ten diastereomers were fully optimized at the B3LYP/6-31G(d) level of theory (Table S9). The most plausible isomerization pathway from HH-1 to HH-2 consists of an inversion of the three helicities labeled H4−6 with a lowest total activation energy of ΔG‡ = 28.5 kcal·mol−1 (Figure 10). The inversion at H1−3, which furnishes ent-HH-2 (ent-D3-II), represents an alternative pathway (Figure S21). The first step starts from the inversion of inner helicene H4 to afford C1-II (ΔG‡ = 14.3 kcal·mol−1). We were surprised that the activation energy for the inversion of the inner helicene (H4) is the lowest among the four possible helicities (H2−5), irrespective of the severe repulsion from the surrounding helicities. The second step includes the inversion of an outer helicity (H5) to furnish C2-III (ΔG‡ = 18.4 kcal·mol−1). The final step consists of an inversion of the inner helicene (H6) to generate D3-II, which is the rate-determining step of the overall

(1)

where [HH-1]0 is the initial ratio of the integration of [HH-1]0, and [HH-1]t is the ratio of [HH-1] at a time t during the conversion relative to the residual methyl signal of C6D5CD2H (Figure S16). Based on these data, an Arrhenius plot and an Eyring plot could be constructed: ln k = −Ea /RT + ln A

(3)

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Figure 11. Energy diagram for the racemization of HH-2 (D3-II).

conversion energy. The calculated racemization energy (ΔG‡ = 36.3 kcal·mol−1) is comparable to that experimentally obtained [ΔG‡ = 35.4(8) kcal·mol−1]. It should also be noted that the racemization of HH-1 proceeds more easily via intermediates C1-II and ent-C1-II (ΔG‡ = 17.8 kcal·mol−1), which is lower than the inversion of [5]helicene (ΔG‡ = 24.1 kcal·mol−1).23 Thus, the optical resolution of HH-1 (C2-IV) with a chiral column should be difficult, which is consistent with our unsuccessful efforts.

inversion process. Although both the C1-II and C2-III intermediates could not be detected by 1H NMR measurements during the thermal isomerization, they can be elucidated by theoretical calculations. Thermal Racemization of HH-2 (D3-II). Kinetic studies on the racemization of (+)-HH-2 were also carried out at elevated temperature, using 1,2,4-trichlorobenzene as the solvent. The decrease of the enantiopurity of HH-2 was monitored by chiral HPLC (Figure S18). Because the reverse reaction from entHH-2 to HH-2 occurs at the end of the racemization, the initial decay of HH-2 (enantiopurity > 70%) was used. The first-order rate constants kr (s−1) in the range of 170−190 °C were estimated using the following equation: ln([( +)‐HH‐2]t /[( +)‐HH‐2]0 ) = −k rt

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CONCLUSIONS We have reported the synthesis of hexapole helicenes that consist of [5]helicenes by Pd-catalyzed [2+2+2]cyclizations. The structures of HH-1 (C2-IV) and HH-2 (D3-II) were determined by single-crystal X-ray diffraction analysis, which revealed a profound distortion for the saddle-shaped isomer (HH-1), commensurate with the highest degree of twisting angle per benzene unit (35.7°) reported so far. Photophysical and electrochemical studies in combination with DFT calculations indicated narrow HOMO−LUMO gaps due to the accumulated [5]helicene subunits. Experimental and theoretical kinetic studies revealed an appropriate thermal isomerization process from HH-1 (C2-IV) to HH-2 (D3-II) among the 20 possible transition states based on a 6-fold helicity and 20 stereoisomers involving ten pairs of enantiomers. It can thus be concluded that the assembling of multiple helicities by [2+2+2]cycloadditions of helicenyl arynes significantly deformes the π-systems.

(4)

where [(+)-HH-2]0 is the initial ratio of the integration of [(+)-HH-2], and [(+)-HH-2]t is the ratio of [(+)-HH-2] at a certain time t during the racemization. Arrhenius and Eyring plots were constructed from using the experimental data in eqs 2 and 3. These plots provided the activation parameters ΔH‡ [35.0(5) kcal·mol−1], ΔS‡ [−1.5(11) cal·mol−1·K−1], Ea [35.9(5) kcal·mol−1], and ΔG‡ [35.4(8) kcal·mol−1] at 298 K (Figure S19). The most plausible pathway for the racemization of HH-2 is shown in Figure 11. To achieve the racemization of HH-2, it should be necessary to invert all six helicities, as all ten diastereomers of HHs are chiral isomers. Initially, the conversion should proceed via HH-2 (D3-II) → C2-III → C1-II → HH-1 (C2-IV) in reverse order relative to the thermal isomerization from HH-1 to HH-2 with an activation energy of 36.3 kcal·mol−1.22 The loss of the initial chiral information occurs via the inversion of the outer (H1) and inner (H4) helicities among C1-II, HH-1, ent-HH-1, and ent-C1-II as the conversion energies (22.1 and 25.5 kcal·mol−1) are smaller than that from HH-2 to HH-1 (36.3 kcal·mol−1). Finally, the transformation from ent-C1-II to ent-HH-2 occurs in the same manner as that from C1-II to HH-2, requiring the same

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b07113. Experimental procedures; characterization data; theoretical calculations on HHs and related compounds (PDF) 18519


Article


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Crystallographic data for HH-1 and HH-2 (CIF)

AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Soichiro Watanabe: 0000-0002-1227-3079 Shoko Kikkawa: 0000-0002-9390-5671 Ken Kamikawa: 0000-0002-7844-4993 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Grant-in-Aid for Scientific Research on Innovative Areas “Precisely Designed Catalysts with Customized Scaffolding” (#JP16H01039 to K.K.) from the JSPS (Japan), a Grant-in-Aid for Scientific Research (B) (#15H0370), a Grant-in-Aid for Challenging Exploratory Research (16K13956 to K.K.) from MEXT (Japan), and the MEXT-supported Program for the Strategic Research Foundation at Private Universities 2012-2016 (S1201034 to Toho Univ.) K.K. also thanks the Naito Foundation and the Yamada Science Foundation. The authors thank Prof. Ikuko Miyahara (Osaka City University), Dr. Shin-ichiro Kato (The University of Shiga Prefecture), and Dr. Hiroyasu Sato (Rigaku Corp.) for help with the X-ray crystallographic analysis. Prof. Hideki Fujiwara (Osaka Prefecture University) is gratefully acknowledged for providing access to CV, UV−vis, and fluorescence spectroscopy instrumentation. The authors also thank Prof. Satoshi Minakata and Prof. Youhei Takeda (Osaka University) for the measurement of absolute quantum yield and photoluminescence lifetime values, Dr. Daisuke Fujiwara (Osaka Prefecture University) for the measurement of CD spectra, as well as Prof. Toshio Asada (Osaka Prefecture University) and Prof. Takeaki Iwamoto (Tohoku University) for help with theoretical calculations. All theoretical calculations were carried out at the Research Center for Computational Science (Japan).

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Synthesis, Structures, and Properties of Hexapole Helicenes

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