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Rhodium-catalyzed direct oxidative cross-coupling of 2-aryl pyridine with benzothiazoles
Tetrahedron 70 (2014) 6474e6481
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Rhodium-catalyzed direct oxidative cross-coupling of 2-aryl pyridine with benzothiazoles Hui Liu, Huan Xu, Yu Yuan * College of Chemistry and Chemical Engineering, Yangzhou University, 225002 Yangzhou, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 May 2014 Received in revised form 1 July 2014 Accepted 7 July 2014 Available online 10 July 2014
A rhodium-catalyzed cross-coupling reaction of 2-aryl pyridine and benzothiazoles via dual CeH bond functionalization has been developed in the presence of copper salts. The reaction system provides a new approach to heterobiaryl species, which are ubiquitous in pharmaceuticals and nature products. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Rhodium 2-Aryl pyridine Benzothiazoles Dehydrogenation Cross-coupling
1. Introduction
2. Results and discussion
Heterobiaryl species are usually very important compounds in biological, pharmaceutical, and material sciences due to the construction of backbone of pharmaceuticals and nature products. The traditional method for synthesis of these compounds is crosscoupling reactions of aryl halides with heteroarylmetals or arylmetals with heteroaryl halides catalyzed by different metals.1 In the past decades, several efficient catalytic systems have been developed in the direct arylation of heterocycle CeH bonds with aryl halides employing Pd,2 Rh,3 Ru,4 Ir,5 Fe,6 Co,7 Ni,8 and Cu.9 A more efficient protocol for the synthesis would be crossdehydrogenative-coupling of arenes and heteroarenes directly by using palladium,10 copper,11 rhodium,12 and ruthenium13 as the catalyst. In these cases, several heteroarene derivatives have been studied to achieve this goal, such as oxazoles,14 thiazoles,15 imidazoles,16 furans,17 thiophenes,18 pyrroles.19 Among these research, Miura and co-workers developed CeH functionalization of 2-aryl pyridine with benzoxazole in the presence of copper salt in good yields.14a You’s group realized this transformation by using (RhCp*Cl2)2 as the catalyst in the presence of copper salt.12j In continuing our efforts in transition metalcatalyzed CeH functionalization,20 herein, we report rhodium (II) acetate catalyze direct cross-dehydrogenative-coupling reaction of 2-aryl pyridine with benzothiazoles in the presence of Cu(OAc)2 (Fig. 1).
Initially, the cross-coupling reaction of 2-phenylpyridine (1a) with benzothiozole (2a) was carried out in the presence of 3 equiv Cu(OAc)2 in N,N-dimethylacetamide (DMA) at 160 C for 9 h under an argon atmosphere, forming the desired product (3aa) in 15% yields (Table 1, entry 1). Interestingly, while catalytic rhodium acetate dimmer and Xantphos (entry 4) were used in the system, the yield was increased to 42%. The other ligands (entries 5e7) also could improve the reactivity of rhodium species and the best ligand is triphenylphosphine (entry 7). Usually, CuI is an efficient reagent in the coupling reaction of benzothiozoles,21 which was chosen as an additive. It was found that 0.5 equiv CuI was employed in this reaction under former conditions to obtain the product 3aa in 81% yield (entry 11). While the amount of CuI was increased to 1 equiv the yield could be achieved to 90% without diheteroarylated product, which showed the good regioselectivity and higher yield compared with the literature.14a (entry 12). Whereas only 30% yield was obtained in the reaction using CuCl as the additive (entry 13). It was shown that the Cu(OAc)2 was necessary for the reaction since the absence of Cu(OAc)2 induced the decrease of the yield (58%) even 1 equiv CuI was added under the conditions (entry 14). Moreover, the reaction could not carry out without two copper salts using rhodium acetate as the catalyst (entry 15). With a highly active catalytic system in hand, the scope of 2phenylpyridines was investigated. As expected, a variety of substituted 2-phenylpyridines 1 were reacted with
* Corresponding author. E-mail address: [email protected] (Y. Yuan). http://dx.doi.org/10.1016/j.tet.2014.07.022 0040-4020/Ó 2014 Elsevier Ltd. All rights reserved.
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Fig. 1. The work on dehydrogenative cross-coupling of heteroarenes.
Table 1 Optimization of the reaction conditionsa
Entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 a b c d e
Cat. (mol %) Pd(OAc)2 (10) RhCl(PPh3)3 (10) [Rh(OAc)2]2 (5) [Rh(OAc)2]2 (5) [Rh(OAc)2]2 (5) [Rh(OAc)2]2 (5) [Rh(OAc)2]2 (5) [Rh(OAc)2]2 (5) [Rh(OAc)2]2 (5) [Rh(OAc)2]2 (5) [Rh(OAc)2]2 (5) [Rh(OAc)2]2 (5) [Rh(OAc)2]2 (5) [Rh(OAc)2]2 (5)
L (mol %)
Additive
Xantphosc (20) Xantphos (20) 1,10-Phenanthroline (20) 2,20 -Bipyridine (20) PPh3 (20) PPh3 (20) PPh3 (20) PPh3 (20) PPh3 (20) PPh3 (20) PPh3 (20) PPh3 (20) PPh3 (20)
CuI(0.5) CuI(1) CuCl(1) CuI(1)
Solvents
Yield (%)b
DMA DMA DMA DMA DMA DMA DMA Mesitylene DMSO DMF DMA DMA DMA DMA DMA
15 0 18 42 34 24 53 17 46 36 81 90 30 58d Tracee
Reaction conditions: 1a (0.2 mmol), 2a (0.6 mmol), catalyst, copper acetate (0.6 mmol), additive, solvent (2.0 mL), 160 C, 9 h. Yield after purification. 4,5-Bis-diphenylphosphanyl-9,9-dimethyl-9H-xanthene. Without 3 equiv Cu(OAC)2. Without Cu(OAc)2 and CuI.
benzothiazole 2a to evaluate the substitution effects in 2phenylpyridine. The electro-rich substituent on the aryl ring of 2-aryl pyridine showed good reactivities in the reaction. 94% Yield of products could be achieved using 2-(4-methoxyphenyl) pyridine 1e as the substrate (Table 2, entry 5). In contrast, electro-deficient substituent on the aryl ring of 2-aryl pyridine hindered the reaction, which induced to form moderate yield of corresponding product (entries 7e9). Fortunately, the chloro group was compatible with the reaction conditions in 62% yield (entry 10).
Next, various benzothiazole derivatives worked well under the optimal conditions. A range of functional groups could be tolerated in this reaction, such as alkyl, methoxy, chloro, nitro, and ester group. Among these cases, the electro-rich substituent on the phenyl ring of benzothiozoles benefited from the reaction (Table 3, entries 1 and 2), whereas the electro-deficient substituents affected the reaction efficient (entries 3 and 4). However, the reaction was turned less active while the substituted benzothiazole with nitro group (entry 5) was employed in the transformation. Unfortunately, benzoxazole was not a nice substrate under the
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Table 2 Reaction of 2-phenylpyridine derivatives with benzothiazolea
Entry
1
2
2-Phenylpyridine 1
1a
1b
3
1c
4
1d
5
Yield (%)b
Product 3
3aa
90
3ba
78
3ca
53
3da
1e
3ea
1f
3fa
89
94
81
6
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Table 2 (continued ) Entry
7
2-Phenylpyridine 1
Yield (%)b
Product 3
1g
3ga
76
74
8
1h
9
1i
10
1j
3ha
3ia
72
62
3ja
a b
Reaction conditions: 1a (0.2 mmol), 2 (0.6 mmol), [Rh(OAc)2]2 (5 mol %), PPh3 (20 mol %), Cu(OAc)2 (3 equiv), CuI (1 equiv), DMA (2.0 mL), 160 C, 9 h. Yield after purification.
conditions, and the corresponding product 3ag was formed only in 60% yield (entry 6). A hypothetical mechanism for the oxidative cross-coupling of 2aryl pyridine with benzothiazoles is depicted in Scheme 1. First, rhodium (II) acetate was oxidized to Rh (III) complex22 in the presence of Cu(OAc)2 and PPh3, which is the real active species, through the chelate-directed CeH activation of 2-aryl pyridine 1 form the intermediate A. Then rhodium complex A reacted with copper complex B, which was confirmed by a previous report about the reaction of benzothiazole 2 with CuI,15b,23 to produce intermediate C via transmetalation. The desired product 3 was obtained while the intermediate C underwent reductive elimination. Finally, the formed Rh (I) species was oxidized by Cu(OAc)2 to regenerated Rh (III) to fulfill the catalytic cycle. Meanwhile, the copper acetate converted to Cu (0), which could be found after reaction.
method provided a concise access to heteroarene-containing biaryl structure, which could be applied in pharmacy and organic synthesis. 4. Experimental section 4.1. General methods 1 H NMR and 13C NMR spectra were obtained with a Bruker AVANCE 600 spectrometer in CDCl3 with TMS as an internal standard. Chemical shifts (d) are reported in parts per million, and coupling constants (J) are in Hertz (Hz). The following abbreviations were used to explain the multiplicities: s¼singlet, d¼doublet, t¼triplet, q¼quartet, m¼multiplet, br¼broad. FTIR spectra were obtained with a Cary 610/670 spectrophotometer. HRMS were obtained on a Finnigan MAT8430 instrument. All reactions were monitored by TLC. Flash column chromatograph was carried out using 300e400 mesh silica gel at medium pressure.
3. Conclusions 4.2. Typical procedure In conclusion, we have developed an efficient rhodium-catalyzed cross-dehydrogenative-coupling of 2-aryl pyridine with benzothiazoles in moderate to good yields in the presence of copper salts. This
[Rh(OAc)2]2 (4.4 mg, 0.01 mmol), 2-phenylpyridine 1a (31 mg, 0.2 mmol), PPh3 (10.4 mg, 0.04 mmol), CuI (38.2 mg, 0.2 mmol),
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Table 3 Reaction of 2-phenylpyridine with benzothiazole derivativesa
Entry
1
Benzothiazole 2
Yield (%)b
Product 3
2
2c
3
81
3ab
2b
3ac
3ad
2d
85
93
4
2e
3ae
91
5
2f
3af
85
6
60
2g 3ag
a b
Reaction conditions: 1a (0.2 mmol), 2 (0.6 mmol), [Rh(OAc)2]2 (5 mol %), PPh3 (20 mol %), Cu(OAc)2 (3 equiv), CuI (1 equiv), DMA (2.0 mL), 160 C, 9 h. Yield after purification.
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Scheme 1. Plausible mechanism for the oxidative cross-coupling of 2-aryl pyridine with benzothiazoles.
Cu(OAc)2 (108.6 mg, 0.6 mmol), N,N-dimethylacetamide (2.0 mL) were placed inside a 10 mL Schlenk tube equipped with a reflux condenser. Then, the temperature was risen to 160 C. Benzothiazole 2a (81 mg, 0.6 mmol) was separated into five portions and each portion was added within 1 h. After finishing addition, the solution was stirred under argon atmosphere at 160 C for another 4 h. After cooling to room temperature, the reaction mixture was quenched with ethyl acetate (15 mL) and washed with water (30 mL). The aqueous phase was extracted twice with EtOAc (310 mL), and the combined organic layer was dried over anhydrous Na2SO4. After evaporation of the solvents in vacuo, the crude product was purified by column chromatography on silica gel (petroleum ether/EtOAc¼3/1) to give desire product 3aa as yellow oil; yield: 51.8 mg (90%). 4.2.1. 2-[2-(Pyridin-2-yl)phenyl] benzothiazole (3aa). Yield: 90%. Yellow oil. 1H NMR (600 MHz, DMSO-D6) d 8.52 (d, J¼4.8 Hz, 1H), 8.03e7.90 (m, 3H), 7.78 (t, J¼11.6 Hz, 1H), 7.71e7.59 (m, 3H), 7.41 (m, 4H); 13C NMR (150 MHz, DMSO-D6) d 167.0, 157.6, 152.6, 149.2, 140.4, 136.6, 135.8, 132.2, 130.6, 130.4, 130.4, 128.7, 126.2, 125.2, 124.5, 122.8, 122.7, 121.9; IR (KBr) n¼3059, 1585, 1562, 1522, 1457, 1434, 1312, 1268, 1218, 1022, 964, 794, 759, 690, 616, 437 cm1; HRMS: calcd for C18H12N2S: 288.0721, found: 288.0725. 4.2.2. 2-(4-Methyl-2-(pyridin-2-yl)phenyl)benzothiazole (3ba).Yield: 78%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.63 (d, J¼3.3 Hz, 1H), 7.96 (d, J¼8.1 Hz, 1H), 7.85 (d, J¼7.9 Hz, 1H), 7.72 (d, J¼8.0 Hz, 1H), 7.54 (td, J¼7.7, 1.7 Hz, 1H), 7.45 (s, 1H), 7.43e7.39 (m, 1H), 7.34 (dd, J¼7.8, 0.8 Hz, 1H), 7.32e7.27 (m, 1H), 7.26e7.23 (m, 1H), 7.20 (m, 1H), 2.45 (s, 3H); 13C NMR (150 MHz, CDCl3) d 167.9, 158.6, 153.4, 149.7, 140.8, 140.6, 136.6, 136.2, 131.5, 130.8, 130.1, 129.7, 126.1, 125.4, 125.0, 123.4, 122.5, 121.5, 21.6; IR (KBr) n¼3060, 2972, 2901, 1559, 1456, 1430, 1313, 1263, 1065, 962, 761,
728, 692 cm1; HRMS: calcd for C19H14N2S: 302.0878, found: 302.0876. 4.2.3. 2-(2,4-Dimethyl-6-(pyridin-2-yl)phenyl)benzothiazole (3ca). Yield: 53%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.54e8.50 (m, 1H), 8.05 (d, J¼8.2 Hz, 1H), 7.74 (d, J¼7.8 Hz, 1H), 7.48e7.43 (m, 1H), 7.37 (s, 1H), 7.36e7.31 (m, 2H), 7.19e7.14 (m, 2H), 7.02 (m, 1H), 2.42 (s, 3H), 2.28 (s, 3H); 13C NMR (150 MHz, CDCl3) d 167.8, 158.6, 153.3, 149.4, 141.2, 140.0, 138.1, 136.9, 135.9, 131.6, 129.6, 128.5, 126.0, 125.1, 124.7, 123.5, 121.9, 121.6, 21.5, 20.5; IR (KBr) n¼3058, 2922, 2856, 1586, 1565, 1520, 1456, 1436, 1313, 1277, 1219, 1086, 1038, 861, 792, 759, 687 cm1; HRMS: calcd for C20H16N2S: 316.1034, found: 316.1039. 4.2.4. 2-(3-Methoxy-2-(pyridin-2-yl)phenyl)benzothiazole (3da).Yield: 89%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.64 (d, J¼4.7 Hz, 1H), 7.94 (d, J¼8.2 Hz, 1H), 7.72e7.64 (m, 3H), 7.49 (dd, J¼10.7, 5.5 Hz, 1H), 7.41e7.37 (m, 1H), 7.34 (d, J¼7.8 Hz, 1H), 7.26 (m, 2H), 7.11 (d, J¼8.3 Hz, 1H), 3.78 (s, 3H); 13C NMR (150 MHz, CDCl3) d 167.3, 157.6, 155.6, 153.5, 149.7, 136.7, 136.3, 134.7, 129.9, 126.9, 126.1, 125.1, 123.5, 122.7, 122.7, 121.4, 113.1, 56.3; IR (KBr) n¼3060, 2930, 2851, 1592, 1495, 1461, 1431, 1380, 1312, 1260, 1083, 992, 800, 759, 664 cm1; HRMS: calcd for C19H14N2OS: 318.0827, found: 318.0831. 4.2.5. 2-(5-Methoxy-2-(pyridin-2-yl)phenyl)benzothiazole (3ea).Yield: 94%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.59 (d, J¼4.2 Hz, 1H), 8.00 (d, J¼8.1 Hz, 1H), 7.74 (d, J¼7.9 Hz, 1H), 7.58 (d, J¼8.5 Hz, 1H), 7.51 (t, J¼7.7 Hz, 1H), 7.48e7.41 (m, 2H), 7.32 (t, J¼7.5 Hz, 1H), 7.21 (d, J¼7.8 Hz, 1H), 7.18e7.14 (m, 1H), 7.11 (dd, J¼8.5, 2.0 Hz, 1H), 3.91 (s, 3H); 13C NMR (150 MHz, CDCl3) d 167.9, 159.9, 158.2, 153.5, 149.7, 136.7, 136.2, 134.0, 133.5, 132.2, 126.2, 125.4, 125.3, 123.6, 122.1, 121.6, 116.9, 115.3, 55.9; IR (KBr) n¼3059,
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2923, 2853, 1603, 1502, 1461, 1428, 1318, 1265, 1239, 1172, 1044, 1018, 886, 788, 758 cm1; HRMS: calcd for C19H14N2OS: 318.0827, found: 318.0826. 4.2.6. 2-(5-(Benzyloxy)-2-(pyridin-2-yl)phenyl)benzothiazole (3fa). Yield: 81%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.59 (d, J¼4.5 Hz, 1H), 8.00 (d, J¼8.1 Hz, 1H), 7.74 (d, J¼7.9 Hz, 1H), 7.57 (dd, J¼10.2, 5.5 Hz, 2H), 7.52 (m, 1H), 7.47e7.42 (m, 3H), 7.39 (t, J¼7.5 Hz, 2H), 7.33 (t, J¼7.3 Hz, 2H), 7.21 (d, J¼7.8 Hz, 1H), 7.19e7.15 (m, 2H), 5.18 (s, 2H); 13C NMR (150 MHz, CDCl3) d 167.8, 159.0, 158.1, 153.3, 149.7, 136.7, 136.2, 134.0, 133.7, 132.3, 128.9, 128.3, 127.9, 127.8, 126.2, 125.3, 125.3, 123.6, 122.3, 122.2, 121.6, 121.5, 117.4, 116.5, 70.5; IR (KBr) n¼3061, 2925, 2857, 1602, 1562, 1499, 1459, 1429, 1381, 1318, 1288, 1235, 1126, 1042, 1015, 893, 816, 759, 731, 697 cm1; HRMS: calcd for C25H18N2OS: 394.1140, found: 394.1141. 4.2.7. 2-(2-(Pyridin-2-yl)-5-(trifluoromethyl)phenyl)benzothiazole (3ga). Yield: 76%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.64 (d, J¼4.5 Hz, 1H), 8.27 (s, 1H), 8.01 (d, J¼8.2 Hz, 1H), 7.81 (d, J¼8.0 Hz, 1H), 7.79e7.73 (m, 2H), 7.60 (t, J¼7.7 Hz, 1H), 7.45 (t, J¼7.6 Hz, 1H), 7.37e7.25 (m, 3H); 13C NMR (150 MHz, CDCl3) d 165.9, 157.1, 153.2, 145.0, 136.7, 136.4, 133.7, 131.4, 127.34 (q, J¼148.5 Hz), 125.6, 125.2, 123.8, 123.1, 121.6; IR (KBr) n¼3061, 1587, 1522, 1492, 1457, 1404, 1331, 1260, 1211, 1129, 1085, 1037, 899, 792, 758, 728, 704 cm1; HRMS: calcd for C19H11F3N2S: 356.0595, found: 356.0590. 4.2.8. Methyl 4-(benzothiazol-2-yl)-3-(pyridin-2-yl)benzoate (3ha). Yield: 74%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.62 (d, J¼4.1 Hz, 1H), 8.29 (s, 1H), 8.18 (d, J¼8.0 Hz, 1H), 8.07 (d, J¼8.1 Hz, 1H), 7.99 (d, J¼8.2 Hz, 1H), 7.75 (d, J¼8.0 Hz, 1H), 7.62 (t, J¼7.6 Hz, 1H), 7.44 (t, J¼7.6 Hz, 1H), 7.34 (t, J¼7.6 Hz, 2H), 7.28e7.24 (m, 1H), 3.94 (s, 3H); 13C NMR (150 MHz, CDCl3) d 166.5, 166.5, 157.7, 153.3, 149.9, 140.9, 136.9, 136.7, 136.6, 132.1, 131.8, 131.1, 129.8, 126.4, 125.6, 125.3, 123.8, 122.9, 121.6, 52.6; IR (KBr) n¼3060, 2960, 2870, 1691, 1590, 1494, 1462, 1314, 1259, 1093, 1019, 798 cm1; HRMS: calcd for C20H14N2O2S: 346.0776, found: 346.0778. 4.2.9. 1-(3-(Benzothiazol-2-yl)-4-(pyridin-2-yl)phenyl)ethanone (3ia). Yield: 72%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.63 (d, J¼4.6 Hz, 1H), 8.52 (s, 1H), 8.15 (dd, J¼8.0, 1.4 Hz, 1H), 8.00 (d, J¼8.2 Hz, 1H), 7.76 (d, J¼8.0 Hz, 2H), 7.59 (t, J¼7.7 Hz, 1H), 7.45 (t, J¼7.8 Hz, 1H), 7.34 (t, J¼7.6 Hz, 1H), 7.29 (d, J¼7.8 Hz, 1H), 7.24 (t, J¼6.3 Hz, 1H), 2.69 (s, 3H); 13C NMR (150 MHz, CDCl3) d 197.3, 166.7, 157.5, 153.4, 145.0, 144.9, 137.3, 136.7, 136.4, 133.5, 131.4, 131.2, 129.7, 126.4, 125.5, 125.2, 123.9, 123.1, 121.6, 27.0; IR (KBr) n¼3060, 2924, 1687, 1600, 1498, 1543, 1431, 1357, 1314, 1252, 1206, 1095, 1025, 904, 790, 758, 694 cm1; HRMS: calcd for C20H14N2OS: 330.0827, found: 330.0824. 4.2.10. 2-(5-Chloro-2-(pyridin-2-yl)phenyl)benzothiazole (3ja).Yield: 62%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.61 (d, J¼4.7 Hz, 1H), 7.99 (dd, J¼7.9, 5.1 Hz, 2H), 7.74 (d, J¼8.0 Hz, 1H), 7.60e7.52 (m, 3H), 7.44 (t, J¼7.5 Hz, 1H), 7.33 (t, J¼7.6 Hz, 1H), 7.23 (m, 2H); 13C NMR (150 MHz, CDCl3) d 166.2, 157.4, 153.3, 149.9, 139.2, 136.7, 136.3, 135.0, 134.4, 132.2, 130.7, 130.4, 126.4, 125.5, 125.3, 123.8, 122.8, 121.6; IR (KBr) n¼3060, 1590, 1494, 1457, 1428, 1393, 1314, 1255, 1099, 1037, 895, 782, 759, 729, 691 cm1; HRMS: calcd for C18H11ClN2S: 322.0331, found: 322.0329. 4. 2.11. 4-Methyl-2 -(2-(pyridin -2-yl)phenyl)benz othiazole (3ab). Yield: 81%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.65 (s, 1H), 7.99 (s, 1H), 7.71e7.52 (m, 5H), 7.37e7.18 (m, 4H), 2.60 (s, 3H); 13 C NMR (150 MHz, CDCl3) d 166.2, 159.1, 153.1, 149.6, 140.8, 133.6, 133.1, 130.9, 130.9, 130.2, 128.9, 126.6, 125.2, 125.1, 122.3, 118.9, 18.4; IR (KBr) n¼3057, 2922, 2852, 1584, 1560, 1520, 1464, 1425, 1315,
1266, 1217, 1062, 1022, 969, 884, 760 cm1; HRMS: calcd for C19H14N2S: 302.0878, found: 302.0877. 4.2.12. 6-Methoxy-2-(2-(pyridin-2-yl)phenyl)benzothiazole (3ac). Yield: 85%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.59 (d, J¼4.2 Hz, 1H), 8.00 (d, J¼8.1 Hz, 1H), 7.74 (d, J¼7.9 Hz, 1H), 7.58 (d, J¼8.5 Hz, 1H), 7.51 (t, J¼7.7 Hz, 1H), 7.48e7.41 (m, 2H), 7.32 (t, J¼7.5 Hz, 1H), 7.21 (d, J¼7.8 Hz, 1H), 7.18e7.14 (m, 1H), 7.11 (dd, J¼8.5, 2.0 Hz, 1H), 3.91 (s, 3H); 13C NMR (150 MHz, CDCl3) d 167.9, 159.9, 158.2, 153.3, 149.7, 136.7, 136.2, 134.0, 133.5, 132.2, 126.2, 125.4, 125.3, 123.6, 122.1, 121.6, 116.9, 115.3, 55.9; IR (KBr) n¼3061, 2931, 1723, 1602, 1559, 1464, 1435, 1318, 1265, 1226, 1118, 1025, 892, 827, 794, 758, 702 cm1; HRMS: calcd for C19H14N2OS: 318.0827, found: 318.0832. 4.2.13. Methyl 2-(2-(pyridin-2-yl)phenyl)benzothiazole-5carboxylate (3ad). Yield: 93%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.61 (s, 1H), 8.59 (d, J¼4.7 Hz, 1H), 7.98 (dd, J¼18.6, 8.0 Hz, 2H), 7.79 (d, J¼8.4 Hz, 1H), 7.59 (m, 4H), 7.31 (d, J¼7.8 Hz, 1H), 7.24e7.21 (m, 1H), 3.94 (s, 3H); 13C NMR (150 MHz, CDCl3) d 169.18, 167.13, 158.35, 153.29, 149.74, 141.37, 140.91, 136.39, 132.52, 130.93, 130.9, 130.8, 129.0, 128.5, 125.8, 125.2, 125.1, 122.7, 121.4, 52.5; IR (KBr) n¼3059, 2954, 2851, 1720, 1586, 1562, 1462, 1432, 1303, 1243, 1149, 1092, 904, 794, 759, 694 cm1; HRMS: calcd for C20H14N2O2S: 346.0776, found: 346.0778. 4. 2.14 . 5-Chloro-2-(2-(pyridin-2-yl)phenyl) benz othiazol e (3ae). Yield: 91%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.59 (d, J¼4.5 Hz, 1H), 7.93 (dd, J¼7.8, 1.5 Hz, 2H), 7.66e7.56 (m, 4H), 7.54 (m, 1H), 7.29 (dd, J¼8.5, 1.9 Hz, 1H), 7.22 (m, 1H); 13C NMR (150 MHz, CDCl3) d 169.8, 158.4, 154.2, 149.7, 141.5, 140.9, 136.4, 134.9, 132.6, 132.2, 130.9, 130.7, 129.0, 125.7, 125.1, 123.3, 122.6, 122.3; IR (KBr) n¼3060, 1590, 1494, 1457, 1428, 1393, 1314, 1255, 1099, 1037, 895, 782, 759, 729, 691 cm1; HRMS: calcd for C18H11ClN2S: 322.0331, found: 322.0333. 4 . 2 .15 . 6 - N i t r o - 2 - ( 2 - ( p y r i d i n - 2 - yl ) p h e n yl ) b e n z o t h i a z o l e (3af). Yield: 85%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.68 (d, J¼2.3 Hz, 1H), 8.57 (d, J¼4.4 Hz, 1H), 8.29 (dd, J¼8.9, 2.3 Hz, 1H), 8.02 (t, J¼8.4 Hz, 2H), 7.66 (td, J¼7.7, 1.8 Hz, 1H), 7.62 (d, J¼3.9 Hz, 2H), 7.60e7.55 (m, 1H), 7.37 (d, J¼7.8 Hz, 1H), 7.28e7.24 (m, 1H); 13C NMR (150 MHz, CDCl3) d 173.8, 158.1, 157.0, 149.8, 145.0, 141.1, 136.9, 136.7, 132.1, 131.4, 131.0, 129.1, 125.0, 123.7, 122.9, 121.7, 118.2; IR (KBr) n¼3061, 1566, 1513, 1463, 1439, 1339, 1261, 968, 794, 748, 692 cm1; HRMS: calcd for C18H11N3O2S: 333.0572, found: 333.0578. 4.2.16. 2-[2-(Pyridin-2-yl)phenyl]benzoxazole (3ag). Yield: 60%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.54 (d, J¼4.8 Hz, 1H), 8.12 (dd, J¼7.7, 1.1 Hz, 1H), 7.71e7.59 (m, 4H), 7.55 (td, J¼7.6, 1.3 Hz, 1H), 7.33 (d, J¼7.8 Hz, 1H), 7.31e7.24 (m, 3H), 7.23e7.20 (m, 1H); 13C NMR (150 MHz, CDCl3) d 163.8, 158.8, 150.9, 149.5, 142.0, 141.3, 136.4, 131.3, 131.1, 130.9, 128.8, 126.6, 125.1, 124.5, 123.9, 122.3, 120.4, 110.7; IR (KBr) n¼3060, 1586, 1564, 1453, 1344, 1302, 1242, 1074, 1031, 925, 795, 747, 679 cm1; HRMS: calcd for C18H12N2O: 272.0950, found: 272.0949. Acknowledgements This work was supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.tet.2014.07.022.
H. Liu et al. / Tetrahedron 70 (2014) 6474e6481
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Rhodium-catalyzed direct oxidative cross-coupling of 2-aryl pyridine with benzothiazoles Hui Liu, Huan Xu, Yu Yuan * College of Chemistry and Chemical Engineering, Yangzhou University, 225002 Yangzhou, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 May 2014 Received in revised form 1 July 2014 Accepted 7 July 2014 Available online 10 July 2014
A rhodium-catalyzed cross-coupling reaction of 2-aryl pyridine and benzothiazoles via dual CeH bond functionalization has been developed in the presence of copper salts. The reaction system provides a new approach to heterobiaryl species, which are ubiquitous in pharmaceuticals and nature products. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Rhodium 2-Aryl pyridine Benzothiazoles Dehydrogenation Cross-coupling
1. Introduction
2. Results and discussion
Heterobiaryl species are usually very important compounds in biological, pharmaceutical, and material sciences due to the construction of backbone of pharmaceuticals and nature products. The traditional method for synthesis of these compounds is crosscoupling reactions of aryl halides with heteroarylmetals or arylmetals with heteroaryl halides catalyzed by different metals.1 In the past decades, several efficient catalytic systems have been developed in the direct arylation of heterocycle CeH bonds with aryl halides employing Pd,2 Rh,3 Ru,4 Ir,5 Fe,6 Co,7 Ni,8 and Cu.9 A more efficient protocol for the synthesis would be crossdehydrogenative-coupling of arenes and heteroarenes directly by using palladium,10 copper,11 rhodium,12 and ruthenium13 as the catalyst. In these cases, several heteroarene derivatives have been studied to achieve this goal, such as oxazoles,14 thiazoles,15 imidazoles,16 furans,17 thiophenes,18 pyrroles.19 Among these research, Miura and co-workers developed CeH functionalization of 2-aryl pyridine with benzoxazole in the presence of copper salt in good yields.14a You’s group realized this transformation by using (RhCp*Cl2)2 as the catalyst in the presence of copper salt.12j In continuing our efforts in transition metalcatalyzed CeH functionalization,20 herein, we report rhodium (II) acetate catalyze direct cross-dehydrogenative-coupling reaction of 2-aryl pyridine with benzothiazoles in the presence of Cu(OAc)2 (Fig. 1).
Initially, the cross-coupling reaction of 2-phenylpyridine (1a) with benzothiozole (2a) was carried out in the presence of 3 equiv Cu(OAc)2 in N,N-dimethylacetamide (DMA) at 160 C for 9 h under an argon atmosphere, forming the desired product (3aa) in 15% yields (Table 1, entry 1). Interestingly, while catalytic rhodium acetate dimmer and Xantphos (entry 4) were used in the system, the yield was increased to 42%. The other ligands (entries 5e7) also could improve the reactivity of rhodium species and the best ligand is triphenylphosphine (entry 7). Usually, CuI is an efficient reagent in the coupling reaction of benzothiozoles,21 which was chosen as an additive. It was found that 0.5 equiv CuI was employed in this reaction under former conditions to obtain the product 3aa in 81% yield (entry 11). While the amount of CuI was increased to 1 equiv the yield could be achieved to 90% without diheteroarylated product, which showed the good regioselectivity and higher yield compared with the literature.14a (entry 12). Whereas only 30% yield was obtained in the reaction using CuCl as the additive (entry 13). It was shown that the Cu(OAc)2 was necessary for the reaction since the absence of Cu(OAc)2 induced the decrease of the yield (58%) even 1 equiv CuI was added under the conditions (entry 14). Moreover, the reaction could not carry out without two copper salts using rhodium acetate as the catalyst (entry 15). With a highly active catalytic system in hand, the scope of 2phenylpyridines was investigated. As expected, a variety of substituted 2-phenylpyridines 1 were reacted with
* Corresponding author. E-mail address: [email protected] (Y. Yuan). http://dx.doi.org/10.1016/j.tet.2014.07.022 0040-4020/Ó 2014 Elsevier Ltd. All rights reserved.
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Fig. 1. The work on dehydrogenative cross-coupling of heteroarenes.
Table 1 Optimization of the reaction conditionsa
Entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 a b c d e
Cat. (mol %) Pd(OAc)2 (10) RhCl(PPh3)3 (10) [Rh(OAc)2]2 (5) [Rh(OAc)2]2 (5) [Rh(OAc)2]2 (5) [Rh(OAc)2]2 (5) [Rh(OAc)2]2 (5) [Rh(OAc)2]2 (5) [Rh(OAc)2]2 (5) [Rh(OAc)2]2 (5) [Rh(OAc)2]2 (5) [Rh(OAc)2]2 (5) [Rh(OAc)2]2 (5) [Rh(OAc)2]2 (5)
L (mol %)
Additive
Xantphosc (20) Xantphos (20) 1,10-Phenanthroline (20) 2,20 -Bipyridine (20) PPh3 (20) PPh3 (20) PPh3 (20) PPh3 (20) PPh3 (20) PPh3 (20) PPh3 (20) PPh3 (20) PPh3 (20)
CuI(0.5) CuI(1) CuCl(1) CuI(1)
Solvents
Yield (%)b
DMA DMA DMA DMA DMA DMA DMA Mesitylene DMSO DMF DMA DMA DMA DMA DMA
15 0 18 42 34 24 53 17 46 36 81 90 30 58d Tracee
Reaction conditions: 1a (0.2 mmol), 2a (0.6 mmol), catalyst, copper acetate (0.6 mmol), additive, solvent (2.0 mL), 160 C, 9 h. Yield after purification. 4,5-Bis-diphenylphosphanyl-9,9-dimethyl-9H-xanthene. Without 3 equiv Cu(OAC)2. Without Cu(OAc)2 and CuI.
benzothiazole 2a to evaluate the substitution effects in 2phenylpyridine. The electro-rich substituent on the aryl ring of 2-aryl pyridine showed good reactivities in the reaction. 94% Yield of products could be achieved using 2-(4-methoxyphenyl) pyridine 1e as the substrate (Table 2, entry 5). In contrast, electro-deficient substituent on the aryl ring of 2-aryl pyridine hindered the reaction, which induced to form moderate yield of corresponding product (entries 7e9). Fortunately, the chloro group was compatible with the reaction conditions in 62% yield (entry 10).
Next, various benzothiazole derivatives worked well under the optimal conditions. A range of functional groups could be tolerated in this reaction, such as alkyl, methoxy, chloro, nitro, and ester group. Among these cases, the electro-rich substituent on the phenyl ring of benzothiozoles benefited from the reaction (Table 3, entries 1 and 2), whereas the electro-deficient substituents affected the reaction efficient (entries 3 and 4). However, the reaction was turned less active while the substituted benzothiazole with nitro group (entry 5) was employed in the transformation. Unfortunately, benzoxazole was not a nice substrate under the
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Table 2 Reaction of 2-phenylpyridine derivatives with benzothiazolea
Entry
1
2
2-Phenylpyridine 1
1a
1b
3
1c
4
1d
5
Yield (%)b
Product 3
3aa
90
3ba
78
3ca
53
3da
1e
3ea
1f
3fa
89
94
81
6
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Table 2 (continued ) Entry
7
2-Phenylpyridine 1
Yield (%)b
Product 3
1g
3ga
76
74
8
1h
9
1i
10
1j
3ha
3ia
72
62
3ja
a b
Reaction conditions: 1a (0.2 mmol), 2 (0.6 mmol), [Rh(OAc)2]2 (5 mol %), PPh3 (20 mol %), Cu(OAc)2 (3 equiv), CuI (1 equiv), DMA (2.0 mL), 160 C, 9 h. Yield after purification.
conditions, and the corresponding product 3ag was formed only in 60% yield (entry 6). A hypothetical mechanism for the oxidative cross-coupling of 2aryl pyridine with benzothiazoles is depicted in Scheme 1. First, rhodium (II) acetate was oxidized to Rh (III) complex22 in the presence of Cu(OAc)2 and PPh3, which is the real active species, through the chelate-directed CeH activation of 2-aryl pyridine 1 form the intermediate A. Then rhodium complex A reacted with copper complex B, which was confirmed by a previous report about the reaction of benzothiazole 2 with CuI,15b,23 to produce intermediate C via transmetalation. The desired product 3 was obtained while the intermediate C underwent reductive elimination. Finally, the formed Rh (I) species was oxidized by Cu(OAc)2 to regenerated Rh (III) to fulfill the catalytic cycle. Meanwhile, the copper acetate converted to Cu (0), which could be found after reaction.
method provided a concise access to heteroarene-containing biaryl structure, which could be applied in pharmacy and organic synthesis. 4. Experimental section 4.1. General methods 1 H NMR and 13C NMR spectra were obtained with a Bruker AVANCE 600 spectrometer in CDCl3 with TMS as an internal standard. Chemical shifts (d) are reported in parts per million, and coupling constants (J) are in Hertz (Hz). The following abbreviations were used to explain the multiplicities: s¼singlet, d¼doublet, t¼triplet, q¼quartet, m¼multiplet, br¼broad. FTIR spectra were obtained with a Cary 610/670 spectrophotometer. HRMS were obtained on a Finnigan MAT8430 instrument. All reactions were monitored by TLC. Flash column chromatograph was carried out using 300e400 mesh silica gel at medium pressure.
3. Conclusions 4.2. Typical procedure In conclusion, we have developed an efficient rhodium-catalyzed cross-dehydrogenative-coupling of 2-aryl pyridine with benzothiazoles in moderate to good yields in the presence of copper salts. This
[Rh(OAc)2]2 (4.4 mg, 0.01 mmol), 2-phenylpyridine 1a (31 mg, 0.2 mmol), PPh3 (10.4 mg, 0.04 mmol), CuI (38.2 mg, 0.2 mmol),
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Table 3 Reaction of 2-phenylpyridine with benzothiazole derivativesa
Entry
1
Benzothiazole 2
Yield (%)b
Product 3
2
2c
3
81
3ab
2b
3ac
3ad
2d
85
93
4
2e
3ae
91
5
2f
3af
85
6
60
2g 3ag
a b
Reaction conditions: 1a (0.2 mmol), 2 (0.6 mmol), [Rh(OAc)2]2 (5 mol %), PPh3 (20 mol %), Cu(OAc)2 (3 equiv), CuI (1 equiv), DMA (2.0 mL), 160 C, 9 h. Yield after purification.
H. Liu et al. / Tetrahedron 70 (2014) 6474e6481
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Scheme 1. Plausible mechanism for the oxidative cross-coupling of 2-aryl pyridine with benzothiazoles.
Cu(OAc)2 (108.6 mg, 0.6 mmol), N,N-dimethylacetamide (2.0 mL) were placed inside a 10 mL Schlenk tube equipped with a reflux condenser. Then, the temperature was risen to 160 C. Benzothiazole 2a (81 mg, 0.6 mmol) was separated into five portions and each portion was added within 1 h. After finishing addition, the solution was stirred under argon atmosphere at 160 C for another 4 h. After cooling to room temperature, the reaction mixture was quenched with ethyl acetate (15 mL) and washed with water (30 mL). The aqueous phase was extracted twice with EtOAc (310 mL), and the combined organic layer was dried over anhydrous Na2SO4. After evaporation of the solvents in vacuo, the crude product was purified by column chromatography on silica gel (petroleum ether/EtOAc¼3/1) to give desire product 3aa as yellow oil; yield: 51.8 mg (90%). 4.2.1. 2-[2-(Pyridin-2-yl)phenyl] benzothiazole (3aa). Yield: 90%. Yellow oil. 1H NMR (600 MHz, DMSO-D6) d 8.52 (d, J¼4.8 Hz, 1H), 8.03e7.90 (m, 3H), 7.78 (t, J¼11.6 Hz, 1H), 7.71e7.59 (m, 3H), 7.41 (m, 4H); 13C NMR (150 MHz, DMSO-D6) d 167.0, 157.6, 152.6, 149.2, 140.4, 136.6, 135.8, 132.2, 130.6, 130.4, 130.4, 128.7, 126.2, 125.2, 124.5, 122.8, 122.7, 121.9; IR (KBr) n¼3059, 1585, 1562, 1522, 1457, 1434, 1312, 1268, 1218, 1022, 964, 794, 759, 690, 616, 437 cm1; HRMS: calcd for C18H12N2S: 288.0721, found: 288.0725. 4.2.2. 2-(4-Methyl-2-(pyridin-2-yl)phenyl)benzothiazole (3ba).Yield: 78%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.63 (d, J¼3.3 Hz, 1H), 7.96 (d, J¼8.1 Hz, 1H), 7.85 (d, J¼7.9 Hz, 1H), 7.72 (d, J¼8.0 Hz, 1H), 7.54 (td, J¼7.7, 1.7 Hz, 1H), 7.45 (s, 1H), 7.43e7.39 (m, 1H), 7.34 (dd, J¼7.8, 0.8 Hz, 1H), 7.32e7.27 (m, 1H), 7.26e7.23 (m, 1H), 7.20 (m, 1H), 2.45 (s, 3H); 13C NMR (150 MHz, CDCl3) d 167.9, 158.6, 153.4, 149.7, 140.8, 140.6, 136.6, 136.2, 131.5, 130.8, 130.1, 129.7, 126.1, 125.4, 125.0, 123.4, 122.5, 121.5, 21.6; IR (KBr) n¼3060, 2972, 2901, 1559, 1456, 1430, 1313, 1263, 1065, 962, 761,
728, 692 cm1; HRMS: calcd for C19H14N2S: 302.0878, found: 302.0876. 4.2.3. 2-(2,4-Dimethyl-6-(pyridin-2-yl)phenyl)benzothiazole (3ca). Yield: 53%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.54e8.50 (m, 1H), 8.05 (d, J¼8.2 Hz, 1H), 7.74 (d, J¼7.8 Hz, 1H), 7.48e7.43 (m, 1H), 7.37 (s, 1H), 7.36e7.31 (m, 2H), 7.19e7.14 (m, 2H), 7.02 (m, 1H), 2.42 (s, 3H), 2.28 (s, 3H); 13C NMR (150 MHz, CDCl3) d 167.8, 158.6, 153.3, 149.4, 141.2, 140.0, 138.1, 136.9, 135.9, 131.6, 129.6, 128.5, 126.0, 125.1, 124.7, 123.5, 121.9, 121.6, 21.5, 20.5; IR (KBr) n¼3058, 2922, 2856, 1586, 1565, 1520, 1456, 1436, 1313, 1277, 1219, 1086, 1038, 861, 792, 759, 687 cm1; HRMS: calcd for C20H16N2S: 316.1034, found: 316.1039. 4.2.4. 2-(3-Methoxy-2-(pyridin-2-yl)phenyl)benzothiazole (3da).Yield: 89%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.64 (d, J¼4.7 Hz, 1H), 7.94 (d, J¼8.2 Hz, 1H), 7.72e7.64 (m, 3H), 7.49 (dd, J¼10.7, 5.5 Hz, 1H), 7.41e7.37 (m, 1H), 7.34 (d, J¼7.8 Hz, 1H), 7.26 (m, 2H), 7.11 (d, J¼8.3 Hz, 1H), 3.78 (s, 3H); 13C NMR (150 MHz, CDCl3) d 167.3, 157.6, 155.6, 153.5, 149.7, 136.7, 136.3, 134.7, 129.9, 126.9, 126.1, 125.1, 123.5, 122.7, 122.7, 121.4, 113.1, 56.3; IR (KBr) n¼3060, 2930, 2851, 1592, 1495, 1461, 1431, 1380, 1312, 1260, 1083, 992, 800, 759, 664 cm1; HRMS: calcd for C19H14N2OS: 318.0827, found: 318.0831. 4.2.5. 2-(5-Methoxy-2-(pyridin-2-yl)phenyl)benzothiazole (3ea).Yield: 94%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.59 (d, J¼4.2 Hz, 1H), 8.00 (d, J¼8.1 Hz, 1H), 7.74 (d, J¼7.9 Hz, 1H), 7.58 (d, J¼8.5 Hz, 1H), 7.51 (t, J¼7.7 Hz, 1H), 7.48e7.41 (m, 2H), 7.32 (t, J¼7.5 Hz, 1H), 7.21 (d, J¼7.8 Hz, 1H), 7.18e7.14 (m, 1H), 7.11 (dd, J¼8.5, 2.0 Hz, 1H), 3.91 (s, 3H); 13C NMR (150 MHz, CDCl3) d 167.9, 159.9, 158.2, 153.5, 149.7, 136.7, 136.2, 134.0, 133.5, 132.2, 126.2, 125.4, 125.3, 123.6, 122.1, 121.6, 116.9, 115.3, 55.9; IR (KBr) n¼3059,
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2923, 2853, 1603, 1502, 1461, 1428, 1318, 1265, 1239, 1172, 1044, 1018, 886, 788, 758 cm1; HRMS: calcd for C19H14N2OS: 318.0827, found: 318.0826. 4.2.6. 2-(5-(Benzyloxy)-2-(pyridin-2-yl)phenyl)benzothiazole (3fa). Yield: 81%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.59 (d, J¼4.5 Hz, 1H), 8.00 (d, J¼8.1 Hz, 1H), 7.74 (d, J¼7.9 Hz, 1H), 7.57 (dd, J¼10.2, 5.5 Hz, 2H), 7.52 (m, 1H), 7.47e7.42 (m, 3H), 7.39 (t, J¼7.5 Hz, 2H), 7.33 (t, J¼7.3 Hz, 2H), 7.21 (d, J¼7.8 Hz, 1H), 7.19e7.15 (m, 2H), 5.18 (s, 2H); 13C NMR (150 MHz, CDCl3) d 167.8, 159.0, 158.1, 153.3, 149.7, 136.7, 136.2, 134.0, 133.7, 132.3, 128.9, 128.3, 127.9, 127.8, 126.2, 125.3, 125.3, 123.6, 122.3, 122.2, 121.6, 121.5, 117.4, 116.5, 70.5; IR (KBr) n¼3061, 2925, 2857, 1602, 1562, 1499, 1459, 1429, 1381, 1318, 1288, 1235, 1126, 1042, 1015, 893, 816, 759, 731, 697 cm1; HRMS: calcd for C25H18N2OS: 394.1140, found: 394.1141. 4.2.7. 2-(2-(Pyridin-2-yl)-5-(trifluoromethyl)phenyl)benzothiazole (3ga). Yield: 76%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.64 (d, J¼4.5 Hz, 1H), 8.27 (s, 1H), 8.01 (d, J¼8.2 Hz, 1H), 7.81 (d, J¼8.0 Hz, 1H), 7.79e7.73 (m, 2H), 7.60 (t, J¼7.7 Hz, 1H), 7.45 (t, J¼7.6 Hz, 1H), 7.37e7.25 (m, 3H); 13C NMR (150 MHz, CDCl3) d 165.9, 157.1, 153.2, 145.0, 136.7, 136.4, 133.7, 131.4, 127.34 (q, J¼148.5 Hz), 125.6, 125.2, 123.8, 123.1, 121.6; IR (KBr) n¼3061, 1587, 1522, 1492, 1457, 1404, 1331, 1260, 1211, 1129, 1085, 1037, 899, 792, 758, 728, 704 cm1; HRMS: calcd for C19H11F3N2S: 356.0595, found: 356.0590. 4.2.8. Methyl 4-(benzothiazol-2-yl)-3-(pyridin-2-yl)benzoate (3ha). Yield: 74%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.62 (d, J¼4.1 Hz, 1H), 8.29 (s, 1H), 8.18 (d, J¼8.0 Hz, 1H), 8.07 (d, J¼8.1 Hz, 1H), 7.99 (d, J¼8.2 Hz, 1H), 7.75 (d, J¼8.0 Hz, 1H), 7.62 (t, J¼7.6 Hz, 1H), 7.44 (t, J¼7.6 Hz, 1H), 7.34 (t, J¼7.6 Hz, 2H), 7.28e7.24 (m, 1H), 3.94 (s, 3H); 13C NMR (150 MHz, CDCl3) d 166.5, 166.5, 157.7, 153.3, 149.9, 140.9, 136.9, 136.7, 136.6, 132.1, 131.8, 131.1, 129.8, 126.4, 125.6, 125.3, 123.8, 122.9, 121.6, 52.6; IR (KBr) n¼3060, 2960, 2870, 1691, 1590, 1494, 1462, 1314, 1259, 1093, 1019, 798 cm1; HRMS: calcd for C20H14N2O2S: 346.0776, found: 346.0778. 4.2.9. 1-(3-(Benzothiazol-2-yl)-4-(pyridin-2-yl)phenyl)ethanone (3ia). Yield: 72%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.63 (d, J¼4.6 Hz, 1H), 8.52 (s, 1H), 8.15 (dd, J¼8.0, 1.4 Hz, 1H), 8.00 (d, J¼8.2 Hz, 1H), 7.76 (d, J¼8.0 Hz, 2H), 7.59 (t, J¼7.7 Hz, 1H), 7.45 (t, J¼7.8 Hz, 1H), 7.34 (t, J¼7.6 Hz, 1H), 7.29 (d, J¼7.8 Hz, 1H), 7.24 (t, J¼6.3 Hz, 1H), 2.69 (s, 3H); 13C NMR (150 MHz, CDCl3) d 197.3, 166.7, 157.5, 153.4, 145.0, 144.9, 137.3, 136.7, 136.4, 133.5, 131.4, 131.2, 129.7, 126.4, 125.5, 125.2, 123.9, 123.1, 121.6, 27.0; IR (KBr) n¼3060, 2924, 1687, 1600, 1498, 1543, 1431, 1357, 1314, 1252, 1206, 1095, 1025, 904, 790, 758, 694 cm1; HRMS: calcd for C20H14N2OS: 330.0827, found: 330.0824. 4.2.10. 2-(5-Chloro-2-(pyridin-2-yl)phenyl)benzothiazole (3ja).Yield: 62%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.61 (d, J¼4.7 Hz, 1H), 7.99 (dd, J¼7.9, 5.1 Hz, 2H), 7.74 (d, J¼8.0 Hz, 1H), 7.60e7.52 (m, 3H), 7.44 (t, J¼7.5 Hz, 1H), 7.33 (t, J¼7.6 Hz, 1H), 7.23 (m, 2H); 13C NMR (150 MHz, CDCl3) d 166.2, 157.4, 153.3, 149.9, 139.2, 136.7, 136.3, 135.0, 134.4, 132.2, 130.7, 130.4, 126.4, 125.5, 125.3, 123.8, 122.8, 121.6; IR (KBr) n¼3060, 1590, 1494, 1457, 1428, 1393, 1314, 1255, 1099, 1037, 895, 782, 759, 729, 691 cm1; HRMS: calcd for C18H11ClN2S: 322.0331, found: 322.0329. 4. 2.11. 4-Methyl-2 -(2-(pyridin -2-yl)phenyl)benz othiazole (3ab). Yield: 81%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.65 (s, 1H), 7.99 (s, 1H), 7.71e7.52 (m, 5H), 7.37e7.18 (m, 4H), 2.60 (s, 3H); 13 C NMR (150 MHz, CDCl3) d 166.2, 159.1, 153.1, 149.6, 140.8, 133.6, 133.1, 130.9, 130.9, 130.2, 128.9, 126.6, 125.2, 125.1, 122.3, 118.9, 18.4; IR (KBr) n¼3057, 2922, 2852, 1584, 1560, 1520, 1464, 1425, 1315,
1266, 1217, 1062, 1022, 969, 884, 760 cm1; HRMS: calcd for C19H14N2S: 302.0878, found: 302.0877. 4.2.12. 6-Methoxy-2-(2-(pyridin-2-yl)phenyl)benzothiazole (3ac). Yield: 85%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.59 (d, J¼4.2 Hz, 1H), 8.00 (d, J¼8.1 Hz, 1H), 7.74 (d, J¼7.9 Hz, 1H), 7.58 (d, J¼8.5 Hz, 1H), 7.51 (t, J¼7.7 Hz, 1H), 7.48e7.41 (m, 2H), 7.32 (t, J¼7.5 Hz, 1H), 7.21 (d, J¼7.8 Hz, 1H), 7.18e7.14 (m, 1H), 7.11 (dd, J¼8.5, 2.0 Hz, 1H), 3.91 (s, 3H); 13C NMR (150 MHz, CDCl3) d 167.9, 159.9, 158.2, 153.3, 149.7, 136.7, 136.2, 134.0, 133.5, 132.2, 126.2, 125.4, 125.3, 123.6, 122.1, 121.6, 116.9, 115.3, 55.9; IR (KBr) n¼3061, 2931, 1723, 1602, 1559, 1464, 1435, 1318, 1265, 1226, 1118, 1025, 892, 827, 794, 758, 702 cm1; HRMS: calcd for C19H14N2OS: 318.0827, found: 318.0832. 4.2.13. Methyl 2-(2-(pyridin-2-yl)phenyl)benzothiazole-5carboxylate (3ad). Yield: 93%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.61 (s, 1H), 8.59 (d, J¼4.7 Hz, 1H), 7.98 (dd, J¼18.6, 8.0 Hz, 2H), 7.79 (d, J¼8.4 Hz, 1H), 7.59 (m, 4H), 7.31 (d, J¼7.8 Hz, 1H), 7.24e7.21 (m, 1H), 3.94 (s, 3H); 13C NMR (150 MHz, CDCl3) d 169.18, 167.13, 158.35, 153.29, 149.74, 141.37, 140.91, 136.39, 132.52, 130.93, 130.9, 130.8, 129.0, 128.5, 125.8, 125.2, 125.1, 122.7, 121.4, 52.5; IR (KBr) n¼3059, 2954, 2851, 1720, 1586, 1562, 1462, 1432, 1303, 1243, 1149, 1092, 904, 794, 759, 694 cm1; HRMS: calcd for C20H14N2O2S: 346.0776, found: 346.0778. 4. 2.14 . 5-Chloro-2-(2-(pyridin-2-yl)phenyl) benz othiazol e (3ae). Yield: 91%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.59 (d, J¼4.5 Hz, 1H), 7.93 (dd, J¼7.8, 1.5 Hz, 2H), 7.66e7.56 (m, 4H), 7.54 (m, 1H), 7.29 (dd, J¼8.5, 1.9 Hz, 1H), 7.22 (m, 1H); 13C NMR (150 MHz, CDCl3) d 169.8, 158.4, 154.2, 149.7, 141.5, 140.9, 136.4, 134.9, 132.6, 132.2, 130.9, 130.7, 129.0, 125.7, 125.1, 123.3, 122.6, 122.3; IR (KBr) n¼3060, 1590, 1494, 1457, 1428, 1393, 1314, 1255, 1099, 1037, 895, 782, 759, 729, 691 cm1; HRMS: calcd for C18H11ClN2S: 322.0331, found: 322.0333. 4 . 2 .15 . 6 - N i t r o - 2 - ( 2 - ( p y r i d i n - 2 - yl ) p h e n yl ) b e n z o t h i a z o l e (3af). Yield: 85%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.68 (d, J¼2.3 Hz, 1H), 8.57 (d, J¼4.4 Hz, 1H), 8.29 (dd, J¼8.9, 2.3 Hz, 1H), 8.02 (t, J¼8.4 Hz, 2H), 7.66 (td, J¼7.7, 1.8 Hz, 1H), 7.62 (d, J¼3.9 Hz, 2H), 7.60e7.55 (m, 1H), 7.37 (d, J¼7.8 Hz, 1H), 7.28e7.24 (m, 1H); 13C NMR (150 MHz, CDCl3) d 173.8, 158.1, 157.0, 149.8, 145.0, 141.1, 136.9, 136.7, 132.1, 131.4, 131.0, 129.1, 125.0, 123.7, 122.9, 121.7, 118.2; IR (KBr) n¼3061, 1566, 1513, 1463, 1439, 1339, 1261, 968, 794, 748, 692 cm1; HRMS: calcd for C18H11N3O2S: 333.0572, found: 333.0578. 4.2.16. 2-[2-(Pyridin-2-yl)phenyl]benzoxazole (3ag). Yield: 60%. Yellow oil. 1H NMR (600 MHz, CDCl3) d 8.54 (d, J¼4.8 Hz, 1H), 8.12 (dd, J¼7.7, 1.1 Hz, 1H), 7.71e7.59 (m, 4H), 7.55 (td, J¼7.6, 1.3 Hz, 1H), 7.33 (d, J¼7.8 Hz, 1H), 7.31e7.24 (m, 3H), 7.23e7.20 (m, 1H); 13C NMR (150 MHz, CDCl3) d 163.8, 158.8, 150.9, 149.5, 142.0, 141.3, 136.4, 131.3, 131.1, 130.9, 128.8, 126.6, 125.1, 124.5, 123.9, 122.3, 120.4, 110.7; IR (KBr) n¼3060, 1586, 1564, 1453, 1344, 1302, 1242, 1074, 1031, 925, 795, 747, 679 cm1; HRMS: calcd for C18H12N2O: 272.0950, found: 272.0949. Acknowledgements This work was supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.tet.2014.07.022.
H. Liu et al. / Tetrahedron 70 (2014) 6474e6481
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