One-pot synthesis of tryptophols with mesoporous MCM-41 silica catalyst functionalized with sulfonic acid groups

Microporous and Mesoporous Materials 143 (2011) 73–77

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One-pot synthesis of tryptophols with mesoporous MCM-41 silica catalyst functionalized with sulfonic acid groups Xiaoyan Sheng, Jianrong Gao, Liang Han ⇑, Yixia Jia, Weijian Sheng State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, Zhejiang University of Technology, Hang Zhou 310014, PR China

a r t i c l e

i n f o

Article history: Received 25 September 2010 Received in revised form 19 January 2011 Accepted 7 February 2011 Available online 4 March 2011 Keywords: MCM-41 Sulfonic group Functionalization Tryptophols Fischer indole synthesis

a b s t r a c t Mesoporous MCM-41 silica functionalized with sulfonic acid groups (SO3H-MCM-41) was prepared through the condensation of MCM-41 with benzyl alcohol followed with sulfonation, which was characterized by XRD, FT-IR and nitrogen adsorption analyses. One-pot Fischer indole synthesis of tryptophols under the catalysis of SO3H-MCM-41 was reported with phenylhydrazine hydrochlorides and 2,3-dihydrofuran. The acidity and amount of sulfur loading were surveyed and also were the catalytic performance of SO3H-MCM-41. The results showed that higher amount of sulfur loading leads to higher acidity and consequently better yields of tryptophols. Moreover, SO3H-MCM-41 displayed good shapeselectivity in Fischer indole synthesis and inhibited the production of bulky byproduct polyindoles which were generally found in traditional acid catalysis. A series of tryptophols were synthesized with SO3HMCM-41 in better yields than those of conventional methods reported. Ó 2011 Elsevier Inc. All rights reserved.

1. Introduction Tryptophols are a kind of indoles bearing a C-3 hydroxyethyl side chain, which are found in a variety of natural sources [1,2]. It was implicated that the production of tryptophol through tryptophan metabolism may play a key role in one of the pathophysiological mechanisms that provoke sleeping sickness upon infection by trypanosomes [3]. Therefore many tryptophols are of pharmaceutical importance. For example, 7-ethyltryptophol (7-Et) is a vital intermediate for synthesizing the Etodolac, a non-steroidal antiinflammatory drugs (NSAID) [4]; while Pemedolac, one of the most potent analgesics known and exhibits anti-inflammatory activity similar to that of Etodolac, is prepared from 2-phenyl-2-(3-indolyl)-1-ethanol [5]. In addition, the esters of 5-methoxytryptophol posses anticholinergic activities [6]. A lot of methods were used to synthesize tryptophols, such as Fischer indole methodology [7]; the reduction of indole acetic acid derivatives [8], or of 3-substituted-dioxindoles using a borane tetrahydrofuran complex [9]; the ring opening of epoxides by indole compounds under the catalysis of Bi(OTf)3 [10], or by the reaction with epoxides under pressure in the presence of Yb(OTf)3 [11]; the domino reaction of aryl hydrazines and silyl-protected x-(hydroxyoalkyl)alkynes [12]. However, Fischer indole synthesis of tryptophols remains to be one of the most versatile and widely employed methodologies, which was proceeded through the reaction of aldehydes or ketones with aryl hydrazines to obtain the corresponding ⇑ Corresponding author. Tel.: +86 0571 88320891; fax: +86 0571 88320544. E-mail address: [email protected] (L. Han). 1387-1811/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2011.02.008

hydrazones followed with [3,3]-sigmatropic rearrangement. Generally, strong mineral acids, such as H2SO4, HCl or Lewis acids like ZnCl2 and FeCl3 are used for Fischer indole synthesis. However, this typical homogeneous catalysis system suffer from reactor corrosion, waste neutralization, difficult separations and the inability for reuse, which is associated with a number of environmental problems. Another major drawback of Fischer indole synthesis of tryptophols is that tryptophols tend to form the bulky byproduct polyindoles under acid catalysis, which leads to low yields of tryptophols (Scheme 1). Therefore, it is imperative to develop a better catalytic process to avoid major environmental hazards and improve the yields of tryptophols. MCM-41 is a well-known ordered mesoporous (alumino) silicates and its micelle-templated silicas contain uniform channels with tunable diameters in the range of 1.5–10 nm and a greater uniformity of acid sites than other amorphous materials [13]. The application of MCM-41 in shape-selective catalysis has attracted great interest owing to its ability to discriminate between molecules solely on the basis of their size [14,15]. Though the acidity of MCM-41 is not as strong as that of liquid mineral acid and conventional Lewis acid, grafting with sulfonic acid groups seems to be a good solution to this problem. Recently MCM-41 functionalized with sulfonic acid groups (SO3H-MCM-41) has been designed to catalyze many organic reactions and after grafted it not only obtains improved acidity but also at the same time retains high surface areas and tunable pore diameters. For example, SO3H-MCM41 display preferable activity and selectivity in esterification or acylation reactions [16,17], etherification reactions [18], Friedel– Crafts alkylation [19], and isomerization [20].

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HO

OH

2.2. Preparation of sulfuric acid (50%)-mounted silica (H2SO4-silica) catalyst

HO

H+ N H

N N H H polyindoles

tryptophol

Silica gel (10 g, 200–400 mesh) was suspended in dry diethyl ether (50 mL) and H2SO4 (3 mL) was added. The mixture was stirred for 30 min at room temperature. The solvent was evaporated and the white solid H2SO4-silica was obtained after dried at 110 °C for 3 h.

n

Scheme 1. Formation of polyindole with tryptophol under acid catalysis.

Pore size of SO3H-MCM-41 is in the range between the small molecule tryptophol and the bulky polyindole. The formation of bulky polyindoles can be inhibited by SO3H-MCM-41 catalyst and shape-selectivity in Fischer indole synthesis may create better yields of tryptophols. Therefore, SO3H-MCM-41 is expected to exhibit both enough acidity and good shape selectivity to tryptophol synthesis. Covalent anchoring of the sulfonic acid groups to the MCM-41 surface could be achieved either by a direct synthesis route or via an indirect post synthetic anchoring technique followed by an oxidation step to generate the sulfonic acid groups [21,22]. However, these immobilization methods of sulfonic acid groups need the expensive reagent mercaptoalkyl trialkoxysilane, which inhibit the practical application of SO3H-MCM-41. Recently, Chen et al. reported another method for preparing SO3H-MCM-41 by covalently anchoring benzyl group onto the hydroxyl groups on the silica surface followed with sulfonation in a post-synthesis step [23]. This method was characterized by simple process, cheap reagent and satisfied acidity. Base on this report, we prepared SO3H-MCM-41 and reported its catalysis role in one-pot synthesis of tryptophols from phenylhydrazine hydrochlorides and 2,3-dihydrofuran (Scheme 2).

2. Experimental

2.3. Preparation of MCM-41 functionalized with sulfonic acid groups [23] MCM-41 was synthesized by hydrothermal process [24]. MCM41 (1.0 g) was added into the solution of benzyl alcohol (5 mL) in toluene (20 mL) and the mixture was stirred at reflux for 12 h. After filtrated and dried at 60 °C for 12 h, the white solid was soaked in the solution of ClSO3H in dry CHCl3 (20 mL) and the reactant was refluxing for 2 h. After filtrated and washed with dry CHCl3, the solid was dried in vacuum at 60 °C for 5 h to obtain MCM-41 functionalized with sulfonic acid groups, which was designed as SO3H-MCM-41-x, where x indicates the ratio of the volume of ClSO3H to the MCM-41 mass (mL
g 1). 2.4. Synthesis of tryptophols 2-Ethylphenyl hydrazine hydrochloride (4.5 g, 0.026 mol) was dissolved in the co-solvent of EtOH (50 mL) and H2O (130 mL). Dihydrofuran (1.8 g, 0.026 mol) was added drop by drop and the reactant was stirred for 80 min at 68 °C. Then the catalyst (0.225 g) was added and the solution was stirred at 90 °C for 4 h under the protection of nitrogen. The solvent was evaporated and the residue was extracted with toluene. After the toluene was removed, the residue was purified by column chromatography with PE/EtOAc (2:1, v/v) to obtain the product. A series of tryptophols were synthesized by the same method and the structures of these compounds were confirmed by 1H NMR and MS.

2.1. Chemicals and instruments 3. Results and discussion Small-angle X-ray diffraction (XRD) patterns were recorded with a PANalytical X’Pert PRO diffractometer using Cu radiation (60 kV and 55 mA). The FT-IR spectra were recorded within 400– 4000 cm 1 region by a Nicolet 6700 FT-IR spectrophotometer using the KBr pellet technique. The nitrogen adsorption–desorption isotherms were measured at 77 K on a Quantachrome NovaWin2 analyzer. All samples were outgassed for 12 h at 200 °C under vacuum (10 3 Torr) in the degassing port of the adsorption analyzer. The specific surface areas of the samples were calculated using BET model and pore size distributions were calculated using BJH method form adsorption branches. The catalyst’s acid amount was surveyed by titration method: the sample (0.05 g) was added into salt water (15 mL, 2 mol/L) and its acid amount was measured by NaOH (0.01 mol/L) titration.

OH NHNH2HCl R

+

SO3H-MCM-41 O

R

1

2 2a R=7-CH2CH3 2b R=H 2c R=5-CH3

N H

2d R=5-CH3O 2e R=5-Cl

Scheme 2. Synthesis of tryptophols catalyzed by SO3H-MCM-41.

3.1. Preparation and characterization of SO3H-MCM-41 Benzyl-incorporated sample was prepared by etherifying the hydroxyl group on freshly calcined MCM-41 samples with benzyl alcohol in toluene. Then the benzene ring in benzyl-incorporated sample was sulfonated by ClSO3H to obtain SO3H-MCM-41 (Scheme 3). The amount of sulfonic acid groups immobilized on MCM-41 varied with the ratio of ClSO3H to MCM-41, which leads to the different acidity of SO3H-MCM-41 (Table 1). The high ratio of ClSO3H to MCM-41 which means more sulfonic acid groups immobilized on MCM-41 leads to high acid amount of SO3HMCM-41. SO3H-MCM-41-0.6 exhibits the highest acid amount 8.2 mmol/g, however, further increase of ClSO3H amount does not improve the acid amount any more. Moreover, to further examine the presence of sulfonic acid groups, the amount of sulfur loading was determined by elemental analysis. Typically, a 0.2– 4.1 mmol
g 1 sulfur loading can be obtained by adjustment of the amounts of ClSO3H in the sulfonation step. For all samples the amount of sulfur loading was found to be correlated with the acid amount. The higher the amount of sulfur loading, the higher the acid amount. Pore structure parameters of SO3H-MCM-41 and ungrafted MCM-41 derived from the nitrogen adsorption–desorption isotherms at 77 K are listed in Table 2. It can be seen that the specific surface area and pore volume of SO3H-MCM-41 are less than MCM-41 and decreased continuously with higher reacting amount

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OH OH OH

O CH2

benzyl alcohol toluene reflux,12 h

O CH2

ClSO3H

OH

dry CHCl3 reflux, 2h

O CH2

SO3H

OH O CH2

MCM-41

SO3H

SO3H-MCM-41-x Scheme 3. Preparation of SO3H-MCM-41.

Table 1 Acidity and amount of sulfur loading of SO3H-MCM-41 with different amount of ClSO3H.

MCM-41 SO3H-MCM-41-0.2 SO3H-MCM-41-0.4 SO3H-MCM-41-0.5 SO3H-MCM-41-0.6 SO3H-MCM-41-0.8

Reacting amount of chlorosulfonic acid (mL
g 1)

Acid amount (mmol
g 1)

Surful content (mmol
g 1)

0 0.2 0.4 0.5 0.6 0.8

0.8 1.2 5.7 6.8 8.2 8.2

0 0.2 2.5 3.8 4.1 –

9000

100

8000

relative intensity (a.u.)

Sample

10000

7000 6000 5000 4000

110 200

3000

MCM-41

2000

SO3H-MCM-41-0.6

1000

Table 2 Pore structure parameters of MCM-41 and SO3H-MCM-41 derived from the N2 adsorption–desorption isotherms. Sample

VBJH (cm3/g)a

DBJH (Å)b

SBET (m2/g)c

MCM-41 SO3H-MCM-41-0.2 SO3H-MCM-41-0.4 SO3H-MCM-41-0.5 SO3H-MCM-41-0.6

0.84 0.600 0.661 0.267 0.273

36 16 18 14 14

1080 747.2 727.2 372.7 393

0 2

4

6

8

10

2θ (degree) Fig. 1. XRD patterns of MCM-41 and SO3H-MCM-41-0.6.

100 b c

BJH pore volume. BJH average pore diameter. BET specific surface area.

of ClSO3H. It was due to the anchoring of the organics and the different loading amount of sulfonic acid groups [25,26]. On the other hand it was noted that average pore diameter centered at ca. 16 ± 2 Å and were shrinking sharply when compared with MCM41. This observed decrease in the pore size, as compared with ungrafted MCM-41, may be attributed to the close packing of the benzylsulfonic acid chains protruding from the walls into the channels [27]. Moreover, when the ratio of ClSO3H to MCM-41 was increased to more than 0.4 ml/g, the specific surface area and pore volume of SO3H-MCM-41-0.5 and SO3H-MCM-41-0.6 are far less than SO3H-MCM-41-0.2 and SO3H-MCM-41-0.4. It seemed that except for more amounts of sulfonic acid groups immobilized, the partial collapse of SO3H-MCM-41 was happened under the strong acid condition caused by the presence of large amount of ClSO3H. However, the ordered structure of SO3H-MCM-41 remained intact, which was supported by XRD results. The power X-ray diffraction analyses performed on MCM-41 and SO3H-MCM-41-0.6 were compared in Fig. 1. These patterns feature distinct Bragg peaks in the 2h range of 0.8–2.5°, which can be indexed as (1 0 0), (1 1 0) and (2 0 0) reflections of a twodimensional hexagonal structure of MCM-41 material [28]. The presence of three diffraction peaks indicates that the crystallographic ordering of the mesopores in SO3H-MCM-41-0.6 is retained after grafted. Also a noticeable change of SO3H-MCM-410.6 in the intensity ratios of these reflections (1 0 0, 1 1 0, 2 0 0) was observed and they are lower than that of MCM-41, implying that the introduction of organic groups would reduce the order degree of pore structure.

80

Transmittance (%)

a

60

40

MCM-41 SO3H-MCM-41-0.6

20

0

4000

3500

3000

2500

2000

1500

1000

-1

Wavelength (cm ) Fig. 2. FT-IR spectra of MCM-41 and SO3H-MCM-41-0.6.

Moreover, the formation of MCM-41 structure is evidenced by the IR bands located at 3425 cm 1 (mas O–H), 1089 cm 1 (mas Si–O), 807 cm 1 (ms Si–O), 463 cm 1 (d Si–O–Si) (m represents stretching, d bending, s symmetric, and as asymmetric vibrations, Fig 2). SO3H-MCM-41-0.6 exhibits additional diffraction peaks corresponding to the benzene ring and sulfonic acid group. The peaks at 2936 cm 1 (m C–H of benzyl), 1504 cm 1 (m benzene), 878 cm 1 (d benzene) and 855 cm 1 (d benzene) indicate the existence of benzene ring; while the characteristic peaks of sulfonic acid group are occurred at 1169 cm 1 (m SO2) and 574 cm 1 (m S–O). 3.2. Synthesis of tryptophols Synthesis of 7-Et was selected as a model experiment to survey the reaction condition (Table 3). Firstly, conventional Lewis acid

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X. Sheng et al. / Microporous and Mesoporous Materials 143 (2011) 73–77

Table 3 Synthesis of 7-Eta.

OH NHNH 3Cl + CH2 CH3

SO3 H-MCM-41 O

N Et

H 2a

1a Entry

Catalyst

Reactant ratiob

Catalyst amount (g/g 2-Ethylphenyl hydrazine hydrochloride)

Temperaturec (°C)

Yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

H2SO4 ZnCl2 FeCl3 H2SO4–SiO2 MCM-41 SO3H-MCM-41-0.2 SO3H-MCM-41-0.4 SO3H-MCM-41-0.5 SO3H-MCM-41-0.6 SO3H-MCM-41-0.6 SO3H-MCM-41-0.6 SO3H-MCM-41-0.6 SO3H-MCM-41-0.6 SO3H-MCM-41-0.6 SO3H-MCM-41-0.6 SO3H-MCM-41-0.6

1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:12 1:1.5 1:2

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.05 0.15 0.05 0.05 0.05

90 90 90 90 90 90 90 90 90 40 60 90 90 90 90 90

31 30 31 17 5 15 20 30 50 20 20 50 48 49 50 50

a Reaction condition: 2-Ethylphenyl hydrazine hydrochloride (0.02 mol) with 2,3-dihydrofuran was stirred in the solvent of EtOH (50 mL) and H2O (130 mL) for 80 min at 68 °C. After the catalyst was added, the mixture was heating for 4 h under the nitrogen protection. b Reactant ratio: 2-Ethylphenyl hydrazine hydrochloride: 2,3-dihydrofuran. c Reaction temperature after the addition of catalyst.

Table 4 SO3H-MCM-41-0.6 catalyzed the synthesis of tryptophols.

NHNH 3 Cl + R

SO 3H-MCM-41

OH

R

O

N H 2

1 Entry

Phenylhydrazine hydrochloride 1

1

Tryptophols 2

NHNH3Cl CH2CH3 1a

2

N H

N H

3

NHNH3Cl H3C

H3CO

5

Cl 1e

43

OH

47

23

53

32

59

33.5

44

42.3

OH

2c

OH

H3CO N H

1d NHNH3Cl

50

H3C N H

NHNH3Cl

OH

2b

1c

4

Reference [7] Yield (%)

2a

NHNH3Cl

1b

Yield (%)

2d OH

Cl N H

2e

X. Sheng et al. / Microporous and Mesoporous Materials 143 (2011) 73–77

and H2SO4 were used to catalyze 7-Et synthesis and compared with solid acids. It can be seen that homogeneous catalysis of 7-Et synthesis was superior to heterogeneous catalysis and the yields of ZnCl2, FeCl3 and H2SO4 catalytic systems were higher than those of solid acid catalysts H2SO4-SiO2 and MCM-41 (Table 3, Entry 1– 5). However, grafted MCM-41 gave better yields than ungrafted and the reaction yield was improved with the increase of acid amount of SO3H-MCM-41. SO3H-MCM-41-0.6 gave higher yield 50% than H2SO4 (Table 3, Entry 6–9). The HPLC analysis showed that H2SO4 catalyzed reaction produced a lot of polyindoles which can be hardly found in SO3H-MCM-41 catalyzed reaction. It may be attributed to the shape-selectivity of mesoporous zeolite that prevents the production of polyindoles. When the reaction temperatures are lower than 90 °C, SO3H-MCM-41-0.6 can also catalyze Fischer indole synthesis but give comparatively lower yield (Table 3, Entry 10–11). It affords a comparable yield when the catalyst amount was reduced to 0.05 (g/g 2-ethylphenyl hydrazine hydrochloride); while it was unfavorable when the catalyst amount was further increased to 0.15 (g/g 2-ethylphenyl hydrazine hydrochloride) (Table 3, Entry 12–13). In addition, increasing the mole ratio of 2,3-dihydrofuran was unhelpful to the yield improvement (Table 3, Entry 14–16). Therefore, optimized conditions involved reaction of 2-ethylphenyl hydrazine hydrochloride 1a with 2,3dihydrofuran (1 equiv) under the catalysis of SO3H-MCM-41-0.6 (0.05 g/g 2-ethylphenyl hydrazine hydrochloride) at 90 °C, which gave 7-ethyltryptophol 2a in 50% yield (Table 3, Entry 9). To investigate the scope of one-pot Fischer indole synthesis with SO3H-MCM-41-0.6 catalyst, different tryptophols with varying substitutents were prepared (Table 4). The results indicate that SO3H-MCM-41-0.6 can successfully catalyze Fischer indole synthesis of tryptophols from phenylhydrazine hydrochlorides bearing electron-donating group (Table 4, Entry 3–4) or electron-withdrawing group (Table 4, Entry 5) with 2,3-dihydrofuran. These yields of tryptophols synthesized with SO3H-MCM-41-0.6 were better than that of the reference [7,29,30] reported.

4. Conclusion SO3H-MCM-41 was prepared through the condensation of MCM-41 with benzyl alcohol followed with sulfonation by ClSO3H. The more the reacting amount of ClSO3H the higher the acid amount of SO3H-MCM-41. SO3H-MCM-41-0.6, which has the highest acid amount 8.2 mmol/g, exhibits satisfied shape-selectivity and better catalytic performance to one-pot Fischer indole synthesis of tryptophols than H2SO4 and H2SO4-SiO2. A series of tryptophols were synthesized successfully under the catalysis of SO3HMCM-41-0.6 in better yields than those of conventional methods reported. This novel catalytic process has distinct advantages over literature precedent due to comparable yields, easier post-reaction work-up and reduced waste water.

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