Synthesis and Physical Properties of Polyfluorinated

Letter Cite This: Org. Lett. 2018, 20, 5973−5976

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Synthesis and Physical Properties of Polyfluorinated Cycloparaphenylenes Sigma Hashimoto, Eiichi Kayahara, Yoshiyuki Mizuhata, Norihiro Tokitoh, Katsuhiko Takeuchi, Fumiyuki Ozawa, and Shigeru Yamago* Institute for Chemical Research, Kyoto University, Kyoto, 611-0011, Japan

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S Supporting Information *

ABSTRACT: Fluorinated [6]- and [9]cycloparaphenylene (CPP) derivatives, 8F-[6]CPP and 12F-[9]CPP, were synthesized based on the previous synthesis of the parent CPPs. While the reductive aromatization conditions used in the final step of the synthesis of the parent CPPs did not work for the fluorinated compounds, the use of PBr3 and SnCl2 in acetonitrile successfully accomplished the desired transformation. The structures of F-CPPs were determined by singlecrystal X-ray analysis. Photo- and electrochemical analyses and host−guest chemistry revealed the effects of the introduction of fluorine atoms. oop-shaped π-conjugated molecules have attracted considerable attention because of their intriguing molecular structures, which give them unique physical properties and potential applications in materials science.1 In particular, cycloparaphenylenes (CPPs, Figure 1a), the

However, no fluorine-containing CPP derivatives have been reported to date.13 Herein, we report the first synthesis of fluorine-substituted CPPs, 8F-[6]CPP and 12F-[9]CPP, which are [6]- and [9]CPP derivatives with 8 and 12 embedded fluorine atoms, respectively (Figure 1b). The synthesis relied on the method used to prepare the parent CPPs14 in which metal-mediated assembly of three-ring unit 1 with a cis-1,4dihydroxy-2,3,5,6-tetrafluoro-2,5-cyclohexadiene-1,4-diyl group and subsequent reductive aromatization of the 1,4-diyl moiety are the key steps (Scheme 1). While the reductive aromatization did not proceed under the reported conditions used for the synthesis of the parent CPPs, the use of a mixture of PBr3 and SnCl2 reported by Koert15 was found to be highly suitable for the desired reaction. Photophysical and electrochemical measurements as well as host−guest interaction clarified the unique physical properties of 8F-[6]CPP and 12F[9]CPP. Requisite building block 1 was synthesized from tetrafluoro1,4-benzoquinone and 4-bromolithiobenzene, which was prepared in situ from 1,4-dibromobenzene and BuLi. The addition took place selectively in a cis fashion (>90%) and gave dibromide 1a (R = H) in 81% yield, and the hydroxyl groups of 1a were protected with triethylsilyl (TES) groups in 83% yield. Then, resulting 1b (R = TES) was cyclooligomerized using a Yamamoto coupling14b via the addition of Ni(cod)2 (2.0 equiv; cod refers to cycloocta-1,5-diene) and 2,2′bipyridyl (2.0 equiv) in refluxing THF. After a routine workup, cyclic dimer 2b and trimer 3b were isolated in 6% and 28% yields, respectively. The TES groups of 2b and 3b were quantitatively deprotected by tetrabutylammonium fluoride to give 2a and 3a.

H

Figure 1. [N]Cycloparaphenylene and its fluorinated derivative.

simplest cyclic structural unit of armchair carbon nanotubes, have occupied a central position among such molecules after their recent bottom-up organic syntheses by Jasti and Bertozzi et al.,2 Itami et al.,3 Yamago et al.,4 and others.5 These syntheses have also clarified the distinctive physical properties of these compounds, such as their size-dependent photophysical6 and redox properties7 and size-complementary host− guest chemistry.8 The introduction of heteroatom(s) into π-conjugated molecules is an important molecular design strategy for tuning their electronic properties for use in organic electronics.9 Hence, the syntheses of CPP derivatives with embedded boron, nitrogen, oxygen, silicon, sulfur, chlorine, bromine, or iodine atoms have been reported to date.10 Fluorine is the most electronegative element, yet its van der Waals radius is similar to that of hydrogen (1.47 vs 1.20 Å, respectively).11 Therefore, fluorine has been widely used to modulate the electronic properties of organic electronic materials such as semiconductors without inducing notable structural changes.12 © 2018 American Chemical Society

Received: August 24, 2018 Published: September 13, 2018 5973

DOI: 10.1021/acs.orglett.8b02715 Org. Lett. 2018, 20, 5973−5976

Letter

Organic Letters

The structures of 8F-[6]CPP and 12F-[9]CPP were unambiguously determined by X-ray diffraction experiments (Figure 2). A single-crystal of 8F-[6]CPP was obtained by slow

Scheme 1. Synthesis of 8F-[6]CPP and 12F-[9]CPP

Figure 2. ORTEP drawings and crystal packing structures of (a, b) 8F-[6]CPP and (c, d) 12F-[9]CPP. The thermal ellipsoids are drawn at 50% probability. The hydrogen atoms and solvent molecules are omitted for clarity. Several H−F distances (Å) are given in blue.

vapor diffusion of n-hexane into a solution of 8F-[6]CPP in dichloromethane at room temperature, and crystals of 12F[9]CPP were obtained by recrystallization from chloroform. 8F-[6]CPP forms an alternating zigzag structure with dihedral angles between the two paraphenylene units of 24°−36°, while 12F-[9]CPP has a helical arrangement with dihedral angles of 4°−46°. The basic structures are identical to that of the parent CPPs, but the average dihedral angles of the fluorinated CPPs are slightly larger than those of the parent CPPs ([6]CPP = 26°17 and [9]CPP = 24°).18 Both C6H4 and C6F4 units have benzenoid characteristics based on the average C−C bond lengths (see the Supporting Information (SI)), and these values are also similar to those of the parent CPPs. In the crystal packing, 8F-[6]CPP and 12F-[9]CPP molecules adopt herringbone and columnar-like arrangements, respectively, with a short distance between the fluorine and hydrogen atoms (2.4−2.7 Å, Figure 2b,d); these distances are less than the sum of the van der Waals radii of F and H. These results indicate the presence of fluorine−hydrogen interactions in the solid state.19 The photophysical properties of 8F-[6]CPP and 12F[9]CPP were next examined. In its UV−vis absorption spectrum in CHCl3 (Figure 3), 8F-[6]CPP shows characteristic absorptions at λmax = 327 nm with a molar extinction coefficient (ε) = 3.81 × 104 M−1·cm−1 and a weak shoulder peak that extends to approximately 450 nm. Similarly, 12F[9]CPP shows a single absorption at λmax = 324 nm with ε = 3.27 × 104 M−1·cm−1 and a weak shoulder at approximately 360 nm. These absorption maxima of the fluorinated CPPs were slightly blue-shifted compared with those of the parent nonfluorinated CPPs. In the fluorescence measurements, 8F[6]CPP showed no fluorescence, which was similar to what was observed for [6]CPP.17 On the other hand, 12F-[9]CPP showed fluorescence at 471 nm with a quantum yield (ΦF) of 0.14. The wavelength is shorter than that of [9]CPP (497 nm), and the ΦF is significantly lower than that of [9]CPP (0.70). These properties are similar to those of other CPP derivatives

Alternatively, 2b was selectively synthesized by platinummediated dimerization;10m,14a 1b was converted to bisstannane 4, which was treated with PtCl2(cod) (1.0 equiv) in THF at 80 °C for 24 h. While bisplatinum complex 5 could not be fully characterized due to its poor solubility, treatment of the crude mixture with triphenylphosphine (5.0 equiv) in toluene at 90 °C for 19 h gave 2b in 54% yield (two steps). Cyclic dimer 2a was subjected to reductive aromatization by using H2SnCl4,16 but no reaction was observed. Aromatization under strong reducing conditions using sodium naphthalenide17 gave only complex mixtures. Through extensive screening of aromatization conditions, we found that a mixture of excess PBr3 (10 equiv) and SnCl2 (8.0 equiv) in THF, which was used to aromatize electron-deficient 1,4-dihydroxy2,5-cyclohexadienes,15 gave the desired 8F-[6]CPP in 51% yield. The yield increased to 90% when the reaction was carried out in acetonitrile with a decreased amount of PBr3 (4.0 equiv) and SnCl2 (4.0 equiv). Cyclic trimer 3a was also inert to H2SnCl4 but aromatized to 12F-[9]CPP in excellent yield (95%) with PBr3 (6.0 equiv) and SnCl2 (6.0 equiv) in acetonitrile. These conditions are thus applicable for the aromatization of other various electron-deficient substrates. The structures of 8F-[6]CPP and 12F-[9]CPP were initially estimated from NMR spectroscopy. In their 1H NMR spectra at 25 °C in CDCl3, both compounds showed two doublet peaks at 7.53 and 7.78 ppm (1J = 8.8 Hz) for 8F-[6]CPP and at 7.56 and 7.61 ppm (1J = 8.8 Hz) for 12F-[9]CPP. In their 13 C and 19F NMR spectra, only six peaks and one peak, respectively, were observed for both compounds. In their mass spectra acquired with fast atom bombardment ionization, the molecular ion peaks of 8F-[6]CPP and 12F-[9]CPP were observed at m/z 600.1124 and 900.1686, respectively. All results are consistent with the putative structures of 8F[6]CPP and 12F-[9]CPP. 5974

DOI: 10.1021/acs.orglett.8b02715 Org. Lett. 2018, 20, 5973−5976

Letter

Organic Letters

Table 1. Redox Properties and Orbital Energies of 8F[6]CPP, [6]CPP, 12F-[9]CPP, and [9]CPP E1/2 ox a

compound

(V)

8F-[6]CPP [6]CPP 12F-[9]CPP [9]CPP

0.85 0.28 1.23c 0.72

E1/2 red

HOMO

LUMO

(V)a

(eV)b

(eV)b

−1.79 −2.25 −2.06, −2.20c −2.45

−5.43 −4.92 −5.61 −5.13

−2.66 −2.11 −2.34 −2.03

V vs ferrocene/ferrocenium couple; Bu4NPF6 (0.10 mol L−1) in dichloromethane (oxidation) and THF (reduction). bData obtained from DFT calculations at the B3LYP/6-31G* level of theory. cEp, determined by dpv. a

Figure 3. UV−vis absorption and fluorescence spectra of 8F-[6]CPP, 12F-[9]CPP, [6]CPP, and [9]CPP in CHCl3.

V for oxidation and reduction, respectively). Notably, the rate of the electron-transfer process in the oxidation of 8F-[6]CPP appears faster than that of [6]CPP based on the peak width and shape of the waves.20 12F-[9]CPP exhibits an irreversible oxidation and two quasireversible reduction waves at Ep = 1.23, − 2.06, and −2.20 V (vs Fc/Fc+, determined by differential pulse voltammetry). These values are significantly higher than those of [9]CPP (0.72 and −2.45 V for oxidation and reduction, respectively). The results are consistent with the calculated HOMO and LUMO energies, in which the introduction of fluorine significantly lowers the energies of these orbitals relative to those of the parent CPPs due to the electron-withdrawing effects of the fluorine atoms. The effect of fluorine was further investigated in the host− guest chemistry. We have already reported that [N]CPP selectively interacts with [N + 5]CPP, giving the [N + 5]CPP⊃[N]CPP complex,8c but the interaction between two CPPs is very weak. For example, the association constant, Ka, between [6]CPP and [11]CPP is 540 ± 10 L·mol−1 in 1,1,2,2tetrachloroethane-d2 at 25 °C. Titration experiments between 8F-[6]CPP and [11]CPP under identical conditions revealed that these two CPPs formed a 1:1 complex, [11]CPP⊃8F[6]CPP, with a Ka of 1533 ± 171 L·mol−1. This value is approximately three times higher than that of [11]CPP⊃[6]CPP. This result is supported by theoretical calculations at the wB97X-D/6-31G(d) level of theory; the enthalpy of the formation of a complex involving 8F-[6]CPP is more exothermic than that of the parent [6]CPP (−216 kJ·mol−1 vs −194 kJ·mol−1).8c The optimized structure is essentially the same as that of the parent complex; both show a planetary orbit structure. The results indicate that the fluorine atoms have virtually no effect on the structure, although several short interactions between H−H and F−H atoms were observed in the current complex (see the SI). The Mulliken charges of the carbons of [11]CPP in [11]CPP⊃8F-[6]CPP became more negative than those in the parent complex. These results suggest that the increase in the polarization between the host and guest is responsible for the observed increase in the interactions; however, the increased van der Waals interactions between H−H and F−H would also contribute to the stabilization. In summary, we have successfully synthesized 8F-[6]CPP and 12F-[9]CPP, which are the first fluorinated CPP derivatives reported. A structural study suggested the existence of intermolecular F−H interactions in the solid state. While the structures and photophysical properties are rather insensitive to the introduction of fluorine atoms, the redox properties and host−guest interactions are significantly altered. These results clearly reveal that the introduction of fluorine atoms can

with electron-withdrawing substituents, such as carboxyl groups.10j Time-dependent density functional theory (TD-DFT) calculations indicated that the strongest absorption bands of 8F-[6]CPP and 12F-[9]CPP are derived from the sum of the HOMO−2 to LUMO, HOMO−1 to LUMO, HOMO to LUMO+1, and HOMO to LUMO+2 transitions (HOMO and LUMO refer to highest occupied and lowest unoccupied molecular orbitals, respectively), and the results are the same as those of the parent CPPs.7a,17 The weak shoulder peaks were assigned to forbidden HOMO−LUMO transitions. Cyclic voltammograms (CVs) of 8F-[6]CPP and 12F[9]CPP were measured in 0.10 mol·L−1 Bu4NPF6 solutions in dichloromethane (for oxidation) and THF (for reduction) (Figure 4 and Table 1). 8F-[6]CPP showed one reversible oxidation wave and one reversible reduction wave with halfwave oxidation potentials, E1/2, of 0.85 and −1.79 V [vs ferrocene/ferrocenium couple (Fc/Fc+)], respectively. These values are much higher than those of [6]CPP (0.28 and −2.25

Figure 4. Cyclic voltammograms of 8F-[6]CPP, [6]CPP, 12F[9]CPP, and [9]CPP and differential pulse voltammograms (DPV) of 12F-[9]CPP. 5975

DOI: 10.1021/acs.orglett.8b02715 Org. Lett. 2018, 20, 5973−5976

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Organic Letters

Chem., Int. Ed. 2013, 52, 13722. (c) Fujitsuka, M.; Tojo, S.; Iwamoto, T.; Kayahara, E.; Yamago, S.; Majima, T. J. Phys. Chem. Lett. 2014, 5, 2302. (d) Toriumi, N.; Muranaka, A.; Kayahara, E.; Yamago, S.; Uchiyama, M. J. Am. Chem. Soc. 2015, 137, 82. (e) Talipov, M. R.; Jasti, R.; Rathore, R. J. Am. Chem. Soc. 2015, 137, 14999. (f) Kayahara, E.; Kouyama, T.; Kato, T.; Yamago, S. J. Am. Chem. Soc. 2016, 138, 338. (8) (a) Iwamoto, T.; Watanabe, Y.; Sadahiro, T.; Haino, T.; Yamago, S. Angew. Chem., Int. Ed. 2011, 50, 8342. (b) Iwamoto, T.; Watanabe, Y.; Takaya, H.; Haino, T.; Yasuda, N.; Yamago, S. Chem. - Eur. J. 2013, 19, 14061. (c) Hashimoto, S.; Iwamoto, T.; Kurachi, D.; Kayahara, E.; Yamago, S. ChemPlusChem 2017, 82, 1015. (9) (a) Wang, X.; Sun, G.; Routh, P.; Kim, D.-H.; Huang, W.; Chen, P. Chem. Soc. Rev. 2014, 43, 7067. (b) Stępień, M.; Gońka, E.; Ż yła, M.; Sprutta, N. Chem. Rev. 2017, 117, 3479. (10) Boron, Silicon: (a) Kubota, N.; Segawa, Y.; Itami, K. J. Am. Chem. Soc. 2015, 137, 1356. Boron, Bromine, Iodine: (b) Kayahara, E.; Qu, R.; Yamago, S. Angew. Chem., Int. Ed. 2017, 56, 10428. Nitrogen: (c) Matsui, K.; Segawa, Y.; Itami, K. Org. Lett. 2012, 14, 1888. (d) Darzi, E. R.; Hirst, E. S.; Weber, C. D.; Zakharov, L. N.; Lonergan, M. C.; Jasti, R. ACS Cent. Sci. 2015, 1, 335. (e) Van Raden, J. M.; Darzi, E. R.; Zakharov, L. N.; Jasti, R. Org. Biomol. Chem. 2016, 14, 5721. (f) Kuroda, Y.; Sakamoto, Y.; Suzuki, T.; Kayahara, E.; Yamago, S. J. Org. Chem. 2016, 81, 3356. Oxygen: (g) 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. (h) Huang, C.; Huang, Y.; Akhmedov, N. G.; Popp, B. V.; Petersen, J. L.; Wang, K. K. Org. Lett. 2014, 16, 2672. (i) Miyauchi, Y.; Johmoto, K.; Yasuda, N.; Uekusa, H.; Fujii, S.; Kiguchi, M.; Ito, H.; Itami, K.; Tanaka, K. Chem. - Eur. J. 2015, 21, 18900. (j) Hayase, N.; Miyauchi, Y.; Aida, Y.; Sugiyama, H.; Uekusa, H.; Shibata, Y.; Tanaka, K. Org. Lett. 2017, 19, 2993. (k) Li, S.; Aljhdli, M.; Thakellapalli, H.; Farajidizaji, B.; Zhang, Y.; Akhmedov, N. G.; Milsmann, C.; Popp, B. V.; Wang, K. K. Org. Lett. 2017, 19, 4078. (l) Sala, P. D.; Talotta, C.; Caruso, T.; De Rosa, M.; Soriente, A.; Neri, P.; Gaeta, C. J. Org. Chem. 2017, 82, 9885. (m) Kayahara, E.; Sun, L.; Onishi, H.; Suzuki, K.; Fukushima, T.; Sawada, A.; Kaji, H.; Yamago, S. J. Am. Chem. Soc. 2017, 139, 18480. (n) Lu, D.; Zhuang, G.; Jia, H.; Wang, J.; Huang, Q.; Cui, S.; Du, P. Org. Chem. Front. 2018, 5, 1446. Sulfur: (o) Kayahara, E.; Zhai, X.; Yamago, S. Can. J. Chem. 2017, 95, 351. Chlorine: (p) Ishii, Y.; Matsuura, S.; Segawa, Y.; Itami, K. Org. Lett. 2014, 16, 2174. (11) Brédas, J. L.; Heeger, A. J. Chem. Phys. Lett. 1994, 217, 507. (12) (a) Tang, M. L.; Bao, Z. Chem. Mater. 2011, 23, 446. (b) Leclerc, N.; Chávez, P.; Ibraikulov, O.; Heiser, T.; Lévêque, P. Polymers 2016, 8, 11. (13) Theoretical study of fluorinated CPPs was already reported: Rio, J.; Erbahar, D.; Rayson, M.; Briddon, P.; Ewels, C. P. Phys. Chem. Chem. Phys. 2016, 18, 23257. (14) (a) Kayahara, E.; Patel, V. K.; Xia, J.; Jasti, R.; Yamago, S. Synlett 2015, 26, 1615. (b) Kayahara, E.; Cheng, Y.; Yamago, S. Chem. Lett. 2018, 47, 1108. (15) (a) Schwaben, J.; Münster, N.; Breuer, T.; Klues, M.; Harms, K.; Witte, G.; Koert, U. Eur. J. Org. Chem. 2013, 2013, 1639. (b) Schwaben, J.; Münster, N.; Klues, M.; Breuer, T.; Hofmann, P.; Harms, K.; Witte, G.; Koert, U. Chem. - Eur. J. 2015, 21, 13758. (c) Glöcklhofer, F.; Lunzer, M.; Stöger, B.; Fröhlich, J. Chem. - Eur. J. 2016, 22, 5173. (16) Patel, V. K.; Kayahara, E.; Yamago, S. Chem. - Eur. J. 2015, 21, 5742. (17) Xia, J.; Jasti, R. Angew. Chem., Int. Ed. 2012, 51, 2474. (18) Segawa, Y.; Š enel, P.; Matsuura, S.; Omachi, H.; Itami, K. Chem. Lett. 2011, 40, 423. (19) Dalvit, C.; Vulpetti, A. Chem. - Eur. J. 2016, 22, 7592. (20) Kayahara, E.; Fukayama, K.; Nishinaga, T.; Yamago, S. Chem. Asian J. 2016, 11, 1793.

effectively control the morphology and tune the electronic properties of cyclic conjugated molecules, which are important for applications in organic electronic materials.

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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.8b02715. Full details of experimental procedures, characterization data, NMR spectra, and computational details (PDF) Accession Codes

CCDC 1863800−1863801 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.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Eiichi Kayahara: 0000-0003-1663-5273 Yoshiyuki Mizuhata: 0000-0001-5301-0024 Fumiyuki Ozawa: 0000-0002-3937-4435 Shigeru Yamago: 0000-0002-4112-7249 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by the Japan Society for the Promotion of Science KAKENHI Grant No. 16H06352 (S.Y.). Computation was supported by the Super Computer Laboratory, Institute for Chemical Research, Kyoto University.

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REFERENCES

(1) (a) Schröder, A.; Mekelburger, H.-B.; Vögtle, F. Top. Curr. Chem. 1994, 172, 179. (b) Kawase, T.; Kurata, H. Chem. Rev. 2006, 106, 5250. (c) Tahara, K.; Tobe, Y. Chem. Rev. 2006, 106, 5274. (2) Jasti, R.; Bhattacharjee, J.; Neaton, J. B.; Bertozzi, C. R. J. Am. Chem. Soc. 2008, 130, 17646. (3) Takaba, H.; Omachi, H.; Yamamoto, Y.; Bouffard, J.; Itami, K. Angew. Chem., Int. Ed. 2009, 48, 6112. (4) Yamago, S.; Watanabe, Y.; Iwamoto, T. Angew. Chem., Int. Ed. 2010, 49, 757. (5) (a) Hitosugi, S.; Nakanishi, W.; Yamasaki, T.; Isobe, H. Nat. Commun. 2011, 2, 492. (b) Nishiuchi, T.; Feng, X.; Enkelmann, V.; Wagner, M.; Müllen, K. Chem. - Eur. J. 2012, 18, 16621. (c) Jiang, H.W.; Tanaka, T.; Mori, H.; Park, K. H.; Kim, D.; Osuka, A. J. Am. Chem. Soc. 2015, 137, 2219. (d) Mysliwiec, D.; Kondratowicz, M.; Lis, T.; Chmielewski, P. J.; Stępień, M. J. Am. Chem. Soc. 2015, 137, 1643. (e) 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. L.; Nuckolls, C. J. Am. Chem. Soc. 2015, 137, 9982. (6) (a) Fujitsuka, M.; Cho, D. W.; Iwamoto, T.; Yamago, S.; Majima, T. Phys. Chem. Chem. Phys. 2012, 14, 14585. (b) Segawa, Y.; Fukazawa, A.; Matsuura, S.; Omachi, H.; Yamaguchi, S.; Irle, S.; Itami, K. Org. Biomol. Chem. 2012, 10, 5979. (7) (a) Iwamoto, T.; Watanabe, Y.; Sakamoto, Y.; Suzuki, T.; Yamago, S. J. Am. Chem. Soc. 2011, 133, 8354. (b) Kayahara, E.; Kouyama, T.; Kato, T.; Takaya, H.; Yasuda, N.; Yamago, S. Angew. 5976

DOI: 10.1021/acs.orglett.8b02715 Org. Lett. 2018, 20, 5973−5976