Introduction

Substantial research attention has been drawn to the newly emerged multi-component reactions (MCRs) in recent years (Safari et al. 2014; Rakhtshah and Salehzadeh 2016; Tayebee et al. 2014). This is because MCRs can offer the possibility of obtaining divers and complex organic molecules from simple building blocks in a single step (Nasr Esfahani et al. 2016; Gupta et al. 2016). Using MCRs in organic synthesis, especially when they are performed under solvent-free conditions sometimes also called as zipper reactions, can practically minimize the reaction times, unwanted by-products, energy, and environmental pollutions (Zhang et al. 2010). The newly developed MCRs, therefore, have been frequently used for the synthesis of some valuable compounds such as 1-amidoalkyl-2-naphthols. There are many reasons for the importance of 1-amidoalkyl-2-naphthol derivatives, including their potentially useful biological and pharmacological properties (Maleki et al. 2016; Moghanian et al. 2014; Taghrir et al. 2016: Kiasat et al. 2013). 1-Amidoalkyl-2-naphthols are commonly prepared via condensation reaction of aliphatic and/or aromatic aldehydes, 2-naphthol, and acetonitrile or amides in the presence of heterogeneous acid catalyst. Several catalysts, either Lewis or Brønsted acids, have been used to initiate this condensation reaction, such as graphene oxide (Gupta et al. 2016), sulfamic acid (Shaterian et al. 2008a, b), FeCl3@SiO2 (Shaterian et al. 2008a, b), heteropoly acids (Khabazzadeh et al. 2009), Fe(HSO4)3 (Shaterian et al. 2008a, b), K5CoW12O40·3H2O (Nagarapu et al. 2007), and Cu(II) acetylacetonate (Khairnar et al. 2016).

A desirable catalyst in any organic reaction has to be environmentally benign, easily recoverable, cost effective, and highly efficient at mild conditions. Most of the so far used catalysts for the synthesis of 1-amidoalkyl-2-naphthols do not meet all these clear-cut criteria. They suffer from some drawbacks, such as long reaction times, low product yields, the use of strongly acidic media, toxic or corrosive reagents, and complex equipment or experimental conditions. Therefore, designing of new catalysis system for the preparation of medicinally important amidoalkyl-2-naphthols, which has the least disadvantages, still remains a challenge (Nandi et al. 2009; Nagarapu et al. 2007; Kantevari et al. 2007; Singha et al. 2015; Zhu et al. 2012; Chen et al. 2013).

On the other hand, graphene, one of the thinnest materials with two-dimensional carbon sheet structure, has inspired enormous interest in diverse fields including catalytic chemistry. The extraordinary and unique properties of graphene make it one of the most favorable material for wide interesting applications (Sheshmani and Amini 2013; Georgakilas et al. 2012; Su et al. 2013; Allen et al. 2009; Guo et al. 2012; Gemeay et al. 2017; Rayati et al. 2017; Li et al. 2013; Rondinone et al. 1999; Chandekar and Kant 2017; Kumar et al. 2013). Moreover, combining of graphene with a magnetic material, such as CoFe2O4, will provide an excellent means for separation of graphene composites via magnetic decantation as well as reducing the agglomeration tendency of graphene sheets (Yan et al. 2010; Xiong et al. 2014; Rakjumar and Rao 2008; Sofia et al. 2009; Zhang et al. 2009). Magnetic nanoparticles (MNPs) deposited on the surface of graphene sheets can provide a suitable inorganic support for designing and constructing novel nanocatalysts. The fascinating feature of nanocatalysts containing MNPs is their ease of separation from the reaction media by applying an external magnet, which eliminates the need for tedious filtration or centrifugation processes. Another benefit of the MNPs presence in a solid catalyst is providing the required high surface area for desired functionalization (Dupont et al. 2002; Virtanen et al. 2010; Hosseini and Asadnia 2012; Jiang et al. 2009; Zheng et al. 2009; Polshettiwar et al. 2011; Lee et al. 2007; Majidi et al. 2006).

We herein report the synthesis of a new composite consisted of Cu(II) complex immobilized on magnetized reduced graphene oxide (RGO) and its application as catalyst for the synthesis of 1-amidoalkyl-2-naphthols. Three-component one-pot condensations of aldehydes, 2-naphthol, and amides were performed in the presence of this prepared nanocatalyst, under solvent-free conditions to give 1-amidoalkyl-2-naphthol derivatives. The synthesized composite, in which the Cu(II) center acts as Lewis acid, exhibited high catalytic activity in these condensation reactions. To the best of our knowledge, this is the first Cu(II) complex supported on magnetic RGO which is used as catalyst for the preparation of 1-amidoalkyl-2-naphthol derivatives.

Experimental

Graphene oxide (GO) was prepared and purified by the Hummers’ method (Rakjumar and Rao 2008) and converted to RGO using sodium borohydride (NaBH4) as reducing agent (Sofia et al. 2009). The RGO/CoFe2O4 composite was synthesized by dispersing of 0.4 g of RGO into 100 mL of deionized water with sonication for 2 h, and to this suspension, 4.04 g of Fe(NO3)3·9H2O and 1.45 g of Co(NO3)2·6H2O, dissolved in 25 mL of water, was added. After adjusting the pH of the mixture at 11–12 with 2 M NaOH and addition of 1.0 g of polyvinylpyrrolidone (PVP), as surfactant, it was heated at 80 °C for 3 h under continuous magnetic stirring. The obtained nanocomposite was collected by applying a permanent magnet and washed with hot water-ethanol and finely powdered after being dried in an oven at 60 °C for 24 h. In the next step, 1.0 g of RGO/CoFe2O4 composite was dispersed in 50 mL of dry toluene using an ultrasonic bath to produce a uniform suspension, and to this solution, 1.0 mL (5.5 mmol) of 3-chloropropyltrimethoxysilane (CPTMS) was added. The mixture was then stirred for 48 h at 80 °C. Finally, the obtained solid product was washed with toluene and dried in an oven to yield CPTMS functionalized RGO/CoFe2O4 (designated as RGO/CoFe2O4/Si-Cl). The Cu(II) complex was immobilized on the surface of RGO/CoFe2O4/Si-Cl as follows: 0.75 g (5.5 mmol) of 4-pyridinecarboxylic acid hydrazide, as ligand (Patel et al. 2013), was added to 1.0 g of well-dispersed RGO/CoFe2O4/Si-Cl in 40 mL of ethanol and the mixture was stirred for 24 h at 60 °C. The resulting solid was mixed with CuCl2·2H2O (0.93 g, 5.5 mmol) in 40 mL of ethanol and stirred for 24 h at 60 °C. The obtained product which is Cu(II) complex anchored on the surface of RGO/CoFe2O4 (designated as RGO/CoFe2O4@Cu(II)) was washed with 60 mL of ethanol (three times each time with 20 mL) and dried in an oven at 60 °C for 24 h.

Synthesis of 1-amidoalkyl naphthols catalyzed by RGO/CoFe2O4@Cu(II)

Of RGO/CoFe2O4@Cu(II), as nanocatalyst, 0.03 g was added to a mixture of 2-naphthol (1 mmol), aldehyde (1 mmol), and amide/urea (1.2 mmol). The mixture was magnetically stirred under solvent-free conditions in an oil bath at 120 °C for a certain period of time. The reaction was stopped when the TLC (hexane/ethyl acetate, 2:5) indicated complete consumption of the starting substrates. After completion, the resultant solid product was washed with hot ethanol and the nanocatalyst was separated by an external magnet and reused as such in new experiment. The obtained solid product of 1-amidoalkyl-2-naphthol was purified by recrystallization in aqueous ethanol solution to give pure product which was identified by comparison of their spectroscopic and physical data with those reported for known samples. The sequential procedure for the synthesis of RGO/CoFe2O4@Cu(II) nanocatalyst is presented in Fig. 1.

Fig. 1
figure 1

Schematic representation of step-by-step preparation of RGO/CoFe2O4@Cu(II) nanocatalyst

Full size image

Results and discussion

The Cu(II) complex supported on RGO/CoFe2O4, as a heterogeneous nanocatalyst, was characterized with FT-IR, XRD, SEM, EDX, TEM, BET, TGA, and VSM techniques.

In order to confirm the presence of the copper complex on the surface of RGO/CoFe2O4, FT-IR spectra of the as-synthesized materials were obtained and are shown in Fig. 2. The broad band at about 3420–3480 cm−1 was assigned to O-H stretching vibration in all the synthesized compounds. Comparison the spectra of RGO/CoFe2O4 and RGO/CoFe2O4@Si-Cl in Fig. 2A and B reveals an additional strong band at 1000–1200 cm−1 corresponding to the characteristic absorption of Si-O bonds in the latter composite. In the FT-IR spectrum of the RGO/CoFe2O4@Cu(II), the C=O stretching vibration has shifted to lower frequency (1721 cm−l) after coordination of the ligand to copper metal (Fig. 2D). In all these spectra, peaks at about 595 and 421 cm−l were observed which are attributed to the Fe3+-O stretching vibrations of cobalt ferrite.

Fig. 2
figure 2

FT-IR spectra of RGO/CoFe2O4 (A), RGO/CoFe2O4@Si-Cl (B), RGO/CoFe2O4@Si-4-Pyridin carbohydrazide (C), and RGO/CoFe2O4@Cu(II) (D)

Full size image

Figure 3 displays the PXRD patterns of CoFe2O4, RGO/CoFe2O4@Si-Cl, and RGO/CoFe2O4@Cu(II) nano materials. The PXRD pattern of CoFe2O4 (Fig. 3A) shows peaks at 2θ = 30°, 35°, 43°, 54°, 57°, and 63° expected for spinel cubic structure and all these peaks are seen in the PXRD patterns of other synthesized samples (Fig. 3B, C). This indicates that the crystallinity and morphology of CoFe2O4 are preserved during the grafting process. In the PXRD pattern of RGO/CoFe2O4@Si-Cl, peaks at 2θ = 14° and 2θ = 17° are also observed which can be assigned to RGO and silica components, respectively (Sahoo et al. 2013; Tang et al. 2012; Chang et al. 2013). The slight shift observed for these two peaks is probably due to their interactions with magnetic nanoparticles core. The crystallite size of pure CoFe2O4 was 25 nm as estimated by using the well-known Debye-Scherrer formula but increased to 45 nm in the final nanocatalyst.

Fig. 3
figure 3

PXRD patterns of CoFe2O4 (A), RGO/CoFe2O4@Si-Cl (B), and RGO/CoFe2O4@Cu(II) (C)

Full size image

The SEM and TEM images of RGO/CoFe2O4@Cu(II) are depicted in Fig. 4. The CoFe2O4 nanoparticles, which are well-resolved with almost spherical shapes, anchored with the copper complex can be clearly seen on the surface of RGO (Fig. 4a). Also the transmission electron microscopy (TEM) image of the as-prepared RGO/CoFe2O4@Cu(II) (Fig. 4b) clearly shows that CoFe2O4@Cu(II) on the surface of RGO has almost spherical-shaped morphology. The image indicates that CoFe2O4@Cu(II) (dark spots) was grafted on RGO, and the particles exhibit size distribution of 25–30 nm.

Fig. 4
figure 4

SEM (a) and TEM (b) images of RGO/CoFe2O4@Cu(II)

Full size image

The composition of the as-fabricated RGO/CoFe2O4@Cu(II) nanocomposite was further affirmed by EDX analysis. The EDX spectrum of this composite is shown in Fig. 5 which indicates the presence of copper, as well as other elements such as C, O, N, Cl, and Si existed in the composite. The copper content of the complex supported on RGO/CoFe2O4 was also measured by ICP analysis and was found to be about 1.7%. The chloride content of RGO/CoFe2O4@Cu(II) was determined by potentiometric titration with silver nitrate. The amount of Cl in this composite was found to be about 2.06% and the potentiometric titration curve is shown in Fig. S2. According to the results obtained from ICP measurement and potentiometric titration, the Cu/Cl mole ratio of the composite is approximately 1:2.

Fig. 5
figure 5

EDX spectrum of a RGO/CoFe2O4@Si-4-Pyridin carbohydrazide; b RGO/CoFe2O4@Cu(II)

Full size image

The magnetic properties of CoFe2O4 and RGO/CoFe2O4@Cu(II) were studied by a vibrating sample magnetometer (VSM), and the hysteresis loops are shown in Fig. 6. The VSM curve for RGO/CoFe2O4@Cu(II) nanocomposite shows that the value of saturation magnetization (Ms) is 28.5 emu g−1 which is much less than the Ms value of pure CoFe2O4 (61 emu g−1). The decrease of Ms may be due to the entrapment of CoFe2O4 NPs with the nonmagnetic components including silica and the copper complex. The magnetization of the RGO/CoFe2O4@Cu(II)composite, however, is still sufficiently high to ensure its separation and manipulation under an external magnetic field.

Fig. 6
figure 6

Hysteresis loops of CoFe2O4 (a) and RGO/CoFe2O4@Cu(II) (b)

Full size image

The as-fabricated RGO/CoFe2O4@Cu(II) was also studied by thermogravimetric analysis and its TGA curve is given in Fig. 7. A total weight loss of 47% for the nanocatalyst was observed on heating to 600 °C. This study has been performed at temperature range of 60–1000 °C in air atmosphere. Degassing and solvent evaporation at low temperatures accounts for approximately 4% of the weight loss. The second stage is from 120 to 280 °C with a weight loss of 6%, is due to the release of the organic ligand in the Cu(II) complex. The main weight loss which occurs at temperatures from 280 to 450 °C corresponds to the combustion of residue organic ligands as well as decomposition of the composite (37%). DTA analysis of RGO/CoFe2O4@Cu(II) is shown in Fig. S1.

Fig. 7
figure 7

The TGA thermogram of RGO/CoFe2O4@Cu(II)

Full size image

Nitrogen adsorption/desorption is a common method for characterization of mesoporous materials which provides information about the specific surface area, average pore diameter, and pore volume. N2 adsorption-desorption isotherm and BJH pore plot of the as-prepared nanocatalyst are shown in Fig. 8. The BET surface area, pore volume, and pore diameter of RGO/CoFe2O4@Cu(II) are found to be 60.011 m2 g−1, 0.25 cm3, and 16.16 nm, respectively. The observed isotherm for RGO/CoFe2O4@Cu(II) corresponds to type(III)adsorption isotherm, which can also explain the formation of multilayer structure for the nanocomposite.

Fig. 8
figure 8

N2 adsorption/desorption isotherm and BJH-Plot of RGO/CoFe2O4@Cu(II)

Full size image

Evaluation of catalytic activity of RGO/CoFe2O4@Cu(II)

The capability of RGO/CoFe2O4@Cu(II) nanocomposite as a heterogeneous catalyst was evaluated in the synthesis of 1-amidoalkyl-2-naphthol derivatives. To find optimized conditions for the reaction, various parameters, including catalyst dose, type of solvent, and different temperatures, were probed. The model reaction, condensation of benzaldehyde with 2-naphthol and acetamide, was checked with two different amounts of nanocatalyst (0.015 and 0.03 g) and better result was obtained with 0.03 g dose of catalyst, under solvent-free conditions. This reaction proceeds only in the presence of the catalyst because no new product was detected in the absence of this catalyst (see Table 1). The reaction was also studied in different solvents, such as EtOH, CH3CN, CH2Cl2, CHCl3, and n-hexane to compare with the solvent-free conditions. It was found that solvent-free conditions give better results in respect of reaction times and yields (Table 2). The effect of temperature was also checked and based on the obtained results 120 °C was selected for all the examined reactions (see Table 1). The condensation of various amides and aldehydes with 2-naphthol under the optimum conditions was then carried out in the presence of RGO/CoFe2O4@Cu(II) as catalyst. In all the examined reactions, aminoalkylnaphthols were found to be the only products and no other by-products were observed.

Table 1 Effect of different catalysts, amount of catalyst, and temperature range on the preparation of amidoalkyl naphthol derivatives under solvent-free conditions a
Full size table
Table 2 Effect of solvent on the synthesis of amidoalkyl naphthol derivatives
Full size table

In this study, it was also found that aromatic aldehydes containing either electron-donating or electron-withdrawing groups give the target product with high yield. However, aromatic aldehydes with electron-donating groups (OH, N(CH3)2, CH3) required longer reaction times compared to those aldehydes bearing electron-withdrawing groups (Cl, NO2, Br). These observations are in agreement with the previously reported findings [1–7]. Some of these condensation reactions were performed using urea or benzamide instead of acetamide and a set can be seen in Table 3, longer reaction times were needed for these substrates to give substantial yields.

Table 3 Three-component synthesis of amidoalkylnaphthol derivatives using 0.03 g of nanocatalysta
Full size table

In Table 4, the catalytic activity of the as-prepared nanocatalyst was compared with other recently studied catalysts for the synthesis of amidoalkylnaphthol derivatives. The data show that the catalyst of the present work is superior to the other reported catalysts with respect to reaction times and reaction conditions. The superiority of our introduced nanocatalyst will become more evident when its magnetic property is taken into account which provides a facile and efficient separation for the catalyst in the recycling process.

Table 4 Comparison of efficiency of the RGO/CoFe2O4@Cu(II) with other catalysts
Full size table

Recycling and reusability of the catalyst

Facile recovery is considered as an important advantage of the heterogeneous catalysts in organic chemistry and industry. In order to test the as-made catalyst reusability, the synthesis of amidoalkylnaphthol derivatives was achieved in the presence of catalytic amount of RGO/CoFe2O4@Cu(II) under optimized reaction conditions. After completion of the reaction, the catalyst was isolated by applying an external magnetic force, followed by washing it for three times with ethanol. After drying the isolated catalyst at 100 °C for 2 h, it was reused in a new reaction with fresh substrates. The results of reusability tests of this catalyst are presented in Table 5, which indicates the recovered catalyst could be reused for at least five successive times with almost no change in its catalytic performance. The amount of copper was also measured in the recycled nanocatalyst after five runs by ICP-AES analysis. It was found that the copper content of the catalyst remained almost unchanged during the catalytic reactions indicating no detectable leaching of the catalyst into the solution. SEM and TEM images of RGO/CoFe2O4@Cu(II), after reusing it for five runs, are presented in Fig. S3, and the PXRD pattern of the same sample is depicted in Fig. S4. These analyses clearly revealed that the morphology and the structure of the used catalyst, after five successive catalytic reactions, are almost the same as fresh catalyst.

Table 5 Reusability of RGO/CoFe2O4@Cu(II) nanocatalyst
Full size table

Mechanism for the catalysis reactions

Based on the previously reported research (Nasr esfahani et al. 2016; Zhang et al. 2010) and our observation, a plausible mechanism for the formation of amidoalkylnaphthols from 2-naphthol, acetamide and aldehyde was proposed which is presented in Fig. 9. According to this mechanism, the copper complex can act as a Lewis acid which activates the carbonyl group of the aldehyde to form intermediate (I). A subsequent nucleophilic attack by 2-naphthol on the latter intermediate will result in the formation of intermediate (II). Finally, a Michael reaction occurs through the addition of amide or urea to the last intermediate will lead to the production of 1-amidoalkyl-2-naphthol derivative product.

Fig. 9
figure 9

Mechanism for the formation of amidoalkyl naphthols catalyzed by RGO/CoFe2O4@Cu(II)

Full size image

Conclusion

In brief, a straightforward and facile procedure was used to immobilize a Cu(II) complex onto magnetized reduced graphene to construct a novel nanocatalyst. This composite was prepared in order to be employed as catalyst for the synthesis of 1-amidoalkyl-2-naphthols using multicomponent condensation reactions. It was shown that the as-synthesized nanocatalyst can efficiently catalyze the condensation of 2-naphthol, aldehydes, and amides to give excellent yields of 1-amidoalkyl-2-naphthol derivatives. According to the obtained results and the reaction conditions, the current catalysis system can be considered as an efficient, rapid, and green procedure for the synthesis of the biologically and pharmaceutically important substituted amidoalkylnaphthols.