NOTE

* E-mail: jhli@hunnu.edu.cn; Tel.: 0086-0731-8872576; Fax: 0086-0731-8872101 Received and revised May 19, 2010; accepted July 19, 2010. Project supported by the New Century Excellent Talents in University (No. NCET-06-0711), Hunan Normal University (No. 20801), the National

Natural Science Foundation of China (No. 20872112) and Fok Ying Tung Education Foundation (No. 101012).

2318 © 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chin. J. Chem. 2010, 28, 2318—2322

Synthesis of Alkyl-Aryl Ethers by Copper-catalyzed Etherization Reactions of Aryl Fluorides with

Tetraalkylammonium Bromides and H2O

Wang, Feng(王峰) Tang, Boxiao(唐伯孝) Xie, Yexiang*(谢叶香) Li, Jinheng*(李金恒)

Key Laboratory of Chemical Biology & Traditional Chinese Medicine Research (Ministry of Education), Hunan Normal University, Changsha, Hunan 410081, China

Synthesis of alkyl aryl ethers via copper-catalyzed etherizations of electron-deficient aryl fluorides with quater-nary ammonium bromides and water has been developed. In the presence of Cu(OAc)2, POPh3 (L4) and Cs2CO3, a variety of electron-deficient aryl fluorides underwent the reaction with quaternary ammonium bromides and H2O in moderate to good yields. The mechanism was also discussed.

Keywords copper, POPh3, aryl fluoride, quaternary ammonium bromide, alkyl aryl ether, etherization

Introduction

Alkyl aryl ethers are important class of constituents in a tremendous range of natural products and bioactive molecules as well as are valuable intermediates in or-ganic synthesis.1 There are three common transforma-tions for their synthesis, including (1) nucleophilic sub-stitution reactions of activated aryl halides with alco-hols,2 (2) copper-catalyzed Ullmann reactions of aryl halides with alcohols,3 and (3) palladium-catalyzed cross-couplings of aryl halides with alcohols.4 However, all these methods must be performed under basic condi-tions to form alkoxides, which are sensitive to air and water. In addition, harmful organic solvents are required in all cases. Wandless and co-workers,2c for instance, have reported that electron-deficient aryl fluorides could undergo the etherization reaction with aliphatic alcohols smoothly in THF to prepare alkyl aryl ethers in moder-ate to good yiels. However, stronger bases, such as KOBu-t or KHMDS [potassium bis(trimethylsilyl)- amide], were required. For these reasons, the develop-ment of a new and environmentally benign route to the synthesis of alkyl aryl ethers is still significant. Here, we wish to report a novel protocol for the preparation of alkyl aryl ethers via copper-catalyzed etherization reac-tions of aryl fluorides with tetraalkylammonium bro-mides and H2O (Eq. 1). To the best of our knowledge,

there is no report on the use of tetraalkylammonium bromides combined with water as nucleophiles for this process to synthesize alkyl aryl ethers.5

Results and discussion

In our recent report on the Cu2O-catalyzed Stille cross-coupling, 1-n-butoxy-4-nitrobebzene (3aa) was observed in a 20% yield when 1-fluoro-4-nitrobenzene (1a) was employed as the substrate, P(o-tol)3 (L1) as the ligand, KF as the base and (n-Bu)4NBr (TBAB, 2a) as the medium (Entry 1 in Table 1).6 These prompted us to optimize the conditions to improve the yield of 3aa. Initially, a number of bases, including KF, Cs2F, Cs2CO3, K2CO3 and KOH, were examined, and Cs2CO3 provided the best results. In the presence of Cu2O, L1 and Cs2CO3, treatment of substrate 1a with 2a and H2O afforded the corresponding product 3aa in a 48% yield (Entry 2). We then turned our efforts on testing copper salts as the catalysts (Entries 2—5). The results showed that Cu(OAc)2 was the most effective catalyst in terms of yield (61% yield, Entry 5), and no reaction was ob-served without copper salts. Subsequently, a series of ligands, such as PPh3 (L2), P(2,6-diMeOC6H3)2 (L3), POPh3 (L4) and bipyridine (L5), were screened, and POPh3 (L4) was superior to the other ligands (Entries 6—8). In the presence of Cu(OAc)2, POPh3 (L4) and Cs2CO3, the yield of 3aa was enhanced to 80% (Entry 8). It is noteworthy that the reaction performed in air atmosphere gives a low yield (Entry 9). In addition, no reaction was observed in the presence of solvents, such as hexane, toluene, THF, MeCN, DMF, DMSO and

Synthesis of Alkyl-Aryl Ethers by Copper-catalyzed Etherization Reactions

Chin. J. Chem. 2010, 28, 2318—2322 © 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cjc.wiley-vch.de 2319

Et3N. The results also disclosed that the amount of wa-ter affected the reaction to some extent, and the yield of 3aa was reduced either in the absence of H2O or in the presence of 6 equiv. of H2O (Entries 8 and 10). It is worth noting that a moderate yield of 3aa was still iso-lated without Cu(OAc)2 or/and POPh3 (L4) (Entries 11 and 12). Finally, the loading of TBAB and temperature effects were also investigated (Entries 13— 16). It turned out that 6 equiv. of TBAB at 145 ℃ provided the best results (Entry 8). We found that a mixture of products was observed when the reaction was conducted at 180 ℃ without Cu and POPh3 (Entry 13). It was happy to observe that a good yield of 3aa was still ob-tained from the reaction of 2 mmol of substrate 1a (En-try 16). However, no reaction was observed using (n-Bu)4NCl or (n-Bu)4NI instead of TBAB (Entries 17 and 18).

Table 1 Copper-catalyzed cross-coupling of 1-fluoro-4-nitro-benzene (1a) with TBAB (2a) and H2O

a

Entry [Cu] Ligand Yieldb/%

1c Cu2O P(o-tol)3 (L1) 20

2 Cu2O P(o-tol)3 (L1) 48

3 CuI (L1) 18

4 CuBr2 (L1) 43

5 Cu(OAc)2 (L1) 61

6 Cu(OAc)2 PPh3 (L2) Trace

7 Cu(OAc)2 P(2,6-diMeOC6H3)2 (L3) 32

8 Cu(OAc)2 POPh3 (L4) 80

9d Cu(OAc)2 (L4) 25

10e Cu(OAc)2 (L4) 56

11 ⎯ (L4) 50

12 ⎯ ⎯ 48

13f ⎯ ⎯ Mixture

14g Cu(OAc)2 (L4) 25

15h Cu(OAc)2 (L4) 49

16i Cu(OAc)2 (L4) 82

17j Cu(OAc)2 (L4) Trace

18k Cu(OAc)2 (L4) Trace a Reaction conditions: 1a (0.5 mmol), H2O (3 equiv.), TBAB (6 equiv.), [Cu] (10 mol%), ligand (20 mol%) and Cs2CO3 (2 equiv.) at 145 ℃ for 24 h. b Isolated yield. c KF (2 equiv.) instead of Cs2CO3.

d Under air atmosphere. e H2O (6 equiv.). f At 180 ℃ in high-pressure reaction vessel. g At 120 ℃. h TBAB (3 equiv.). i 1a (2 mmol). j (n-Bu)4NCl (6 equiv.) instead of TBAB. k (n-Bu)4NI (6 equiv.) instead of TBAB.

Subsequently, a variety of aryl fluorides were sur-veyed to investigate scope of the reaction under the

standard conditions (Table 2). We were delighted to find that nitro-substituted aryl fluorides 1b—1h af-forded satisfactory results in the presence of Cu(OAc)2, L4 and Cs2CO3, and fluorides bearing chloro, butoxy and trifluoromethyl substituents were tolerated well (Entries 1—7). Interestingly, the reaction of substrate 1e with TBAB and H2O gave a moderate yield together with the occurrence of displacement reaction (Entry 4). It was pleased to discover that difluorobenzenes 1g and 1h were selectively reacted with TBAB and H2O under the standard conditions to provide the mono-butoxy- substituted products 3ga and 3ha in good yields (Entries 6 and 7). Unfortunately, a mixture of products was ob-served from the reaction of 1,3,5-trifluoro-2-nitroben-zene 1i (Entry 8). We found that the yields of the target products were reduced using fluorobenzonitriles 1j and 1k as the substrates (Entries 9 and 10). For instance, only 30% yield of the desired product was isolated when 4-fluorobenzonitrile (1j) was treated with TBAB, H2O•Cu(OAc)2, L4 and Cs2CO3 after 36 h (Entry 9). However, attempt to etherization of 1-chloro-2,4- dinitrobenzene with TBAB and H2O failed (Entry 11). It is noteworthy that other aryl fluorides, including 1-fluoro-4-(trifluoromethyl)benzene (1l), 1-(4-fluoro-phenyl)-ethanone (1m), fluorobenzene (1n) and 4-fluoroanisole (1o), are not suitable substrates for the reaction under the standard conditions.

Screening the etherization reaction of substrate 1a with several commercially available quaternary ammo-nium bromides (2) is listed in Scheme 1. Quaternary ammonium bromides, such as (n-C3H7)4NBr (2c), (n-C7H15)4NBr (2d), (n-C8H17)4NBr (2e), (benzyl)- (Et)3NBr (2f), (benzyl)(n-C4H9)3NBr (2g), and (ally)(n-C4H9)3NBr (2h) except Et4NBr (2b), all per-formed the reaction successfully under the standard conditions. It is worth noting that both (benzyl)(Et)3NBr (2f) and (benzyl)(n-C4H9)3NBr (2g) selectively afford the same product 3af alone, whereas (ally)(n-C4H9)3NBr (2h) gave a mixture of two products 3ah and 3aa (molar ratio: 1∶3).

Scheme 1 Etherizations of substrate 1a with commercially available quaternary ammonium bromides (2)

A working mechanism was proposed as outlined in Scheme 2.1-4 Substrate 2 readily undergoes the radical reaction to afford the radical intermediate A under heating and Cu conditions,5,7 followed by performing the reaction according to the SNAr mechanism.2-4 There are at least two roles of Cu(OAc)2 in the reaction: (i) stability of the radical intermediate A, and (ii) catalyst for the SNAr reaction.

Wang et al.NOTE

2320 www.cjc.wiley-vch.de © 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chin. J. Chem. 2010, 28, 2318—2322

Table 2 Copper-catalyzed cross-coupling of aryl fluorides (1) with TBAB (2a) and H2Oa

Entry Substrate t/h Product Isolated yieldb/%

1

24

87

2

45

96

3

46

64

4

48

38

5

36

90

6

46

60

7

40

86

8

48 Mixture

N.D.b

9

36

30

10

48

35

11

48

trace

a Reaction conditions: 1 (0.5 mmol), H2O (3n equiv.), 2a (6n equiv.), Cu(OAc)2•H2O (10n mol%), L4 (20n mol%) and Cs2CO3 (2n equiv.) at 145 ℃. n＝number of fluoro and methoxy groups. b Not determined.

Synthesis of Alkyl-Aryl Ethers by Copper-catalyzed Etherization Reactions

Chin. J. Chem. 2010, 28, 2318—2322 © 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cjc.wiley-vch.de 2321

Scheme 2 A possible mechanism

In summary, we describe here the first example of constructing C—O bond via copper-catalyzed reaction of the electron-deficient aryl fluorides with quaternary ammonium bromides and water. Work to probe the de-tailed mechanism and apply the reaction in organic syn-thesis is currently underway.

Experimental

The NMR spectroscopy was performed on an INOVA-400 (Varian) spectrometer operating at 400 MHz (1H NMR) and 100 MHz (13C NMR). TMS (tetramethylsilane) was used an internal standard and CDCl3 was used as the solvent. Mass spectrometric analysis was performed on GC-MS analysis (SHIMA-DZU GCMS-QP2010).

Typical experimental procedure for the copper- catalyzed etherization reaction

A mixture of aryl fluoride 1 (0.5 mmol), H2O (3n equiv.), R4NBr 2 (6n equiv.), Cu(OAc)2•H2O (10n mol%), L4 (20n mol%) and Cs2CO3 (2n equiv.) (n＝number of fluoro and methoxy groups) was stirred at 145 ℃ under argon atmosphere for the indicated time in Tables 1 and 2 and Scheme 1 until complete con-sumption of starting material as monitored by TLC. Af-ter the reaction was finished, diethyl ether was poured into the mixture, then washed with water, dried with anhydrous Na2SO4 and evaporated under vacuum. The residue was purified by flash column chromatography (hexane or hexane/ethyl acetate) to afford the desired coupled product.

1-Butoxy-4-nitrobenzene (3aa)7 Slight yellow oil; 1H NMR (CDCl3, 400 MHz) δ: 8.18 (d, J＝9.2 Hz, 2H), 6.94 (d, J＝9.2 Hz, 2H), 4.06 (t, J＝6.0 Hz, 2H), 1.84—1.77 (m, 2H), 1.54—1.48 (m, 2H), 0.99 (t, J＝7.2 Hz, 3H); 13C NMR (CDCl3, 100 MHz) δ: 164.1, 141.2, 125.8, 114.3, 68.5, 30.9, 19.1, 13.7; LRMS (EI) m/z (%): 195 (M＋, 59), 139 (96), 140 (80), 123 (32), 109 (55), 56 (87), 41 (100).

1-Butoxy-2-chloro-4-nitrobenzene (3ba)8 Color-less oil; 1H NMR (CDCl3, 400 MHz) δ: 8.29 (s, 1H), 8.15 (d, J＝9.2 Hz, 1H), 6.97 (d, J＝8.8 Hz, 1H), 4.14 (t, J＝6.4 Hz, 2H), 1.90—1.84 (m, 2H), 1.58—1.53 (m, 2H), 1.00 (t, J＝7.2 Hz, 3H); 13C NMR (CDCl3, 100 MHz) δ: 159.8, 140.9, 126.0, 124.0, 123.4, 111.6, 69.6, 30.8, 19.1, 13.7; LRMS (EI) m/z (%): 231 (M＋

＋2, 9), 229 (M＋, 28), 175 (19), 173 (51), 145 (8), 143 (27), 129 (3), 127 (9), 101 (4), 99 (11), 63 (17), 56 (100).

1-Butoxy-2-phenyl-4-nitrobenzene (3ca) Slight yellow solid, m.p. 81—82 ℃ (uncorrected); 1H NMR

(CDCl3, 400 MHz) δ: 8.24—8.19 (m, 2H), 7.53 (d, J＝7.2 Hz, 2H), 7.45—7.37 (m, 3H), 7.01 (d, J＝8.0 Hz, 1H), 4.09 (t, J＝6.0 Hz, 2H), 1.84—1.77 (m, 2H), 1.54—1.48 (m, 2H), 0.93 (t, J＝7.2 Hz, 3H); 13C NMR (CDCl3, 100 MHz) δ: 161.1, 141.2, 136.3, 131.3, 129.4, 128.1, 127.8, 126.4, 124.7, 111.4, 68.9, 30.8, 19.1, 13.7; LRMS (EI) m/z (%): 271 (M＋, 37), 215 (100); HRMS (EI) calcd for C16H17NO3 (M ＋ ) 271.1208, found 271.1208.

2,4-Dibutoxy-1-nitrobenzene (3da) Colorless oil; 1H NMR (CDCl3, 400 MHz) δ: 7.96 (d, J＝8.8 Hz, 1H), 6.50 (s, 1H), 6.47 (d, J＝9.2 Hz, 1H), 4.07 (t, J＝6.0 Hz, 2H), 4.02 (t, J＝6.4 Hz, 2H), 1.86—1.77 (m, 4H), 1.55—1.49 (m, 4H), 1.01—0.96 (m, 6H); 13C NMR (CDCl3, 100 MHz) δ: 164.2, 155.2, 132.8, 128.2, 104.9, 100.6, 69.2, 68.4, 31.0, 30.9, 19.1, 13.7; LRMS (EI) m/z (%): 267 (M＋, 26), 211 (9), 155 (100); HRMS (EI) calcd for C14H21NO4 (M

＋) 267.1471, found 267.1470. 1-Butoxy-2-nitro-4-(trifluoromethyl)benzene (3fa)

Colorless oil; 1H NMR (CDCl3, 400 MHz) δ: 8.10 (s, 1H), 7.77 (d, J＝8.8 Hz, 1H), 7.18 (d, J＝8.8 Hz, 1H), 4.18 (t, J＝10.4 Hz, 2H), 1.87—1.83 (m, 2H), 1.55—1.49 (m, 2H), 0.99 (t, J＝7.6 Hz, 3H); 13C NMR (CDCl3, 100 MHz) δ: 154.8, 130.8 (d, J＝3.1 Hz, 1C), 124.5, 123.2 (d, J＝3.8 Hz, 1C), 122.6, 112.3, 114.6, 69.9, 30.8, 19.0, 13.6; LRMS (EI) m/z (%): 263 (M＋, 6), 207 (15), 188 (6), 56 (100); HRMS (EI) calcd for C11H12F3NO3 (M

＋) 263.0769, found 263.0767. 4-Butoxy-2-fluoro-1-nitrobenzene (3ga)9 Color-

less oil; 1H NMR (CDCl3, 400 MHz) δ: 7.94, 7.92 (dd, J＝5.6 Hz, 5.6 Hz, 1H), 6.77 (d, J＝10.4 Hz, 1H), 6.73—6.79 (m, 1H), 4.09 (t, J＝6.4 Hz, 2H), 1.86—1.82 (m, 2H), 1.56—1.51 (m, 2H), 0.99 (t, J＝7.6 Hz, 3H); 13C NMR (CDCl3, 100 MHz) δ: 167.2, 164.7, 155.1 (d, J＝11.4 Hz, 1C), 128.2 (d, J＝11.4 Hz, 1C), 107.3 (d, J＝12.8 Hz, 1C), 102.3 (d, J＝26.7 Hz, 1C), 70.0, 31.2, 19.3, 14.0; LRMS (EI) m/z (%): 213 (M＋, 17), 157 (31), 141 (10), 127 (16), 56 (100).

1-Butoxy-2-fluoro-4-nitrobenzene (3ha) Color-less oil; 1H NMR (CDCl3, 400 MHz) δ: 8.05 (d, J＝8.8 Hz, 1H), 7.97 (d, J＝10.8 Hz, 1H), 7.03 (t, J＝8.0 Hz, 1H), 4.10 (t, J＝6.4 Hz, 2H), 1.86—1.82 (m, 2H), 1.56—1.51 (m, 2H), 0.99 (t, J＝7.6 Hz, 3H); 13C NMR (CDCl3, 100 MHz) δ: 153.0 (d, J＝9.8 Hz, 1C), 152.4, 149.9, 120.8 (d, J＝3.1 Hz, 1C), 112.7 (d, J＝1.5 Hz, 1C), 112.1 (d, J＝2.8 Hz, 1C), 69.5, 30.8, 19.0, 13.7; LRMS (EI) m/z (%): 213 (M＋, 12), 143 (23), 57 (100); HRMS (EI) calcd for C10H12FNO3 (M＋ ) 213.0801, found 213.0801.

4-Butoxybenzonitrile (3ja)7 Colorless oil; 1H NMR (CDCl3, 400 MHz) δ: 7.57 (d, J＝8.8 Hz, 2H), 6.93 (d, J＝9.2 Hz, 2H), 4.00 (t, J＝6.4 Hz, 2H), 1.81—1.77 (m, 2H), 1.52—1.47 (m, 2H), 0.96 (t, J＝7.2 Hz, 3H); 13C NMR (CDCl3, 100 MHz) δ: 162.4, 133.9, 119.3, 115.1, 103.6, 68.1, 30.9, 19.1, 13.7; LRMS (EI) m/z (%): 175 (M＋, 22), 119 (100).

2,6-Dibutoxybenzonitrile (3ka)10 Colorless oil; 1H NMR (CDCl3, 100 MHz) δ: 7.37 (t, J＝8.0 Hz, 1H),

Wang et al.NOTE

2322 www.cjc.wiley-vch.de © 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chin. J. Chem. 2010, 28, 2318—2322

6.50 (d, J＝8.4 Hz, 2H), 4.05 (t, J＝6.8 Hz, 4H), 1.83—1.80 (m, 4H), 1.56—1.50 (m, 4H), 0.98 (t, J＝7.2 Hz, 6H); 13C NMR (CDCl3, 100 MHz) δ: 162.3, 134.4, 114.0, 104.0, 91.8, 68.8, 30.9, 19.1, 13.8; LRMS (EI) m/z (%): 247 (M＋, 22), 191 (81), 135 (100).

1-Nitro-4-propoxybenzene (3ac)11 Slight yellow oil; 1H NMR (CDCl3, 400 MHz) δ: 8.18 (d, J＝9.2 Hz, 2H), 8.94 (d, J＝9.2 Hz, 2H), 4.01 (t, J＝6.4 Hz, 2H), 1.88—1.81 (m, 2H), 1.06 (t, J＝7.2 Hz, 3H); 13C NMR (CDCl3, 100 MHz) δ: 164.3, 141.3, 126.0, 114.4, 70.3, 22.4, 10.4; LRMS (EI) m/z (%): 181 (M＋, 43), 139 (100).

1-(Heptyloxy)-4-nitrobenzene (3ad)12 Slight yel-low oil; 1H NMR (CDCl3, 400 MHz) δ: 8.20 (d, J＝8.8 Hz, 2H), 6.95 (d, J＝9.2 Hz, 2H), 4.06 (t, J＝6.4 Hz, 2H), 1.85—1.81 (m, 2H), 1.49—1.44 (m, 2H), 1.37—1.20 (m, 6H), 0.87 (t, J＝7.2 Hz, 3H); 13C NMR (CDCl3, 100 MHz) δ: 164.3, 141.3, 125.9, 114.4, 68.9, 31.9, 29.2 (2C), 25.9, 22.6, 14.1; LRMS (EI) m/z (%): 237 (M＋, 21), 139 (12), 123 (15), 109 (23), 98 (18), 70 (18), 57 (100).

1-Nitro-4-(octyloxy)benzene (3ae)12 Slight yel-low oil; 1H NMR (CDCl3, 400 MHz) δ: 8.19 (d, J＝9.6 Hz, 2H), 6.94 (d, J＝9.2 Hz, 2H), 4.05 (t, J＝6.4 Hz, 2H), 1.84—1.80 (m, 2H), 1.46—1.42 (m, 2H), 1.35—1.29 (m, 8H), 0.88 (t, J＝7.2 Hz, 3H); 13C NMR (CDCl3, 100 MHz) δ: 164.3, 141.2, 125.8, 114.3, 68.8, 31.7, 29.2, 29.1, 28.9, 25.9, 22.6, 14.0; LRMS (EI) m/z (%): 251 (M＋, 100).

1-((4-Nitrophenoxy)methyl)benzene (3af)13 Slight yellow solid, m.p. 103.2—103.7 ℃ (uncorrected, lit.13 m.p. 102—105 ℃); 1H NMR (CDCl3, 400 MHz) δ: 8.18 (d, J＝9.2 Hz, 2H), 7.42—7.36 (m, 5H), 7.01 (d, J＝9.2 Hz, 2H), 5.15 (s, 2H); 13C NMR (CDCl3, 100 MHz) δ: 163.8, 141.7, 135.6, 128.9, 128.3, 127.6, 126.0, 114.9, 70.7; LRMS (EI) m/z (%): 229 (M＋, 3), 108 (3), 91 (100).

1-(Allyloxy)-4-nitrobenzene (3ah)14 Slight yellow oil; 1H NMR (CDCl3, 400 MHz) δ: 8.21 (d, J＝9.6 Hz, 2H), 6.99 (d, J＝9.2 Hz, 2H), 6.10—6.00 (m, 1H), 5.44, 5.35 (dd, J＝13.2, 10.4 Hz, 2H), 4.66 (d, J＝5.2 Hz, 2H); 13C NMR (CDCl3, 100 MHz) δ: 163.4, 141.3, 131.7, 125.7, 118.4, 114.5, 69.2; LRMS (EI) m/z (%): 179 (M＋, 14), 149 (3), 109 (1), 63 (7), 41 (100).

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(E0903262 Zhao, C.)