1 Introduction

Benzopyrans and their derivatives are known to show several pharmacological properties such as spasmolytic, diuretic, antianaphylactin, antisterility and are used as anticancer agents.[1] The polyfunctionalized benzopyrans are also used as cosmetics, pigments and biodegradable agrochemicals.[2] The benzylidene malonitrile derivatives have been found to posses inhibitory activity to HER2,[3] EGFR,[4] IGF1R[5] and have been used for treatment of cancer.[6] Many synthetic procedures have been reported for tetrahydro benzo pyrans and benzylidene malonitriles.

The tetrahydro benzo pyrans have been previously synthesized by two or three-component condensations including the use of catalysts like potassium phosphate,[7] ZnO-beta zeolite,[8] Ce(SO4)2.4H2O,[9] s-proline,[10] caro’s acid-silica gel,[11] hexadecyl trimethyl ammonium bromide,[12] sulphonic acid functionalized silica,[13] tetra butyl ammonium bromide,[14] rare earth perfluorooctanoate,[15] basic quaternary ammonium salt,[16] phenyl boronic acid,[17] LiBr,[18] TEAA,[19] PEG- 400,[20] basic ionic liquid,[21] amines[22] and (NH4)2HPO4.[23] The benzylidene malonitriles were synthesized using catalysts such as calcium oxide,[24] TEBA,[25] PEG,[26] base,[27] NH2SO3NH4,[28] MgBr2.OEt2,[29] organo-base mediation,[30] quaternary ammonium salts,[31] Na2S/ Al2O3,[32] mpg-C3N4,[33] mesoporous base,[34] zirconia,[35] amine supported on silica gel[36] and MgC2O4/SiO2.[37]

Recently, metal oxide nanoparticles in the form of nanocatalyst have emerged as viable alternatives to conventional materials in various fields of chemistry and attracted marvelous interest of chemists. Metal oxide nanoparticles are known to be promising heterogeneous catalysts in a variety of organic transformations.[38] Nanoparticles have the potential for improving the efficiency, selectivity and yield of catalytic processes. In particular, the PbO nanoparticles provide higher surface to volume ratio in the reaction. Higher selectivity of PbO nanoparticles towards reaction proceeds through less waste and fewer impurities, which could lead to safer technique and reduced environmental impact. PbO nanoparticles have been investigated as catalysts in the organic reactions including the Paal–Knorr reaction and oxidative coupling of methane.[39, 40] Thus, the significant catalytic property with operational simplicity, high reactivity, environmental friendliness, reduced reaction times and reusability of PbO nanoparticles have prompted us to employ as catalyst for multicomponent reaction.

In view of the importance of PbO nanoparticles as a catalyst in organic synthesis, we report here a simple solvent-free synthesis of tetrahydrobenzo pyrans and benzylidene malonitriles. In this study, PbO nanoparticles were synthesized by hydrothermal method and characterized by IR, XRD, BET Surface area, SEM, EDAX and TEM with SAED techniques. In view of emerging importance of heterogeneous catalyst, we wish to explore the applications of PbO nanoparticles as a catalyst in organic synthesis.

2 Experimental

The synthesized PbO nanoparticles were characterized by FTIR using Shimadzu 8400 s instrument. XRD pattern was recorded using Phillips-1710 diffractometer with Cu–kα radiation (λ = 1.54 Å). The surface area was recorded with the help of Quantachrome Autosorb Automated Gas Sorption System. SEM and EDAX were recorded using JEOL-JEM-6360 microscope. TEM was recorded with SAED using CM-200 Philips Microscope. Melting points were determined using open capillary tubes on Veego melting point apparatus and are uncorrected. FTIR spectra were recorded on Shimadzu 8400 s spectrometer using KBr pellets. 1H NMR spectra were recorded on a Bruker Advance II 400 MHz spectrometer in DMSO-d 6 with TMS as an internal standard.

2.1 Synthesis of PbO nanoparticles

A mixture of citric acid (2.5 mmol) and sodium hydroxide (10 ml, 0.1 N) in distilled water was added to a magnetically stirred methanolic solution of lead nitrate (2 mmol). The reaction mixture was stirred for 2 h at room temperature. The white polycrystalline product was filtered, washed with distilled water and dried at 110°C for 2 h. The solid product was calcinized at 500°C for 2 h. Over the course of this process, the white PbO nanoparticles turned pale yellow colour.

2.2 Synthesis of tetrahydro benzo pyran derivatives

A mixture of aromatic aldehydes (1 mmol), malononitrile (1 mmol), dimedone (1 mmol) and PbO nanoparticles (50 mg) were ground at a room temperature with a mortar and pestle. The reaction was monitored by thin-layer chromatography (TLC). After completion of reaction, the product was washed with distilled water. The crude product was dried and recrystallized from ethanol to afford pure compounds with high yield (table 1; scheme 1).

Table 1 PbO nanoparticles catalysed synthesis of tetrahydro benzo pyrans.
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Scheme 1
scheme 1

Synthesis of tetrahydro benzo pyrans.

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2.3 Spectroscopic data

2.3a Compound (table 1, 4a):

IR (KBr, cm − 1): 3392, 3319, 3182, 2954, 2192, 1681, 1656, 1360 cm. − 1 1H NMR (400 MHz, DMSO-d 6): 0.94 (s, 3H, CH3), 1.04 (s, 3H, CH3), 2.08 (d, J = 16.0 Hz, 1H), 2.23 (d, J = 16.0 Hz, 1H), 2.50 (m, 2H, CH2), 4.11 (s, 1H), 7.06 (s, br, 2H, NH2), 7.19 (m, 3H, ArH), 7.33 (m, 2H). MS (m/z) = 294 (M + 1).

2.3b Compound (table 1, 4b):

IR (KBr, cm − 1): 3303, 3041, 2994, 2246, 1653, 1613, 1490, 850 cm. − 1 1H NMR (400 MHz, DMSO-d 6): 1.07 (s, 3H, CH3), 1.12 (s, 3H, CH3), 2.26 (s, 2H, CH2), 2.46–2.48 (m, 2H, CH2), 4.43 (s, 1H, CH), 6.58 (s, 2H, NH2), 7.18–7.28 (m, 4H, ArH). MS (m/z) = 328 (M + 1).

2.3c Compound (table 1, 4c):

IR (KBr, cm − 1): 3400, 3300, 3040, 2990, 2240, 1680, 1510, 840 cm. − 1 1H NMR (400 MHz, DMSO-d 6): 1.03 (s, 3H, CH3), 1.11 (s, 3H, CH3), 2.21 (d, J = 16.0 Hz, 1H), 2.22 (d, J = 16.0 Hz, 1H), 2.43 (s, 2H), 3.77 (s, 3H), 4.36 (s, 3H), 4.55 (s, 2H), 6.84 (d, J = 8.7 Hz, 2H), 7.15 (d, J = 8.7 Hz, 2H). MS (m/z) = 324 (M + 1).

2.3d Compound (table 1, 4e):

IR (KBr, cm − 1): 3394, 3323, 3213, 2970, 2193, 1683, 1523, 1365 cm. − 1 1H NMR (400 MHz, DMSO-d 6): 0.99 (s, 3H, CH3), 1.06 (s, 3H, CH3), 2.14 (d, J = 16.0 Hz, 1H), 2.30 (d, J = 16.0 Hz, 1H), 2.53–2.57 (m, 2H, CH2), 4.39 (s, 1H), 7.24 (s, 2H, NH2), 7.48 (d, 2H, J = 8.4 Hz, ArH), 8.21 (s, J = 8.4 Hz, 2H, ArH). MS (m/z) = 339 (M + 1).

2.4 Synthesis of benzylidene malonitrile derivatives (scheme 2)

A mixture of aromatic aldehydes (1 mmol), malononitrile (1 mmol) and PbO nanoparticles (40 mg) was ground in the presence of sunlight at room temperature. The reaction was monitored by TLC. After completion of reaction, the crude product was washed with distilled water, dried and recrystallized from alcohol to afford pure product (table 2).

Table 2 PbO nanoparticles catalysed synthesis of benzylidene malonitriles.
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Scheme 2
scheme 2

Synthesis of benzylidene malonitriles.

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2.5 Spectroscopic data

2.5a Compound (table 2, 3c):

IR (KBr, cm − 1): 3107, 2225, 1945, 1610, 1595, 1529, 1479 cm. − 1 1H NMR (400 MHz, DMSO-d 6): 7.86 (s, 1H), 8.31 (s, 1H, = CH), 8.33–8.45 (m, 3H). MS (m/z) = 200 (M + 1).

2.5b Compound (table 2, 3d):

IR (KBr, cm − 1): 3350, 3026, 2223, 1569, 1511 cm. − 1 1H NMR (400 MHz, DMSO-d 6): 6.91 (d, J = 6.4 Hz, 2H, ArH), 7.89 (s, 2H, ArH), 7.97 (s, 1H, = CH). MS (m/z) = 171 (M + 1).

2.5c Compound (table 2, 3e):

IR (KBr, cm − 1): 2231, 1607 cm. − 1 1H NMR (400 MHz, DMSO-d 6): 8.12 (d, 2H, J = 8.8 Hz, ArH), 8.31 (d, J = 8.8 Hz, 2H, ArH), 8.50 (s, 1H, = CH). MS (m/z) = 200 (M + 1).

2.5d Compound (table 2, 3g):

IR (KBr, cm − 1): 2229, 1584 cm. − 1 1H NMR (400 MHz, DMSO-d 6): 7.51 (d, 2H, J = 8.5 Hz, ArH), 7.70 (s, 1H, = CH), 7.85 (d, 2H, J = 8.5 Hz, ArH). MS (m/z) = 189 (M + 1).

3 Results and discussion

The FT-IR spectra of PbO nanoparticles show bands at 574, 642 and 844 cm − 1 due to Pb–O vibrations (figure 1). The absorption band around 3400 cm − 1 is due to the presence of water molecules. The XRD pattern (figure 2) suggests that the PbO nanoparticles contain 111, 002, 200, 210, 022, 222, 311 and 131 crystal planes. XRD pattern of the synthesized PbO confirmed the formation of a single orthorhombic structure (JCPDS Card No. 76–1796) with space group Pca 21 (29). Sharp diffraction peaks indicated good crystallinity. The broadening of peaks indicated that the particles are in nano regime and are in good agreement with observed SEM images. The average particle size of PbO nanoparticles was determined using Debye–Scherer formula [41] and was found to be 69 nm.

Figure 1
figure 1

FTIR spectrum of nanocrystalline PbO catalyst.

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Figure 2
figure 2

XRD pattern of nanocrystalline PbO catalyst.

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The SEM image (figure 3) showed the morphology and size of the synthesized PbO nanoparticles which, suggested the surface of the PbO nanoparticles are spongy and discrete in appearance. The elemental analysis (EDAX) confirmed the material contain Pb and O elements (figure 4). The TEM image revealed that the synthesized PbO material is orthorhombic with several hexagonal shaped crystallites (figure 5). The dark spot in the TEM micrograph (figure 5) can be alluded to synthesized PbO nanoparticles as SAED pattern associated with such spots reveals occurrence of PbO in total agreement with the XRD data. The average size of the PbO nanocrystallite by TEM was found to be 69 nm.

Figure 3
figure 3

SEM image of nanocrystalline PbO catalyst.

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Figure 4
figure 4

EDAX spectrum of nanocrystalline PbO catalyst.

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Figure 5
figure 5

(a) TEM image of nanocrystalline PbO catalyst. (b) SAED image of nanocrystalline PbO catalyst.

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The N2 adsorption–desorption isotherms and BJH pore size distribution of PbO nanoparticles (figure 6) revealed that the samples have typical IV N2 adsorption–desorption isotherms with H1 hysteresis which indicated that the sample reserve the cylindrical mesopores. The BJH pore size distribution demonstrated that all the samples have a narrow pore diameter range. Based on the N2 adsorption–desorption isotherms, the specific surface area (SBET) of PbO nanoparticles obtained from BET method was 31.99 m2/g, the average pore volume (VP) and pore diameter (dp) were 0.02256 cc/g and 30.86 Å (figure 6).

Figure 6
figure 6

BET surface area and pore size of nanocrystalline PbO catalyst.

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3.1 Catalytic results

In continuation of our work on the synthesis of heterocyclic molecules using nanoparticles for cyclization and condensation reactions.[42, 43] We report here facile synthesis of tetrahydrobenzo pyrans and benzylidene malonitriles by grinding under solvent-free condition using PbO nanoparticles. To optimize the reaction condition for synthesis of tetrahydrobenzopyrans, benzaldehyde, malononitrile and dimedone were used as a reactant.

In order to verify the role of grinding, the reaction mixture was left over night when it was observed that reaction remained incomplete and in the absence of catalyst does not proceed even after grinding. Under the optimized reaction conditions, a range of substituted tetrahydrobenzo pyran derivatives were synthesized by grinding at room temperature (table 1) using PbO as catalyst. In order to study the scope of the reaction, several substituted aromatic aldehydes with electron-donating as well as electron-withdrawing groups were employed. The reaction proceeded smoothly with good yields. The aromatic aldehydes with hydroxyl group required longer reaction time and gave lower yield. The heterocyclic aldehydes also reacted smoothly to give corresponding derivatives.

Similarly, for the synthesis of benzylidene malonitriles, benzaldehyde and malononitrile were used as model reaction to optimize the reaction condition. As mentioned above, the role of grinding was important for completeness of reaction. Under the optimized reaction conditions a range of substituted benzylidene malonitrile derivatives were synthesized by grinding at room temperature (table 2). The reactions proceeded smoothly for electron-withdrawing and electron-donating aromatic aldehydes with high yields. The aromatic aldehydes with hydroxyl groups and chloro groups at ortho position required longer reaction time and gave lower yields. The strong electron-donating aromatic aldehydes reacted quickly with high yields.

The role of catalyst was also studied for the model reaction with different amount of catalysts such as 10, 20, 30, 40, 50, 60 and 70 mg. It was observed that 50 mg of catalyst was sufficient to promote the reaction and greater amount of the catalyst did not improve the yields (table 3). In the synthesis of benzylidene malononitriles, to study the amount of catalyst required, the model reaction of benzaldehyde and malononitrile (scheme 2) was used with different amount of catalysts such as 10, 20, 30, 40, 50, 60 and 70 mg. It was found that the use of 40 mg of the catalyst was sufficient to promote the reaction (table 3).

Table 3 Effect of amount of catalyst on the reaction.
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To study the reusability of catalyst, it was separated after the completion of the reaction, washed with acetone and dried at 100°C and reused in the model reaction for four consecutive runs (table 4). Further, recycled catalyst was characterized by different analytical techniques. We observed that the particle size and crystal morphology of reused PbO nanoparticles was nearly the same. After every use, a little loss of catalytic activity was observed which may be attributed to microscopic change in the structure of the catalyst.

Table 4 Recycling of catalyst.
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4 Conclusions

We have developed a convenient, efficient protocol for one-pot synthesis of tetrahydrobenzo pyrans and benzylidene malonitriles in the presence of nanocrystalline PbO catalyst by grinding at room temperature. The attractive features of this procedure are simple work-up, mild reaction condition, short reaction time, excellent yield, solvent-free reaction and utilization of nanoparticles as a reusable catalyst.