Synthesis and Redox Properties of Pyrrole- and Azulene-Fused

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

pubs.acs.org/OrgLett

Synthesis and Redox Properties of Pyrrole- and Azulene-Fused Azacoronene Yoshiki Sasaki,† Masayoshi Takase,*,† Tetsuo Okujima,† Shigeki Mori,‡ and Hidemitsu Uno*,† †

Graduate School of Science and Engineering, Ehime University, Matsuyama 790-8577, Japan Advanced Research Support Center, Ehime University, Matsuyama 790-8577, Japan

‡

Org. Lett. Downloaded from pubs.acs.org by UNIV OF TEXAS AT DALLAS on 03/05/19. For personal use only.

S Supporting Information *

ABSTRACT: Synthesis of an azacoronene, in which both pyrrole and azulene moieties are circularly fused, was achieved just in three steps. This new azacoronene exhibited multistep reversible oxidations under electrochemical and chemical conditions. Formation of an aromatic 22π-electron conjugation and a tropylium cation (6π-electron conjugation) in the dicationic state was revealed by the single-crystal X-ray crystallographic analysis as well as the nucleus-independent chemical shift (NICS) and anisotropy of the induced current density (ACID) calculations.

S

ynthesis and characterization of polycyclic aromatic hydrocarbons (PAHs) with well-defined structures is one of the important research areas to explore next-generation materials.1,2 Similarly, investigation of polycyclic heteroaromatic molecules (PHAs) is also of importance in terms of constructing novel πelectron systems. The incorporation of heteroatoms likely offers other synthetic opportunities that could not be possible with PAHs and promotes π-electron delocalization, leading to the stabilization of charges and spins. Based on this viewpoint, several kinds of pyrrole-embedded PHAs have been reported recently, which showed drastically changed physical properties.3−6 In one of the pioneering approaches, pyrrole-fused azacoronene, i.e., hexapyrrolohexaazacoronene (HPHAC) 1a, was synthesized and characterized, where six pyrrole rings were fused to a coronene core (Figure 1).4 Due to the circularly connected pyrrole rings, oxidized species were reversibly obtained, exhibiting global aromaticity in the dicationic state by forming a 22π-electron conjugation. This stable redox property is retained for analogue 2, in which one pyrrole is replaced by a dialkoxybenzene.5 The HPHAC analogue 2 forms several oxidized species reversibly, but possesses weaker global aromaticity than 1a in its dicationic state owing to the slight inhibition of the cyclic π-conjugation caused by the replaced sixmembered ring. In this study, azulene-fused azacoronene 3 (AzAC) was designed and synthesized, since azulene can be regarded as a carbon analogue of pyrrole because of its five-membered ring.7 The inherent electron-rich nature of the five-membered ring of azulene would favor its oxidative coupling (Scholl reaction) with hexaarylbenzene to produce a disc-like structure. Moreover, the oxidized form of 3 can be stabilized by the electron-deficient seven-membered ring of azulene by forming an aromatic tropylium cation, as shown in Figure 1. Although several kinds © XXXX American Chemical Society

Figure 1. Molecular structures of previously reported HPHACs 1a and 1b, their dialkoxybenzene-containing analogue 2, and azulenecontaining analogue 3 (AzAC).

of azulene-containing π-systems have been reported to date, PAH analogues that contain azulene, except for porphyrinoids, are scarcely reported.8,9 Carbocations based on hydrocarbons are generally very reactive. In the case of unsubstituted azulene, 1-, 3-, 5-, and 7positions are electron-rich and strongly favor electrophilic Received: February 8, 2019

A

DOI: 10.1021/acs.orglett.9b00515 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

features of the azulene moiety. Moreover, a negligible photoluminescence is observed for 3a. To further investigate the electronic properties, DFT calculations were performed for 3b, taking 1b as a reference (Figure 3). The HOMO of 3b localizes not only in the

substitution to build carbon−carbon bonds. Positions 1 and 3 are structurally analogous to the α-positions in pyrrole and, thus, facilitate the intramolecular Scholl reaction between pyrrole and azulene moieties. In fact, DFT calculations for 6b support localized MOs from HOMO to HOMO−4 at those positions (Figure S14 in the Supporting Information (SI)). Introduction of a tert-butyl group on the 6-position enhances both the kinetic and thermodynamic stabilities of azulene. Based on these predictions, AzAC 3 was synthesized in three steps from the reported Bpin-azulene 4 (Scheme 1).10 Precursor 6a was Scheme 1. Synthetic Procedures for AzAC 3 and Its Dication 32+

Figure 3. MO diagrams of 3b (left) and 1b (right) calculated at the B3LYP/6-31G(d,p) level of theory.

peripheral pyrrole rings but also in the azulene moiety, which is quite similar to that in 1b. On the other hand, LUMO and LUMO+1 are concentrated in the azulene moiety as opposed to that in 1b, where they are delocalized over the whole molecule, suggesting the partial intramolecular charge transfer (ICT) character of 3b. In fact, positive solvatochromism was slightly observed for 3b in cyclohexane, toluene, THF, dichloromethane, carbon disulfide, and pyridine (Figure S5 and Table S1 in the SI). According to the time-dependent (TD)-DFT calculations, the weak, broad absorption band centered at around 800 nm is mainly attributed to the HOMO → LUMO transition with a low oscillator strength (Table S3(a) in the SI). The HOMO−LUMO gap of 3b is much smaller than that of 1b because of the lowered LUMO level. The redox properties of 3a were investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) (Table 1 and Figure S6 in the SI). As observed for 1a and other HPHAC-related compounds, the CV of 3a exhibited three reversible oxidation waves under the diluted condition (0.09 mM), with a relatively large difference (0.66 V) between the second and third oxidation potentials (ΔEox3−ox2). This value is

prepared by the Suzuki−Miyaura cross-coupling of 4 with bromopentafluorobenzene, resulting in coupling adduct 5, followed by its nucleophilic aromatic substitution (SNAr) reaction with 3,4-diethylpyrrole. Scholl reaction of 6a with FeCl3 gave 3a smoothly in a moderate yield. The UV−vis−NIR spectra of 3a, 5, and 6a in toluene are shown in Figure 2. Compounds 5 and 6a exhibit weak, broad absorptions from 720 to 440 nm with intense peaks at 369 and 389 nm, respectively. AzAC 3a displays a drastically perturbed spectrum with similar broadened bands up to 950 nm owing to the effective π-conjugation, which reflects the distinctive

Table 1. Oxidation Potentials (Eox) of 3a, 1a, and 2 in CH2Cl2 (vs Fc/Fc+)

3a 1a4 25

Figure 2. Absorption spectra of 3a, 5, and 6a in toluene. B

E1/2ox1 (V)

E1/2ox2 (V)

E1/2ox3 (V)

ΔE1/2ox3−ox2 (V)

−0.43 0.04 0.03

−0.07 0.23 0.22

0.59 1.23 0.88

0.66 1.00 0.66


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Organic Letters exactly the same as that of dialkoxybenzene-substituted 25 and smaller than that of HPHAC 1a, indicating that the dications are stabilized (Table 1). Notably, when the concentration was increased (0.47 mM), the first oxidation wave became broad, suggesting the formation of a mixed-valence (MV) complex between a neutral species and a radical cation.11 To study the redox properties of 3a, its oxidative titration with antimony pentachloride was conducted. When the oxidant was added incrementally to the CH2Cl2 solution of 3a, the color of the solution gradually changed from brown to turquoise and then to blue in two steps, with several isosbestic points in the absorption spectra, respectively (Figure S7 in the SI). In contrast to the above-mentioned MV complex formation, these spectral changes strongly support the sequential oxidation from a neutral species to a radical cation and then to a dication. Neither a disproportion reaction of two radical cations into a neutral species and a dication nor a π-dimer formation between two radical cations was observed under the measurement conditions. Figure 4 illustrates the spectra of 3•+ and 32+, which, in

Figure 5. X-ray crystal crystallographic structure of 3a2+[PF6−]2 (top and side views) with thermal ellipsoids drawn at 50% probability level.

Table 2. Averaged Bond Lengths of 3a2+, 1a, and 1a2+ in the Xray Crystal Structures [Å]a 3a2+[PF6−]2 1a2+[SbCl6−]24 1a4

Cα−Cα

Cα−Cβ

Cβ−Cβ

1.447(6) 1.45(4) 1.480(9)

1.426(4) 1.42(3) 1.390(7)

1.402(5) 1.40(5) 1.44(1)

a

Averaged values are shown with standard deviations calculated by the following equation: {∑(xi-)2/(n-1)}1/2

moiety of 3a2+ is longer than that in pristine azulene (1.397(1) Å), with a smaller bond length alternation (BLA) of the sevenmembered ring; the averaged bond lengths are 1.390(4) Å and 1.391(21) Å for 3a2+ and pristine azulene, respectively (Table S2 in the SI). These findings are indicative of the formation of an aromatic tropylium cation as observed in the case of azuliporphyrin.7b,8 Thereafter, nucleus independent chemical shift (NICS) calculations12 were performed to evaluate the aromatic character of 3b and 3b2+ (Figure 6). The NICS values, especially

Figure 4. Absorption spectra of 3a•+ and 3a2+ in CDCl3 (2.34 × 10−5 M).

agreement with the TD-DFT results, cover the whole visible and NIR regions up to approximately 2500 and 2000 nm, respectively (Figure S12 and Table S3(b,c) in the SI). The radical cation 3•+ is ESR-active and shows a broad but flat signal (g = 2.0024) due to the delocalization of its spin (Figure S8 in the SI). To investigate the fundamental properties, dication 32+ was prepared by the treatment of 3 with AgPF6 in CH2Cl2. The 1H NMR resonances of the azulene moiety of 3a2+[PF6−]2 appeared at 9.26 and 7.70 ppm with considerable downfield shifts of 0.70 and 0.64 ppm, respectively (Figure S9 in the SI). A similar trend was also observed for the ethyl groups at the periphery. These shifts are ascribable to the decrease in the electron density of both the azacoronene core and the seven-membered ring of azulene by forming the resonance structures depicted in Figure 1. Vapor diffusion of hexane into the chlorobenzene solution of 3a2+[PF6−]2 resulted in its single crystals. In its molecular structure, as determined by the X-ray diffraction analysis, 3a2+[PF6−]2 adopted a slightly distorted structure (Figure 5). The mean plane deviation (MPD) of the azacoronene core is 0.157 Å, which is larger than that of HPHAC 1a (0.050 Å). Table 2 summarizes the selected bond lengths of Cα−Cα, Cα− Cβ, and Cβ−Cβ of pyrrole moieties of 3a2+, 1a2+, and 1a, respectively. As observed for HPHAC, the quinoidal resonance structures contribute significantly to stabilize the cationic charges at the HPHAC core. In addition, the averaged bond length of 1.428(1) Å of C1−C8a and C3−C3a in the azulene

Figure 6. NICS(0) and (NICS(1)zz) values of 3b and 3b2+ calculated by GIAO/HF/6-311+G(d,p)//B3LYP/6-31G(d,p) level of theory. Point “J” refers to the point on the opposite side of “A”, at the same distance from the center point “E”.

NICS(1)zz, at points C, E, F, and G are drastically decreased in the dicationic state (3b2+), indicating the appearance of global aromaticity. A similar phenomenon can be seen at points D, H, and I, although the value at D increased slightly. However, the increase in the NICS values of the dicationic states becomes evident for point B. This is probably due to the antishielding effect from both the azacoronene core with global aromaticity and the aromatic tropylium cation. The NICS value at point A is expected to decrease owing to the aromaticity of the tropylium cation. However, the value increased, meaning that the global aromaticity based on the 22π-cyclic conjugation was strongly affected even at point A. In fact, this phenomenon is also C


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Organic Letters

(2) (a) Takeya, J.; Yamagishi, M.; Tominari, Y.; Hirahara, R.; Nakazawa, Y.; Nishikawa, T.; Kawase, T.; Shimoda, T.; Ogawa, S. Appl. Phys. Lett. 2007, 90, 102120. (b) Kelley, T. W.; Boardman, L. D.; Dunbar, T. D.; Muyres, D. V.; Pellerite, M. J.; Smith, T. P. J. Phys. Chem. B 2003, 107, 5877. (3) (a) Chen, F.; Hong, Y. S.; Shimizu, S.; Kim, D.; Tanaka, T.; Osuka, A. Angew. Chem., Int. Ed. 2015, 54, 10639. (b) Nagata, Y.; Kato, S.; Miyake, Y.; Shinokubo, H. Org. Lett. 2017, 19, 2718. (c) Yokoi, H.; Hiraoka, Y.; Hiroto, S.; Sakamaki, D.; Seki, S.; Shinokubo, H. Nat. Commun. 2015, 6, 8215. (d) Ito, S.; Tokimaru, Y.; Nozaki, K. Angew. Chem., Int. Ed. 2015, 54, 7256. (e) Higashibayashi, S.; Pandit, P.; Haruki, R.; Adachi, S.; Kumai, R. Angew. Chem., Int. Ed. 2016, 55, 10830. (f) Gońka, E.; Chmielewski, P. J.; Lis, T.; Stępień, M. J. Am. Chem. Soc. 2014, 136, 16399. (g) Ż yła, M.; Gońka, E.; Chmielewski, P. J.; Cybińska, J.; Stępień, M. Chem. Sci. 2016, 7, 286. (h) Ż yłaKarwowska, M.; Zhylitskaya, H.; Cybińska, J.; Lis, T.; Chmielewski, P. J.; Stępień, M. Angew. Chem., Int. Ed. 2016, 55, 14658. (4) Takase, M.; Enkelmann, V.; Sebastiani, D.; Baumgarten, M.; Müllen, K. Angew. Chem., Int. Ed. 2007, 46, 5524. (5) Takase, M.; Narita, T.; Fujita, W.; Asano, M. S.; Nishinaga, T.; Benten, H.; Yoza, K.; Müllen, K. J. Am. Chem. Soc. 2013, 135, 8031. (6) Oki, K.; Takase, M.; Mori, S.; Shiotari, A.; Sugimoto, Y.; Ohara, K.; Okujima, T.; Uno, H. J. Am. Chem. Soc. 2018, 140, 10430. (7) (a) Xin, H.; Gao, X. ChemPlusChem 2017, 82, 945. (b) Lash, T. D. Acc. Chem. Res. 2016, 49, 471. (c) Ito, S.; Morita, N. Eur. J. Org. Chem. 2009, 2009, 4567. (8) For examples of azulene-containing porphyrinoids, see: (a) Lash, T. D.; Chaney, S. T. Angew. Chem., Int. Ed. Engl. 1997, 36, 839. (b) Sprutta, N.; Świderska, M.; Latos-Grażyński, L. J. Am. Chem. Soc. 2005, 127, 13108. (c) Kurotobi, K.; Kim, K. S.; Noh, S. B.; Kim, D.; Osuka, A. Angew. Chem., Int. Ed. 2006, 45, 3944. (d) Zhang, Z.; Ferrence, G. M.; Lash, T. D. Org. Lett. 2009, 11, 101. (e) Muranaka, A.; Yonehara, M.; Uchiyama, M. J. Am. Chem. Soc. 2010, 132, 7844. (f) Okujima, T.; Kikkawa, T.; Nakano, H.; Kubota, H.; Fukugami, N.; Ono, N.; Yamada, H.; Uno, H. Chem. - Eur. J. 2012, 18, 12854. (g) Sprutta, N.; Wełnic, M.; Białek, M. J.; Lis, T.; Szterenberg, L.; LatosGrażyński, L. Chem. - Eur. J. 2016, 22, 6974. (h) Lash, T. D.; Fosu, S. C.; Smolczyk, T. J.; AbuSalim, D. I. J. Org. Chem. 2018, 83, 12619. (9) (a) Ito, S.; Nomura, A.; Morita, N.; Kabuto, C.; Kobayashi, H.; Maejima, S.; Fujimori, K.; Yasunami, M. J. Org. Chem. 2002, 67, 7295. (b) Xin, H.; Ge, C.; Yang, X.; Gao, H.; Yang, X.; Gao, X. Chem. Sci. 2016, 7, 6701. (c) Koide, T.; Takesue, M.; Murafuji, T.; Satomi, K.; Suzuki, Y.; Kawamata, J.; Terai, K.; Suzuki, M.; Yamada, H.; Shiota, Y.; Yoshizawa, K.; Tani, F. ChemPlusChem 2017, 82, 1010. (d) Murai, M.; Iba, S.; Ota, H.; Takai, K. Org. Lett. 2017, 19, 5585. (e) Konishi, A.; Morinaga, A.; Yasuda, A. Chem. - Eur. J. 2018, 24, 8548. (f) Uehara, K.; Mei, P.; Murayama, T.; Tani, F.; Hayashi, H.; Suzuki, M.; Aratani, N.; Yamada, H. Eur. J. Org. Chem. 2018, 2018, 4508. (g) Jiang, Q.; Tao, T.; Phan, H.; Han, Y.; Gopalakrishna, T. Y.; Herng, T. S.; Li, G.; Yuan, L.; Ding, J.; Chi, C. Angew. Chem., Int. Ed. 2018, 57, 16737. (10) (a) Kurotobi, K.; Miyauchi, M.; Takakura, K.; Murafuji, T.; Sugihara, T. Eur. J. Org. Chem. 2003, 2003, 3663. (b) Narita, M.; Murafuji, T.; Yamashita, S.; Fujinaga, M.; Hiyama, K.; Oka, Y.; Tani, F.; Kamijo, S.; Ishiguro, K. J. Org. Chem. 2018, 83, 1298. (11) (a) Nakamura, K.; Takashima, T.; Shirahata, T.; Hino, S.; Hasegawa, M.; Mazaki, Y.; Misaki, Y. Org. Lett. 2011, 13, 3122. (b) Takase, M.; Yoshida, N.; Nishinaga, T.; Iyoda, M. Org. Lett. 2011, 13, 3896. (12) (a) Schleyer, P. v. R.; Maerker, C.; Dransfeld, A.; Jiao, H.; Hommes, N. J. R. v. E. J. Am. Chem. Soc. 1996, 118, 6317. (b) Chen, Z.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; Schleyer, P. v. R. Chem. Rev. 2005, 105, 3842. (13) (a) Herges, R.; Geuenich, D. J. Phys. Chem. A 2001, 105, 3214. (b) Geuenich, D.; Hess, K.; Köhler, F.; Herges, R. Chem. Rev. 2005, 105, 3758.

observed at point J, where the value moderately increased from the neutral species to the dication. To support this explanation, anisotropy of the induced current density (ACID) calculations13 were further performed for 3b and 3b2+. In both the molecules, there is a competition between the differently oriented currents on the outer rim versus those on the individual inner rings. In the neutral form, the intensities of the two opposite currents are about the same, leading to the cancellation of their induced magnetic fields. However, in the dicationic state, the outer rims of the azacoronene core and the seven-membered ring appear stronger, exhibiting two aromatic cyclic conjugations (Figure S13 in SI). In summary, pyrrole- and azulene-fused AzAC 3 was successfully synthesized just in three steps including the oxidative coupling between pyrrole and the five-membered ring of azulene. Similar to the parent HPHAC 1, 3 also displayed stable redox properties. Dication 32+ was successfully isolated, and the structure was unambiguously revealed. Its crystal structure analysis with the aid of NICS and ACID calculations demonstrated the existence of a 22π-electron conjugation at the azacoronene core and an aromatic tropylium cation. To achieve effective cojugation, which is often essential for the creation of organic functional materials, the control of charge resonance is an important design guide. The present contribution would lead to further study on PAH chemistry. Study on such systems containing charge resonances in PHA is currently underway in our laboratory.

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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/acs.orglett.9b00515. Experimental procedures and spectroscopic data, results of theoretical calculations, and crystallographic data (PDF) Accession Codes

CCDC 1896067 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Masayoshi Takase: 0000-0002-9737-9779 Shigeki Mori: 0000-0001-6731-2357 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research (C) (JP16K05698) from MEXT, Japan. We thank Prof. Ohara (Ehime Univ.) for the ESR measurements.

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REFERENCES

(1) Wu, J.; Pisula, W.; Müllen, K. Chem. Rev. 2007, 107, 718. D


Synthesis and Redox Properties of Pyrrole- and Azulene-Fused

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