Green and efficient synthesis of pyranopyrazoles using [bmim][OH⁻] as an ionic liquid catalyst in water under microwave irradiation and investigation of their antioxidant activity

Abstract

An imidazole-based ionic liquid, 1-butyl-3-methylimidazolium hydroxide ([bmim][OH⁻]), was utilized as a novel heterogeneous catalyst for the synthesis of pyranopyrazoles. The target pyranopyrazole compounds were isolated, purified and fully characterized. This organo-solid catalyst was used as a novel, mild and fully developed catalysts for the cascade synthesis of colorful pyrano[2,3-c]-pyrazoles via the three component reaction of various benzaldehydes, pyrazolone and malononitrile in the presence of the aforementioned catalyst in a microwave oven at a power of 230 W. Here, we attempt to present a simple and reliable method for the preparation of this type of compound that is environmentally friendly and operationally simple with high yields, short reaction times and an efficient workup method. The antioxidant activity of the products was evaluated using a 2,2-azinobis(3-ethylbenzothiazoline-sulfonate) (ABTS) assay. The antioxidant activity of the synthesized products was compared with both vitamin E and ascorbic acid. The products showed a higher antioxidant activity using this method. Products 4o, 4n, 4a, 4e and 4h were much more active than ascorbic acid and products 4o, 4n and 4a were much more active than vitamin E in the ABTS assay. The high antioxidant activity of product 4o could be due to the longer conjugated system which can stabilize the free radical by resonance through a longer system.

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Introduction

In recent years, finding creative ways to reduce environmental pollution has been the goal of many organic chemists. Research has been focused on reducing or eliminating the use of toxic solvents to minimize damage to humans and the environment. To achieve this goal, water-based organic synthesis has been an interesting field for many organic chemists. Using water instead of common organic solvents which are mostly toxic is a suitable idea for reducing environmental pollution.^1,2 Other than this great advantage, using water can also affect the rate and selectivity of organic reactions. The synthesis of organic compounds using an ionic liquid as a catalyst and water as a solvent shows extra advantages such as energy conservation, a short reaction time and the avoidance of using volatile and toxic organic solvents.^3–5 Ionic liquids are molten salts with melting points at or below ambient temperature.^6–8 The preparation and use of ionic liquids as catalysts in organic reactions shows distinct advantages over traditional processes such as high thermal stability, negligible vapor pressure, high ionic conductivity, the elimination of the need for volatile organic solvents and short reaction times.^9,10

Most organic reactions are heated using traditional heat equipment such as oil baths, sand baths and heating jackets. In these heating systems, a temperature gradient can develop within the sample and overheating can lead to product, substrate and reagent decomposition.

In contrast, microwave heating can be very rapid, producing heat profiles that are not easily accessible by other heating systems.¹¹

The main advantage of using microwave assisted organic synthesis is the shorter reaction times. The rate of the reaction can be described by the Arrhenius equation (eqn (1)).

K = Ae^−ΔG/RT (1)

In eqn (1), there are principally two ways to increase the rate of a chemical reaction. First, the pre-exponential factor A, which describes the molecular mobility and depends on the frequency of the vibrations of the molecules at the reaction interface; for example, water, which has a large dipole moment, heats readily.^12,13 However, some authors have suggested that microwave irradiation produces an alteration in the exponential factor by affecting the free energy of activation, ΔG.¹⁴

The pyranopyrazole nucleus is an important class of biologically active heterocycles. There are four possible isomers of pyranopyrazole: pyrano[2,3-c]pyrazole, pyrano[4,3-c]pyrazole, pyrano[3,2-c]pyrazole and pyrano[3,4-c]-pyrazole. The pyrano[2,3-c]pyrazoles (Fig. 1) are a fertile source of biologically important molecules.¹⁵ Compounds containing this moiety have received significant attention from many pharmaceutical, color and organic chemists. Compounds including pyranopyrazoles have various biological activities, such as antimicrobial, analgesic, vasodilator, anticancer, anti-inflammatory, molluscicidal and anti-fungicidal activities, and also these compounds are used as cosmetics and pigments.^16–22.

Fig. 1 Pyrano[2,3-c] pyrazole.

In the present work, we report the synthesis of pyrano[2,3-c]-pyrazoles via the three component reaction of various benzaldehydes, pyrazolone and malononitrile in the presence of the basic ionic liquid 1-methyl-3-butylimidazoliumhydroxide ([bmim][OH⁻]) as a catalyst in water. We examined a wide variety of aldehydes with various substituents primarily to establish the significant synthesis of colorful pyrano[2,3-c]-pyrazoles via a three component reaction and secondarily to determine the catalytic importance of this ionic liquid for this type of reaction. Three component reactions have allowed us to develop an unlimited diversity of synthetic dyes and pigments to complement the colorful world that life has provided.

Experimental

Material

Solvents and reagents were obtained from Fluka, Merck and Aldrich chemical companies and were used without further purification. The purity determination of the substrate and reaction monitoring were accomplished using thin layer chromatography (TLC) on silica-gel polygram SILG/UV 254 plates.

Instrumentation

Fourier transform infrared (FT-IR) spectra were recorded on a PerkinElmer spectrum BX series. ¹H nuclear magnetic resonance (NMR) and ¹³C NMR spectra were determined on Bruker AV-400 and Bruker Avance Spectro spin 100 MHz spectrometers using trimethylsilane (TMS) as the internal standard and deuterated dimethyl sulfoxide (DMSO-d₆) or CDCl₃ as the solvent (Fig. 2). Melting points were recorded on a Büchi B-545 apparatus in open capillary tubes. All the reactions were carried out in a microwave oven.

Fig. 2 ¹H NMR spectra of [bmim][OH⁻].

Preparation of [bmim][OH⁻]

The [bmim][OH⁻] was prepared according to the literature.^23,24 To a solution of 1-methyl-3-n-butylimidazolium chloride (10 mmol) in dry CH₃CN, solid KOH (10 mmol) was added, and the mixture was stirred vigorously at room temperature for 24 h. The precipitated KCl was filtered off and the residual liquid was evaporated at reduced pressure. The residual viscous liquid was washed with Et₂O (20 × 3 mL) and dehydrated under vacuum to yield a clear honey-colored syrup. Spectroscopic data for [bmim][OH⁻] are as follows:

FT-IR (KBr, cm⁻¹) ν_max: 3423 (OH), 3144 (CH ring), 3074 (CH alkyl), 2959 (CH alkyl), 2870 (CH alkyl), 1666 (C=N), 1601 and 1460 (C=C), 1572 (C–C) cm⁻¹; ¹H NMR (400 MHz, DMSO-d₆): δ = 1.444 (3H, s broad, CH₃), 1.872 (4H, m), 4.511 (3H, s, N–CH₃), 5.170 (2H, t, J = 8 Hz, N–CH₂), 7.42 (1H, d, J = 7.2), 7.56 (1H, d, J = 7.2), 9.70 (1H, s, CH) ppm.

General procedure for the preparation of pyrano[2,3-c]pyrazole derivatives under MW irradiation

A mixture of the aryl aldehyde (1 mmol), pyrazolone (1 mmol) and malononitrile (1.1 mmol) in the presence of [bmim][OH⁻] (5 mol%) was irradiated in a microwave oven at a power of 230 W. The progress of the reaction was monitored by TLC (EtOAc : hexane 7 : 1). After completion of the reaction, the reaction mixture was cooled to room temperature, filtered off, washed with small amounts of water (10 mL) and then recrystallized from EtOH to give the pure products 4a–4n.

Spectral data for some of the pyranopyrazoles

6-Amino-3-methyl-4-(p-tolyl)-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile (4a). Mp 215–217 °C; FT-IR (KBr, cm⁻¹) ν_max: 3406, 3315, 3188, 2191, 1646, 1600; ¹H NMR (400 MHz, DMSO-d₆) δ ppm 1.819 (s, 3H, CH₃), 3.872 (s, 3H, CH₃), 4.619 (s, 1H, CH), 6.935 (s, 2H, NH₂), 7.396 (d, 2H, J = 8.4 Hz, ArH), 8.099 (d, 2H, J = 8.4 Hz, ArH), 11.866 (s, 1H, NH). Mass spectrometry (electron ionization) (MS(EI)): exact mass: (M⁺): calcd 254.1419; found 254.1420.

6-Amino-4-(4-methoxyphenyl)-3-methyl-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile (4g). Mp 215–216 °C; FT-IR (KBr, cm⁻¹) ν_max: 3483, 3249, 3122, 2190, 1643, 1600; ¹H NMR (400 MHz, DMSO-d₆) δ ppm 1.885 (s, 3H, CH₃), 3.859 (s, 3H, CH₃), 4.620 (s, 1H, CH), 6.944 (s, 2H, NH₂), 7.396 (d, 2H, J = 8.4 Hz, ArH), 8.099 (d, 2H, J = 8.4 Hz, ArH), 11.899 (s, 1H, NH). MS(EI): exact mass: (M⁺): calcd 282.1117; found 282.1124.

6-Amino-4-(4-chlorophenyl)-3-methyl-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile (4h). Mp 226–228 °C; FT-IR (KBr, cm⁻¹) ν_max: 3483, 3357, 3221, 2210, 1634, 1600; ¹H NMR (400 MHz, DMSO-d₆) δ ppm 1.871 (s, 3H, CH₃), 4.614 (s, 1H, CH), 6.997 (s, 2H, NH₂), 7.358 (d, 2H, J = 8.4 Hz, ArH), 8.093 (d, 2H, J = 8.4 Hz, ArH), 11.899 (s, 1H, NH). MS(EI): exact mass: (M⁺): calcd 282.1117; found 282.1125.

6-Amino-4-(2,4-dichlorophenyl)-3-methyl-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile (4i). Mp 223–225 °C; FT-IR (KBr, cm⁻¹) ν_max: 3478, 3246, 3117, 2188, 1640, 1594; ¹H NMR (400 MHz, DMSO-d₆) δ ppm 1.738 (s, 3H, CH₃), 4.587 (s, 1H, CH), 7.025 (s, 1H, ArH), 7.126 (s, 2H, NH₂), 7.617 (d, 2H, J = 8.4 Hz, ArH), 7.690 (d, 2H, J = 8.4 Hz, ArH), 11.874 (s, 1H, NH). MS(EI): exact mass: (M⁺): calcd 320.0232; found 320.0236.

6-Amino-4-(4-fluorophenyl)-3-methyl-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile (4j). Mp 236–238 °C; FT-IR (KBr, cm⁻¹) ν_max: 3389, 3304, 3168, 2187, 1644, 1598; ¹H NMR (400 MHz, DMSO-d₆) δ ppm 1.865 (s, 3H, CH₃), 4.619 (s, 1H, CH), 6.935 (s, 2H, NH₂), 7.433 (t, 1H, J = 8.8 Hz, ArH), 8.283 (m, 1H, ArH), 11.800 (s, 1H, NH). MS(EI): exact mass: (M⁺): calcd 270.0917; found 270.0920.

6-Amino-3-methyl-4-(4-nitrophenyl)-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile (4m). Mp 230–232 °C; FT-IR (KBr, cm⁻¹) ν_max: 3473, 3275, 3137, 2186, 1641, 1598; ¹H NMR (400 MHz, DMSO-d₆) δ ppm 1.844 (s, 3H, CH₃), 4.619 (s, 1H, CH), 6.935 (s, 2H, NH₂), 7.408 (d, 2H, J = 8.4 Hz, ArH), 8.099 (d, 2H, J = 8.4 Hz, ArH), 12.00 (s, 1H, NH). MS(EI): exact mass: (M⁺): calcd 297.0862; found 297.0871.

6-Amino-4-(3-bromophenyl)-3-methyl-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile (4n). Mp 189–190 °C; FT-IR (KBr, cm⁻¹) ν_max: 3389, 3166, 2185, 1642, 1596; ¹H NMR (400 MHz, DMSO-d₆) δ ppm 1.772 (s, 3H, CH₃), 4.558 (s, 1H, CH), 6.950 (s, 2H, NH₂), 7.381 (t, 1H, ArH), 7.585 (d, 2H, J = 8.4 Hz, ArH), 7.884 (d, 2H, J = 8.4 Hz, ArH), 8.348 (s, 1H, ArH), 11.874 (s, 1H, NH). MS(EI): exact mass: (M⁺): calcd 330.0116; found 330.0121.

4,4′-(1,4-Phenylene)bis(5-amino-3-methyl-1,4-dihydropyrano[2,3-c]pyrazole-6-carbonitrile) (4o). Mp 258–260 °C. ¹H NMR (400 MHz, DMSO-d₆): δ (ppm) 1.75 (s, 6H, CH₃), 4.58 (s, 2H, CH), 6.87 (s, 4H, NH₂), 7.12 (s, 4H, ArH), 12.1 (s, NH). ¹³C NMR (100 MHz, DMSO-d₆): δ (ppm) 10.18, 36.42, 57.67, 98.18, 121.28, 128.12, 136.07, 143.24, 155.18, 129.01, 161.30 (Fig. 3 & 4).

Fig. 3 FT-IR spectrum of 4,4′-(1,4-phenylene)bis(5-amino-3-methyl-1,4-dihydropyrano[2,3-c]pyrazole-6-carbonitrile), 4o.

Fig. 4 ¹H NMR spectrum of 4,4′-(1,4-phenylene)bis(5-amino-3-methyl-1,4-dihydropyrano[2,3-c]pyrazole-6-carbonitrile), 4o.

Biology

ABTS assay. The 2,2-azinobis(3-ethylbenzothiazoline-sulfonate) (ABTS) assay of compounds was evaluated according to the literature.²⁵ A solution of ABTS (7.4 mM) in methanol (MeOH) and a solution of potassium persulfate (K₂S₂O₈) (2.6 mM) as an oxidizing agent in MeOH were mixed in equal volumes and allowed to react for 12 h in the dark at room temperature to produce the ABTS radical cation (ABTS˙⁺) stock solution. Then, the resultant stock solution was diluted with MeOH to give an absorbance of 1.1 ± 0.02 at 734 nm. A series of sample solution at concentrations of 4000, 2000, 1000, 500, 250, 125 μg mL⁻¹ in MeOH was prepared by two-fold serial dilution. Then, 150 mL of the sample solution was added to 3.0 mL of the ABTS˙⁺ solution, this mixture was shaken and incubated in the dark for 2 h. Then, the absorbance of each solution was recorded at 734 nm and MeOH was used as the blank.

The IC₅₀ values (the concentration of compounds to scavenge 50% of ABTS) of each compound for the ABTS assay were calculated by plotting the inhibition percentage against the concentration of the samples and the results were expressed in μM.

Results and discussion

Recently, the preparation, isolation and characterization of novel heterocyclic compounds became an important part of our continuing research program.^26–32 In continuation of our previous studies, we were interested in the synthesis of pyrano[2,3-c]-pyrazoles via 3-MCRs (three multicomponent reactions), as well as the catalytic importance of [bmim][OH⁻] in increasing the yield of the products. [bmim][OH⁻] was efficiently prepared according to the appropriate procedure (Scheme 1).

Scheme 1 The preparation of [bmim][OH⁻].

Our results suggest that the [bmim][OH⁻] basic catalyst significantly enhances the rate and the yield of the products under green conditions (Scheme 2).

Scheme 2 [bmim][OH⁻] catalyzed the synthesis of pyrano[2,3-c]pyrazoles derivatives.

Initially the optimization of the reaction conditions for the preparation of compound 4h from the reaction of 4-chlorobenzaldehyde, malononitrile and pyrazolone was investigated and the results are tabulated in Table 1. The optimized conditions are shown in entry 3 (Table 1).

Table 1 Effect of temperature, amount of the catalyst and solvent on the synthesis of 4h in the presence of [bmim][OH⁻]

Entry	Catalyst amount (mol%)	Solvent	Temperature (°C)	Time (min)	Yield (%)
1	1	H2O	MW	5	44
2	3	H2O	MW	5	67
3	5	H2O	MW	5	92
4	8	H2O	MW	5	94
5	5	EtOH	MW	5	84
6	5	DMF	MW	5	75
7	5	CH2Cl2	MW	6	67
8	5	EtOH/H2O (1/1)	MW	5	95
9	5	H2O	Reflux	20	67
10	5	H2O	Reflux	30	74
11	5	H2O	Reflux	40	87
12	5	H2O	Reflux	50	92
13	5	H2O	Reflux	60	93
14	5	Solvent free	MW	60	34

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Moreover, the proposed catalytic mechanism is depicted in Scheme 3.^33,34 Initially the yellowish arylidenemalononitrile was formed via a Knoevenagel reaction of activated malononitrile and activated aromatic aldehyde in a quantitative yield. The formation of the dihydropyrano-[2,3-c]-pyrazole is proposed to involve the following tandem reactions:

Scheme 3 Proposed mechanism for the synthesis of pyrano[2,3-c]pyrazoles derivatives in the presence of [bmim][OH⁻].

Pyrazolone formation after tautomerization of 3-methyl-2-pyrazoline-5-one, Michael addition of pyrazolone to arylidenemalononitrile, followed by cyclization and tautomerization (Scheme 3).

Various aromatic aldehydes were reacted with malononitrile and pyrazolone under the optimized reaction conditions to provide the corresponding colourful products 4a–4o in high yields and in short reaction times. The obtained results are summarized in Table 2 (entries 1–14).

Table 2 Preparation of pyrano[2,3-c]pyrazoles derivatives using [bmim][OH⁻]as the catalyst

Entry	Structure	Time (min)	Yield (%)	Mp (°C)		Product	Picture
				Found	Reported (ref.)
1		4	93	215–217	215 (ref. 35)	4a
2		7	78	241–243	243 (ref. 35)	4b
3		5	81	255–257	250 (ref. 35)	4c
4		3	95	212–214	217 (ref. 35)	4d
5		4	91	210–211	209 (ref. 36)	4e
6		5	90	290–291	295 (ref. 37)	4f
7		4	92	215–216	212 (ref. 37)	4g
8		5	80	226–228	230 (ref. 35)	4h
9		5	79	223–225	221 (ref. 37)	4i
10		4	85	236–238	247 (ref. 42)	4j
11		4	91	211	213 (ref. 35)	4k
12		4	89	208–209	202 (ref. 37)	4l
13		8	69	230–232	235 (ref. 36)	4m
14		5	77	189–190	194 (ref. 37)	4n
15		12	87	258–260	This work	4o

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In order to show the advantage of this method, the efficiency of [bmim][OH⁻] as a catalyst in the synthesis of pyrano[2,3-c]pyrazoles is compared with some reported catalysts in Table 3 (entries 1–7).

Table 3 Compared performance of various catalysts with [bmim][OH⁻] in the synthesis of pyrano[2,3-c]pyrazoles

Entry	Catalyst/conditions	Time (min)	Yield (%)	Ref.
1	[(CH2)4SO3HMIM][HSO4]/solvent-free, rt	30	85	38
2	Triethyl amine/H2O, rt	30	77	39
3	Sodium benzoate/H2O, rt	35	86	40
4	Free catalyst/H2O, reflux	180	90	41
5	NaOH/H2O, reflux	60	95	42
6	Cesium fluoride (CsF)/EtOH, reflux	230	87	10
7	[bmim][OH^−]/H2O, MW	3–12	95	This work

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Furthermore, [bmim][OH⁻] for the first time was used in the preparation of new mono and specially bis pyrano[2,3-c]pyrazole derivatives (entry 1–15 Table 2). Using this ionic liquid as a catalyst, the obtained products were much more purified and the isolated product was more reproducible than when for instance NaOH was used as catalyst. After completion of the reaction, the reaction mixture was cooled to room temperature and was filtered off, washed with small amounts of H₂O and was recrystallized from EtOH to give a pure bis-product 4o (entry 15, Table 2). The structure of the compound was characterized using infrared (IR), ¹H NMR and ¹³C NMR spectra. The IR spectrum of this compound shows two peaks at 3232 and 3477 cm⁻¹ related to the NH₂ stretching vibrations. A sharp peak at 2191 cm⁻¹ is related to the C≡N stretching vibrations and a peak at 1639 cm⁻¹ is related to the C=N stretching vibration (Fig. 3). The structure of 4o is further confirmed by the ¹H and ¹³C NMR spectrum, which precisely indicate an appropriate number of peaks corresponding to the right structure (Fig. 4 & 5).

Fig. 5 ¹³C NMR spectrum of 4,4′-(1,4-phenylene)bis(5-amino-3-methyl-1,4-dihydropyrano[2,3-c]pyrazole-6-carbonitrile), 4o.

Biology

Antioxidant activity. Some of the products were screened for their in vitro antioxidant activity. The ABTS radical cation is widely used as a rapid, simple, and inexpensive method to evaluate the antioxidant ability of compounds. The antioxidant activity of compounds was evaluated using a colorimetric ABTS method. When a compound acts as an antioxidant, it scavenges free radicals and leads to a decrease in the absorption band at 734 nm for ABTS solutions. Moreover, potential antioxidant activity leads to a rapid decrease in absorbance.

ABTS is also frequently used in the food industry and by agricultural researchers to measure the antioxidant capacities of foods. ABTS is converted to its radical cation (ABTS˙⁺) by the addition of sodium persulfate or potassium persulfate as an oxidant. The scavenging activity of ABTS˙⁺ is considered to be an electron transfer reaction.⁴³ The ABTS radical cation is blue in color, when it reacts with an antioxidant compound the blue color changes to light blue or colorless.

The antioxidant activities of the compounds were screened at the concentrations of 125–4000 μg mL⁻¹ at 734 nm for the ABTS assay. Also, the IC₅₀ values were calculated by plotting the radical scavenging activity against concentration and obtaining a line equation. Ascorbic acid and vitamin E were used as the standards. The investigation of antioxidant activity revealed that all the selected compounds showed potent to moderate radicals scavenging activity when compared to ascorbic acid and vitamin E as the standards. As depicted in Fig. 6, the radical scavenging activity of the products was dose dependent.

Fig. 6 ABTS radical-scavenging activity.

In addition, the antioxidant activity of the synthesized products 4a, 4d, 4e, 4h, 4k, 4l, 4n and 4o was compared with both vitamin E and ascorbic acid. The IC₅₀ values of the selected products were calculated (Fig. 7). The IC₅₀ values were in the range of 104–3133 μM for the ABTS assay. Products 4o, 4n, 4a, 4e and 4h were much more active than ascorbic acid and products 4o, 4n and 4a were much more active than vitamin E in the ABTS assay. Product 4l showed a weaker activity and other compounds showed moderate activity than both vitamin E and C. The high antioxidant activity of product 4o could be due to the longer conjugated system which can stabilize the free radical by resonance through a longer system.

Fig. 7 IC₅₀ values of selected products using ABTS assays.

Conclusions

In summary, we have introduced [bmim][OH⁻] as a new and efficient catalyst for the synthesis of pyrano[2,3-c]pyrazoles derivatives. The simplicity of the catalyst preparation, the easy work-up procedure, high reaction rates and excellent yields are the most important advantages of the present method. The antioxidant activity of the products was evaluated using the ABTS assay. The products showed higher antioxidant activity using this method. Products 4o, 4n, 4a, 4e and 4h were much more active than ascorbic acid and products 4o, 4n and 4a were much more active than vitamin E in the ABTS assay. Product 4l showed a weaker activity than both ascorbic acid and vitamin E in the ABTS assay. The high antioxidant activity of bisproduct 4o was related to the extended conjugated system which can stabilize the free radical via resonance through a longer system.

Acknowledgements

This study was supported in part by the Research Committee of the University of Guilan.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra17594e

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