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Microwave assisted solvent-free synthesis of some imine derivatives.

1. Introduction

Compounds containing the -C=N- (azomethine group) structure are known as Schiff bases, usually synthesized from the condensation of primary amines and active carbonyl groups [1]. The reaction is acid-catalyzed and is generally carried out by refluxing the carbonyl compound and amine, with an azeotroping agent if necessary, and separating the water as formed [2]. Schiff bases are well known for their biological applications as antibacterial, antifungal, anticancer, and antiviral agents; furthermore, they have been used as intermediates in medical substrates and as ligands in complex formation with some metal ions [1,3]. The synthesis of imines was firstly reported by Hugo Schiff in 1864 and they have been known since then [4]. The imine compounds have been prepared using molecular sieves [5, 6], infrared irradiation [7], Mg[(Cl[O.sub.4]).sub.2] [8], [P.sub.2][O.sub.5]/Si[O.sub.2] [9], Zn[Cl.sub.2] [10], CaO under microwave power [11], ethyl lactate as a tunable solvent [12], K10 clay [13], Ti[Cl.sub.4] [14], alumina [15], Ce[Cl.sub.3] x 7[H.sub.2]O [16], ultrasound irradiation [17], polymer-supported [18], nanotube Ti[O.sub.2] (in sunlight) [19], and Ti[(OEt).sub.4] [20, 21].

The present work reveals the comparative aspects of condensation of some aromatic amines with aldehyde derivatives using microwave and conventional methods. The amine and aldehyde compounds as starting materials, [beta]-ethoxyethanol ([beta]-EE) as wetting reagent and microwave power as an effective source of heating are used in this study. The corresponding imine compounds were prepared in high yields and short reaction times using this effective and environment friendly method.

2. Results and Discussion

In this work, we synthesized quickly and efficiently a series imine derivatives (1a-7d) by condensation of some aromatic amines (1-7) and aldehyde derivatives such as salicylaldehyde (a), p-chlorobenzaldehyde (b), p-methoxybenzaldehyde (c) and cinnamaldehyde (d) under microwave-assisted solvent-free conditions using [beta]-EE as wetting reagent. [beta]-EE that is a polar molecule quickly absorbs microwaves and therefore heats up and heats around effectively. As a result, [beta]-EE, which increases the polarity of the reaction medium, has an active role in the heating of the reaction medium by microwaves.

The general reaction was summarized in Scheme 1. In addition, we tested the effect of different microwave power such as 180, 360, 600, 900 W and detected the microwave power of 180 W and 360 W are more appropriate choices for the reaction. Hence, the optimum microwave reaction conditions were determined using 180 and 360 W microwave power and neat and wetting with [beta]-EE for compound 1a. The highest reaction yield (94%) and shorter reaction time (1.5 min.) were obtained at 360 W microwave power with [beta]-EE. In the absence of wetting reagent, the reaction yields are 81% for 1.5 minutes and 88% for 5 minutes. It is understood that the reaction yield was increased by wetting reagent that increases the polarity of the reaction medium. The optimization of microwave-assisted conditions was given in Table 1.

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In addition, we have synthesized some imine derivatives (1a, 4a, 1b, 4b, 1c, 4c, 1d, 4d) under classical conditions using a solvent (azeotropic) and traditional heating source (hot plate) for comparison with microwave conditions. Firstly, synthesis of compound 1a under the classical reaction conditions was tested atvarious periods of time (30, 60, 120, 240, 1300 min) to determine the optimum reaction time, which was determined as 120 minutes. Information on the optimization of the reaction time of compound 1a for classic conditions was presented in Table 2.

The compounds 1a, 4a, 1b, 4b, 1c, 4c, 1d, and 4d were obtained at 76%, 73%, 63%, 79%, 67%, 73%, 67%, and 75% yields, respectively, in 120 minutes under the classical reaction conditions, whereas in microwave conditions, the corresponding reaction yields were 94%, 97%, 95%, 98%, 91%, 97%, 95%, and 96%, respectively, in 1.0-1.5 minutes. All the results such as reactions time, yields, and melting points of the compounds were presented in Table 3. In addition, the yields of microwave and conventional reaction conditions for some imines were expressed graphically in Figure 1.

3. Conclusion

In conclusion, we have developed a simple microwave assisted solvent-free method for the synthesis of imines using a wetting reagent ([beta]-ethoxyethanol). The method works well for the reaction of this type amines and aldehydes. Because, we have synthesized imine compounds for good yields and fast reaction times in our method. Our method has some advances such as higher reaction yields, shorter reaction times, and green reaction conditions than classical requirements used of a solvent and conventional heat source (like hot plate) and other some microwave methods used of catalyst and solid supports. In addition, some imines were synthesized for the first time by us with this study.

4. Experimental

All of the chemicals were obtained from commercial sources or prepared according to standard methods. Melting points were determined with an Electrothermal 9100 apparatus. The infrared absorption spectrum of the compounds has been recorded in the region at 4000 and 400 [cm.sup.-1] range using a Bruker Vertex 80/80 v spectrophotometer using KBr pressed pellet technique at room temperature. The [sup.1]H NMR and [sup.13]C NMR spectra were recorded using a Bruker AC 200 NMR 200 MHz spectrometer in CD[Cl.sub.3]-[d.sub.1] and DMSO-[d.sub.6] using TMS as the internal standard. Elemental analyses were conducted at the METU Central Laboratory using a LECO, CHNS-932. All syntheses were carried out in A Bosh HMT 812 C modified microwave oven.

General Procedure for Preparation of 1a-7d. Aldehyde (1 mmoL), amine (1 mmoL) and two drops of [beta]-ethoxyethanol as a wetting reagent were mixed in a beaker. Then the beaker was placed in the microwave oven and was exposed to microwave irradiation (360 W). The product obtained by reaction was washed with cold ethanol. Then, it was recrystallized in ethanol. The physical and spectra data of the compounds 3a, 2b, 3b, 2c, 3c, 2d, and 3d are as follows.

N-(Salicylidene)-2-hydroxy-4-methylaniline (3a). M.p. 199-202[degrees]C. IR: 3033, 2965, 2916, 2858, 1612 (C=N), 1592, 1527, 1486, 1422, 1278, 1221, 1141, 1113, 1012, 869, 788, 754, 720, 622, 587, 556 [cm.sup.-1]. [sup.1]H NMR (DMSO-[d.sub.6]): [delta] 8.94 (s, 1H, N=CH), 13.90 (s, 1H, OH), 9.65 (s, 1H, OH). [sup.13]C NMR (DMSO-[d.sub.6]): [delta] 160.54, 160.36, 150.82, 137.56, 132.36, 132.25, 131.92, 120.17, 119.46, 119.14, 118.46, 116.93, 116.48. Elemental Anal. Calcd. for [C.sub.14][H.sub.13]N[O.sub.2]: C, 73.99; H, 5.77; N, 6.16. Found: C, 72.81; H, 5.64; N, 6.16%.

N-(p-Chlorobenzylidene)-2-hydroxy-5-chloroaniline (2b). M.p. 123-125[degrees]C. IR: 3367, 3086, 3071, 3059, 2910, 2894, 1626 (C=N), 1588, 1569, 1478, 1424, 1370, 1274, 1234, 1194, 1154, 1087, 1011, 911, 857, 815, 799 (C-Cl), 697, 658, 608, 548, 533 [cm.sup.-1]. [sup.1]H NMR (CD[Cl.sub.3]): [delta] 8.66 (s, 1H, N=CH), 7.91 (d, J = 8.4 Hz, 2H, Ar), 7.54 (d, J = 8.4 Hz, 2H, Ar), 7.34 (s, 1H, Ar), 7.24 (d, J = 8.6 Hz, 1H, Ar), 7.02 (d, J = 8.6 Hz, 1H, Ar). [sup.13]C NMR (CD[Cl.sub.3]): [delta] 156.75 (N=C), 150.94, 138.21, 135.86, 133.87, 130.06, 129.28, 128.77, 125.07, 116.19, 116.16. Elemental Anal. Calcd. for [C.sub.13][H.sub.9][Cl.sub.2]NO: C, 58,67; H, 3,41; N, 5,26. Found: C, 57.74; H, 3.42; N, 5.26%.

N-(p-Chlorobenzylidene)-2-hydroxy-4-methylaniline (3b). M.p. 134-137[degrees]C. IR: 3367, 3063, 3049, 3028, 2907, 2857, 1629 (C=N), 1576, 1569, 1497, 1370, 1344, 1295, 1242, 1169, 1154, 1080, 1011, 947, 874, 857, 828, 808 (C-Cl), 793, 733, 687, 610, 591, 579 [cm.sup.-1]. [sup.1]H NMR (CD[Cl.sub.3]): [delta] 8.72 (s, 1H, N=CH), 7.92 (d, J = 8.3 Hz, 2H, Ar), 7.53 (d, J = 8.4 Hz, 2H, Ar), 7.29 (d, J = 8.1 Hz, 1H, Ar), 6.92 (s, 1H, Ar), 6.80 (d, J = 8.0 Hz, 1H, Ar), 2.42 (s, 3H, C[H.sub.3]). [sup.13]C NMR (CD[Cl.sub.3]): [delta] 154.09 (N=C), 152.34, 139.81, 137.40, 134.54, 132.60, 129.72, 129.16, 120.93, 115.67,115.36,21.48. Elemental Anal. Calcd. for [C.sub.14][H.sub.12]ClNO: C, 68.44; H, 4.92; N, 5.70. Found: C, 67.64; H, 4.89; N, 5.69%.

N-(p-Methoxybenzylidene)-2-hydroxy-5-chloroaniline (2c). M.p. 86-88[degrees]C. IR: 3323, 3017, 2970, 2954, 2927, 2840, 1626 (C=N), 1599, 1569, 1511, 1453, 1312, 1250, 1165, 1027, 804, 650, 594 [cm.sup.-1]. [sup.1]H NMR (CD[Cl.sub.3]): [delta] 9.88 (s, 1H, OH), 8.55 (s, 1H, N=CH), 7.85 (d, J = 8.5 Hz, 2H, Ar), 7.23 (s, 1H, Ar), 7.11 (d, J = 8.6 Hz, 1H, Ar), 6.99 (d, J = 8.5 Hz, 2H, Ar), 6.90 (d, J = 8.6 Hz, 1H, Ar), 3.88 (s, 3H, OC[H.sub.3]). [sup.13]C NMR (CD[Cl.sub.3]): [delta] 162.92, 157.76 (N=C), 150.70, 136.69, 130.85, 128.49, 127.81, 124.91, 116.08, 115.77, 114.33, 55.49. Elemental Anal. Calcd. for [C.sub.14][H.sub.12]ClN[O.sub.2]: C, 64.25; H, 4.62; N, 5,35. Found: C, 62.92; H, 4.57; N, 5.45%.

N-(p-Methoxybenzylidene)-2-hydroxy-4-methylaniline (3c). M.p. 86-87[degrees]C. IR: 3344, 3066, 3018, 2974, 2934, 2907, 2840, 1622 (C=N), 1593, 1567, 1509, 1481, 1423, 1378, 1245, 1162, 1026, 969, 909, 861, 835, 809, 761, 666, 617, 559 [cm.sup.-1]. [sup.1]H NMR (CD[Cl.sub.3]): [delta] 8.59 (s, 1H, N=CH), 7.83 (d, J = 8.7 Hz, 2H, Ar), 7.16 (d, J = 8.1 Hz, 1H, Ar), 6.97 (d, J = 8.7 Hz, 2H, Ar), 6.81 (s, 1H, Ar), 6.68 (d, J = 7.8 Hz, 1H, Ar), 3.86 (s, 3H, OC[H.sub.3]), 2.31 (s, 3H, C[H.sub.3]). [sup.13]C NMR (CD[Cl.sub.3]): [delta] 162.52, 155.25 (N=C), 152.02, 138.71, 133.68, 130.36, 129.13, 120.74, 115.34, 115.26, 114.30, 55.45, 21.41. Elemental Anal. Calcd. for [C.sub.15][H.sub.15]N[O.sub.2]: C, 74.67; H, 6.27; N, 5.81. Found: C, 74.63; H, 6.41; N, 6.21%.

N-(Cinnamylidene)-2-hydroxy-5-chloroaniline (2d). M.p. 91-93[degrees]C. IR: 3344, 3064, 3013, 2964, 2911, 2881, 2839, 1621 (C=N), 1577, 1501, 1483, 1445, 1366, 1240, 1154, 1082, 979, 837, 804, 742, 683, 661, 593 [cm.sup.-1] [sup.1]H NMR (CD[Cl.sub.3]): [delta] 8.41 (d, J = 8.2 Hz, 1H, N=CH), 7.54 (d, J = 5.2, 2H, Ar), 7.43-7.00 (m, 5H, Ar & CH=CH), 7.29 (s, 1H, Ar), 7.22 (d, J = 8.1Hz, 1H, Ar), 6.90 (d, J = 8.6Hz, 1H, Ar). [sup.13]C NMR (CD[Cl.sub.3]): [delta] 159.36, 150.95 (N=C), 145.23, 136.47, 135.35, 129.99, 128.99, 128.36, 128.11, 127.69, 124.94, 115.99, 115.86. Elemental Anal. Calcd, for [C.sub.15][H.sub.12]ClNO: C, 69.91; H, 4.69; N, 5.43. Found: C, 68.80; H, 4.72; N, 5.36%.

N-(Cinnamylidene)-2-hydroxy-4-methylaniline (3d). M.p. 97-98[degrees]C. IR: 3350, 3061, 3012, 2963, 2912, 2880, 2840, 1624 (C=N), 1577, 1541, 1498, 1447, 1368, 1257, 1157, 1092, 975, 836, 804, 749, 686, 630, 552 [cm.sup.-1] [sup.1]H NMR (CD[Cl.sub.3]): [delta] 8.57 (d, J = 6.6 Hz, 1H, N=CH), 7.66 (dd, J = 6.0,1.9 Hz, 2H, Ar), 7.54-7.12 (m, 5H, Ar & CH=CH), 730 (d, J = 79 Hz, 1H, Ar), 6.93 (s, 1H, Ar), 6.80 (d, J = 8.1 Hz, 1H, Ar), 2.43 (s, 3H, Ar). [sup.13]C NMR (CD[Cl.sub.3]): [delta] 156.85, 152.28 (N=C), 143.26, 139.38, 135.71, 133.20, 129.53, 128.92, 128.73, 127.46, 120.75, 115.48, 115.04, 21.46. Elemental Anal. Calcd, for [C.sub.16][H.sub.15]NO: C, 80.98; H, 6.37; N, 5,26. Found: C, 79.22; H, 6.18; N, 5.90%.

http://dx.doi.org/10.1155/2014/816487

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

The authors would like to thank the Middle East Technical University analysis Center (Turkiye) for elemental analyses and Ozgur Ozdamar (Assistant Professor) and Hasan Saral (Assistant Professor) for the analysis, NMR, and IR of some imine compounds.

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Yunus Bekdemir (1) and Kursat Efil (1,2)

(1) Department of Molecular Biology and Genetics, Faculty of Arts and Science, Canik Basari University, 55080 Samsun, Turkey

(2) Department of Chemistry, Faculty of Arts and Sciences, Ondokuz Mayis University, 55139 Samsun, Turkey

Correspondence should be addressed to Yunus Bekdemir; bekdemir@basari.edu.tr

Received 2 January 2014; Revised 23 February 2014; Accepted 24 February 2014; Published 26 March 2014

Academic Editor: Joseph E. Saavedra

TABLE 1: The optimization of microwave conditions for compound
1a.

Watt              Microwave method             Yield (%)

       Time (min)     Reaction conditions

180        5        Wetting (with [beta]-EE)      83
360        5        Wetting (with [beta]-EE)      93
360       1.5       Wetting (with [beta]-EE)      94
360        5                  Neat                88
360       1.5                 Neat                81

TABLE 2: The optimization of classical conditions for compound 1a.

Time (min)           Classical method

             Yield (%)   Reaction conditions

30              64             Reflux
60              73             Reflux
120             76             Reflux
240             75             Reflux
1300 *          74             Reflux

Procedure: 1 mmol aldehyde (10 mL of ethanol) and 1 mmol amine
(10 mL of ethanol) are mixed a balloon and then boiled under
reflux on a hot plate.

* 300 min (heated) + 1000 min (unheated).
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Author:Bekdemir, Yunus; Efil, Kursat
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Date:Jan 1, 2014
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