Letter pubs.acs.org/OrgLett
Synthesis, Properties, and Semiconducting Characteristics of BF2 Complexes of β,β-Bisphenanthrene-Fused Azadipyrromethenes Wanle Sheng,† Yu-Qing Zheng,‡ Qinghua Wu,† Yayang Wu,† Changjiang Yu,† Lijuan Jiao,*,† Erhong Hao,† Jie-Yu Wang,*,‡ and Jian Pei‡ †
Laboratory of Functional Molecular Solids, Ministry of Education, School of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, China ‡ Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Peking 100080, China S Supporting Information *
ABSTRACT: Three novel π-extended BF2 complexes of β,β-bisphenanthrenefused azadipyrromethenes containing nine fused rings have been synthesized on the basis of a tandem Suzuki coupling reaction on readily available 2,6dibromoazaBODIPYs followed by an intramolecular oxidative aromatic coupling mediated by iron(III) chloride. These resultant BF2 complexes exhibit strong absorption (extinction coefficients up to 2.4 × 105 M−1 cm−1) and emission in the near-infrared (NIR) range (790−816 nm) with excellent photo and thermal stabilities. The hole mobility of the thin-film field-effect transistors of these dyes fabricated by a solution process reaches up to 0.018 cm2 V−1 s−1.
as π-functional materials.6 In contrast, the synthesis of [α]-fusion azaBODIPYs containing large aromatic rings and their [β]-fusion analogues (even β,β-bisbenzo-fused azaBODIPYs B, Figure 1) remains a challenge.7 Oxidative ring fusion strategy has recently been used for the construction of perylene- and porphyrin-fused BODIPYs.8 However, no regioselectivity issue has been involved. Recent synthetic progress in the preparation of (aza)BODIPYs facilitates the facile installation of various aromatic moieties, for example, phenyls onto the 2,6-positions to form 1,2,3,5,6,7-hexaphenylazaBODIPYs 3 (Figure 1). We rationalized that the regioselective fusion of the 2,3,5,6-tetraphenyl moieties on hexaphenylazaBODIPY 3 via an oxidative ring-fusion reaction9 may straightforwardly generate the desired β,β-bisphenanthrene-fused azaBODIPY chromophore 1 (Scheme 1). Herein, we report the regioselective straightforward synthesis of a set of β,βbisphenanthrene azaBODIPYs 1, their characterizations via Xray analysis, optical, stability, and DFT calculations, and the preliminary investigation of their hole-charge-transport performance as promising organic field electron transfer (OFET) materials. 2,6-DibromoazaBODIPY 2a was formed in nearly quantitative yield from the regioselective bromination of readily available azaBODIPY 4a using a literature procedure (Scheme 1)2b and was further applied for the Suzuki coupling reaction with 3,5dimethoxyphenylboronic acid to smoothly afford the desired hexaphenylazaBODIPY 3a in 78% isolated yield. Surprisingly,
N
ear-infrared (NIR) dyes with absorption and/or emission lying in the range of 700−2000 nm have found important applications in biomedicine (e.g., as bioimaging and therapeutic agents) and in material science (as various electronic devices).1 Boron azadipyrromethenes (azaBODIPYs) have received extensive research interest lately due to their remarkable optical and electrical properties, especially the good photostability and the strong tunable absorption at a wavelength above 650 nm.2−4 As demonstrated by α,α-bisbenzo-fused azaBODIPY A (Figure 1) and its pyrazine-fused analogues,5 aromatic ring fusion at the peripheral positions of the chromophore generally brings the desired structure rigidity and strong intermolecular π−π interactions in the solid state, which facilitates their applications
Figure 1. Chemical structures of [α]- and [β]-fusion azaBODIPYs A and B and the general concept for the construction of β,βbisphenanthrene fused azaBODIPY 1 from hexaphenylazaBODIPY 3 in this work. © 2017 American Chemical Society
Received: April 13, 2017 Published: May 22, 2017 2893
DOI: 10.1021/acs.orglett.7b01133 Org. Lett. 2017, 19, 2893−2896
Letter
Organic Letters
excellent isolated yields (93−96%) using the same procedure described for 1a from readily available azaBODIPY 4b (Scheme 1) and were characterized by NMR and HRMS. As expected, the installation of dodecyloxy chains at the 1,7-phenyl moieties of the azaBODIPY chromophore greatly improves the solubility of the dyes. In great contrast to azaBODIPY 1a, which shows poor solubility in common organic solvents, both azaBODIPYs 1b and 1c show good solubility in common organic solvents, like dichloromethane, THF, and toluene. AzaBODIPYs 3a−c show nearly identical strong absorption centered at around 683 nm and moderate fluorescence emission centered at 716 nm, respectively, in toluene (Figure 3 and Table
Scheme 1. Synthesis of azaBODIPYs 1
the subsequent reaction of azaBODIPY 3a with 10 equiv of FeCl3 in CH2Cl2/CH3NO2 at room temperature afforded exclusively only one product in nearly qualitative yield (96%). The HRMS (MALDI) analysis clearly shows that four protons have been eliminated from this reaction. 1H NMR analysis further confirms that this generated product is a 2-fold-fusion product. Fortunately, we were able to unambiguously identify the fusion position via the X-ray analysis result (Figure 2b), which clearly shows that it is β,β-bisphenanthrene-fused azaBODIPY 1a.
Figure 3. Overlaid normalized absorption (solid lines) and fluorescence emission (dotted lines) of 3a (black lines) and 1a (red lines) in toluene.
Table 1. Photophysical Properties of 3a,b and 1a−c at Room Temperature in Toluene dyes
λabsmax (nm)
λemmax (nm)
ε (M−1 cm−1)
Φa
Stokes shift (cm−1)
3a 3b 3c 1a 1b 1c
683 683 684 804 804 790
716 716 718 816 816 807
76200 81800 86500 215900 235100 208100
0.14 0.11 0.03 0.13 0.17 0.05
700 700 700 200 200 300
a
Fluorescence quantum yields were obtained by using 1,7-diphenyl3,5-dimethoxyphenylazadipyrromethene (ϕ = 0.36 in chloroform) as the reference compound for 3a−c, and Indocyanine Green (ICG) (ϕ = 0.12 in DMSO) for 1a−c. The standard errors are less than 10%.
Figure 2. Top (and side) views of the X-ray crystal structures of (a) 3a and (b) 1a and (c) the crystal packing structure of 1a. Hydrogen atoms are omitted for clarity. Key: C, light gray; N, blue; B, dark yellow; F, light green; O, red.
1). In comparison with 3a−c, this ring fusion brings more than 100 nm red-shifts in both the absorption and emission bands, a 2fold increase of the extinction coefficients (up to 2.3 × 105 M−1 cm−1), a narrowed absorption band (fwhm decreased from 1364 to 448 cm−1), and a decrease of the Stokes shift (up to 11 nm) in these [β]-fusion azaBODIPYs 1a−c. This red-shift of the spectra is in good agreement with the DFT calculations and may be attributed to the slightly stabilized LUMO and the destabilized HOMO levels caused by ring fusions (Figure 4). Variation of the alkoxy substituents at the 1,7-phenyls and the alkyl substituents at the 2,6-phenyl moieties has negligible influence on the absorption and emission spectra of these azaBODIPYs 1a−c (Table 1). The small Stokes shift observed for azaBODIPYs 1a− c indicates the high rigid symmetrical structure of the chromophore. Although the fluorescence quantum yields are moderate for azaBODIPYs 1a (0.13), 1b (0.17), and 1c (0.05) in
X-ray crystals of 3a and azaBODIPY 1a suitable for X-ray analysis were obtained from the slow diffusion of methanol into their chloroform solutions. Both dyes show almost planar azadipyrromethene cores (Figure 2). The central chelated BF2 ring of 3a disordered over two orientations. The small dihedral angle (0.916°) between the two pyrrole units in azaBODIPY 1a indicates that this ring fusion causes little structural disruption of the planar structure of the azadipyrromethene core. The dihedral angles between the 1,7-phenyl moieties and the azadipyrromethene core are between 54 and 59° for 3a, which were slightly changed to 44−55° in 1a (Table S1). To demonstrate the versatility of this highly regioselective [β]ring-fusion reaction, azaBODIPYs 1b and 1c were synthesized in 2894
DOI: 10.1021/acs.orglett.7b01133 Org. Lett. 2017, 19, 2893−2896
Letter
Organic Letters
obvious substituent-dependent high-field-effect-mobility (0.018 cm2 V−1 s−1 for 1b and 1.32 × 10−4 cm2 V−1 s−1 for 1c, Figure 5
Figure 4. Frontier molecular orbitals and the energies of 3a (left) and 1a (right).
Figure 5. Transfer (a) and output (b) characteristics of the thin film FET device based on 1b.
toluene, it was still impressive when considering the >800 nm emission wavelength of these dyes. Electrochemical properties of azaBODIPYs 1a−c were investigated by cyclic voltammetry (CV) (Figure S9). Each of these three dyes shows two reversible oxidation and two reversible reduction waves. AzaBODIPYs 1a and 1b show nearly identical oxidation and reduction waves to each other, with the oxidation and reduction waves at 0.87 and −0.38 V, respectively (vs Fc/Fc+). The oxidation and reduction waves for 1c were slightly different from 1a and 1b, located at 0.97 and −0.35 V. The HOMO and LUMO energy levels estimated from the halfwave value for azaBODIPY 1b are +5.21 and −3.90 eV, and the values for 1c are +5.31 and −3.93 eV, respectively, which match well with their optical bands.10 The significantly lowered LUMO levels of 1b and 1c are comparable to many representative n-type semiconducting materials (such as perylene bisimides and fullerenes).11 In addition, in the crystal packing structure (Figure 1c), azaBODIPY 1a showed a well-ordered (a slipped head-to-tail) two-dimensional structure due to π−π and dipole−dipole interactions.12,13 The planes of the two molecules are almost parallel to each other, with a 3.90 Å mean distance between the two neighboring cores. Although no azaBODIPY based small molecules have been used in OFET, we envisioned that this packing mode may favor the charge transport process in this organic framework.12a Moreover, to demonstrate the potential application of these two dyes as semiconducting materials, the thermal characteristics of these compounds were studied by thermogravimetric analysis. No obvious weight loss was observed until 310 °C (Figure S7). Their remarkably high thermostability indicates their good potential as a novel air-stable semiconductor. The semiconducting properties for 1b and 1c were investigated by field-effect transistor (FET) measurements. A top-gate/bottom-contact (TG/BC) device configuration was used to fabricate FETs. The semiconducting layer was deposited by spin-coating the toluene solution (4 mg/mL) of either 1b or 1c on a patterned Au/SiO2/Si substrate at 1500 rpm for 30 s. After thermal annealing of the film for 5 min at 110 °C, a CYTOP solution was spin-coated as the dielectric layer, and an aluminum layer was thermally evaporated as the gate electrode. Interestingly, the OFET devices of these two dyes did not show the expected n-type transport properties. By contrast, only p-type transport characteristics were observed for them under ambient conditions with a > 103 current on/off ratio and an
and Figure S10). The absence of the expected n-type transport characters in 1b and 1c may be attributed to the concentrated LUMO distribution, which hampers the intermolecular electronic overlap. To understand the substituent-dependent field-effect mobility of 1b and 1c, atomic force microscopy (AFM) and X-ray diffraction studies were performed on the thin film (deposited on Si/SiO2 substrates) of these dyes. Both dyes show similar out-ofplane distances, which were measured to be 21.06 and 21.63 Å, respectively. But the thin film of 1b seems to be more in ordered as indicated by the stranger diffraction signal. The grain size revealed by AFM indicates that the different p-type transport properties may associate with the crystallinity of the semiconducting layer (Figures S11 and S12). This hole mobility of 1b is much higher than those of previously reported small molecular BODIPY dyes14a−d and tetrapyrrole-based dyes14e,f and is even comparable to those of BODIPY-containing polymers (Figure S13).15 In summary, we have developed a straightforward approach to a class of unprecedented β,β-aromatic-ring-fused azaBODIPY chromophore. The bottom-up selective oxidative ring-fusion reactions readily provided 1,2,3,5,6,7-hexaphenylazaBODIPY derivatives. These resultant π-extended systems show highly spectral selectivity in the NIR region (with narrow absorption and emission bands), large extinction coefficients, good thermostability, and interesting semiconducting properties. The substituent-dependent p-type transport characteristics under ambient conditions were achieved with the hole mobility of the thin film devices fabricated by solution process up to 0.018 cm2 V−1 s−1. These results demonstrate the promising optoelectronic applications of these β,β-bisphenanthrene-fused azaBODIPYs.
■
ASSOCIATED CONTENT
S Supporting Information *
Experimental details, tables, additional spectra and CIF files. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b01133. Experimental details, tables, and additional spectra (PDF) X-ray data for 1a (CIF) X-ray data for 3a (CIF) 2895
DOI: 10.1021/acs.orglett.7b01133 Org. Lett. 2017, 19, 2893−2896
Letter
Organic Letters
■
(6) (a) Gsänger, M.; Bialas, D.; Huang, L.; Stolte, M.; Würthner, F. Adv. Mater. 2016, 28, 3615. (b) Stepien, M.; Gonka, E.; Zyla, M.; Sprutta, N. Chem. Rev. 2017, 117, 3479. (c) Zhylitskaya, H.; Cybinska, J.; Chmielewski, P.; Lis, T.; Stepien, M. J. Am. Chem. Soc. 2016, 138, 11390. (7) On the other hand, several β-fused BODIPYs are available: (a) Hayashi, Y.; Obata, N.; Tamaru, M.; Yamaguchi, S.; Matsuo, Y.; Saeki, A.; Seki, S.; Kureishi, Y.; Saito, S.; Yamaguchi, S.; Shinokubo, H. Org. Lett. 2012, 13, 866. (b) Heyer, E.; Retailleau, P.; Ziessel, R. Org. Lett. 2014, 16, 2330. (c) Chua, M. H.; Huang, K.; Xu, J.; Wu, J. Org. Lett. 2015, 17, 4168. (d) Gobo, Y.; Yamamura, M.; Nakamura, T.; Nabeshima, T. Org. Lett. 2016, 18, 2719. (e) Wakamiya, A.; Murakami, T.; Yamaguchi, S. Chem. Sci. 2013, 4, 1002. (f) Ni, Y.; Zeng, W.; Huang, K.; Wu, J. Chem. Commun. 2013, 49, 1217. (8) (a) Jiao, C.; Huang, K.; Wu, J. Org. Lett. 2011, 13, 632. (b) Jiao, C.; Zhu, L.; Wu, J. Chem. - Eur. J. 2011, 17, 6610. (c) Zeng, L.; Jiao, C.; Huang, X.; Huang, K.; Chin, W.; Wu, J. Org. Lett. 2011, 13, 6026. (9) (a) Wu, J.; Pisula, W.; Müllen, K. Chem. Rev. 2007, 107, 718. (b) Grzybowski, M.; Skonieczny, K.; Butenschön, H.; Gryko, D. Angew. Chem., Int. Ed. 2013, 52, 9900. (c) Narita, A.; Wang, X.; Feng, X.; Müllen, K. Chem. Soc. Rev. 2015, 44, 6616. (d) Nowak-Król, A.; Gryko, D. T. Org. Lett. 2013, 15, 5618. (e) Krzeszewski, M.; Gryko, D. T. J. Org. Chem. 2015, 80, 2893. (f) Chou, C.; Saito, S.; Yamaguchi, S. Org. Lett. 2014, 16, 2868. (10) Wong, W.-Y.; Wang, X.-Z.; He, Z.; Djurii, A. B.; Yip, C.-T.; Cheung, K.-Y.; Wang, H.; Mak, C. S. K.; Chan, W.-K. Nat. Mater. 2007, 6, 521. (11) (a) Ostrick, J. R.; Dodabalapur, A.; Torsi, L.; Lovinger, A. J.; Kwock, E. W.; Miller, T. M.; Galvin, M.; Berggren, M.; Katz, H. E. J. Appl. Phys. 1997, 81, 6804. (b) Yamada, K.; Takeya, J.; Takenobu, T.; Iwasa, Y. Appl. Phys. Lett. 2008, 92, 253311. (c) Itaka, K.; Yamashiro, M.; Yamaguchi, J.; Haemori, M.; Yaginuma, S.; Matsumoto, Y.; Kondo, M.; Koinuma, H. Adv. Mater. 2006, 18, 1713. (12) (a) Wang, C.; Dong, H.; Hu, W.; Liu, Y.; Zhu, D. Chem. Rev. 2012, 112, 2208. (b) Takacs, C. J.; Sun, Y.; Welch, G. C.; Perez, A.; Liu, X.; Wen, W.; Bazan, G. C.; Heeger, A. J. J. Am. Chem. Soc. 2012, 134, 16597. (c) Huang, L.; Stolte, M.; Bürckstümmer, H.; Würthner, F. Adv. Mater. 2012, 24, 5750. (d) Wang, X.; Lin, H.; Lei, T.; Yang, D.; Zhuang, F.; Wang, J.; Pei, J. Angew. Chem., Int. Ed. 2013, 52, 3117. (e) Lv, A.; Stolte, M.; Würthner, F. Angew. Chem., Int. Ed. 2015, 54, 10512. (13) (a) Kumar, S.; Ho, M.; Tao, Y. Org. Lett. 2016, 18, 4876. (b) Nakano, M.; Sawamoto, M.; Yuki, M.; Takimiya, K. Org. Lett. 2016, 18, 3770. (c) Ie, Y.; Uchida, A.; Kawaguchi, N.; Nitani, M.; Tada, H.; Kakiuchi, F.; Aso, Y. Org. Lett. 2016, 18, 4320. (d) Rao, M. R.; Black, H. T.; Perepichka, D. F. Org. Lett. 2015, 17, 4224. (e) Miyamoto, F.; Nakatsuka, S.; Yamada, K.; Nakayama, K.; Hatakeyama, T. Org. Lett. 2015, 17, 6158. (f) Chen, W.; Tian, K.; Song, X.; Zhang, Z.; Ye, K.; Yu, G.; Wang, Y. Org. Lett. 2015, 17, 6146. (g) Ye, Q.; Chang, J.; Shi, X.; Dai, G.; Zhang, W.; Huang, K.-W.; Chi, C. Org. Lett. 2014, 16, 3966. (14) (a) Bura, T.; Leclerc, N.; Fall, S.; Lévêque, P.; Heiser, T.; Retailleau, P.; Rihn, S.; Mirloup, A.; Ziessel, R. J. Am. Chem. Soc. 2012, 134, 17404. (b) Poe, A.; Della Pelle, A. M.; Subrahmanyam, A. V.; White, W.; Wantz, G.; Thayumanavan, S. Chem. Commun. 2014, 50, 2913. (c) Singh, S.; Venugopalan, V.; Krishnamoorthy, K. Phys. Chem. Chem. Phys. 2014, 16, 13376. (d) Liu, W.; Tang, A.; Chen, J.; Wu, Y.; Zhan, C.; Yao, J. ACS Appl. Mater. Interfaces 2014, 6, 22496. (e) Gao, D.; Zhang, X.; Kong, X.; Chen, Y.; Jiang, J. ACS Appl. Mater. Interfaces 2015, 7, 2486. (f) Lu, G.; Kong, X.; Ma, P.; Wang, K.; Chen, Y.; Jiang, J. ACS Appl. Mater. Interfaces 2016, 8, 6174. (15) (a) Popere, B. C.; Pelle, A. M. D.; Thayumanavan, S. Macromolecules 2011, 44, 4767. (b) Usta, H.; Yilmaz, M. D.; Avestro, A.; Boudinet, D.; Denti, M.; Zhao, W.; Stoddart, J. F.; Facchetti, A. Adv. Mater. 2013, 25, 4327. (c) Yoshii, R.; Yamane, H.; Nagai, A.; Tanaka, K.; Taka, H.; Kita, H.; Chujo, Y. Macromolecules 2014, 47, 2316. (d) Debnath, S.; Singh, S.; Bedi, A.; Krishnamoorthy, K.; Zade, S. S. J. Phys. Chem. C 2015, 119, 15859.
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Changjiang Yu: 0000-0002-9509-7778 Lijuan Jiao: 0000-0002-3895-9642 Jian Pei: 0000-0002-2222-5361 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work is supported by the National Nature Science Foundation of China (21372011, 21402001, and 21302009) and the Nature Science Foundation of Anhui Province (1508085J07). The numerical calculations in this paper have been done on the supercomputing system in the Supercomputing Center of USTC.
■
REFERENCES
(1) (a) Yuan, L.; Lin, W.; Zheng, K.; He, L.; Huang, W. Chem. Soc. Rev. 2013, 42, 622. (b) Pansare, V. J.; Hejazi, S.; Faenza, W. J.; Prud’homme, R. K. Chem. Mater. 2012, 24, 812. (c) Fischer, G. M.; Daltrozzo, E.; Zumbusch, A. Angew. Chem., Int. Ed. 2011, 50, 1406. (d) Ding, Y.; Tang, Y.; Zhu, W.; Xie, Y. Chem. Soc. Rev. 2015, 44, 1101. (e) Lei, Z.; Li, X.; Luo, X.; He, H.; Zheng, J.; Qian, X.; Yang, Y. Angew. Chem., Int. Ed. 2017, 56, 2979. (2) (a) Loudet, A.; Burgess, K. Chem. Rev. 2007, 107, 4891. (b) Ulrich, G.; Ziessel, R.; Harriman, A. Angew. Chem., Int. Ed. 2008, 47, 1184. (c) Boens, N.; Leen, V.; Dehaen, W. Chem. Soc. Rev. 2012, 41, 1130. (d) Lu, H.; Mack, J.; Yang, Y.; Shen, Z. Chem. Soc. Rev. 2014, 43, 4778. (e) Ni, Y.; Wu, J. Org. Biomol. Chem. 2014, 12, 3774. (f) Zhao, J.; Xu, K.; Yang, W.; Wang, Z.; Zhong, F. Chem. Soc. Rev. 2015, 44, 8904. (3) (a) Killoran, J.; Allen, L.; Gallagher, J. F.; Gallagher, W. M.; O’Shea, D. F. Chem. Commun. 2002, 1862. (b) McDonnell, S. O.; Hall, M. J.; Allen, L. T.; Byrne, A.; Gallagher, W. M.; O’Shea, D. F. J. Am. Chem. Soc. 2005, 127, 16360. (c) McDonnell, S. O.; O’Shea, D. F. Org. Lett. 2006, 8, 3493. (d) Wu, D.; O’Shea, D. F. Org. Lett. 2013, 15, 3392. (e) Loudet, A.; Bandichhor, R.; Burgess, K.; Palma, A.; McDonnell, S. O.; Hall, M. J.; O’Shea, D. F. Org. Lett. 2008, 10, 4771. (f) Ge, Y.; O’Shea, D. F. Chem. Soc. Rev. 2016, 45, 3846. (4) (a) Coskun, A.; Yilmaz, M. D.; Akkaya, E. U. Org. Lett. 2007, 9, 607. (b) Bellier, Q.; Pegaz, S.; Aronica, C.; Guennic, B. L.; Andraud, C.; Maury, O. Org. Lett. 2011, 13, 22. (c) Gresser, G.; Hartmann, H. M.; Wrackmeyer, K. L.; Riede, M. Tetrahedron 2011, 67, 7148. (d) Bellier, Q.; Dalier, F.; Jeanneau, E.; Maury, O.; Andraud, C. New J. Chem. 2012, 36, 768. (e) Bhuniya, S.; Lee, H.; Jeon, H. M.; Han, J. H.; Lee, J. H.; Park, N.; Maiti, S.; Kang, C.; Kim, J. S. Chem. Commun. 2013, 49, 7141. (f) Jiao, L.; Wu, Y.; Wang, S.; Hu, X.; Zhang, P.; Yu, C.; Cong, K.; Meng, Q.; Hao, E.; Vicente, M. G. H. J. Org. Chem. 2014, 79, 1830. (g) Guo, S.; Ma, L.; Zhao, J.; Kucukoz, B.; Karatay, A.; Hayvali, M.; Yaglioglu, H. G.; Elmali, A. Chem. Sci. 2014, 5, 489. (h) Ziegler, C. J.; Chanawanno, K.; Hasheminsasab, A.; Zatsikha, Y. V.; Maligaspe, E.; Nemykin, V. N. Inorg. Chem. 2014, 53, 4751. (i) Jiang, X.; Xi, D.; Sun, C.; Guan, J.; He, M.; Xiao, L. Tetrahedron Lett. 2015, 56, 4868. (5) (a) Donyagina, V. F.; Shimizu, S.; Kobayashi, N.; Lukyanets, E. A. Tetrahedron Lett. 2008, 49, 6152. (b) Lu, H.; Shimizu, S.; Mack, J.; Shen, Z.; Kobayashi, N. Chem. - Asian J. 2011, 6, 1026. (c) Liu, H.; Mack, J.; Guo, Q.; Lu, H.; Kobayashi, N.; Shen, Z. Chem. Commun. 2011, 47, 12092. (d) Gresser, G.; Hummert, M.; Hartmann, H.; Leo, K.; Riede, M. Chem. - Eur. J. 2011, 17, 2939. (e) Zheng, W.; Wang, B.; Li, C.; Zhang, J.; Wan, C.; Huang, J.; Liu, J.; Shen, Z.; You, X. Angew. Chem., Int. Ed. 2015, 54, 9070. (f) Majumdar, P.; Mack, J.; Nyokong, T. RSC Adv. 2015, 5, 78253. 2896
DOI: 10.1021/acs.orglett.7b01133 Org. Lett. 2017, 19, 2893−2896
