English

• 1.   Introduction

  Aromatic nitro compounds are important intermediates which have been used in the synthesize of dynamites, engineering plastics, dyes, pharmaceuticals and perfumes.^[1] The traditional routes of nitration reaction usually use HNO₃ or HNO₃/H₂SO₄ as nitrification reagent. But these reaction conditions are harsh and have many serious problems of environmental pollution. Hence, more attention have been attracted in developing methods to perform nitration under milder conditions, for example, the iso-nitration of many arylboronic acids, ^[2] carboxylic acids, ^[3] aryl halides and pseudohalides.^[4] In the past decade, C—H functionalization has emerged as a powerful tool for regioselective formation of C—C, C—O, C—N and C—halogen bonds. Most importantly, developments of new strategies for the direct C—H nitration have attract more attention. Palladium and copper salts have been shown to be effective catalysts for this C—H activated nitration. AgNO₃^[5] and t-butyl nitrite^[6] are good nitro source but the price is high. Recently, many significant progress has been made in nitrification reagents such as NO₂, ^[7] bismuth nitrate^[8] and Fe(NO₃)₃•9H₂O.^[9]

  Azobenzenes have attracted much interest due to their applications as pharmaceuticals, ^[10] liquid crystals, ^[11] molecular switches, ^[12] femtosecond fluorescence.^[13] Among these, the 2-(phenyldiazenyl)aniline scaffold is an important motif found in a variety of compounds, including dyes, ^[14] chiroptical molecular switches, ^[15] pharmaceutical drugs, ^[16] photochemical materials^[17] and functional materials such as azobenzene-functionalized PVAms.^[18] Many representative compounds of 2-(phenyldiazenyl)aniline are shown in Figure 1. In addition, the 2-nitroaniline also is key fine chemical intermediates for the generation of many valuable compounds, such as 1, 2-diamines^[19] and benzimidazoles.^[20] In recent years, various nitrating agents, such as t-BuONO^{[6b, 6c]} and NO₂, ^[7] have been utilized in regioselective nitration of azobenzenes. But, direct o-nitrification of azobenzenes with cobalt nitrate (Co(NO₃)₂•6H₂O) has not been reported. As part of our continuing efforts on the C—X bonds formation, ^[21] herein we disclose a highly practical procedure to build C-2 nitro of aromatic azo compounds in one step fromazobenzenes with cobalt nitrate via C—H activation. Also, it is an important method to synthesize 2-nitroaniline or 2-aminoaniline by hydrolysis reaction of the above product.

  Figure 1

  

  Figure 1.  Some representative compounds of O-aminoazobenzene

  下载: 全尺寸图片 幻灯片

  2.   Results and discussion

  We initiated our investigation on the model reaction of 1, 2-di-p-tolyldiazene (1a) with Co(NO₃)₂•6H₂O to optimize the reaction parameters (Table 1). To our delight, the o-nitrification took place in the presence of Pd(OAc)₂ (10 mol%) and K₂S₂O₈ in dichloroethane (DCE) under N₂ for 12 h, the desired product was acquired in 93% yield (Entry 1, Table 1). Thus, Cu(OAc)₂, Pd(CH₃CN)₂Cl₂, PdCl₂, Pd, Pd(PPh₃)₂Cl₂ and Pd(PPh₃)₄ were tested to catalyze this reaction, in which Pd(OAc)₂ gave the best result (Entries 1~7, Table 1). Secondly, the effects of oxidants were examined. It was found that dicumyl peroxide (DCP) could give moderate yield of 2a (Entry 12, Table 1) while other oxidants, such as tert-butyl peroxybenzoate (TBPB), tert-butyl hydroperoxide (TBHB) and di-t-butyl peroxide (DTBP), showed low efficiency (Entries 9~11, Table 1). Without oxidants, the reaction could not take place at all (Entry 8, Table 1). DCE was found to be a more effective solvent compared to tetrahydrofuran (THF), N, N-dimethyl- formamide (DMF), 1, 4-dioxane, toluene and dimethyl sulfoxide (DMSO) (Entries 13~16, Table 1), although toluene provided relatively 60% yield (Entry 17, Table 1). Then, a series of nitrites were surveyed, and Co(NO₃)₂ was proven to be the best choice (Entries 18~26, Table 1). Subsequently, when the catalyst loading was reduced to 5 mol%, the yield decreased to 82% (Entry 27, Table 1). The increase of the amount of Co(NO₃)₂ beyond 2 equiv. did not affect the outcome of the reaction (Entry 28, Table 1). When the temperature was reduced to 100 ℃ or increased to 120 ℃, the yield decreased to 77% and 81% (Entries 29, 30, Table 1). Finally, shortening or delaying the reaction times decreased the yield of 2a (Entries 1, 31, 32, Table 1). Control experiments revealed that the presence of air had no adverse effect on the reaction outcome (Entry 33, Table 1). From the results shown above, the optimized nitration conditions were identified as follows: 1a (0.5 mmol), Co(NO₃)₂•6H₂O (1.2 equiv.), K₂S₂O₈ (1.2 equiv.), 10 mol% of Pd(OAc)₂ as the catalyst, and DCE (2.0 mL) as the solvent, at 110 ℃ under an N₂ atmosphere for 12 h.

  Table 1

  

  Table 1.  Optimization of reaction conditions^a

  下载: 导出CSV

  Entry	Catalyst	Oxidant	Nitro source	Solvent	Yieldb/%
  1	Pd(OAc)2	K2S2O8	Co(NO3)2•6H2O	DCE	93
  2	Cu(OAc)2	K2S2O8	Co(NO3)2•6H2O	DCE	n.r.
  3	Pd(CH3CN)2Cl2	K2S2O8	Co(NO3)2•6H2O	DCE	41
  4	PdCl2	K2S2O8	Co(NO3)2•6H2O	DCE	25
  5	Pd/C (10%)	K2S2O8	Co(NO3)2•6H2O	DCE	71
  6	Pd(Ph3P)2Cl2	K2S2O8	Co(NO3)2•6H2O	DCE	75
  7	Pd(Ph3P)4	K2S2O8	Co(NO3)2•6H2O	DCE	69
  8	Pd(OAc)2	—	Co(NO3)2•6H2O	DCE	n.r.
  9	Pd(OAc)2	TBPB	Co(NO3)2•6H2O	DCE	35
  10	Pd(OAc)2	TBHP	Co(NO3)2•6H2O	DCE	15
  11	Pd(OAc)2	DTBP	Co(NO3)2•6H2O	DCE	25
  12	Pd(OAc)2	DCP	Co(NO3)2•6H2O	DCE	65
  13	Pd(OAc)2	K2S2O8	Co(NO3)2•6H2O	THF	Trace
  14	Pd(OAc)2	K2S2O8	Co(NO3)2•6H2O	DMF	Trace
  15	Pd(OAc)2	K2S2O8	Co(NO3)2•6H2O	1, 4-Dioxane	10
  16	Pd(OAc)2	K2S2O8	Co(NO3)2•6H2O	DMSO	Trace
  17	Pd(OAc)2	K2S2O8	Co(NO3)2•6H2O	Toluene	60
  18	Pd(OAc)2	K2S2O8	Ca(NO3)2	DCE	23
  19	Pd(OAc)2	K2S2O8	Fe(NO3)2	DCE	57
  20	Pd(OAc)2	K2S2O8	Fe(NO3)3	DCE	29
  21	Pd(OAc)2	K2S2O8	NaNO2	DCE	Trace
  22	Pd(OAc)2	K2S2O8	KNO3	DCE	Trace
  23	Pd(OAc)2	K2S2O8	AgNO2	DCE	78
  24	Pd(OAc)2	K2S2O8	Fe(NO3)3•9H2O	DCE	36
  25	Pd(OAc)2	K2S2O8	Bi(NO3)3	DCE	70
  26	Pd(OAc)2	K2S2O8	Cu(NO3)2	DCE	43
  27	Pd(OAc)2	K2S2O8	Co(NO3)2•6H2O	DCE	82c
  28	Pd(OAc)2	K2S2O8	Co(NO3)2•6H2O	DCE	93d
  29	Pd(OAc)2	K2S2O8	Co(NO3)2•6H2O	DCE	77e
  30	Pd(OAc)2	K2S2O8	Co(NO3)2•6H2O	DCE	81f
  31	Pd(OAc)2	K2S2O8	Co(NO3)2•6H2O	DCE	67g
  32	Pd(OAc)2	K2S2O8	Co(NO3)2•6H2O	DCE	89h
  33	Pd(OAc)2	K2S2O8	Co(NO3)2•6H2O	DCE	81i
  a Reaction conditions: 1a (0.5 mmol), nitro source (1.2 equiv.), K2S2O8 (1.2 equiv.), catalyst (10 mol%) and solvent (2.0 mL) under N2 atmosphere at 110 ℃ for 12 h, unless otherwise noted. b Isolated yields. c Pd(OAc)2 (5 mol%). d 2 equiv. of Co(NO3)2•6H2O was used. e 100 ℃. f 120 ℃. g 8 h. h 24 h. i Air atmosphere.

  The representative results of the nitration of various azobenzenes with Co(NO₃)₂•6H₂O under the influence of K₂S₂O₈ in DCE at 110 ℃ for 12 h were summarized in Table 2. A series of azobenzenes were allowed to react with Co(NO₃)₂, affording the corresponding 2-nitro aromatic azo compounds in moderate to good yields. (E)-1, 2-Di-p-tol- yldiazene (1a) reacted with Co(NO₃)₂•6H₂O under these conditions to give (E)-1-(4-methyl-2-nitrophenyl)-2-(p- tolyl)diazene (2a) in 93% yield. Similarly, such as (E)-1, 2-bis(4-ethylphenyl)diazene, (E)-1, 2-bis(4-isopropyl- phenyl)diazene, (E)-1, 2-diphenyldiazene, (E)-1, 2-bis(4- methoxyphenyl)diazene also gave good yields (2b, 2c, 2d, 2e). However, (E)-1, 2-bis(4-chlorophenyl)diazene, (E)-dimethyl azobenzene-4, 4'-dicarboxylate which substituted with electron-withdrawing group afforded lower yields (2f, 2g). Surprisingly, meta-substituted azobenzenes showed excellent regioselectivity towards C—H nitration under the reaction conditions (2h, 2i). Also, the azobenzene with two electron-donating groups (CH₃) gave moderate yield (2j). Meanwhile, 1, 2-bis(2-chlorophenyl)diazene, the ortho-substituted azobenzene afforded a lower yield because of electron-withdrawing and the steric effect (2k). Additionally, the reactions of unsymmetrical azobenzenes also proceeded smoothly and gave the products in good yields (2l, 2m) which could be determined by ¹H NMR. Next, (E)-4-((4-aminophenyl)diazenyl)phenol and (E)-4-(phenyldiazenyl)aniline can not be nitrified under this reaction condition (2n, 2o). Furthermore, other directing group such as 2-phenylpyridine was also nitrified (2p).

  Table 2

  

  Table 2.  Substrate scope^a

  下载: 导出CSV

  a Conditions: 1 (0.5 mmol), K2S2O8 (1.2 equiv.), Co(NO3)2•6H2O (1.2 equiv.), Pd(OAc)2 (10 mol%) and solvent (2.0 mL) under N2 atmosphere at 110 ℃ for 12 h. The yields are of the isolated products.

  On the basis of previous related studies and reported literature, ^{[2b, 5d, 6b, 6c, 7]} a catalytic cycle of this Pd-catalyzed direct ortho nitration of azobenzenes with Co(NO₃)₂•6H₂O via C—H activation was illustrated in Scheme 1. In step (ⅰ), the coordination of azo in azobenzene (1a) with Pd(OAc)₂ forms a cyclopalladated intermediate A. In step (ⅱ), oxidative addition of active radicals NO₂ to A provides Pd(Ⅲ) species B by K₂S₄O₈. In step (ⅲ), the Pd(Ⅲ) species B are oxidized to form Pd(Ⅳ) intermediate C. In step (ⅳ), C—N bond formation afford the final product 2a via reductive elimination of C with regeneration of Pd(Ⅱ) to start the next cycle.

  Scheme 1

  

  Scheme 1.  Plausible reaction mechanism

  下载: 全尺寸图片 幻灯片

  3.   Conclusions

  In summary, we have developed a simple method for direct o-nitration of azoarenes with Co(NO₃)₂•6H₂O via C(sp²)—H activation. The reaction proceeds under mild conditions and shows a broad substrate scope. It provides a convenient method to synthesize o-nitroazobenzenes. And we can also get o-aminoazobenzene, 2-nitroaniline and 2-aminoaniline by reduction or hydrolysis reaction of the above product.

  4.   Experimental section

  4.1   General information

  ¹H NMR, ¹³C NMR spectra were recorded on a 500 MHz spectrometer in CDCl₃ with shifts referenced to SiMe₄. IR spectra were recorded on an FTIR spectrophotometer. Melting points were determined by using a local hot-stage melting point apparatus and were uncorrected. Mass spectra were recorded on LC-MS and on HRMS (ESI-TOF analyzer) equipment. All reactions were run under air atmosphere in Schlenk tubes. DCE, THF, DMF, 1, 4-dioxane, toluene and DMSO analytical grade were not distilled before use. Commercial K₂S₂O₈ and azobenzenes were used without purification.

  4.2   General procedure for palladium-catalyzed direct o-nitrification of azobenzenes with Co(NO₃)₂• 6H₂O via C—H activation

  Azoic compounds (0.5 mmol), Co(NO₃)₂•6H₂O (1.2 equiv.), K₂S₂O₈ (1.2 equiv.), Pd(OAc)₂ (10 mol%) in DCE (2 mL) were mixed and vigorously stirred under atmosphere at 110 ℃ for 12 h. After cooled down to room temperature and concentrated in vacuum, the residue was purified by flash chromatography on a short silica gel to afford corresponding product.

  (E)-1-(4-Methyl-2-nitrophenyl)-2-(p-tolyl)diazene (2a): Orange-red solid, 93% yield. m.p. 93.2~95.2 ℃ (lit.^[7] 89~91 ℃); ¹H NMR (500 MHz, CDCl₃) δ: 7.73 (d, J＝8.2 Hz, 2H), 7.60 (s, 1H), 7.52 (d, J＝8.2 Hz, 1H), 7.36 (d, J＝8.0 Hz, 1H), 7.22 (d, J＝8.0 Hz, 2H), 2.40 (s, 3H), 2.35 (s, 3H); ¹³C NMR (126 MHz, CDCl₃) δ: 149.63, 146.70, 142.15, 141.85, 140.37, 132.46, 128.83, 123.08, 122.51, 117.07, 20.57, 20.18; HRMS (ESI⁺) calcd for C₁₄H₁₄N₃O₂ [M+H]⁺ 256.1081, found 256.1083.

  (E)-1-(4-Ethyl-2-nitrophenyl)-2-(4-ethylphenyl)diazene (2b): Orange-red solid, 83% yield. m.p. 121.3~123.3 ℃. ¹H NMR (500 MHz, CDCl₃) δ: 7.86~7.82 (m, 2H), 7.73~7.69 (m, 1H), 7.63~7.60 (m, 1H), 7.47 (dd, J＝8.2, 1.8 Hz, 1H), 7.33 (d, J＝8.4 Hz, 2H), 2.78 (dd, J＝10.0, 5.3 Hz, 2H), 2.72 (dd, J＝9.5, 5.7 Hz, 2H), 1.31~1.27 (m, 6H); ¹³C NMR (126 MHz, CDCl₃) δ: 150.85, 149.11, 147.81, 147.57, 143.41, 132.38, 128.68, 123.66, 122.99, 118.23, 28.93, 28.50, 15.34, 14.99; HRMS (ESI⁺) calcd for C₁₆H₁₈N₃O₂ [M+H]⁺ 284.1394, found 284.1391.

  (E)-1-(4-Isopropyl-2-nitrophenyl)-2-(4-isopropylphenyl)- diazene (2c): Brown-red liquid, 84% yield. ¹H NMR (500 MHz, CDCl₃) δ: 7.87~7.84 (m, 2H), 7.74 (d, J＝1.8 Hz, 1H), 7.63~7.61 (m, 1H), 7.51 (dd, J＝8.3, 1.8 Hz, 1H), 7.38~7.36 (m, 2H), 3.06~2.98 (m, 2H), 1.33~1.31 (m, 6H), 1.29 (d, J＝6.9 Hz, 6H); ¹³C NMR (126 MHz, CDCl₃) δ: 153.65, 152.19, 150.95, 147.77, 143.58, 131.10, 127.26, 123.68, 121.70, 118.31, 34.25, 33.99, 23.79, 23.59; HRMS (ESI⁺) calcd for C₁₈H₂₂N₃O₂ [M+H]⁺ 312.1707, found 312.1703.

  (E)-1-(2-Nitrophenyl)-2-phenyldiazene (2d): Orange-red solid, 87% yield. m.p. 61~63 ℃ (lit.^[7] 67~68 ℃); ¹H NMR (500 MHz, CDCl₃) δ: 7.85~7.79 (m, 3H), 7.55 (d, J＝3.9 Hz, 2H), 7.46~7.41 (m, 4H); ¹³C NMR (126 MHz, CDCl₃) δ: 151.37, 146.45, 144.40, 132.02, 131.27, 129.38, 128.20, 122.99, 122.57, 117.39; HRMS (ESI⁺) calcd for C₁₂H₁₀N₃O₂ [M+H]⁺ 228.0768, found 228.0763.

  (E)-1-(4-Methoxy-2-nitrophenyl)-2-(4-methoxyphenyl)- diazene (2e): Brown-red solid, 76% yield. m.p. 82~84 ℃ (lit.^[7] 86~88 ℃); ¹H NMR (500 MHz, CDCl₃) δ: 7.81 (dd, J＝9.5, 2.5 Hz, 2H), 7.71~7.68 (m, 1H), 7.24 (d, J＝2.7 Hz, 1H), 7.07 (dd, J＝9.0, 2.7 Hz, 1H), 6.92 (dd, J＝9.4, 2.4 Hz, 2H), 3.85 (s, 3H), 3.81 (d, J＝2.2 Hz, 3H); ¹³C NMR (126 MHz, CDCl₃) δ: 161.66, 159.77, 145.94, 137.71, 124.36, 120.90, 120.51, 118.39, 117.86, 113.30, 107.04, 93.72, 55.14, 54.61; HRMS (ESI⁺) calcd for C₁₄H₁₄N₃O₄ [M+H]⁺ 288.0979, found 288.0977.

  (E)-1-(4-Chloro-2-nitrophenyl)-2-(4-chlorophenyl)- diazene (2f): Orange-red solid, 73% yield. m.p. 112.6~114.6 ℃ (lit.^[6c] 108~109 ℃); ¹H NMR (500 MHz, CDCl₃) δ: 7.80 (t, J＝6.6 Hz, 1H), 7.78~7.72 (m, 2H), 7.58 (dd, J＝9.4, 4.8 Hz, 1H), 7.56~7.51 (m, 1H), 7.43~7.36 (m, 2H); ¹³C NMR (126 MHz, CDCl₃) δ: 150.63, 148.10, 143.21, 138.83, 136.70, 133.09, 129.63, 124.95, 124.18, 119.47; HRMS (ESI⁺) calcd for C₁₂H₈Cl₂N₃O₂ [M+H]⁺ 295.9988, found 295.9985.

  Methyl (E)-4-((4-(methoxycarbonyl)phenyl)diazenyl)-3- nitrobenzoate (2g): Orange-red solid, 71% yield. m.p. 156~158 ℃; ¹H NMR (500 MHz, CDCl₃) δ: 8.62 (s, 1H), 8.35 (d, J＝8.1 Hz, 1H), 8.21 (d, J＝8.4 Hz, 2H), 7.99 (d, J＝8.3 Hz, 2H), 7.70 (d, J＝8.3 Hz, 1H), 4.01 (s, 3H), 3.97 (s, 3H); ¹³C NMR (126 MHz, CDCl₃) δ: 166.15, 164.42, 154.62, 147.81, 134.13, 133.49, 132.50, 130.78, 125.63, 123.63, 122.98, 118.73, 53.00, 52.52; HRMS (ESI⁺) calcd for C₁₆H₁₄N₃O₆ [M+H]⁺ 344.0877, found 344.0875.

  (E)-1-(5-Methyl-2-nitrophenyl)-2-(m-tolyl)diazene (2h): Brown-red liquid, 86% yield. ¹H NMR (500 MHz, CDCl₃) δ: 7.86 (d, J＝8.2 Hz, 1H), 7.73 (dd, J＝5.5, 5.0 Hz, 2H), 7.42~7.39 (m, 1H), 7.36 (s, 1H), 7.34~7.31 (m, 2H), 2.48 (s, 3H), 2.44 (d, J＝6.4 Hz, 3H); ¹³C NMR (126 MHz, CDCl₃) δ: 152.59, 146.06, 144.66, 139.22, 133.00, 130.60, 129.04, 124.30, 123.52, 121.33, 118.66, 114.77, 21.58, 21.32; HRMS (ESI⁺) calcd for C₁₄H₁₄N₃O₂ [M+H]⁺ 256.1081, found 256.1085.

  (E)-1-(5-Chloro-2-nitrophenyl)-2-(3-chlorophenyl)- diazene (2i): Orange-red solid, 84% yield. m.p. 135~137 ℃ (lit.^[7] 138~140 ℃).¹H NMR (500 MHz, CDCl₃) δ: 7.97~7.90 (m, 1H), 7.90~7.85 (m, 1H), 7.84 (dt, J＝7.5, 1.5 Hz, 1H), 7.64~7.58 (m, 1H), 7.57~7.53 (m, 1H), 7.52 (dt, J＝7.9, 1.6 Hz, 1H), 7.48 (t, J＝7.5 Hz, 1H); ¹³C NMR (126 MHz, CDCl₃) δ: 151.85, 144.89, 144.62, 138.56, 134.43, 131.49, 129.47, 129.35, 124.57, 121.94, 121.89, 117.45; HRMS (ESI⁺) calcd for C₁₂H₈Cl₂N₃O₂ [M+H]⁺ 295.9988, found 295.9983.

  (E)-1-(3, 5-Dimethyl-2-nitrophenyl)-2-(3, 5-dimethyl- phenyl)diazene (2j): Orange-red solid, 83% yield. m.p. 119~121 ℃ (lit.^[6b] 102~104 ℃); ¹H NMR (500 MHz, CDCl₃) δ: 7.42 (d, J＝14.7 Hz, 2H), 7.35 (s, 1H), 7.08 (s, 1H), 7.05 (s, 1H), 2.33~2.29 (m, 12H); ¹³C NMR (126 MHz, CDCl₃) δ: 151.51, 147.26, 142.26, 139.79, 137.82, 132.85, 132.55, 129.09, 120.25, 113.99, 20.29, 20.16, 15.80. HRMS (ESI⁺) calcd for C₁₆H₁₈N₃O₂ [M+H]⁺ 284.3383, found 284.3385.

  (E)-1-(2-Chloro-6-nitrophenyl)-2-(2-chlorophenyl)diazene (2k): Orange-red solid, 68% yield. m.p. 96~98 ℃ (lit.^[6b] 89~91 ℃); ¹H NMR (500 MHz, CDCl₃) δ: 7.80 (dd, J＝8.1, 1.2 Hz, 1H), 7.75 (ddd, J＝8.1, 2.7, 1.4 Hz, 2H), 7.59 (dd, J＝8.0, 1.2 Hz, 1H), 7.51~7.45 (m, 2H), 7.40~7.36 (m, 1H); ¹³C NMR (126 MHz, CDCl₃) δ: 148.67, 144.61, 142.60, 136.63, 134.46, 133.68, 130.99, 130.39, 129.19, 127.44, 123.42, 117.86; HRMS (ESI⁺) calcd for C₁₂H₈Cl₂N₃O₂ [M+H]⁺ 295.9988, found 295.9981.

  (E)-1-(4-Methoxyphenyl)-2-(2-nitrophenyl)diazene (2l): Orange-red solid, 68% yield. m.p. 76~78 ℃ (lit.^[22] 77~79 ℃). ¹H NMR (500 MHz, CDCl₃) δ: 7.93 (d, J＝9.0 Hz, 2H), 7.89 (dd, J＝8.1, 0.8 Hz, 1H), 7.66 (dd, J＝9.1, 1.6 Hz, 2H), 7.55~7.51 (m, 1H), 7.01 (d, J＝9.0 Hz, 2H), 3.90 (s, 3H); ¹³C NMR (126 MHz, CDCl₃) δ: 163.16, 147.01, 145.56, 132.91, 129.77, 125.78, 123.96, 118.50, 114.40, 108.19, 55.68; HRMS (ESI⁺) calcd for C₁₃H₁₂N₃O₃ [M+ H]⁺ 258.0873, found 258.0875.

  (E)-1-(4-Methoxy-2-nitrophenyl)-2-(3-methyl-6-nitro- phenyl)diazene (2m): Orange-red solid, 71% yield. m.p. 168~170 ℃; ¹H NMR (500 MHz, CDCl₃) δ: 7.82 (d, J＝8.4 Hz, 1H), 7.70 (d, J＝9.0 Hz, 1H), 7.33~7.31 (m, 3H), 7.12 (dd, J＝9.0, 2.7 Hz, 1H), 3.89 (s, 3H), 2.43 (s, 3H); ¹³C NMR (126 MHz, CDCl₃) δ: 162.25, 150.98, 145.07, 138.69, 131.46, 125.70, 125.41, 124.35, 120.02, 119.09, 118.67, 108.56, 56.36, 21.68; HRMS (ESI⁺) calcd for C₁₄H₁₃N₄O₅ [M+H]⁺ 317.0881, found 317.0885.

  2-(2-Nitrophenyl)pyridine (2p): Light yellow solid, 49% yield. m.p. 41~43 ℃ (lit.^[6c] 35~36 ℃); ¹H NMR (500 MHz, CDCl₃) δ: 8.65 (d, J＝4.4 Hz, 1H), 7.90 (dd, J＝8.1, 1.0 Hz, 1H), 7.80 (td, J＝7.7, 1.8 Hz, 1H), 7.68~7.64 (m, 1H), 7.62 (dd, J＝7.7, 1.5 Hz, 1H), 7.56~7.53 (m, 1H), 7.48 (d, J＝7.8 Hz, 1H), 7.32 (dd, J＝7.6, 4.9 Hz, 1H); ¹³C NMR (126 MHz, CDCl₃) δ: 155.44, 149.60, 149.28, 136.94, 135.21, 132.41, 131.26, 129.24, 124.41, 122.94, 122.72. HRMS (ESI⁺) calcd for C₁₁H₉N₂O₂ [M+H]⁺ 201.0659, found 201.0655.

  Supporting Information   Detailed experimental procedures, characterization data, ¹H NMR and ¹³C NMR spectra of the products. The Supporting Information is available free of charge via the Internet at http://sioc-journal.cn.

• 1. [1]

     (a) Ballini, R. ; Barboni, L. ; Fringuelli, F. ; Palmieri, A. ; Pizzo F. ; Vaccaro, L. Green Chem. 2007, 9, 823.
     (b) Papadopoulou, M. V. ; Trunz, B. B. ; Bloomer, W. D. ; McKenzie, C. ; Wilkinson, S. R. ; Prasittichai, C. ; Brun, R. ; Kaiser, M. ; Torreele, E. J. Med. Chem. 2011, 54, 8214.
     (c) Hou, W. D. ; Wei, Q. ; Liu, G. S. ; Chen, J. ; Guo, J. ; Peng, Y. G. Org. Lett. 2015, 17, 4870.

  2. [2]

     (a) Prakash, G. K. S. ; Panja, C. ; Mathew, T. ; Surampudi, V. ; Petasis, N. A. ; Olah, G. A. Org. Lett. 2004, 6, 2205.
     (b) Manna, S. ; Maity, S. ; Rana, S. ; Agasti, S. ; Maiti, D. Org. Lett. 2012, 14, 1736.

  3. [3]

     Natarajan, P.; Chaudhary, R.; Venugopalan, P. J. Org. Chem. 2015, 80, 10498. doi: 10.1021/acs.joc.5b02133

  4. [4]

     (a) Fors, B. P. ; Buchwald, S. L. J. Am. Chem. Soc. 2009, 131, 12898.
     (b) Yan, G. B. ; Yang, M. H. Org. Biomol. Chem. 2013, 11, 2554.

  5. [5]

     (a) Liu, Y. K. ; Lou, S. J. ; Xu, D. Q. ; Xu, Z. Y. Chem. -Eur. J. 2010, 16, 13590.
     (b) Zhang, L. ; Liu, Z. H. ; Li, H. Q. ; Fang, G. C. ; Barry, B. D. ; Belay, T. A. ; Bi, X. H. ; Liu, Q. Org. Lett. 2011, 13, 6536.
     (c) Zhang, W. ; Lou, S. J. ; Liu, Y. K. ; Xu, Z. Y. J. Org. Chem. 2013, 78, 5932.
     (d) Katayev, D. ; Pfister, K. F. ; Wendling, T. ; Gooßen, L. J. Chem. -Eur. J. 2014, 20, 9902.

  6. [6]

     (a) Kilpatrick, B. ; Heller, M. ; Arns, S. Chem. Commun. 2013, 49, 514.
     (b) Majhi, B. ; Kundu, D. ; Ahammed, S. ; Ranu, B. C. Chem. -Eur. J. 2014, 20, 9862.
     (c) Liang, Y. F. ; Li, X. Y. ; Wang, X. Y. ; Yan, Y. P. ; Feng, P. ; Jiao, N. ACS Catal. 2015, 5, 1956.
     (d) Zhang, W. ; Ren, S. B. ; Zhang, J. ; Liu, Y. K. J. Org. Chem. 2015, 80, 5973.

  7. [7]

     Dong, J. W.; Jin, B.; Sun, P. P. Org. Lett. 2014, 16, 4540. doi: 10.1021/ol502090n

  8. [8]

     Lu, Y.; Li, Y. M.; Zhang, R.; Jin, K.; Duan, C. Y. Tetrahedron 2013, 69, 9422. doi: 10.1016/j.tet.2013.08.076

  9. [9]

     Sadhu, P.; Alla, S. K.; Punniyamurthy, T. J. Org. Chem. 2015, 80, 8245. doi: 10.1021/acs.joc.5b01021

  10. [10]

      (a) Banghart, M. R. ; Mourot, A. ; Fortin, D. L. ; Yao, J. Z. ; Kramer, R. H. ; Trauner, D. Angew. Chem., Int. Ed. 2009, 48, 9097.
      (b) Takahashi, H. T. ; Ishioka, T. ; Koiso, Y. ; Sodeoka, M. ; Hashimoto, Y. Biol. Pharm. Bull. 2000, 23, 1387.

  11. [11]

      Roy, D.; Fragiadakis, D.; Roland, C. M.; Dabrowski, R.; Dziaduszek J.; Urban, S. J. Chem. Phys. 2014, 140, 530.

  12. [12]

      (a) Ferri, V. ; Elbing, M. ; Pace, G. ; Dickey, M. D. ; Zharnikov, M. ; Samor, P. ; Mayor, M. ; Rampi, M. A. Angew. Chem., Int. Ed. 2008, 47, 3407.
      (b) Puntoriero, F. ; Ceroni, P. ; Balzani, Vi. ; Bergamini, G. ; Vögtle, F. J. Am. Chem. Soc. 2007, 129, 10714.

  13. [13]

      Chang, C. W.; Lu, Y. C.; Wang, T. T.; Diau, E. W. G. J. Am. Chem. Soc. 2004, 126, 10109. doi: 10.1021/ja049215p

  14. [14]

      (a) Zarei, A. ; Hajipour, A. R. ; Khazdooz, L. ; Mirjalili, B. F. ; Chermahini, A. N. Dyes Pigm. 2009, 81, 240.
      (b) Bafana, A. ; Devi, S. S. ; Chakrabarti, T. Environ. Rev. 2011, 19, 350.

  15. [15]

      Feringa, B. L.; Delden, R. A. V.; Koumura, N.; Geertsema, E. M. Chem. Rev. 2000, 100, 1789. doi: 10.1021/cr9900228

  16. [16]

      (a) Jones, P. ; Altamura, S. ; Boueres, J. ; Ferrigno, F. ; Fonsi, M. ; Giomini, C. ; Lamartina, S. ; Monteagudo, E. ; Ontoria, J. M. ; Orsale, M. V. ; Palumbi, M. C. ; Pesci, S. ; Roscilli, G. ; Scarpelli, R. ; Fademrecht, C. S. ; Toniatti, C. ; Rowley, M. J. Med. Chem. 2009, 52, 7170.
      (b) Loddo, R. ; Novelli, F. ; Sparatore, A. ; Tasso, B. ; Tonelli, M. ; Boido, V. ; Sparatore, F. ; Collu, G. ; Delogu, I. ; Giliberti, G. ; Colla, P. L. Bioorg. Med. Chem. 2015, 23, 7024.
      (c) Nykaza, T. V. ; Harrison, T. S. ; Ghosh, A. ; Putnik, R. A. ; Radosevich, A. T. J. Am. Chem. Soc. 2017, 139, 6839.

  17. [17]

      (a) Bellotto, S. ; Reuter, R. ; Heinis, C. ; Wegner, H. A. J. Org. Chem. 2011, 76, 9826.
      (b) Bandara, H. M. D. ; Friss, T. R. ; Enriquez, M. M. ; Isley, W. ; Incarvito, C. ; Frank, H. A. ; Gascon, J. ; Burdette, S. C. J. Org. Chem. 2010, 75, 4817.

  18. [18]

      Hofmann, K.; Brumm, S.; Mende, C.; Nagel, K.; Seifert, A.; Roth, I.; Schaarschmidt, D.; Lang, H.; Spange, S. New J. Chem. 2012, 36, 1655. doi: 10.1039/c2nj40313g

  19. [19]

      (a) Cantillo, D. ; Baghbanzadeh, M. ; Kappe, C. O. Angew. Chem., Int. Ed. 2012, 51, 10190.
      (b) Shi, Q. ; Lu, R. ; Jin, K. ; Zhang, Z. ; Zhao, D. Green Chem. 2006, 8, 868.
      (c) Vass, A. ; Dudas, J. ; Toth, J. ; Varma, R. S. Tetrahedron Lett. 2001, 42, 5347.

  20. [20]

      (a) Nguyen, T. B. ; Ermolenko, L. ; Al-Mourabit, A. J. Am. Chem. Soc. 2012, 135, 118.
      (b) Kim, J. ; Kim, J. ; Lee, H. ; Lee, B. M. ; Kim, B. H. Tetrahedron 2011, 67, 8027.

  21. [21]

      (a) Xia, C. C. ; Wei, Z. J. ; Yang, Y. ; Yu, W. B. ; Liao, H. X. ; Shen, C. ; Zhang, P. F. Chem. -Asian J. 2016, 11, 360.
      (b) Shen, H. Y. ; Shen, C. ; Wang, A. M. ; Zhang, P. F. Catal. Sci. Technol. 2015, 5, 2065.
      (c) Xia, C. C. ; Wei, Z. J. ; Shen, C. ; Xu, J. ; Yang, Y. ; Su, W. K. ; Zhang, P. F. RSC Adv. 2015, 5, 52588.
      (d) Xia, C. C. ; Wang, K. ; Xu, J. ; Wei, Z. J. ; Shen, C. ; Duan, G. Y. ; Zhu, Q. ; Zhang, P. F. RSC Adv. 2016, 6, 37173.
      (e) Xia, C. C. ; Wang, K. ; Xu, J. ; Shen, C. ; Sun, D. ; Li, H. S. ; Wang, G. D. ; Zhang, P. F. Org. Biomol. Chem. 2017, 15, 531.
      (f) Fu, X. P. ; Wei, Z. J. ; Xia, C. C. ; Shen, C. ; Xu, J. ; Yang, Y. ; Wang, K. ; Zhang, P. F. Catal. Lett. 2017, 147, 400.

  22. [22]

      Rosevear, J.; Wilshire, J. F. K. Aust. J. Chem. 1987, 40, 1663. doi: 10.1071/CH9871663

• 

  Figure 1  Some representative compounds of O-aminoazobenzene

  下载: 全尺寸图片 幻灯片
  

  Scheme 1  Plausible reaction mechanism

  下载: 全尺寸图片 幻灯片

  Table 1.  Optimization of reaction conditions^a

  Entry	Catalyst	Oxidant	Nitro source	Solvent	Yieldb/%
  1	Pd(OAc)2	K2S2O8	Co(NO3)2•6H2O	DCE	93
  2	Cu(OAc)2	K2S2O8	Co(NO3)2•6H2O	DCE	n.r.
  3	Pd(CH3CN)2Cl2	K2S2O8	Co(NO3)2•6H2O	DCE	41
  4	PdCl2	K2S2O8	Co(NO3)2•6H2O	DCE	25
  5	Pd/C (10%)	K2S2O8	Co(NO3)2•6H2O	DCE	71
  6	Pd(Ph3P)2Cl2	K2S2O8	Co(NO3)2•6H2O	DCE	75
  7	Pd(Ph3P)4	K2S2O8	Co(NO3)2•6H2O	DCE	69
  8	Pd(OAc)2	—	Co(NO3)2•6H2O	DCE	n.r.
  9	Pd(OAc)2	TBPB	Co(NO3)2•6H2O	DCE	35
  10	Pd(OAc)2	TBHP	Co(NO3)2•6H2O	DCE	15
  11	Pd(OAc)2	DTBP	Co(NO3)2•6H2O	DCE	25
  12	Pd(OAc)2	DCP	Co(NO3)2•6H2O	DCE	65
  13	Pd(OAc)2	K2S2O8	Co(NO3)2•6H2O	THF	Trace
  14	Pd(OAc)2	K2S2O8	Co(NO3)2•6H2O	DMF	Trace
  15	Pd(OAc)2	K2S2O8	Co(NO3)2•6H2O	1, 4-Dioxane	10
  16	Pd(OAc)2	K2S2O8	Co(NO3)2•6H2O	DMSO	Trace
  17	Pd(OAc)2	K2S2O8	Co(NO3)2•6H2O	Toluene	60
  18	Pd(OAc)2	K2S2O8	Ca(NO3)2	DCE	23
  19	Pd(OAc)2	K2S2O8	Fe(NO3)2	DCE	57
  20	Pd(OAc)2	K2S2O8	Fe(NO3)3	DCE	29
  21	Pd(OAc)2	K2S2O8	NaNO2	DCE	Trace
  22	Pd(OAc)2	K2S2O8	KNO3	DCE	Trace
  23	Pd(OAc)2	K2S2O8	AgNO2	DCE	78
  24	Pd(OAc)2	K2S2O8	Fe(NO3)3•9H2O	DCE	36
  25	Pd(OAc)2	K2S2O8	Bi(NO3)3	DCE	70
  26	Pd(OAc)2	K2S2O8	Cu(NO3)2	DCE	43
  27	Pd(OAc)2	K2S2O8	Co(NO3)2•6H2O	DCE	82c
  28	Pd(OAc)2	K2S2O8	Co(NO3)2•6H2O	DCE	93d
  29	Pd(OAc)2	K2S2O8	Co(NO3)2•6H2O	DCE	77e
  30	Pd(OAc)2	K2S2O8	Co(NO3)2•6H2O	DCE	81f
  31	Pd(OAc)2	K2S2O8	Co(NO3)2•6H2O	DCE	67g
  32	Pd(OAc)2	K2S2O8	Co(NO3)2•6H2O	DCE	89h
  33	Pd(OAc)2	K2S2O8	Co(NO3)2•6H2O	DCE	81i
  a Reaction conditions: 1a (0.5 mmol), nitro source (1.2 equiv.), K2S2O8 (1.2 equiv.), catalyst (10 mol%) and solvent (2.0 mL) under N2 atmosphere at 110 ℃ for 12 h, unless otherwise noted. b Isolated yields. c Pd(OAc)2 (5 mol%). d 2 equiv. of Co(NO3)2•6H2O was used. e 100 ℃. f 120 ℃. g 8 h. h 24 h. i Air atmosphere.

  下载: 导出CSV

  Table 2.  Substrate scope^a

  a Conditions: 1 (0.5 mmol), K2S2O8 (1.2 equiv.), Co(NO3)2•6H2O (1.2 equiv.), Pd(OAc)2 (10 mol%) and solvent (2.0 mL) under N2 atmosphere at 110 ℃ for 12 h. The yields are of the isolated products.

  下载: 导出CSV
•