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Catalysis Communications 10 (2009) 833–837 Contents lists available at ScienceDirect Catalysis Communications journal homepage: www.elsevier.com/locate/catcom Bronsted acidic ionic liquid as an efficient and reusable catalyst for transesterification of b-ketoesters Ziyauddin S. Qureshi, Krishna M. Deshmukh, Malhari D. Bhor, Bhalchandra M. Bhanage * Department of Chemistry, Institute of Chemical Technology (Autonomous), University of Mumbai, N. Parekh Marg, Matunga, Mumbai – 400 019, India a r t i c l e i n f o Article history: Received 2 August 2008 Received in revised form 28 November 2008 Accepted 7 December 2008 Available online 27 December 2008 a b s t r a c t Halogen free Bronsted acidic ionic liquid N-methyl-2-pyrrolidone hydrogen sulfate has been efficiently used as a catalyst for the transesterification of b-ketoesters with variety of alcohols. The ionic liquid shows high catalytic activity and reusability with good to excellent yields of the desired products. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Bronsted acidic ionic liquid b-Ketoesters Alcohols N-Methyl-2-pyrrolidone hydrogen sulfate 1. Introduction Transesterification of b-ketoesters is one of the important reaction for the synthesis of esters and have variety of applications in pharmaceutical, agrochemical, chemical and polymer industries [1,2]. Transesterification is an equilibrium process and several methods have been reported for the transesterification of b-ketoesters using various bases, enzymes and antibodies [1,3]. Traditionally such transformations are carried out in presence of an acidic or basic catalyst to promote the rate of reaction. In this regard several protic acidic [4–6], and basic catalysts [7–11], have been reported in the literature. Solid Lewis acid catalysts such as distannoxanes [12], InI3 [13], Mo–ZrO2 [14], Ti(IV) alkoxides [3b], Zinc–I2 [15], and Nb2O5 [16] are also developed for the transesterification of b-ketoesters. However, many of these methods are not greener and developing a greener protocol for this transformation is still a challenging task. In recent years, ionic liquids (ILs) have been considered as a promising greener reaction media, which overcomes the disadvantages of both traditional molecular solvents and melt salts. These ILs are non-flammable, thermally stable and exhibits negligible vapor pressure (non-volatile) and offer the potential for recyclability. These properties make them advantageous and have found to be widely used in catalytic and non-catalytic reactions [17]. Recent trends in the field of ionic liquid chemistry are the catalytic applications of task-specific ILs to the organic transformations. * Corresponding author. Tel.: +91 22 24145616; fax: +91 22 24145614. E-mail address: [email protected] (B.M. Bhanage). 1566-7367/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2008.12.048 Transesterification of methyl acetoacetate (MAA) with alcohols was first reported by Ming et al. using ionic liquid-regulated sulfamic acid [18]. Bronsted acidic ionic liquid NH2SO3H [C3MIm]Cl shows satisfactory conversion rate and selectivity for transesterification, however it suffers from disadvantages like IL is used as solvent and it is required in a large amount (i.e., 10 g of ionic liquid and 1 g of sulfamic acid), moreover the halogen content makes it less attractive from greener prescriptive. Hence, the review of literature suggests that there are still challenges in developing a greener protocol for transesterification of b-ketoesters. Herein we report, a halogen free Bronsted acidic IL N-methyl-2pyrrolidone hydrogen sulfate as an efficient and reusable catalyst for transesterification of b-ketoesters (Scheme 1). The IL has several important features over prevalent transesterification systems such as (1) ½NMPþ HSO 4 shows better activity in catalytic amount, (2) Simpler synthetic procedure with high reproducibility, (3) Nmethyl-2-pyrrolidone as a source of cation is more economical and commercially available as compared to other imidazolium/ pyridinium counter parts. 2. Experimental 2.1. General All the chemicals were received from M/s S.D. Fine Chemicals Ltd., India and used without further purification. The reactions were carried out in a round bottom flask fitted with a distillation condenser for the removal of methanol/ethanol during reaction progress. 834 Z.S. Qureshi et al. / Catalysis Communications 10 (2009) 833–837 O N+ H HSO4- O 1 OR O 2 + R OH O O OR2 + R1OH o 80 C Scheme 1. Transesterification of b-ketoester using ½NMPþ HSO 4. 2.2. Synthesis of ionic liquids In this study, several ILs were synthesized according to the procedures reported in the literature [19]. These are included 1-butyl3-methylimidazolium p-toluenesulfonic acid ([BMIm]+Tsa), 1-butyl-3-methylimidazolium hydrogen sulfate ([BMIm]+HSO4), 1methylimidazolium hydrogen sulfate ([HMIm]+HSO4) and Nmethyl-2-pyrrolidone hydrogen sulfate ([NMP]+HSO4) (Scheme 2). 2.3. Characterization methods The IR spectra of ionic liquids were recorded on Buck Scientific M-500 IR Spectrometer using KBr pellets in the frequency range of 1750–1300 cm1 using pyridine as a probe for acidity measurement. The 1H NMR spectrum of the ionic liquid was recorded immediately after dissolution of complex in D2O with TMS as reference (400 MHz). 2.4. A typical experimental procedure for transesterification of bketoesters MAA (0.58 g, 5 mmol), 1-Butanol (0.48 g, 6.5 mmol) and [NMP]+HSO4 (0.049 g, 5 mol%) were placed in a 25 mL round bottom flask fitted with a distillation condenser. The reaction mixture was heated to 80 °C for desired time and then cooled to room temperature. MAA conversion as well as product formation was monitored by gas chromatography (30 m 0.32 mm1D-0.25 lm BP10). A small amount of water (2 5 mL) was added to the reaction mixture and transesterified product was extracted in toluene (3 10 mL). The residue was chromatographed on silica gel column, eluted with a mixture of ethyl acetate-pet ether, to afford pure b-ketoesters. The identities of the products were confirmed by using GC–MS (Shimadzu GC–MS QP 2010) as well as using authentic samples. 3. Results and discussion 3.1. Catalyst characterization The ionic liquid catalyst was characterized by spectroscopic techniques such as IR, 1H NMR. 3.1.1. Characterization of ionic liquid using infrared spectroscopy The Bronsted acidity of the ionic liquid was determined using pyridine as a probe molecule by monitoring the band range of 1350–1600 cm1 arising from its ring vibration modes [20,21]. The IR spectrum of pyridine shows a band at 1480 cm1 (Fig. 1a). N + H2SO4 O N+ H HSO4O Scheme 2. Synthesis of ionic liquid. Fig. 1. IR spectra: (a) neat pyridine, (b) pyridine with [NMP]+HSO4, (c) pyridine with [HMIm]+HSO4, (d) pyridine with [BMIm] +HSO4, (e) pyridine with [BMIm]+pTSA. Note: only IR of (b) shows notable peaks for Bronsted acidity at 1550 cm1. However, slight shift was observed in wave number after mixing pyridine with ionic liquid. The spectrum of ionic liquids shows a band at 1550 cm1 indicating the presence of Bronsted acid sites due to the formation of pyridinium ions (Fig. 1b–e). 3.1.2. Characterization of ionic liquid using 1H NMR [NMP]+HSO4: 1H NMR (400 MHz, CDCl3-d1, TMS): d 1.85 (m, 2H, J = 6.1 Hz), 2.28 (t, 2H, J = 7.4 Hz), 2.61 (s, 3H, J = 6.1 Hz), 3.32 (t, 2H, J = 7.1 Hz), 8.26 (bs, 1H). [BMIm]+HSO4: 1H NMR (400 MHz, D2O-d2, TMS): d 0.91 (t, 3H, J = 7.5 Hz), 1.25(m, 2H, J = 7.1 Hz), 1.75 (m, 2H, J = 7.4 Hz), 3.86 (s, 3H), 4.18 (t, 2H, J = 7.0 Hz), 7.74 (m, 2H), 7.79 (s, 1H), 9.25 (s, 1H). 3.2. Influence of various catalysts Several Bronsted acidic ionic liquids and solid acid catalysts were tested for their catalytic activity in transesterification of bketoesters with various alcohols. The prepared ILs is viscous clear liquid at room temperature. They are partially miscible in esters, miscible in water and alcohol and can be conveniently separated whereas solid acid catalysts were easily filtered. Transesterification of MAA with 1-butanol to give butyl acetoacetate using [NMP]+HSO4 as a catalyst was chosen as a model reaction for study. Various ILs and solid acid catalysts were screened for the transesterification of MAA with 1-butanol and the results are summarized in Table 1. It was found that [NMP]+HSO4 gave 80% conversion of MAA with 98% selectivity to transesterified product at 80 °C (entry 4), which indicates that it is an efficient catalyst compared to that of other Bronsted acidic ionic liquids and solid acid catalysts (entry, 5 and 6) in terms of conversion, selectivity and recyclability. 835 Z.S. Qureshi et al. / Catalysis Communications 10 (2009) 833–837 Table 1 Influence of catalysts for transesterification of MAA with 1-butanola. Entry Catalyst [BMIm] p-TSA 1 - O Conversionb (%) Selectivityc (%) Yieldd(%) 75 93 70 OH 81 90 73 OH 83 89 74 OH 80 98 79 OH 78 79 62 OH 35 71 25 Alcohol b-ketoester + O OH O O [BMIm] +HSO4 2 O O O [HMIm]+HSO4 3 O O O [NMP] +HSO4 4 O O O 5 O Amberlyst-15 O O 6 O Montmorillonite-K10 O b,c,d Conversion, selectivity and yield are based on GC analysis. Reaction conditions: MAA (5 mmol), 1-butanol (6.5 mmol), IL (5 mol%), reaction time = 3 h, temp. = 80 °C. d Yield of transesterified product. a Various reaction parameters such as reaction time, temperature and IL concentration were studied using Bronsted acidic IL [NMP]+HSO4 as an optimized catalyst (Table 2). The results for the transesterification of MAA with 1-butanol demonstrate that chemical equilibrium is reached within 3 h at 80 °C (entry 1). Increase in the reaction time and temperature does not offer any significant advantage (entries 2–5). MAA was found to undergo cyclization at higher temperature to form cyclic esters and prolonged reaction time leading to the formation of acylated esters and ethers as a side products with a change in color of the reaction mass (colorless to green). Lowering the temperature decreases the conversion of MAA without affecting the selectivity performance (entry 6). IL with various concentrations were also studied (entries 1, 7 and 8) and it was found that 5 mol% of IL is sufficient for increasing the conversion rate and yield. The reaction was also studied in absence of IL (entry 9) which gives 30% conversion indicating the significant role of IL as a catalyst for transesterification reaction. In order to have general applicability of the reaction, IL was further explored for transesterification of other b-ketoesters with a variety of alcohols. The results were summarized in Table 3. It was observed that IL [NMP]+HSO4 has a very high activity and excellent conversion/selectivity were obtained. b-Ketoesters and alcohols (1:1.2) were smoothly transesterified in high yields with easy separation of IL and corresponding products. A wide range of structurally varied open chain, cyclic and aromatic carboxylic esters underwent transesterification with different alcohols under mild reaction conditions. Transesterification of MAA with higher aliphatic alcohols gave the yield of ester in the range of 79–91% (entries 1–5); similar results were also obtained in case of transesterification of ethyl acetoacetate EAA (entries 12–14). Transesterification of aromatic/cyclic ester and secondary alcohols was found to be sluggish as compared to that of aliphatic esters/alcohols i.e., the transesterification of MAA and EAA gave lower yield (entries 6–8, and 15). A series of aliphatic, cyclic and aromatic b-ketoesters were efficiently transesterified to the desired products with good yields (entries 17–19). All the reactions were analyzed by GC and were run till optimum conversion of bketoesters has been reached so as to have maximum yield of transeterified products. Hence, reaction time varies depending upon the nature of the substrate. Transesterification of unsaturated alcohols is a challenging one due to its facile decarboxylation rearrangement, i.e., Coroll rearrangement [22]. However, [NMP]+HSO4 was found to give good results in transesterification of b-ketoesters with several unsaturated alcohols (entries 9–11 and 16). Table 2 Influence of various parameters on transesterification of MAA with 1-butanol. 3.4. Reusability of catalyst 3.3. Influence of time, temperature and catalyst concentration Entry MAA: 1-butanol: IL (mmol:mmol:mol%) Time/ (h) Temp./ (oC) Conversiona (%) Selectivityb (%) Yieldc (%) 1 2 3 4 5 6 7 8 9 5: 5: 5: 5: 5: 5: 5: 5: 5: 3 6 9 12 3 3 3 3 3 80 80 80 80 100 70 80 80 80 80 85 89 92 94 70 75 80 30 98 96 94 76 69 78 97 98 86 79 82 84 70 65 55 73 79 26 a,b,c 6.5: 6.5: 6.5: 6.5: 6.5: 6.5: 6.5: 6.5: 6.5: 5 5 5 5 5 5 2.5 10 - Conversion, selectivity and yield are based on GC analysis. In order to have greener protocol, recycling experiment were conducted for Bronsted acidic IL [NMP]+HSO4. IL from the reaction mixture was separated by adding water (2 5 mL) and the upper organic phase was decanted. The aqueous solution containing IL was dried in vacuum. The reactor containing the recovered IL was then charged with MAA and 1-butanol again. The process is followed for four successive cycles and slight decrease in the catalytic activity of IL was observed after fourth cycle Fig. 2. The slight decrease in yield might be ascribed to the slight deactivation of [NMP]+HSO4. Thus, [NMP]+HSO4 is found to be a recyclable catalyst for transesterification of b-ketoesters. 836 Z.S. Qureshi et al. / Catalysis Communications 10 (2009) 833–837 Table 3 [NMP]+HSO4 catalyzed transesterification of b-ketoesters with different alcoholsa. Entry 1 Alcohol b-ketoester O O Time (h) Conversionb (%) Selectivityc (%) Yieldd (%) 3 80 98 79 OH O O O 2 (CH2)3 OH 3 93 96 90 (CH2)4 OH 3 94 94 89 (CH2)5 OH 3 92 94 91 3 88 95 84 3.5 87 85 74 4 94 94 85 3.5 89 90 76 3 91 95 87 3 95 94 90 3 92 93 86 3 83 93 78 O O O 3 O O O 4 O O O OH 5 O O O 6 OH O O O 7 OH O O O 8 OH O O O OH 9 O O O 10 OH O O O 11 OH O O O OH 12 O O O 13 (CH2)3 OH 3 94 90 85 (CH2)5 OH 3 93 90 84 3.5 91 76 70 3 94 85 80 3.5 89 97 87 3.5 90 93 84 3.5 88 94 83 O O O 14 O O O 15 OH O O O 16 OH O O O 17 O OH O O 19 O O b,c,d OH O 18 a (CH2)3 O (CH2)5 OH Conversion, selectivity and yield are based on GC analysis. Reaction conditions: methyl acetoacetate (5 mmol), 1-butanol (6.5 mmol), IL (5 mol%), temp. = 80 °C, all products confirmed by GC–MS. Z.S. Qureshi et al. / Catalysis Communications 10 (2009) 833–837 Acknowledgement 100 Yield (%) 80 837 80 78 76 75 The financial assistance from Indira Gandhi Centre for Atomic Research (IGCAR) Kalpakkam, India is kindly acknowledged. 60 References 40 [1] (a) J. Otera, Chem. Rev. 93 (1993) 1449; (b) S. Benetti, R. Romagoli, Chem. Rev. 95 (1995) 1065. [2] R. Clemens, Chem. Rev. 86 (1986) 241. [3] (a) T. Fujita, M. Tanaka, Y. Norimine, H. Suemune, J. Org. Chem. 62 (1997) 3824; (b) G. Shapiro, M. Marzi, J. Org. Chem. 62 (1997) 7096. [4] C.E. Rehberg, C.H. Fisher, J. Am. Chem. Soc. 66 (1944) 1203. [5] C.E. Rehberg, W.A. Faucette, C.H. Fisher, J. Am. Chem. Soc. 66 (1944) 1723. [6] J.K. Haken, J. Appl. Chem. 13 (1963) 168. [7] R.W. Taft Jr., M.S. Newman, F.H. Verhoek, J. Am. Chem. Soc. 72 (1950) 4511. [8] J.H. Billman, W.T. Smith Jr., J.L. Rendall, J. Am. Chem. Soc. 69 (1947) 2058. [9] L. Osipow, F.D. Snell, W.C. York, A. Finchler, Ind. Eng. Chem. 48 (1956) 1459. [10] J.C. Gilbert, T.A. Kelly, J. 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Commun. 9 (2008) 89. [20] E.P. Parry, J. Catal. 2 (1963) 371. [21] Y.I. Yang, Y. Kou, Chem. Commun. (2004) 226. [22] W. Kamel, A.C. Cope, J. Am. Chem. Soc. 65 (1943) 1992. 20 0 1 2 3 Recycle number 4 Fig. 2. Recyclability of catalyst: reaction conditions: MAA (5 mmol), 1-butanol (6.5 mmol), IL (5 mol%), reaction time = 3 h, temp. = 80 °C, yield is based on GC analysis. 4. Conclusion In conclusion, the transesterification of b-ketoester with the variety of alcohols is demonstrated using the IL [NMP]+HSO4 as a novel Bronsted acid catalyst. It has several advantages; (1) The IL [NMP]+HSO4 shows better catalytic activity and high yields, (2) The economical aspects in the preparation of [NMP]+HSO4 makes it more attractive, (3) The halogen free IL is more advantageous and solvent free conditions meets the greener aspects in catalysis, (4) [NMP]+HSO4 could be easily recycled after separation.