Chitosan-EDTA-Cellulose network as a green, recyclable and multifunctional biopolymeric organocatalyst for the one-pot synthesis of 2-amino-4H-pyran derivatives

In this research, cellulose grafted to chitosan by EDTA (Cs-EDTA-Cell) bio-based material is reported and characterized by a series of various methods and techniques such as FTIR, DRS-UV–Vis, TGA, FESEM, XRD and EDX analysis. In fact, the Cs-EDTA-Cell network is more thermally stable than pristine cellulose or chitosan. There is a plenty of both acidic and basic sites on the surface of this bio-based and biodegradable network, as a multifunctional organocatalyst, to proceed three-component synthesis of 2-amino-4H-pyran derivatives at room temperature in EtOH. The Cs-EDTA-Cell nanocatalyst can be easily recovered from the reaction mixture by using filtration and reused for at least five times without significant decrease in its catalytic activity. In general, the Cs-EDTA-Cell network, as a heterogeneous catalyst, demonstrated excellent catalytic activity in an environmentally-benign solvent to afford desired products in short reaction times and required simple experimental and work-up procedure compared to many protocols using similar catalytic systems.

www.nature.com/scientificreports/ On the other hand, multicomponent reactions (MCRs) have been considered as a superior synthetic strategy and an important subset of domino reactions in recent years. MCRs have received considerable attention from both academia and industry because of their various benefits over traditional multistep protocols in the synthesis of novel or complex heterocyclic scaffolds, especially in medicinal chemistry and pharmaceutical industry [41][42][43] . In addition, MCRs are valuable in organic synthesis because of their green chemical aspects such as high atom economy and low waste generation [44][45][46] . MCRs are highly convergent and require minimum time and effort to afford desired products. All of these merits can be intensified by using heterogeneous catalysts under mild reaction conditions 32,[47][48][49][50] .

Results and discussion
Characterization of the Cs-EDTA-Cell network (1). FTIR spectra of chitosan, cellulose and Cs-EDTA-Cell network (1) have been compared in Fig. 2. According to the FTIR spectra of chitosan (Fig. 2a) and cellulose (Fig. 2b), the wide band at ~ 3500-3200 cm −1 is related to intramolecular hydrogen bonds due to O-H stretching or N-H stretching vibrations. A peak found at ~ 2911 cm −1 is attributed to the C-H stretching of saccharide rings of both chitosan and cellulose. Furthermore, absorption band observed at 1660 cm −1 is assigned to the residual acetamide groups in the backbone of chitosan. On the other hand, The CH 2 bending or CH 3 symmetrical deformations of both chitosan and cellulose are appeared at ~ 1423-1375 cm −1 . The asymmetric stretching of the C-O-C bridge were confirmed by the presence of bands at ~ 1150 cm −1 . The broad signals at 1066 and 1028 cm −1 correspond to the C-O stretching. On the other hand, the FTIR spectra of Cs-EDTA-Cell network (1, Fig. 2c) show the absorption peaks at 1735 cm −1 , 1685 cm −1 , 1627 cm −1 corresponding to ester, acid and amide groups, respectively. All of these data demonstrate successful grafting of the cellulose to chitosan by using EDTA to form Cs-EDTA-Cell network (1). Figure 3 shows the diffuse reflectance UV-Visible (DRUV) spectra of chitosan, cellulose and Cs-EDTA-Cell network (1), respectively. In fact, chitosan and especially cellulose show very simple DRUV spectra. Hence, the www.nature.com/scientificreports/ characteristic absorption near 220 nm correspond to the pristine chitosan ( Fig. 3a-b). The DRUV absorption spectra of pure chitosan and cellulose exhibited maximum absorption peak (λ max ) around 220 nm. Furthermore, EDTA dianhydride cross-links chitosan and cellulose backbones and forms new amide and ester functional groups on reaction with amine groups of chitosan or the hydroxyl groups on both chitosan and cellulose. Consequently, the λ max in Cs-EDTA-Cell network (1) shifted to 230 nm due to formation of the new amide and esteric functional groups. Thermal stability of the bio-based Cs-EDTA-Cell network (1) was evaluated under air atmosphere at the range of 50-800 °C and a rate of 5 °C/min compared to chitosan and cellulose (Fig. 4). The TGA curves of the Cs-EDTA-Cell network and cellulose showed that their total weight losses were around 95%. In fact, commercial chitosan is less thermally stable than cellulose and demonstrates a two-stage weight losses starting at 200 and 290 °C. On the other hand, cellulose demonstrate higher thermal stability than chitosan and its decomposition starts at about 285 °C. However, the most of weight loss of cellulose (86%) occurs up to 405 °C in contrast to chitosan, which about of 32% of its mass has remained at this temperature. Interestingly, Cs-EDTA-Cell network (1) was found to be more thermally stable than its moieties, especially commercial chitosan 68 and cellulose 69 . Indeed, the most mass loss of the Cs-EDTA-Cell network (1, 56%) was occurred at about 220-405 °C that can be attributed to the decomposition of both chitosan and cellulose moieties as well as EDTA linker. This characteristic is very important in designing and application of reusable heterogeneous catalytic systems, which require recycling and subsequent consecutive heating during catalytic cycles or thermal reactivation.
The surface morphologies of the Cs-EDTA-Cell network (1) determined by FESEM analysis have been shown in Fig. 5. Indeed, pure chitosan 70 and cellulose 71 have smooth and layered surface morphologies according to the literature data. By comparing of the obtained FESEM images with surface morphologies of both pure chitosan and cellulose, it can be concluded that chitosan and cellulose are successfully grafted together by the EDTA linker. The average size of the spherical particles was measured to be about 28-57 nm in the Cs-EDTA-Cell network (1).
The XRD pattern of Cs-EDTA-Cell network (1) was compared to chitosan, cellulose and EDTA reference cards (      www.nature.com/scientificreports/ respectively ( Fig. 7a-b). Indeed, EDX analysis of Cs-EDTA-Cell network (1) confirmed clearly the presence of C, N and O atoms (Fig. 7c). As can be seen from the obtained results by the EDX spectra, the presence of C, O and N peaks can be ascribed to the structure of chitosan and cellulose biopolymers as well as EDTA. Since EDTA contains two nitrogen atoms in its structure, increasing of the percentage of N in the Cs-EDTA-Cell network (1) indicates that cellulose has been successfully grafted to chitosan by the EDTA linker.  it was investigated in the one-pot three-component synthesis of 2-amino-3-cyano-4H-pyran derivatives as a heterogeneous catalyst. First, to find the optimized conditions, the reaction of ethyl acetoacetate (2), 4-chlorobenzaldehyde (3a), malononitrile (4) was selected as the model reaction. The effect of various parameters such as the amount of catalyst loading, solvent, temperature, and time on the model reaction was examined. The obtained results is shown in Table 1. In the absence of any catalyst and under solvent-free conditions at 100 °C, only a trace amount of product ethyl 6-amino-4-(4-chlorophenyl)-5-cyano-2-methyl-4H-pyran-3-carboxylate (5a) was obtained after two hours (  13,14). Hence, 0.01 g of Cs-EDTA-Cell loading in ETOH at room temperature was selected as the optimized reaction conditions and used in further experiments to develop the scope of this protocol for the synthesis of other derivatives of 2-amino-3-cyano-4H-pyran scaffold 5a-l. The required time for other aromatic and heteroaromatic derivatives of aldehydes 4a-l when Cs-EDTA-Cell nanomaterial (1) was employed, as a heterogeneous organocatalyst, under optimized conditions are summarized in Table 2. All studied aldehydes 4a-l were smoothly involved in the optimized conditions to afford corresponding 2-amino-3-cyano-4H-pyran scaffold 5a-l. In addition, aldehydes bearing electron-withdrawing groups (entries 1-7) generally afforded the desired products in higher yields and short reaction times compared to benzaldehyde or aldehydes having electron-withdrawing groups (entries [8][9][10][11][12]. Mechanistic aspects of the synthesis of 2-amino-3-cyano-4H-pyran derivatives (5a-l) catalyzed by Cs-EDTA-Cell network (1). A plausible mechanism has been proposed for the reaction of ethyl acetoacetate (2), aromatic aldehydes (3a-l) and malononirile (4) in the presence of Cs-EDTA-Cell network (1, Fig. 8). According to this mechanism, both carboxylic acid and hydroxyl functional groups as well as amine basic sites of the multifunctional organocatalyst 1 with proper geometry are the main active sites that influence and  www.nature.com/scientificreports/ proceed different reaction steps. On the other hand, the by-product of the reaction is water molecules, which they can be absorbed by the Cs-EDTA-Cell network (1) efficiently in the EtOH solvent and accelerate the reaction.

Investigating of the reusability of Cs-EDTA-Cell network (1) in the synthesis of 2-amino-3-cy-
ano-4H-pyran derivative 5a. Finally, the recyclability of the catalyst 1 was examined under the optimized conditions. The results are shown in Fig. 9. Consecutive experiments for the model reaction between ethyl acetoacetate (2), 4-chlorobenzaldehyde (3a) and malononirile (4) under the optimized conditions were run and the recycled catalyst was used for the next experiment after activation. As shown in Fig. 9, the catalyst was recycled five times with only 10% loss in the model reaction yield. These data demonstrate proper stability of the heterogeneous Cs-EDTA-Cell catalyst (1) for the synthesis of 2-amino-3-cyano-4H-pyran derivatives 5.

Comparison of the catalytic activity of Cs-EDTA-Cell network (1) for the synthesis of 2-amino-3-cyano-4H-pyran derivatives 5a with other catalytic systems.
To show the merits of the synthesis of 2-amino-3-cyano-4H-pyran derivatives catalyzed by Cs-EDTA-Cell network (1), its catalytic performance, as a heterogeneous multifunctional catalyst, has been compared with the previous catalytic systems for preparation of 5a. The results have been summarized in Table 3. Comparison of the results shows that the catalytic activity of Cs-EDTA-Cell network (1) is superior to the most of introduced protocols in terms of catalyst loading, obtained yields, simplicity of the catalyst preparation process, and required temperature and time.

Conclusions
In this study, we designed and developed the thermally stable Cs-EDTA-Cell network, as a heterogeneous multifunctional organocatlyst, based on the most abundant natural biopolymers including chitosan and cellulose using the EDTA cross-linker. The Cs-EDTA-Cell network was prepared in one-pot and simple procedure and then characterized adequately by various microscopic or spectroscopic methods as well as analytical techniques. The new Cs-EDTA-Cell catalyst was investigated in the synthesis of 2-amino-4H-pyran derivatives at room temperature via a one-pot, multicomponent reaction between ethyl acetoacetate aromatic aldehydes, and malononirile in EtOH as a green solvent. Several advantages of this protocol are broad scope of substrates using low catalyst loading, high to excellent yields of the desired products, and short reaction times as well as simple work-up procedure and reusability of the catalyst for at least five consecutive runs.    Determination of the acidity of Cs-EDTA-Cell network (1). The acidity of the Cs-EDTA-Cell catalyst (1) was calculated by the back-titration. In this procedure, the Cs-EDTA-Cell (0.1 g), NaCl (0.1 g) and NaOH (0.1 M, 2 ml) were added to 7 mL of distilled water and stirred at room temperature for 24 h. During this time, all acidic protons, which released from the Cs-EDTA-Cell network (1) were completely neutralized by the hydroxide ions of NaOH. Then, two drops of the phenolphthalein indicator aqueous solution were added to the mixture and the color of the solution changed to pink. Eventually, the excess of hydroxide ions was titrated using HCl solution (0.1 M, 1.11 ml) to turn the color of the obtained mixture into colorless. This point indicates the neutral PH and demonstrates that the acidity of Cs-EDTA-Cell (1) is about 0.89 mmol.g -1 .

Experimental section
General procedure for the synthesis of 2-amino-4H-pyran 5a-l derivatives catalyzed by Cs-EDTA-Cell network (1). In a 5 ml round-bottomed flask, a mixture of ethyl acetoacetate (2, 1 mmol), aromatic aldehyde (3, 1 mmol), malononitrile (4, 1 mmol) and Cs-EDTA-Cell (1, 0.01 g) were added to EtOH (3 ml) in a 25 ml round-bottomed flask. The obtained mixture was mechanically stirred at room temperature for appropriate time indicated in Table 2. After completion of the reaction, which was confirmed by thin layer chromatography (TLC), hot EtOH (5 ml) was added and stirring continued at room temperature for 10 min. The catalyst 1 was separated easily by filtration and the filtrate was allowed at room temperature to precipitate pure product. The recycled catalyst 1 was kept in an oven at 50 °C for one hour and then reused for the next runs.
Physical and spectral data for the selected compounds 5a, 5c, 5j and 5 k. Ethyl Table 3. Comparative results of the catalytic activity of Cs-EDTA-Cell network (1) for the synthesis of 5a.