A simple and efficient procedure for the Knoevenagel condensation catalyzed by [MeHMTA]BF 4 ionic liquid

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1 Indian Journal of Chemistry Vol. 54B, September 2015, pp A simple and efficient procedure for the Knoevenagel condensation catalyzed by [MeHMTA]BF 4 ionic liquid Sanjoy Keithellakpam, Nimalini Moirangthem & Warjeet S Laitonjam* Department of Chemistry, Manipur University, Canchipur, Imphal , India warjeetlaitonjam@yahoo.com Received 22 September 2014; accepted (revised) 7 April 2015 Hexamethylenetetramine-based ionic liquid, 1-methylhexamethylenetetraminium tetrafluoroborate, has been used as a catalyst for a simple and efficient method for Knoevenagel condensation of active methylene compounds and various carbonyl compounds affording products of very high yields within short duration. This method is very simple, clean and avoids hazardous organic solvents. Most of the products require no further purification. In this process, after stirring for a few minutes, the product is isolated in pure form for both solid and liquid products. The catalyst is easily recovered and can be recycled. Keywords: Active methylene compounds, carbonyl compounds, hexamethylenetetramine-base, Ionic liquids, knoevenagel condensation, recyclable. In recent years ionic liquids have emerged as environmentally benign solvents and catalysts for many important organic reactions 1. The Knoevenagel condensation reaction is a useful reaction in organic synthesis that has been employed for carbon-carbon bond formation. It is usually performed in organic solvents in presence of common bases such as ammonia, primary or secondary amines and their salts 2. A number of catalysts have been used for the Knoevenagel condensation. A few of them includes Al 2 O 3 3, CdI 4 2, Rare-earth exchanged NaY zeolite 5, NH 4 OAc 6, Zirconium tetrachloride-sio 7 2, Lewis acidic ionic liquids [Bmim]Cl.xAlCl 3 and [Bpy]-Cl.xAlCl 8 3, ethylenediammonium diacetate in ionic liquids 9, K 2 O-Al 2 O 10 3, CaO 11, triphenylphosphane 12, Ti(O-i-Pr) 13 4, ethylammoniumnitrate 14, triethylbenzylammonium chloride 15, 1-aminoethyl-3-methylimidazolium hexafluorophosphate 16, I 2 -K 2 CO 17 3, 2-hydroxyethylammoniumacetate 18, methoxy propylamine acetate 19, Knoevenagel Condensation catalyzed by 1,1,3,3-Tetramethylguanidium Lactate 20, Sodium carbo-nate 21, Piperidinium acetate 22, Proline-functionalized polyacrylonitrile fiber 23. However, these methods have some limitations, such as using hazardous solvents, high catalyst loading or non-recoverable catalysts that sometimes contain toxic metals. Therefore, there is a great need for new catalytic methods that do not have these problems. In continuation of our work 24, herein we report hexamethylenetetramine-based ionic liquids as simple, efficient and environmentally friendly catalyst for Knoevenagel condensation reaction of various carbonyl compounds with active methylene compounds at room temperature in water, providing excellent yields. The catalysts can be recycled many times without much loss of activity. Results and Discussion The catalyst was prepared with modification according to the procedure reported previously 25. The Knoevenagel condensation reaction of benzaldehyde and malononitrile was carried out in the presence of [MeHMTA]BF 4 as the catalyst. The reaction was very fast and the reaction mixture solidified as soon as the catalyst was added (Entry 1, Table I). The effect of the concentration of the catalyst was also studied (Entries 1-4, Table I). When the amount of the catalyst used was changed, we found that 15 mol% of the catalyst gave quantitative yield of the product (Entry 3, Table I). When the amount of the catalyst, [MeHMTA]BF 4 was further reduced to 1.0 mol%, the reaction required more time to complete; however, the yields were still very high. Next, the Knoevenagel condensation reactions of benzaldehyde and malononitrile were studied by using various catalysts, such as, [MeHMTA]PF 6, and [MeHMTA]I. It was found that the reaction proceeded efficiently but there was slight change in reaction time (Entries 5-6, Table I). However, in the absence of these catalysts, the Knoevenagel reaction was very slow and the yield also very low (Entry 7, Table I). Thus, [MeHMTA]BF 4 (15 mol%) was taken as the catalyst of choice for the Knoevenagel condensation reactions. The Knoevenagel condensation reactions of various carbonyl compounds with active methylene compounds were examined in presence of [MeHMTA]BF 4 (15 mol%) and the products were isolated with excellent yields (Table II). In general, Knoevenagel condensation reactions of various carbonyl compounds with malononitrile were faster than ethylcyanoacetate.

2 1158 INDIAN J. CHEM., SEC B, SEPTEMBER 2015 This may be because of strong electron withdrawing nature of nitrile groups compare to ester group. The formation of the condensation products catalyzed by [MeHMTA]BF 4 was described by the following plausible mechanism (Scheme I). The recyclability of the catalyst was also examined (Table III). Once product 3a had been filtered from the reaction mixture, excess water from the ionic liquid was evaporated under reduced pressure, and the catalyst was reused for the same reaction. The catalyst did not show much reduction of activity even after the sixth run. All reactions were completed in 1-2 min affording % yields. Experimental Section All the reagents were commercial reagents of A.R. grade and were used without further purification. All the commercial solvents were used after distillation. Melting points were determined by using Veego melting point apparatus-i and were uncorrected. 1 H and 13 C Table I Knoevenagel condensation of benzaldehyde and malononitrile in different concentration and in different ionic liquids a Entry Catalyst Time (min) Yields (%) f 1 b [MeHMTA]BF 4 5s 82 2 c [MeHMTA]BF 4 30s 89 3 d [MeHMTA]BF e [MeHMTA]BF d [MeHMTA]PF 6 1h 92 6 [MeHMTA]I d 4h 85 7 Neat 24h 20 a Reaction conditions: benzaldehyde (2 mmol), malononitrile (2 mmol), catalyst [MeHMTA]BF 4, RT; b Catalyst 30 mol% (145.2 mg); c Catalyst 20 mol% (96.8 mg); d Catalyst 15 mol% (72.6 mg); e Catalyst 5 mol% (24.2 mg); f Isolated yield of the product. NMR spectra were recorded on a Bruker AV III 500 MHz or Varian 400 MHz or 300 MHz spectrometer in CDCl 3 or D 2 O. 1 H and 13 C NMR chemical shifts (δ) in ppm were downfield from tetramethylsilane. Elemental analyses were done in Perkin Elmer CHNS/O Analyzer Synthesis of catalyst The catalyst was prepared with modification according to the procedures reported previously 25. To a solution of 1-methylhexamethylenetetraminium iodide (3.5 g, 12.4 mmol) in 20 ml of water, solid NaBF 4 (1.36 g, 12.4 mmol) was added and the mixture was stirred vigorously at RT for 1 hr. The precipitate, NaI Table II Knoevenagel condensation catalyzed by [MeHMTA]BF 4 a Entry 1 W 1 W 2 Product Time (min) Yield (%) b 1 PhCHO CN CN 3a PhCHO CN CO 2 Et 3b MeOPhCHO CN CN 3c MeOPhCHO CN CO 2 Et 3d NO 2 PhCHO CN CN 3e NO 2 PhCHO CN CO 2 Et 3f ClPhCHO CN CN 3g ClPhCHO CN CO 2 Et 3h NO 2 PhCHO CN CN 3i NO 2 PhCHO CN CO 2 Et 3j C 2 H 5 CHO CN CN 3k C 3 H 7 CHO CN CN 3l Furfural CN CN 3m Cyclohexanone CN CN 3n 5 99 a Reaction conditions: 1 (2 mmol), 2 (2 mmol), catalyst [MeHMTA]BF 4 (72.6 mg, 15 mol%), RT; b Isolated yield of the product. Scheme I

3 NOTES 1159 Table III Recycling of the catalyst [MeHMTA]BF 4 in the Knoevenagel condensation of benzaldehyde and malononitrile a Run Time (min) Yield (%) b a Reaction conditions: 1a (2 mmol), 2a (2 mmol), catalyst ([MeHMTA]BF 4, 15 mol%), RT b Isolated yield of 3a was removed by filtration and the resulting filtrate was evaporated at reduced pressure. The colourless solid so obtained was washed with diethyl ether (3 20 ml) and dried to give pure ionic liquid, [MeHMTA]BF 4. 1 H NMR (500 MHz, D 2 O): δ 2.7 (s, 1 N + -CH 3 ), 4.8 (dd, 3 N-CH 2 N), 5.2 (s, 3 N + -CH 2 -N); 13 C NMR (125 MHz, D 2 O): δ 43.5 (N + -CH 3 ), 70.0 (N-CH 2 -N), 80.0 (N + -CH 2 -N); MS: m/z (%) 242 (100) [M + ]. Anal. Calcd for C 7 H 15 BF 4 N 4 : C, 34.74; H, 6.25; N, Found: C, 34.77; H, 6.20; N, 23.21%. General procedure for Knoevenagel condensation To a well stirred mixture of carbonyl compound, 1 (2.0 mmol) and active methylene compound, 2 (2.0 mmol), ionic liquid [MeHMTA]BF 4 (72.6 mg, 15 mol% of the substrate) was added and stirred at RT. The formation of the products was monitored by TLC. After completion of the reaction, the reaction mixture was filtered, washed with water (3 5 ml) and dried to obtain the products. In general, no further purification was required for solid product. However, for liquid mixture, ethyl acetate (2.0 ml) was added to the reaction mixture. The organic phase was dried with anhydrous MgSO 4 and evaporated. In some cases, the crude product was purified by column chromatography over silica gel to afford the pure product. All the products were previously reported and were characterized by melting point determination, 1 H and 13 C NMR spectral data. The ionic liquid catalyst was recovered from water and reused for the subsequent reactions. Selected data for the products are given below. 2-(Phenylmethylene)malononitrile, 3a: White solid, m.p C (lit C) (Ref 20); IR (KBr): 3032, 2224, 1591, 756 cm -1 ; 1 H NMR (400 MHz, CDCl 3 ): δ 7.5 (t, J = 8.0, 2H), 7.6 (t, J = 7.2, 2H), 7.7 (s, 1H), 7.9 d, J = 7.6, 2H); MS: m/z (%) 154 (100) [M + ]. Anal. Calcd for C 10 H 6 N 2 : C, 77.91; H, 3.92; N, Found: C, 77.80; H, 3.83; N, 18.09%. Ethyl-(E)-2-cyano-3-phenyl-2-propeonate, 3b: Colourless crystal, m.p C (lit C) (Ref 18); IR (KBr): 3032, 2983, 2222, 1726, 1604, 769 cm -1 ; 1 H NMR (400 MHz, CDCl 3 ): δ 1.4 (t, J = 7.2, 3H), 4.4 (q, J = 7.2, 2H), (m, 3H), 8.0 (d, J = 7.6, 2H), 8.3 (s, 1H); MS: m/z (%) 201 (100) [M + ]. Anal. Calcd for C 12 H 11 NO 2 : C, 71.63; H, 5.51; N, Found: C, 71.53; H, 5.41; N, 6.87%. 2-(4-Methoxyphenylmethylene)malononitrile, 3c: Yellow crystalline solid, C (lit C) (Ref 12); IR (KBr) 3028, 2850, 2222, 1604, 1570, 1450, 1155, 833 cm -1 ; 1 H NMR (300 MHz, CDCl 3 ): δ 3.9 (s, 3H), 7.01 (d, J = 8.0 Hz, 2H), 7.94 (d, J = 8.5, 2H), 8.15 (s, 1H), 8.7 (s,1h); 13 C NMR (75 MHz, CDCl 3 ): δ 55.7, 83.4, 113.5, 114.2, 124.5, 129.2, 158.8, 163.5; MS: m/z (%) 184 (100) [M + ]. Anal. Calcd for C 11 H 8 N 2 O: C, 71.73; H, 4.38; N, Found: C, 71.64; H, 4.35; N, 15.13%. Ethyl (E)-2-cyano-3-(4-methoxyphenyl)-2-propenoate, 3d: Pale yellow crystalline solid, m.p C (lit C) (Ref 18); IR (KBr): 3024, 2843, 2216, 1714, 1566, 1020, 839 cm -1 ; 1 H NMR (300 MHz, CDCl 3 ): δ 1.4 (t, J = 7.2, 3H), 3.9 (s, 3H), 4.4 (q, J = 6.9, 2H), 7.0 (d, J = 9, 2H), 8.0 (d, J = 9.0, 2H), 8.2 (s, 1H); 13 C NMR (75 MHz, CDCl 3 ): δ 14.1, 55.5, 62.3, 99.2, 114.7, 116.2, 124.2, 133.5, 154.3, 163.0, 163.7; MS: m/z (%) 231 (100) [M + ]. Anal. Calcd for C 13 H 13 NO 3 : C, 67.52; H, 5.67; N, Found: C, 67.56; H, 5.56; N, 5.95%. 2-(4-Nitrophenylmethylene)malononitrile, 3e: Yellow solid, m.p C (lit 161 C) (Ref 14); IR (KBr) 3084, 2227, 1595, 1535 cm -1 ; 1 H NMR (300 MHz, CDCl 3 ) δ H 7.8 (s, 1H), 8.0 (d, J 7.2, 2H), 8.20 (d, J 7.2, 2H); 13 C NMR (75MHz,CDCl 3 ) δ C 82.2, 112.8, 126.0, 129.2, 130.0, 134.8, 142.5, 159.7; MS: m/z (%) = 199 (100) [M + ]. Anal. Calcd for: C 10 H 5 N 3 O 2 : C, 60.31; H, 2.53; N, 21.10%. Found: C, 60.3; H, 2.59; N, 21.5%. Ethyl (E)- 3-(4-nitrophenyl)-2-cyano-2- propenoate, 3f: Dull yellow solid, m.p C (lit C)(Ref 20); IR (KBr) 3092, 2988, 2224, 1722, 1609, 1589, 1491, 831 cm -1 ; 1 H NMR (300 MHz, CDCl 3 ) δ H 1.40 (t, J 7.2, 3H), 4.4 (q, J 7.2, 2H), 7.9 (d, J 8.5, 2H), 8.2(d, J 8.5, 2H), 8.3 (s, 1H); MS: m/z (%) = 246 (100) [M + ]. Anal. Calcd for: C 12 H 10 N 2 O 4 : C, 58.54; H, 4.09; N, 11.38%. Found: C, 58.50; H, 4.00; N, 11.30%. 2-(4-Chlorophenylmethylene)malononitrile, 3g: Colourless solid, m.p C (lit 161 C)(Ref 20); IR (KBr) 3034, 2227, 1585, 823 cm -1 ; 1 H NMR (300 MHz, CDCl 3 ) δ H 7.5 (d, J 8.4, 2H), 7.7 (s, 1H), 7.9 (d,

4 1160 INDIAN J. CHEM., SEC B, SEPTEMBER 2015 J 8.4, 2H); 13 C NMR (75MHz,CDCl 3 ) δ C 83.2, 112.3, 113.4, 129.2, 130.0, 131.8, 141.1, 158.3; MS: m/z (%) = 188 (100) [M + ]. Anal. Calcd for: C 10 H 5 ClN 2 : C, 63.68; H, 2.67; N, 14.85%. Found: C, 63.59; H, 2.60; N, 14.74%. Ethyl-(E)-3-(4-chlorophenyl)-2-cyano-2-propenoate, 3h: White crystalline solid, m.p C (lit 89-90); IR (KBr): 3035, 2989, 2224, 1726, 1610, 1589, 1491, 831 cm -1 ; 1 H NMR (400 MHz, CDCl 3 ): δ 1.4 (t, J = 7.2, 3H), 4.42 (q, J = 7.2, 2H), 7.49 (d,j = 8.5, 2H), 8.0 (d, J = 8.6, 2H), 8.3 (s, 1H); MS: m/z (%) 235 (100) [M + ]. Anal. Calcd for C 12 H 10 ClNO 2 : C, 61.16; H, 4.28 N, Found: C, 61.10; H, 4.30; N, 5.89%. 2-(3-Nitrophenylmethylene)malononitrile, 3i: Yellow crystalline solid, m.p C (lit C) (Ref 20); IR (KBr): 3088, 2229, 1602, 1529, 823 cm -1 ; 1 H NMR (300 MHz, CDCl 3 ): δ 7.8 (t, J = 7.8, 1H), 7.9 (s, 1H), 8.3 (d, J = 7.8, 1H), 8.5 (d, J = 8.4, 1H), 8.7 (s,1h); 13 C NMR (75 MHz, CDCl 3 ): δ 86.7, 111.6, 112.6, 125.5, 128.2, 130.9, 131.9, 137.3, 148.6, 157.0; MS: m/z (%) 199 (100) [M + ]. Anal. Calcd for C 10 H 5 N 3 O 2 : C, 60.31; H, 2.53; N, Found: C, 60.30; H, 2.55; N, 21.09%. Ethyl(E)-3-(3-nitrophenyl)-2-cyanoacrylate, 3j: White solid, m.p C (lit ) (Ref 20); IR (KBr): 3095, 2987, 2225, 1720, 1609, 1570, 1356, 763 cm -1 ; 1 H NMR (400 MHz, CDCl 3 ): δ 1.43 (t, J = 7.2 Hz, 3H), 4.45 (q, J = 7.2 Hz, 2H), 7.77 (t, J = 8.0 Hz, 1H), 8.30 (s, 1H), (m, 2H), 8.71 (s, 1H); MS: m/z (%) 246 (100) [M + ]. Anal. Calcd for C 12 H 10 N 2 O 4 : C, 58.54; H, 4.09 N, Found: C, 58.50; H, 4.14; N, 11.29%. 2-Propylidenemalononitrile, 3k: Yellow oil, IR (KBr): 2972, 2197, 1571, 1471 cm -1 ; 1 H NMR (300 MHz, CDCl 3 ): δ 7.32 (t, J = 7.8, 1H), 2.60 (quintet, J = 7.2, 2H), 1.15 (t, J = 7.2, 3H); 13 C NMR (75 MHz, CDCl 3 ): δ 170.1, 114.2, 108.6, 89.4, 25.2, 13.1; MS: m/z (%) 106 (100) [M + ]. Anal. Calcd for C 6 H 6 N 2 : C, 67.90; H, 5.70; N, Found: C, 67.86; H, 5.72; N, 26.32%. 2-Butylidenemalononitrile, 3l: Light yellow oil, IR (KBr): 2937, 2229,1605 cm -1 ; 1 H NMR (300 MHz, CDCl 3 ): δ 7.26 (t, J = 7.7, 1H), (m, 2H), (m, 2H), 0.98 (t, J = 7.2, 3H); 13 C NMR (75 MHz, CDCl 3 ) δ 12.6, 23.5, 32.2, 85.0, 110.7, 113.0, 169.0; MS: m/z (%) 20 (100) [M + ]. Anal. Calcd for C 7 H 8 N 2 : C, 69.97; H, 6.71; N, Found: C, 69.91; H, 6.76; N, 23.21%. 2-(Furylmethylene)malononitrile, 3m: Solid, m.p C. IR (KBr): 3036, 2950, 2226, 1604, 1450, 760 cm -1 ; 1 H NMR (400 MHz, CDCl 3 ): δ 6.71 (dd, 1H, J = 1.6, 3.7 Hz), 7.37 (d, 1H, J = 3.7 Hz), 7.46 (d, 1H, J = 1.6 Hz), 7.89 (s, 1H); 13 C NMR (100 MHz, CDCl 3 ): δ 74.5, 111.0, 112.9, 113.5, 114.6, 123.0, 145.0, 148.0; MS m/z (%) 144 (100) [M + ]. Anal. Calcd for C 8 H 4 N 2 O: C, 66.67; H, 2.80; N, Found: C, 66.61; H, 2.85; N, 19.32%. 2-(Cyclohexylmethylene)malononitrile, 3n: Colurless oil, IR (KBr): , 1593 cm -1 ; 1 H NMR (400 MHz, CDCl 3 ): δ 7.21 (d, J = 9.8, 1H), (m, 1H), (m, 4H), (m, 6H); 13 C NMR (100 MHz, CDCl 3 ): δ 24.4, 26.0, 30.4, 43.0, 87.5, 102.5, 114.0, 170.4; MS m/z (%) 160 (100) [M + ]. Anal. Calcd for C 10 H 12 N 2 : C, 74.97; H, 7.55; N, Found: C, 74.90; H, 7.58; N, 17.41%. Conclusion In conclusion, it has been demonstrated that readily available 1-methylhexamethylenetetraminium tetrafluoroborate ionic liquid, [MeHMTA]BF 4 behaves as recyclable catalyst for the Knoevenagel condensation of various carbonyl compounds with active methylene compounds affording excellent yields in short durations. This method exhibits a simple, clean, and green manner, mild conditions and avoids hazardous organic solvents. Most of the products required no further purification. In this process, after stirring for a few minutes, the product was isolated in pure form for both solid and liquid products. The catalyst was easily recovered and could be reused more than six times without much loss of activity. The procedure can be applied for large-scale syntheses also. Thus, we have developed an improved process which offers several advantages over other processes and would contribute to environ-mentally friendly and safer processes. Acknowledgements One of the authors (SK) acknowledges the University Grants Commission, New Delhi for BSR-fellowship; and the Guwahati University, the SAIF, IIT Madras and the CIF, IIT Guwahati, India for the spectral data. References 1 (a) Welton T, Chem Rev, 99, 1999, 2071; (b) Wassercheid P & Keim W, Angew Chem Int Ed, 39, 2000, 3772; (c) Dupont J, de Souza R F & Suarez P A Z, Chem Rev, 102, 2002, 3667; (d) Wassercheid P & Welton T, Ionic Liquids in Synthesis, (Wiley-VCH, Weinheim), Jones G, Organic reactions (Wiley, New York), 15, 1967, 204; (b) Tietze L F, Chem Rev, 96, 1996, Texiey-Boullet F & Foucaud A, Tetrahedron Lett, 23, 1982, Prajapati D & Sandhu J S, J Chem Soc, Perkin Trans 1, 1993, 739.

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