Nanocrystalline-cellulose-supported acidic ionic liquid carrying SO3H functional group was prepared using nanocrystalline cellulose, imidazole and 1,4-butane sultone as the source chemicals. The prepared nanocrystalline-cellulose-supported ionic liquid catalyst was characterized by AFM and SEM and its catalytic activity in the reaction of resorcinol with ethyl acetoacetate was tested in a solvent-free condition. The effects of reaction time, reaction temperature, and the ratio of catalyst on the conversion of resorcinol were investigated. A variety of coumarin derivatives were obtained in good yield in the absence of solvent.
Coumarins are very significant heterocyclic compounds in organic synthesis. These compounds are known as benzo-2-pyrone derivatives which are principally found in plants. Most of coumarin derivatives show some useful bioactivities. These compounds are widely used for pharmaceutical and agricultural applications and fragrance [
Among the biopolymers, cellulose and its derivatives are extensively used in chemicals and bioapplications. They are also applied as support for synthesis of organic compounds because cellulose and its derivatives are biodegradable, environmentally safe, widely abundant in nature, and easy to handle.
On the other hand, because of the unique physical and chemical properties of nanoscale materials especially in catalytic reactions, synthesis and application of nanostructured materials are an important field in nanoscience and technology. Through these, nanocrystalline cellulose (NCC) has attracted a great deal of interest in the nanocomposite field due to its intensive properties such as nanosize dimension, high surface area, and unique morphology [
Therefore, in this project, we have prepared and applied nanocrystalline-cellulose-supported sulfonic acid ionic liquid as a highly efficient catalyst in promotion of the Pechmann reaction for convenient synthesis of coumarins in solvent free conditions.
Chemicals were purchased from Merck and Fluka chemical companies. 1H NMR and 13C NMR spectra were recorded on Bruker Avance DRX 400 MHz spectrometer. IR spectra were recorded on a Bruker FTIR spectrometer. Melting points were measured on an Electrothermal 9100 apparatus. Characterization of cellulose whiskers was performed using scanning electron microscopy (SEM) (XL30, Philips, the Netherlands) at 30 kV and atomic force microscopy (AFM) Nanosurf Easy Scan 2 Flex AFM (tapping mode).
NCC suspension was produced according to the already reported procedure with slight modification [
In a round-bottom flask equipped with condenser, 1 N NaOH (2 mL) was added to a suspension of prepared NCC (2 g). The system was homogenized for 10 min on an ice bath (
IR (KBr): 989, 1060, 2881, 2995, 3388 cm−1.
A mixture of imidazole (0.34 g), epoxidized nanocrystalline cellulose (0.9 g), and THF (2 mL) was placed in a round-bottom flask and stirred magnetically for 12 h at 60°C. The resulting mixture was separated by simple filtration and washed with THF which afforded 1.2 g of imidazole functionalized NCC.
IR (KBr): 1100, 1160, 1370, 1634, 2920, 3352 cm−1.
Elemental analysis: found; C (41.91%), H (4.45%), N (10.47%); calculated (based on monofunctionalized glucopyranose repeating units of cellulose); C (59.87%), H (6.4%), N (17.46%).
SO3H-functionalized Bronsted acidic ionic liquid was prepared in the laboratory according to the procedure outlined in the literature [
The Pechmann reaction.
IR (KBr): 1040, 1180, 1349, 1450, 1632, 2900, 3320 cm−1.
In a typical experiment, nanocrystalline-cellulose-supported acidic ionic liquid which was prepared based on some literature (Scheme
13C NMR (100 MHz, CDCl3):
IR (KBr): 1064, 1238, 1543, 1705, 3020 cm−1.
13C NMR (100 MHz, CDCl3):
IR (KBr): 1060, 1225, 1590, 1680, 3100 cm−1.
13C NMR (100 MHz, CDCl3):
IR (KBr): 1055, 1225, 1565, 1693, 3010, 3412 cm−1.
13C NMR (100 MHz, CDCl3):
IR (KBr): 1070, 1146, 1212, 1248, 1378, 1579, 1636, 1704, 2920, 2970 cm−1.
13C NMR (100 MHz, DMSO):
IR (KBr): 1068, 1284, 1510, 1725, 2927, 3070 cm−1.
13C NMR (100 MHz, CDCl3):
IR (KBr): 1052, 1238, 1570, 1688, 3012, 3312, 3468 cm−1.
13C NMR (100 MHz, DMSO):
IR (KBr): 629, 807, 1006, 1064, 1186, 1337, 1480, 1524, 1653, 2925, 3217 cm−1.
13C NMR (100 MHz, DMSO):
IR (KBr) 1044, 1275, 1572, 1750, 2922, 3012 cm−1.
13C NMR (100 MHz, CDCl3):
IR (KBr) 1044, 1215, 1514, 1630, 3052 cm−1.
13C NMR (100 MHz, DMSO):
IR (KBr): 1130, 1353, 1460, 1616, 1733, 2939, 2992, 3024 cm−1.
Fourier transform infrared spectroscopy (FT-IR) is a useful technique for studying polysaccharides chemistry. Therefore, FT-IR analysis was performed to investigate the functionalization reaction. The spectrum of cellulose is well known in the literature which shows the characteristic vibrational bands including strong broad OH stretching (3300–4000 cm−1), C–H stretching of methine and methylene groups (2800–3000 cm−1), and the alcoholic C–O and glycosidic bond vibrations around 1100–1150 cm−1 [
As seen in Figure
(a) Epoxidized NCC, (b) imidazole-NCC, and (c) NCC-supported ionic liquid.
Comparison of IR spectra of imidazole-containing NCC (Figure
In Figure
Characterization of cellulose nanocrystals was also performed using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The size distribution of the cellulose nanocrystals was determined by AFM. It has been widely used to provide valuable and rapid indication of surface topography of NCC under ambient conditions.
Figures
Atomic force microscope (AFM) image of NCC.
Morphology of this nanocomposite was studied with a scanning electron microscope (SEM). A representative SEM image of the cellulose-nanocrystals-supported ionic liquid is shown in Figure
Scanning electron microscopy (SEM) of nanocrystalline-cellulose-supported ionic liquid.
Chemically, coumarins can be synthesized by a range of methods such as the Perkin, Pechmann reaction, Knoevenagel condensation, Reformatsky, Wittig, and catalytic cyclization reactions [
We herein report an ecofriendly, facile, and efficient methodology for the synthesis of 4-methyl-2H-chromen-2-ones using a nanosized acidic catalyst. This method involves the convenient synthesis of substituted coumarins by treatment of ethyl acetoacetate and substituted phenol using NCC supported sulfonic acid ionic liquid as an environmentally safe and efficient nanocomposite in catalytic amount to promote the reaction under solvent-free condition at 80°C (Scheme
Preparation of catalyst.
To establish the optimum condition on this reaction, first of all, various amounts of catalyst were examined using ethyl acetoacetate and resorcinol as model compounds. The effect of ratio of catalyst on resorcinol conversion and product selectivity was studied at 80°C using 5 to 20% of the catalyst while keeping the ethyl acetoacetate/resorcinol molar ratio at 1 : 1 (Table
Effect of catalyst ratio on Pechmann reaction.
Entry | Catalysta | Time (min) | Yieldb (%) |
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1 | 5% | 37 | 90 |
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3 | 15% | 12 | 90 |
4 | 20% | 20 | 85 |
The excellent yield was obtained with 10% of the catalyst. Next, to find the best solvent, the reaction of ethylacetoacetate and resorcinol using 10% of catalyst was monitored at 80°C in different solvents, namely, n-Hexane, THF, CH2Cl2, and CH3CN, and also in solvent-free condition. The results are summarized in Table
Effect of solvent on condensation of ethyl acetoacetate with resorcinol.
Entry | Solvent | Time (min) | Yield (%) |
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2 | THF | 30 | 90 |
3 | CH3CN | 35 | 95 |
4 | CH2Cl2 | 12 | 95 |
5 | n-Hexane | 20 | 90 |
It was found that solvent-free condition stands out as the choice, with its fast conversion, high yield, and no side product, such as chromones formation [
And finally the influence of reaction temperature on conversion of resorcinol was investigated. The reaction was studied at various temperatures from room temperature up to 100°C and the results were summarized in Table
Effect of reaction temperature on resorcinol conversion.
Entry | Temperature (°C) | Time | Yield (%) |
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1 | r.t | 24 (h) | 92 |
2 | 50 | 5 (h) | 89 |
3 | 70 | 35 (min) | 80 |
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5 | 100 | 18 (min) | 80 |
Using optimized reaction parameters, a variety of 4-methyl-2H-chromen-2-ones were synthesized from ethyl acetoacetate and corresponding phenols (Table
Preparation of coumarins using NCC-supported sulfonic acid ionic liquida.
Entry | Reactant | Product | Time | Yield (%)b | m.p (°C) | Lit.c |
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1 |
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24 (h) | trace | 77–79 | 79–81 [ |
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2 |
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20 (min) | 95 | 185 | 182–184 [ |
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3 |
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6.5 (h) | 60 | 244 | 241-242 [ |
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4 |
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6 (h) | 20 | 130–132 | 130-131 [ |
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5 |
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5 (h) | 80 | 161-162 | 158–160 [ |
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6 |
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1.5 (h) | 75 | 221–224 | 220–224 [ |
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7 |
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20 (min) | 85 | 230–233 | 234-235 [ |
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8 |
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5 (h) | 94 | 154–156 | 153–155 [ |
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9 |
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10 (h) | 40 | 180–182 | 182-183 [ |
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10 |
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24 (h) | N.R | — | — |
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11 |
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18 (min) | 98 | 172–175 | 170–172 [ |
Proposed mechanistic pathway for naphthols.
The reaction condition is relatively mild and workup procedure is simple. The catalyst was separated by filtration and after washing with petroleum ether reused for the new experiments. The reaction conversion for the model reaction after 3 times recycling and reuse of the catalyst gradually reduced to 68%. In conclusion, we have found that NCC-supported acidic ionic liquid is an efficient catalyst for solid phase synthesis of coumarins via the Pechmann reaction. The significant features of the method are (a) operation simplicity, (b) mild reaction condition, and (c) use of an environmentally benign and reusable catalyst.
The authors are grateful to the research councils of Mazandaran University for the financial support.