Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Rhodium-Catalyzed Synthesis, Crystal Structures, and Photophysical Properties of [6]Cycloparaphenylene Tetracarboxylates Norihiko Hayase,† 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
Downloaded via UNIV OF MELBOURNE on April 11, 2019 at 13:13:04 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information *
ABSTRACT: The synthesis of C2-symmetrical [6]cycloparaphenylene (CPP) tetracarboxylates has been achieved via macrocyclization by the rhodium-catalyzed intermolecular stepwise cross-alkyne cyclotrimerization and subsequent reductive aromatization. The 1H NMR spectra of the thus-obtained C2-symmetrical [6]CPP-tetracarboxylates revealed that the rotation of unsubstituted benzene rings is slow at room temperature. These [6]CPPs formed columnar packing structures, and their absorption maxima were significantly blue-shifted compared to that of nonsubstituted [6]CPP due to the presence of four electron-withdrawing ester moieties.
C
ycloparaphenylenes (CPPs) have attracted much attention because of their potential applications in materials science.1,2 The systematic synthesis of the CPPs with various sizes3 revealed that ring sizes of the CPPs change physical properties and crystal-packing motifs. For physical properties, decreasing the number of benzene rings decreases the HOMO−LUMO gap, which is opposite to linear p-phenylene oligomers.4 For crystal-packing motifs, very small [6]CPP forms a columnar structure in the crystalline state,3g,5 while other sizes of the CPPs do not form the columnar structure in the crystalline state. Not only ring sizes but also substituents of the CPPs change their properties.6−15 For physical properties, Itami reported the synthesis of donor−acceptor CPP derivatives containing an anthraquinone or a tetracyanoanthraquinodimethane moiety that show solvatochromic properties.7−9 For crystal-packing motifs, our research group reported the synthesis of C3-symmetrical [12]CPP-hexacarboxylate, which forms the nanotube-like columnar packing structures in the crystalline state and on the flat solid surface, via rhodium-catalyzed intermolecular cross-cyclotrimerization10,11 followed by reductive aromatization (Figure 1, top left).12 After this report, Wang reported the synthesis of more strained C3-symmetrical [9]CPP-hexacarboxylate via the nickel-mediated homocoupling followed by oxidative aromatization (Figure 1, top right).13 Recently, our research group succeeded in the synthesis of C4-symmetrical [8]CPP-octacarboxylates via the rhodium-catalyzed stepwise intermolecular cross-cyclotrimerization followed by reductive aromatization (Figure 1, bottom left).14 These carboxylated [9]- and [8]CPPs also form © XXXX American Chemical Society
Figure 1. Carboxylated CPPs with various ring sizes.
the columnar packing structures in the crystalline state.12−14 In this paper, we disclose the rhodium-catalyzed synthesis, crystal Received: March 6, 2019
A
DOI: 10.1021/acs.orglett.9b00820 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
Single crystals of [6]CPP-octacarboxylates 1aa and 1bb that are suitable for X-ray crystal structure analyses were easily grown by vapor diffusion of n-hexane into solutions of 1aa and 1bb in EtOAc at room temperature under air. Their unimolecular and packing structures are shown in Figures 2
structures, and photophysical properties of highly strained [6]CPP-tetraboxylates (Figure 1, bottom right). The synthesis of [6]CPP-tetracarboxylates 1 is shown in Scheme 1. The stepwise 1,2-addition of lithium (trimethysilyl)Scheme 1. Synthesis of [6]CPP-tetracarboxylates 1aa and 1bb
Figure 2. X-ray crystallographic analysis of 1aa: Unimolecular structures [top view (a) and side view (b)] and packing structures [top view (c) and side view (d)].
acetylide and 4-((triisopropylsilyl)ethynyl)phenyllithium, derived from 3, to p-benzoquinone (2) afforded disilylated diyne 4. Dimethylation followed by desilylation afforded monosilylated diyne 5. The cationic rhodium(I)/H8−BINAP complexcatalyzed cross-cyclotrimerization of 5 with di-tert-butyl acetylenedicarboxylate (6a) followed by desilylation with TBAF (tetrabutylammonium fluoride) afforded U-shaped terminal diyne 7a. In 7a, two terminal alkyne moieties are very close to each other. Therefore, 7a reacted with one molecule of 6a in the presence of the same rhodium(I) catalyst to give [6]CPP precursor 8aa. The subsequent reductive aromatization of 8aa with sodium naphthalenide afforded tetra-tert-butyl [6]CPP-tetracarboxylate 1aa. Similarly, tetramethyl [6]CPP-tetracarboxylate 1bb was also synthesized by using U-shaped terminal diyne 7b and dimethyl acetylenedicarboxylate (6b) instead of 7a and 6a, although the overall yield of 1bb was lower than that of 1aa. The yields of 8aa and 8bb were lower than those of 7a and 7b as a result of the strain-relieving rearrangement of the 1,4-dimethoxycyclohexadiene templates12 catalyzed by the rhodium(I) complex as well as the intermolecular oligomerization of 7a and 7b. The 1 H NMR spectra revealed that the rotation of unsubstituted benzene rings is slow at room temperature on the time scale of the 1H NMR measurement, and thus, the broadening of the unsubstituted benzene protons was observed.
Figure 3. X-ray crystallographic analysis of 1bb: unimolecular structures [top view (a) and side view (b)] and packing structures [side view (c) and top view (d)].
and 3. The diameters of 1aa and 1bb are 7.96−8.27 and 8.07− 8.24 Å, respectively, and thus, these [6]CPPs are almost circular (Figures 2a and 3a). Torsion angles of neighboring phthalate and benzene rings of 1aa and 1bb are 31.9−38.0° and 31.9−36.7°, respectively (Figures 2b and 3b), which are significantly smaller than those of [12]CPP-(CO2t-Bu)6 (47.6°)12a and [8]CPP-(CO2t-Bu)8 (42.8−47.9°).14 Importantly, two phthalate moieties of 1aa and 1bb point in the opposite direction to the CPP ring (Figures 2b and 3b). This feature is in sharp contrast to the crystal structures of [12]CPP-(CO2t-Bu)612a and [8]CPP-(CO2t-Bu)8,14 in which all phthalate moieties point in the same direction, whereas, as B
DOI: 10.1021/acs.orglett.9b00820 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters with [12]CPP-(CO2t-Bu)612a and [8]CPP-(CO2t-Bu)8,14 the packing motifs of 1aa and 1bb show the columnar packing structures with channel structures inside (Figures 2c,d and 3c,d). In the columnar structure of 1bb, there are two intercolumn interactions: one between the ester moieties and one between the unsubstituted benzenes in which the CH−π interaction (2.82 Å) is observed (Figure 2c). On the contrary, in the columnar structure of 1aa, no intercolumn interaction is observed; instead, the intracolumn CH−O interaction between the hydrogen atom of the benzene ring and the oxygen atom of the carbonyl group is observed (Figure 3c). The UV/vis absorption and fluorescence properties of 1aa and 1bb were compared with those of nonsubstituted [6]CPP. Absorption maxima of 1aa (311 nm) and 1bb (309 nm) in CHCl3 were significantly blue-shifted compared to that of nonsubstituted [6]CPP (340 nm)3f due to the presence of four electron-withdrawing ester moieties (Figures S1 and S2). As with nonsubstituted [6]CPP,3f no fluorescence was observed in both 1aa and 1bb.16 To understand the electronic structures of 1bb, a DFT calculation was performed using Gaussian 09 at the B3LYP/631G(d) level. Figure 4 shows the pictorial representations of
Figure 5. Energy diagrams for MOs of nonsubstituted [6]CPP (left)3f and 1bb (right). Two-way arrows represent HOMO−LUMO gaps.
and thus, this transition corresponds to the maximum absorption around 309 nm. In summary, we disclosed the synthesis of C2-symmetrical [6]CPP-tetracarboxylates via macrocyclization by the rhodium-catalyzed intermolecular stepwise cross-alkyne cyclotrimerization and subsequent reductive aromatization. The 1H NMR spectra of the thus-obtained C2-symmetrical [6]CPPtetracarboxylates revealed that the rotation of unsubstituted benzene rings is slow at room temperature on the time scale of the 1H NMR measurement. These [6]CPPs formed columnar packing structures, as with previously reported carboxylated CPPs. In tetra-tert-butyl [6]CPP-tetracarboxylate, intercolumn interactions were observed; on the contrary, intracolumn interactions were observed in tetramethyl [6]CPP-tetracarboxylate. Absorption maxima of [6]CPP-tetracarboxylates in CHCl3 were significantly blue-shifted compared to that of nonsubstituted [6]CPP due to the presence of four electronwithdrawing ester moieties. [6]CPP-tetracarboxylates showed no fluorescence, which is the same as nonsubstituted [6]CPP. Further applications of the rhodium-catalyzed alkyne cyclotrimerization to the synthesis of functionalized CPPs and related cyclic π-conjugated compounds are underway in our laboratory.
Figure 4. Pictorial representation of six frontier molecular orbitals of 1bb, calculated at the B3LYP/6-31G(d) level of theory.
six frontier molecular orbitals (MOs) of 1bb. The HOMO and LUMO are delocalized throughout the ring, but LUMO+1 is spread over the entire molecule including the ester moieties and LUMO+2 is localized on the phthalate moieties. Such contribution of the phthalate moieties for LUMO+1 and LUMO+2 was not observed in [8]CPP-(CO2Me)8.14 In the CPPs, it is well-known that HOMO increases and LUMO decreases as ring size decreases, but HOMO−2 and HOMO− 1 decrease and LUMO+1 and LUMO+2 increase.4 Increased LUMO+1 and LUMO+2 energy levels in 1bb account for the above characteristic molecular orbitals. A comparison of the energy diagram of nonsubstituted [6]CPP to that of 1bb is shown in Figure 5. The energies of all orbitals from HOMO−2 to LUMO+2 decreased compared to those of nonsubstituted [6]CPP. This feature has the same tendency as [8]CPP-(CO2Me)8.14 Besides, the HOMO− LUMO transition is forbidden (f = 0), which is consistent with nonsubstituted [6]CPP.3f,4 The transition from HOMO− 1 to LUMO has the largest oscillator strength (f = 0.4218),
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b00820. Experimental procedures and compound characterization data (PDF) Accession Codes
CCDC 1900774−1900775 contain 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
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. C
DOI: 10.1021/acs.orglett.9b00820 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
■
[8]Cycloparaphenylene from a Square-Shaped Tetranuclear Platinum Complex. Angew. Chem., Int. Ed. 2010, 49, 757−759. [7]CPP: (e) Sisto, T. J.; Golder, M. R.; Hirst, E. S.; Jasti, R. Selective Synthesis of Strained [7]Cycloparaphenylene: An Orange-Emitting Fluorophore. J. Am. Chem. Soc. 2011, 133, 15800−15802 [6]CPP: . (f) Xia, J.; Jasti, R. Synthesis, Characterization, and Crystal Structure of [6]Cycloparaphenylene Angew. Angew. Chem., Int. Ed. 2012, 51, 2474−2476 [5]CPP: . (g) Evans, P. J.; Darzi, E. R.; Jasti, R. Efficient room-temperature synthesis of a highly strained carbon nanohoop fragment of buckminsterfullerene. Nat. Chem. 2014, 6, 404−408. (4) Segawa, Y.; Fukazawa, A.; Matsuura, S.; Omachi, H.; Yamaguchi, S.; Irle, S.; Itami, K. Combined experimental and theoretical studies on the photophysical properties of cycloparaphenylenes. Org. Biomol. Chem. 2012, 10, 5979−5984. (5) The formation of the columnar structure in the crystalline state was also observed in a thiophene-containing CPP ([4]cyclo-1,4phenylene-2′,5′-thienylene) and a fluorinated nanohoop. For a thiophene-containing CPP, see: (a) Ito, H.; Mitamura, Y.; Segawa, Y.; Itami, K. Thiophene-Based, Radial π-Conjugation: Synthesis, Structure, and Photophysical Properties of Cyclo-1,4-phenylene-2’,5′thienylenes. Angew. Chem., Int. Ed. 2015, 54, 159−163. For a fluorinated nanohoop, see: (b) Leonhardt, E. J.; Van Raden, J. M.; Miller, D.; Zakharov, L. N.; Alemán, B.; Jasti, R. A Bottom-Up Approach to Solution-Processed, Atomically Precise Graphitic Cylinders on Graphite. Nano Lett. 2018, 18, 7991−7997. (6) For a review of functionalized CPPs, see: Tran-Van, A.-F.; Wegner, H. A. Nano-rings with a handle − Synthesis of substituted cycloparaphenylenes. Beilstein J. Nanotechnol. 2014, 5, 1320−1333. (7) Kuwabara, T.; Orii, J.; Segawa, Y.; Itami, K. Curved Oligophenylenes as Donors in Shape-Persistent Donor−Acceptor Macrocycles with Solvatofluorochromic Properties. Angew. Chem., Int. Ed. 2015, 54, 9646−9649. (8) For other examples of donor−acceptor CPP derivatives, see: (a) Darzi, E. R.; Hirst, E. S.; Weber, C. D.; Zakharov, L. N.; Lonergan, M. C.; Jasti, R. Synthesis, Properties, and Design Principles of Donor−Acceptor Nanohoops. ACS Cent. Sci. 2015, 1, 335−342. (b) Van Raden, J. M.; Darzi, E. R.; Zakharov, L. N.; Jasti, R. Synthesis and characterization of a highly strained donor−acceptor nanohoop. Org. Biomol. Chem. 2016, 14, 5721−5727. (c) Ball, M.; Fowler, B.; Li, P.; Joyce, L. A.; Li, F.; Liu, T.; Paley, D.; Zhong, Y.; Li, H.; Xiao, S.; Ng, F.; Steigerwald, M. F.; Nuckolls, C. Chiral Conjugated Corrals. J. Am. Chem. Soc. 2015, 137, 9982−9987. (9) Our research group also reported the donor−acceptor CPP derivatives with solvatochromic properties. See: Nishigaki, S.; Fukui, M.; Kawauchi, S.; Sugiyama, H.; Uekusa, H.; Shibata, Y.; Tanaka, K. Synthesis, Structures, and Photophysical Properties of Alternating Donor−Acceptor Cycloparaphenylenes. Chem. - Eur. J. 2017, 23, 7227−7231. (10) (a) Tanaka, K.; Shirasaka, K. Highly Chemo- and Regioselective Intermolecular Cyclotrimerization of Alkynes Catalyzed by Cationic Rhodium(I)/Modified BINAP Complexes. Org. Lett. 2003, 5, 4697−4699. (b) Tanaka, K.; Toyoda, K.; Wada, A.; Shirasaka, K.; Hirano, M. Chemo- and Regioselective Intermolecular Cyclotrimerization of Terminal Alkynes Catalyzed by Cationic Rhodium(I)/Modified BINAP Complexes: Application to One-Step Synthesis of Paracyclophanes. Chem. - Eur. J. 2005, 11, 1145−1156. (11) For recent reviews of the rhodium-catalyzed alkyne cyclotrimerization, see: (a) Shibata, Y.; Tanaka, K. In Rhodium Catalysis in Organic Synthesis: Methods and Reactions; Tanaka, K., Ed.; WileyVCH: Weinheim, 2019; Chapter 9. (b) Shibata, Y.; Tanaka, K. Rhodium-Catalyzed [2 + 2+2] Cycloaddition of Alkynes for the Synthesis of Substituted Benzenes: Catalysts, Reaction Scope, and Synthetic Applications. Synthesis 2012, 44, 323−350. (c) TransitionMetal-Mediated Aromatic Ring Construction; Tanaka, K., Ed.; Wiley: Hoboken, 2013; Chapter 4. (d) Tanaka, K. Cationic Rhodium(I)/ BINAP-Type Bisphosphine Complexes: Versatile New Catalysts for Highly Chemo-, Regio-, and Enantioselective [2 + 2+2] Cycloadditions. Synlett 2007, 2007, 1977−1993.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hidehiro Uekusa: 0000-0001-9809-1411 Yu Shibata: 0000-0003-1017-0436 Ken Tanaka: 0000-0003-0534-7559 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This research was supported in part by a Grant-in-Aid for Scientific Research on Innovative Areas “π-System Figuration: Control of Electron and Structural Dynamism for Innovative Functions” (No. JP26102004) from the Japan Society for the Promotion of Science (JSPS, Japan) and ACT-C (No. JPMJCR1122YR) from the Japan Science and Technology Agency (JST, Japan). We thank Takasago International Corp. for the gift of H8−BINAP and Umicore for generous support in supplying the rhodium complex.
■
REFERENCES
(1) For selected recent reviews of CPPs, see: (a) Majewski, M. A.; Stępień, M.; Bowls; Hoops; Saddles. Synthetic Approaches to Curved Aromatic Molecules. Angew. Chem., Int. Ed. 2019, 58, 86−116. (b) Wu, D.; Cheng, W.; Ban, X.; Xia, J. Cycloparaphenylenes (CPPs): An Overview of Synthesis, Properties, and Potential Applications. Asian J. Org. Chem. 2018, 7, 2161−2181. (c) Segawa, Y.; Yagi, A.; Matsui, K.; Itami, K. Design and Synthesis of Carbon Nanotube Segments. Angew. Chem., Int. Ed. 2016, 55, 5136−5158. (d) Darzi, E. R.; Jasti, R. The dynamic, size-dependent properties of [5]− [12]cycloparaphenylenes. Chem. Soc. Rev. 2015, 44, 6401−6410. (e) Lewis, S. E. Cycloparaphenylenes and related nanohoops. Chem. Soc. Rev. 2015, 44, 2221−2304. (f) Yamago, S.; Kayahara, E.; Iwamoto, T. Organoplatinum-Mediated Synthesis of Cyclic πConjugated Molecules: Towards a New Era of Three-Dimensional Aromatic Compounds. Chem. Rec. 2014, 14, 84−100. (g) Omachi, H.; Segawa, Y.; Itami, K. Synthesis of Cycloparaphenylenes and Related Carbon Nanorings: A Step toward the Controlled Synthesis of Carbon Nanotubes. Acc. Chem. Res. 2012, 45, 1378−1389. (h) Sisto, T. J.; Jasti, R. Overcoming Molecular Strain: Synthesis of [7]Cycloparaphenylene. Synlett 2012, 23, 483−489. (i) Itami, K. Toward controlled synthesis of carbonnanotubes and graphenes. Pure Appl. Chem. 2012, 84, 907−916. (j) Bunz, U. H. F.; Menning, S.; Martín, N. para-Connected Cyclophenylenes and Hemispherical Polyarenes: Building Blocks for Single-Walled Carbon Nanotubes? Angew. Chem., Int. Ed. 2012, 51, 7094−7101. (2) For seminal works, see: (a) Jasti, R.; Bhattacharjee, J.; Neaton, J. B.; Bertozzi, C. R. Synthesis, Characterization, and Theory of [9]-, [12]-, and [18]Cycloparaphenylene: Carbon Nanohoop Structures. J. Am. Chem. Soc. 2008, 130, 17646−17647. (b) Takaba, H.; Omachi, H.; Yamamoto, Y.; Bouffard, J.; Itami, K. Selective Synthesis of [12]Cycloparaphenylene. Angew. Chem., Int. Ed. 2009, 48, 6112− 6116. (3) For the first synthesis of each size of CPPs, see the following. [18]-, [12]-, and [9]CPPs: (a) See ref 2a. [16]-, [15]-, and [14] CPPs: (b) Omachi, H.; Matsuura, S.; Segawa, Y.; Itami, K. A Modular and Size-Selective Synthesis of [n]Cycloparaphenylenes: A Step toward the Selective Synthesis of [n,n] Single-Walled Carbon Nanotubes. Angew. Chem., Int. Ed. 2010, 49, 10202−10205. [13]-, [11]-, and [10]CPPs: (c) Iwamoto, T.; Watanabe, Y.; Sakamoto, Y.; Suzuki, T.; Yamago, S. Selective and Random Syntheses of [n]Cycloparaphenylenes (n = 8−13) and Size Dependence of Their Electronic Properties. J. Am. Chem. Soc. 2011, 133, 8354−8361. [8] CPP: (d) Yamago, S.; Watanabe, Y.; Iwamoto, T. Synthesis of D
DOI: 10.1021/acs.orglett.9b00820 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters (12) (a) Miyauchi, Y.; Johmoto, K.; Yasuda, N.; Uekusa, H.; Fujii, S.; Kiguchi, M.; Ito, H.; Itami, K.; Tanaka, K. Concise Synthesis and Facile Nanotube Assembly of a Symmetrically Multifunctionalized Cycloparaphenylene. Chem. - Eur. J. 2015, 21, 18900−18904. (b) Nishigaki, S.; Miyauchi, Y.; Noguchi, K.; Ito, H.; Itami, K.; Shibata, Y.; Tanaka, K. Macrocyclization by Rhodium-Catalyzed Cross-Cyclotrimerization of L-Shaped Diynes with Di-tert-butyl Acetylenedicarboxylate: Effect of Bent Linkers of Diynes. Eur. J. Org. Chem. 2016, 2016, 4668−4673. (13) Li, S.; Huang, C.; Thakellapalli, H.; Farajidizaji, B.; Popp, B. V.; Petersen, J. L.; Wang, K. K. Syntheses and Structures of Functionalized [9]Cycloparaphenylenes as Carbon Nanohoops Bearing Carbomethoxy and N-Phenylphthalimido Groups. Org. Lett. 2016, 18, 2268−2271. (14) Hayase, N.; Miyauchi, Y.; Aida, Y.; Sugiyama, H.; Uekusa, H.; Shibata, Y.; Tanaka, K. Synthesis of [8]Cycloparaphenyleneoctacarboxylates via Rh-Catalyzed Stepwise Cross-Alkyne Cyclotrimerization. Org. Lett. 2017, 19, 2993−2996. (15) For other examples of the synthesis of symmetrically heteroatom-substituted CPPs, see: (a) Huang, C.; Huang, Y.; Akhmedov, N. G.; Popp, B. V.; Petersen, J. L.; Wang, K. K. Functionalized Carbon Nanohoops: Synthesis and Structure of a [9]Cycloparaphenylene Bearing Three 5,8-Dimethoxynaphth-1,4-diyl Units. Org. Lett. 2014, 16, 2672−2675. (b) Tran-Van, A.-F.; Huxol, E.; Basler, J. M.; Neuburger, M.; Adjizian, J.-J.; Ewels, C. P.; Wegner, H. A. Synthesis of Substituted [8]Cycloparaphenylenes by [2 + 2 + 2] Cycloaddition. Org. Lett. 2014, 16, 1594−1597. (c) Kuroda, Y.; Sakamoto, Y.; Suzuki, T.; Kayahara, E.; Yamago, S. Tetracyclo(2,7carbazole)s: Diatropicity and Paratropicity of Inner Regions of Nanohoops. J. Org. Chem. 2016, 81, 3356−3363. (d) Kayahara, E.; Zhai, X.; Yamago, S. Synthesis and physical properties of [4]cyclo-3,7dibenzo[b,d]thiophene and its S,S-dioxide. Can. J. Chem. 2017, 95, 351−356. (e) Farajidizaji, B.; Huang, C.; Thakellapalli, H.; Li, S.; Akhmedov, N. G.; Popp, B. V.; Petersen, J. L.; Wang, K. K. Synthesis and Characterization of Functionalized [12]Cycloparaphenylenes Containing Four Alternating Biphenyl and Naphthyl Units. J. Org. Chem. 2017, 82, 4458−4464. (f) Kayahara, E.; Qu, R.; Yamago, S. Bromination of Cycloparaphenylenes: Strain-Induced Site-Selective Bis-Addition and Its Application for Late-Stage Functionalization. Angew. Chem., Int. Ed. 2017, 56, 10428−10432. (g) Kayahara, E.; Sun, L.; Onishi, H.; Suzuki, K.; Fukushima, T.; Sawada, A.; Kaji, H.; Yamago, S. Gram-Scale Syntheses and Conductivities of [10]Cycloparaphenylene and Its Tetraalkoxy Derivatives. J. Am. Chem. Soc. 2017, 139, 18480−18483. (h) Hashimoto, S.; Kayahara, E.; Mizuhata, Y.; Tokitoh, N.; Takeuchi, K.; Ozawa, F.; Yamago, S. Synthesis and Physical Properties of Polyfluorinated Cycloparaphenylenes. Org. Lett. 2018, 20, 5973−5976. (i) White, B. M.; Zhao, Y.; Kawashima, T. E.; Branchaud, B. P.; Pluth, M. D.; Jasti, R. Expanding the Chemical Space of Biocompatible Fluorophores: Nanohoops in Cells. ACS Cent. Sci. 2018, 4, 1173−1178. (16) For the detailed explanation for the rapid drop in fluorescence quantum efficiencies of smaller CPPs, see ref 1d.
E
DOI: 10.1021/acs.orglett.9b00820 Org. Lett. XXXX, XXX, XXX−XXX
