HETEROCYCLES

Note | Special issue | Vol. 80, No. 2, 2010, pp. 1479-1488
Received, 6th August, 2009, Accepted, 11th September, 2009, Published online, 15th September, 2009.
DOI: 10.3987/COM-09-S(S)107

■ Oxidative Dimerization of 4-Methoxynaphthylamines in the Presence of Semiconductors

Tetsuya Takeya,* Yosuke Takahashi, Iwao Okamoto, and Osamu Tamura*

Showa Pharmaceutical University, 3-3165, Higashi-tamagawagakuen, Machida, Tokyo 194-8543, Japan

Abstract

Three types of 4-methoxynaphthylamines 4a-c were oxidized by treatment with metal oxides under molecular oxygen (O2). 4-Methoxy-1-naphthylamine 4a and 4,6-dimethoxy-1-naphthylamine 4b, on treatment with TiO2 under O2, gave mainly 2-amino-1,4-naphthoquinone derivatives 5a and 5b, respectively whereas 4,8-dimethoxy-1-naphthylamine 4c afforded an unique carbazole 6c as the major product.

Most of semiconductor-mediated transformation of organic molecules are initiated by photo-irradiation,1-10 namely photo-catalytic reaction as well as electochemistry using semiconductors as electrodes.11,12 In stark contrast, we recently reported that oxidative dimerization of naphthols and phenols mediated by semiconductors and molecular oxygen (O2).13-15 In this context, we would like to report herein reactions of 4-methoxynaphthylamines by treatment with semiconductor and O2.

Preparation of the starting 4-methoxynaphthylamines is outlined in Scheme 1. O-Methylation of 1-naphthols 1a-c by Me2SO4 or MeI gave 1-methoxynaphthalenes 2a-c, which were treated with nitric acid and acetic anhydride to provide 4-methoxy-1-nitronaphthalenes 3a-c. Finally, reduction of nitro groups in 3a-c by hydrogenation afforded 4-methoxynaphthylamines 4a-c.

Before synthetic experiments, we measured cyclic voltammetry of 4a-c to examine their oxidation potentials (Eoxs) (Table 1). The measurements showed low oxidation potentials, ca. 0.5 V for 4a and 4b and 0.33 V for 4c in CH2Cl2. The oxidation potentials (0.37 V and 0.23 V for 4a and 4c, respectively) in MeCN were further lower. These experiments implied that 4-methoxy-1-naphthylamines 4a-c readily underwent single electron transfer (SET) reaction to give radical cation species.

Oxidation of 4-methoxynaphthylamine 4a was first examined with various metal oxides, SnO2, ZrO2, NbO5, and TiO2 in oxygen-saturated solvents (Table 2, entries 1-8). As a result, it was found that use of TiO2 shortened reaction time both in CH2Cl2 and in MeCN to give oxidative dimerized product, aminoquinone 5a, as the major product along with small amount of another dimerized product, carbazole 6a (entries 7 and 8). Next, oxidative dimerization of 4a with TiO2 in various solvents was conducted at room temperature (entries 9-14). The reaction did not proceed at room temperature in MeNO2, DMF, and dioxane probably due to reduced reactivity of TiO2 by coordination with the solvent (entries 10-12). Use of aromatic solvents improved the reaction (entries 13-15). Thus, the reaction in benzene completed even at room temperature within 3 h to afford 5a in 50% yield and a small amount (4%) of 6a (entry 13). Considering toxicity of benzene, toluene was examined as the solvent (entries 14 and 15). Although the reaction in toluene seemed to be slightly less efficient than that in benzene (entry 14), mild heating at 60°C accelerated the reaction to give 5a and 6a in comparable yield to that of entry 13 (entriy 15).

We next examined oxidation reaction of 4b by using TiO2 and O2 (Table 3). In the case of 4b, both MeCN and toluene could be used for oxidation reaction. Thus, mild heating 4b with TiO2 in oxygen-saturated solvents, MeCN and toluene afforded aminoquinone 5b in 36% and 38% yields, respectively along with small amount of carbazole 6b (entries 2 and 4).

4,8-Dimethoxynaphthyl-1-amine (4c) having the lowest oxidation potential indicated different distribution of the products (Table 4). Naphthylamine 4c reacted with TiO2 in oxygen-saturated MeCN at room temperature to give equal amount (23% yield, each) of aminoquinone 5c and carbazole 6c (entry 1). Use of higher reaction temperature led to increase of aminoquinone 5c (entry 2). Oxidation reaction of 4c in toluene showed reversal of distribution of product (entry 3). Thus, reaction of 4c at room temperature in toluene for 1.5 h provided carbazole 6c in 63% yield accompanied by aminoquinone 5c in 27% yield.

Although the detailed mechanism for the formation of 5 and 6 remains unclear, it may involve radical cation A (Scheme 2). Thus, the electron-rich 1-naphthylamine 4 causes SET reaction to generate radical cation A, which subsequently looses a proton to lead radical B. SET reaction of radical B gives cation C, which undergoes hydrolysis to provide quinone D. Finally, nucleophilic addition of 4 to quinone D followed by oxidation affords aminoquinone 5. On the other hand, long-lived radical cation A can react with another 4 to give intermediate E, which causes cyclization to yield 6 via intermediates F and G. Accordingly, electron-rich 4c tends to afford carbazole 6c. Use of toluene, a non-polar solvent, might stabilize A by suppressing SET reaction of B into C leading to quinone 5, and hence carbazole 6c became the main product (Table 4, entry 3).

EXPERIMENTAL
All melting points are uncorrected. Infrared (IR) spectra were recorded with a JASCO IR-700 spectrometer, and 1H- and 13C-NMR spectra with JEOL JNM-AL300 with tetramethylsilane as an internal standard. Mass spectra were recorded on a JEOL JMS-D300 or Shimadzu QP-5000 spectrometer. Merck Kieselgel 60 (230–400 mesh), Wako silica gel C-200 were used for column chromatography. Although compounds 3a and 3c are known,17 their preparation and characterization are presented here.

1-Methoxynaphthalene (2a)
Dimethyl sulfate (420 mL, 4.5 mol) was added dropwise to a stirred mixture of 1-naphthol (1a, 400 g, 2.8 mol), tetra-n-butylammonium bromide (116 g, 0.36 mol) in THF (760 mL) and a solution of KOH (500 g, 8.9 mol) in water (300 mL) at 0 °C, and the mixture was further stirred for 1.5 h at the same temperature. To the mixture was added a 10% aqueous solution of NaOH (500 mL). The whole was extracted with AcOEt (1.5 L x 3), and organic layer was washed with water and brine, dried (MgSO4), and concentrated in vacuo. The crude product was purified by column chromatography on silica gel (CH2Cl2) to give 2a (420 g, 95%) as an oil. IR (KBr) cm-1: 1580. 1H-NMR (300 MHz, CDCl3) δ: 4.00 (3H, s), 6.81 (1H, dd, J = 1.5, 7.0 Hz), 7.35-7.51 (4H, m), 7.79 (1H, m), 8.26 (1H). 13C-NMR (75 MHz, CDCl3) δ: 55.2, 103.7, 120.1, 121.9, 125.1, 125.5, 125.8, 126.3, 127.4, 134.4, 155.3. LR-MS m/z: 158 (M+). HR-MS m/z: 158.0734 (Calcd for C11H10O: 158.0732).

1-Methoxy-4-nitronaphthalene (3a)
A mixture of nitric acid (51 mL, 0.56 mol) in acetic anhydride (100 mL, 0.98 mol) was added dropwise to a solution of 2a (92 g, 0.63 mol) in acetic anhydride (280 mL, 2.7 mol) at 0 °C, and the resulting mixture was stirred at the same temperature for 4 h. To the mixture was added ice-cold water, and the mixture was stirred for 5 h. The precipitates were collected by filtration and purified by column chromatography on silica gel (n-hexane-AcOEt, 1:20) to give 3a (64 g, 50%) as yellow needles. mp 85.0-87.0 °C (CH2Cl2-hexane). IR (KBr) cm-1: 1625. 1H-NMR (300 MHz, CDCl3) δ: 4.11 (3H, s), 6.83 (1H, d, J = 8.8 Hz), 7.60 (1H, m, 6 or 7-H), 7.74 (1H, m, 6 or 7-H), 8.38 (1H, dd, J = 0.7, 1.0, 8.5 Hz, 5 or 8-H), 8.41 (1H, d, J = 8.6 Hz, 3-H), 8.79 (1H, dd, J = 0.7, 0.9, 8.7 Hz). 13C NMR (75 MHz, CDCl3) δ: 56.0, 101.6, 122.5, 123.0, 125.1, 126.2, 126.4, 127.0, 129.7, 138.6, 160.3. LR-MS m/z: 203 (M+). HR-MS m/z: 203.0599 (Calcd for C11H9NO3: 203.0582).

4-Methoxy-1-naphthylamine (4a)
A mixture of 3a (1.00 g, 4.9 mmol) and 10% Pd-C (200 mg) in THF was stirred at room temperature for 3 h under an atmosphere of hydrogen. The mixture was filtered through a pad of Celite, and the filtrate was concentrated in vacuo. The crude product was purified by column chromatography on silica gel (n-hexane-AcOEt, 2:1) to give 4a (721 mg, 85%) as crystals. mp 38.0-40.0 °C (Et2O-n-hexane). IR (KBr) cm-1: 3360, 3185. 1H NMR (300 MHz, CDCl3) δ: 3.94 (3H, s), 6.67 (1H, d, J = 8.1 Hz), 6.72 (1H, d, J = 8.1 Hz), 7.49 (2H, m), 7.82 (1H, m), 8.24 (1H, m). 13C-NMR (75 MHz, CDCl3) δ: 55.8, 104.4, 109.6, 120.9, 122.5, 125.2, 125.2, 125.6, 126.1, 135.2, 149.2. LR-MS m/z: 173 (M+). HR-MS m/z: 173.0855 (Calcd for C11H11NO: 173.0841).

1,7-Dimethoxynaphthalene (2b)
Using a procedure similar to that for the preparation of 2a, treatment of 1,7-naphthalenediol 1b (15.0 g, 94 mmol) with dimethyl sulfate (97 mL, 1.0 mol), tetra-n-butylammonium bromide (3.90 g, 12 mmol), and KOH (116 g, 2.1 mol) in THF-water (1:2, 150 mL) gave crude product, which was purified by column chromatography on silica gel (n-hexane-AcOEt, 1:10) to afford 2b (17 g, 95%) as an oil. IR (KBr) cm-1: 1630, 1605, 1510. 1H-NMR (300 MHz, CDCl3) δ: 3.75 (3H, s), 3.85 (3H, s), 6.76 (1H, d, J = 7.7 Hz), 7.00 (1H, dd, J = 2.6, 9.5 Hz), 7.10 (1H, br t, J = 7.9 Hz), 7.24 (1H, d, J = 8.1 Hz), 7.40 (1H, d, J = 2.6 Hz), 7.60 (1H, d, J = 8.9 Hz). 13C-NMR (75 MHz, CDCl3) δ: 55.7, 55.9, 101.3, 105.4, 119.8, 120.9, 124.5, 127.5, 130.1, 130.9, 155.5, 158.5. LR-MS m/z: 188 (M+). HR-MS m/z: 188.0813 (Carcd for C12H12O2: 188.0837).

1,7-Dimethoxy-4-nitronaphthalene (3b)
Using a procedure similar to that for the preparation of 3a, treatment of 2b (5.00 g, 27 mmol) with nitric acid (2.1 mL, 24 mmol) in acetic anhydride (16 mL) gave crude product, which was purified by column chromatography on silica gel (n-hexane-AcOEt, 1:20) to afford 3b (2.1 g, 35%). IR (KBr) cm-1: 1605. 1H-NMR (300 MHz, CDCl3) δ: 3.95 (3H, s), 4.09 (3H, s), 6.77 (1H, d, J = 8.6 Hz), 7.35 (1H, dd, J = 2.7, 9.5 Hz), 7.60 (1H, d, J = 2.8 Hz), 8.24 (1H, d, J = 8.8 Hz), 8.68 (1H, d, J = 9.5 Hz). 13C-NMR (75 MHz, CDCl3) δ: 55.4, 56.3, 101.1, 102.3, 122.0, 122.2, 124.5, 125.3, 127.1, 139.3, 158.0, 159.3. LR-MS m/z: 233 (M+). HR-MS m/z: 233.0684 (Carcd for C12H11NO4: 233.0688).

4,6-Dimethoxy-1-naphthylamine (4b)
Using a procedure similar to that for the preparation of 4a, treatment of 3b (1.50 g, 6.4 mmol) with 10% Pd-C (300 mg) in THF (10 mL) under hydrogen gave crude product, which was purified by column chromatography on silica gel (n-hexane-AcOEt, 1:10) to afford 4b (1.10 g, 88%). mp 77.0-79.0 °C (Et2O-n-hexane). IR (KBr) cm-1: 3385, 3215. 1H-NMR (300 MHz, CDCl3) δ: 3.94 (6H, br s), 6.57 (1H, d, J = 8.1 Hz), 6.66 (1H, d, J = 8.1 Hz), 7.15 (1H, dd, J = 2.8, 9.2 Hz), 7.54 (1H, d, J = 2.6 Hz), 7.73 (1H, d, J = 9.2 Hz). 13C-NMR (75 MHz, CDCl3) δ: 55.3, 55.8, 100.9, 105.3, 107.6, 117.9, 120.4, 122.8, 127.2, 135.4, 148.3, 157.4. LR-MS m/z: 203 (M+). HR-MS m/z: 203.0947 (Carcd for C12H13NO2: 203.0946).

4,8-Dimethoxy-1-naphthylamine (2c)
A mixture of 1,5-naphthalenediol (1c, 5.00 g, 31 mmol), methyl iodide (9.7 mL, 0.16 mol), and K2CO3 in DMF (50 mL) was stirred at room temperature for 2 h. The mixture was poured into ice-cold water (50 mL), and the resulting precipitates were collected by filtration to give sufficiently pure 2c (4.7 g, 80%) as granules. mp 181.0-183.0 °C (CH2Cl2-n-hexane). IR (KBr) cm-1: 1595. 1H-NMR (300 MHz, CDCl3) δ: 3.99 (6H, s), 6.84 (2H, d, J = 7.7 Hz), 7.37 (2H, br t, J = 8.3 Hz), 7.83 (2H, d, J = 8.8 Hz). 13C-NMR (75 MHz, CDCl3) δ: 55.5, 104.5, 114.2, 125.1, 126.6, 155.2. LR-MS m/z: 188 (M+). HR-MS m/z: 188.0826 (Calcd for C12H12O2: 188.0837).

1,5-Dimethoxy-4-nitronaphthalene (3c)
Using a procedure similar to that for the preparation of 3a, treatment of 2c (1.00 g, 5.3 mmol) with nitric acid (0.4 mL, 4.7 mol) in acetic anhydride (8 mL) gave crude product, which was purified by column chromatography on silica gel (n-hexane-AcOEt, 1:20) to afford 3c (540 mg, 43%) as granules. mp 166.0-168.0 °C (CH2Cl2-hexane). IR (KBr) cm-1: 1595, 1530, 1515. 1H-NMR (300 MHz, CDCl3) δ: 3.92 (3H, s), 4.04 (3H, s), 6.75 (1H, d, J = 8.3 Hz, 2-H), 7.02 (1H, dd, J = 0.7, 7.8 Hz), 7.50 (1H, br t, J = 8.2 Hz), 7.50 (1H, d, J = 8.3 Hz, 3-H), 7.91 (1H, dd, J = 0.9, 8.6 Hz). 13C-NMR (75 MHz, CDCl3) δ: 56.0, 56.1, 102.3, 108.4, 114.7, 117.2, 127.2, 132.4, 141.0, 153.7, 157.1. LR-MS m/z: 233 (M+). HR-MS m/z: 233.0701 (Calcd for C12H11NO4: 233.0688).

4,8-Dimethoxy-1-naphthylamine (4c)
Using a procedure similar to that for the preparation of 4a, treatment of 3c (1.00 g, 4.3 mmol) with 10% Pd-C (200 mg) in THF (60 mL) under hydrogen gave crude product, which was purified by column chromatography on silica gel (n-hexane-CH2Cl2, 1:1) to afford 4c (742 mg, 85%) as grnules. mp 158.0-160.0 °C (CH2Cl2-n-hexane). IR (KBr) cm-1: 3495, 3380. 1H-NMR (300 MHz, CDCl3) δ: 3.91 (3H, s), 3.96 (3H, s), 6.52 (1H, d, J = 8.3 Hz), 6.70 (1H, d, J = 8.3 Hz), 6.77 (1H, dd, J = 0.9, 7.7 Hz), 7.30 (1H, br t, J = 8.1 Hz), 7.80 (1H, dd, J = 1.0, 8.5 Hz). 13C-NMR (75 MHz, CDCl3) δ: 55.7, 55.1, 104.8, 106.5, 109.1, 115.0, 116.1, 125.2, 128.4, 138.1, 147.3, 157.4. LR-MS m/z: 203 (M+). HR-MS m/z: 203.0978 (Calcd for C12H13NO2: 203.0946).

2-(4-Methoxynaphthalen-1-ylamino)-1,4-naphthoquinone (5a) and 5,8-dimethoxy-13H-dibenzo[a,i]- carbazole (6a)
Oxygen was sufficiently bubbled through a mixture of 4a (50 mg, 0.29 mmol) and TiO2 (1.00 g, 0.63 mmol) in toluene (15 mL). The mixture was heated under oxygen in a sealed tube at 60 °C for 0.5 h. The mixture was filtered through a pad of Celite, and the filtrate was concentrated in vacuo. The crude product was chromatographed on silica gel (n-hexane-CH2Cl2, 5:1) to give 5a (23.0 mg, 48%), 6a (2.8 mg, 6%).
5a: Red granules, mp 199.0-201.0 °C (CH2Cl2-n-hexane). IR (KBr) cm-1: 3375, 1630, 1600. 1H-NMR (300 MHz, CDCl3) δ: 4.05 (3H, s), 6.83 (1H, d, J = 8.3 Hz), 7.38 (1H, t, J = 8.1 Hz), 7.54 (2H, m), 7.58 (1H, s), 7.67 (1H, dt, J = 1.5, 7.5 Hz), 7.75 (1H, dd, J = 1.5, 7.5 Hz), 7.80 (1H, m), 8.08 (1H, dd, J = 1.5, 7.5 Hz), 8.12 (1H, dd, J = 1.5, 7.5 Hz), 8.33 (1H, m). 13C-NMR (75 MHz, CDCl3) δ: 55.8, 103.2, 103.4, 121.8, 122.9, 123.9, 125.3, 126.0, 126.2, 126.3, 126.4, 127.5, 130.0, 130.6, 132.2, 133.5, 134.9, 147.5, 155.0, 182.3, 183.7. LR-MS m/z: 329 (M+). HR-MS m/z: 329.1029 (Calcd for C21H15NO3: 329.1052).
6a: Colorless sylup. IR (KBr) cm-1: 3315. 1H-NMR (DMSO-d6) δ: 4.16 (6H, s), 7.60 (2H, t, J = 7.3 Hz), 7.76 (2H, t, J = 7.3 Hz), 7.81 (2H, s), 8.34 (2H, d, J = 7.3 Hz), 8.71 (2H, d, J = 7.3 Hz), 12.40 (1H, s). LR-MS m/z: 327 (M+).

7-Methoxy-2-(4,6-dimethoxynaphthalen-1-ylamino)-1,4-naphthoquinone (5b) and 3,5,8,10-tetra- methoxy-13H-dibenzo[a,i]carbazole (6b)
(a) Using a procedure similar to that for the preparation of 5a and 6a, treatment of 4b (50 mg, 0.25 mmol) with TiO2 (1.00 g, 0.63 mmol) in MeCN (15 mL) at 70 °C under oxygen gave crude product, which was purified by column chromatography on silica gel (n-hexane-CH2Cl2, 2:1) to afford 5b (17.5 mg, 36%) and 6b (2.9 mg, 6%).
(b) The same reaction in toluene at 60 °C afforded 5b (18.5 mg, 38%) and 6b (1.0 mg, 2%).
5b: IR (KBr) cm-1: 3330, 1665, 1630. 1H-NMR (300 MHz, CDCl3) δ: 3.950 (3H, s), 3.954 (3H, s), 4.04 (3H, s), 5.78 (1H, s), 6.81 (1H, d, J = 8.2 Hz), 7.11 (1H, dd, J = 2.6, 8.6 Hz), 7.19 (1H, dd, J = 2.6, 9.3 Hz), 7.22 (1H, d, J = 8.4 Hz), 7.55 (1H, d, J = 2.6 Hz), 7.60 (1H, d, J = 2.6 Hz), 7.65 (1H, s), 7.71 (1H, d, J = 9.3 Hz), 8.10 (1H, d, J = 8.4 Hz). 13C-NMR (75 MHz, CDCl3) δ: 55.4, 55.7, 55.9, 101.2, 102.7, 103.9, 109.8, 118.6, 119.9, 121.1, 123.6, 123.8, 125.1, 125.5, 127.4, 129.0, 136.2, 147.8, 153.8, 157.9, 165.1, 180.8, 183.3. LR-MS m/z: 389 (M+). HR-MS m/z: 389.1286 (Carcd for C23H19NO5: 389.1263).
6b: Blue purple powder, mp 257.0-261.0 °C (CH2Cl2-n-hexane). IR (KBr) cm-1: 3400. 1H-NMR (300 MHz, CDCl3) δ: 3.82 (6H, s), 4.00 (6H, s), 7.14 (2H, dd, J = 2.6, 9.0 Hz), 7.55 (2H, s), 7.61 (2H, d, J = 2.6 Hz), 8.30 (2H, d, J = 9.0 Hz), 11.21 (1H, s). 13C-NMR (75 MHz, CD2Cl2) δ: 55.5, 55.9, 97.1, 103.0, 117.1, 117.3, 118.0, 121.8, 125.7, 128.7, 149.5, 157.0. LR-MS m/z: 387 (M+). HR-MS m/z: 387.1502 (Carcd for C24H21NO4: 387.1470).

8-Methoxy-2-(4,8-dimethoxynaphthalen-1-ylamino)-1,4-naphthoquinone (5c) and 1,5,8,12-tetra- methoxy-13H-dibenzo[a,i]carbazole (6c)
Using a procedure similar to that for the preparation of 5a and 6a, treatment of 4c (50 mg, 0.25 mmol) with TiO2 (1.00 g, 0.63 mmol) in toluene (15 mL) at room temperature under oxygen gave crude product, which was purified by column chromatography on silica gel (CH2Cl2) to afford 5c (13.1 mg, 27%) and 6c (30.5 mg, 63%).
5c: Purple granules, mp 209.0-211.0 °C (CH2Cl2-n-hexane). IR (KBr) cm-1: 3450, 1670, 1625. 1H-NMR (300 MHz, CDCl3) δ: 4.01 (3H, s), 4.06 (3H, s), 4.09 (3H, s), 6.54 (1H, s), 6.83 (1H, d, J = 8.6 Hz), 6.94 (1H, d, J = 7.7 Hz), 7.22 (1H, d, J = 8.3 Hz, 7-H), 7.407 (1H, t, J = 8.2), 7.409 (1H, d, J = 8.4 Hz), 7.68 (1H, t, J = 8.1 Hz), 7.80 (1H, d, J = 8.1 Hz), 7.92 (1H, d, J = 8.4 Hz), 10.14 (1H, s). 13C-NMR (75 MHz, CDCl3) δ: 55.8, 56.2, 56.5, 100.0, 104.0, 107.0, 115.3, 115.9, 118.5, 118.8, 119.6, 119.7, 125.9, 127.4, 128.3, 135.9, 136.0, 146.0, 152.5, 156.1, 160.1, 180.9, 183.6. LR-MS m/z: 389 (M+). HR-MS m/z: 389.1270 (Calcd for C23H19NO5: 389.1263).
6c: Colorless crysrtals, mp over 300 °C (CH2Cl2-n-hexane). IR (KBr) cm-1: 3490. 1H-NMR (300 MHz, DMSO-d6) δ: 4.10 (6H, s), 4.26 (6H, s), 7.25 (2H, d, J = 7.7 Hz), 7.49 (2H, t, J = 8.1 Hz), 7.85 (2H, s), 7.90 (2H, d, J = 8.1 Hz), 11.27 (1H, s). 13C-NMR (75 MHz, DMSO-d6) δ: 55.9, 56.2, 98.0, 106.0, 113.2, 115.0, 116.7, 124.6, 125.4, 126.1, 148.6, 155.1. LR-MS m/z: 387 (M+). HR-MS m/z: 387.1463 (Calcd for C24H21NO4: 387.1470).

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