CH coupling of pentafluorophenyl lithium

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Journal of Organometallic Chemistry 867 (2018) 278e283

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Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem

Direct C-Li/C-H coupling of pentafluorophenyl lithium with azines An atom- and step-economical strategy for the synthesis of polyfluoroaryl azaaromatics* Mikhail V. Varaksin a, b, Timofey D. Moseev a, Valery N. Charushin a, b, Oleg N. Chupakhin a, b, * a b

Ural Federal University, 19 Mira Str., 620002 Ekaterinburg, Russia Institute of Organic Synthesis, Ural Branch of the Russian Academy of Sciences, 22 S. Kovalevskaya Str., 620041 Ekaterinburg, Russia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 October 2017 Accepted 16 January 2018 Available online 31 January 2018

2 The SH N methodology has successfully been applied for the direct C(sp )-H functionalization of azaaromatics through the C-Li/C-H coupling of pentafluorophenyl lithium with azines and their N-oxides. As a result, a number of novel fluorinated biheterocyclic ensembles, that are of interest in the design of bioactive molecules and advanced materials, have been prepared in good to excellent yields under rather mild condition. © 2018 Elsevier B.V. All rights reserved.

Keywords: C-H functionalization C-C coupling Pentafluorophenyl lithium Azines Azine-N-Oxides

1. Introduction One of the challenges for modern synthetic chemistry is the development of pot, atom, and step economical (PASE) methods to obtain organic molecules through the direct formation of novel carbon-carbon bonds [1]. An enhanced interest in these methods is due to the fact that a great deal of practically useful organic compounds are structurally based on a bi(hetero)aryl motif [2]. In the series of bi(hetero)aryls, fluorine-containing compounds are of special value due to a crucial role of fluorine atom (atoms), affecting dramatically their physical and chemical properties, as well as biological activities [3]. In particular, being compared with a non-fluorinated analogue, a polyfluoroaryl substituent proved to possess a much stronger electron-withdrawing effect, increasing significantly the photoluminescence efficiency, minimizing the self-quenching effect, and lowering the HOMO/LUMO energy levels [4]. Due to these features, a fluorinated bi(hetero)aryl motif is widely presented in chemicals used in both life and material

*

The paper is dedicated to Prof. Irina P. Beletskaya on the occasion of her 85th birthday. * Corresponding author. Ural Federal University, 19 Mira Str., 620002 Ekaterinburg, Russia. E-mail address: [email protected] (O.N. Chupakhin). https://doi.org/10.1016/j.jorganchem.2018.01.020 0022-328X/© 2018 Elsevier B.V. All rights reserved.

sciences, namely in agrochemicals and organic compounds of medicinal interest (antibiotics, anti-inflammatories, anti-hypertensives, anticancer and antifungal drugs), as well as in polymers, liquid crystals, molecular electronic devices, such as field-effect transistors (FETs) and organic light-emitting diodes (OLEDs) [5]. Therefore, the development of atom- and stage-efficient methods for incorporating a polyfluoroaryl moiety into (aza)aromatic substrates is undoubtedly to be a key challenge in modern synthetic chemistry. One of the promising approaches to perform C-C couplings of fluoroarenes with azines is based on using the C(sp2)-H functionalization [6] in the series of p-electron deficient arenes and heteroarenes, including the methodology of nucleophilic substitution of hydrogen (SH N) [7]. The green chemistry [8] tended approach, based on the direct C-Li/C-H couplings of pentafluorophenyl lithium with azines, has been chosen as the main strategy, enabling us to obtain a wide range of polyfluoroaryl substituted azaaromatics in good to excellent yields under mild conditions, without any catalysis by transition metals. 2. Results and discussion It is well-known that pentafluorobenzene 1 undergoes lithiation selectively by action of n-BuLi or other strong bases to give pentafluorophenyl lithium 2 [9], which is relatively stable due to the

M.V. Varaksin et al. / Journal of Organometallic Chemistry 867 (2018) 278e283

electron withdrawing effect of fluorine groups in the aromatic ring. In this study, an elucidation of the reactivity of pentafluorophenyl lithium in transition metal-free C-C couplings with azines and their activated forms, N-oxides, has been carried out for the first time. It should be noted that no direct C-Li/C-H coupling reactions of pentafluorophenyllithium with heteroarenes have so far been described. As a result of these stage- and atom-economic C(sp2)-H functionalizations of azines, a wide variety of novel bifunctional organic molecules, bearing both polyfluoroaryl and heterocyclic scaffolds, have successfully been obtained. According to the modern concept, the nucleophilic substitution of hydrogen (SH N) [7] is a two-step process that can be implemented in two ways, either either “Addition - Oxidation” SH N(AO) or the “Addition - Elimination” SH N(AE) protocols (Table 1). In case of C-H functionalization of 1,2,4-triazines, the stable pentafluorophenyl substituted dihydrotriazine 4 is formed as the intermediate sHadduct at the first stage in 63% yield. In order to convert 4 into the corresponding SH N product, a number of oxidants, such as DDQ, oChloranil, and p-Chloranyl, have been tested to find out the optimal reaction conditions. It has finally been found that the best yield of the target product (63%) can be achieved using DDQ. In case of C-H modifications of mono-, di-, and triazine-N-oxides, lithium compound 2 is added to the C¼NþeO- bond of azine N-oxides to afford unstable N-hydroxy sH-adducts 7a-d at the first stage. Intermediates 7a-d can then be converted into a variety of products 5, 8a-c and 9, depending on azaaromatic structures and reaction conditions used for the second step (aromatization). Thus, pentafluorophenyl azines 5, 8a-c have been found to be formed in 54e78% yields through the eliminative aromatization. Studying the reactivity of triazines, the C-C coupling conditions have also been optimized by using various deoxygenating agents such as AcCl, Ac2O, and TFFA. The highest yield of triazine 5 (65%) was shown when the reaction mixture had been treated with AcCl as a deoxygenating reagent. Additionally, in order to obtain the C-C coupling products of azine-N-oxides with retention of N-oxide function, the features of oxidative aromatization have been studied as well. It has been found that pentafluorophenyl azine 9 is the only product of the reaction of 3,6-diphenyl-1,2,4-triazine-4-oxide 6d. The use of mono- and diazine-N-oxides in the deoxygenative coupling with pentafluorobenzene lithium 2 has been observed to give no desired biheterocyclic products, the reaction mass being a mixture of starting materials and degradation products. Pentafluorophenyl-modified azaheterocycles obtained contain both the novel compounds 4, 5, 8c, 9 and the known ones 8a and 8b. It should be noted that the described pentafluorophenylcontaining quinoline 8a and quinoxaline 8b have earlier been obtained from chloroazines or/and azinyl tosylate by using Pd(II)- or Cu(I)-catalyzed C-C coupling reactions. Contrary to that, the SH N approach used at the present work is based on nucleophilic attack of pentafluorophenyl lithium (generated in situ) on the azine C(sp2)-H bond. It enables one to carry out analogous C-C coupling reactions in full accordance with the principles of pot and stage efficiency. In other words, the use of SH N reactions provides comparable yields of the desired products without a prior incorporation of chlorine (or other auxiliary groups) into azine and catalysis by transition metals, such as palladium and copper (Table 1, Entries 4 and 5). The compounds obtained have been characterized by elemental analysis, IR, 1H, 13C, 19F NMR, as well as the data of mass spectrometry. The IR, 1H, 13C, and 19F NMR spectra have been found to be in a full compliance with the proposed structures. In particular, the characteristic absorption bands, corresponding to the stretching vibrations of C¼N groups, have been registered at n 15961655 cm1 in the IR spectra for compounds 4, 5, 8, 9. The IR spectrum of dihydro compound 4 contains also the characteristic NH

279

stretching band at n 3066 cm1. The mass spectra of all compounds measured exhibit the corresponding molecular ion peaks. In the 1H NMR spectra of fluorinated heterocyclic compounds 4, 5, 8, and 9, the resonance signals of (hetero)aryl substituents are observed at d 7.35e8.51 ppm, while in the 13C NMR spectra of the same compounds the corresponding signals are exhibited at d 122.7e160.8 ppm. In the 19F NMR spectra, the fluorine nuclei resonate as three detached multiplet signals at d 162.3 e (134.4) ppm. For instance, the 1H NMR spectra of pentafluoro-containing dihydrotriazine 4 (a), triazine 5 (b), and triazine-N-oxide 9 (c) are presented in Fig. 1. Notably, in the 1H NMR spectra of the sH-adduct 4, the signal of proton attached to the sp3-hybridized carbon C(sp3)eH is observed at d 6.41 ppm, while the NH proton resonates at d 11.48 ppm. In the 13C NMR spectra of dihydro triazine 4, the carbon resonance C(sp3)eH signal is observed at d 46.4 ppm; no signals in these fields have been found in the spectra of heteroaromatic analogue 5. The structures of polyfluroaryl-substituted heterocycles have also been proved by X-ray analysis (Fig. 2). The appropriate crystals of 8c were obtained by crystallization from a mixture of heptane/ CH2Cl2, 1:1. In accordance with the X-ray analysis data, the mono crystals of 8c proved to belong to the space group P121/n1 (monoclinic crystal system). No considerable deviations in bond lengths and bond angles from the standard values [12] have been observed. Pentafluorobenxene and phthalazine rings are nearly planar, and deflections of atoms from the mean square plane are less than 0.021 Å. The angle between the median planes of the pentafluorobenzene and phthalazine rings is 78.09 . 3. Conclusion In summary, a pot, atom and step economic (PASE) method has been developed, based on exploiting the green chemistry-oriented SH N methodology. A number of biheterocyclic compounds, bearing both pentafluorophenyl and azaaromatic fragments, linked to each other through the C-C bonds, have been obtained in good to excellent yields. The SH N approach used enables the fluorinated bi(hetero)arene ensembles to be obtained from pentafluorobenzene and azines in situ. The compounds synthesized seem to be of interest in drug and agrochemical design, as well as they can possibly be used in molecular electronic devices and other advanced materials. 4. Experimental section 4.1. General experimental methods The 1H NMR (400 MHz) and 13C NMR (100 MHz), 19F (376 MHz) spectra were recorded using TMS as the internal standard and DMSO-d6 as a deuterated solvent. The X-ray diffraction analysis was performed on a diffractometer, equipped with CDD detector (Mo KR graphite-monochromated radiation, l ¼ 1.54184 Å, u-scanning technique, the scanning step was 1 and the exposure time per frame was 10 s at 295(2) K. Analytical absorption correction was used in the reflection intensity integration [13]. The structure was solved by the direct method and refined applying full matrix leastsquares versus F2hkl with anisotropic displacement parameters for all non-hydrogen atoms using the SHELX97 program package [14]. All hydrogen atoms were located in different electron density maps and refined using a riding model with fixed thermal parameters. The mass spectra were recorded on a mass spectrometer with sample ionization by electron impact (EI). The IR spectra were recorded using a Fourier-transforminfrared spectrometer equipped with a diffuse reflection attachment. The elemental analysis was carried out on a CHNS/O analyzer. The course of the reactions was

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Table 1 Synthesis of pentaflurophenyl-modified azaheterocycles 5, 8, and 9.

Entry

Starting azine/azine-N-oxide

C-C Coupling Product

Isolated yield (%)

Remarks Known/new

Described yield (%)

1

63

new

e

2

63(i) 57(ii) 51(iii)

new

e

3

65(iv) 40(v) trace(vi)

new

4

78(iv)

knowna

82e90 [10,11]

5

71(iv)

knownb

73

6

54

new

e

(iv)

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281

Table 1 (continued ) Entry

Starting azine/azine-N-oxide

C-C Coupling Product

Isolated yield (%)

62 55 53

7

(i)

(i)

Remarks Known/new

Described yield (%)

new

e

(ii) (iii)

DDQ, (ii) o-Chloranil, (iii) p-Chloranil, (iv) AcCl, (v) Ac2O, (vi) TFAA. Pd(II)- or Cu(I)-Catalyzed C-C coupling of 2-chloroquinoline and pentafluorobenzene. b Pd(II)-Catalyzed C-C coupling of of quinozalin-2-yl-tosylate and pentafluorobenzene. a

monitored by TCL on 0.25 mm silica gel plates (60F 254). Column chromatography was performed on silica gel (60, 0.035e0.070 mm (220e440 mesh)). n-BuLi (1.6 M solution in hexane), DDQ, ochloranil, p-chloranil, AcCl, Ac2O, TFAA, quinoline-N-oxide 6a, quinoxaline-N-oxide 6b, and phthalazine-N-oxide 6c were purchased. 3,6-Diphenyl-1,2,4-triazine 3 [15] and 3,6-diphenyl-1,2,4triazine-4-oxide 6d [16] were prepared according to literature procedures.

4.2. General method for the synthesis of 5-(perfluorophenyl)-3,6diphenyl-4,5-dihydro-1,2,4-triazine (4) and the characterization data To a vigorously stirring solution of pentafluorobenzene 1 (1.00 mmol, 0.1 mL) in dry THF (3 mL) at 78 C under argon, a 1.6 M solution of n-BuLi in hexane (1.1 mmol, 0.688 mL) was added

dropwise The mixture then was stirred for 40 min at 78  C and a solution of 3,6-diphenyl-1,2,4-triazine 3 (1.1 mmol, 0.256 mg) in anhydrous THF (15 mL) was added. The mixture was allowed to warm up to ambient temperature and stirred for additional 1 h. The resulted mixture was subjected to a silica gel column chromatography with a mixture of hexane/EtOAc, 6:4 as an eluent. The formed eluate was finally concentrated to dryness under reduced pressure. Yield: 252 mg (63%), mp ¼ 235e240  C. Rf 0.2 (hexane/EtOAc, 6:4). 1Н NMR (DMSO-d6): d 6.41 (s, 1H); 7.35e7.42 (m, 3H); 7.43e7.52 (m, 3H); 7.63e7.68 (m, 2H); 7.83e7.88 (m, 2H); 11.48 (s, 1H) ppm. 13C NMR (DMSO-d6): d 46.4; 115.6e116.0 (m); 125.1; 126.3; 128.2; 128.5; 129.3; 130.7; 132.1; 134.4; 135.5e136.0 (m); 137.2; 138.1e138.5 (m); 141.2e141.5 (m); 143.1e143.6 (m); 145.6e146.0 (m); 150.4 ppm. 19F NMR (DMSO-d6): 161.9 e (161.8) (m); 154.6 e (154.5) (m); 144.2 e (144.1) (m) ppm. IR (DRA): n 3066, 2923, 1654, 1577, 1504, 1416, 1324, 1262, 1173, 1118, 1007, 962, 804, 703, 619, 585, 524 cm1. MS (EI): m/z 401 [M]þ. Anal. Calcd for C21H12F5N3: C, 62.85; H, 3.01; F, 23.67; N, 10.47. Found: C, 62.46; H, 3.38; N, 10.07. 4.3. General method for the synthesis of polyfluoric azaheterocycles (5, 8a-c) and the characterization data To a vigorously stirring solution of pentafluorobenzene 1 (1.00 mmol, 0.1 mL) in dry THF (3 mL) at 78  C under argon, a 1.6 M solution of n-BuLi in hexane (1.1 mmol, 0.688 mL) was added dropwise. The mixture was stirred for 40 min at 78  C and a solution of the corresponding azine-N-oxide (1.1 mmol) in anhydrous THF (10 mL) was added. The mixture was allowed to warm up to 0  C and the corresponding acylating agent (1.3 mmol) was added.

Fig. 1. The 1H NMR spectra of 5-pentafluorophenyl compounds 4, 5, and 9 based on 1,2,4-triazine scaffold in DMSO-d6 at 295 K.

Fig. 2. Molecular structure of 1-(perfluorophenyl)phthalazine 8c (CCDC 1582106). Selected bond distances (Å) and angles ( ): C(1)eC(11), 1.514; C(1)eC(10), 1.424; C(1)e N(2), 1.287; N(2)eN(3), 1.391; N(3)eC(4), 1.300; C(4)eC(5), 1.418; C(5)eC(10), 1.405; C(5)eC(6), 1.406; C(6)eC(7), 1.322; C(7)eC(8), 1.414; C(8)eC(9), 1.372; C(9)eC(10), 1.389; C(11)-C(12), 1.381, C(12)-C(13), 1.387; C(13)-C(14), 1.330; C(14)-C(15), 1.353; C(15)-C(16), 1.364; C(16)-C(11), 1.374; C(11)eC(1)eC(10), 119.46 ; C(11)eC(1)eN(2), 114.91.

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Then the mixture was allowed to warm up to ambient temperature and stirred for additional 1 h. The resulted mixture was subjected to silica gel column chromatography with a mixture of EtOAc/hexane as an eluent and the formed eluate was concentrated to dryness under reduced pressure. 4.3.1. 5-(Perfluorophenyl)-3,6-diphenyl-1,2,4-triazine (5) Yield: 259 mg (65%), mp ¼ 185e190  C. Rf 0.1 (hexane/EtOAc, 6:4). 1Н NMR (DMSO-d6): d 7.44e7.57 (m, 3H); 7.61e7.70 (m, 5H); 8.47e8.51 (m, 2H) ppm. 13C NMR (DMSO-d6): d 110.5e111.0 (m); 127.7; 128.0; 128.6; 128.9; 130.1; 131.9; 133.2; 133.4; 135.7e136.1 (m); 138.2e138.6 (m); 140.3e140.8 (m); 142.2e142.6 (m); 144.8e145.0 (m); 145.2; 156.5; 160.8 ppm. 19F NMR (DMSOd6): 160.6 e (160.4) (m); 150.1 e (150.0) (m); 140.6 e (140.5) (m) ppm. IR (DRA): n 2924, 1650, 1518, 1492, 1389, 1111, 1023, 836, 763, 691, 667, 555 cm1. MS (EI): m/z 399 [M]þ. Anal. Calcd for C21H10F5N3: C, 63.16; H, 2.52; N, 10.52. Found: C, 62.86; H, 2.91; N, 10.31. 4.3.2. 2-(Perfluorophenyl)quinoxaline (8a) Yield: 230 mg (78%), mp ¼ 145e150  C. Rf 0.2 (hexane/EtOAc, 6:4). 1Н NMR (DMSO-d6): d 7.98e8.02 (m, 2H), 8.18e8.24 (m, 2H), 9.21 (s, 1H) ppm. 13C NMR (DMSO-d6): d 112.0e112.5 (m); 128.9; 129.1; 131.1; 131.5; 135.8e136.3 (m); 138.4e138.8 (m); 139.9e140.2 (m); 141.1; 141.3; 142.3; 143.0e143.3 (m); 145.5e145.7 (m); 145.8 ppm. 19F NMR (DMSO-d6): 161.7 e (161.5) (m); 152.3 e (152.2) (m); 143.2 e (143.1) (m) ppm. IR (DRA): n 3400, 2368, 2256, 1650, 1571, 1484, 1428, 1322, 1195, 1125, 1024, 996, 796, 732 cm1. MS (EI): m/z 296 [M]þ. Anal. Calcd for C14H5F5N2: C, 56.77; H, 1.70; N, 9.46. Found: C, 56.82; H, 1.93; N, 9.56. 4.3.3. 2-(Perfluorophenyl)quinoline (8b) Yield: 209 mg (71%), mp ¼ 215e220  C. Rf 0.2 (hexane/EtOAc, 6:4). 1Н NMR (DMSO-d6): d 7.69e7.8 (m, 2H); 7.83e7.89 (m, 1H); 8.07e8.12 (m, 2H); 8.59 (d, 1H, J ¼ 8.44) ppm. 13C NMR (DMSO-d6): d 114.9e115.4 (m); 122.7; 127.0; 127.6; 127.8; 128.8; 130.2; 135.6e136.1 (m); 137.1; 138.1e138.6 (m); 139.1e139.4 (m); 142.5e142.9 (m); 145.1e145.4 (m); 146.4; 147.3 ppm. 19F NMR (DMSO-d6): 162.3 e (162.1) (m); 154.2 e (154.1) (m); 143.7 e (143.6) (m) ppm. IR (DRA): 3400, 2924, 2339, 1969, 1650, 1495, 1465, 1322, 1195, 1024, 986, 923, 732 n cm1. MS (EI): m/z 295 [M]þ. Anal. Calcd for C15H6F5N: C, 61.03; H, 2.05; N, 4.74. Found: C, 60.78; H, 1.93; N, 4.95. 4.3.4. 1-(Perfluorophenyl)phthalazine (8c) Yield: 160 mg (54%), mp ¼ 140e145  C. Rf 0.1 (hexane/EtOAc, 6:4). 1Н NMR (DMSO-d6): d 7.82e7.87 (m, 1H); 7.99e8.12 (m, 2H); 8.29e8.33 (m, 1H); 9.79 (s, 1H) ppm. 13C NMR (DMSO-d6): d 110.1e110.7 (m); 124.5; 125.3; 126.0; 127.2; 133.8; 134.2; 135.9e136.4 (m); 138.4e138.9 (m); 140.2e140.5 (m); 142.5e143.3 (m); 145.3e145.6 (m); 148.6; 152.3 ppm. 19F NMR (DMSOd6): 161.2 e (161.1) (m); 152.8 e (152.7) (m); 140.5 e (140.4) (m) ppm. IR (DRA): n 2937, 1655, 1577, 1562, 1526, 1496, 1354, 1396, 1281, 1196, 1151, 982, 809, 754, 562 cm1. MS (EI): m/z 296 [M]þ. Anal. Calcd for C14H5F5N2: C, 56.77; H, 1.70; N, 9.46. Found: C, 56.63; H, 1.85; N, 9.41. 4.4. General method for the synthesis and characterization data of 5-(perfluorophenyl)-3,6-diphenyl-1,2,4-triazine-4-oxide (9) To a vigorously stirring solution of pentafluorobenzene (1.00 mmol, 0.1 mL) in dry THF (3 mL) at 78  C under argon, a 1.6 M solution of n-BuLi in hexane (1.1 mmol, 0.688 mL) was added dropwise. The mixture was stirred for 40 min at 78  C and a solution of the corresponding 3,6-diphenyl-1,2,4-triazine-4-oxide 6d

(1.1 mmol) in anhydrous THF (17 mL) was added. The mixture was allowed to warm up to 0  C and a THF solution (5 mL) of the corresponding oxidative agent (1.35 mmol) in THF (5 mL) was added. Then the mixture was allowed to warm up to ambient temperature and stirred for additional 1 h. The resulted mixture was filtered off through Al2O3, washed with EtOAc and the eluate was concentrated to dryness under reduced pressure. The product 9 was purified by silica gel column chromatography with the EtOAc/hexane mixture as an eluent, the resulted eluate was concentrated to dryness under reduced pressure. Yield: 257 mg (62%), mp ¼ 145e150  C. Rf 0.5 (hexane/EtOAc, 6:4). 1Н NMR (DMSO-d6): d 7.44e7.49 (m, 2H); 7.51e7.56 (m, 3H); 7.57e7.66 (m, 3H); 8.20e8.26 (m, 2H) ppm. 13C NMR (DMSO-d6): d 103.4e103.9 (m); 127.9; 128.3; 128.4; 128.6; 129.6; 130.4; 131.5; 132.5; 133.2; 135.8e136.2 (m); 138.2e138.7 (m); 141.0e141.2 (m); 142.6e148.8 (m); 145.0e145.3 (m); 156.2; 157.5 ppm. 19F NMR (DMSO-d6): 160.7e (160.6) (m); 148.5 e (148.4) (m); 134.5e (134.4) (m) ppm. IR (DRA): n 3060, 1739, 1596, 1499, 1441, 1361, 1312, 1158, 1079, 1017, 945, 847, 740, 635, 590 cm1. MS (EI): m/z 415 [M]þ. Anal. Calcd for C21H10F5N3: C, 60.73; H, 2.43; N, 10.12; O, 3.85 Found: C, 60.41; H, 2.71; N, 10.38. 4.5. Method for the synthesis of 5-(perfluorophenyl)-3,6-diphenyl1,2,4-triazine (5) from 5-(perfluorophenyl)-3,6-diphenyl-4,5dihydro-1,2,4-triazine by oxidative protocol (4) To a vigorously stirring solution of 5-(perfluorophenyl)-3,6diphenyl-4,5-dihydro-1,2,4-triazine 4 (1.1 mmol) in EtOAc (10 mL) a corresponding oxidative agent (1.3 mmol) in EtOAc (2 mL) was added and the mixture was refluxed for 30 min. The resulted mixture was subjected to Al2O3 column cromatography with a mixture of hexane/EtOAc, 6:4 as an eluent and the formed eluate was finally concentrated to dryness under reduced pressure. Acknowledgements The research was financially supported by the Russian Science Foundation (Project No. 14-13-01177). Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.jorganchem.2018.01.020. References [1] (a) Y.H. Zhang, G.F. Shi, J.-Q. Yu (Eds.), Carbonecarbon s-Bond Formation via CeH Bond Functionalization, in Comprehensive Organic Synthesis, second ed., Elsevier, Oxford, 2014, pp. 1101e1209; (b) R.V. Hoffman, Organic Chemistry: an Intermediate Text, second ed., John Wiley & Sons, Inc, 2004; (c) C.-J. Li (Ed.), From CeH to CeC Bonds: Cross-dehydrogenative Coupling, Royal Society of Chemistry, Cambridge, UK, 2015; (d) S.H. Cho, J.Y. Kim, J. Kwak, S. Chang, Recent advances in the transition metal-catalyzed twofold oxidative CeH bond activation strategy for CeC and CeN bond formation, Chem. Soc. Rev. 40 (2011) 5068e5083, https://doi.org/ 10.1039/c1cs15082k; (e) C.S. Yeung, V.M. Dong, Catalytic dehydrogenative cross-coupling: forming carbon-carbon bonds by oxidizing two carbon-hydrogen bonds, Chem. Rev. 111 (2011) 1215e1292, https://doi.org/10.1021/cr100280d. [2] (a) D. Zhao, J. You, C. Hu, Recent progress in coupling of two heteroarenes, Chem. Eur J. 17 (2011) 5466e5492, https://doi.org/10.1002/chem.201003039; (b) Y. Yang, J. Lan, J. You, Oxidative C-H/C-H coupling reactions between two (Hetero)arenes, Chem. Rev. 117 (2017) 8787e8863, https://doi.org/10.1021/ acs.chemrev.6b00567; (c) M. Simonetti, D.M. Cannas, I. Larrosa, Biaryl synthesis via CeH bond actirez (Ed.), Advances in Organomevation: strategies and methods, in: P.J. Pe tallic Chemistry, vol. 67, Elsevier, Amsterdam, 2017, pp. 299e399; (d) G.J.P. Perry, I. Larrosa, Recent progress in decarboxylative oxidative crosscoupling for biaryl synthesis, Eur. J. Org Chem. 2017 (2017) 3517e3527,

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