Introduction

Recently, multi-component, one-pot synthesis have become one of the most attractive reactions due to their vast applications such as high selectivity, mild reaction condition and construction of several bonds in a single operation. These reactions are widely applied in pharmaceutical chemistry for producing different structures and combinatorial libraries for drug discovery [1]. 2-amino benzo[h]chromene are some examples of multi-component reactions. 4H-chromenes and fused 4H-chromene derivatives are attractive, because they generally show biological properties such as anti-microbial, anti-oxidant, anti-tumor, hypotensive and antiproliferation activities [2–6].

In recent years, there has been growing interest in finding inexpensive and effective solid acid nano catalyst such as nanocrystalline TiO2–HClO4 [7], nano-TiCl4·SiO2 [8–10] nano-SnCl4·SiO2 [11, 12] nano-BF3·SiO2 [13–15] HClO4–SiO2 nanoparticles [16] and nano-silica sulfuric acid [17–23]. Sawdust is one of the most common materials that used for various chemical industries such as removing pollutants from water [24], the color removal from textile industry [25], the removing cationic, anionic, and disperse dyes from aqueous solution [26] and the CuFe2O4/sawdust composite was used for removal of cyanine acid blue from aqueous solution [27].

The sawdust consists of cellulose, lignin, and hemicelluloses. Cellulose is composed of a long chain of glucose molecules, lignin is a complex polymer composed of phenylpropane units, and hemicelluloses are branched polymers composed of xylose, arabinose, galactose, mannose, and glucose [28–31]. The lignocellulosic material of sawdust includes a wide variety of hydroxyl groups that can be used as active sites for the preparation of solid acid catalysts.

In this study, the sawdust has been used as adsorbent for the preparation of nano-sawdust–BF3 whose average size is small and is well distributed. The presence of new functional groups on the surface of sawdust–BF3 resulted in a dramatic increase of surface polarity and acidity, thereby improving the catalytic efficiency of the nano-sawdust–BF3.

Recently some conditions and catalysts have been applied for the 2-amino benzo[h]chromene synthesis [32–38]. Although many of the reported methods are effective, but, some of them suffer from disadvantages such as harsh reaction conditions, use of hazardous solvents and toxic metals, long reaction times, complex working and purification procedures, long volume of catalyst loading and moderate yields. Therefore, the development of a simple, mild, and efficient method is still needed. In continuation of our previous research on the use of solid nano catalyst in organic synthesis [14–21], the nano-sawdust–BF3 as a new catalyst has been applied for the synthesis of 2-amino benzo[h]chromene derivatives.

Results and discussions

In continuation of our investigation into the application of solid acids in organic synthesis, we studied the application of sawdust as a green, inexpensive and available surface to synthesis of solid acid nano catalyst. In this study, sawdust–BF3 nanoparticles were prepared and characterized. The catalytic activity of nanoparticles was investigated for synthesis of 2-amino benzo[h]chromene derivatives, by the condensation of an aldehyde 1a–k, malononitrile 2 and 2-naphthol 3 (Scheme 1).

Scheme 1
scheme 1

Synthesis of 2-amino benzo[h]chromene derivatives in the presence of nano-sawdust–BF3 as catalyst

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Figure 1 shows the scanning electron microscopy (SEM) image of sawdust. The morphology and size of sawdust and sawdust–BF3 was observed by SEM images. After adding the BF3 to sawdust, the particles will be smaller and more homogeneous. As shown in Fig. 2, the size of nano-sawdust–BF3 is below 100 nm.

Fig. 1
figure 1

SEM micrograph of sawdust

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Fig. 2
figure 2

SEM micrograph of nano-sawdust–BF3

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The results of EDX analyses of the sawdust and sawdust–BF3 are given in the following Table 1.

Table 1 Chemical analysis of sawdust and sawdust–BF3
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The EDX analysis show how the different elements are distributed. Chemical analysis of sawdust gave carbon and oxygen as the major elements (Fig. 3). The elemental analysis by EDX identified the high peak of fluorine and boron in addition to other elements in the nano-sawdust–BF3 (Fig. 4). Presence of fluorine and boron in the EDX, indicates that the chemical interaction of boron trifluoride etherate with the surface of sawdust.

Fig. 3
figure 3

EDX of sawdust

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Fig. 4
figure 4

EDX of nano-sawdust–BF3

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In order to determine the optimum quantity of nano-sawdust–BF3, the reaction of 2-naphthol, malononitrile, and benzaldehyde was carried out under reflux in ethanol using different quantities of nano-sawdust–BF3. As shown in Table 2, 0.02 g of nano-sawdust–BF3 gives an excellent yield in 20 min. The above reaction was also examined in various solvents (Table 2, entries 1–4). Most of these solvents required a longer time and gave low yields, and the best results were obtained when ethanol was used (Table 2, entry 2). An interesting feature of this method is that the reagent can be regenerated at the end of the reaction and can be used several times without losing its activity. To recover the catalyst, after completion of the reaction, catalyst was separated and washed with ethanol and then dries the solid residue. This process repeated for two cycles and the yield of product 4a did not change significantly (Table 2, entries 8, 9).

Table 2 Optimization of the reaction conditions for synthesis of 4a
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To study the scope of the reaction, a series of aldehydes with 2-naphthol and malononitrile were examined by nano-sawdust–BF3 as catalyst. The results are shown in Table 3. In all cases, aromatic aldehyde substituted with either electron-donating or electron-withdrawing groups underwent the reaction smoothly and formed products in approving yields.

Table 3 Synthesis of 2-amino benzo[h]chromene
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The proposed mechanism for the formation of nano-sawdust–BF3 catalyzed 2-amino benzo[h]chromene is illustrated in Scheme 2. Condensation of aldehyde 1 and malononitrile 2 in the presence of the acidic catalyst of nano-sawdust–BF3 (A+) produce an intermediate 5. Then the Michael addition of 2-naphthol 3 to intermediate 5 would furnish intermediate 6. Finally, the product 4 was obtained by an intramolecular cyclization and tautomerism.

Scheme 2
scheme 2

Plausible mechanism for the formation of 2-amino benzo[h]chromene derivatives

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Conclusions

The present investigation shows that nano-sawdust–BF3 a capable nanocatalyst to be used for 2-amino benzo[h]chromene synthesis via one-pot reaction of aldehydes, malononitrile, and 2-naphthol. Nano-sawdust–BF3 was successfully prepared and characterized using EDX and SEM. Prominent among the advantages of this method are such as shorter reaction times, simple work-up, affords excellent yield, and re-usable for a number of times without appreciable loss of activity. The present method does not involve any hazardous organic solvent. Therefore, this procedure could be classified as green chemistry.

Methods

Instrumentation

Melting points were determined with an Electrothermal 9100 apparatus. IR spectra were recorded on a Shimadzu IR-470 spectrometer. 1H NMR spectra were obtained using Bruker Avance 400. The morphologies of the nanoparticles were observed using FESEM of a MIRA3 TESCAN microscope with an accelerating voltage of 15 kV. The EDX analysis was done using a SAMx-analyser. The chemicals for this work were purchased from Merck or Fluka chemical companies.

Synthesis of nano-sawdust–BF3

The sawdust was collected from sawmill in Farrokhi city, Iran. Sawdust was washed several times to remove adhering dirt and then dried at 60 °C for 24 h. The dried sawdust was ground to pass through a 1 mm sieve and labeled as sawdust [27]. The nano-sawdust–BF3 was prepared by combination of BF3·OEt2 (0.6 g, 4.2 mmol) drop by drop over 10 min via a syringe to sawdust powder (0.4 g) in a 50 ml flask include 5 ml diethyl ether at room temperature. The reaction mixture was stirred and then after 30 min, the ashy powder was separated and dried in an oven at 60 °C for 4 h and pulverized at the mortar. The size of particles was obtained below 100 nm using SEM.

General procedure for the synthesis 2-amino benzo[h]chromenes

Nano-sawdust–BF3 (0.02 g) was added to a stirred mixture of the aromatic aldehyde (1 mmol), malononitrile (1.2 mmol), and 2-naphthol (1 mmol) in EtOH (5 mL). The materials were mixed and refluxed for the appropriate time. The progress of the reaction was followed by TLC (n-hexane:ethyl acetate 5:1). After completion of the reaction, the mixture was filtered to remove the catalyst. After evaporation of the solvent, the crude product was re-crystallized from hot ethanol to obtain the pure compound.

3-Amino-1-phenyl-1H-benzo[f]chromene-2-carbonitrile (4a)

Mp: 278–280 °C; IR (KBr, cm−1): 3427, 3342, 2188, 1642, 1590, 1421; 1H NMR (CDCl3, 400 MHz) δ = 4.57 (s, 2H), 5.24 (s,1H), 7.20 (t, J = 7.36 Hz, 3H), 7.25 (d, J = 8.88 Hz, 2H), 7.38 (q, J = 3.36 Hz, 2H), 7.68 (q, J = 3.44 Hz, 2H), 7.77–7.81 (m, 2H) ppm.

3-Amino-1-(4-chlorophenyl)-1H-benzo[f]chromene-2-carbonitrile (4b)

Mp: 205–207 °C; IR (KBr,cm−1): 3424, 3325, 2192, 1653, 1590, 1405; 1H NMR (CDCl3, 400 MHz) δ = 4.61 (s, 2H), 5.23 (s, 1H), 7.11 (d, J = 8.44 Hz, 1H), 7.25(d, J = 8.88 Hz, 2H), 7.24 (q, J = 8.52 Hz, 2H),7.40 (q, J = 3.36 Hz, 2H), 7.61 (q, J = 5.8 Hz, 1H), 7.78–7.82 (m, 2H) ppm.

3-Amino-1-(2-chlorophenyl)-1H-benzo[f]chromene-2-carbonitrile (4c)

Mp: 264–265 °C;IR (KBr, cm−1): 3462, 3367, 2180, 1658, 1571, 1410; 1H MR (CDCl3, 400 MHz) δ = 4.62 (s, 2H), 5.88 (s, 1H), 6.90 (q, J = 1.76 Hz,1H), 7.02-7.11 (m, 2H), 7.23(s, 1H), 7.38-7.45 (m, 3H), 7.65 (d, J = 8.12 Hz, 1H), 7.77–7.80 (m, 2H) ppm.

3-Amino-1-(4-fluorophenyl)-1H-benzo[f]chromene-2-carbonitrile (4d)

Mp: 234–236 °C; IR (KBr, cm−1): 3458, 3358, 2187, 1665, 1594, 1411; 1H NMR (CDCl3, 400 MHz) δ = 4.66 (s, 2H), 5.26 (s, 1H), 6.96 (t, J = 8.4 Hz, 2H), 7.18 (q, J = 5.2 Hz, 2H), 7.28 (s, 1H), 7.42 (q, J = 3.2 Hz, 2H), 7.64 (t, J = 4.2 Hz, 1H), 7.83 (t, J = 6.0 Hz, 2H) ppm.

3-Amino-1-(3-nitrophenyl)-1H-benzo[f]chromene-2-carbonitile (4e)

Mp: 235–237 °C; IR (KBr,cm-1): 3465, 3349, 2190, 1658, 1593, 1529, 1411; 1H NMR (DMSO,400 MHz) δ = 5.60 (s, 1H), 7.13 (s, 2H), 7.36–7.47 (m, 3H), 7.56 (t, J = 8.48 Hz,1H), 7.64 (d, J = 7.8 Hz, 1H), 7.84 (d, J = 7.84 Hz, 1H), 7.91–8.06 (m, 4H) ppm.

3-Amino-1-(4-nitrophenyl)-1H-benzo[f]chromene-2-carbonitrile (4f)

Mp: 182–184 °C; IR (KBr, cm−1): 3435, 3348, 2191, 1654, 1590, 1518, 1412; 1H NMR (CDCl3, 400 MHz) δ = 5.20 (s, 1H), 7.13 (s, 2H), 7.37 (d, J = 8.96 Hz, 1H), 7.44–7.49 (m, 4H), 7.79 (t, J = 7.16 Hz, 1H), 7.92–7.98 (m, 2H), 8.15 (d, J = 8.67 Hz, 2H) ppm.

3-Amino-1-(4-methoxyphenyl)-1H-benzo[f]chromene-2-carbonitrile (4g)

Mp: 191–193 °C; IR (KBr,cm−1): 3437, 3354, 2186, 1648, 1590, 1421; 1H NMR (CDCl3, 400 MHz) δ = 3.72 (s, 3H), 4.59 (s, 2H), 5.19 (s, 1H), 6.79 (d, J = 8.6 Hz, 2H), 7.10 (d, J = 6.08 Hz, 2H), 7.23 (d, J = 8.97 Hz, 1H), 7.37–7.39 (m, 2H), 7.69 (t, J = 5.96 Hz, 1H), 7.79 (d, J = 8.48 Hz, 2H) ppm.

3-Amino-1-(4-hydroxy-3-methoxyphenyl)-1H-benzo[f]chromene-2-carbonitrile (4 h)

Mp: 249–250 °C; IR (KBr, cm−1): 3438, 3335, 2188, 1659, 1587, 1410; 1H NMR(CDCl3, 400 MHz) δ = 3.85 (s, 3H), 4.57 (s, 2H), 5.21(s, 1H), 5.49 (s, 1H), 6.60 (d, J = 8.4 Hz, 1H), 6.78 (t, J = 5.6 Hz, 2H), 7.29–7.31 (m, 1H), 7.42–7.44 (m, 2H), 7.72–7.74 (m, 1H),7.83 (d, J = 8.8 Hz, 2H) ppm.

3-Amino-1-(4-chloro-3-nitrophenyl)-1H-benzo[f]chromene-2-carbonitrile (4i)

Mp: 184–186 °C; IR (KBr, cm−1): 3454, 3369, 2196, 1678, 1589, 1415, 1357; 1H NMR (DMSO,400 MHz) δ = 5.55 (s, 1H), 7.15 (s, 2H), 7.37 (d, J = 8.96 Hz, 1H), 7.42–7.49 (m, 3H), 7.66 (d, J = 8.36 Hz, 1H), 7.82 (d, J = 8.16 Hz, 1H), 7.92–7.99 (m, 3H) ppm.

3-Amino-1-(4-hydroxyphenyl)-1H-benzo[f]chromene-2-carbonitrile (4j)

Mp: 288–290 °C; IR (KBr, cm−1): 3454, 3346, 3182, 2196, 1653, 1589, 1415; 1H NMR (DMSO, 400 MHz) δ = 5.15 (s, 1H), 6.62 (d, J = 8.4 Hz, 2H), 6.87–6.99 (m, 3H), 7.16 (s, 1H), 7.29–7.45 (m, 3H), 7.82–7.91 (m, 3H), 9.27 (s, 1H) ppm.

3-Amino-1-(pyridine-2-yl)-1H-benzo[f]chromene-2-carbonitrile (4 k)

Mp: 172–174 °C; IR (KBr, cm−1): 3458, 3349, 2186, 1662, 1593, 1406; 1H NMR (DMSO, 400 MHz) δ = 5.04 (s, 1H), 7.09 (d, J = 8.5 Hz, 1H), 7.19 (s, 2H), 7.47–7.56 (m, 3H), 7.64 (t, J = 7.9 Hz, 1H), 7.74 (d, J = 6.6 Hz, 1H), 7.89 (d, J = 8.1 Hz, 1H), 8.07 (s, 1H), 8.13 (d, J = 11.2 Hz, 1H), 8.24 (d, J = 8.5 Hz, 1H) ppm.