Clean and Selective Oxidation of Alcohols Catalyzed by Ion-Supported TEMPO in Water

Tetrahedron 62 (2006) 556–562 Clean and selective oxidation of alcohols catalyzed by ion-supported TEMPO in water Weixing Qian, Erlei Jin, Weiliang Bao* and Yongmin Zhang Department of Chemistry, Xi Xi Campus, Zhejiang University, Hangzhou, Zhejiang 310028, People’s Republic of China Received 25 March 2005; revised 9 October 2005; accepted 11 October 2005 Available online 22 November 2005 Abstract—Three different types of ion-supported TEMPO catalysts are synthesized and their catalytic activity in the chemoselective oxidation of alcohols is investigated. These new catalysts show high catalytic activity in water and can be reused for the next run by extraction of products. Recycling experiments exhibit that ion-supported TEMPO can be reused up to five times without loss of catalytic activity. This system offers a very clean, convenient, environmentally benign method for the selective oxidation of alcohols. q 2005 Elsevier Ltd. All rights reserved. 1. Introduction Chemoselective oxidation of alcohols to carbonyl compounds is an important reaction in organic synthesis and many methods for this transformation have been documented in the literature in view of its importance.1 Recently the use of metal-free catalysts for selective oxidation of organic compounds is attracting more and more attention because these metal-free catalysts are beneficial from both economic and environmental viewpoints. Moreover, they are readily able to tether to a support covalently and obviate the problem of metal leaching.2 In the area of metal-free catalytic alcohol oxidations stable free nitroxyl radicals, such as 2,2,6,6-tetramethylpiperidine-1oxyl (TEMPO), play an increasingly important role in organic synthesis.3 Usually, efficient methods for the transformation of alcohols to carbonyl compounds or carboxylic acids under mild conditions include the use of 1 mol% of TEMPO as a catalyst and a stoichiometric amount of a terminal oxidant. Many different terminal oxidants have been developed in this reaction including sodium hypochlorite, 4 [bis(acetoxy)iodo]benzene, 5 m-CPBA,6 sodium bromite,7 trichloroisocyanuric acid,8 oxone,9 iodine10 and oxygen in combination with CuCl11 or NaNO2.12 Although these oxidants are successful for efficient alcohol oxidation, separation of the TEMPO from products needs tedious workup procedures. To simplify the product isolation and catalyst recovery the use of polymersupported catalysts seems alternative. Various polymersupported TEMPO or its derivatives have been synthesized Keywords: Alcohols; Oxidation; Ion-supported TEMPO; Catalysis; Hypervalent iodine reagent. * Corresponding author. Tel./fax: C86 571 88911554; e-mail: wlbao@css.zju.edu.cn 0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2005.10.022 either based on inorganic13 or organic supports.4a,b,14 This polymer-supported TEMPO, however, in some cases will result in decreasing activity or extending reaction time after recycling.4a,15 In the last decade ionic liquids have attracted considerable attention as an alternative reaction medium, which represents interesting properties such as high thermal stability, negligible vapor pressure, high loading capacity and easy recyclability. Various chemical reactions can be performed in ionic liquids.16 One of the attractive features of ionic liquids in organic synthesis is that the structures with the cationic or anionic components can be modified according to requirement, so that they can be adapted to special applications. Recently, increasing attention has been focused on the use of ionic liquids as a means of immobilizing catalysts, facilitating products separation and providing an alternative to recycle the catalysts, and several publications that present their potential in catalysis have been demonstrated.17 More recently, a TEMPOderived task-specific ionic liquid for oxidation of alcohols by the Anelli protocol has been described.18 In this paper, we wish to report three different types of ion-supported TEMPO catalysts for the oxidation of alcohols by an ion-supported hypervalent iodine reagent 1-(4-diacetoxyiodobenzyl)-3methyl imidazolium tetrafluoroborate19 [dibmin]C[BF4]K in water and examine their catalytic activity in different cases. 2. Results and discussion The route for the synthesis of ion-supported TEMPO catalysts was depicted in Scheme 1. Starting from the commercially available 4-hydroxyl-TEMPO (1), reaction W. Qian et al. / Tetrahedron 62 (2006) 556–562 557 Scheme 1. Synthesis of ion-supported catalysts 4, 6, and 7. Reaction conditions: (i) 1.0 equiv NaH, 1.5 equiv 1,4-dibromobutane, acetone, rt, 25%; (ii) 1.1 equiv 1-methylimidazole, CH3CN, 60 8C, 97%; (iii) 1.5 equiv NaBF4, acetone, refluxing, 86%; (iv) 1.1 equiv 2-chloroacetyl chloride, 1.1 equiv pyridine, CH2Cl2, 5 8C to rt, 80%; (v) 1.1 equiv 1-methylimidazole, CH3CN, 60 8C, 98%; (vi) 1.5 equiv NaBF4, acetone, refluxing, 85%; (vii) 1.1 equiv imidazole, 1.1 equiv K2CO3, acetone, rt; (viii) 1.1 equiv NaBF4, 82%. with 1,4-dibromobutane (2) as a linker in acetone afforded 4-(2,2,6,6-tetramethyl-1-oxyl-4-piperidoxyl)butyl bromide (3) in 25% yield.20 Quaternization of 1-methylimidazole with 3 and subsequent anion exchange with NaBF4 gave the desired ion-supported TEMPO catalyst (4) in 97 and 86% yields, respectively. To raise the yield in the first step, catalyst (6) was synthesized with chloroacetyl chloride (5) as a linker instead of 1,4-dibromobutane 2 and the yield reached to 80%. The rest steps were analogous to the catalyst 4. As to symmetrical catalyst (7), we wanted to make imidazolium cation have higher loading capacity, reaction of imidazole with 2 equiv bromide 3 in the presence of K2CO3 and subsequent metathesis with NaBF4 in one pot afforded the ion-supported diradical 7 in good yield, which was purified by silica gel chromatography. These ion-supported catalysts are insoluble in low polar organic solvents such as ethers or hexanes but are soluble in CH2Cl2 and highly soluble in water and ionic liquids. They are ideal candidates for aqueous homogeneous catalysis. After three ion-supported TEMPO catalysts were obtained, the catalyst 4 was chosen as a candidate to investigate its catalytic activity in combination with various terminal oxidants in water.21 Table 1 listed the results. The ion-supported TEMPO catalyst 4 proved to be an effective one for the selective oxidation of 4-methoxylbenzyl alcohol except for oxidant peracetic acid, giving 4methoxylbenzyl aldehyde in good to excellent yields and short reaction times under mild conditions (a 1:1.2 ratio of alcohol/oxidant). It showed similar catalytic activity to that of free TEMPO (Table 1, entry 6). Using PhI(OAc)2 as a terminal oxidant, the reaction did not work well because of poor solubility of PhI(OAc)2 in water. Both I2 and NaOCl showed effective oxidants. The reaction proceeded fast and gave excellent yields. However, in the case of I2, a slight Table 1. Oxidation of 4-methoxybenzyl alcohol catalyzed by an ion-supported TEMPO 4 in water Entry Oxidant Time (min) Yield (%)a 1 2 3 4 5 6 CH3CO3H PhI(OAc)2 I2 NaOCl [dibmim]C[BF4]K [dibmim]C[BF4]K 600 120 40 3 6 6 25 70 98 96 98 97b a b Isolated yields after chromatographic purification unless otherwise noted. Free TEMPO was used as a catalyst. 558 W. Qian et al. / Tetrahedron 62 (2006) 556–562 excess of I2 would contaminate the products, and in the TEMPO-bleach protocol introduced by Anelli et al. CH2Cl2 usually was used as a biphasic system4c and chlorinated byproducts could be formed.9 After screening several terminal oxidants listed in Table 1, we found [dibmin]C[BF4]K to be suitable for the reaction conducted in water, which based on the following reasons: (a) readily dissolving in water due to its ion-type structure; (b) acting as a role of phase transfer catalyst (PTC) to some extent to facilitate substrate dissolving in water; (c) most part of the oxidant, which was reduced easily precipitated out from water and recovered by filtration after the products extracted by Et2O (the oxidant can be reoxidized by peracetic acid). The ion-supported TEMPO catalyst 4 remained in filtrate, which could be reused for next run after reloading the substrates and the oxidant [dibmin]C[BF4]K. Next, we examined the oxidation of a variety of primary and secondary alcohols in the presence of 5 mol% of ion-supported TEMPO (4, 6 or 7) catalysts employing 1.2 mol equiv of [dibmin]C[BF4]K as a terminal oxidant. Under these conditions, of all most alcohols were quantitatively oxidized to carbonyl compounds within 330 min in good to excellent yields (Table 2). Benzylic primary alcohols (Table 2, entries 1–4 and 6) gave excellent yields of the corresponding aldehydes in very short reaction times (6–40 min) without any noticeable overoxidation to the carboxylic acids. This methodology is mild and compatible with several functional groups other than the hydroxyl group. For example, the ester linkage of ethyl 4-(hydroxymethyl)benzoate (Table 2, entry 6) remained intact. Cinnamyl alcohol (Table 2, entry 5) was oxidized to the corresponding aldehyde in 97% yield and no doublebond addition product was found. For the oxidation of benzylic secondary alcohols (Table 2, entries 12–15), the reaction afforded excellent yields but required 15–120 min for completion. The oxidation was relatively slow for the aliphatic primary and secondary alcohols (Table 2, entries 7–11), but the good yields could be obtained by extending reaction times. Notably, from the results, as expected, obtained in experiment, the catalytic activity for the catalyst 4 is the same as that for catalyst 6 (Table 2, entries 2, 5 and 13). The symmetric catalyst 7 gives the same yields as catalyst 4 or 6 in almost half reaction times and exhibits catalytic activities two times greater than catalyst 4 or 6 alone under similar conditions (Table 2, entries 5, 7–9 and 11), which implies this ion-supported catalyst with higher loading capacity would be more economic and potential in organic synthesis. Table 2. Oxidation of alcohols catalyzed by ion-supported TEMPO catalysts (4, 6 or 7) using [dibmim]C[BF4]K as terminal oxidant in water Entry Substrate Product Catalyst Time (min) Yield (%)a 1 4 6 97b 2 4 6 6 6 98 98 3 4 15 97 4 4 10 97b 5 4 6 7 60 60 20 98 97 98 6 4 40 90 7 4 240 86 7 90 85 4 7 4 7 240 120 330 180 95b 93 85b 88 10 4 300 86 11 4 7 300 120 80 78 12 4 120 90b 13 4 6 30 30 99 98 14 4 30 98 15 4 15 99 8 9 a b Isolated yields after chromatographic purification unless otherwise noted. Yields were determined by GC. W. Qian et al. / Tetrahedron 62 (2006) 556–562 To examine whether the catalytic activity of ion-supported TEMPO 6 decreases in recycling we compared it with the catalyst 4 under the same conditions described above using 4-methoxybenzyl alcohol as a substrate. The results are listed in Table 3. It was found that almost constant yields were obtained in five subsequent runs for both catalysts. No induction period was observed upon recycling of catalysts 4 and 6.4a It suggested that the ester linkage of catalyst 6 probably was stable in reaction processes. Table 3. Recyclability of ion-supported TEMPO catalysts 4 and 6 in the oxidation of 4-methoxylbenzyl alcohol Catalyst Cycle (yield %)a 1 2 3 4 5 4 6 98 98 96 98 98 97 97 94 a 97 96 559 pressure and water (30 mL) was added to dissolve the solid. The aqueous layer was then extracted with methylene chloride (15 mL!3). The combined organic extracts were dried over Na2SO4 and concentrated under vacuum to a small volume. The resulting concentrated solution was separated by flash chromatography (10:1, petroleum/ethyl acetate) to obtain red oil 1.15 g (25%); 1H NMR (CDCl3, ppm): dZ1.17 (s, 6H), 1.23 (s, 6H), 1.47 (t, 3J(H,H)Z 11.6 Hz, 2H), 1.68–1.73 (m, 2H), 1.90–1.96 (m, 4H), 3.42–3.47 (m, 4H), 3.50–3.57 (m, 1H); 13C NMR (CDCl3, ppm): dZ20.8, 28.6, 29.7, 31.7, 33.8, 44.4, 59.7, 67.0, 70.4; IR (KBr): nZ2973, 2937, 1459, 1376, 1363, 1245, 1177, 1104 cmK1; MS (ESI): m/z (%): 306 (23.0) [MC], 308 (21.6) [MCC2], 135 (93.2) [C4H8BrC], 137 (92.1) [C4H8BrCC2], 55 (100.0) [C4HC]. Anal. Calcd for 7 C13H25BrNO2: C 50.82, H 8.20, N 4.56. Found: C 50.82, H 8.25, N 4.19. Reaction time 7 min. 3. Conclusion In conclusion, we have successfully developed three ionsupported TEMPO catalysts and examined their catalytic activities. These low molecular weight catalysts displayed the same selectivities and activities as those of free TEMPO. The symmetric catalyst 7 noticeably increased the rate of oxidation of alcohols. Their combination with an ionsupported oxidant [dibmin]C[BF4]K afforded an easy workup and environmentally benign catalyst system for mild and selective oxidation of primary and secondary alcohols to carbonyl compounds. Both catalysts and oxidant could be recovered and recycled, and almost no waste was produced. 4. Experimental 4.1. General 1 H and 13C NMR spectra were determined in CDCl3 or DMSO-d6 on a Bruker 400 MHz spectrometer with TMS as the internal standard. IR spectra were recorded on a Bruker Vector-22 infrared spectrometer. C, H, N were analyzed on a Carlo Erba 1110 elemental analyzer. GC–MS analyses were performed on a Hewlett-Packard 5973 instrument (column: HP-5 30 m!0.25 mm!0.25 mm). All melting points were uncorrected. The columns were handpacked with silica gel H60 (w400). Reactions were carried out under atmosphere. 4.2. Synthesis of catalysts (4, 6, and 7) The bromide, chloride, and tetrafluoroborate salts of ionsupported TEMPO were prepared by modification of published literature procedures.22 4.2.1. 4-(2,2,6,6-Tetramethyl-1-oxyl-4-piperidoxyl)butyl bromide. To a solution of 4-hydroxy-TEMPO (2.58 g, 0.015 mol) in anhydrous acetone (25 mL) was added NaH (0.6 g, 0.015 mol) and the resulting slurry stirred for 10 min at room temperature. 1,4-Dibromobutane (4.86 g, 0.0225 mol) was then added and stirred for 3 h at room temperature. The acetone was removed under reduced 4.2.2. 1-Methyl-3-(4-(2,2,6,6-tetramethyl-1-oxyl-4-piperidoxyl)butyl)imidazolium bromide. Under stirring, 0.922 g (3 mmol) of 4-(2,2,6,6-tetramethyl-1-oxyl-4-piperidoxyl)butyl bromide was slowly added to a solution of 0.295 g (3.6 mmol) of 1-methylimidazole in 1 mL of acetonitrile. The mixture was stirred for 6 h at 60 8C. After cooling to room temperature, 5 mL ether was added to the mixture causing precipitation of 1-methyl-3-(4-(2,2,6, 6-tetramethyl-1-oxyl-4-piperidoxyl)butyl)imidazolium bromide as a red solid. This solid was recovered by filtration and washed with ether twice. The yield was 1.13 g (97%). Mp: 71–73 8C; 1H NMR (DMSO-d6, ppm): dZ1.03 (s, 6H), 1.07 (s, 6H), 1.22–1.24 (m, 2H), 1.42–1.47 (m, 2H), 1.78–1.82 (m, 4H), 3.40 (t, 3J(H,H)Z6.4 Hz, 2H), 3.49–3.54 (m, 1H), 3.86 (s, 3H), 4.19 (t, 3J(H,H)Z7.2 Hz, 2H), 7.74 (s, 1H), 7.80 (s, 1H), 9.19 (s, 1H); 13C NMR (DMSO-d6, ppm): dZ20.9, 26.6, 27.1, 32.8, 36.2, 45.0, 49.0, 58.3, 66.8, 70.4, 122.6, 124.0, 136.9; IR (KBr): nZ3087, 2974, 2938, 1632, 1572, 1463, 1363, 1171, 1097 cmK1; MS (ESI): m/z (%): 309 (8.5) [MCKBrK], 124 (100.0) [C9HC ], 123 (66.4) [C7HC ], 81 (59.8) 16 15 [C4H5NC], 55 (65.8) [C3H5NC], 42 (53.6) [C2H4NC]. 2 Anal. Calcd for C17H31BrN3O2: C 52.44, H 8.03, N 10.79. Found: C 52.09, H 8.09, N 10.98. 4.2.3. 1-Methyl-3-(4-(2,2,6,6-tetramethyl-1-oxyl-4-piperidoxyl)butyl)imidazolium tetrafluoroborate. The 1-methyl-3-(4-(2,2,6,6-tetramethyl-1-oxyl-4-piperidoxyl)butyl)imidazolium bromide obtained above 1.17 g (3 mmol) was added to a suspension of NaBF4 (0.495 g, 4.5 mmol) in acetone (5 mL). The mixture was then stirred under the refluxing for 72 h. The sodium bromide precipitate was removed by filtration and the filtrate concentrated by rotary evaporation. The yield was 1.02 g (86%), red viscous oil; 1H NMR (DMSO-d6, ppm): dZ1.05 (s, 6H), 1.08 (s, 6H), 1.25 (t, 3J(H,H)Z10.8 Hz, 2H), 1.43–1.47 (m, 2H), 1.81–1.87 (m, 4H), 3.41 (t, 3J(H,H)Z6.4 Hz, 2H), 3.50–3.55 (m, 1H), 3.85 (s, 3H), 4.18 (t, 3J(H,H)Z7.2 Hz, 2H), 7.68 (s, 1H), 7.74 (s, 1H), 9.06 (s, 1H); 13C NMR (DMSO-d6, ppm): dZ20.8, 26.6, 27.1, 32.7, 36.1, 44.9, 49.1, 58.5, 66.9, 70.3, 122.6, 124.0, 136.9; IR (KBr): nZ3159, 2937, 2938, 1575, 1465, 1364, 1170, 1060 cmK1; MS (ESI): m/z (%): 309 (1.0) [M C KBFK], 137 (78.2) [C 8H 13N C], 83 (100.0) 4 2 [C4H7NC], 55 (70.2) [C3H5NC], 42 (99.0) [C2H4NC]. 2 560 W. Qian et al. / Tetrahedron 62 (2006) 556–562 Anal. Calcd for C17H31BF4N3O2: C 51.53, H 7.89, N 10.60. Found: C 51.15, H 7.96, N 10.62. 4.2.4. 1,3-Bis(4-(2,2,6,6-tetramethyl-1-oxyl-4-piperidoxyl)butyl)imidazolium tetrafluoroborate. To a suspension of 0.245 g (3.6 mmol) of imidazole and 0.7 g potassium carbonate in acetone (10 mL) was added 1.84 g (6 mmol) of 4-(2,2,6,6-tetramethyl-1-oxyl-4-piperidoxyl)butyl bromide, and the mixture was stirred under the refluxing for 48 h. And then 0.396 g (3.6 mmol) NaBF4 was added to the mixture to continue stirring for 72 h. The sodium bromide precipitate was removed by filtration and the filtrate concentrated by rotary evaporation. The yield was 1.50 g (82%), red viscous oil; 1H NMR (DMSO-d6, ppm): dZ1.04 (s, 12H), 1.07 (s, 12H), 1.22–1.26 (m, 4H), 1.37–1.42 (m, 4H), 1.70–1.76 (m, 4H), 1.82–1.86 (m, 4H), 3.37 (t, 3J(H,H)Z6.4 Hz, 4H), 3.48–3.52 (m, 2H), 3.96 (t, 3J(H,H)Z7.2 Hz, 4H), 7.61 (s, 2H), 8.96 (s, 1H); 13C NMR (DMSO-d6, ppm): dZ20.9, 27.0, 28.1, 30.0, 32.5, 32.8, 45.0, 46.2, 56.2, 58.3, 67.0, 68.9, 70.3, 119.6, 129.1, 137.6; IR (KBr): nZ2973, 2935, 2871, 1510, 1464, 1363, 1147, 1108 cmK1; MS (ESI): m/z (%): 141 (43.2) [C9H19NC], 124 (100.0) [C9HC]. Anal. 16 Calcd for C29H53BF4N4O4: C 57.24, H 8.78, N 9.21. Found: C 56.95, H 8.96, N 9.52. 4.2.5. 2,2,6,6-Tetramethyl-1-oxyl-4-piperidinyl 2-chloroacetate. To a solution of 4.3 g (0.025 mol) 4-hydroxyTEMPO and 3.11 g (0.0275 mol) 2-chloroacetyl chloride in dichloromethane (30 mL) was slowly added 2.18 g (0.0275 mol) of pyridine under cooling, and the mixture was standing over night. The precipitate was removed by filtration and the filtrate was washed with water (30 mL), 10% NaHCO3, then 2 mol dilute HCl and finally water (30 mL). The solution was dried over Na2SO4 and concentrated under vacuum to give red solid 4.98 g (80%). Mp: 49–51 8C; 1H NMR (CDCl3, ppm): dZ1.22 (s, 6H), 1.25 (s, 6H), 1.68 (t, 3J(H,H)Z11.6 Hz, 2H), 1.94–1.98 (m, 2H), 4.02 (s, 2H), 5.11–5.16 (m, 1H); 13C NMR (CDCl3, ppm): dZ20.4, 31.8, 41.0, 43.5, 59.2, 69.1, 166.8; IR (KBr): nZ2978, 2954, 1753, 1464, 1319, 1177, 1136 cmK1; MS (ESI): m/z (%): 248 (14.5) [MC], 250 (5.2) [MCC2], 124 (53.9) [C9HC], 109 (100), 41 (82.3). Anal. 16 Calcd for C11H19ClNO3: C 53.12, H 7.70, N 5.63. Found: C 52.99, H 7.73, N 5.54. 4.2.6. 1-Methyl-3-(2-oxo-2-(2,2,6,6-tetramethyl-1-oxyl-4piperidoxyl)ethyl)imidazolium chloride. (This procedure is similar to the preparation of bromide above). Mp: 225–227 8C; 1H NMR (DMSO-d6, ppm): dZ1.08 (s, 6H), 1.11 (s, 6H), 1.51 (t, 3J(H,H)Z11.6 Hz, 2H), 1.88–1.94 (m, 2H), 3.92 (s, 3H), 5.02–5.07 (m, 1H), 5.28 (s, 2H), 7.77 (s, 2H), 9.22 (s, 1H); 13C NMR (DMSO-d6, ppm): dZ20.7, 32.5, 36.3, 43.8, 49.9, 58.4, 69.3, 123.7, 124.1, 138.1, 166.9; IR (KBr): nZ2977, 1747, 1633, 1464, 1223, 1177 cmK1; MS (ESI): m/z (%): 124 (34.3) [C6H8N2OC], 82 (100.0) [C4H6NC], 56 (85.1) [C2H4NC], 55 (91.2) [C4HC], 42 2 2 7 (84.2) [C2H4NC]. Anal. Calcd for C15H25ClN3O3: C 54.46, H 7.62, N 12.70. Found: C 54.44, H 7.75, N 12.61. 4.2.7. 1-Methyl-3-(2-oxo-2-(2,2,6,6-tetramethyl-1-oxyl-4piperidoxyl)ethyl)imidazolium tetrafluoroborate. (This procedure is similar to the preparation of tetrafluoroborate above). Red oil; 1H NMR (DMSO-d6, ppm): dZ1.15 (s, 6H), 1.24 (s, 6H), 1.53 (t, 3J(H,H)Z11.2 Hz, 2H), 1.90–1.94 (m, 2H), 3.90 (s, 3H), 5.02–5.08 (m, 1H), 5.21 (s, 2H), 7.71 (s, 2H), 9.05 (s, 1H); 13C NMR (DMSO-d6, ppm): dZ20.7, 32.3, 36.3, 43.6, 49.9, 58.6, 69.3, 1237, 124.1, 138.1, 166.8; IR (KBr): nZ2978, 2940, 1753, 1580, 1467, 1225, 1179, 1061 cmK1; MS (ESI): m/z (%): 98 (68.2) [C5H8NOC], 82 (46.2) [C4H6NC], 55 (94.3) [C4HC], 41 (100.0) [C2H3NC]. 2 7 Anal. Calcd for C15H25BF4N3O3: C 47.14, H 6.59, N 10.99. Found: C 46.86, H 6.76, N 10.63. 4.3. General procedure for alcohol oxidations [dibmim]C[BF4]K (353 mg, 0.6 mmol) was added to a solution of alcohol (0.5 mmol) in 1.5 g of water containing 5% mmol of ion-supported TEMPO. The reaction mixture was stirred at 30 8C for a given time. The mixture was extracted with ether (3!8 mL) and concentrated under reduced pressure. The filtrate could be reused for next run after the oxidant in reduced form was removed by filtration. The product was purified by flash column chromatography using petroleum ether and Et2O as eluent. All products were commercially available and identified by comparison of the isolated products with authentic samples. 4.3.1. Benzaldehyde. Oil; 1H NMR (CDCl3, ppm): dZ 7.52–7.56 (m, 2H; Ar-H), 7.62–7.65 (m, 1H; Ar-H), 7.88–7.90 (m, 2H; Ar-H), 10.03 (s, 1H; CHO); IR (neat): nZ1703 cmK1 (C]O); MS (70 eV): m/z (%): 106 (100) [MC], 105 (94) [MCKH], 77 (92) [C6HC]. 5 4.3.2. 4-Methoxybenzaldehyde. Oil; 1H NMR (CDCl3, ppm): dZ3.90 (s, 3H; CH3), 7.01 (d, 3J(H,H)Z7.6 Hz, 2H; Ar-H), 7.85 (d, 3J(H,H)Z7.6 Hz, 2H; Ar-H), 9.89 (s, 1H; CHO); IR (neat): nZ1685 cmK1 (C]O); MS (70 eV): m/z (%): 136 (76) [MC], 135 (100) [MCKH]. 4.3.3. 4-Chlorobenzaldehyde. Mp: 46–47 8C; 1H NMR (CDCl3, ppm): dZ7.53 (d, 3J(H,H)Z8.4 Hz, 2H; Ar-H), 7.83 (d, 3J(H,H)Z8.4 Hz, 2H; Ar-H), 9.99 (s, 1H; CHO); IR (KBr): nZ1702 cmK1 (C]O); MS (70 eV): m/z (%): 139 (100) [MC], 141 (33) [MCC2]. 4.3.4. Furaldehyde. Oil; 1H NMR (CDCl3, ppm): dZ 6.61–6.63 (m, 1H), 7.27–7.30 (m, 1H), 7.70–7.71 (m, 1H), 9.67 (s, 1H; CHO); IR (neat): nZ1675 cmK1 (C]O); MS (70 eV): m/z (%): 96 (100) [MC], 95 (94) [MCKH], 39 (58) [C3HC]. 3 4.3.5. Cinnamaldehyde. Oil; 1H NMR (CDCl3, ppm): dZ 6.73 (dd, 3J(H,H)Z8.0, 15.6 Hz, 1H; CH), 7.44–7.45 (m, 3H; Ar-H), 7.49 (d, 3J(H,H)Z15.6 Hz, 1H; CH), 7.56–7.58 (m, 2H; Ar-H), 9.71 (d, 3J(H,H)Z8.0 Hz, 1H; CH); IR (neat): nZ1679 cmK1 (C]O); MS (70 eV): m/z (%): 132 (73) [MC], 131 (100) [MCKH], 103 (56) [C8HC]. 7 4.3.6. Ethyl 4-formylbenzoate. Mp: 156–158 8C; 1H NMR (CDCl3, ppm): dZ1.43 (t, 3J(H,H)Z6.4 Hz, 3H; CH3), 4.42 (q, 3J(H,H)Z6.4 Hz, 2H; CH2), 7.96 (d, 3J(H,H)Z8.4 Hz, 2H; Ar-H), 8.21 (d, 3J(H,H)Z8.4 Hz, 2H; Ar-H), 10.11 (s, 1H; CHO); IR (KBr): nZ3058, 2981, 2924, 1711, 1274, 1102, 733 cmK1; MS (70 eV): m/z (%): 178 (3.3) [MC], 149 (100) [C9H9OC]. 2 W. Qian et al. / Tetrahedron 62 (2006) 556–562 4.3.7. 3-Phenylpropanal. Oil; 1H NMR (CDCl3, ppm): dZ 2.77 (t, 3J(H,H)Z7.2 Hz, 2H; CH2), 2.95 (t, 3J(H,H)Z 7.2 Hz, 2H; CH2), 7.18–7.22 (m, 3H; Ar-H), 7.27–7.31 (m, 2H; Ar-H), 9.80 (s, 1H; CHO); IR (film): nZ3029, 2928, 2826, 1726, 1497, 1454, 747, 701 cmK1; MS (70 eV): m/z (%): 134 (58.0) [MC], 92 (75.3) [C7HC], 91 (100.0) 8 [C7HC]. 7 4.3.8. Hexanal. Oil; 1H NMR (CDCl3, ppm): dZ0.93 (t, 3 J(H,H)Z7.2 Hz, 3H; CH3), 1.27–1.36 (m, 4H; CH2), 1.51–1.58 (m, 2H; CH2), 2.42 (t, 3J(H,H)Z7.2 Hz, 2H; CH2), 9.77 (s, 1H; CHO); IR (film): nZ2960, 2932, 1718, 1460 cmK1; MS (70 eV): m/z (%): 56 (82.1) [C4HC], 44 8 (100.0) [C2H4OC]. 4.3.9. Hexan-2-one. Oil; 1H NMR (CDCl3, ppm): dZ0.91 (t, 3J(H,H)Z7.2 Hz, 3H; CH3), 1.27–1.36 (m, 2H; CH2), 1.52–1.60 (m, 2H; CH2), 2.14 (s, 3H; CH3), 2.43 (t, 3 J(H,H)Z7.2 Hz, 2H; CH2); IR (film): nZ2962, 2936, 1718, 1360, 1169 cmK1; MS (70 eV): m/z (%): 100 (7.8) [MC], 58 (49.1) [C4HC], 43 (100.0) [C2H3OC]. 10 4.3.10. 4-Hydroxycyclohexanone. Oil; 1H NMR (CDCl3, ppm): dZ1.95–2.01 (m, 2H), 2.03–2.08 (m, 2H), 2.28–2.37 (m, 2H), 2.58–2.65 (m, 2H), 3.80 (s, 1H), 4.19–4.23 (m, 1H); IR (KBr): nZ3386 (OH), 1707 cmK1 (C]O); MS (70 eV): m/z (%): 114 (90.0) [MC], 57 (78.4) [C3H5OC], 55 (100.0) [C3H3OC]. 4.3.11. 4-Methylcyclohexanone. 1H NMR (CDCl3, ppm): dZ1.02 (d, 3J(H,H)Z6.8 Hz, 3H; CH3), 1.37–1.48 (m, 2H), 1.86–1.94 (m, 1H; CH), 1.98–2.02 (m, 2H), 2.34–2.38 (m, 4H); IR (neat): nZ1714 cmK1 (C]O); MS (70 eV): m/z (%): 112 (43) [MC], 55 (100) [C3H3OC]. 4.3.12. Acetophenone. 1H NMR (CDCl3, ppm): dZ2.59 (s, 3H; CH3), 7.43–7.47 (m, 2H; Ar-H), 7.54–7.57 (m, 1H; ArH), 7.95 (d, 3J(H,H)Z8.8 Hz, 2H; Ar-H); IR (neat): nZ 1686 cmK1 (C]O); MS m/z 120 (MC). MS (70 eV): m/z (%): 120 (25) [MC], 105 (100) [C7H5OC], 77 (73) [C6HC]. 5 4.3.13. 4-Methoxyacetophenone. Mp: 37–398C; 1H NMR (CDCl3, ppm): dZ2.56 (s, 3H; CH3), 3.87 (s, 3H; CH3), 6.94 (d, 3J(H,H)Z8.4 Hz, 2H; Ar-H), 7.94 (d, 3J(H,H)Z 8.4 Hz, 2H; Ar-H); IR (KBr): nZ1677 cmK1 (C]O); MS m/z 150 (MC). MS (70 eV): m/z (%): 150 (29) [MC], 135 (100) [C8H7OC]. 2 4.3.14. 4-Bromoacetophone. Mp: 50–518C; 1H NMR (CDCl3, ppm): dZ2.59 (s, 3H), 7.61 (d, 3J(H,H)Z8.4 Hz, 2H; Ar-H), 7.82 (d, 3J(H,H)Z8.4 Hz, 2H; Ar-H); IR (KBr): nZ1676 cmK1 (C]O); MS m/z 198 (MC), 200 (MC2). MS (70 eV): m/z (%): 198 (29) [MC], 200 (27) [MCC2], 183 (99) [C7H4OBrC], 185 (100) [C7H4OBrC]. 4.3.15. 2-Hydroxy-1,2-diphenylethanone. Mp: 132– 134 8C; 1H NMR (CDCl3, ppm): dZ4.56 (d, 3J(H,H)Z 6.0 Hz, 1H; OH), 5.96 (d, 3J(H,H)Z6.0 Hz, 1H; CH), 7.27–7.34 (m, 5H; Ar-H), 7.38–7.42 (m, 2H; Ar-H), 7.50–7.54 (m, 1H; Ar-H), 7.91–7.93 (m, 2H; Ar-H); IR (KBr): nZ3417 (OH), 1680 cmK1 (C]O); MS (70 eV): m/z (%): 212 (3) [MC], 105 (100) [C7H5OC]. 561 Acknowledgements This work was financially supported by the Natural Science Foundation of China (No. 20225309). References and notes 1. (a) Choudhary, V. R.; Chaudhari, P. A.; Narkhede, V. S. Catal. Commun. 2003, 4, 171–175. (b) Smith, M. B.; March, J. March’s Advanced Organic Chemistry: Reaction, Mechanism, and Structure, 5th ed.; Wiley-Interscience: New York, 2001. (c) Larock, R. C. Comprehensive Organic Transformations, 2nd ed.; Wiley-VCH: New York, 1999. (d) Muzart, J. Chem. Rev. 1992, 92, 113–140. 2. Adam, W.; Saha-Moller, C. R.; Ganeshpure, P. A. Chem. Rev. 2001, 101, 3499. 3. De Nooy, A. E. J.; Besemer, A. C.; Van Bekkum, H. Synthesis 1996, 1153–1175. 4. 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