1,3-thiazoles possess many biological properties such as anticancer , antimicrobial , anti-inflammatory , and anti-candida . Finding effective methods for the preparation of 1,3-thiazoles is a principal challenge. The synthesis of 1,3-thiazole derivatives have been reported in the presence of different catalysts including DBU , HClO4-SiO2 , Bi(SCH2COOH)3 , [Et3NH][HSO4] , and Ytterbium(III) Triflate . However, some of the reported methods endure drawbacks including long reaction times, harsh reaction conditions, use of toxic and non-reusable catalyst. To avoid the existing limitations, the exploration of an efficient catalyst with high catalytic activity and short reaction times for the preparation of 1,3-thiazoles is still favored. The feasibility of achieving one-pot multi-component synthesis under ultrasonic irradiation with a heterogeneous catalyst could improve the reactions rates and shorten the reactions times. The modifying cross-linked polyacrylamides make them attractive objects in chemistry and polymer science [10-12]. Sulfonated polyacrylamides have unique characteristics such as high strength, hydrophilicity, and proton conductivity [13-14]. Recently, magnetic nanoparticles (MNPs) have been successfully utilized to immobilize enzymes, polymers, transition metal catalysts and organocatalysts [15-16]. The ultrasound approach decreases time, and increases yields of products by creating the activation energy in micro surroundings [17-18]. In the current study, we investigate an easy and rapid method for the synthesis of thiazole-2(3H)-thione through three-component reactions of phenacyl bromide or 4-methoxyphenacyl bromide, carbon disulfide and primary amine using cross-linked sulfonated polyacrylamide (Cross-PAA-SO3H) attached to nano-Fe3O4, as an efficient catalyst under ultrasonic irradiations (Scheme 1).
Chemicals and apparatus
NMR spectra were obtained on a Bruker spectrometer with CDCl3 as the solvent and TMS as an internal standard. Chemical shifts (δ) are given in ppm and coupling constants (J) are given in Hz. FT-IR spectra were recorded with KBr pellets by a Magna-IR, spectrometer 550 Nicolet. CHN compositions were measured by Carlo ERBA Model EA 1108 analyzer. Powder X-ray diffraction (XRD) was performed on a Philips diffractometer of X’pert Company with monochromatized Cu Kα radiation (λ = 1.5406 Å). Microscopic morphology of products was visualized by SEM (MIRA3). The thermogravimetric analysis (TGA) curves are registered using a V5.1A DUPONT 2000. The magnetic measurement of samples was performed in a vibrating sample magnetometer (VSM) (Meghnatis Daghigh Kavir Co.; Kashan Kavir; Iran).
Preparation of Cross-linked Sulfonated Polyacrylamide (Cross-PAA-SO3H):
In a round-bottom flask (200 mL) equipped with magnetic stirrer and condenser, 5 gr of acrylamid (AAM) (70 mmol) and 5.17 gr of 2-acryloylamino-2-methylpropane-sulfonic acid (25 mmol) (AAMPS), (approximately AAM/AAMPS (3/1)) and 0.77 gr of N,N-methylene-bis-acrylamid (NNMBA) (5 mmol), as a crosslinking agent and benzoyl peroxide, as an initiator, were added to 80 mL EtOH under reflux condition for 5 h. After completion of reaction, the white precipitate was formed, filtered, washed and dried in a vacuum oven at 70ºC for 12 h. The weight of polymer was 10.1 gr with the yield of 91.8%. This catalyst was characterized with infrared spectroscopy and back titration acid-base to confirm sulfonation and determine accurate sulfonation levels. Acidic capacity of this catalyst was estimated to be 1.1 mmol /gr.
Preparation of cross-linked solfonated polyacrylamide@nano-Fe3O4
1 gr of synthesized polymers was poured in 100 mL round bottom flask under stirring at room-temperature, then 50 mL HCl (0.4 M) was added to it. Our target molecules was synthesized by magnetic nanocatalyst with mass ratio polymer/nano-Fe3O4 = 2/1. So, 0.43 gr (2.1 mol) FeCl2.4H2O and 1.17 gr (2×2.1) FeCl3.6 H2O were added and the mixture was stirred until dissolved completely (flask1). In another 500 ml round-bottom flask, No 2, 400 mL aqueous solution of NH3 (0.7M) was poured under argon gas. Then, flask 1 was added to flask 2 immediately. Nanocatalyst was filtered and washed with water (2×25 mL) and dried in an oven at 50 o C.
General procedure for the synthesis of 1,3-thiazoles
A mixture of primary amine (1.0 mmol) and carbon disulfide (1.0 mmol) in ethanol (8 mL) was stirred for 5 min, and then phenacyl bromide or 4-methoxyphenacyl bromide (1.0 mmol) and Cross-PAA-SO3H attached to nano-Fe3O4 (5 mg) were added and sonicated at 40 W power for the appropriate times (monitored by TLC). After completion of the reaction, the nanocatalyst was easily separated using an external magnet. The solvent was evaporated and the solid obtained was washed with EtOH to get pure product. The characterization data of the compounds are given below.
3-Benzyl-4-phenyl-1,3-thiazole-2(3H)-thione (4a): Colorless viscous oil; FT-IR (KBr): = 3102, 3005, 1602, 1479, 1202 cm-1; 1H NMR (250 MHz, CDCl3): δ 4.90 (s, 2H, CH2), 6.03 (s, 1H, CH of alkene), 6.95–7.36 (m, 10H, CH, ArH). 13C NMR (62.5 MHz, CDCl3): δ 47.24, 98.85, 127.06, 127.42, 128.52, 128.55, 129.08, 133.32, 137.45, 154.85, 178.37, 197.18. MS (EI, 70 eV): m/z (%) = 283 (5), 267(68), 181(7), 91 (100), 77 (4), 65 (12), 45 (4). Anal. Calcd. for C16H13NS2 (283): C, 67.81; H, 4.62; N, 4.94. Found: C, 67.70; H, 4.52; N, 4.73 %.
3-(3,4-dichlorobenzyl)-4-phenyl-1,3-thiazole-2(3H)-thione (4b): Colorless viscous oil; FT-IR (KBr): = 3152, 3004, 1628, 1603, 1477, 1302, 1104 cm-1. 1H NMR (250 MHz, CDCl3): δ 4.83 (s, 2H, CH2), 6.05 (CH of alkene), 6.75–7.97 (m, 8H, CH of ArH). 13C NMR (62.5 MHz, CDCl3): δ 46.07, 99.25, 126.72, 128.65, 128.74, 129.35, 129.66, 133.54, 130.50, 135.38, 136.62, 137.21, 172.70, 194.15. Anal. Calcd. for C16H11Cl2NS2 (350): C, 54.55; H, 3.15; N, 3.98. Found: C, 54.36; H, 3.05; N, 3.84 %.
3 - ( 2- Naphthyl methyl ) - 4 - phenyl - 1, 3 - thiazole 2(3H)- thione (4c): Colorless viscous oil; FT-IR (KBr): = 3102, 3009, 1652, 1605, 1479, 1204 cm-1. 1H NMR (250 MHz, CDCl3): δ 3.95 (s, 2H, CH2), 6.12 (s, 1H, CH of alkene), 6.92–7.97 (m, 12H, CH of ArH). 13C NMR (62.5 MHz, CDCl3): δ 45.35, 99.05, 123.77, 125.32, 125.84, 126.34, 128.06, 128.68, 128.75, 133.54, 122.52, 129.28, 131.50, 135.08, 172.44, 194.16. Anal. Calcd. for C20H15NS2 (333): C, 72.03; H, 4.53; N, 4.20. Found: C, 72.05; H, 4.40; N, 4.15 %.
3-(2-Furyl methyl)-4-phenyl-1,3-thiazole-2(3H)-thione (4d): Colorless viscous oil; FT-IR (KBr): = 3105, 3002, 1653, 1607, 1474, 1202 cm-1. 1H NMR (250 MHz, CDCl3): δ 4.84 (s, 2H, CH2), 6.10 (s, 1H, CH of alkene), 6.22 (1H, CH of furan), 7.25–8.05 (m, 7H, CH of ArH and CH of furan). 13C NMR (62.5 MHz, CDCl3): δ 44.25, 98.32, 109.52, 110.83, 127.08, 128.76, 129.58, 142.12, 144.54, 147.92, 155.44, 192.18. Anal. Calcd. for C14H11NOS2 (273): C, 61.51; H, 4.06; N, 5.12. Found: C, 61.46; H, 4.04; N, 5.09 %.
3-(4-Fluorobenzyl)-4-phenyl-1,3-thiazole-2(3H)-thione (4e): Colorless viscous oil; FT-IR (KBr): = 3153, 3005, 1628, 1604, 1473, 1302, 1108 cm-1. 1H NMR (250 MHz, CDCl3): δ 4.85 (s, 2H, CH2), 6.05 (s, 1H, CH of alkene), 6.85 (d, 2H, J = 6.8 Hz, CH arom), 7.02–7.59 (m, 5H, CH of ArH), 7.98 (d, 2H, J = 7.5 Hz, CH of ArH).13C NMR (62.5 MHz, CDCl3): δ 46.45, 99.08, 114.53, 128.67, 128.78, 133.51, 129.05, 135.46, 137.57, 153.28, 159.50, 194.19. Anal. Calcd. for C16H12FNS2 (301): C, 63.76; H, 4.01; N, 4.65. Found: C, 63.60; H, 4.04; N, 4.42 %.
3-(2-Methoxybenzyl)-4-phenyl-1,3-thiazole-2(3H) -thione (4f): Colorless viscous oil; FT-IR (KBr): = 3150, 3000, 1650, 1600, 1470, 1200, 1100 cm-1. 1H NMR (250 MHz, CDCl3): δ 3.62 (s, 3H, OCH3), 4.90 (s, 2H, CH2), 6.03 (s, 1H, CH of alkene), 6.71–7.98 (m, 9H, CH, ArH).13C NMR (62.5 MHz, CDCl3): δ 42.56, 55.05, 98.47, 110.05, 120.53, 128.38, 128.46, 128.55, 128.78, 129.05, 133.54, 127.12, 135.45, 156.35, 194.14. Anal. Calcd. for C17H15NOS2 (313): C, 65.14; H, 4.82; N, 4.47. Found: C, 65.03; H, 4.74; N, 4.35%.
3-(4-Methylbenzyl)-4-(4-methoxyphenyl)-1,3-thiazole-2(3H)-thione (4g): Colorless viscous oil; FT-IR (KBr): = 3156, 3008, 1648, 1612, 1475, 1206, 1108 cm-1. 1H NMR (250 MHz, CDCl3): δ 2.23 (s, 3H, CH3), 3.86 (s, 3H, OCH3), 4.95 (s, 2H, CH2), 5.98 (s, 1H, CH of alkene), 6.82–7.35 (m, 8H, CH of ArH).13C NMR (62.5 MHz, CDCl3): δ 21.35, 48.54, 55.95, 98.68, 115.38, 123.42, 125.64, 130.65, 131.25, 132.59, 139.25, 160.20, 174.25, 183.56. Anal. Calcd. for C18H17NOS2 (327): C, 66.02; H, 5.23; N, 4.28;. Found: C, 65.90; H, 5.14; N, 4.12 %.
3-benzyl-4-(4-methoxyphenyl)-1,3-thiazole-2 (3H)-thione (4h): Colorless viscous oil; FT-IR (KBr): = 3157, 3012, 1645, 1616, 1478, 1209, 1107 cm-1. 1H NMR (250 MHz, CDCl3): δ 3.89 (s, 3H, OCH3), 5.25 (s, 2H, CH2), 6.28 (s, 1H, CH of alkene), 6.85–7.39 (m, 9H, CH of ArH).13C NMR (62.5 MHz, CDCl3): δ 48.50, 55.37, 99.86, 110.55, 114.54, 122.54, 128.38, 129.54, 132.86, 137.54, 145.68, 160.85, 185.36. MS (EI, 70 eV): m/z (%) = 313 (M). Anal. Calcd. for C17H15NOS2 (313): C, 65.14; H, 4.82; N, 4.47; Found: C, 65.02; H, 4.56; N, 4.34; %.
3-(2-Furyl methyl)-4-(4-methoxyphenyl)-1,3-thiazole-2(3H)-thione (4i): Colorless viscous oil; FT-IR (KBr): = 3144, 3012, 1658, 1615, 1478, 1209, 1112 cm-1. 1H NMR (250 MHz, CDCl3): δ 3.88 (s, 3H, OCH3), 4.82 (s, 2H, CH2), 5.98 (2H, CH of furan), 6.20 (s, 1H, CH of alkene), 6.75–7.42 (m, 5H, CH of furan and CH of ArH). 13C NMR (62.5 MHz, CDCl3): δ 41.35, 55.34, 98.36, 108.35, 110.35, 118.35, 122.54, 130.22, 138.54, 142.35, 150.65, 161.25, 178.25. Anal. Calcd. for C15H13NO2S2 (303): C, 59.38; H, 4.32; N, 4.62; Found: C, 59.15; H, 4.14; N, 4.42%.
3-(2-methoxybenzyl)-4-(4-methoxyphenyl)-1,3-thiazole-2(3H)-thione (4j): Colorless viscous oil; FT-IR (KBr): = 3142, 3010, 1654, 1611, 1472, 1205, 1116 cm-1. 1H NMR (250 MHz, CDCl3): δ 3.68 (s, 3H, OCH3), 3.84 (s, 3H, OCH3), 4.89 (s, 2H, CH2), 6.05 (s, 1H, CH of alkene), 6.72–7.53 (m, 8H, CH of ArH). 13C NMR (62.5 MHz, CDCl3): δ 43.54, 56.45, 56.48, 98.45, 110.25, 115.28, 120.54, 122.54, 125.85, 125.64, 128.54, 130.42, 138.20, 158.64, 160.24, 172.54. Anal. Calcd. for C18H17NO2S2 (343): C, 62.94; H, 4.99; N, 4.08; Found: C, 62.72; H, 4.70; N, 3.91. %.
RESULTS AND DISCUSSION
Characterization of the nanocatalyst
In this study, we synthesized the cross-linked sulfonated polyacrylamide (Cross-PAA-SO3H) with simultaneous radical co-polymerization in the presence of initiator and cross-linking agent. The FT-IR absorbance spectra of the dried cross-linked sulfonated polyacrylamide (poly AAM-co-AAMPS), Fe3O4 and Cross-PAA-SO3H@nano-Fe3O4 are shown in Fig. 1. AAM is abbreviation of acrylamide, and AAMPS is abbreviation of 2-acrylamido-2-methylpropanesulfonic acid. The N–H stretching vibration of the amide groups in AAm and AAMPS and overlapping O–H stretching vibration of sulfonic acid group in AAMPS are observed in the region 3100–3500 cm−1. The strong absorption band in the 1658 cm−1 can be attributed to the stretching vibrations of C=O groups in both AAm and AAMPS. Secondary amide band of AAMPS unit has a peak in 1545 cm−1. The sharp peak at 1042 cm −1 is related to sulfonic acid (–SO3H) group. The symmetric band of SO2 is observed in 1178-1216 cm −1. The band at 1453 cm−1 is assigned to the stretching vibration of the C–N bond (amide) and the asymmetric bending of the C–H bond in methyl groups of AMPS. Table 1 gives the main characteristic peak assignment of the FT-IR spectra. Also a schematic illustration of the reaction is shown in Scheme 2. The absence of the olefinic band at 1620–1635 cm −1 confirms that there is no residual monomer in the system. The results in Fig. 1 (c) suggest the integration of Fe3O4 NPs and Cross-PAA-SO3H.
The particle size and morphology of Cross-PAA-SO3H@nano-Fe3O4 were determined by Scanning Electronic Microscopy (SEM). The statistic of results from SEM images clearly demonstrates that the average size of Cross-PAA-SO3H@nano-Fe3O4 is about 7-25 nanometers (Fig. 2).
Fig. 3 exhibits the powder X-ray diffraction (XRD) pattern. The pattern agrees well with the reported pattern for Fe3O4 (JCPDS No. 75-1609). The particle size diameter (D) of the nanoparticles has been calculated by the Debye–Scherrer equation (D = Kλ/β cos θ), where β FWHM (full-width at half-maximum or half-width) is in radian and θ is the position of the maximum of the diffraction peak. K is the so-called shape factor, which usually takes a value of about 0.9, and λ is the X-ray wavelength (1.5406Å for CuKα). All the strong peaks appeared at 2θ = 30.08°, 35.40°, 43.17°, 53.59°, 57.20°, 62.86°, and 74.02° can be easily indexed to nano-Fe3O4. The crystallite size of Cross-PAA-SO3H@nano-Fe3O4 calculated by the Debye–Scherer equation is about 20-25 nm, in good agreement with the result obtained by SEM.
An EDS (energy dispersive X-ray) spectrum of Cross-PAA-SO3H@nano-Fe3O4 (Fig. 4) exhibits that the elemental compositions are carbon, oxygen, sulfur, iron and nitrogen.
The magnetic attributes of nano-Fe3O4 and Cross-PAA-SO3H@nano-Fe3O4 were given with the help of a vibrating sample magnetometer (VSM) (Fig. 5). The amounts of saturation-magnetization for nano-Fe3O4 and Cross-PAA-SO3H@nano-Fe3O4 are 47.2 emu/g and 26.8 emu/g. These results illustrate that the magnetization property is reduced by coating and functionalization.
Thermogravimetric analysis (TGA) evaluates the thermal stability of the Cross-PAA-SO3H@nano-Fe3O4. These nanoparticles display proper thermal stability without a significant decrease in weight (Fig. 6). The weight loss at temperatures below 200ºC is owing to the removal of physically adsorbed solvent and surface hydroxyl groups. The curve shows a weight loss about 20% from 250 to 600ºC, resulting from the decomposition of the organic spacer attaching to the nano-Fe3O4 surface.
Catalytic behaviors of Cross-PAA-SO3H@nano-Fe3O4 for the synthesis of 1,3-thiazoles
Initially, the conditions for the synthesis of 3- alkyl -4 – phenyl -1,3 –thiazole -2(3H) -thione derivatives were optimized by the reaction of phenacyl bromide, carbon disulfide and benzyl amine as a model reaction. The model reactions were performed by CAN, NaHSO4, InCl3, ZrO2, p-TSA, nano-Fe3O4 and Cross-PAA-SO3H@nano-Fe3O4. The reactions were tested using various solvents including ethanol, acetonitrile, water, and dimethylformamide. The best results were gained in EtOH. The reaction gave convincing results in the presence of cross-PAA-SO3H@nano-Fe3O4 (5 mg) under ultrasonic irradiation (Table 2). We studied the feasibility of the reaction by selecting some representative substrates (Table 2). To investigate the extent of this catalytic process, phenacyl bromide or 4-methoxyphenacyl bromide, carbon disulfide and primary amine were elected as substrates. Seeking of the reaction scope demonstrated that various primary amines can be utilized in this method (Table 3).
Scheme 3 displays a proposed mechanism for this reaction in the presence of cross-PAA-SO3H@nano-Fe3O4 as the catalyst. Initially the nucleophilic attack by amines on a carbon disulfide generates intermediate (I). The next step involves nucleophilic attack of intermediate (I) on the methylene carbon of phenacyl bromide, leading to intermediate (II), and then ring closure by intramolecular attack of nitrogen at the carbonyl carbon to afford the 3-alkyl-4-phenyl-1,3- thiazole-2(3H)-thione deri-vatives. In this mechanism, the surface atoms of cross-PAA-SO3H@nano-Fe3O4activate the C=S and C=O groups for better reaction with nucleophiles.
The reusability of Cross-PAA-SO3H@nano-Fe3O4 was studied for the reactions of phenacyl bromide, carbon disulfide and benzyl amine. It was found that the product yields are reduced to a small extent on each reuse (run 1, 93%; run 2, 93%; run 3, 92%; run 4, 92%; run 5, 91%; run 6, 90%;). After completion of the reaction, the nanocatalyst was separated by an external magnet. The catalyst was washed four times with ethanol and dried at room temperature for 18 h. The catalyst could be reused for six times with a minimal loss of activity. The morphology and particle size of Cross-PAA-SO3H@nano-Fe3O4 were investigated by scanning electron microscopy (SEM) before use and after six times reuse with images shown in Fig. 7. The SEM of Cross-PAA-SO3H@nano-Fe3O4 after reaction showed identical shape. The morphology of the nanoparticles remained unchanged before and after reaction. We believe that this is also the possible reason for the extreme stability of the nanoparticles presented herein.
In conclusion, we have reported an efficient way for the synthesis of 3-alkyl-4-phenyl-1,3-thiazole-2(3H)-thione derivatives using cross-PAA-SO3H@nano-Fe3O4 under ultrasonic irradiation. The method offers several advantages including easy availability, high yields, shorter reaction time, reusability of the catalyst, and low catalyst loading.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.