Synthesis and Structure of a Propeller-Shaped Polycyclic Aromatic

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

Synthesis and Structure of a Propeller-Shaped Polycyclic Aromatic Hydrocarbon Containing Seven-Membered Rings Kazuya Kawai,† Kenta Kato,† Lingqing Peng,‡ Yasutomo Segawa,*,§,† Lawrence T. Scott,‡ and Kenichiro Itami*,†,§,∥ †

Graduate School of Science, Nagoya University, Chikusa, Nagoya 464-8602, Japan Department of Chemistry, Boston College, Chestnut Hill, Massachusetts 02467-3860, United States § JST, ERATO, Itami Molecular Nanocarbon Project, Chikusa, Nagoya 464-8602, Japan ∥ Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Chikusa, Nagoya 464-8602, Japan ‡

S Supporting Information *

ABSTRACT: The synthesis and structure of a C3-symmetric propeller-shaped polycyclic aromatic hydrocarbon that bears three seven-membered rings is reported. The synthesis was accomplished in three steps from benzo[c]naphtho[2,1-p]chrysene, including a Pd-catalyzed intramolecular C−H arylation for the formation of the seven-membered rings. The combination of the helicities (P/ M) of the three seven-membered-ring moieties and three [4]helicene moieties affords 24 possible conformers, and two relatively stable conformations were observed by 1H NMR spectroscopy.

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the difficulties associated with the formation of sevenmembered rings at late stages of the PAH synthesis. The early stage introduction of seven-membered rings into PAHs has been reported5 (for example, III in 2015).5a There are two reports on the synthesis of π-extended PAHs, in which sevenmembered rings are generated at the final step. In 2013, Durola and co-workers reported an unexpected rearrangement product containing a seven-membered ring (IV) via the Scholl reaction6 of tris(phenanthrophenyl)benzene.7 Our group has reported the synthesis of warped nanographene (V) via the Scholl reaction of pentakis(2-biphenylyl)corannulene.8 However, despite these numerous studies, a general method for the formation of seven-membered rings in PAHs remains elusive.9 To investigate the chemistry of negatively curved PAHs, such methods for the formation of seven-membered rings are highly desirable. Herein, we report the synthesis and structure of a C3symmetric propeller-shaped PAH that bears three sevenmembered rings and three [4]helicene moieties (1, Figure 2). This is the first example of the formation of a seven-membered ring in a PAH by intramolecular C−H arylation. A single-crystal X-ray diffraction analysis of 1 revealed a propeller-shaped structure and a gear-like dimeric packing structure. Two conformations of 1 were observed in solution at room temperature, and these interconverted upon heating to 140 °C. A density functional theory (DFT) study revealed the structure of the two conformations of 1 and the interconversion pathway between them.

olycyclic aromatic hydrocarbons (PAHs) that bear sevenmembered rings have garnered interest on account of their dynamic behavior, electronic properties, and packing modes derived from their negatively curved structures.1 Pioneering studies in this research area include the synthesis of hexa[7]circulene (I, Figure 1) by Jessup and co-workers in 19752 and the synthesis of [7]circulene (II) by Yamamoto and co-workers in 1983.3 Although several related molecules have since been reported,4 the synthesis of negatively curved PAHs had not been investigated until very recently, probably due to

Figure 1. Previously reported PAHs containing seven-membered rings. © XXXX American Chemical Society

Received: February 8, 2018

A

DOI: 10.1021/acs.orglett.8b00477 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

formation of seven-membered rings by cyclization, a simplified starting material 6 was subjected to the same cyclization conditions as a model reaction. In contrast to the reaction of 4, only a product that contains a six-membered ring (7) was obtained in 79% yield, and the seven-membered ring compound 8 was not observed (Scheme 1b). Considering these results, we conclude that the unusual formation of a seven-membered ring in 4 must be more favored than the formation of a six-membered ring, probably due to the steric repulsion between the [6]helicene moieties in the intermediates that would lead to 5. The threefold symmetric structure of 1 was unambiguously determined by X-ray crystallography. Single crystals of 1 suitable for X-ray crystallography were obtained from a solution of 1 in CS2. Two equivalents of CS2 per 1 were incorporated. As shown in Figure 3a, 1 exhibits a C3-symmetric propeller-

Figure 2. Structure and components of 1.

The synthesis of 1 was achieved in two steps from 2, which was prepared using a previously reported procedure.10 A Suzuki coupling reaction of 2 with 2-boryl-2′-chlorobiphenyl (3) catalyzed by Pd(OAc)2 and PPh3 afforded 4 (68%; Scheme 1a), Scheme 1. (a) Synthesis of 1; (b) Model Reaction for the Formation of Six- Or Seven-Membered Rings by a Direct Intramolecular Couplinga

Figure 3. (a) ORTEP drawing of 1 with thermal ellipsoids set to 50% probability; all hydrogen atoms and solvent molecules are omitted for clarity. (b) Dihedral angle between the outer (blue) and the inner (red) benzene rings. (c, d) Packing structure of 1.

shaped structure, in which three seven-membered-ring moieties are twisted in the same chiral sense. The twisting angle between the outer benzene rings (blue in Figure 3b) and the central benzene ring (red in Figure 3b) is 75.6°. In the packing structure (Figure 3c, d), the enantiomers of 1 (green and gray) are meshed with each other to form a gear-like dimeric structure, and the dimers are aligned two-dimensionally on the ab plane. In contrast to the simple C3-symmmetric structure of 1 in the crystalline state, the 1H NMR spectrum of 1 in deuterated 1,1,2,2-tetrachloroethane at room temperature showed a ca. 3:2 mixture of symmetric and relatively unsymmetric components, which fused to a single component upon heating to 140 °C. To gain insight into the conformational isomers of 1, a density functional theory (DFT) study was carried out at the B3LYP/631G(d) level of theory. Similar to previously reported multiple helicenes,13 compound 1 has many possible conformations as local minima. The combinations of the helicity (P/M) of the three [4]helicene moieties and the three seven-membered ring moieties of 1 (Figure 4a) afford 12 pairs of enantiomers, i.e. 24 isomers (A−L and A*−L* where X and X* are the pair of

a

Abbreviations: Bpin = 4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl, DMAc = N,N-dimethylacetamide.

which was subsequently subjected to a catalytic intramolecular C−H arylation reaction using PdCl2(PCy3)2, Cs2CO3, and pivalic acid11 to afford 1 in 26% yield. The threefold cyclization product 1 was observed only when pivalic acid was added. To the best of our knowledge, this is the first example of the formation of seven-membered rings in PAHs by a Pd-catalyzed direct coupling reaction.12 Although we initially expected the formation of three six-membered rings from the cyclization of 4, the corresponding product 5 was not observed under the reaction conditions. To gain insight into the generality of the B

DOI: 10.1021/acs.orglett.8b00477 Org. Lett. XXXX, XXX, XXX−XXX

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Figure 4. (a) Description of the helicities of [4]helicene and seven-membered-ring moieties. (b) All possible conformers of 1 (A−L) with relative Gibbs free energy (ΔG in kcal·mol−1). (c) Favored and disfavored combinations of the [4]helicene moieties. All calculations were performed at the B3LYP/6-31G(d) level of theory.

enantiomers) in total. We optimized the structures of 12 conformations (A−L), and the calculated Gibbs free energy values (kcal·mol−1) relative to that of the most stable conformer A (and A*), which is the conformer observed in the crystalline state, are summarized in Figure 4b. The relative energies clearly increase with increasing number of the “disfavored” conformation (Figure 4c): zero [A (0.0), B (0.5)], one [C (4.3), D (4.3), E (6.5), F (6.6)], two, [G (8.4), H (8.8), I (9.9), J (12.2)], and three [K (13.5), L (not obtained)]. Conformer L could not be obtained as a local minimum, presumably due to the instability arising from three disfavored moieties.14 Considering the low difference in energy between A and B (0.5 kcal·mol−1), B should be the minor conformer observed in the 1H NMR spectrum of 1 at room temperature. An extensive DFT study revealed the lowest interconversion pathway between A and B, i.e., A → TSAC → C → TSBC → B, wherein TSAC and TSBC correspond to the transition states (TSs) of the inversion of the helicities of the seven-membered ring and [4]helicene moiety, respectively, and the energy barriers (18.7 kcal·mol−1) are similar to each TS value of the pristine unit. Similarly, the racemization barrier of A was estimated to be 19.3 kcal·mol−1,15 which implies that chiral separation and isolation of 1 should be difficult. The thermodynamic behavior of 1 and the pristine benzo[c]naphtho[2,1-p]chrysene (9) are summarized in Figure 5. In contrast to 1, the minor conformer of 9 (9b) could not be observed in the 1H NMR spectrum, and this difference was ascribed to the energy barrier of interconversion between the PPP conformation (A or 9a) and the PPM conformation (B or 9b). Given that the inversion of the axial chirality of the biphenyl moiety (TSAC) represents the rate-determining step, it seems feasible to consider the three biphenyl moieties in 1 as “stoppers,” which delay the rate of interconversion between A and B, and, as a result, B can be observed. This result is important especially for stabilizing the helicity of helicene moieties. To gain insight into the effect of the seven-membered rings on the π-conjugation system of 1, the photophysical properties of 1 were measured as shown in Figure 6. Compound 10, the dechlorinated derivative of 4, was chosen as a reference compound. It was clearly seen that absorption and fluorescence spectra of 1 were shifted to a higher wavelength region compared with those of 10 reflecting the π-elongation effect of the seven-membered rings of 1. The fluorescence quantum

Figure 5. Thermodynamic behavior of 1 (a) and 9 (b). Energy values were calculated at the B3LYP/6-31G(d) level of theory.

Figure 6. UV−vis absorption (solid line) and fluorescence (broken line) of 1 and reference compound 10.

yield of 1 (ΦF = 0.10) was higher than that of 10 (ΦF = 0.03). DFT calculation also indicated the slightly narrower HOMO− LUMO gap of 1 (3.43 eV) compared with that of 10 (3.58 eV; see the Supporting Information for details), which may cause the bathochromic shift. C

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G. Polycyclic Aromatic Hydrocarbons; Wiley-VCH: New York, 1997. (c) Segawa, Y.; Ito, H.; Itami, K. Nat. Rev. Mater. 2016, 1, 15002. (d) Segawa, Y.; Maekawa, T.; Itami, K. Angew. Chem., Int. Ed. 2015, 54, 66. (e) Narita, A.; Wang, X.-Y.; Feng, X.; Mullen, K. Chem. Soc. Rev. 2015, 44, 6616. (2) Jessup, P. J.; Reiss, J. A. Tetrahedron Lett. 1975, 16, 1453. (3) Yamamoto, K.; Harada, T.; Nakazaki, M.; Naka, T.; Kai, Y.; Harada, S.; Kasai, N. J. J. Am. Chem. Soc. 1983, 105, 7171. (4) Yamamoto, K.; Saitho, Y.; Iwaki, D.; Ooka, T. Angew. Chem., Int. Ed. Engl. 1991, 30, 1173. (5) (a) Cheung, K. Y.; Xu, X.; Miao, Q. J. J. Am. Chem. Soc. 2015, 137, 3910. (b) Gu, X.; Li, H.; Shan, B.; Liu, Z.; Miao, Q. Org. Lett. 2017, 19, 2246. (c) Márquez, I. R.; Fuentes, N.; Cruz, C. M.; PuenteMuñoz, V.; Sotorrios, L.; Marcos, M. L.; Choquesillo-Lazarte, D.; Biel, B.; Crovetto, L.; Gómez-Bengoa, E.; González, M. T.; Martin, R.; Cuerva, J. M.; Campaña, A. G. Chem. Sci. 2017, 8, 1068. (d) Pun, S. H.; Chan, C. K.; Luo, J.; Liu, Z.; Miao, Q. Angew. Chem., Int. Ed. 2018, 57, 1581. (6) Grzybowski, M.; Skonieczny, K.; Butenschön, H.; Gryko, D. T. Angew. Chem., Int. Ed. 2013, 52, 9900. (7) Pradhan, A.; Dechambenoit, P.; Bock, H.; Durola, F. J. J. Org. Chem. 2013, 78, 2266. (8) (a) Kawasumi, K.; Zhang, Q.; Segawa, Y.; Scott, L. T.; Itami, K. Nat. Chem. 2013, 5, 739. (b) Kato, K.; Segawa, Y.; Scott, L. T.; Itami, K. Chem. - Asian J. 2015, 10, 1635. (9) Fu, W. C.; Wang, Z.; Chan, W. T. K.; Lin, Z.; Kwong, F. Y. Angew. Chem., Int. Ed. 2017, 56, 7166. (10) (a) Hagen, S.; Scott, L. T. J. J. Org. Chem. 1996, 61, 7198. (b) Hagen, S.; Bratcher, M. S.; Erickson, M. S.; Zimmermann, G.; Scott, L. T. Angew. Chem., Int. Ed. Engl. 1997, 36, 406. (11) (a) Chang, N.-H.; Mori, H.; Chen, X.-C.; Okuda, Y.; Okamoto, T.; Nishihara, Y. Chem. Lett. 2013, 42, 1257. (b) Chang, N.-H.; Chen, X.-C.; Nonobe, H.; Okuda, Y.; Mori, H.; Nakajima, K.; Nishihara, Y. Org. Lett. 2013, 15, 3558. (12) For the all-sp2-carbon seven-membered-ring formations of heteroatom-containing π-systems by palladium-catalyzed direct coupling reactions, see: (a) Hong, D.; Zhu, Y.-X.; Li, Y.; Lin, X.-F.; Lu, P.; Wang, Y.-G. Org. Lett. 2011, 13, 4668. (b) Brenner, M.; Mayer, G.; Terpin, A.; Steglich, W. Chem. - Eur. J. 1997, 3, 70. (c) Danel, K. S.; Uchacz, T.; Karelus, M. ARKIVOC 2011, 9, 272. (13) Review: (a) Li, C.; Yang, Y.; Miao, Q. Chem. Asian J. 2018, DOI: 10.1002/asia.201800073. Representative examples of multiple carbohelicenes: (b) Kato, K.; Segawa, Y.; Scott, L. T.; Itami, K. Angew. Chem., Int. Ed. 2018, 57, 1337. (c) Hosokawa, T.; Takahashi, Y.; Matsushima, T.; Watanabe, S.; Kikkawa, S.; Azumaya, I.; Tsurusaki, A.; Kamikawa, K. J. J. Am. Chem. Soc. 2017, 139, 18512. (d) Berezhnaia, V.; Roy, M.; Vanthuyne, N.; Villa, M.; Naubron, J.-V.; Rodriguez, J.; Coquerel, Y.; Gingras, M. J. J. Am. Chem. Soc. 2017, 139, 18508. (e) Saito, H.; Uchida, A.; Watanabe, S. J. J. Org. Chem. 2017, 82, 5663. (f) Fujikawa, T.; Segawa, Y.; Itami, K. J. J. Am. Chem. Soc. 2016, 138, 3587. (g) Yang, Y.; Yuan, L.; Shan, B.; Liu, Z.; Miao, Q. Chem. - Eur. J. 2016, 22, 18620. (h) Arslan, H.; Uribe-Romo, F. J.; Smith, B. J.; Dichtel, W. R. Chem. Sci. 2013, 4, 3973. (i) Xiao, S.; Kang, S. J.; Wu, Y.; Ahn, S.; Kim, J. B.; Loo, Y.-L.; Siegrist, T.; Steigerwald, M. L.; Li, H.; Nuckolls, C. Chem. Sci. 2013, 4, 2018. (j) Bennett, A. A.; Kopp, M. R.; Wenger, E.; Willis, A. C. J. J. Organomet. Chem. 2003, 667, 8. (k) Peña, D.; Cobas, A.; Pérez, D.; Guitián, E.; Castedo, L. Org. Lett. 2000, 2, 1629. (l) Barnett, L.; Ho, D. M.; Baldridge, K. K.; Pascal, R. A., Jr. J. Am. Chem. Soc. 1999, 121, 727. (m) Peña, D.; Peŕez, D.; Guitiań, E.; Castedo, L. Org. Lett. 1999, 1, 1555. (n) Hacker, N. P.; McOmie, J. F. W.; Meunier-Piret, J.; Van Meerssche, M. J. J. Chem. Soc., Perkin Trans. 1 1982, 19. (14) By using a scan method, the energy value for L was estimated to be ca. 14 kcal·mol−1 relative to that of A. See the Supporting Information for details. (15) An enantiomerization pathway from A to A*: A → TSAC → C → TSBC → B → TSBF → F → TSFB* → B* → TSB*C* → C* → TSA*C* → A*. See the Supporting Information for details.

In summary, we have synthesized and structurally characterized a C3-symmetric propeller-shaped PAH 1 that contains three seven-membered rings. This is the first example of the formation of seven-membered rings in a PAH by a Pd-catalyzed intramolecular C−H arylation reaction. A single-crystal X-ray diffraction analysis of 1 revealed a propeller-shaped structure and a gear-like dimeric packing structure. The optimization of the 24 possible conformations of 1 showed that the stability of each conformer is strongly correlated with the number of “disfavored” local conformations. Moreover, we discovered that the three seven-membered-ring moieties in 1 work as stabilization units for the helicity of the [4]helicene moieties. Further investigations into the physical properties of the heptagon−helicene hybrid π-systems are currently in progress in our laboratory.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b00477. Experimental details, spectra of new compounds (PDF) Cartesian coordinates of optimized structures (XYZ) Accession Codes

CCDC 1823019 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] (Y.S.). *E-mail: [email protected] (K.I.). ORCID

Yasutomo Segawa: 0000-0001-6439-8546 Lawrence T. Scott: 0000-0003-3496-8506 Kenichiro Itami: 0000-0001-5227-7894 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the ERATO program from JST (JPMJER1302 to K.I.), the Funding Program for KAKENHI from MEXT (16K05771 to Y.S.), a grant-in-aid for Scientific Research on Innovative Areas “π-Figuration” (17H05149 to Y.S.), the Noguchi Institute (to Y.S.), and the US National Science Foundation (CHE-0414066 to L.T.S.). K.K. thanks the IGER Program in Green Natural Sciences, Nagoya University and is grateful for a JSPS fellowship for young scientists. Calculations were performed using the resources of the Research Center for Computational Science, Okazaki, Japan. ITbM is supported by the World Premier International Research Center (WPI) Initiative, Japan.

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REFERENCES

(1) (a) Fragments of Fullerenes and Carbon Nanotubes: Designed Synthesis, Unusual Reactions, and Coordination Chemistry; Petrukhina, M. A., Scott, L. T., Eds.; Wiley-VCH: Weinheim, 2011. (b) Harvey, R. D

DOI: 10.1021/acs.orglett.8b00477 Org. Lett. XXXX, XXX, XXX−XXX