Facile and efficient one-pot synthesis of 2-arylbenzoxazoles using hydrogen tetrachloroaurate as catalyst under oxygen atmosphere

Facile and efficient one-pot synthesis of 2-arylbenzoxazoles using hydrogen tetrachloroaurate as catalyst under oxygen atmosphere first-author †E-mail:ykuiliu@zjut.edu.cngreensyn@zjut.edu.cn †E-mail:ykuiliu@zjut.edu.cngreensyn@zjut.edu.cn ykuiliu@zjut.edu.cn corresp ‡Corresponding Author ‡Corresponding Author †E-mail:ykuiliu@zjut.edu.cngreensyn@zjut.edu.cn †E-mail:ykuiliu@zjut.edu.cngreensyn@zjut.edu.cn ykuiliu@zjut.edu.cn State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, Zhejiang University of Technology, Hangzhou 310014, China State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, Zhejiang University of Technology, Hangzhou 310014, China In this paper, we presented a novel method for the facile and efficient one-pot synthesis of 2-arylbenzoxazoles, which were directly synthesized from 2-aminophenol and aldehydes catalyzed by hydrogen tetrachloroaurate (HAuCl 4 ·4H 2 O) under an oxygen atmosphere with anhydrous tetrahydrofuran (THF) as solvent or in solvent-free condition. The results show that this method could bring excellent yields as high as 96%. THF was proven to be the best choice among several solvents screened and the reaction was tolerated with a variety of aromatic aldehydes possessing electron-donating or withdrawing groups. The advantages of the present method lie in catalytic process using economic and environmentally benign dioxygen as oxidant. 2-Arylbenzoxazoles 2-Aminophenol Aldehydes Hydrogen tetrachloroaurate One-pot synthesis INTRODUCTION 1 Nowadays gold is no doubt the “star metal” in chemistry because there has been a focus on gold-catalyzed organic transformations in recent years (Hashmi, 2003 2003 ; 2004 2004 ; 2007 2007 ; Hashmi and Hutching, 2006 2006 ; Muzart, 2008 2008 ; Li et al., 2008 2008 ; Arcadi, 2008 2008 ). Among various new transformations catalyzed by gold, those involving nucleophilic additions to C-C multiple bonds (alkynes, alkenes, or allenes) and catalytic C-H bond functionalization have been intensively studied (Widenhoefer and Han, 2006 2006 ; Jiménez-Núňez and Echavarren, 2007 2007 ; Shen, 2008 2008 ; Skouta and Li, 2008 2008 ; Díaz-Requejo and Pérez, 2008 2008 ). In contrast, gold-catalyzed oxidation chemistry, in particular gold as a catalyst for selective oxidation reactions with economic and environmentally benign oxidants, such as dioxygen or hydrogen peroxide, has been less developed (de Vos and Sels, 2005 2005 ). So far, representative oxidation transformations are mainly limited in oxidation of monoxide (Haruta et al., 1987 1987 ; Valden et al., 1998 1998 ; Mallat and Baiker, 2004 2004 ), alcohols (Arcadi and Giuseppe, 2004 2004 ; Guan et al., 2005 2005 ; Tsunoyama et al., 2005 2005 ; Choudhary et al., 2007 2007 ; Li et al., 2007 2007 ; Miyamura et al., 2007 2007 ; Kanaoka et al., 2007 2007 ), amines (Lazar and Angelici, 2006 2006 ), and sulfides (Boring et al., 2001 2001 ; Yuan and Bian, 2007 2007 ), epoxidations of olefins (Min and Friend, 2007 2007 ), oxidative cleavage of carbon-carbon multiple bonds (Liu et al., 2006 2006 ; Xing et al., 2006 2006 ), oxidation of alkanes (Shul′pin et al., 2001 2001 ), etc. In addition, the field of gold-catalyzed oxidation chemistry is mostly dominated by heterogeneous catalysts, while homogeneous catalysts still represent the much smaller part. Therefore, it is still desirable to develop new oxidation transformations involving homogeneous gold catalysts. Benzoxazoles are an important class of heterocycles, because they have exhibited a variety of biological activities including antimicrobial and antitumor properties (Deluca and Kerwin, 1997 1997 ; Temiz et al., 1998 1998 ; Sato et al., 1998 1998 ), thus leading them to becoming appealing targets in drug discovery (Evindar and Batey, 2006 2006 ). Besides, benzoxazoles are also found applicable in material science as photochromic agents and laser dyes (Heynderickx et al., 2003 2003 ; El′tsov, 1990 1990 ; Trost and Flemming, 1991 1991 ; Reiser et al., 1972 1972 ). There are two general strategies for synthesizing 2-arylbenzoxazoles based on substituted 2-aminophenols as starting materials. One is treatment of 2-aminophenols with carboxylic acid derivatives in the presence of strong acids in harsh conditions (Terashima et al., 1982 1982 ). The other is via photochemical, oxidative, and radical cyclization of phenolic Schiff bases derived from the condensation of 2-aminophenols and aldehydes (Tauer and Grellmann, 1981 1981 ; Wilfred and Taylor, 2004 2004 ; Kawashita et al., 2003 2003 ; Park et al., 2002 2002 ). For oxidative cyclization, various oxidants such as 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) (Chang et al., 2002 2002 ), Mn(OAc) 3 (Varma and Kumar, 1998 1998 ), PhI(OAc) 2 (Varma et al., 1997 1997 ), ThClO 4 (Park et al., 1996 1996 ), BaMnO 4 (Srivastava and Venkataramani, 1988 1988 ), NiO 2 (Nakagawa et al., 1964 1964 ), Pb(OAc) 4 (Stephens and Bower, 1949 1949 ), and I 2 (Moghaddam et al., 2006 2006 ) have been used. However, most of these oxidants are not economic or environmentally benign; furthermore, they are usually required in stoichiometric amounts relative to the substrates. Therefore, to develop more economic and environmental-friendly catalysts to replace conventional stoichiometric oxidants is currently needed. Recently, Kawashita et al.( 2003 2003 ) and Kidwai et al.( 2006 2006 ) reported utilization of activated carbon (using 50% (w/w) of catalyst at 120 °C) and Cu-nanoparticles (using 10 mol% of catalyst at 80~100 °C) as catalysts under oxygen atmosphere for the one-pot synthesis of 2-arylben-zoxazoles from 2-aminophenols and aldehydes. As part of our continued interest in synthesis of useful heterocycles (Xu et al., 2007 2007 ), we wish to report here a more efficient and mild synthesis of 2-arylbenzoxazoles in one-pot manner using only 2 mol% of hydrogen tetrachloroaurate as catalyst at 66 °C under oxygen atmosphere with anhydrous tetrahydrofuran (THF) as solvent or in solvent-free condition. MATERIALS AND METHODS 1 A representative procedure for the one-pot synthesis of 2-arylbenzoxazoles 3 is described as follows: A three-necked flask (25 ml) was charged with 2-aminophenol 1 (1.0 mmol), benzaldehyde 2a (1.0 mmol), HAuCl 4 ·4H 2 O (8.0 mg, 0.02 mmol), and THF (2 ml). The resulting mixture was stirred at 40 °C for 30 min. Then the flask was equipped with a balloon filled with dioxygen and the reaction mixture was heated to 66 °C and stirred for another 6 h under oxygen atmosphere. The progress of the reaction was monitored by gas chromatography-mass spectrometry (GC-MS) and thin-layer chromatography (TLC). Upon completion, the resulting mixture was filtered to remove gold catalyst, the filtrate was diluted with distill water (10 ml) and extracted with ethyl acetate (15 ml×2). The organic layer was dried over anhydrous MgSO 4 . After evaporation of solvent under vacuum, the residue was purified by chromatography using hexane/ethyl acetate (9:1, v/v) as eluent. RESULTS AND DISCUSSION 1 Initially, phenolic Schiff base 2-(benzylidene-amino)phenol 4a was chosen as model substrate for investigating the oxidative cyclization by using gold catalysts (Fig. 1 1 , Table 1 1 ). In the absence of a gold catalyst, the reaction could hardly take place (entry 1). The oxidative cyclization did proceed to give the desired product 3a in 66% yield when 4a was treated with of 2 mol% of AuCl 3 under an oxygen atmosphere in anhydrous THF at reflux for 6 h (entry 2). Under the catalysis of AuCl and NaAuCl 4 ·2H 2 O, the yield of 3a increased to be 76% and 88%, respectively (entries 3 and 4). So far, the best result was obtained by using HAuCl 4 ·4H 2 O as catalyst, which gave 3a in 98% yield (entry 5). We also tried to use hydrogen peroxide as oxidant; however, the desired 2-phenylbenzoxale was obtained in quite low yield due to the happening of a hydrolysis of the Schiff base 4a (entry 6). Note that the reaction could still proceed smoothly to give 3a in moderate yield even under air (entry 7). In the reaction, gold catalyst may act as a bifunctional catalyst; namely, it serves as an acid catalyst to help the cyclization process via activation of the imine group (Yang et al., 2007 2007 ) as well as an oxidative catalyst for the dehydrogenation of the in situ generated dihydrobenzoxazole with dioxygen (Zhu and Angelici, 2007 2007 ). Reaction condition: catalyst HAuCl 4 ·4H 2 O (2 mol%), O 2 (balloon, 101 325 Pa), THF (2 ml), reflux, 6 h Reaction condition: 4a (1 mmol), catalyst (0.02 mmol), THF (2 ml), O 2 (balloon, 101 325 Pa), reflux, 6 h; Isolated yield; Aqueous 30% (v/v) H 2 O 2 (2 mmol) was used as oxidant; The reaction was carried out under air Entry Catalyst Yield (%) b b 1 None <5 2 AuCl 3 66 3 AuCl 76 4 NaAuCl 4 ·2H 2 O 88 5 HAuCl 4 ·4H 2 O 98 6 HAuCl 4 ·4H 2 O c c 35 7 HAuCl 4 ·4H 2 O d d 80 Encouraged by the success of gold-catalyzed oxidative cyclization of 4a , we next examined the direct synthesis of 2-arylbenzoxazole 3a from 2-aminophenol 1 and benzaldehyde 2a catalyzed by HAuCl 4 ·4H 2 O (Fig. 2 2 ). The one-pot procedure involved the first treatment of 1 and 2a (molar ratio: 1:1) with 2 mol% of HAuCl 4 ·4H 2 O in THF at 40 °C for 30 min followed by elevating the reaction temperature to 66 °C (at reflux) for another 6 h under an oxygen atmosphere, and finally gave desired product 3a in 96% yield (Table 2 2 , entry 1). In this section, the effect of solvents on the reaction was particularly investigated (Table 2 2 ). It was found that the reaction proceeded equally well even in solvent-free condition (95%, entry 2). The yield of 3a decreased very much when a mixture of THF and water (4:1, v/v) was used as reaction medium (entry 3). CH 2 Cl 2 , CHCl 3 , dimethyl sulphoxide (DMSO), and N , N -dimethyl formamide (DMF) were proven to be not good choice for the reaction (entries 9~12), although the step for the formation of Schiff base 4a could proceed well. Using CH 3 OH, EtOH, m -xylene, CH 3 CN, and 1,2-dichloroethane (1,2-DCE) as solvent, 3a could be obtained in moderate to good yields (from 75%~92%, entries 4~8). Reaction conditions: (1) catalyst HAuCl 4 ·4H 2 O (2 mol%), 40 °C, 30 min; (2) O 2 (balloon 101 325 Pa), solvent, 66 °C, 6 h All reactions were carried out by the first treatment of 1 (1 mmol) and 2a (1 mmol) with HAuCl 4 ·4H 2 O (0.02 mmol) in solvent (2 ml) at 40 °C for 30 min, followed by elevating the reaction temperature to 66 °C under an oxygen atmosphere; Isolated yield THF: tetrahydrofuran; 1,2-DCE: 1,2-dichloroethane; DMSO: dimethyl sulphoxide; DMF: N , N -dimethyl formamide Entry Solvent Time (h) Yield (%) b b 1 THF 6 96 2 Free 6 95 3 THF-H 2 O (4:1, v/v) 6 3 4 CH 3 OH 6 75 5 EtOH 6 80 6 m -xylene 12 85 7 CH 3 CN 6 87 8 1,2-DCE 12 92 9 CH 2 Cl 2 12 45 10 CHCl 3 12 63 11 DMSO 12 18 12 DMF 12 54 Finally, a range of aromatic and heteroaromatic aldehydes was tested in the one-pot synthesis of 2-arylbenzoxazoles 3 from aminophenol 1 and aldehydes 2 catalyzed by HAuCl 4 ·4H 2 O in THF or solvent-free condition (Fig. 3 3 , Table 3 3 ). In most cases, the reaction proceeded smoothly to give 3 in good to excellent yields. Aromatic aldehydes possessing electron-withdrawing groups (entries 6~8) generally gave better yield of 3 than those containing electron-donating groups (entries 2~4). Surprisingly, the reaction became a little messed when a nitro-substituted aromatic aldehyde was used and decreased the yield of 3 (entries 9~11). Reaction conditions: (1) HAuCl 4 ·4H 2 O (2 mol%), 40 °C, 30 min; (2) O 2 (balloon, 101 325 Pa), THF or solvent-free, 66 °C All reactions were carried out by the first treatment of 1 (1 mmol) and 2 (1 mmol) with HAuCl 4 ·4H 2 O (0.02 mmol) in THF (2 ml) or solvent-free condition at 40 °C for 30 min followed by elevating the reaction temperature to 66 °C under an oxygen atmosphere; Isolated yield; THF as solvent; Solvent-free condition Entry R Product Time (h) Yield (%) b b 1 4-C 6 H 5 ( 2a ) 3a 6 96 c c 2 4-CH 3 C 6 H 4 ( 2b ) 3b 6 90 d d 3 4-CH 3 OC 6 H 4 ( 2c ) 3c 6 88 c c 4 3,4-OCH 2 OC 6 H 3 ( 2d ) 3d 6 87 c c 5 1-Naphthyl ( 2e ) 3e 6 93 c c 6 4-ClC 6 H 4 ( 2f ) 3f 6 97 c c 7 4-BrC 6 H 4 ( 2g ) 3g 6 95 c c 8 4-CF 3 C 6 H 4 ( 2h ) 3h 6 98 d d 9 2-NO 2 C 6 H 4 ( 2i ) 3i 9 76 c c 10 3-NO 2 C 6 H 4 ( 2j ) 3j 6 83 c c 11 4-NO 2 C 6 H 4 ( 2k ) 3k 12 44 c c 12 2-Furyl ( 2l ) 3l 12 84 c c 13 2-Thiophenyl ( 2m ) 3m 9 96 c c CONCLUSIONS AND EXPERIMENTAL DETAILS 1 In summary, we described an efficient protocol for preparing 2-arylbenzoxazoles via oxidative cyclization of in situ generated phenolic Schiff bases catalyzed by HAuCl 4 ·4H 2 O under an oxygen atmosphere. The advantages of the present method lie in catalytic process using economic and environmentally benign dioxygen as oxidant, relatively low catalyst loading, one-pot manner, mild reaction conditions, and good yields. General remarks: All the starting chemicals were commercial products (Alderich or J & K Chemical). Melting points were measured on a Büchi B-545 and uncorrected. 1 H NMR (nuclear magnetic resonance) spectra were obtained on a Bruker AVANCE III 500 (500 MHz) instrument in CDCl 3 using tetramethylsilane (TMS) as internal standard. Chemical shifts ( δ ) are expressed in ×10 −6 and coupling constants ( J ) are given in Hz. GC-MS experiments were performed with an Agilent 6890N GC system equipped with a 5973N mass-selective detector. 2-Phenylbenzoxazole ( 3a ): white crystals, m.p. 102.1~102.2 °C [102 °C (Varma et al., 1997 1997 )]; 1 H NMR (500 MHz, CDCl 3 ): δ (×10 −6 ) 7.33~7.37 (m, 2H), 7.51~7.54 (m, 3H), 7.56~7.59 (m, 1H), 7.77~7.79 (m, 1H), 8.25~8.27 (m, 2H). 2-(4′-Methylphenyl)benzoxazole ( 3b ): white crystals, m.p. 112.3~112.8 °C [113~114 °C (Varma et al., 1997 1997 )]; 1 H NMR (500 MHz, CDCl 3 ): δ (×10 −6 ) 2.44 (s, 3H), 7.32~7.35 (m, 4H), 7.56~7.58 (m, 1H), 7.75~7.77 (m, 1H), 8.15 (d, 2H, J =8.5Hz). 2-(4′-Methoxyphenyl)benzoxazole ( 3c ): white crystals, m.p. 99.5~99.8 °C [101 °C (Varma et al., 1997 1997 )]; 1 H NMR (500 MHz, CDCl 3 ): δ (×10 −6 ) 3.90 (s, 3H), 7.03 (d, 2H, J =9.0 Hz), 7.30~7.36 (m, 2H), 7.55~7.57 (m, 1H), 7.74~7.75 (m, 1H), 8.21 (d, 2H, J =9.0 Hz). 2-(3′,4′-Piperonyl)benzoxazole ( 3d ): gray crystals, m.p. 150.8~151.2 °C [150~152 °C (Kidwai et al., 2006 2006 )]; 1 H NMR (500 MHz, CDCl 3 ): δ (×10 −6 ) 6.07 (s, 2H), 6.94 (d, 1H, J =8.0 Hz), 7.32~7.34 (m, 2H), 7.54~7.56 (m, 1H), 7.70 (s, 1H), 7.72~7.74 (m, 1H), 7.83 (d, 1H, J =9.5 Hz). 2-(Naphthyl)benzoxazole ( 3e ): white crystals, m.p. 104.3~104.4 °C [104 °C (Tauer and Grellmann, 1981 1981 )]; 1 H NMR (500 MHz, CDCl 3 ): δ (×10 −6 ) 7.38~7.41 (m, 2H), 7.57~7.66 (m, 4H), 7.69~7.90 (m, 2H), 8.03 (d, 1H, J =8.0 Hz), 8.43 (d, 1H, J =8.0 Hz), 9.47 (d, 1H, J =9.0 Hz). 2-(4′-Chlorophenyl)benzoxazole ( 3f ): white crystals, m.p. 151.4~151.5 °C [147 °C (Varma et al., 1997 1997 )]; 1 H NMR (500 MHz, CDCl 3 ): δ (×10 −6 ) 7.35~7.39 (m, 2H), 7.51 (d, 2H, J =8.5 Hz), 7.58~7.60 (m, 1H), 7.77~7.79 (m, 1H), 8.20 (d, 2H, J =8.5 Hz). 2-(4′-Bromophenyl)benzoxazole ( 3g ): white crystals, m.p. 155.4~155.5 °C [157~158 °C (Evindar and Batey, 2006 2006 )]; 1 H NMR (500 MHz, CDCl 3 ): δ (×10 −6 ) 7.36~7.38 (m, 2H), 7.58~7.60 (m, 1H), 7.68 (d, 2H, J =7.5 Hz), 7.76~7.79 (m, 1H), 8.13 (d, 2H, J =7.5 Hz). 2-(4′-Trifluoromethylphenyl)benzoxazole ( 3h ): white crystals, m.p. 140.3~140.4 °C [143~145 °C (Lewis et al., 2004 2004 )]; 1 H NMR (500 MHz, CDCl 3 ): δ (×10 −6 ) 7.37~7.42 (m, 2H), 7.59~7.62 (m, 1H), 7.77~7.82 (m, 3H), 7.37 (d, 2H, J =9 Hz). 2-(2′-Nitrophenyl)benzoxazole ( 3i ): pale yellow crystals, m.p. 101.5~101.6 °C [104~105 °C (Bougrin et al., 1998 1998 )]; 1 H NMR (500 MHz, CDCl 3 ): δ (×10 −6 ) 7.37~7.42 (m, 2H), 7.56~7.58 (m, 1H), 7.66~7.70 (m, 1H), 7.72~7.82 (m, 1H), 7.89 (d, 1H, J =9.5 Hz), 8.14 (d, 1H, J =9.5 Hz). 2-(3′-Nitrophenyl)benzoxazole ( 3j ): pale yellow crystals, m.p. 211.5~211.6 °C [211.5~212 °C (Nakagawa et al., 1964 1964 )]; 1 H NMR (500 MHz, CDCl 3 ): δ (×10 −6 ) 7.40~7.45 (m, 2H), 8.64~7.66 (m, 1H), 7.74 (t, 1H, J =8.0 Hz), 7.82~7.84 (m, 1H), 8.38~8.40 (m, 1H), 8.59~8.61 (m, 1H), 9.10 (m, 1H). 2-(4′-Nitrophenyl)benzoxazole ( 3k ): yellow crystals, m.p. 264.1~264.2 °C [266~268 °C (Varma et al., 1997 1997 )]; 1 H NMR (500 MHz, CDCl 3 ): δ (×10 −6 ) 7.41~7.46 (m, 2H), 7.64 (d, 1H, J =7.0 Hz), 7.84 (d, 1H, J =7.0 Hz), 8.39 (d, 2H, J =8.5 Hz), 8.44 (d, 2H, J =8.5 Hz). 2-(2-Furyl)benzoxazole ( 3l ): white crystals, m.p. 88.1~88.5 °C [89~90 °C (Kidwai et al., 2006 2006 )]; 1 H NMR (500 MHz, CDCl 3 ): δ (×10 −6 ) 6.61 (dd, 1H, J 1 =3.5 Hz, J 2 =4.0 Hz), 7.28 (dd, 1H, J 1 =3.5 Hz, J 2 =2.0 Hz), 7.33~7.37 (m, 2H), 7.55~7.57 (m, 1H), 7.67 (t, 1H, J =2.0 Hz), 7.73~7.76 (m, 1H). 2-(2-Thiophenyl)benzoxazole ( 3m ): white crystals, m.p. 108.1~108.2 °C [108 °C (Kidwai et al., 2006 2006 )]; 1 H NMR (500 MHz, CDCl 3 ): δ (×10 −6 ) 7.18~7.20 (m, 1H), 7.33~7.36 (m, 2H), 7.51~7.56 (m, 2H), 7.71~7.75 (m, 1H), 7.92 (dd, 1H, J 1 =3.5 Hz, J 2 =4.0 Hz). 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