Letter pubs.acs.org/OrgLett
Synthesis of [8]Cycloparaphenylene-octacarboxylates via RhCatalyzed Stepwise Cross-Alkyne Cyclotrimerization Norihiko Hayase,† Yuta Miyauchi,† Yukimasa Aida,† Haruki Sugiyama,‡ Hidehiro Uekusa,‡ Yu Shibata,† and Ken Tanaka*,† †
Department of Chemical Science and Engineering and ‡Department of Chemistry, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8550, Japan S Supporting Information *
ABSTRACT: The synthesis of C4- and C2-symmetrical [8]cycloparaphenylene (CPP)-octacarboxylates has been achieved via macrocyclization by the rhodium-catalyzed intermolecular stepwise cross-cyclotrimerization and subsequent reductive aromatization. The C4-symmetrical octa-tertbutyl [8]CPP-octacarboxylate forms a dimer in which eight ester moieties face each other. The dimers are aligned so as to make a one-dimensional column with a channel structure inside. Both absorption and fluorescence maxima of [8]CPPoctacarboxylates in CHCl3 were significantly blue-shifted compared to those of [8]CPP due to the presence of eight electronwithdrawing ester moieties.
C
Scheme 1
ycloparaphenylenes (CPPs) have attracted considerable attention because of their fascinating structures, unique properties, and potential applications in materials science.1 One of the most interesting property of CPPs is the ring-size effect in frontier molecular orbitals. Decreasing the number of benzene rings decreases the HOMO−LUMO gap. This feature is in sharp contrast with linear p-phenylene oligomers, in which increasing the number of benzene rings decreases the HOMO− LUMO gap. Following the first synthesis of [9]-, [12]-, and [18]CPPs by Jasti and Bertozzi in 20082 and Itami in 2009,3 highly strained [5]−[8]CPPs have been successfully synthesized.4 For the application of CPPs in materials science, many efforts have been made to the synthesis of functionalized CPPs. Thus, various symmetrically heteroatom-substituted CPPs have been synthesized by Wang,5 Wegner,6 and Yamago.7 However, available functional groups were limited to less polar ones such as alkoxy groups. Recently, our research group reported the synthesis of a symmetrically multifunctionalized CPP, C3symmetrical hexa-tert-butyl [12]CPP-hexacarboxylate 1, via macrocyclization by the rhodium-catalyzed intermolecular cross-cyclotrimerization8,9 of a diyne, possessing two benzene units, with di-tert-butyl acetylenedicarboxylate and subsequent reductive aromatization (Scheme 1).10,11 After this report, Wang reported the synthesis of a more strained C3-symmetrical hexamethyl [9]CPP-hexacarboxylate, via macrocyclization by the nickel-mediated homocoupling and subsequent oxidative aromatization.12 To synthesize more strained and densely substituted CPPcarboxylates, we attempted the rhodium-catalyzed intermolecular cross-cyclotrimerization of a diyne, possessing no benzene units, with di-tert-butyl acetylenedicarboxylate, while not macrocycles but a substituted benzobarrelene was generated © 2017 American Chemical Society
(Scheme 2).13 In this paper, we disclose the synthesis of highly strained C4- and C2-symmetrical [8]CPP-octacarboxylates via macrocyclization by the rhodium-catalyzed intermolecular stepwise cross-cyclotrimerization and subsequent reductive aromatization. Received: April 24, 2017 Published: May 17, 2017 2993
DOI: 10.1021/acs.orglett.7b01231 Org. Lett. 2017, 19, 2993−2996
Letter
Organic Letters
mass spectrometry). The 1H and 13C NMR spectra revealed that the structures of these CPPs are highly symmetrical. A single crystal of [8]CPP-octacarboxylate 7aa, that is suitable for X-ray crystal structure analysis, was easily grown by vapor diffusion of n-hexane into a solution of 7aa in EtOAc at room temperature under air. The crystal structure of 7aa revealed that 7aa forms a dimer around the inversion center, in which eight ester moieties face each other (the red and magenta pair, and green and blue pair in Figure 1bc). In the dimer, the ester moieties engage with each other using the dispersion force interaction16 among tert-butyl groups (Figure S1). Diameter of the ring of 7aa is 9.9−11.2 Å (Figure S2), which is significantly smaller that that of 1 (16.4 Å).10a Torsion angles of neighboring phthalate and benzene rings of 7aa were 42.8− 47.9° (Figure S3), which are close to that of 1 (47.6°),10a but significantly larger than that of neighboring two benzene rings of nonsubstituted [8]CPP (30.7°).17 The dimers of 7aa are aligned along the crystallographic b-axis so as to make a column with a channel structure inside. In the column, there are gaps between the dimers, in which the ester groups of adjacent columns and solvent molecules are filled. Thus, the adjacent columns with concave-convex shape fit together like gears to be closely packed by dispersion force intermolecular interactions (Figure 1). As seen in the previous reports, inclusion of solvents within the ring of CPPs was frequently observed.4i,6,10,11 Indeed, disordered solvent molecules (EtOAc and nhexane) were incorporated within the ring of 7aa, although the precise assignment of the solvent molecules could not be made. The UV/vis absorption and fluorescence properties of 7aa, 7ba, and 7bb were compared with those of nonsubstituted [8]CPP (Table 1 and Figures S4−6).4j Both absorption and fluorescence maxima of 7aa, 7ba, and 7bb in CHCl3 were significantly blue-shifted compared to those of [8]CPP due to the presence of eight electron-withdrawing ester moieties. The absolute fluorescence quantum yields (ΦF) of 7aa, 7ba, and 7bb were 0.013−0.018, which are lower than that of [8]CPP (ΦF = 0.1).4h To understand the electronic structures of 7aa, a DFT calculation was performed using Gaussian 09 at the B3LYP/631G(d) level. Figure 2 shows the pictorial representations of six frontier molecular orbitals (MOs) of 7aa. The HOMO and LUMO are delocalized over the entire ring, whereas four other orbitals rather delocalize to two opposite sides. A comparison of the energy diagram of nonsubstituted [8]CPP to that of 7aa is shown in Figure 3. In 7aa, the orbitals of HOMO−1 and HOMO−2 as well as those of LUMO+1 and LUMO+2 are pseudo-degenerated. This feature is similar to [8]CPP.4k On the other hand, in comparison with [8]CPP, energy levels of HOMO, HOMO−1, and HOMO−2 of 7aa decrease significantly (0.26−0.45 eV), but that of LUMO is almost unchanged, and those of LUMO+1 and LUMO+2 decrease slightly (0.24−0.33 eV) presumably due to the presence of eight electron-withdrawing ester moieties, which results in decreasing electron density to decrease the HOMO and LUMO energies and enlarging torsion angles of neighboring benzene rings to decrease the HOMO energy and increase the LUMO energy.17 Thus, increasing the HOMO/LUMO gap (3.86 eV for 7aa vs 3.41 eV for [8]CPP) corresponds to the blue-shifted absorption maximum (301 nm for 7aa vs 340 nm for [8]CPP, Table 1). In summary, we disclosed the synthesis of C4- and C2symmetrical [8]CPP-octacarboxylates via macrocyclization by the rhodium-catalyzed intermolecular stepwise cross-cyclo-
Scheme 2
In order to avoid the formation of the undesired benzobarrelene, we designed the stepwise cross-cyclotrimerization strategy as shown in Scheme 3. The intermolecular crossScheme 3
cyclotrimerization of diyne 2,14 in which one terminal position is protected with the triisopropylsilyl group, and di-tert-butyl acetylenedicarboxylate (3a) in the presence of the cationic rhodium(I)/H8−BINAP catalyst proceeded at room temperature to give protected diyne 4a. Deprotection with TBAF (tetrabutylammonium fluoride) afforded terminal diyne 5a. The second intermolecular cross-cyclotrimerization of 5a and 3a afforded [8]CPP precursor 6aa. The subsequent reductive aromatization of 6aa with lithium naphthalenide15 afforded C4symmetrical octa-tert-butyl [8]CPP-octacarboxylate 7aa. In a similar way, C4-symmetrical octamethyl [8]CPP-octacarboxylate 7bb was also synthesized by using dimethyl acetylenedicarboxylate (3b) instead of 3a, although the overall yield of 7bb was very low. Importantly, C2-symmetrical tetra-tert-butyl and tetramethyl [8]CPP-octacarboxylate 7ba could be synthesized by using 3b in the first rhodium-catalyzed cross-cyclotrimerization and 3a in the second rhodium-catalyzed cross-cyclotrimerization. Compounds 7aa, 7ba, and 7bb were identified by 1 H and 13C NMR spectroscopy and HRMS (high-resolution 2994
DOI: 10.1021/acs.orglett.7b01231 Org. Lett. 2017, 19, 2993−2996
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Organic Letters
Table 1. Comparison of Photophysical Properties of [8]CPP-Octacarboxylates 7aa, 7ba, 7bb, and [8]CPPa CPP 7aa 7ba 7bb [8] CPP
UV absorption λmax (nm)
fluorescence λmax (nm) (excitation wavelength (nm))
ϕF (excitation wavelength (nm))
301 300 287 340b
403 (301) 406 (300) 406 (287) 533b
0.014 (280) 0.013 (300) 0.018 (300) 0.1c
Measured in CHCl3 at 25 °C unless otherwise indicated. At 1.0 × 10−5 M. bData of ref 4j. cData of ref 4h.
a
Figure 2. Pictorial representation of six frontier molecular orbitals of 7aa, calculated at the B3LYP/6-31G(d) level of theory.
Figure 3. Energy diagrams for MOs of nonsubstituted [8]CPP (left) and 7aa (right). Two-way arrows represent HOMO−LUMO gaps.
channel filled with solvent molecules. Both absorption and Figure 1. Packing structure of 7aa determined by X-ray crystallographic analysis: (a) viewed along the columnar direction; (b) viewed along the row direction (b) in (a). (c) Viewed along the row direction (c) in (a).
fluorescence maxima of [8]CPP-octacarboxylates in CHCl3 were significantly blue-shifted compared to those of [8]CPP due to the presence of eight electron-withdrawing ester
trimerization and subsequent reductive aromatization. C4symmetrical octa-tert-butyl [8]CPP-octacarboxylate forms a dimer in which eight ester moieties face each other. The dimers are aligned so as to make a column with one-dimensional
moieties. Future works will focus on further synthesis and utilization of a variety of symmetrically multifunctionalized cycloparaphenylenes. 2995
DOI: 10.1021/acs.orglett.7b01231 Org. Lett. 2017, 19, 2993−2996
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Organic Letters
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(5) Huang, C.; Huang, Y.; Akhmedov, N. G.; Popp, B. V.; Petersen, J. L.; Wang, K. K. Org. Lett. 2014, 16, 2672. (6) Tran-Van, A.-F.; Huxol, E.; Basler, J. M.; Neuburger, M.; Adjizian, J.-J.; Ewels, C. P.; Wegner, H. A. Org. Lett. 2014, 16, 1594. (7) (a) Kuroda, Y.; Sakamoto, Y.; Suzuki, T.; Kayahara, E.; Yamago, S. J. Org. Chem. 2016, 81, 3356. (b) Kayahara, E.; Zhai, X.; Yamago, S. Can. J. Chem. 2017, 95, 351. (8) (a) Tanaka, K.; Shirasaka, K. Org. Lett. 2003, 5, 4697. (b) Tanaka, K.; Toyoda, K.; Wada, A.; Shirasaka, K.; Hirano, M. Chem. - Eur. J. 2005, 11, 1145. (9) For reviews, see: (a) Tanaka, K. Synlett 2007, 2007, 1977. (b) Shibata, Y.; Tanaka, K. Synthesis 2012, 44, 323. (c) TransitionMetal-Mediated Aromatic Ring Construction; Tanaka, K., Ed.; Wiley: Hoboken, 2013; Chapter 4, p 127. (10) (a) Miyauchi, Y.; Johmoto, K.; Yasuda, N.; Uekusa, H.; Fujii, S.; Kiguchi, M.; Ito, H.; Itami, K.; Tanaka, K. Chem. - Eur. J. 2015, 21, 18900. (b) Nishigaki, S.; Miyauchi, Y.; Noguchi, K.; Ito, H.; Itami, K.; Shibata, Y.; Tanaka, K. Eur. J. Org. Chem. 2016, 2016, 4668. (11) Our research group recently reported the synthesis of alternating donor-acceptor cycloparaphenylenes via the rhodiumcatalyzed intermolecular cross-cyclotrimerization. See: Nishigaki, S.; Fukui, M.; Kawauchi, S.; Sugiyama, H.; Uekusa, H.; Shibata, Y.; Tanaka, K. Chem. - Eur. J. 2017, 23, x. (12) Li, S.; Huang, C.; Thakellapalli, H.; Farajidizaji, B.; Popp, B. V.; Petersen, J. L.; Wang, K. K. Org. Lett. 2016, 18, 2268. (13) Miyauchi, Y.; Shibata, Y.; Tanaka, K. Chem. Lett. 2016, 45, 86. (14) Sankararaman, S.; Srinivasan, M. Org. Biomol. Chem. 2003, 1, 2388. (15) The use of sodium naphthalenide instead of lithium naphthalenide lowered the yield of 7aa. For the optimization of the reductive aromatization step, see the Supporting Information. (16) For a review, see: Wagner, J. P.; Schreiner, P. R. Angew. Chem., Int. Ed. 2015, 54, 12274. (17) Segawa, Y.; Fukazawa, A.; Matsuura, S.; Omachi, H.; Yamaguchi, S.; Irle, S.; Itami, K. Org. Biomol. Chem. 2012, 10, 5979.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b01231. Experimental procedures and compound characterization data (PDF) X-ray crystallographic file for (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Ken Tanaka: 0000-0003-0534-7559 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported partly by ACT-C (No. JPMJCR1122YR) from the Japan Science and Technology Agency (Japan) and Grants-in-Aid for Scientific Research (Nos. 15H03775 and 26102004) from the Ministry of Education, Culture, Sports, Science and Technology (Japan). We thank Prof. Keisuke Suzuki and Prof. Ken Ohmori (Tokyo Institute of Technology) for collecting the 13C NMR spectrum of 7bb, Prof. Takashi Ishizone (Tokyo Institute of Technology) for his valuable advice for the preparation of sodium and lithium naphthalenides, and Mr. Shuhei Nishigaki (Tokyo Institute of Technology) for his assistance for the analysis of the packing structures of the octa-tert-butyl [8]CPP-octacarboxylate. We also thank Takasago International Corp. for the gift of H8− BINAP and Umicore for generous support in supplying the rhodium complex.
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REFERENCES
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DOI: 10.1021/acs.orglett.7b01231 Org. Lett. 2017, 19, 2993−2996
