- Email: [email protected]
Highly efficient heterogenous catalyst for acylation of alcohols and amines using natural ferrous chamosite
Applied Clay Science 43 (2009) 425–434
Contents lists available at ScienceDirect
Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / c l a y
Highly efficient heterogenous catalyst for acylation of alcohols and amines using natural ferrous chamosite B. Sreedhar a,⁎, R. Arundhathi a, M. Amarnath Reddy a, G. Parthasarathy b a b
Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology (Council of Scientific and Industrial Research), Hyderabad-500007, India National Geophysical Research Institute (Council of Scientific and Industrial Research), Hyderabad 500007, India
a r t i c l e
i n f o
Article history: Received 29 April 2008 Received in revised form 18 September 2008 Accepted 8 October 2008 Available online 17 October 2008
a b s t r a c t An efficient acylation of alcohols and amines employing carboxylic acid as an acylating agent using ferrous chamosite-natural marine clay as the heterogeneous and reusable catalyst has been realized for the first time, through the tuning of the reaction by simple exposure of the reactants to conventional heating or ultrasound irradiation in the absence of any additive under ligand and solvent free conditions. © 2008 Elsevier B.V. All rights reserved.
Keywords: Chamosite Natural clay Acylation Acylating agent Ultrasound irradiation
1. Introduction Acylation of alcohols and amines is of enormous interest in organic synthesis as it provides a useful and efficient protection protocol in a multistep synthetic process (Green and Wuts, 1999). Moreover, this reaction has biological significance because of the presence of alcoholic hydroxyl and amino groups in a variety of biologically active compounds that necessitates the manipulation of the chemical reactivity of these functional groups during the synthesis of multifunctional synthetic targets possessing one or more of these groups. 12% of the total chemical reactions involved in the synthesis of drugs frequently use acylation reactions in the preparation of drug candidate molecules (Carey et al., 2006). Acylation is usually carried out by treatment of an alcohol or amine with acetyl chloride or acetic anhydride in the presence of an acid or a base catalyst in a suitable organic solvent, although acetic anhydride is the most commonly used as it is less toxic. Basic catalysts such as 4-(dimethylamino)pyridine (DMAP) (Steglich and Hofle, 1969; Vedejs et al., 1993; Vedejs and Mackay, 2001), tributyl-phosphines (Vedejs and Diver, 1993) and acidic catalysts like Cu(OTf)2 (Sarvanan and Singh,1999), Me3SiOTf (Procopious et al.,1998), Sc(OTf)3 (Ishihara et al., 1996; Lee et al., 2002), In(OTf)3 (Chauhan et al., 1999), Bi(OTf)3 (Orita et al., 2001), LiClO4 (Nakae et al., 2001), Gd(OTf)3 (Alleti et al., 2005a,b), iodine (Phukan, 2004), bromo dimethyl sulfonium bromide (Khan et al., 2005), lanthanide(III) tosylates (Parac-Vogt et al., 2005), CoCl2.6H2O (Velusamy et al., 2005), ZrOCl2.8H2O (Gosh et al., 2005), catalyze acylation reactions with acid chloride or anhy⁎ Corresponding author. Tel.: +91 40 27193510; fax: +91 40 27160921. E-mail address: [email protected] (B. Sreedhar). 0169-1317/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2008.10.001
dride as the acylating agent under homogenous conditions. Use of homogenous catalysts poses serious problems, such as difficulty in the separation and recovery of the catalyst, disposal of the spent catalyst, corrosion problems. Development of easily separable and reusable solid catalyst having high activity for the acylation reaction is of great practical importance. Solid acid catalysts viz., commercial HY zeolites and montmorillonite K-10 or KSF clay and ZnO have been reported for the acylation of alcohols with acetic anhydride (Ballini et al., 1998; Choudary et al., 2000; Choudary et al., 2001a,b; Sarvari and Sharghi, 2005; Tammaddon et al., 2005). Though, acylation of alcohols can also be brought about by the action of Lewis acid reagents in conjunction with carboxylic acids, the Lewis acid is destroyed in the workup procedure resulting in substantial waste production (Izumi et al., 1995; Shiina and Mukaiyama, 1994; Kumar and Chattopadhyay, 1987). Some heterogenized homogenous catalysts have also been reported for the acylation of alcohols and amines (Sreedhar et al., 2003; Choudary et al., 2004; Alleti et al., 2005a,b; Chakraborti and Gulhane, 2003). Most of the methods require longer reaction times, use of halogenated solvents and expensive moisture-sensitive toxic reagents. Apart from these difficulties, the above methods do not satisfy the requirements of green synthesis due to the inability to recovery and reuse of the catalyst. Ultrasound accelerated chemical reactions are well known and proceed via the formation and adiabatic collapse of transient cavitation bubbles (Gaplovsky et al., 2000; Suslick, 1988; Deshmukh et al., 2001). Ultrasound irradiation can be utilized as an alternative energy source for organic reactions ordinarily accomplished by heating. It increases the reaction rate many folds when compared with conventional reaction conditions. It is also known to accelerate diverse types of organic reactions and is established as an important technique in organic
426
B. Sreedhar et al. / Applied Clay Science 43 (2009) 425–434
Fig. 1. Acylation of alcohols (1 mmol) with acetic acid (2 mmol) under conventional heating and sonochemical conditions.
synthesis (Lauche, 1998). Many reactions have been conducted with homogenous and heterogeneous catalysts, in which the rate of the reaction was accelerated on sonication. Several reports on versatile reactions carried out using ultra-sonication include low temperature Heck reaction with Pd/C (Ambulgekar et al., 2005). Reformatsky reactions, Sonogashira coupling and sonochemical preparation of ionic liquids (Ross et al., 2004; Namoodiri and Varma, 2002; Gholap et al., 2005). In continuation of our studies on sonochemical reactions, (Sreedhar et al., 2005; Sreedhar and Reddy, 2007) we herein report conventional as well as ultrasound accelerated time and energy saving protocol for O, N-acylation of alcohols and amines, respectively with acetic acid using natural clay, a reusable and economic solid catalyst. This approach requires shorter reaction times under ultrasound irradiation in contrast to several hours needed under conventional heating conditions and avoids the use of organic solvents as the reaction medium (Figs. 1 and 2). Complete characterization of the catalyst, chamosite — a naturally occurring clay mineral, has been carried out. 2. Experimental 2.1. Materials and methods Naturally occurring clay used in this study as a reusable catalyst, was collected from the quaternary marine sediments deposited near Kudiamozhi, Tuticorin District, Tamil Nadu. The composition determined by X-ray fluorescence (XRF) of the natural clay is Fe2+3[Fe2+2Al] [Si3AlO10] [OH]−8 chamosite, a chlorite group mineral. The structure of this clay consists of alternate layers of tetrahedral-octahedral-tetrahedral silicate layer (sometimes called the 2:1 silicate layer, talc layer). The t-o-t layer is itself made up of two infinite sheets of six-member rings of SiO4 and AlO4 tetrahedra. The interlayer and the t-o-t layer are bound together by electrostatic and hydrogen-bonding forces. The powder XRD data on the chamosite yielded a monoclinic unit cell with a = 0.5383(5) nm, b =0.9292(10) nm, c = 1.4200(10) nm and β =97.60 (Dana et al., 1997). The natural clay catalyst, thus obtained unlike other 2:1 clay minerals, has an interlayer space filled by a cation comprised of Fe2+3[Fe2+2 Al] (Si3Al)O10[OH]−8. This Fe2+3[Fe2+2 Al] (Si3Al)O10[OH]−8 unit is more commonly referred to as the brucite like layer, due to its closer structural resemblance to the brucite structure, and has been well characterized by various techniques such as FT-IR, XPS, TEM, XRD, TPD, TGA/DTG and BET surface area. Infrared spectra were recorded on a Thermo Nicolet Nexus 670 FT-IR spectrometer as KBr pellets. The major element analyses were done on a Philips XRF machine with an overall accuracy better than ±6%. X-ray photoelectron spectra were recorded on a KRATOS AXIS 165 with dual
anode (Mg and Al) apparatus using the Mg Kα anode. The particle size and external morphology of the samples were observed on a PHILIPS TECNAI F12 FEI Transmission Electron Microscopy (TEM). X-ray powder diffraction (XRD) data were collected on a Simens/D-5000 diffractometer using Cu Kα radiation. Thermogravimetric (TG) was carried out on TGA/SDTA Mettler Toledo 851e system using an open alumina crucible containing samples weighing about 8–10 mg with a linear heating rate of 10 °C min− 1 in nitrogen atmosphere. Temperature Programmed Desorption (TPD) studies were conducted on an Autochem 2910 Micromeritics, USA) instrument. Nitrogen was used as purge gas for all these measurements. NMR spectra were recorded on a Varian Gemini (200 MHz), Bruker Avance (300 MHz) and Varian Unity (400 MHz) spectrometers. Chemical shifts (δ) are reported in parts per million, using TMS as an internal standard and CDCl3 as solvent. The specific surface area of the catalyst samples was estimated using the N2 adsorption isotherm at 350 °C by the multipoint BET method using an Autosorb-1 Quantachrom instrument. Melting points were recorded on a Barnstead electrothermal 9200 instrument and are uncorrected. All the known compounds were characterized by comparing their physical data with those in the literature (Choudary et al., 2000; Narender et al., 2000a,b). All the reactants were commercially available and used without further purification. 2.2. Typical experimental procedure In a typical procedure, benzyl alcohol/amine (1 mmol) and glacial acetic acid (2 mmol) corresponding to a 1:2 molar ratio (1:2 alcohol/ amine:carboxylic acid) were subjected to ultra-sonic irradiation in the presence of 10 mg of catalyst for about 5–7 min. After completion of the reaction as monitored by TLC, the reaction mixture was filtered and the filtrate was concentrated under reduced pressure using rotavapor to obtain the crude product. The crude product was then purified by column chromatography using 60–120 silica gel and hexane: ethylacetate (80:20) as the eluent to obtain the pure product. The product was analyzed by 1H NMR, while the catalyst was washed with ethyl acetate and dried in an oven at 65 °C overnight and then reused. The same procedure is followed for other substrates for O- and N-acylation and all the products gave satisfactorily 1H NMR and mass spectral data in comparison with those reported in the literature (Shrini et al., 2003; Krishna Mohan et al., 2006; Ranu et al., 2003). 2.3. Characterization of the products Spectral data of some representative compounds. N-(4-fluoro-phenyl )-acetamide (Table 4, entry 7).
Fig. 2. Acylation of amines (1 mmol) with acetic acid (2 mmol) under conventional heating and sonochemical conditions.
B. Sreedhar et al. / Applied Clay Science 43 (2009) 425–434
427
Fig. 3. FT-IR spectra of (a) fresh chamosite-natural marine clay and (b) used catalyst after 5th cycle.
Yield 99%, white solid mp 152 to 154 °C, 1H NMR (CDCl3, 200 MHz) δ 9.61–9.50 (s,b,1H), 7.58–7.51 (m,2H), 6.97–6.88 (m,2H), 2.05 (s,3H). 13 C NMR (CDCl3, 400 MHz) δ 173.64, 164.54, 140.02, 126.17, 119.94, 28.82. LSI-MS: 81, 91, 107, 111, 112, 136, 137, 138, 153, 154, 155. Expected mol. wt: 154, observed mol. wt, MS(ESI): 155 (M + H) . Acetic acid-3-phenoxy-benzyl ester (Table 2, entry 7). Yield 95 %, oily liquid, 1H NMR (CDCl3, 200 MHz) δ 7.39–7.22 (m,10H), 2.18–2.07 (s,3H). 13C NMR (CDCl3, 300 MHz) δ 170.30, 43.28, 143.07, 128.73, 128.24, 127.92, 27.48, 126.81, 126.40, 74.42, 20.95. Expected mol. wt: 242, observed mol. wt, MS(ESI): 265(M + Na). 3. Results and discussion
Fig. 5. XRD spectra for (a) fresh chamosite-natural marine clay, (b) used chamositenatural marine clay catalyst after 5th cycle.
1005 cm−1 which can be assigned to the complexation of ferric ion with clay mineral. The same pattern with almost negligible deformities is observed in used clay catalyst as shown in Fig. 3a. 3.1.2. X-ray photoelectron spectroscopy The XPS survey scan of fresh and used natural clay catalysts showed binding energy peaks at 285 eV, 535 eV, 75 eV, 100 eV and 710 eV which are characteristic of the presence of elements carbon (C1s), oxygen (O 1s), aluminum (Al 2p), silica (Si 2p) and iron (Fe 2p), respectively. Peaks due to other elements which are in lower
3.1. Characterization 3.1.1. FT-IR spectroscopy The catalyst before and after use were studied by FT-IR spectroscopy to probe structural modifications before and after the reaction process and are represented in Fig. 3. The IR spectra showed strong absorption bands in the range 3400–3600 cm−1 due to the hydroxyl (OH)-stretching vibration and the corresponding deformation mode appeared around 1630 cm−1 in both the fresh and used catalyst. The IR spectra also exhibits strong absorbance band near 1036 cm−1 and
Fig. 6. TPD thermogram of chamosite-natural marine clay catalyst.
Fig. 4. XPS high resolution narrow scans of Fe 2p for (a) fresh chamosite-natural marine clay, and (b) used chamosite-natural marine clay catalyst after 5th cycle.
Peak No.
Retention (s)
Temperature at maximum (°C)
Volume (ml/g at STP)
Peak concentration (%)
Flags
1 2
905.7 841.5
342.8 T1 461.7 T2
10.81547 98.57702
0.36 3.32
S S
428
B. Sreedhar et al. / Applied Clay Science 43 (2009) 425–434 Table 1 Effect of the amount of ferrous chamosite-natural marine clay catalyst for acylation of benzyl alcohol with acetic acida Entry
1 2 3 4
Catalyst (mg)
Time Conventional (h)b
Sonochemical (min)c
Conventional (h)b
Conversion (%) Sonochemical (min)c
25 25 15 10
0.5 0.25 2.5 4.0
7 4 5 5
73 59 40 45
100 98 98 98
Reaction conditions: a Benzyl alcohol (1 mmol), acetic acid (2 mmol), reaction temperature 100 °C. b Under conventional heating conditions. c Under sonochemical conditions.
appeared at 12.6 A° remained unchanged after the reaction which indicates that there is no change in the structure of the natural clay during the course of the reaction.
Fig. 7. TEM images of chamosite-natural marine clay catalyst.
concentrations could not be identified. The high resolution narrow scans of the fresh and used catalyst are shown in Fig. 4. The observed binding energy peaks for Fe 2p3/2 and Fe 2p1/2 in both the fresh and used clay catalyst at 710.89 eV and 724.49 eV and 710.58 eV and 723.96 eV, respectively clearly indicate that iron is in +2 oxidation state in both the fresh and used clay catalyst. 3.1.3. XRD analysis XRD data were collected on a Simens/D-5000 diffractometer using Cu Kα radiation. The X-ray diffraction of the powder samples of the catalysts before and after reaction (Fig. 5) hardly differ in the range 2θ = 3–65°. The observed d001 basal spacing of the natural clay
3.1.4. TPD analysis Porosity and surface area measurements of samples were performed on a Micromeritics ASAP2020 automated gas adsorption system. All the samples were outgassed at 350 °C under vacuum prior to N2 adsorption. The acidic property of the catalyst was determined by temperature-programmed desorption of ammonia (NH3-TPD). The experiments were carried out in the range of 27 to 800 °C in a fixedbed flow microreactor (i.d., 6.0 mm; 500 mm). The temperature in the catalyst bed was measured by a thermocouple located in a quartz capillary immersed in the catalyst bed. The molecules desorbing from sample were monitored online by a quadrapole mass spectrometer (Quantachrome Autosorb-IC). Prior to the NH3-TPD experiments, sample (153 mg) was outgassed at 200 °C for 1 h in a flow of dry helium (50 mL/min). Then the sample was purged in a helium flow until a constant baseline level was attained. Then it was adsorbed with 10% NH3–He (75 mL/min) at 80 °C for an hour. Desorption was carried out with a linear heating rate (80 °C/min) in a flow of He (75 mL/min). From Fig. 6, it can be seen that for the clay sample, two desorption peaks appear one in the low-temperature region (b 350 °C) and the other in the high-temperature region (N 450 °C). In general, the desorption peaks at low-temperature may be assigned to the presence of weak acidic sites and the high-temperature desorption peaks to the
Fig. 8. TGA/DTG thermogram of chamosite-natural marine clay catalyst.
B. Sreedhar et al. / Applied Clay Science 43 (2009) 425–434
429
Table 2 Acylation of various benzyl alcohols with acetic acid using ferrous chamosite-natural marine clay catalyst
a
Reaction conditions: Benzyl alcohol (1 mmol), acetic acid (2 mmol), reaction temperature 100 °C. Under conventional heating conditions.
strong acidic sites. The area under the envelope for each sample represents the total acidic sites. For the natural clay sample, the high temperature peak area is obviously higher than that of the low temperature peak. These results suggest that the number of strongly acidic sites is much greater than that of weak acidic sites on the
lb
Under sonochemical conditions.
surface of the natural clay. Considering the difference in surface area of the studied sample, the peak area results must be compared on the basis of the acidic sites present per unit surface area. The total desorption peak area normalized to per unit surface area are listed in a table below Fig. 6. From the table, it can be concluded that the catalyst
430
B. Sreedhar et al. / Applied Clay Science 43 (2009) 425–434
Table 3 Acylation of secondary alcohols with acetic acid using ferrous chamosite-natural marine clay catalyst
a
Reaction conditions: Benzyl alcohol (1 mmol), acetic acid (2 mmol), reaction temperature 100 °C. Under conventional heating conditions.
possess higher acidic sites per unit surface area than the number of less acidic sites per unit surface area. 3.1.5. TEM analysis of the natural marine clay chamosite The natural clay sample has been studied by transmission electron microscopy. Three micrographs of the material are represented in Fig. 7. They showed the presence of stacking of clay layers distributed in a disorganized way. Such arrangements are typical of delaminated clays. Interlayer spacings were not uniform and varied from 3 to 10 Å, showing that the distribution of the iron cationic species within the particles is inhomogeneous. The selected area electron diffraction pattern shows that these particles are crystalline in nature. 3.1.6. TGA/DTG analysis The TGA and DTG profiles of the fresh natural clay sample showed three step decomposition pattern (Fig. 8). Initially the observed weight loss can be attributed to the loss of adsorbed water on the surface whereas the weight loss at higher temperature ie., between the temperature range ~ 200 °C and 400 °C is due to the removal of water molecules located in between the layers of the clay and also due to the presence of iron and/or aluminum in the form of hydroxides. 3.1.7. BET surface area and chemical analysis Surface area by BET technique of the natural clay was found to be ~ 52.2 m2/g. The composition by wt.% of the metal oxides namely, Fe2O3, Al2O3, MgO, CaO, SiO2, TiO2, MnO present in chamosite-natural marine clay was found to be 8.88, 28.04, 0.45, 0.32, 42.3, 1.17, 0.12, respectively with other organic ingredients comprising of 14.52%. 3.2. Application of natural clay in acylation reaction To explore the generality and scope of this natural clay as a heterogeneous catalyst for organic transformations, we initially performed the acylation of benzyl alcohol with acetic acid under three different reaction conditions namely ambient temperature, conventional heating and ultrasound irradiation. The reaction at ambient temperature in the presence of 25 mg of catalyst gave the acylated product in 30% yield in 7 h and further increasing the time period did not enhance the rate of conversion. On the other hand, when the reaction was conducted at higher temperatures (90–100 °C),
b
Under sonochemical conditions.
73% of conversion within 0.5 h (Table 1, entry 1) was observed. Though the conventional heating gave encouraging results with shorter reaction time, an increase in the efficiency of the catalyst activity was noticed under sonication (7–10 min) 100% conversion. To optimize the amount of catalyst required under sonication, the reaction was conducted with different amounts of the catalyst and as can be seen from Table 1, 10 mg of the catalyst is optimal for the reaction under ultrasonication (Table 1, entry 4). After completion of the reaction, the catalyst (clay) was recovered and reused for nearly up to 5 cycles with negligible loss in its activity. However, the acylation of benzyl alcohol failed to proceed in the absence of the catalyst. 3.2.1. Results and discussion To study the applicability of the catalyst for the acylation reactions, a wide range of structurally and electronically varied alcohols were subjected to reaction with acetic acid and corresponding acetates were obtained in excellent yields by this procedure and the results are presented in Table 2. Various electron-rich, sterically hindered benzyl alcohols underwent reaction with acetic acid under conventional and sonochemical conditions to afford the corresponding acylated products in good yields (Table 2, entries 1–7). When aryl thio substituted alcohol (Table 2, entry 8) was treated with acetic acid the corresponding Oacylated derivative was obtained in excellent yield. The excellent ‘electrophilic activation’ of the catalyst was seen for alcohols having electron withdrawing groups that decrease nucleophilic property of the hydroxyl group (Table 2, entries 9–14). The requirement of heating or sonication for the nitro substituted benzyl alcohols which reduced the nucleophilicity of the hydroxyl group (Table 2, entries 13 and 14) and 4hydroxy-3-methoxy benzyl alcohol (Table 2 entry 10) provided the corresponding O-acylated products in 70–90% yield within 7 min under sonication. It was found that aryl alkyl alcohols undergo reaction at a relatively faster rate (Table 2, entry 15). It is observed that primary alcohols undergo acylation at a shorter period of time compared to secondary alcohols but it is impressive to note that there is no byproduct formation in both the alcohols. Similarly, secondary alcohols when subjected to reaction with acetic acid the corresponding Oacylated products were obtained in good yields and the results are summarized in Table 3. Sterically hindered diphenyl methanol, cinnamyl alcohol and cyclohexanol also gave the acylated products in high yields (Table 3, entries 3–5). We next planned to evaluate the efficiency of the
B. Sreedhar et al. / Applied Clay Science 43 (2009) 425–434
431
Table 4 Acylation of different amines with acetic acid using ferrous chamosite-natural marine clay catalyst
a
Reaction conditions: Amine (1 mmol), acetic acid (2 mmol), reaction temperature 100 °C. Under conventional heating conditions.
b
Under sonochemical conditions.
432
B. Sreedhar et al. / Applied Clay Science 43 (2009) 425–434
Fig. 9. Plausible mechanism for the acylation of alcohols with acetic acid using chamosite-natural marine clay catalyst.
clay catalyst for acylation of amines, so a series of amines were subjected to acylation reaction and the results are given in Table 4. Aniline and substituted anilines on reaction with acetic acid gave the corresponding amides in excellent yields in shorter reaction times under solvent-free conditions (Table 4, entries 1–4 and entries 6–9). The presence of nitro substituent at para position of aniline makes the amino group poorly nucleophillic and the presence of the nitro group adjacent to the amino group (ortho position) makes the amino group sterically hindered and thus, the corresponding products are obtained in less yields (Table 4, entries 8 and 9). Heterocyclic amine on reaction with acetic acid also gave the product in good yields (Table 4, entry 10). Cyclohexyl amine, benzyl amine, substituted benzyl amines and phenethyl amine underwent reaction with acetic acid and the corresponding acetamides were obtained in good yields (Table 4, entries 11–15). It is also observed that when both hydroxyl and amino groups are present in the substrate, the catalyst selectively acylates amino group rather than hydroxyl group (Table 4, entries 16–17) which was confirmed when there was no reaction with phenolic hydroxyl groups (Table 2, entry 10). 3.2.2. Mechanism Acylation of alcohols generally proceeds via an acyl-oxygen cleavage bimolecular (AAC2) mechanism; it can be expected that the rate of esterification could be affected due to the transient complexation of the metal ion with the carbonyl group. According to the reaction mechanism Fig. 9, the density of the Brønsted acid sites in the acid-treated natural chamosite clay, specially chosen for the studies, is increased over natural clay because the in-
creased number of broken edges resulting from the broken layers favour the formation of esters assumed to proceed via the AAC2 mechanistic pathway. The presence of metal ion in the natural chamosite clay not only enhances the Lewis acidity but also Brønsted acidity which is generated from the interstitial cation–aqua complex of the metal ion as described, because the interaction of the metal–aqua complex formed in the natural chamosite clay with water present/formed during the reaction enables the generation of hydronium ion (Brønsted acidity) and metal hydroxide as described in the reversible reaction (Laura,1976 and Pinnavaia, 1983). The formation of the right admix of Brønsted and Lewis acid sites is able to facilitate the esterification reaction as described in the reversible reaction.
The consumption of Brønsted acid further shifts towards the right side to generate more of Brønsted acidity. Water as a by-product is formed during the reaction, but is not expected to affect the acidity of the catalyst and thereby catalyst activity. However, the excess water present in the system could not affect the acidic sites as it forms a binary mixture with acetic acid used as a solvent, apart from acting as an acylating agent in the present reaction. Indeed, the hydrated cations of the chamosite clay are more reactive, since the interlayer acidity increases with increasing ratio of charge to ionic radius of the cation. Moreover, the Fe3+ ion has a lower redox potential which can polarize the carboxylic acid more strongly. The sustained and the long
B. Sreedhar et al. / Applied Clay Science 43 (2009) 425–434
433
Fig. 10. Plausible mechanism for the acylation of amines with acetic acid chamosite-natural marine clay catalyst.
lasting activity of the catalyst over the number of cycles is ascribed to the generation of the Brønsted acid site as described. Moreover, the cationic aqua-complexes located in the interlayers of natural clay have been countered with an array of negative layer of charges that do not allow the cationic complex to go out of the interlayers. This is also attributed to the long lasting activity of the catalyst for the number of cycles. In case of amines, acylation by the mechanistic pathway is assumed to proceed via the free radical mechanism. The plausible mechanism involving a free radical is described in Fig. 10. However, both the metals involve one electron redox reaction. Initially, the active catalyst, Fe2+clay(1) is formed by the reduction of Fe3+-clay, presumably with the amine. The generation of a carbonium free radical (2) is likely facilitated by Fe2+-clay (1) which in turn is oxidized to give carbonium ion Fe3+-clay (3). The carbocation finally reacts with the amine to give
amide with the simultaneous reformation of the catalyst and water (Figs. 9 and 10). 3.3. Reusability of natural clay For any heterogeneous catalyst, it is important to know its ease of separation and possible reusability, which was tested for the acylation of 4-flouroaniline with acetic acid. The catalyst was separated by simple filtration and washed with ethyl acetate followed by water, finally acetone and air-dried. The recovered catalyst was used in the next run and almost consistent activity was noticed for five consecutive cycles. Next to check whether the reaction was occurring mainly due to the leached metal or supported catalyst, the reaction was terminated after a small conversion (2 min, 10% conversion) and the catalyst was filtered off by hot filtration and the reaction was continued with
Table 5 Comparison of acylation using different catalysts Entry
Catalyst
Mol (%)
Time (h)
Acylating agent
Temperature (°C)
Yield (%)
Reference
1 2 3 4 5a 6b 7c
FeAC Fe-PILLC Fe-(Cr-PILLC) Cr-(Fe-PILLC) Fe-mont Fe-mont Fe-chamosite
0.1 – – – 0.1 0.1 0.15 0.08
2 1 1 1 3 6.5 0.5 0.08
AcOH AcOH AcOH AcOH AcOH PhAcOH AcOH AcOH
116 60 60 60 116 116 110 Sonication
99 80 81 80 97 100 99 99
Sreedhar et al. (2003) Akcay (2004) Akcay (2004) Akcay (2004) Choudary et al. (2001a,b) Kantam et al. (2001)
a b c
Acylation of amines. Acylation of alcohols. Reaction conditions as exemplified in the experimental procedure.
gPresent work
434
B. Sreedhar et al. / Applied Clay Science 43 (2009) 425–434
the filtrate for required period of time. Almost no change in the conversion was observed. This confirmed that no active species leached during the reaction (Table 5), shows comparison of different catalysts used for acylation reaction. 4. Conclusion In conclusion we found that chamosite, a naturally occurring marine clay, serves as an efficient and reusable, heterogenous catalyst for the acylation of arylalkyl alcohols and amines. The use of catalytic amounts of clay catalyst, acetic acid as acylating agent, shorter reaction times and selective acylation process shows the superiority of this methodology. The acylations are, in general reasonably fast and clean without any side products. The results indicate that the process of acylation with this clay catalyst affords excellent possible atom economy when accounted with respect to substrate, product and acylating reagent. Acknowledgements We wish to thank the DST for financial support under the project GAP-0152 and R.A. thanks the Department of Science and Technology (DST), India, for a research fellowship. References Akcay, M., 2004. The catalytic acylation of alcohols with acetic acid by using Lewis acid character pillared clays. Applied Catalysis A: Chemical 191, 157–160. Alleti, R., Perambuduru, M., Samantha, S., Reddy, V.P., 2005a. Gadolinium triflate: an efficient and convenient catalyst for acetylation of alcohols. Journal of Molecular Catalysis A: Chemical 226, 57–59. Alleti, R., Oh, W.S., Perambuduru, M., Afrasiabi, Z., Sinn, E., Reddy, V.P., 2005b. Gadolinium triflate immobilized in imidazolium based ionic liquids: a recyclable catalyst and green solvent for acetylation of alcohols and amines. Green Chemistry 7, 203–206. Ambulgekar, G.V., Bhanage, B.M., Samant, S.D., 2005. Low temperature recyclable catalyst for Heck reactions using ultrasound. Tetrahedron Letters 46, 2483–2485. Ballini, R., Bosica, G., Carloni, S., Ciralli, L., Maggi, R., Sartori, G., 1998. Zeolite HSZ-360 as a new reusable catalyst for the direct acetylation of alcohols and phenols under solventless conditions. Tetrahedron Letters 39, 6049–6052. Dana, J.D., Dana, E.S., 1997. Dana's New Minerology, John Wiley, New York, 1820. Carey, J.S., Laffan, D., Thomson, C., Williams, M.T., 2006. Analysis of the reactions used for the preparation of drug candidate molecules. Organic and Biomolecular Chemistry 4, 2337–2347. Chakraborti, A.K., Gulhane, R., 2003. Perchloric acid adsorbed on silica gel as a new, highly efficient, and versatile catalyst for acetylation of phenols, thiols, alcohols, and amines. Chemical Communication 1896–1897. Chauhan, K.K., Frost, C.G., Love, I., Waite, D., 1999. Indium triflate: an efficient catalyst for acylation reactions. Synlett 1743–1744. Choudary, B.M., Bhaskar, V., Kantam, M.L., Koteswara Rao, K., Raghavan, K.V., 2000. Acylation of alcohols with carboxylic acids via the evolution of compatible acidic sites in montmorillonites. Green Chemistry 2, 67–70. Choudary, B.M., Bhaskar, V., Kantam, M.L., Koteswara Rao, K., Raghavan, K.V., 2001a. Acylation of amines with carboxylic acids: the atom economic protocol catalysed by Fe(III)-Montmorillonite. Catalysis Letters 74, 207–211. Choudary, V.R., Jana, S.K., Patil, N.S., 2001b. Acylation of benzene over clay and mesoporous Si-MCM-41 supported InCl3, GaCl3 and ZnCl2 catalysts. Catalysis Letters 76, 235–239. Choudary, V.R., Patil, K., Jana, S.K., 2004. Acylation of aromatic alcohols and phenols over InCl3/montmorillonite K-10 catalysts. Journal of Chemical Science 116, 175–177. Deshmukh, R.R., Rajagopal, R., Srinivasan, K.V., 2001. Ultrasound promoted C–C bond formation: Heck reaction at ambient conditions in room temperature ionic liquids. Chemical Communication 1544–1545. Gaplovsky, A., Gaplovsky, M., Toma, S., Luche, J.-L., 2000. Ultrasound effects on the photopinacolization of benzophenone. Journal of Organic Chemistry 65, 8444–8447. Gholap, A.R., Venkatesan, K., Pasricha, R., Daniel, T., Lahoti, R.J., Srinivasan, K.V., 2005. Copper and ligand-free sonogashira reaction catalyzed by Pd(0) nanoparticles at ambient conditions under ultrasound irradiation. Journal of Organic Chemistry 70, 4869–4872. Gosh, R., Maiti, S., Chakraborty, A., 2005. Facile catalyzed acylation of alcohols, phenols, amines and thiols based on ZrOCl2·8H2O and acetyl chloride in solution and in solvent-free conditions. Tetrahedron Letters 46, 147–151. Green, T.W., Wuts, P.G.M., 1999. Protective Groups in Organic Synthesis, 3rd ed. Wiley, New York. Izumi, J., Shiina, I., Mukaiyama, T., 1995. An efficient esterification reaction between equimolar amounts of free carboxylicacids and alcohols by the combined use of
octamethylcyclotetrasiloxane and a catalytic amount of titanium tris (trifluoromethane- sulfonate). Chemistry Letters 141–142. Ishihara, K., Kubota, M., Kurihara, H., Yamamoto, H., 1996. Scandium trifluromethane sulfonate as an extremely active Lewis acid catalyst in acylation of alcohols with acid anhydrides. Journal of Organic Chemistry 61, 4560–4567. Kantam, M.L., Bhaskar, V., Choudary, B.M., 2001. Direct condensation of carboxylic acids with alcohols: the atom economic protocol catalysed by Fe(III)-montmorillonite. Catalysis Letters 78, 185–188. Khan, A.T., Islam, S., Majee, A., Chattopadhyy, T., Gosh, S., 2005. Bromodimethyl sulfonium bromide: a useful reagent for acylation of alcohols, phenols, amines, thiols, thiophenols and 1,1-diacylation of aldehydes under solvent free conditions. Journal of Molecular Catalysis A: Chemical 239, 158–165. Krishna Mohan, K.V.V., Narender, N., Kulkarni, S.J., 2006. Zeolite catalyzed acylation of alcohols and amines with acetic acid under microwave irradiation. Green Chemistry 8, 368–372. Kumar, A.K., Chattopadhyay, T.K., 1987. Catalytic role of diorganotin dichloride in esterification of carboxylic acids. Tetrahedron Letters 28, 3713–3714. Laura, R.D., 1976. Surface acidity of imogolite and allophane. Clay Minerals 11, 331–334. Lee, J.C., Tai, C.A., Hung, S.C., 2002. Sc(OTf)3-catalyzed acetolysis of 1,6-anhydro-βhexopyranoses and solvent-free per-acetylation of hexoses. Tetrahedron Letters 43, 851–855. Luche, J.L., 1998. Reactivity and selectivity under microwaves in organic chemistry. Relation with medium effects and reaction mechanisms. Synthetic Organic Sonochemistry. Plenum Press, New York. Nakae, Y., Kusaki, I., Sato, T., 2001. Lithium perchlorate catalyzed acetylation of alcohols under mild reaction conditions. Synlett 1584–1586. Namoodiri, V.V., Varma, R.S., 2002. Solvent-free sonochemical preparation of ionic liquids. Organic Letters 4, 3161–3163. Narender, N., Srinivasu, P., Kulkarni, S.J., Raghavan, K.V., 2000a. Liquid phase acylation of alcohols with acetic acid over zeolites. Synthetic Communication 30, 1887–1893. Narender, N., Srinivasu, P., Kulkarni, S.J., Raghavan, K.V., 2000b. Liquid phase acylation of amines with acetic acid over HY zeolite. Green Chemistry 2, 104–105. Orita, A., Tanahashi, C., Kakuda, A., Otera, J., 2001. Highly powerful and practical acylation of alcohols with acid anhydride catalyzed by Bi(OTf)3. Journal of Organic Chemistry 66, 8926–8934. Parac-Vogt, T.N., Deleersnyder, K., Binnemans, K., 2005. Lanthanide(III) tosylates as new acylation catalysts. European Journal of Organic Chemistry 1810–1815. Pinnavaia, T.J., 1983. Intercalated clay catalysts. Science 220, 365–371. Phukan, P., 2004. Iodine as an extremely powerful catalyst for the acetylation of alcohols under solvent-free conditions. Tetrahedron Letters 45, 4785–4787. Procopious, P.A., Baugh, S.P.D., Flack, S.S., Inglis, G.G.A., 1998. An extremely powerful acylation reaction of alcohols with acid anhydrides catalyzed by trimethylsilyl trifluromethane sulfonate. Journal of Organic Chemistry 63, 2342–2347. Ranu, C.B., Dey, S.S., Hajra, A., 2003. Highly efficient acylation of alcohols, amines and thiols under solvent-free and catalyst-free conditions. Green Chemistry 5, 44–46. Ross, N.A., MacGregor, R.R., Bartsch, R.A., 2004. Synthesis of β-lactams and βaminoesters via high intensity ultrasound-promoted Reformatsky reactions. Tetrahedron 60, 2035–2041. Sarvanan, P., Singh, V.K., 1999. An efficient method for acylation reactions. Tetrahedron Letters 40, 2611–2614. Sarvari, M.H., Sharghi, H., 2005. Zinc oxide (ZnO) as a new, highly efficient, and reusable catalyst for acylation of alcohols, phenols and amines under solvent free conditions. Tetrahedron 61, 10903–10907. Shiina, I., Mukaiyama, T., 1994. A novel method for the preparation of macrolides from w- hydroxycarboxylic acids. Chemistry Letters 677–680. Shrini, F.M., Zolfigol, M.A., Abedini, M., Salehi, P., 2003. Al(HSO4)3 catalyzed acetylation and formylation of alcohols. Bulletin of Korean Chemical Society 24, 1683–1685. Sreedhar, B., Reddy, P.S., 2007. Sonochemical synthesis of 1,4-disubstituted 1,2,3triazoles in aqueous medium. Synthetic Communication 37, 805–812. Sreedhar, B., Bhaskar, V., Sridhar, Ch., Srinivas, T., Laszlo, Kotai, Klara, Szentmihalyi, 2003. Acylation of alcohols and amines with carboxylic acids: a first report catalyzed by iron(III) oxide-containing activated carbon. Journal of Molecular Catalysis A: Chemical 191, 141–147. Sreedhar, B., Reddy, P.S., Veda Prakash, B., Ravindra, A., 2005. Ultrasound-assisted rapid and efficient synthesis of propargylamines. Tetrahedron Letters 46, 7019–7022. Steglich, W., Hofle, G., 1969. N,N-dimethyl-4-pyridinamine, a very effective acylation catalyst. Angewandte Chemie International Edition in English, 8, 981. Suslick, K.S., 1988. Ultrasound: Its Chemical, Physical and Biological Effects. VCH: Weinheim, New York. Tammaddon, F., Amrollahi, M.A., Sharafat, L., 2005. A green protocol for chemoselective O-acylation in the presence of zinc oxide as a heterogeneous, reusable and ecofriendly catalyst. Tetrahedron Letters 46, 7841–7844. Vedejs, E., Diver, S.T., 1993. Tributylphosphine: a remarkable acylation catalyst. Journal of American Chemical Society 115, 3358–3359. Vedejs, E., Mackay, J.A., 2001. Kinetic resolution of allylic alcohols using a chiral phosphine catalyst. Organic Letters 3, 535–536. Vedejs, E., Bennet, N.S., Conn, L.M., Diver, S.T., Gingras, M., Lin, S., Oliver, P.M., Peterson, M.J., 1993. Tributylphosphine-catalyzed acylations of alcohols: scope and related reactions. Journal of Organic Chemistry 58, 7286–7288. Velusamy, S., Borpuzari, S., Punniyamurthy, T., 2005. Cobalt(II)-catalyzed direct acetylation of alcohols with acetic acid. Tetrahedron 61, 2011–2015.
Contents lists available at ScienceDirect
Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / c l a y
Highly efficient heterogenous catalyst for acylation of alcohols and amines using natural ferrous chamosite B. Sreedhar a,⁎, R. Arundhathi a, M. Amarnath Reddy a, G. Parthasarathy b a b
Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology (Council of Scientific and Industrial Research), Hyderabad-500007, India National Geophysical Research Institute (Council of Scientific and Industrial Research), Hyderabad 500007, India
a r t i c l e
i n f o
Article history: Received 29 April 2008 Received in revised form 18 September 2008 Accepted 8 October 2008 Available online 17 October 2008
a b s t r a c t An efficient acylation of alcohols and amines employing carboxylic acid as an acylating agent using ferrous chamosite-natural marine clay as the heterogeneous and reusable catalyst has been realized for the first time, through the tuning of the reaction by simple exposure of the reactants to conventional heating or ultrasound irradiation in the absence of any additive under ligand and solvent free conditions. © 2008 Elsevier B.V. All rights reserved.
Keywords: Chamosite Natural clay Acylation Acylating agent Ultrasound irradiation
1. Introduction Acylation of alcohols and amines is of enormous interest in organic synthesis as it provides a useful and efficient protection protocol in a multistep synthetic process (Green and Wuts, 1999). Moreover, this reaction has biological significance because of the presence of alcoholic hydroxyl and amino groups in a variety of biologically active compounds that necessitates the manipulation of the chemical reactivity of these functional groups during the synthesis of multifunctional synthetic targets possessing one or more of these groups. 12% of the total chemical reactions involved in the synthesis of drugs frequently use acylation reactions in the preparation of drug candidate molecules (Carey et al., 2006). Acylation is usually carried out by treatment of an alcohol or amine with acetyl chloride or acetic anhydride in the presence of an acid or a base catalyst in a suitable organic solvent, although acetic anhydride is the most commonly used as it is less toxic. Basic catalysts such as 4-(dimethylamino)pyridine (DMAP) (Steglich and Hofle, 1969; Vedejs et al., 1993; Vedejs and Mackay, 2001), tributyl-phosphines (Vedejs and Diver, 1993) and acidic catalysts like Cu(OTf)2 (Sarvanan and Singh,1999), Me3SiOTf (Procopious et al.,1998), Sc(OTf)3 (Ishihara et al., 1996; Lee et al., 2002), In(OTf)3 (Chauhan et al., 1999), Bi(OTf)3 (Orita et al., 2001), LiClO4 (Nakae et al., 2001), Gd(OTf)3 (Alleti et al., 2005a,b), iodine (Phukan, 2004), bromo dimethyl sulfonium bromide (Khan et al., 2005), lanthanide(III) tosylates (Parac-Vogt et al., 2005), CoCl2.6H2O (Velusamy et al., 2005), ZrOCl2.8H2O (Gosh et al., 2005), catalyze acylation reactions with acid chloride or anhy⁎ Corresponding author. Tel.: +91 40 27193510; fax: +91 40 27160921. E-mail address: [email protected] (B. Sreedhar). 0169-1317/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2008.10.001
dride as the acylating agent under homogenous conditions. Use of homogenous catalysts poses serious problems, such as difficulty in the separation and recovery of the catalyst, disposal of the spent catalyst, corrosion problems. Development of easily separable and reusable solid catalyst having high activity for the acylation reaction is of great practical importance. Solid acid catalysts viz., commercial HY zeolites and montmorillonite K-10 or KSF clay and ZnO have been reported for the acylation of alcohols with acetic anhydride (Ballini et al., 1998; Choudary et al., 2000; Choudary et al., 2001a,b; Sarvari and Sharghi, 2005; Tammaddon et al., 2005). Though, acylation of alcohols can also be brought about by the action of Lewis acid reagents in conjunction with carboxylic acids, the Lewis acid is destroyed in the workup procedure resulting in substantial waste production (Izumi et al., 1995; Shiina and Mukaiyama, 1994; Kumar and Chattopadhyay, 1987). Some heterogenized homogenous catalysts have also been reported for the acylation of alcohols and amines (Sreedhar et al., 2003; Choudary et al., 2004; Alleti et al., 2005a,b; Chakraborti and Gulhane, 2003). Most of the methods require longer reaction times, use of halogenated solvents and expensive moisture-sensitive toxic reagents. Apart from these difficulties, the above methods do not satisfy the requirements of green synthesis due to the inability to recovery and reuse of the catalyst. Ultrasound accelerated chemical reactions are well known and proceed via the formation and adiabatic collapse of transient cavitation bubbles (Gaplovsky et al., 2000; Suslick, 1988; Deshmukh et al., 2001). Ultrasound irradiation can be utilized as an alternative energy source for organic reactions ordinarily accomplished by heating. It increases the reaction rate many folds when compared with conventional reaction conditions. It is also known to accelerate diverse types of organic reactions and is established as an important technique in organic
426
B. Sreedhar et al. / Applied Clay Science 43 (2009) 425–434
Fig. 1. Acylation of alcohols (1 mmol) with acetic acid (2 mmol) under conventional heating and sonochemical conditions.
synthesis (Lauche, 1998). Many reactions have been conducted with homogenous and heterogeneous catalysts, in which the rate of the reaction was accelerated on sonication. Several reports on versatile reactions carried out using ultra-sonication include low temperature Heck reaction with Pd/C (Ambulgekar et al., 2005). Reformatsky reactions, Sonogashira coupling and sonochemical preparation of ionic liquids (Ross et al., 2004; Namoodiri and Varma, 2002; Gholap et al., 2005). In continuation of our studies on sonochemical reactions, (Sreedhar et al., 2005; Sreedhar and Reddy, 2007) we herein report conventional as well as ultrasound accelerated time and energy saving protocol for O, N-acylation of alcohols and amines, respectively with acetic acid using natural clay, a reusable and economic solid catalyst. This approach requires shorter reaction times under ultrasound irradiation in contrast to several hours needed under conventional heating conditions and avoids the use of organic solvents as the reaction medium (Figs. 1 and 2). Complete characterization of the catalyst, chamosite — a naturally occurring clay mineral, has been carried out. 2. Experimental 2.1. Materials and methods Naturally occurring clay used in this study as a reusable catalyst, was collected from the quaternary marine sediments deposited near Kudiamozhi, Tuticorin District, Tamil Nadu. The composition determined by X-ray fluorescence (XRF) of the natural clay is Fe2+3[Fe2+2Al] [Si3AlO10] [OH]−8 chamosite, a chlorite group mineral. The structure of this clay consists of alternate layers of tetrahedral-octahedral-tetrahedral silicate layer (sometimes called the 2:1 silicate layer, talc layer). The t-o-t layer is itself made up of two infinite sheets of six-member rings of SiO4 and AlO4 tetrahedra. The interlayer and the t-o-t layer are bound together by electrostatic and hydrogen-bonding forces. The powder XRD data on the chamosite yielded a monoclinic unit cell with a = 0.5383(5) nm, b =0.9292(10) nm, c = 1.4200(10) nm and β =97.60 (Dana et al., 1997). The natural clay catalyst, thus obtained unlike other 2:1 clay minerals, has an interlayer space filled by a cation comprised of Fe2+3[Fe2+2 Al] (Si3Al)O10[OH]−8. This Fe2+3[Fe2+2 Al] (Si3Al)O10[OH]−8 unit is more commonly referred to as the brucite like layer, due to its closer structural resemblance to the brucite structure, and has been well characterized by various techniques such as FT-IR, XPS, TEM, XRD, TPD, TGA/DTG and BET surface area. Infrared spectra were recorded on a Thermo Nicolet Nexus 670 FT-IR spectrometer as KBr pellets. The major element analyses were done on a Philips XRF machine with an overall accuracy better than ±6%. X-ray photoelectron spectra were recorded on a KRATOS AXIS 165 with dual
anode (Mg and Al) apparatus using the Mg Kα anode. The particle size and external morphology of the samples were observed on a PHILIPS TECNAI F12 FEI Transmission Electron Microscopy (TEM). X-ray powder diffraction (XRD) data were collected on a Simens/D-5000 diffractometer using Cu Kα radiation. Thermogravimetric (TG) was carried out on TGA/SDTA Mettler Toledo 851e system using an open alumina crucible containing samples weighing about 8–10 mg with a linear heating rate of 10 °C min− 1 in nitrogen atmosphere. Temperature Programmed Desorption (TPD) studies were conducted on an Autochem 2910 Micromeritics, USA) instrument. Nitrogen was used as purge gas for all these measurements. NMR spectra were recorded on a Varian Gemini (200 MHz), Bruker Avance (300 MHz) and Varian Unity (400 MHz) spectrometers. Chemical shifts (δ) are reported in parts per million, using TMS as an internal standard and CDCl3 as solvent. The specific surface area of the catalyst samples was estimated using the N2 adsorption isotherm at 350 °C by the multipoint BET method using an Autosorb-1 Quantachrom instrument. Melting points were recorded on a Barnstead electrothermal 9200 instrument and are uncorrected. All the known compounds were characterized by comparing their physical data with those in the literature (Choudary et al., 2000; Narender et al., 2000a,b). All the reactants were commercially available and used without further purification. 2.2. Typical experimental procedure In a typical procedure, benzyl alcohol/amine (1 mmol) and glacial acetic acid (2 mmol) corresponding to a 1:2 molar ratio (1:2 alcohol/ amine:carboxylic acid) were subjected to ultra-sonic irradiation in the presence of 10 mg of catalyst for about 5–7 min. After completion of the reaction as monitored by TLC, the reaction mixture was filtered and the filtrate was concentrated under reduced pressure using rotavapor to obtain the crude product. The crude product was then purified by column chromatography using 60–120 silica gel and hexane: ethylacetate (80:20) as the eluent to obtain the pure product. The product was analyzed by 1H NMR, while the catalyst was washed with ethyl acetate and dried in an oven at 65 °C overnight and then reused. The same procedure is followed for other substrates for O- and N-acylation and all the products gave satisfactorily 1H NMR and mass spectral data in comparison with those reported in the literature (Shrini et al., 2003; Krishna Mohan et al., 2006; Ranu et al., 2003). 2.3. Characterization of the products Spectral data of some representative compounds. N-(4-fluoro-phenyl )-acetamide (Table 4, entry 7).
Fig. 2. Acylation of amines (1 mmol) with acetic acid (2 mmol) under conventional heating and sonochemical conditions.
B. Sreedhar et al. / Applied Clay Science 43 (2009) 425–434
427
Fig. 3. FT-IR spectra of (a) fresh chamosite-natural marine clay and (b) used catalyst after 5th cycle.
Yield 99%, white solid mp 152 to 154 °C, 1H NMR (CDCl3, 200 MHz) δ 9.61–9.50 (s,b,1H), 7.58–7.51 (m,2H), 6.97–6.88 (m,2H), 2.05 (s,3H). 13 C NMR (CDCl3, 400 MHz) δ 173.64, 164.54, 140.02, 126.17, 119.94, 28.82. LSI-MS: 81, 91, 107, 111, 112, 136, 137, 138, 153, 154, 155. Expected mol. wt: 154, observed mol. wt, MS(ESI): 155 (M + H) . Acetic acid-3-phenoxy-benzyl ester (Table 2, entry 7). Yield 95 %, oily liquid, 1H NMR (CDCl3, 200 MHz) δ 7.39–7.22 (m,10H), 2.18–2.07 (s,3H). 13C NMR (CDCl3, 300 MHz) δ 170.30, 43.28, 143.07, 128.73, 128.24, 127.92, 27.48, 126.81, 126.40, 74.42, 20.95. Expected mol. wt: 242, observed mol. wt, MS(ESI): 265(M + Na). 3. Results and discussion
Fig. 5. XRD spectra for (a) fresh chamosite-natural marine clay, (b) used chamositenatural marine clay catalyst after 5th cycle.
1005 cm−1 which can be assigned to the complexation of ferric ion with clay mineral. The same pattern with almost negligible deformities is observed in used clay catalyst as shown in Fig. 3a. 3.1.2. X-ray photoelectron spectroscopy The XPS survey scan of fresh and used natural clay catalysts showed binding energy peaks at 285 eV, 535 eV, 75 eV, 100 eV and 710 eV which are characteristic of the presence of elements carbon (C1s), oxygen (O 1s), aluminum (Al 2p), silica (Si 2p) and iron (Fe 2p), respectively. Peaks due to other elements which are in lower
3.1. Characterization 3.1.1. FT-IR spectroscopy The catalyst before and after use were studied by FT-IR spectroscopy to probe structural modifications before and after the reaction process and are represented in Fig. 3. The IR spectra showed strong absorption bands in the range 3400–3600 cm−1 due to the hydroxyl (OH)-stretching vibration and the corresponding deformation mode appeared around 1630 cm−1 in both the fresh and used catalyst. The IR spectra also exhibits strong absorbance band near 1036 cm−1 and
Fig. 6. TPD thermogram of chamosite-natural marine clay catalyst.
Fig. 4. XPS high resolution narrow scans of Fe 2p for (a) fresh chamosite-natural marine clay, and (b) used chamosite-natural marine clay catalyst after 5th cycle.
Peak No.
Retention (s)
Temperature at maximum (°C)
Volume (ml/g at STP)
Peak concentration (%)
Flags
1 2
905.7 841.5
342.8 T1 461.7 T2
10.81547 98.57702
0.36 3.32
S S
428
B. Sreedhar et al. / Applied Clay Science 43 (2009) 425–434 Table 1 Effect of the amount of ferrous chamosite-natural marine clay catalyst for acylation of benzyl alcohol with acetic acida Entry
1 2 3 4
Catalyst (mg)
Time Conventional (h)b
Sonochemical (min)c
Conventional (h)b
Conversion (%) Sonochemical (min)c
25 25 15 10
0.5 0.25 2.5 4.0
7 4 5 5
73 59 40 45
100 98 98 98
Reaction conditions: a Benzyl alcohol (1 mmol), acetic acid (2 mmol), reaction temperature 100 °C. b Under conventional heating conditions. c Under sonochemical conditions.
appeared at 12.6 A° remained unchanged after the reaction which indicates that there is no change in the structure of the natural clay during the course of the reaction.
Fig. 7. TEM images of chamosite-natural marine clay catalyst.
concentrations could not be identified. The high resolution narrow scans of the fresh and used catalyst are shown in Fig. 4. The observed binding energy peaks for Fe 2p3/2 and Fe 2p1/2 in both the fresh and used clay catalyst at 710.89 eV and 724.49 eV and 710.58 eV and 723.96 eV, respectively clearly indicate that iron is in +2 oxidation state in both the fresh and used clay catalyst. 3.1.3. XRD analysis XRD data were collected on a Simens/D-5000 diffractometer using Cu Kα radiation. The X-ray diffraction of the powder samples of the catalysts before and after reaction (Fig. 5) hardly differ in the range 2θ = 3–65°. The observed d001 basal spacing of the natural clay
3.1.4. TPD analysis Porosity and surface area measurements of samples were performed on a Micromeritics ASAP2020 automated gas adsorption system. All the samples were outgassed at 350 °C under vacuum prior to N2 adsorption. The acidic property of the catalyst was determined by temperature-programmed desorption of ammonia (NH3-TPD). The experiments were carried out in the range of 27 to 800 °C in a fixedbed flow microreactor (i.d., 6.0 mm; 500 mm). The temperature in the catalyst bed was measured by a thermocouple located in a quartz capillary immersed in the catalyst bed. The molecules desorbing from sample were monitored online by a quadrapole mass spectrometer (Quantachrome Autosorb-IC). Prior to the NH3-TPD experiments, sample (153 mg) was outgassed at 200 °C for 1 h in a flow of dry helium (50 mL/min). Then the sample was purged in a helium flow until a constant baseline level was attained. Then it was adsorbed with 10% NH3–He (75 mL/min) at 80 °C for an hour. Desorption was carried out with a linear heating rate (80 °C/min) in a flow of He (75 mL/min). From Fig. 6, it can be seen that for the clay sample, two desorption peaks appear one in the low-temperature region (b 350 °C) and the other in the high-temperature region (N 450 °C). In general, the desorption peaks at low-temperature may be assigned to the presence of weak acidic sites and the high-temperature desorption peaks to the
Fig. 8. TGA/DTG thermogram of chamosite-natural marine clay catalyst.
B. Sreedhar et al. / Applied Clay Science 43 (2009) 425–434
429
Table 2 Acylation of various benzyl alcohols with acetic acid using ferrous chamosite-natural marine clay catalyst
a
Reaction conditions: Benzyl alcohol (1 mmol), acetic acid (2 mmol), reaction temperature 100 °C. Under conventional heating conditions.
strong acidic sites. The area under the envelope for each sample represents the total acidic sites. For the natural clay sample, the high temperature peak area is obviously higher than that of the low temperature peak. These results suggest that the number of strongly acidic sites is much greater than that of weak acidic sites on the
lb
Under sonochemical conditions.
surface of the natural clay. Considering the difference in surface area of the studied sample, the peak area results must be compared on the basis of the acidic sites present per unit surface area. The total desorption peak area normalized to per unit surface area are listed in a table below Fig. 6. From the table, it can be concluded that the catalyst
430
B. Sreedhar et al. / Applied Clay Science 43 (2009) 425–434
Table 3 Acylation of secondary alcohols with acetic acid using ferrous chamosite-natural marine clay catalyst
a
Reaction conditions: Benzyl alcohol (1 mmol), acetic acid (2 mmol), reaction temperature 100 °C. Under conventional heating conditions.
possess higher acidic sites per unit surface area than the number of less acidic sites per unit surface area. 3.1.5. TEM analysis of the natural marine clay chamosite The natural clay sample has been studied by transmission electron microscopy. Three micrographs of the material are represented in Fig. 7. They showed the presence of stacking of clay layers distributed in a disorganized way. Such arrangements are typical of delaminated clays. Interlayer spacings were not uniform and varied from 3 to 10 Å, showing that the distribution of the iron cationic species within the particles is inhomogeneous. The selected area electron diffraction pattern shows that these particles are crystalline in nature. 3.1.6. TGA/DTG analysis The TGA and DTG profiles of the fresh natural clay sample showed three step decomposition pattern (Fig. 8). Initially the observed weight loss can be attributed to the loss of adsorbed water on the surface whereas the weight loss at higher temperature ie., between the temperature range ~ 200 °C and 400 °C is due to the removal of water molecules located in between the layers of the clay and also due to the presence of iron and/or aluminum in the form of hydroxides. 3.1.7. BET surface area and chemical analysis Surface area by BET technique of the natural clay was found to be ~ 52.2 m2/g. The composition by wt.% of the metal oxides namely, Fe2O3, Al2O3, MgO, CaO, SiO2, TiO2, MnO present in chamosite-natural marine clay was found to be 8.88, 28.04, 0.45, 0.32, 42.3, 1.17, 0.12, respectively with other organic ingredients comprising of 14.52%. 3.2. Application of natural clay in acylation reaction To explore the generality and scope of this natural clay as a heterogeneous catalyst for organic transformations, we initially performed the acylation of benzyl alcohol with acetic acid under three different reaction conditions namely ambient temperature, conventional heating and ultrasound irradiation. The reaction at ambient temperature in the presence of 25 mg of catalyst gave the acylated product in 30% yield in 7 h and further increasing the time period did not enhance the rate of conversion. On the other hand, when the reaction was conducted at higher temperatures (90–100 °C),
b
Under sonochemical conditions.
73% of conversion within 0.5 h (Table 1, entry 1) was observed. Though the conventional heating gave encouraging results with shorter reaction time, an increase in the efficiency of the catalyst activity was noticed under sonication (7–10 min) 100% conversion. To optimize the amount of catalyst required under sonication, the reaction was conducted with different amounts of the catalyst and as can be seen from Table 1, 10 mg of the catalyst is optimal for the reaction under ultrasonication (Table 1, entry 4). After completion of the reaction, the catalyst (clay) was recovered and reused for nearly up to 5 cycles with negligible loss in its activity. However, the acylation of benzyl alcohol failed to proceed in the absence of the catalyst. 3.2.1. Results and discussion To study the applicability of the catalyst for the acylation reactions, a wide range of structurally and electronically varied alcohols were subjected to reaction with acetic acid and corresponding acetates were obtained in excellent yields by this procedure and the results are presented in Table 2. Various electron-rich, sterically hindered benzyl alcohols underwent reaction with acetic acid under conventional and sonochemical conditions to afford the corresponding acylated products in good yields (Table 2, entries 1–7). When aryl thio substituted alcohol (Table 2, entry 8) was treated with acetic acid the corresponding Oacylated derivative was obtained in excellent yield. The excellent ‘electrophilic activation’ of the catalyst was seen for alcohols having electron withdrawing groups that decrease nucleophilic property of the hydroxyl group (Table 2, entries 9–14). The requirement of heating or sonication for the nitro substituted benzyl alcohols which reduced the nucleophilicity of the hydroxyl group (Table 2, entries 13 and 14) and 4hydroxy-3-methoxy benzyl alcohol (Table 2 entry 10) provided the corresponding O-acylated products in 70–90% yield within 7 min under sonication. It was found that aryl alkyl alcohols undergo reaction at a relatively faster rate (Table 2, entry 15). It is observed that primary alcohols undergo acylation at a shorter period of time compared to secondary alcohols but it is impressive to note that there is no byproduct formation in both the alcohols. Similarly, secondary alcohols when subjected to reaction with acetic acid the corresponding Oacylated products were obtained in good yields and the results are summarized in Table 3. Sterically hindered diphenyl methanol, cinnamyl alcohol and cyclohexanol also gave the acylated products in high yields (Table 3, entries 3–5). We next planned to evaluate the efficiency of the
B. Sreedhar et al. / Applied Clay Science 43 (2009) 425–434
431
Table 4 Acylation of different amines with acetic acid using ferrous chamosite-natural marine clay catalyst
a
Reaction conditions: Amine (1 mmol), acetic acid (2 mmol), reaction temperature 100 °C. Under conventional heating conditions.
b
Under sonochemical conditions.
432
B. Sreedhar et al. / Applied Clay Science 43 (2009) 425–434
Fig. 9. Plausible mechanism for the acylation of alcohols with acetic acid using chamosite-natural marine clay catalyst.
clay catalyst for acylation of amines, so a series of amines were subjected to acylation reaction and the results are given in Table 4. Aniline and substituted anilines on reaction with acetic acid gave the corresponding amides in excellent yields in shorter reaction times under solvent-free conditions (Table 4, entries 1–4 and entries 6–9). The presence of nitro substituent at para position of aniline makes the amino group poorly nucleophillic and the presence of the nitro group adjacent to the amino group (ortho position) makes the amino group sterically hindered and thus, the corresponding products are obtained in less yields (Table 4, entries 8 and 9). Heterocyclic amine on reaction with acetic acid also gave the product in good yields (Table 4, entry 10). Cyclohexyl amine, benzyl amine, substituted benzyl amines and phenethyl amine underwent reaction with acetic acid and the corresponding acetamides were obtained in good yields (Table 4, entries 11–15). It is also observed that when both hydroxyl and amino groups are present in the substrate, the catalyst selectively acylates amino group rather than hydroxyl group (Table 4, entries 16–17) which was confirmed when there was no reaction with phenolic hydroxyl groups (Table 2, entry 10). 3.2.2. Mechanism Acylation of alcohols generally proceeds via an acyl-oxygen cleavage bimolecular (AAC2) mechanism; it can be expected that the rate of esterification could be affected due to the transient complexation of the metal ion with the carbonyl group. According to the reaction mechanism Fig. 9, the density of the Brønsted acid sites in the acid-treated natural chamosite clay, specially chosen for the studies, is increased over natural clay because the in-
creased number of broken edges resulting from the broken layers favour the formation of esters assumed to proceed via the AAC2 mechanistic pathway. The presence of metal ion in the natural chamosite clay not only enhances the Lewis acidity but also Brønsted acidity which is generated from the interstitial cation–aqua complex of the metal ion as described, because the interaction of the metal–aqua complex formed in the natural chamosite clay with water present/formed during the reaction enables the generation of hydronium ion (Brønsted acidity) and metal hydroxide as described in the reversible reaction (Laura,1976 and Pinnavaia, 1983). The formation of the right admix of Brønsted and Lewis acid sites is able to facilitate the esterification reaction as described in the reversible reaction.
The consumption of Brønsted acid further shifts towards the right side to generate more of Brønsted acidity. Water as a by-product is formed during the reaction, but is not expected to affect the acidity of the catalyst and thereby catalyst activity. However, the excess water present in the system could not affect the acidic sites as it forms a binary mixture with acetic acid used as a solvent, apart from acting as an acylating agent in the present reaction. Indeed, the hydrated cations of the chamosite clay are more reactive, since the interlayer acidity increases with increasing ratio of charge to ionic radius of the cation. Moreover, the Fe3+ ion has a lower redox potential which can polarize the carboxylic acid more strongly. The sustained and the long
B. Sreedhar et al. / Applied Clay Science 43 (2009) 425–434
433
Fig. 10. Plausible mechanism for the acylation of amines with acetic acid chamosite-natural marine clay catalyst.
lasting activity of the catalyst over the number of cycles is ascribed to the generation of the Brønsted acid site as described. Moreover, the cationic aqua-complexes located in the interlayers of natural clay have been countered with an array of negative layer of charges that do not allow the cationic complex to go out of the interlayers. This is also attributed to the long lasting activity of the catalyst for the number of cycles. In case of amines, acylation by the mechanistic pathway is assumed to proceed via the free radical mechanism. The plausible mechanism involving a free radical is described in Fig. 10. However, both the metals involve one electron redox reaction. Initially, the active catalyst, Fe2+clay(1) is formed by the reduction of Fe3+-clay, presumably with the amine. The generation of a carbonium free radical (2) is likely facilitated by Fe2+-clay (1) which in turn is oxidized to give carbonium ion Fe3+-clay (3). The carbocation finally reacts with the amine to give
amide with the simultaneous reformation of the catalyst and water (Figs. 9 and 10). 3.3. Reusability of natural clay For any heterogeneous catalyst, it is important to know its ease of separation and possible reusability, which was tested for the acylation of 4-flouroaniline with acetic acid. The catalyst was separated by simple filtration and washed with ethyl acetate followed by water, finally acetone and air-dried. The recovered catalyst was used in the next run and almost consistent activity was noticed for five consecutive cycles. Next to check whether the reaction was occurring mainly due to the leached metal or supported catalyst, the reaction was terminated after a small conversion (2 min, 10% conversion) and the catalyst was filtered off by hot filtration and the reaction was continued with
Table 5 Comparison of acylation using different catalysts Entry
Catalyst
Mol (%)
Time (h)
Acylating agent
Temperature (°C)
Yield (%)
Reference
1 2 3 4 5a 6b 7c
FeAC Fe-PILLC Fe-(Cr-PILLC) Cr-(Fe-PILLC) Fe-mont Fe-mont Fe-chamosite
0.1 – – – 0.1 0.1 0.15 0.08
2 1 1 1 3 6.5 0.5 0.08
AcOH AcOH AcOH AcOH AcOH PhAcOH AcOH AcOH
116 60 60 60 116 116 110 Sonication
99 80 81 80 97 100 99 99
Sreedhar et al. (2003) Akcay (2004) Akcay (2004) Akcay (2004) Choudary et al. (2001a,b) Kantam et al. (2001)
a b c
Acylation of amines. Acylation of alcohols. Reaction conditions as exemplified in the experimental procedure.
gPresent work
434
B. Sreedhar et al. / Applied Clay Science 43 (2009) 425–434
the filtrate for required period of time. Almost no change in the conversion was observed. This confirmed that no active species leached during the reaction (Table 5), shows comparison of different catalysts used for acylation reaction. 4. Conclusion In conclusion we found that chamosite, a naturally occurring marine clay, serves as an efficient and reusable, heterogenous catalyst for the acylation of arylalkyl alcohols and amines. The use of catalytic amounts of clay catalyst, acetic acid as acylating agent, shorter reaction times and selective acylation process shows the superiority of this methodology. The acylations are, in general reasonably fast and clean without any side products. The results indicate that the process of acylation with this clay catalyst affords excellent possible atom economy when accounted with respect to substrate, product and acylating reagent. Acknowledgements We wish to thank the DST for financial support under the project GAP-0152 and R.A. thanks the Department of Science and Technology (DST), India, for a research fellowship. References Akcay, M., 2004. The catalytic acylation of alcohols with acetic acid by using Lewis acid character pillared clays. Applied Catalysis A: Chemical 191, 157–160. Alleti, R., Perambuduru, M., Samantha, S., Reddy, V.P., 2005a. Gadolinium triflate: an efficient and convenient catalyst for acetylation of alcohols. Journal of Molecular Catalysis A: Chemical 226, 57–59. Alleti, R., Oh, W.S., Perambuduru, M., Afrasiabi, Z., Sinn, E., Reddy, V.P., 2005b. Gadolinium triflate immobilized in imidazolium based ionic liquids: a recyclable catalyst and green solvent for acetylation of alcohols and amines. Green Chemistry 7, 203–206. Ambulgekar, G.V., Bhanage, B.M., Samant, S.D., 2005. Low temperature recyclable catalyst for Heck reactions using ultrasound. Tetrahedron Letters 46, 2483–2485. Ballini, R., Bosica, G., Carloni, S., Ciralli, L., Maggi, R., Sartori, G., 1998. Zeolite HSZ-360 as a new reusable catalyst for the direct acetylation of alcohols and phenols under solventless conditions. Tetrahedron Letters 39, 6049–6052. Dana, J.D., Dana, E.S., 1997. Dana's New Minerology, John Wiley, New York, 1820. Carey, J.S., Laffan, D., Thomson, C., Williams, M.T., 2006. Analysis of the reactions used for the preparation of drug candidate molecules. Organic and Biomolecular Chemistry 4, 2337–2347. Chakraborti, A.K., Gulhane, R., 2003. Perchloric acid adsorbed on silica gel as a new, highly efficient, and versatile catalyst for acetylation of phenols, thiols, alcohols, and amines. Chemical Communication 1896–1897. Chauhan, K.K., Frost, C.G., Love, I., Waite, D., 1999. Indium triflate: an efficient catalyst for acylation reactions. Synlett 1743–1744. Choudary, B.M., Bhaskar, V., Kantam, M.L., Koteswara Rao, K., Raghavan, K.V., 2000. Acylation of alcohols with carboxylic acids via the evolution of compatible acidic sites in montmorillonites. Green Chemistry 2, 67–70. Choudary, B.M., Bhaskar, V., Kantam, M.L., Koteswara Rao, K., Raghavan, K.V., 2001a. Acylation of amines with carboxylic acids: the atom economic protocol catalysed by Fe(III)-Montmorillonite. Catalysis Letters 74, 207–211. Choudary, V.R., Jana, S.K., Patil, N.S., 2001b. Acylation of benzene over clay and mesoporous Si-MCM-41 supported InCl3, GaCl3 and ZnCl2 catalysts. Catalysis Letters 76, 235–239. Choudary, V.R., Patil, K., Jana, S.K., 2004. Acylation of aromatic alcohols and phenols over InCl3/montmorillonite K-10 catalysts. Journal of Chemical Science 116, 175–177. Deshmukh, R.R., Rajagopal, R., Srinivasan, K.V., 2001. Ultrasound promoted C–C bond formation: Heck reaction at ambient conditions in room temperature ionic liquids. Chemical Communication 1544–1545. Gaplovsky, A., Gaplovsky, M., Toma, S., Luche, J.-L., 2000. Ultrasound effects on the photopinacolization of benzophenone. Journal of Organic Chemistry 65, 8444–8447. Gholap, A.R., Venkatesan, K., Pasricha, R., Daniel, T., Lahoti, R.J., Srinivasan, K.V., 2005. Copper and ligand-free sonogashira reaction catalyzed by Pd(0) nanoparticles at ambient conditions under ultrasound irradiation. Journal of Organic Chemistry 70, 4869–4872. Gosh, R., Maiti, S., Chakraborty, A., 2005. Facile catalyzed acylation of alcohols, phenols, amines and thiols based on ZrOCl2·8H2O and acetyl chloride in solution and in solvent-free conditions. Tetrahedron Letters 46, 147–151. Green, T.W., Wuts, P.G.M., 1999. Protective Groups in Organic Synthesis, 3rd ed. Wiley, New York. Izumi, J., Shiina, I., Mukaiyama, T., 1995. An efficient esterification reaction between equimolar amounts of free carboxylicacids and alcohols by the combined use of
octamethylcyclotetrasiloxane and a catalytic amount of titanium tris (trifluoromethane- sulfonate). Chemistry Letters 141–142. Ishihara, K., Kubota, M., Kurihara, H., Yamamoto, H., 1996. Scandium trifluromethane sulfonate as an extremely active Lewis acid catalyst in acylation of alcohols with acid anhydrides. Journal of Organic Chemistry 61, 4560–4567. Kantam, M.L., Bhaskar, V., Choudary, B.M., 2001. Direct condensation of carboxylic acids with alcohols: the atom economic protocol catalysed by Fe(III)-montmorillonite. Catalysis Letters 78, 185–188. Khan, A.T., Islam, S., Majee, A., Chattopadhyy, T., Gosh, S., 2005. Bromodimethyl sulfonium bromide: a useful reagent for acylation of alcohols, phenols, amines, thiols, thiophenols and 1,1-diacylation of aldehydes under solvent free conditions. Journal of Molecular Catalysis A: Chemical 239, 158–165. Krishna Mohan, K.V.V., Narender, N., Kulkarni, S.J., 2006. Zeolite catalyzed acylation of alcohols and amines with acetic acid under microwave irradiation. Green Chemistry 8, 368–372. Kumar, A.K., Chattopadhyay, T.K., 1987. Catalytic role of diorganotin dichloride in esterification of carboxylic acids. Tetrahedron Letters 28, 3713–3714. Laura, R.D., 1976. Surface acidity of imogolite and allophane. Clay Minerals 11, 331–334. Lee, J.C., Tai, C.A., Hung, S.C., 2002. Sc(OTf)3-catalyzed acetolysis of 1,6-anhydro-βhexopyranoses and solvent-free per-acetylation of hexoses. Tetrahedron Letters 43, 851–855. Luche, J.L., 1998. Reactivity and selectivity under microwaves in organic chemistry. Relation with medium effects and reaction mechanisms. Synthetic Organic Sonochemistry. Plenum Press, New York. Nakae, Y., Kusaki, I., Sato, T., 2001. Lithium perchlorate catalyzed acetylation of alcohols under mild reaction conditions. Synlett 1584–1586. Namoodiri, V.V., Varma, R.S., 2002. Solvent-free sonochemical preparation of ionic liquids. Organic Letters 4, 3161–3163. Narender, N., Srinivasu, P., Kulkarni, S.J., Raghavan, K.V., 2000a. Liquid phase acylation of alcohols with acetic acid over zeolites. Synthetic Communication 30, 1887–1893. Narender, N., Srinivasu, P., Kulkarni, S.J., Raghavan, K.V., 2000b. Liquid phase acylation of amines with acetic acid over HY zeolite. Green Chemistry 2, 104–105. Orita, A., Tanahashi, C., Kakuda, A., Otera, J., 2001. Highly powerful and practical acylation of alcohols with acid anhydride catalyzed by Bi(OTf)3. Journal of Organic Chemistry 66, 8926–8934. Parac-Vogt, T.N., Deleersnyder, K., Binnemans, K., 2005. Lanthanide(III) tosylates as new acylation catalysts. European Journal of Organic Chemistry 1810–1815. Pinnavaia, T.J., 1983. Intercalated clay catalysts. Science 220, 365–371. Phukan, P., 2004. Iodine as an extremely powerful catalyst for the acetylation of alcohols under solvent-free conditions. Tetrahedron Letters 45, 4785–4787. Procopious, P.A., Baugh, S.P.D., Flack, S.S., Inglis, G.G.A., 1998. An extremely powerful acylation reaction of alcohols with acid anhydrides catalyzed by trimethylsilyl trifluromethane sulfonate. Journal of Organic Chemistry 63, 2342–2347. Ranu, C.B., Dey, S.S., Hajra, A., 2003. Highly efficient acylation of alcohols, amines and thiols under solvent-free and catalyst-free conditions. Green Chemistry 5, 44–46. Ross, N.A., MacGregor, R.R., Bartsch, R.A., 2004. Synthesis of β-lactams and βaminoesters via high intensity ultrasound-promoted Reformatsky reactions. Tetrahedron 60, 2035–2041. Sarvanan, P., Singh, V.K., 1999. An efficient method for acylation reactions. Tetrahedron Letters 40, 2611–2614. Sarvari, M.H., Sharghi, H., 2005. Zinc oxide (ZnO) as a new, highly efficient, and reusable catalyst for acylation of alcohols, phenols and amines under solvent free conditions. Tetrahedron 61, 10903–10907. Shiina, I., Mukaiyama, T., 1994. A novel method for the preparation of macrolides from w- hydroxycarboxylic acids. Chemistry Letters 677–680. Shrini, F.M., Zolfigol, M.A., Abedini, M., Salehi, P., 2003. Al(HSO4)3 catalyzed acetylation and formylation of alcohols. Bulletin of Korean Chemical Society 24, 1683–1685. Sreedhar, B., Reddy, P.S., 2007. Sonochemical synthesis of 1,4-disubstituted 1,2,3triazoles in aqueous medium. Synthetic Communication 37, 805–812. Sreedhar, B., Bhaskar, V., Sridhar, Ch., Srinivas, T., Laszlo, Kotai, Klara, Szentmihalyi, 2003. Acylation of alcohols and amines with carboxylic acids: a first report catalyzed by iron(III) oxide-containing activated carbon. Journal of Molecular Catalysis A: Chemical 191, 141–147. Sreedhar, B., Reddy, P.S., Veda Prakash, B., Ravindra, A., 2005. Ultrasound-assisted rapid and efficient synthesis of propargylamines. Tetrahedron Letters 46, 7019–7022. Steglich, W., Hofle, G., 1969. N,N-dimethyl-4-pyridinamine, a very effective acylation catalyst. Angewandte Chemie International Edition in English, 8, 981. Suslick, K.S., 1988. Ultrasound: Its Chemical, Physical and Biological Effects. VCH: Weinheim, New York. Tammaddon, F., Amrollahi, M.A., Sharafat, L., 2005. A green protocol for chemoselective O-acylation in the presence of zinc oxide as a heterogeneous, reusable and ecofriendly catalyst. Tetrahedron Letters 46, 7841–7844. Vedejs, E., Diver, S.T., 1993. Tributylphosphine: a remarkable acylation catalyst. Journal of American Chemical Society 115, 3358–3359. Vedejs, E., Mackay, J.A., 2001. Kinetic resolution of allylic alcohols using a chiral phosphine catalyst. Organic Letters 3, 535–536. Vedejs, E., Bennet, N.S., Conn, L.M., Diver, S.T., Gingras, M., Lin, S., Oliver, P.M., Peterson, M.J., 1993. Tributylphosphine-catalyzed acylations of alcohols: scope and related reactions. Journal of Organic Chemistry 58, 7286–7288. Velusamy, S., Borpuzari, S., Punniyamurthy, T., 2005. Cobalt(II)-catalyzed direct acetylation of alcohols with acetic acid. Tetrahedron 61, 2011–2015.