On-line version ISSN 1996-840X
S.Afr.j.chem. (Online) vol.66 Durban Aug. 2013
H.R. Shaterian*; M. Mohammadnia
Department of Chemistry, Faculty of Sciences, University of Sistan and Baluchestan, P.O. Box 98135-674, Zahedan, Iran
The acidic ionic liquids, triethylammonium hydrogen sulfate ([Et3NH]HSO4), 2-pyrrolidonium hydrogen sulfate ([Hnhp]HSO4), 3-methyl-1-sulfonic acid imidazolium chloride ([Msim]Cl), and 1-methylimidazolium hydrogen sulfate ([Hmim]HSO4) catalyzed three-component condensation reaction of aromatic aldehydes, 2-aminobenzothiazole, and β-naphthol to afford corresponding 1-((benzo[d]thiazol-2-ylamino)(aryl)-methyl)naphthalen-2-ol derivatives. The inexpensive and non-toxic ionic liquids can be reused several times without noticeable loss of their activities.
Keywords: Aldehyde, 2-aminobenzothiazole, β-naphthol, ionic liquids, catalyst, green chemistry.
Ionic liquids (ILs), because of their low volatility, non-flammability, capability to dissolve various organic and inorganic compounds, and potentially recyclable properties, have attracted considerable attention as environmentally friendly reaction media in green organic synthesis.1,2
The designs of multi-component reactions (MCRs) for the synthesis of diverse groups of compounds, especially the ones that are biologically active, have commanded great attention in green organic synthesis.3-5 As part of our continuing interest in the development of new synthetic methods in organic synthesis6,7 and multi-component reactions,8 in this paper we report applications of the acidic ionic liquids triethylammonium hydrogen sulfate ([Et3NH]HSO4), 2-pyrrolidonium hydrogen sulfate ([Hnhp]HSO4), 3-methyl-1-sulfonic acid imidazolium chloride ([Msim]Cl), and 1-methylimidazolium hydrogen sulfate ([Hmim]HSO4) as catalysts (Fig. 1) in a three- component reaction for the synthesis of 1-((benzo[d]thiazol-2-ylamino) (aryl)-methyl)naphthalen-2-ol derivatives (Scheme 1).
Triethylammonium hydrogen sulfate9 has been used in the iodination of alcohols,10 2-pyrrolidonium hydrogen sulfate11 can catalyze the synthesis of benzoxanthene derivatives,113-methyl-1-sulfonic acid imidazolium chloride12 has been used in the preparation of 1-amidoalkyl-2-naphthols,13 benzimidazoles,14 and 1-methylimidazolium hydrogen sulfate15 has been applied in the synthesis of 1,1-diacetates15 and 3,4-dihydropyrimidin-2-(1H)-ones.16
2. Results and Discussion
To study the effect of catalyst loading and temperature on the three component condensation reaction for the synthesis of 1-((benzo[d]thiazol-2-ylamino)(4-chloro-phenyl)methyl) naphthalen-2-ol, the reaction of 4-chlorobenzaldehyde, 2-amino-benzothiazole, and β-naphthol in the presence of the ionic liquid, triethylammonium hydrogen sulfate ([Et3NH]HSO4), as catalystwas selected as a model (Table 1). The reaction was carried out with different amounts of triethylammonium hydrogen sulfate as catalyst (0.05,0.07,0.1,0.15,0.2 mmol) and at various temperature (25,50,80 °C) (Table 1). As is shown in Table 1,0.1 mmol of [Et3NH] HSO4 as catalyst at 80 °C afforded the corresponding product in 6 min with 95% of yield. In continuation of our study, optimization conditions of ionic liquids, [Hnhp]HSO4, [Msim]Cl, and [Hmim]HSO4, as catalysts in the mentioned model were investigated. The best results were obtained using 0.06 mmol of [Hnhp]HSO4,0.8 mmol of [Msim]Cl, and 0.08 mmol of [Hmim]HSO4 as catalysts at 80 °C (Table 1).
Next the three-component condensation reaction of aromatic aldehydes, 2-aminobenzothiazole, and β-naphthol, under optimized conditions, for preparation of 1-((benzo[d]thiazol-2-ylamino)(aryl)-methyl)naphthalen-2-ol derivatives was investigated (Table 2). A wide range of substituted and structurally diverse aldehydes was used to synthesize their corresponding products in high to excellent yields using the five Bronsted acidic ionic liquids as catalysts (Table 2).
Furthermore, the aliphatic aldehyde such as n-heptanal instead of benzaldehydes was examined in the reaction. All starting materials in the reaction were intact without formation of any desired product or side products after 10 h (Table 2, Entry 19). This observation is supported by reports on other catalysts in the literature.17
According to a literature survey,18 a suggested mechanism for the formation of the products is shown in Scheme 2.
The recycling of the ionic liquids under solvent-free conditions was also investigated using the model reaction of 4-chloro-benzaldehyde, 2-aminobenzothiazole, and β-naphthol in the presence of [Et3NH]HSO4 (0.1 mmol), [Hnhp]HSO4 (0.06 mmol), [Msim]Cl (0.08 mmol), and [Hmim]HSO4 (0.08 mmol) (Table 2, Entry 2). After completion of the reaction, water was added and the precipitated mixture was filtered off for separation of crude products. After washing the solid products with water completely, the water containing ionic liquid (IL is soluble in water) was evaporated under reduced pressure and ionic liquid was recovered and reused (Fig. 2). The recovered catalysts were reused for four runs without any significant loss of their activities. 1HNMR spectroscopies showed that ionic liquids were not consumed or degraded.
In order to show the accessibility of the present work in comparison with four reported results in the literature, the results for the preparation of 1-((benzo[d]thiazol-2-ylamino) (aryl)-methyl)naphthalen-2-ol derivatives are summarized in Table 3. The results show that acidic ionic liquids such as [Et3NH]HSO4 (0.1 mmol), [ [Hnhp]HSO4 (0.06 mmol), [Msim]Cl (0.08 mmol), and [Hmim]HSO4 (0.08 mmol) under solvent-free conditions relative to Wells-Dawson heteropolyacid (H6P2W18O62-24H2O) (HPAs),17 N-methyl pyrrolidonium hydrogensulfate ([Hnmp]HSO4),18 LiCl,19 sodium dodecyl sulfate (SDS)20 are the most efficient catalysts with respect to the reaction time and the obtained yields.
In conclusion, we have shown that acidic ionic liquids, which can be prepared from commercially available and inexpensive starting materials, catalyzed efficiently synthesis of 1-((benzo[d] thiazol-2-ylamino)(aryl)-methyl)naphthalen-2-ol derivatives under solvent-free conditions. The simplicity of the procedure, eco-friendly, non-volatile, easy handling, safety and reusability of catalyst are the advantages of these methods. These methods not only afford the products in excellent yields but also avoid the problems associated with catalyst cost, and pollution. T he mentioned ionic liquids show priority relative to other catalysts in the literature.
All reagents were purchased from Merck and Aldrich and used without further purification. The acidic ionic liquids such as triethylammonium hydrogen sulfate,9 2-pyrrolidonium hydrogen sulfate,11 3-methyl-1-sulfonic acid imidazolium chloride,12 and 1-methylimidazolium hydrogen sulfate15 were prepared according to literature. All yields refer to isolated products after purification. The NMR spectra were recorded on a Bruker Avance DPX 300 MHz instrument. The spectra were measured in DMSO-d6 relative to TMS (0.00 ppm). IR spectra were recorded on a JASCO FT-IR 460 plus spectrophotometer. Melting points were determined in open capillaries with a BUCHI 510 melting point apparatus. TLC was performed on silica-gel Poly Gram SIL G/UV 254 plates.
3.2 Synthesis of 1-((Benzo[d]thiazol-2-ylamino)(aryl)-methyl) naphthalen-2-ol derivatives under Solvent-free Conditions
The mixture of the aldehydes (10 mmol), 2-aminobenzothiazole (10 mmol), and β-naphthol (10 mmol) and ionic liquids containing [Et3NH][HSO4] (1 mmol), [Hnhp]HSO4 (0.6 mmol), [Msim]Cl (0.8 mmol), or [Hmim]HSO4 (0.8 mmol) as acidic catalysts was stirred at 80 °C for the specific time. After completion of the reaction, it was cooled to room temperature. Then, 5 mL of water was added to the mixture. The ionic liquid was dissolved in water, and filtered for separation of the crude product. The separated product was washed twice with water (2 x 5mL). The crude products were purified by recrystallization from acetone. All of the desired product(s) were characterized by comparison of their physical data with those of known compounds. For recycling the catalysts, after washing mixture of the reaction with water completely, the water containing ionic liquid (IL is soluble in water) was evaporated under reduced pressure and ionic liquid was recovered and reused. The desired pure products were characterized by comparison of their physical data (melting points, IR, 1H and 13C NMR spectra) with those of known compounds.17-20
We are grateful to the University of Sistan and Baluchestan Research Council for the partial support of this research.
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Received 27 Ctober 2012
Revised 26 January 2013
Accepted 7 February 2013