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Supported trifluoromethanesulfonic acid as catalyst in the synthesis of flavone and chromone derivatives
Applied Catalysis A: General 324 (2007) 62–68 www.elsevier.com/locate/apcata
Supported trifluoromethanesulfonic acid as catalyst in the synthesis of flavone and chromone derivatives D.O. Bennardi a,b, G.P. Romanelli a,b,*, J.C. Autino b, L.R. Pizzio a,* a
Centro de Investigacio´n y Desarrollo en Ciencias Aplicadas ‘‘Dr. J.J. Ronco’’ CINDECA, Departamento de Quı´mica, Facultad de Ciencias Exactas, UNLP-CONICET, Calle 47 N8 257, B1900AJK La Plata, Argentina b Ca´tedra de Quı´mica Orga´nica, Facultad de Ciencias Agrarias y Forestales, UNLP, Calles 60 y 119, B1904AAN La Plata, Argentina Received 13 November 2006; received in revised form 23 February 2007; accepted 11 March 2007 Available online 15 March 2007
Abstract Solid acid catalysts based on trifluoromethanesulfonic acid (triflic acid, TFMS) were synthesized using titania with different textural properties as support, obtained by increasing the calcination temperature (Ti100, Ti200, Ti300, and Ti400). They were characterized by FT-IR, DTA-TGA, and BET. The acidic characteristics of the catalysts were determined by potentiometric titration with n-butylamine. According to this technique, the catalysts present very strong acid sites. Their acid strength decreases slightly in the order Ti100 > Ti200 > Ti300 > Ti400, as a result of the partial dehydroxylation that takes place during the thermal treatment of the supports. The amount of trifluoromethanesulfonic acid firmly adsorbed on the support depends on the number of –OH groups available on the titania to be protonated, which decreases with the increase in the calcination temperature. The new catalysts were tested in the cyclization of 1-(2-hydroxyphenyl)-3-phenyl-1,3-propanedione. The reaction experiments were performed using toluene as the solvent at reflux. In all cases, the product (flavone) was obtained with high selectivity. The activity of the catalysts is dependent on the textural properties of the support used to obtain the catalysts and the amount of triflic acid firmly adsorbed on it. The same reaction conditions were applied to the preparation of eight substituted flavones and chromones. Conversions up to 88% were obtained using the Ti100 supported TFMS catalyst. # 2007 Elsevier B.V. All rights reserved. Keywords: Supported trifluoromethanesulfonic acid; Supported triflic acid; Flavones; Chromones; Titania
1. Introduction Compounds containing the chromone skeleton (4H-benzopyran-4-one) (flavones and chromones) are an important class of compounds belonging to the flavonoid group that occur naturally in plants. They are minor constituents of the human diet and have been reported to exhibit a wide range of biological effects [1,2]. These biological properties include anti-inflammatory, antibacterial, antitumor [1], antioxidant [3], anti-HIV [4], vasodilator, antiviral and antiallergenic [5]. Some flavonoids inhibit the histamine release from human basophils and rat mast cells [6]. Moreover, it is known that some
* Corresponding authors. E-mail addresses: [email protected] (G.P. Romanelli), [email protected] (L.R. Pizzio). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.03.009
flavonoids have antifeedant activity against some phytophagous insects and a subterranean termite (Coptotermes sp.) [7,8]. The importance of these compounds has led to the development of various methods for their synthesis: for example, the Allan-Robinson strategy, or from chalcones, and via an intramolecular Wittig strategy [9,10]. One of the most common methods involves acylation of an o-hydroxyacetophenone with an aromatic acid chloride yielding an aryl ester. The ester is then rearranged by a base (the Baker– Venkataraman reaction) to a 1,3-diaryl 1,3-diketone [11]. The latter compound gives a 2-arylchromone on catalyst cyclocondensation. On the other hand, the transformation used for the preparation of such fine chemicals is carried out in very environmentally unfriendly conditions, using homogeneous catalysts such as inorganic acids, Lewis acids, or organic bases that generate large amounts of corrosive and toxic waste from which the catalysts are very difficult to recover. In order to
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overcome these problems, the use of heterogeneous catalysts is a possibility. Different solid acid catalysts such as clays, oxides, zeolites, Al-MCM-41, and supported reagents have been used in this transformation [12]. Trifluoromethanesulfonic acid (triflic acid, CF3SO3H) is known to be a strong acid and it is used in many organic reactions such as Friedel Crafts reactions, polymerization, Koch carbonylation, among others [13]. However, the recovery of the triflic acid from the reaction mixture results in the formation of large amounts of waste [13]. The design of acidcatalyzed reactions over solids has caused a need for further research in this area, and supported triflic acid or materials with –CF2SO3H groups are now becoming available to replace homogeneous acid solutions [14]. Different catalytic systems have been studied in which TFMS is adsorbed on various solids such as acylation of 2-methoxynapthalene [12] and acylation of alcohols over silica embedded-triflate catalysts [15], alkylation of hydroquinone with tert-butanol and alkylation of phenol and naphthols over silica-immobilized triflate derivatives [16,17], benzoylation of toluene with toluoyl chloride over triflic acidfunctionalized mesoporous Zr-TMS catalyst [18], alkylation of isobutane with TFMS absorbed on SiO2 [19], acetalization of ethyl acetoacetate and benzoylation of biphenyl using triflic acid-functionalized mesoporous Zr-TMS catalysts [13], and nitration and esterification of aromatic compounds over CF3SO3H/SiO2 [14]. Mesoporous titania synthesized via sol–gel reactions using urea as a low-cost pore-forming agent [20] presents suitable properties to be used as catalyst support. To the best of our knowledge, there have been no reports in the literature about the use of mesoporous titania as support of trifluoromethanesulfonic acid. In the present work, the synthesis and characterization of trifluoromethanesulfonic acid supported on mesoporous titania prepared using urea as a low-cost, pore-forming agent, via HCl catalyzed sol–gel reactions, is presented. The acidic characteristics of the solids were determined by potentiometric titration with n-butylamine. The catalytic activity of the materials was tested in the synthesis of flavone and chromone derivatives. 2. Experimental 2.1. Synthesis of mesoporous titania Titaniumisopropoxide (Aldrich, 26.7 g) was mixed with absolute ethanol (Merck, 186.6 g) and stirred for 10 min to obtain a homogeneous solution under N2 at room temperature, then 0.33 cm3 of 0.28 M HCl aqueous solution was dropped slowly into the above mixture to catalyze the sol–gel reaction for 3 h. After that, an appropriate amount of urea–alcohol– water (1:5:1 weight ratio) solution was added to the hydrolyzed solution under vigorous stirring to act as template. The amount of added solution was fixed in order to obtain a template concentration of 10% by weight in the final material. The gel was kept in a beaker at room temperature till dryness. The solid was ground into powder and extracted with distilled water for three periods of 24 h, in a system with continuous stirring to
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remove urea. Finally, it was calcined at 100, 200, 300, and 400 8C for 24 h (Ti100, Ti200, Ti300, and Ti400, respectively). 2.2. Synthesis of supported trifluoromethanesulfonic acid catalysts Trifluoromethanesulfonic acid, CF3SO3H (0.01 mol, Alfa Aesar, 99%) was added dropwise to the mixture of mesoporous titania (2 g) and toluene (20 cm3, Merck) at 90 8C under nitrogen atmosphere; then it was further refluxed for 2 h. Next, the sample was cooled, filtered, washed with acetone (Mallinckrodt AR) and dried at 100 8C for 24 h. The solids were extracted with a mixture of dichloromethane and diethyl ether (50%, v/v) (100 cm3 of mixture per gram of catalyst) for three periods of 8 h using a Soxhlet apparatus in order to remove the acid weakly attached to the support. Afterwards, they were dried again at 100 8C for 24 h. The samples were named TriTi100, TriTi200, TriTi300, and TriTi400. The amount of trifluoromethanesulfonic acid retained was determined by elemental analysis of C and S carried out with an EA1108 Elemental Analyzer (Carlo Erba Instruments). 2.3. Support and catalyst characterization The specific surface area of the solids was determined from N2 adsorption–desorption isotherms at liquid-nitrogen temperature. They were obtained using Micromeritics ASAP 2020 equipment. The samples were previously degassed at 100 8C for 2 h. FT-IR spectra of supports and catalysts were obtained in the 400–4000 cm 1 wavenumber range using a Bruker IFS 66 FTIR spectrometer. Powder XRD patterns were recorded on the same samples that had been analyzed by FT-IR. The equipment used to this end was a Philips PW-1732 with built-in recorder, using Cu Ka radiation, nickel filter, 20 mA and 40 kV in the high voltage source, and scanning angle between 5 and 608 of 2u at a scanning rate of 18 per min. The TGA-DTA measurements of the solids were carried out using a Shimadzu DT 50 thermal analyzer. The thermogravimetric and differential thermal analyses were performed under argon or nitrogen respectively, using 25–50 mg samples and a heating rate of 10 8C/min. The studied temperature range was 20–700 8C. The acidity of the solid samples was measured by means of potentiometric titration. The solid (0.05 g) was suspended in acetonitrile (Merck), and stirred for 3 h. The suspension was then titrated with 0.05 N n-butylamine (Carlo Erba) in acetonitrile at 0.05 cm3/min. The electrode potential variation was measured with a Hanna 211 digital pH meter using a double junction electrode. 2.4. Reaction procedure All the starting b-diketones were prepared following a procedure described elsewhere [21]. Initially, we studied the cyclocondensation reaction of 1-(2-hydroxyphenyl)-3-phenylpropanodione to flavone. The reactions were carried out by
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adding the corresponding catalyst (20 mg) to a solution of 0.5 mmol of 1-(2-hydroxyphenyl)-1,3-propanodione in refluxing toluene (3 ml, Baker). The reaction was followed by TLC using TLC aluminum sheets (silica gel 60 F254 Merck). The suspension was stirred at 109 8C for different reaction times. When the reaction time was over, the catalyst was filtered and washed several times with toluene (1 ml). The extracts were combined and washed with 10% sodium hydroxide, then with H2O, and dried with anhydrous sodium sulfate. The organic solution was concentrated in vacuum to give the pure flavone, whose physical and spectral properties matched those reported in the literature. The conversion was expressed as the ratio of moles of product to moles of initial propanodione. In addition, the cyclodehydratation–condensation was followed as a function of time by means of UV-visible (UV– vis) spectroscopy. In order to appropriately obtain the samples, the mixture was centrifuged and diluted with toluene. The absorbance values of the samples at 420 nm and at different reaction times were determined. Their UV-vis spectra were obtained in the range 300–600 nm using a UV/VIS spectrometer LAMBDA 35, Perkin Elmer. The same reaction conditions were used to prepare different substituted flavones and chromones. Products were identified by 1H and 13C NMR with a Bruker instrument, operating at 400 MHz for 1H NMR and 100 MHz for 13C NMR, and by comparison with authentic samples prepared using sulfuric acid as catalysts. 3. Results and discussion 3.1. Support and catalyst characterization The specific surface area (SBET) of the titania support determined from N2 adsorption–desorption isotherms together with the average pore diameter (Dp) obtained from the BJH pore size distribution are shown in Table 1. As can be seen, all the samples are mesoporous with a Dp higher than 3.2 nm. The SBET decreased when the calcination temperature increased (Table 1). Calcination at 300 and 400 8C produced little pore narrowing. Some of the narrowest pores seem to collapse upon evolution of water forming two groups of pore size in the mesopore range. Table 1 Physicochemical properties of titania supports and supported trifluoromethanesulfonic acid catalysts Sample
SBET (m2/g)
DP (nm)
Nw (molecules/g)
NTri (mmol CF3SO3H/g)
Ti100 Ti200 Ti300 Ti400 TriTi100 TriTi200 TriTi300 TriTi400
287 221 159 124 265 209 151 118
3.9 4.0 3.2–5.8 3.9–5.9 3.9 3.9 3.2–5.7 3.9–5.8
1.52 10 21 1.11 10 21 0.51 10 21 0.38 10 21 – – – –
– – – – 0.84 0.53 0.32 0.24
The DTA diagram of Ti100 showed an endothermic peak at 52 8C, associated with the loss of physically adsorbed water, and two poorly developed endothermic peaks at 310 and 370 8C attributed to the partial dehydroxylation of the titania surface. They could be considered as an indication of the presence of two types of hydroxyl groups, usually present at the titania surface [22]. DTA diagrams of Ti200 and Ti300 samples presented features similar to those of Ti100. However, for the Ti400 sample, the endothermic peaks assigned to dehydroxylation were not detected. TGA patterns of the supports showed that dehydration takes place in two main steps. The first step is due to the loss of physically adsorbed water (below 160 8C) and the second to the loss of structural water in the temperature range 160–400 8C. For the Ti100 sample, the number of water molecules released per gram (Nw), estimated from the weight loss ascribed to the second step, is 1.52 1021. Nw decreased when the calcination temperature increased (Table 1), as a result of the dehydroxylation of the titania surface during the thermal treatment of the samples. The amount of acid firmly adsorbed to the support (NTri) calculated from the elemental analysis (Table 1) decreased when the calcination temperature increased. This can be explained if the interaction between the trifluoromethanesulfonic acid and titania is assumed to be of electrostatic type due to the transfer of protons to the –OH groups on the surface of the support. So, as a result of the dehydroxylation of the support that takes place during the thermal treatment, the amount of – OH groups available to be protonated decreases, and hence the amount of adsorbed acid falls. On the other hand, TriTi100, TriTi200, TriTi300, and TriTi400 catalysts presented slightly lower SBET values than those of the solids used as support. From these results, it can be seen that the decrease of SBET seems to be related to the amount of trifluoromethanesulfonic acid adsorbed on the mesoporous titania. The higher the amount of adsorbed acid, the higher the decrease of the support surface area. However, the pore size distribution was practically the same. The FT-IR spectra of titania after being extracted with distilled water did not present any of the characteristic bands of the urea, showing that the template was completely removed by water extraction. The spectrum of the Ti100 sample (Fig. 1a) displayed bands at 1618 and 3427 cm 1 assigned to the OH bending and stretching of water [23] that have been incorporated in the structure of the solid [24]. Also, in the energy interval below 800 cm 1, the band due to Ti–O stretching vibration was observed. FT-IR spectra of Ti200, Ti300, and Ti400 showed features similar to those observed for the Ti100 sample, but the intensity of the bands at 1618 and 3427 cm 1 decreased when the calcination temperature increased, in parallel with the reduction of the SBET. The FT-IR spectrum of the TriTi100 sample (Fig. 1b) displayed bands at 1267, 1183, and 1037 cm 1 in addition to those present in the Ti100 sample. The first two bands are due to the S O stretching mode of the adsorbed trifluoromethanesulfonic acid and the last one is assigned to the C–F stretching [25,26]. These bands are also present in the
D.O. Bennardi et al. / Applied Catalysis A: General 324 (2007) 62–68
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Fig. 1. FT-IR spectra of Ti100 (a), TriTi100 (b), TriTi200 (c), TriTi300 (d), and TriTi400 (e) samples.
spectra of TriTi200, TriTi300, and TriTi400 catalysts (Fig. 1c–e) although their intensity is lower as a result of the lower amount of trifluoromethanesulfonic acid adsorbed on the support. XRD patterns of TriTi100, TriTi200, TriTi300, and TriTi400 samples showed features similar to those of the corresponding support. The position and intensity of the characteristic peaks of anatase phase present in the titania used as support did not change after their impregnation with trifluoromethanesulfonic acid. The pattern of TriTi100 exhibited only the characteristic peaks of anatase phase at 2u = 25.38 (1 0 1), 37.98 (0 0 4), 47.88 (2 0 0), and 54.38. The XRD patterns of TriTi200, TriTi300, and TriTi400 catalysts exhibited the peak at 54.38 splited into two peaks at 54.08 (1 0 5) and 54.98 (2 1 1), as a result of an increment in support crystallinity when the calcination temperature is raised over 100 8C. The acidity measurements of the catalysts by means of potentiometric titration with n-butylamine let us estimate the number of acid sites and their acid strength. As a criterion to interpret the obtained results, it was suggested that the initial electrode potential (Ei) indicates the maximum acid strength of the sites, and the value of meq amine/g solid where the plateau is reached indicates the total number of acid sites [27]. Nevertheless, the end point of the titration given by the inflexion point of the curve is a good measure to carry out a comparison of the acidity of different samples. On the other hand, the acid strength of these sites may be classified according to the following scale [27]: Ei > 100 mV (very strong sites), 0 < Ei < 100 mV (strong sites), 100 < Ei < 0 (weak sites) and Ei < 100 mV (very weak sites). The titration curves of the catalysts are shown in Fig. 2a. The samples presented very strong acid sites, with Ei values in the range 550–700 mV. The number of acid sites determined by potentiometric titration decreased in parallel with the decrease of NTri, the amount of acid firmly adsorbed on the support. Additionally, the acid strength of the catalysts slightly decreased in the following order TriTi100 (673 mV) > TriTi200 (647 mV) > TriTi300 (606 mV) > TriTi400 (586 mV). This
Fig. 2. Potentiometric titration curves of TriTi catalysts (a) and the corresponding supports (b).
reflects the trend followed by Ei values of the bare support (Fig. 2b) that decreases in parallel with the increase of calcination temperature. The support acid strength decreases as a result of the partial dehydroxylation that takes place during the thermal treatment of the solids. 3.2. Catalytic activity The catalytic activity of TriTi100, TriTi200, TriTi300, and TriTi400 catalysts in the cyclization reaction of 1-(2-hydroxyphenyl)-3-phenyl-1,3-propanodione to flavone (Scheme 1) is displayed in Table 2. In all cases, toluene was used as solvent at reflux temperature. In such conditions, the highest conversion was attained using the TriTi100 catalyst (60%). The conversion decreases in the following order: TriTi100 > TriTi200 > TriTi300 > TriTi400, in parallel with the decrease of SBET and NTri. It is also in
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D.O. Bennardi et al. / Applied Catalysis A: General 324 (2007) 62–68 Table 2 Conversion (%) for cyclodehydration reaction of 1-(2-hidroxyphenyl)-3-phenylpropanodione to flavonea
Scheme 1. Synthesis of flavone.
agreement with the decrease of the total number of acid sites estimated by potentiometric titration. In all cases, flavone was the sole product formed. In order to evaluate the possible catalyst solubilization, an additional test was performed. The TriTi100 sample was refluxed in toluene for 2 h, filtered at high temperature and
Catalystb
Conversionc (%)
TriTi100 TriTi200 TriTi300 TriTi400
60 31 18 5
a
Toluene, reflux, 90 min. Catalyst: 20 mg. c Conversion was calculated from the yield (%) of the pure product. (No secondary products were detected.) b
Table 3 Synthesis of flavones and naphthylchromones using TriTi100 catalysta Entry
Diketone
Flavone/chromone
Yield (%)
1
86
2
86
3
88
4
87
5
87
6
82
7
82
a
Toluene, reflux, 360 min. Catalyst: 20 mg.
D.O. Bennardi et al. / Applied Catalysis A: General 324 (2007) 62–68
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or until no changes in the composition of the reaction mixture were observed for TLC. In all the cases, the desired products (flavones, 2-naphtylchromones) were obtained with high selectivity, and almost free of secondary products. The unchanged starting materials were recovered nearly quantitatively. Yields of flavones were similar to those of naphthylchromones (see Table 2). 4. Conclusions
Fig. 3. Conversion of 1-(2-hydroxyphenyl)-1,3-propanodione to flavone as a function of time for: fresh (&), washed (~) and refluxed (&) TriTi100 catalyst.
dried in vacuum till constant weight. The activity of the so treated catalyst was the same as that of the fresh catalyst (94% conversion in 360 min). The refluxed toluene was used as solvent for attempting the reaction without adding the catalyst. After 360 min flavone was not detected, the starting material being quantitatively recovered. The 1-(2-hydroxyphenyl)-3-phenyl-1,3-propanodione conversion obtained using TriTi100 as a function of time is shown in Fig. 3. Conversion was estimated following the consumption of the starting material as a function of time. The reaction time required for reaching the maximum conversion was 360 min. It was observed that for the same reaction time, the conversion obtained when Ti100, Ti200, Ti300, and Ti400 supports were used as catalyst was always lower than 5%. On the other hand, the spent TriTi100 catalyst was washed with toluene and dried at room temperature, and then reused. The change of the conversion with time for the washed catalyst is compared with that of the fresh catalyst in Fig. 3. As can be observed, the maximum conversion is reached at longer times when the reaction is performed with the reused catalyst. The activity lowering of could be due to the adsorption of reagent and/or product onto the catalyst surface. To gain an insight into this behavior, the spent TriTi100 catalyst was refluxed in chloroform, a more polar solvent, for 2 h. The analysis of the extract indicates that the spent catalyst retains flavone. Substituted 1,3-propanodione could not be detected. We found that after this treatment, the activity of the catalyst could be completely restored (Fig. 3). The catalyst treated in this way displayed a similar yield during the second and the third reuse (94 and 92%, respectively). Then the same reaction conditions were applied for different substrates, using the catalyst with better performance for the flavone synthesis. In Table 3, we report the catalytic activity of TriTi100, in the preparation of substituted flavones and 2-naphtylchromones, from 1-(2-hydroxyphenyl)-3-aryl-1,3-propanediones. The experiments were carried out in refluxing toluene in the presence of 20 mg catalyst, till consumption of the b-diketone
Mesoporous titania materials have been prepared using urea as pore-forming agent, via HCl catalyzed sol–gel reactions, and impregnated with trifluoromethanesulfonic acid. The amount of trifluoromethanesulfonic acid firmly adsorbed on the support depends on the number of –OH groups available on the titania to be protonated, which decreases with the increase in the calcination temperature. According to the potentiometric titration measurements, the catalysts present very strong acid sites. Their acid strength decreases slightly as a result of the partial dehydroxylation that takes place during the thermal treatment of the support. The use of these solid catalysts provided interesting yields in the cyclization reaction of 1-(2-hydroxyphenyl)-3-aryl-1,3propanediones to flavones and chromones, also leading to an easy separation and recovering of the catalysts for further use. The activity of the catalysts depends mainly on the amount of triflic acid firmly adsorbed on the support. Moreover, as a significant decrease of the catalytic activity was not observed, they can be recycled without an activity loss. These catalysts are very attractive to be used in the synthesis of flavones and chromones, important compounds for bioactive derivatives. So, they constitute an important contribution to perform acid reactions applying new clean technologies. Acknowledgements The authors are very grateful to Dra. M. Blanco, G. Valle and E. Soto for their contribution and Agencia Nacional de Promocio´n Cientı´fica y Tecnolo´gica (Argentina), Fundacio´n Antorchas, Universidad Nacional de La Plata and CONICET for the financial support. References [1] G.W. Kabalka, A.R. Mereddy, Tetrahedron Lett. 46 (2005) 6315, and references cited therein. [2] S. Martens, A. Mitho¨fer, Phythochemistry 66 (2005) 2399. [3] J. Grassmann, S. Hippeli, E. Eltsner, Plant Physiol. Biochem. 40 (2002) 471. [4] J. Wu, X. Wang, Y. Yic, K. Leeb, Bioorg. Med. Chem. Lett. 13 (2003) 1813. [5] J. Seijas, M. Va´zquez-Tato, R. Carballido-Reboredo, J. Org. Chem. 70 (2005) 2855, and references cited therein. [6] S. Yano, H. Tachibana, K. Yamada, J. Agric. Food Chem. 53 (2005) 1812. [7] M. Morimoto, K. Tanimoto, S. Nakano, T. Ozaki, A. Nakano, K. Komai, J. Agric. Food Chem. 51 (2003). [8] W. Ohmura, S. Doi, M. Aoyama, S. Ohara, S. J. Wood Sci. 46 (2000) 149.
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Supported trifluoromethanesulfonic acid as catalyst in the synthesis of flavone and chromone derivatives D.O. Bennardi a,b, G.P. Romanelli a,b,*, J.C. Autino b, L.R. Pizzio a,* a
Centro de Investigacio´n y Desarrollo en Ciencias Aplicadas ‘‘Dr. J.J. Ronco’’ CINDECA, Departamento de Quı´mica, Facultad de Ciencias Exactas, UNLP-CONICET, Calle 47 N8 257, B1900AJK La Plata, Argentina b Ca´tedra de Quı´mica Orga´nica, Facultad de Ciencias Agrarias y Forestales, UNLP, Calles 60 y 119, B1904AAN La Plata, Argentina Received 13 November 2006; received in revised form 23 February 2007; accepted 11 March 2007 Available online 15 March 2007
Abstract Solid acid catalysts based on trifluoromethanesulfonic acid (triflic acid, TFMS) were synthesized using titania with different textural properties as support, obtained by increasing the calcination temperature (Ti100, Ti200, Ti300, and Ti400). They were characterized by FT-IR, DTA-TGA, and BET. The acidic characteristics of the catalysts were determined by potentiometric titration with n-butylamine. According to this technique, the catalysts present very strong acid sites. Their acid strength decreases slightly in the order Ti100 > Ti200 > Ti300 > Ti400, as a result of the partial dehydroxylation that takes place during the thermal treatment of the supports. The amount of trifluoromethanesulfonic acid firmly adsorbed on the support depends on the number of –OH groups available on the titania to be protonated, which decreases with the increase in the calcination temperature. The new catalysts were tested in the cyclization of 1-(2-hydroxyphenyl)-3-phenyl-1,3-propanedione. The reaction experiments were performed using toluene as the solvent at reflux. In all cases, the product (flavone) was obtained with high selectivity. The activity of the catalysts is dependent on the textural properties of the support used to obtain the catalysts and the amount of triflic acid firmly adsorbed on it. The same reaction conditions were applied to the preparation of eight substituted flavones and chromones. Conversions up to 88% were obtained using the Ti100 supported TFMS catalyst. # 2007 Elsevier B.V. All rights reserved. Keywords: Supported trifluoromethanesulfonic acid; Supported triflic acid; Flavones; Chromones; Titania
1. Introduction Compounds containing the chromone skeleton (4H-benzopyran-4-one) (flavones and chromones) are an important class of compounds belonging to the flavonoid group that occur naturally in plants. They are minor constituents of the human diet and have been reported to exhibit a wide range of biological effects [1,2]. These biological properties include anti-inflammatory, antibacterial, antitumor [1], antioxidant [3], anti-HIV [4], vasodilator, antiviral and antiallergenic [5]. Some flavonoids inhibit the histamine release from human basophils and rat mast cells [6]. Moreover, it is known that some
* Corresponding authors. E-mail addresses: [email protected] (G.P. Romanelli), [email protected] (L.R. Pizzio). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.03.009
flavonoids have antifeedant activity against some phytophagous insects and a subterranean termite (Coptotermes sp.) [7,8]. The importance of these compounds has led to the development of various methods for their synthesis: for example, the Allan-Robinson strategy, or from chalcones, and via an intramolecular Wittig strategy [9,10]. One of the most common methods involves acylation of an o-hydroxyacetophenone with an aromatic acid chloride yielding an aryl ester. The ester is then rearranged by a base (the Baker– Venkataraman reaction) to a 1,3-diaryl 1,3-diketone [11]. The latter compound gives a 2-arylchromone on catalyst cyclocondensation. On the other hand, the transformation used for the preparation of such fine chemicals is carried out in very environmentally unfriendly conditions, using homogeneous catalysts such as inorganic acids, Lewis acids, or organic bases that generate large amounts of corrosive and toxic waste from which the catalysts are very difficult to recover. In order to
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overcome these problems, the use of heterogeneous catalysts is a possibility. Different solid acid catalysts such as clays, oxides, zeolites, Al-MCM-41, and supported reagents have been used in this transformation [12]. Trifluoromethanesulfonic acid (triflic acid, CF3SO3H) is known to be a strong acid and it is used in many organic reactions such as Friedel Crafts reactions, polymerization, Koch carbonylation, among others [13]. However, the recovery of the triflic acid from the reaction mixture results in the formation of large amounts of waste [13]. The design of acidcatalyzed reactions over solids has caused a need for further research in this area, and supported triflic acid or materials with –CF2SO3H groups are now becoming available to replace homogeneous acid solutions [14]. Different catalytic systems have been studied in which TFMS is adsorbed on various solids such as acylation of 2-methoxynapthalene [12] and acylation of alcohols over silica embedded-triflate catalysts [15], alkylation of hydroquinone with tert-butanol and alkylation of phenol and naphthols over silica-immobilized triflate derivatives [16,17], benzoylation of toluene with toluoyl chloride over triflic acidfunctionalized mesoporous Zr-TMS catalyst [18], alkylation of isobutane with TFMS absorbed on SiO2 [19], acetalization of ethyl acetoacetate and benzoylation of biphenyl using triflic acid-functionalized mesoporous Zr-TMS catalysts [13], and nitration and esterification of aromatic compounds over CF3SO3H/SiO2 [14]. Mesoporous titania synthesized via sol–gel reactions using urea as a low-cost pore-forming agent [20] presents suitable properties to be used as catalyst support. To the best of our knowledge, there have been no reports in the literature about the use of mesoporous titania as support of trifluoromethanesulfonic acid. In the present work, the synthesis and characterization of trifluoromethanesulfonic acid supported on mesoporous titania prepared using urea as a low-cost, pore-forming agent, via HCl catalyzed sol–gel reactions, is presented. The acidic characteristics of the solids were determined by potentiometric titration with n-butylamine. The catalytic activity of the materials was tested in the synthesis of flavone and chromone derivatives. 2. Experimental 2.1. Synthesis of mesoporous titania Titaniumisopropoxide (Aldrich, 26.7 g) was mixed with absolute ethanol (Merck, 186.6 g) and stirred for 10 min to obtain a homogeneous solution under N2 at room temperature, then 0.33 cm3 of 0.28 M HCl aqueous solution was dropped slowly into the above mixture to catalyze the sol–gel reaction for 3 h. After that, an appropriate amount of urea–alcohol– water (1:5:1 weight ratio) solution was added to the hydrolyzed solution under vigorous stirring to act as template. The amount of added solution was fixed in order to obtain a template concentration of 10% by weight in the final material. The gel was kept in a beaker at room temperature till dryness. The solid was ground into powder and extracted with distilled water for three periods of 24 h, in a system with continuous stirring to
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remove urea. Finally, it was calcined at 100, 200, 300, and 400 8C for 24 h (Ti100, Ti200, Ti300, and Ti400, respectively). 2.2. Synthesis of supported trifluoromethanesulfonic acid catalysts Trifluoromethanesulfonic acid, CF3SO3H (0.01 mol, Alfa Aesar, 99%) was added dropwise to the mixture of mesoporous titania (2 g) and toluene (20 cm3, Merck) at 90 8C under nitrogen atmosphere; then it was further refluxed for 2 h. Next, the sample was cooled, filtered, washed with acetone (Mallinckrodt AR) and dried at 100 8C for 24 h. The solids were extracted with a mixture of dichloromethane and diethyl ether (50%, v/v) (100 cm3 of mixture per gram of catalyst) for three periods of 8 h using a Soxhlet apparatus in order to remove the acid weakly attached to the support. Afterwards, they were dried again at 100 8C for 24 h. The samples were named TriTi100, TriTi200, TriTi300, and TriTi400. The amount of trifluoromethanesulfonic acid retained was determined by elemental analysis of C and S carried out with an EA1108 Elemental Analyzer (Carlo Erba Instruments). 2.3. Support and catalyst characterization The specific surface area of the solids was determined from N2 adsorption–desorption isotherms at liquid-nitrogen temperature. They were obtained using Micromeritics ASAP 2020 equipment. The samples were previously degassed at 100 8C for 2 h. FT-IR spectra of supports and catalysts were obtained in the 400–4000 cm 1 wavenumber range using a Bruker IFS 66 FTIR spectrometer. Powder XRD patterns were recorded on the same samples that had been analyzed by FT-IR. The equipment used to this end was a Philips PW-1732 with built-in recorder, using Cu Ka radiation, nickel filter, 20 mA and 40 kV in the high voltage source, and scanning angle between 5 and 608 of 2u at a scanning rate of 18 per min. The TGA-DTA measurements of the solids were carried out using a Shimadzu DT 50 thermal analyzer. The thermogravimetric and differential thermal analyses were performed under argon or nitrogen respectively, using 25–50 mg samples and a heating rate of 10 8C/min. The studied temperature range was 20–700 8C. The acidity of the solid samples was measured by means of potentiometric titration. The solid (0.05 g) was suspended in acetonitrile (Merck), and stirred for 3 h. The suspension was then titrated with 0.05 N n-butylamine (Carlo Erba) in acetonitrile at 0.05 cm3/min. The electrode potential variation was measured with a Hanna 211 digital pH meter using a double junction electrode. 2.4. Reaction procedure All the starting b-diketones were prepared following a procedure described elsewhere [21]. Initially, we studied the cyclocondensation reaction of 1-(2-hydroxyphenyl)-3-phenylpropanodione to flavone. The reactions were carried out by
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adding the corresponding catalyst (20 mg) to a solution of 0.5 mmol of 1-(2-hydroxyphenyl)-1,3-propanodione in refluxing toluene (3 ml, Baker). The reaction was followed by TLC using TLC aluminum sheets (silica gel 60 F254 Merck). The suspension was stirred at 109 8C for different reaction times. When the reaction time was over, the catalyst was filtered and washed several times with toluene (1 ml). The extracts were combined and washed with 10% sodium hydroxide, then with H2O, and dried with anhydrous sodium sulfate. The organic solution was concentrated in vacuum to give the pure flavone, whose physical and spectral properties matched those reported in the literature. The conversion was expressed as the ratio of moles of product to moles of initial propanodione. In addition, the cyclodehydratation–condensation was followed as a function of time by means of UV-visible (UV– vis) spectroscopy. In order to appropriately obtain the samples, the mixture was centrifuged and diluted with toluene. The absorbance values of the samples at 420 nm and at different reaction times were determined. Their UV-vis spectra were obtained in the range 300–600 nm using a UV/VIS spectrometer LAMBDA 35, Perkin Elmer. The same reaction conditions were used to prepare different substituted flavones and chromones. Products were identified by 1H and 13C NMR with a Bruker instrument, operating at 400 MHz for 1H NMR and 100 MHz for 13C NMR, and by comparison with authentic samples prepared using sulfuric acid as catalysts. 3. Results and discussion 3.1. Support and catalyst characterization The specific surface area (SBET) of the titania support determined from N2 adsorption–desorption isotherms together with the average pore diameter (Dp) obtained from the BJH pore size distribution are shown in Table 1. As can be seen, all the samples are mesoporous with a Dp higher than 3.2 nm. The SBET decreased when the calcination temperature increased (Table 1). Calcination at 300 and 400 8C produced little pore narrowing. Some of the narrowest pores seem to collapse upon evolution of water forming two groups of pore size in the mesopore range. Table 1 Physicochemical properties of titania supports and supported trifluoromethanesulfonic acid catalysts Sample
SBET (m2/g)
DP (nm)
Nw (molecules/g)
NTri (mmol CF3SO3H/g)
Ti100 Ti200 Ti300 Ti400 TriTi100 TriTi200 TriTi300 TriTi400
287 221 159 124 265 209 151 118
3.9 4.0 3.2–5.8 3.9–5.9 3.9 3.9 3.2–5.7 3.9–5.8
1.52 10 21 1.11 10 21 0.51 10 21 0.38 10 21 – – – –
– – – – 0.84 0.53 0.32 0.24
The DTA diagram of Ti100 showed an endothermic peak at 52 8C, associated with the loss of physically adsorbed water, and two poorly developed endothermic peaks at 310 and 370 8C attributed to the partial dehydroxylation of the titania surface. They could be considered as an indication of the presence of two types of hydroxyl groups, usually present at the titania surface [22]. DTA diagrams of Ti200 and Ti300 samples presented features similar to those of Ti100. However, for the Ti400 sample, the endothermic peaks assigned to dehydroxylation were not detected. TGA patterns of the supports showed that dehydration takes place in two main steps. The first step is due to the loss of physically adsorbed water (below 160 8C) and the second to the loss of structural water in the temperature range 160–400 8C. For the Ti100 sample, the number of water molecules released per gram (Nw), estimated from the weight loss ascribed to the second step, is 1.52 1021. Nw decreased when the calcination temperature increased (Table 1), as a result of the dehydroxylation of the titania surface during the thermal treatment of the samples. The amount of acid firmly adsorbed to the support (NTri) calculated from the elemental analysis (Table 1) decreased when the calcination temperature increased. This can be explained if the interaction between the trifluoromethanesulfonic acid and titania is assumed to be of electrostatic type due to the transfer of protons to the –OH groups on the surface of the support. So, as a result of the dehydroxylation of the support that takes place during the thermal treatment, the amount of – OH groups available to be protonated decreases, and hence the amount of adsorbed acid falls. On the other hand, TriTi100, TriTi200, TriTi300, and TriTi400 catalysts presented slightly lower SBET values than those of the solids used as support. From these results, it can be seen that the decrease of SBET seems to be related to the amount of trifluoromethanesulfonic acid adsorbed on the mesoporous titania. The higher the amount of adsorbed acid, the higher the decrease of the support surface area. However, the pore size distribution was practically the same. The FT-IR spectra of titania after being extracted with distilled water did not present any of the characteristic bands of the urea, showing that the template was completely removed by water extraction. The spectrum of the Ti100 sample (Fig. 1a) displayed bands at 1618 and 3427 cm 1 assigned to the OH bending and stretching of water [23] that have been incorporated in the structure of the solid [24]. Also, in the energy interval below 800 cm 1, the band due to Ti–O stretching vibration was observed. FT-IR spectra of Ti200, Ti300, and Ti400 showed features similar to those observed for the Ti100 sample, but the intensity of the bands at 1618 and 3427 cm 1 decreased when the calcination temperature increased, in parallel with the reduction of the SBET. The FT-IR spectrum of the TriTi100 sample (Fig. 1b) displayed bands at 1267, 1183, and 1037 cm 1 in addition to those present in the Ti100 sample. The first two bands are due to the S O stretching mode of the adsorbed trifluoromethanesulfonic acid and the last one is assigned to the C–F stretching [25,26]. These bands are also present in the
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Fig. 1. FT-IR spectra of Ti100 (a), TriTi100 (b), TriTi200 (c), TriTi300 (d), and TriTi400 (e) samples.
spectra of TriTi200, TriTi300, and TriTi400 catalysts (Fig. 1c–e) although their intensity is lower as a result of the lower amount of trifluoromethanesulfonic acid adsorbed on the support. XRD patterns of TriTi100, TriTi200, TriTi300, and TriTi400 samples showed features similar to those of the corresponding support. The position and intensity of the characteristic peaks of anatase phase present in the titania used as support did not change after their impregnation with trifluoromethanesulfonic acid. The pattern of TriTi100 exhibited only the characteristic peaks of anatase phase at 2u = 25.38 (1 0 1), 37.98 (0 0 4), 47.88 (2 0 0), and 54.38. The XRD patterns of TriTi200, TriTi300, and TriTi400 catalysts exhibited the peak at 54.38 splited into two peaks at 54.08 (1 0 5) and 54.98 (2 1 1), as a result of an increment in support crystallinity when the calcination temperature is raised over 100 8C. The acidity measurements of the catalysts by means of potentiometric titration with n-butylamine let us estimate the number of acid sites and their acid strength. As a criterion to interpret the obtained results, it was suggested that the initial electrode potential (Ei) indicates the maximum acid strength of the sites, and the value of meq amine/g solid where the plateau is reached indicates the total number of acid sites [27]. Nevertheless, the end point of the titration given by the inflexion point of the curve is a good measure to carry out a comparison of the acidity of different samples. On the other hand, the acid strength of these sites may be classified according to the following scale [27]: Ei > 100 mV (very strong sites), 0 < Ei < 100 mV (strong sites), 100 < Ei < 0 (weak sites) and Ei < 100 mV (very weak sites). The titration curves of the catalysts are shown in Fig. 2a. The samples presented very strong acid sites, with Ei values in the range 550–700 mV. The number of acid sites determined by potentiometric titration decreased in parallel with the decrease of NTri, the amount of acid firmly adsorbed on the support. Additionally, the acid strength of the catalysts slightly decreased in the following order TriTi100 (673 mV) > TriTi200 (647 mV) > TriTi300 (606 mV) > TriTi400 (586 mV). This
Fig. 2. Potentiometric titration curves of TriTi catalysts (a) and the corresponding supports (b).
reflects the trend followed by Ei values of the bare support (Fig. 2b) that decreases in parallel with the increase of calcination temperature. The support acid strength decreases as a result of the partial dehydroxylation that takes place during the thermal treatment of the solids. 3.2. Catalytic activity The catalytic activity of TriTi100, TriTi200, TriTi300, and TriTi400 catalysts in the cyclization reaction of 1-(2-hydroxyphenyl)-3-phenyl-1,3-propanodione to flavone (Scheme 1) is displayed in Table 2. In all cases, toluene was used as solvent at reflux temperature. In such conditions, the highest conversion was attained using the TriTi100 catalyst (60%). The conversion decreases in the following order: TriTi100 > TriTi200 > TriTi300 > TriTi400, in parallel with the decrease of SBET and NTri. It is also in
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D.O. Bennardi et al. / Applied Catalysis A: General 324 (2007) 62–68 Table 2 Conversion (%) for cyclodehydration reaction of 1-(2-hidroxyphenyl)-3-phenylpropanodione to flavonea
Scheme 1. Synthesis of flavone.
agreement with the decrease of the total number of acid sites estimated by potentiometric titration. In all cases, flavone was the sole product formed. In order to evaluate the possible catalyst solubilization, an additional test was performed. The TriTi100 sample was refluxed in toluene for 2 h, filtered at high temperature and
Catalystb
Conversionc (%)
TriTi100 TriTi200 TriTi300 TriTi400
60 31 18 5
a
Toluene, reflux, 90 min. Catalyst: 20 mg. c Conversion was calculated from the yield (%) of the pure product. (No secondary products were detected.) b
Table 3 Synthesis of flavones and naphthylchromones using TriTi100 catalysta Entry
Diketone
Flavone/chromone
Yield (%)
1
86
2
86
3
88
4
87
5
87
6
82
7
82
a
Toluene, reflux, 360 min. Catalyst: 20 mg.
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or until no changes in the composition of the reaction mixture were observed for TLC. In all the cases, the desired products (flavones, 2-naphtylchromones) were obtained with high selectivity, and almost free of secondary products. The unchanged starting materials were recovered nearly quantitatively. Yields of flavones were similar to those of naphthylchromones (see Table 2). 4. Conclusions
Fig. 3. Conversion of 1-(2-hydroxyphenyl)-1,3-propanodione to flavone as a function of time for: fresh (&), washed (~) and refluxed (&) TriTi100 catalyst.
dried in vacuum till constant weight. The activity of the so treated catalyst was the same as that of the fresh catalyst (94% conversion in 360 min). The refluxed toluene was used as solvent for attempting the reaction without adding the catalyst. After 360 min flavone was not detected, the starting material being quantitatively recovered. The 1-(2-hydroxyphenyl)-3-phenyl-1,3-propanodione conversion obtained using TriTi100 as a function of time is shown in Fig. 3. Conversion was estimated following the consumption of the starting material as a function of time. The reaction time required for reaching the maximum conversion was 360 min. It was observed that for the same reaction time, the conversion obtained when Ti100, Ti200, Ti300, and Ti400 supports were used as catalyst was always lower than 5%. On the other hand, the spent TriTi100 catalyst was washed with toluene and dried at room temperature, and then reused. The change of the conversion with time for the washed catalyst is compared with that of the fresh catalyst in Fig. 3. As can be observed, the maximum conversion is reached at longer times when the reaction is performed with the reused catalyst. The activity lowering of could be due to the adsorption of reagent and/or product onto the catalyst surface. To gain an insight into this behavior, the spent TriTi100 catalyst was refluxed in chloroform, a more polar solvent, for 2 h. The analysis of the extract indicates that the spent catalyst retains flavone. Substituted 1,3-propanodione could not be detected. We found that after this treatment, the activity of the catalyst could be completely restored (Fig. 3). The catalyst treated in this way displayed a similar yield during the second and the third reuse (94 and 92%, respectively). Then the same reaction conditions were applied for different substrates, using the catalyst with better performance for the flavone synthesis. In Table 3, we report the catalytic activity of TriTi100, in the preparation of substituted flavones and 2-naphtylchromones, from 1-(2-hydroxyphenyl)-3-aryl-1,3-propanediones. The experiments were carried out in refluxing toluene in the presence of 20 mg catalyst, till consumption of the b-diketone
Mesoporous titania materials have been prepared using urea as pore-forming agent, via HCl catalyzed sol–gel reactions, and impregnated with trifluoromethanesulfonic acid. The amount of trifluoromethanesulfonic acid firmly adsorbed on the support depends on the number of –OH groups available on the titania to be protonated, which decreases with the increase in the calcination temperature. According to the potentiometric titration measurements, the catalysts present very strong acid sites. Their acid strength decreases slightly as a result of the partial dehydroxylation that takes place during the thermal treatment of the support. The use of these solid catalysts provided interesting yields in the cyclization reaction of 1-(2-hydroxyphenyl)-3-aryl-1,3propanediones to flavones and chromones, also leading to an easy separation and recovering of the catalysts for further use. The activity of the catalysts depends mainly on the amount of triflic acid firmly adsorbed on the support. Moreover, as a significant decrease of the catalytic activity was not observed, they can be recycled without an activity loss. These catalysts are very attractive to be used in the synthesis of flavones and chromones, important compounds for bioactive derivatives. So, they constitute an important contribution to perform acid reactions applying new clean technologies. Acknowledgements The authors are very grateful to Dra. M. Blanco, G. Valle and E. Soto for their contribution and Agencia Nacional de Promocio´n Cientı´fica y Tecnolo´gica (Argentina), Fundacio´n Antorchas, Universidad Nacional de La Plata and CONICET for the financial support. References [1] G.W. Kabalka, A.R. Mereddy, Tetrahedron Lett. 46 (2005) 6315, and references cited therein. [2] S. Martens, A. Mitho¨fer, Phythochemistry 66 (2005) 2399. [3] J. Grassmann, S. Hippeli, E. Eltsner, Plant Physiol. Biochem. 40 (2002) 471. [4] J. Wu, X. Wang, Y. Yic, K. Leeb, Bioorg. Med. Chem. Lett. 13 (2003) 1813. [5] J. Seijas, M. Va´zquez-Tato, R. Carballido-Reboredo, J. Org. Chem. 70 (2005) 2855, and references cited therein. [6] S. Yano, H. Tachibana, K. Yamada, J. Agric. Food Chem. 53 (2005) 1812. [7] M. Morimoto, K. Tanimoto, S. Nakano, T. Ozaki, A. Nakano, K. Komai, J. Agric. Food Chem. 51 (2003). [8] W. Ohmura, S. Doi, M. Aoyama, S. Ohara, S. J. Wood Sci. 46 (2000) 149.
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