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. 2026 Aug 20. Online ahead of print. doi: 10.1039/d6ra05766g

Amine-functionalized biochar as a green and recyclable nanocatalyst for the efficient synthesis of 1,8-naphthyridine and 2-amino-4H-chromene derivatives

Reza Hazbavi a, Maryam Hajjami a,✉, Mohammad Ali Zolfigol a,✉, Zahra Siahpour a
PMCID: PMC13489792  PMID: 42625578

Abstract

Biochar is a renewable carbon-based material derived from biomass resources, which has attracted widespread attention due to its low production cost, environmental sustainability and high surface modification and functionalization capabilities. However, its potential application as an effective catalyst, especially for green multicomponent organic reactions, is still not fully explored. In this study, an amine-functionalized biochar nanocatalyst (biochar-NH2) is introduced as an effective, cost-effective, and environmentally friendly system for the green synthesis of 1,8-naphthyridine and 2-amino-4H-chromene derivatives. Biochar was prepared by pyrolysis of walnut shells and then modified by nitration and reduction to obtain biochar-NH2. The catalyst was characterized by SEM, EDX, FT-IR, BET and XRD analysis and was evaluated for its catalytic performance in multi-component reactions in aqueous media at ambient temperature. This catalyst exhibited very high efficiency for the synthesis of 1,8-naphthyridine and 2-amino-4H-chromene derivatives under mild conditions and using water as a green solvent. Under the optimized conditions, the model 1,8-naphthyridine derivative was obtained in 98% yield within 20 min using biochar-NH2 (0.04 g, 28.5 wt% relative to 4-chlorobenzaldehyde) in water at 25 °C. Under the same catalyst loading and reaction conditions, the model 2-amino-4H-chromene derivative was obtained in 97% yield within 7 min. The catalyst showed high recyclability and only a slight decrease in its corresponding activity was observed after several cycles of use, which indicates its structural stability and recyclability. Amine-enriched biochar is a highly effective, eco-friendly, and reusable (5-cycle) nanocatalyst for the synthesis of critical pharmaceutical heterocycles. It is a greener substitute for conventional catalysts and it helps researchers to move forward with green synthesis approaches.

Biochar is a renewable carbon-based material derived from biomass resources, which has attracted widespread attention due to its low production cost, environmental sustainability and high surface modification and functionalization capabilities.graphic file with name d6ra05766g-ga.webp

1. Introduction

The many attractive properties of biochar, such as its cheapness, carbon-rich nature and environmental friendliness, are attracting increasing attention from researchers. Biochar can be produced from a variety of renewable biomass sources, including agricultural waste, sludge and sediments. This is in contrast to commercial carbon-based materials such as activated carbon, graphene and carbon nanotubes which have limited and costly raw materials. Biochar is generated through pyrolysis in the absence or limited presence of oxygen. It is an inexpensive method as its raw materials are cheap.1,2 In addition to its economic benefits, this approach is in line with the principles of green chemistry because it emphasizes the use of renewable resources and recycling of waste materials.3,4 Biochar is not only biocompatible, but also cheap and non-toxic. The nanoparticle surface of biochar is decorated with carbonyl, carboxylic acid and hydroxyl functional groups, which allow surface modification and facilitate the immobilization of different catalytic species.5,6 These properties have facilitated the applications of biochar in various fields including water purification,7 3D printing,8 soil remediation,9 CO2 capture,10 and electrochemical systems.11 Despite the great potential of biochar, it has not been explored as a multi-purpose catalyst or a catalytic support in multicomponent reactions. This indicates that further research is needed to find out what it can actually do.

Multicomponent reactions (MCRs) are of great interest in the synthesis of organic compounds since they lead to the direct formation of target products without isolating intermediates thus decreasing the reaction time and energy consumption.12–15

Naphthyridines are key compounds in medicinal chemistry and play an essential role in drug design and the synthesis of biologically noteworthy natural products. In particular, 1,8-disubstituted derivatives of this family have attracted much attention16 because they exhibit a wide range of biological activities, including anti-inflammatory,17 antibacterial,18 antimalarial,19 antihypertensive,20 gastric secretion inhibitory,21 and antitumor22 effects. Koehler first synthesized this compound in 1927, and since then, it has been a key compound in medicinal chemistry.23 Nalidixic acid, with naphthyridine structure, is used specifically to treat urinary tract infections caused by Gram-negative bacteria.24 Gemifloxacin, a member of the naphthyridine class, is used to treat acute bacterial exacerbations of chronic bronchitis and pneumonia.25 Some naphthyridine derivatives, including 1-(2-fluorobenzyl)-3-(2-tolyl)-1,8-naphthyridin-2(1H)-one, are used in the treatment of disorders associated with Alzheimer's disease,26 and Vosaroxin is used to treat acute myeloid leukemia (AML) and ovarian cancer.27 The molecular structures of these compounds are shown in Fig. 1. Recent studies demonstrate the potential of heterogeneous catalytic systems in the synthesis of 1,8-naphthyridine derivatives. Ghiai et al. reported a bifunctional porous organic polymer supported chlorosulfonic acid catalyst for one-pot three-component synthesis of 1,8-naphthyridine derivatives under mild conditions. The designed catalytic system afforded the desired products in good to high yields with good recyclability. However, the synthesis of porous organic polymer based catalyst requires multiple steps of synthesis and functionalization which may increase the complexity and cost of the catalyst preparation.28 In another work, an erbium anchored iminodiacetic acid (IDA)-functionalized CoFe2O4 magnetic nanocomposite was prepared and used as a heterogeneous catalyst for facile synthesis of 1,8-naphthyridine derivatives. The magnetic nanocomposite provided an efficient method for the synthesis of the target products under mild reaction conditions, and it could be conveniently recovered from the reaction mixture using an external magnetic field, which made it easy to reuse. A magnetic CoFe2O4 support decorated with erbium based active sites represented an efficient catalytic system for synthesis of 1,8-naphthyridines. However, the multi-step functionalization and metal anchoring preparation of the catalyst can increase the complexity and cost of the catalyst synthesis. Also, the use of erbium, a rare-earth element, might be a concern on the economic and environmental sustainability of the catalytic system. Therefore, the development of simpler, low-cost, metal-free, and biomass-derived heterogeneous catalysts with facile preparation, efficient catalytic activity, and good recyclability remains highly desirable.29 More recently, Beiranvand et al. (2024) synthesized a MOF catalyst, named Basu-HDI, and used it for the synthesis of new 1,8-naphthyridine derivatives via an anomeric oxidation strategy.30 The anomeric effect (negative hyperconjugation) can promote the formation of the final products via donation of nonbonding electron pairs of heteroatoms into antibonding orbitals of C–H bonds without the direct use of an external oxidizing agent.31,32 This concept has been proposed as a technique for in situ oxidation of organic compounds without an external oxidizing agent. As shown in Fig. 2, stereoelectronic effects play a fundamental role in the redox processes of biologically important compounds, such as NADH/NAD+ and NADPH/NADP+.33 The as-synthesized Basu-HDI MOF can be considered an efficient heterogeneous catalytic system for the synthesis of 1,8-naphthyridine derivatives, providing high yields, short reaction times, low catalyst loading, and good recyclability. However, the synthesis of this MOF requires multistep preparation and post-functionalization procedures which include hydro-solvothermal synthesis followed by reaction with hexamethylene diisocyanate under controlled conditions. The reported reactions were also performed under reflux in ethanol. These considerations underscore the ongoing need for simpler, inexpensive, biomass-derived heterogeneous catalysts capable of operating efficiently at milder conditions and in environmentally benign solvents such as water.

Fig. 1. Some drugs contain a 1,8-naphthyridine nucleus.

Fig. 1

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Fig. 2. Molecular structures of NADPH/NADP+.

Fig. 2

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Numerous natural products, pharmacologically and physiologically active substances, and a wide variety of medications contain heterocyclic compounds that contain 4H-chromene. Derivatives of 2-amino-4H-chromene are known to have a variety of biological properties, including antitumor,34–36 antioxidant,37,38 anti-inflammatory,38,39 antimicrobial,40,41 anti-tuberculosis,42 antiviral,43 antiproliferative,44 antifungal and antibacterial,45 anticoagulant and anti-HIV.46 For the synthesis of 2-amino-4H-chromenes, multicomponent reactions (MCRs) using a wide range of catalysts have attracted much attention as an efficient method in recent years. In this regard, various catalytic materials derived from natural sources, inorganic compounds, and nanostructures has been utilized, including glycine,47 mesolite,48 potassium phthalimide,49 MgFe2O4 nanoparticles,50 POM@Dy-PDA,51 P4VPy-CuI,52 clinoptilolite nanozeolite,53 aqueous lemon peel extract (WELFSA),54 tungstic acid-functionalized SBA-15,55 MIL-101(Cr)-SO3H,56 ionic compound [Et2NH(CH2)2CO2H][AcO],57 {[4,4′-BPyH][C(CN)3]2}9,58 hydrotalcite,59 Fe3O4 magnetic nanoparticles,60 and nanocellulose-OTiCl3.61 These catalytic systems have exhibited good to excellent catalytic activity for the synthesis of 2-amino-4H-chromene derivatives but each of the catalysts has some advantages and limitations. Homogeneous catalysts like glycine and ionic compounds can show good catalytic efficiencies and simple reaction conditions but usually their separation from the reaction mixture and their reuse are difficult. Inorganic and metal-based catalysts such as MgFe2O4 nanoparticles and Fe3O4 magnetic nanoparticles have high catalytic activity and, in some cases convenient magnetic separation. However, the preparation of these catalysts may involve relatively costly or complex procedures and potential environmental concerns with metal-containing materials should also be considered. Moreover, functionalized porous materials, MOFs and zeolites-based catalysts can provide high surface areas and tunable active sites. However, the synthesis and functionalization of these may involve multi-step procedures and relatively expensive reagents. Bio-derived catalysts and catalytic materials, such as aqueous lemon peel extract and nanocellulose-based systems, have the advantages of renewability and environmental compatibility; however, their catalytic performance and structural stability can be strongly dependent on the source and preparation conditions. Therefore, there is a great need for the development of a heterogeneous catalyst with low cost, renewable, functionalized and reusable, and works efficiently under mild conditions and in environmentally friendly solvents.

Considering the disadvantages of the previously reported catalytic systems, there is a need for the development of simple, low-cost, renewable and recyclable heterogeneous catalysts that can catalyze multicomponent reactions under mild and environmentally benign conditions. Therefore, the main objective of this study was to develop an amine-functionalized biochar-based nanocatalyst (biochar-NH2) from renewable walnut-shell biomass and to investigate its catalytic efficiency in the green synthesis of two important classes of heterocyclic compounds, namely 1,8-naphthyridine and 2-amino-4H-chromene derivatives (Scheme 1). The catalytic performance of biochar-NH2 was systematically evaluated under mild reaction conditions, with particular emphasis on the use of water as a green solvent, room-temperature operation, high product yields, short reaction times, and catalyst recyclability. In short, the present work aims to show the feasibility of renewable amine-functionalized biochar as an efficient, green and reusable heterogeneous nanocatalyst for multicomponent organic synthesis.

Scheme 1. Green synthesis of 1,8-naphthyridine and 2-amino-4H-chromene derivatives catalyzed by a biochar-NH2 nanocatalyst.

Scheme 1

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2. Experimental

2.1. Materials and methods

All the materials and solvents were obtained from Merck or Sigma-Aldrich and used without further purification. FT-IR spectra were recorded as KBr pellets using a PerkinElmer Spectrum Version 10.02.00. 1H NMR (500 MHz) and 13C NMR (125 MHz) spectra were recorded on a Bruker spectrometer (δ in ppm) using DMSO-d6 as solvent, with chemical shifts referenced to TMS as internal standard. SEM images were taken on FE-SEM QUANTA 200. The elements were identified by EDAX SILICON DRIFT 2017. The X-ray diffraction (XRD) measurements were carried out with a Bruker D8 Advance instrument.

2.2. Synthesis of biochar

Initially, selected walnut shells obtained from walnuts cultivated in Hamadan Province, Iran, were washed with deionized water and sun-dried at 25 °C for three days. The dried material was then ground in an electric mill and passed through a 1 mm sieve to obtain a uniform particle size. The resulting powder was converted to biochar by slow pyrolysis (heating rate 5 °C min−1 in an airless furnace) at 600 °C for 2 hours.62,63

2.3. Synthesis of biochar-NO2

20 mL of concentrated H2SO4 and 30 mL of concentrated HNO3 were slowly added to a 150 mL round-bottom flask for the nitration of biochar. The flask was placed in an ice-water bath to cool the mixture. Then, 1.5 g of biochar was added to the flask and stirred for 2 h. After nitration, the reaction mixture was cooled to room temperature. The resulting solid (biochar-NO2) was separated by filtration and washed with deionized water and ethanol. The obtained biochar-NO2 was then dried at room temperature for 24 h to remove residual ethanol.

2.4. Synthesis of amine-enriched biochar (biochar-NH2)

1 g of biochar-NO2 was placed in a 50 mL round bottom flask with 10 mL of deionized water and 5 mL of 6 M ammonium hydroxide. It was stirred for 15 min and then 5.8 g of sodium thiosulfate (Na2S2O3) was added and stirred for 20 h. The flask was fitted with a reflux condenser and 20 mL of 5 M glacial acetic acid was added. The mixture was refluxed for 5 h, cooled to room temperature, filtered and washed with deionized water and ethanol. The obtained product was dried at room temperature for 24 h to obtain the modified biochar (biochar-NH2).

2.5. Spectral data for the 1,8-naphthyridine and 2-amino-4H-chromene derivatives

2.5.1. 2,7-Diamino-4-(3-hydroxyphenyl)-1,8-naphthyridine-3-carbonitrile

FT-IR (KBr, cm−1): 3514, 3410, 3326, 3200, 2214, 1645, 1619, 1567, 1548, 1518, 1490, 1447, 1403, 1350, 1247, 1209, 1134, 844, and 781. 1H NMR (500 MHz, DMSO) δppm 9.77 (s, 1H), 7.34 (t, J = 7.9 Hz, 1H), 7.25 (d, J = 8.9 Hz, 1H), 6.98 (s, 2H), 6.92 (d, J = 8.2 Hz, 1H), 6.85–6.69 (m, 4H), 6.44 (d, J = 9.1 Hz, 1H). 1H NMR (500 MHz, D2O) δppm 7.35 (t, J = 7.9 Hz, 1H), 7.25 (d, J = 9.1 Hz, 1H), 6.92 (d, J = 8.4 Hz, 1H), 6.84–6.64 (m, 2H), 6.46 (d, J = 9.1 Hz, 1H). 13C NMR (126 MHz, DMSO) δppm 162.3, 158.9, 158.5, 157.3, 155.0, 135.9, 135.6, 129.7, 119.5, 116.8, 116.0, 115.7, 109.3, 108.2, 87.3.

2.5.2. 2,7-Diamino-4-(2,4-dichlorophenyl)-1,8-naphthyridine-3-carbonitrile

FT-IR (KBr, cm−1): 3441, 3351, 3188, 2215, 1639, 1610, 1571, 1481, 1401, 1300, 1252, 1139, 1103, 1055, 799, 678, 553, 469. 1H NMR (500 MHz, DMSO) δppm 7.88 (d, J = 2.0 Hz, 1H), 7.63 (dd, J = 8.3, 2.1 Hz, 1H), 7.50 (d, J = 8.2 Hz, 1H), 7.12 (s, 2H), 7.01 (d, J = 8.9 Hz, 1H), 6.99 (s, 2H), 6.46 (d, J = 8.9 Hz, 1H). 13C NMR (126 MHz, DMSO) δppm 162.4, 158.7, 158.3, 151.0, 135.0, 134.9, 132.9, 132.6, 132.2, 129.3, 127.7, 116.0, 109.9, 108.1, 87.9.

2.5.3. 2-Amino-4-(4-chlorophenyl)-7-hydroxy-4H-chromene-3-carbonitrile

FT-IR (KBr, cm−1): 3447, 3033, 2229, 1585, 1557, 1489, 1408, 1370, 1291, 1217, 1009, 936, 828, 616, 521, 496. 1H NMR (500 MHz, DMSO) δppm 9.72 (s, 1H), 7.41–7.30 (m, 2H), 7.22–7.13 (m, 2H), 6.91 (s, 2H), 6.77 (d, J = 9.0 Hz, 1H), 6.48 (dd, J = 8.4, 2.3 Hz, 1H), 6.40 (d, J = 2.4 Hz, 1H), 4.65 (s, 1H). 13C NMR (126 MHz, DMSO) δppm 160.2, 157.2, 148.8, 145.3, 131.2, 131.1, 129.8, 129.3, 129.2, 128.5, 120.4, 113.2, 112.4, 102.2, 55.9, 18.5.

2.5.4. 2-Amino-4-(4-bromophenyl)-7-hydroxy-4H-chromene-3-carbonitrile

FT-IR (KBr, cm−1): 3414, 3033, 2228, 1640, 1585, 1556, 1489, 1408, 1370, 1291, 1216, 1096, 1009, 828, 617, 520, 496. 1H NMR (500 MHz, DMSO) δppm 9.72 (s, 1H), 7.53–7.45 (m, 2H), 7.16–7.08 (m, 2H), 6.90 (s, 2H), 6.77 (d, J = 8.9 Hz, 1H), 6.48 (dd, J = 8.4, 2.4 Hz, 1H), 6.39 (d, J = 2.3 Hz, 1H), 4.64 (s, 1H). 13C NMR (126 MHz, DMSO) δppm 160.1, 157.1, 148.7, 145.6, 131.4, 129.8, 129.5, 120.3, 119.6, 113.0, 112.4, 102.1, 55.9, 18.4.

2.6. General procedure for the synthesis of 1,8-naphthyridine derivatives using amine-enriched biochar (biochar-NH2)

In a 25 mL round bottom flask, a mixture of 2,6-diaminopyridine (1 mmol, 0.109 g), aromatic aldehydes (1 mmol), malononitrile (1 mmol, 0.066 g) and biochar-NH2 nanocatalyst (0.04 g) in water (5 mL) as a solvent was stirred at room temperature. The progress of the reaction was followed by TLC (n-hexane/ethyl acetate: 8/2). The catalyst was filtered off after the completion of the reaction. The solvent (water) was removed from the reaction mixture and the pure product was recrystallized from ethanol. The structure was confirmed by FT-IR and NMR Techniques.

2.7. General procedure for the synthesis of 2-amino-4H-chromene derivatives using amine-enriched biochar (biochar-NH2)

Resorcinol (1 mmol, 0.110 g), malononitrile (1 mmol, 0.066 g), aromatic aldehyde (1 mmol), biochar-NH2 (0.04 g) as a nanocatalyst, and water (5 mL) were mixed and stirred at room temperature for an appropriate time. After the reaction, hot ethanol was added to dissolve the organic materials. After the catalyst was separated by filter paper and the solvent evaporated, the product was purified by recrystallization from ethanol. Its structure was confirmed using FT-IR and NMR techniques.

3. Results and discussion

3.1. Characterization of biochar-NH2

After the reduction of the nitro groups of biochar by sodium thiosulfate (Na2S2O3) and the synthesis of the final catalyst (biochar-NH2), SEM, BET, FT-IR, EDX, and XRD methods were employed to verify the structural modification and identify the physicochemical properties of the catalyst. The results of these analyses indicated the successful modification of functional groups, morphological changes, and an increase in the specific surface area of biochar after the reduction process.

3.2. FT‐IR spectra

This technique detects and identifies the functional groups present in nanoparticles. Fig. 3 shows the FT-IR spectrum of biochar, biochar-NO2 and biochar-NH2 as a 500–4000 cm−1 nanocatalyst. According to the FT-IR data of biochar, the broad absorption peaks in the regions of approximately 3431 cm−1 are attributed to the OH stretching vibration, the peak at 1423 cm−1 is attributed to the C Created by potrace 1.16, written by Peter Selinger 2001-2019 C stretching, and the weak peak at 876 cm−1 is attributed to the C–H stretching vibration, in the FT-IR spectrum of biochar-NO2, two vibrational stretching frequencies related to NO2 appeared at 1384 and 1521 cm−1. In the FT-IR spectrum of biochar-NH2, the stretching frequency of the O–H and N–H functional groups is observed in the region of 3438 cm−1. Also, the peak observed in the area of 1630 cm−1 is related to the bending vibrations of the N–H bonds.

Fig. 3. FT-IR comparison of biochar catalysts (blue spectrum), biochar-NO2 (red spectrum) and biochar-NH2 (green spectrum).

Fig. 3

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3.3. SEM images for biochar-NH2 nanocatalyst

SEM images in Fig. 4 show the surface morphology of the biochar-NH2 sample at different magnifications. The sample surface has a porous, rough, and uneven structure due to the chemical modification of biochar and the creation of amine functional groups. Pores and micro-fractures on the surface have caused a significant increase in the specific surface area and the number of active sites. These morphological changes indicate the successful modification of the biochar surface and its increased potential for catalytic applications.

Fig. 4. The SEM images of the biochar-NH2.

Fig. 4

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3.4. XRD pattern for biochar-NH2 nanocatalyst

The XRD pattern of biochar-NH2 (Fig. 5) confirmed the amorphous nature of the material with semi-crystalline carbon domains. A broad diffraction peak at about 2θ = 24° is attributed to the C (002) plane, indicating the disordered stacking of aromatic carbon layers typical of amorphous biochar. A weak and broad peak around 2θ = 43–46° is assigned to the C (100) plane, which is indicative of the presence of some graphitic ordering within the carbon framework. In addition, the low-intensity diffraction feature observed at around 2θ = 12° can be assigned to residual mineral species or structural heterogeneity introduced during the biomass conversion and functionalization processes. Results show that the amine functionalization of biochar does not significantly alter the bulk carbon structure but the surface chemistry in the presence of an amorphous carbon matrix.64

Fig. 5. The XRD patterns of biochar-NH2.

Fig. 5

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3.5. EDS (X-ray energy dispersion spectroscopy) analysis

The EDX analysis (Fig. 6) was used to study the elemental composition of the biochar-NH2 nanocatalyst. EDX spectrum confirmed the presence of nitrogen in the functionalized biochar matrix and confirmed that carbon, nitrogen and oxygen were the main constituent elements of the biochar-NH2 nanocatalyst. Minor signals from other elements were also observed, which are attributed to the sample preparation process and/or trace inorganic impurities and are not thought to contribute significantly to the catalytic activity. Especially the Au signal is related to the gold coating on the sample before SEM-EDX analysis. The minor Al peak might be due to the aluminum foil used in the pyrolysis process. Also, the successful introduction of the nitrogen-containing functional groups was confirmed by FT-IR analysis. X-ray elemental mapping was also conducted to investigate the spatial distribution of the major elements (C, N, and O) in the biochar-NH2 nanocatalyst (Fig. 7). The uniform distribution of these elements in the region studied indicates a homogeneous distribution of these elements in the structure of the catalyst.

Fig. 6. EDX analysis of the biochar-NH2.

Fig. 6

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Fig. 7. Elemental X-ray mapping analysis of the biochar-NH2.

Fig. 7

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3.6. BET analysis of biochar-NH2 nanocatalyst

Adsorption research relies heavily on the Brunauer–Emmett–Teller (BET) methodology, which measures the specific surface area of porous materials, including solids and powders. The BET method is accurate under ideal conditions but sample aggregation (which masks internal surfaces), violation of assumptions, calibration errors and poor sample preparation can all lead to underestimation of the surface area.65

N2 adsorption at 77.35 K (P/P0 = 0.0–1) was used to characterize the adsorption properties of the biochar-NH2 nanocatalyst. The biochar-NH2 nanocatalyst pyrolyzed at 600 °C for two hours had an average pore diameter of 13.0 nm, a specific surface area of 3.2 m2 g−1, and a pore volume of 0.011 cm3 g−1, according to BET analysis. The substance was investigated as an effective heterogeneous catalyst for the synthesis of derivatives of 1,8-naphthyridine and 2-amino-4H-chromene. The interactions with the reactants which are reasons for the catalyst's effectiveness are favored by the amine groups on the biochar surface. The porous structure exhibits a moderate surface area but possesses well-dispersed active sites enabling efficient mass transport and catalytic conversion. The BJH technique was used to measure the adsorbent's pore size distribution. The pore sizes were in the range of 1.2–100 nm with dominance of pore sizes in the mesoporous regime (2–50 nm) and some pores in the micropore (<2 nm) and macropore (>50 nm) ranges. The nitrogen adsorption–desorption isotherm of biochar-NH2 nanoparticles. Its mesoporous nature is confirmed by the type IV isotherm's H3 hysteresis loop (P/P0 = 0.02–0.99).66 The BJH pore size distribution and N2 adsorption–desorption isotherm are provided in Fig. S14 and S15 of the SI, respectively.

3.7. Catalytic activity of biochar-NH2 in the synthesis of 1,8-naphthyridine

First, the catalytic activity of the modified biochar was investigated for the synthesis of 1,8-naphthyridine derivatives. The model reaction was taken as the reaction of 4-chlorobenzaldehyde (1 mmol), malononitrile (1 mmol) and 2,6-diaminopyridine (1 mmol) and the effect of catalyst loading, solvent and temperature was studied (Table 1). The electron-withdrawing chloro substituent makes 4-chlorobenzaldehyde a typical substrate for the catalytic evaluation and it was selected as the model aromatic aldehyde. The model reaction gave the desired product 2,7-diamino-4-(4-chlorophenyl)-1,8-naphthyridine-3-carbonitrile. Initially, catalyst loading was optimized in water at RT. The product was obtained in 35% yield (2 h) in the absence of catalyst (Table 1, entry 8). The effect of surface functionalization was studied by control experiments under the same conditions with biochar, biochar-NO2 and biochar-NH2. Biochar and biochar-NO2 afforded the product in 35 and 43% yields, respectively, whereas biochar-NH2 gave the highest yield of 98% under the same conditions (Table 1, entries 1, 2 and 7). These results suggest that the introduction of amino groups on the surface of biochar significantly improves its catalytic activity. Following that, the model reaction was repeated with 5, 10, 20, 30 and 0.04 g of the catalyst for consecutive runs. As indicated in Table 1, reducing the amount of biochar-NH2 resulted in a decreased product yield (Table 1, entry 3). As a result, 0.04 g of biochar-NH2 has been chosen as the optimal catalyst quantity (Table 1, entry 7). The model reaction was then tested in different solvents with a consistent catalyst amount (0.04 g) under the same identical conditions. Results showed that water yielded the best outcomes compared to acetonitrile, ethyl acetate, acetone, mixed water : ethanol 50 : 50 and n-hexane. The observed solvent effect can be attributed to the differences in the polarity, the dipole moment and the hydrogen-bonding properties of the solvents. Water has a higher dielectric constant and polarity which is better at stabilizing the polar intermediates and transition states in the multicomponent condensation process. In addition, the formation of hydrogen bonds from water helps to activate the reactants and allows efficient mass transfer at the catalyst surface. On the other hand, solvents with less polarity such as n-hexane and ethyl acetate showed lower catalytic performance. Polar aprotic solvents (e.g., acetonitrile and acetone) were less effective than water. The combination of high polarity, environmentally benign nature and favourable hydrogen-bonding interactions makes water the optimal medium for this transformation. Increasing the amounts of 4-chlorobenzaldehyde and malononitrile from two to five equivalents did not result in the formation of the bis-annulated product (Table 1, entries 15–26). This observation may be attributed to electronic deactivation of the remaining amino–carbon reactive site after the first cyclization. In 2,6-diaminopyridine, the two amino groups donate electron density to the pyridine ring through resonance, thereby facilitating the first Michael addition. However, after formation of the 1,8-naphthyridine framework, one amino group is incorporated into the fused ring, and the carbon adjacent to the remaining amino group becomes less nucleophilic. Therefore, the remaining amino–carbon site is not sufficiently reactive to undergo a second Michael addition and subsequent cyclization, even in the presence of excess 4-chlorobenzaldehyde and malononitrile (Scheme 2). The influence of temperature on the model reaction was also investigated. Through the aforementioned research, the best conditions for the synthesis of 1,8-naphthyridines were determined to be in water at room temperature with the addition of 0.04 g of biochar-NH2 (Table 1, entry 7).

Table 1. Effect of various amounts of catalyst, temperature, and solvent in the synthesis of 2,7-diamino-4- (4-chlorophenyl)-1,8-naphthyridine-3-carbonitrile.

Entry Catalyst Catalyst amount (g) Solvent Temperature (°C) Time (min) Yield %
1 Biochar 0.04 H2O 25 120 35
2 Biochar-NO2 0.04 H2O 25 120 43
3 Biochar-NH2 0.005 H2O 25 120 55
4 Biochar-NH2 0.01 H2O 25 120 68
5 Biochar-NH2 0.02 H2O 25 30 75
6 Biochar-NH2 0.03 H2O 25 30 89
7 Biochar-NH2 0.04 H2O 25 20 98
8 No catalyst — H2O 25 120 35
9 Biochar-NH2 0.04 EtOH 25 120 92
10 Biochar-NH2 0.04 EtOH : H2O 25 120 85
11 Biochar-NH2 0.04 Acetonitrile 25 120 50
12 Biochar-NH2 0.04 Ethyl acetate 25 120 57
13 Biochar-NH2 0.04 n-hexane 25 120 58
14 Biochar-NH2 0.04 Acetone 25 120 45
a15 Biochar-NH2 0.04 H2O 25 24 h —
a16 Biochar-NH2 0.04 H2O Reflux 24 h —
a17 Biochar-NH2 0.04 Solvent free 100 24 h —
a18 Biochar-NH2 0.04 H2O 25 24 h —
b19 Biochar-NH2 0.04 H2O Reflux 24 h —
b20 Biochar-NH2 0.04 Solvent free 100 24 h —
C21 Biochar-NH2 0.04 H2O 25 24 h —
c22 Biochar-NH2 0.04 H2O Reflux 24 h —
c23 Biochar-NH2 0.04 Solvent free 100 24 h —
d24 Biochar-NH2 0.04 H2O 25 24 h —
d25 Biochar-NH2 0.04 H2O Reflux 24 h —
d26 Biochar-NH2 0.04 Solvent free 100 24 h —
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a

A mixture of 1 mmol 2,6-diaminopyridine, 2 mmol malononitrile, 2 mmol 4-chlorobenzaldehyde.

b

A mixture of 1 mmol 2,6-diaminopyridine, 3 mmol malononitrile, 3 mmol 4-chlorobenzaldehyde.

c

A mixture of 1 mmol 2,6-diaminopyridine, 4 mmol malononitrile, 4 mmol 4-chlorobenzaldehyde.

d

A mixture of 1 mmol 2,6-diaminopyridine, 5 mmol malononitrile, 5 mmol 4-chlorobenzaldehyde.

Scheme 2. General reaction for the synthesis of 1,8-naphthyridine in the presence of biochar-NH2 as a catalyst.

Scheme 2

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Biochar-NH2 composite catalyzed the three-component condensation of malononitrile, 2,6-diaminopyridine and various aldehydes in water at room temperature, yielding good to outstanding results. The results are shown in Fig. 8. Aldehydes bearing electron withdrawing groups were more reactive than those bearing electron donating groups. The reactivity of aromatic aldehydes was found to be strongly correlated with the electronic properties of substituents. These electronic effects are due to the stabilization or destabilization of the transition state in the nucleophilic addition. The melting points of the prepared products were consistent with the reported data.30 The catalyst showed a broad applicability and efficiency in the synthesis of 1,8-naphthyridine derivatives as the reaction always gave the products in good to excellent yields. The mild reaction conditions, generally room temperature and water as solvent, increase the practicality and attractiveness of this catalytic protocol.

Fig. 8. Synthesis of 1,8-naphthyridine using biochar-NH2. Reaction conditions: 2,6-diaminopyridine (1 mmol), malononitrile (1 mmol), aromatic aldehyde (1 mmol), biochar-NH2 (0.04 g) and H2O (5 mL) at room temperature.

Fig. 8

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3.8. Proposed mechanism for the synthesis of 1,8-naphthyridine

The suggested process for the one-pot synthesis of 1,8-naphthyridines via a three-component domino reaction catalyzed by biochar-NH2 is shown in Fig. 9. Biochar-NH2 activates malononitrile, facilitating Knoevenagel condensation of aldehyde with malononitrile to form intermediate A. 2,6-diaminopyridine undergoes Michael addition to intermediate A, yielding intermediate B. Finally, proton transfer and intramolecular nucleophilic addition within intermediate B produce intermediate C, which then tautomerizes to its corresponding 1,4-dihydropyridine D. Then the intermediate D or 1,4-dihydropyridine is oxidized to its corresponding 1,8-naphthyridine via a cooperative vinylogous anomeric based oxidation.67–69 The solid base biochar-NH2 catalyst facilitates the reaction through Knoevenagel condensation, Michael addition, and proton transfer.28

Fig. 9. Proposed mechanism for the synthesis of 1,8-naphthyridine derivatives using biochar-NH2 as a nanocatalyst.

Fig. 9

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3.9. Catalytic activity of biochar-NH2 in the synthesis of 2-amino-4H-chromene

Scheme 3 shows the synthesis of 2-amino-4H-chromene with biochar-NH2. The catalytic activity of the modified biochar was first investigated in the synthesis of 2-amino-4H-chromene derivatives. 4-Chlorobenzaldehyde (1 mmol), malononitrile (1 mmol) and resorcinol (1 mmol) were used as a model reaction to optimize the reaction parameters (Table 2). Control experiments were performed to clarify the role of the catalyst in the model reaction. After 120 min without catalyst, the desired product was obtained in 48% yield (Table 2, entry 8). Unmodified biochar and biochar-NO2 produced the product in 48 and 52% yields, respectively (Table 2, entries 1 and 2), while biochar-NH2 achieved 97% yield in 7 min (Table 2, entry 7). The results presented here show that the reaction in water is slow without an effective catalyst and that amino functionalization of biochar greatly improves its catalytic activity. The amine groups on the surface can act as basic sites that facilitate the deprotonation of malononitrile and the Novannagel condensation followed by the Michael addition and cyclization steps. Optimization of the catalyst loading showed that increasing the amount of biochar-NH2 increased the product yield and 0.04 g was the best condition (Table 2, entries 7). Water was found to be the best solvent for the reaction compared to acetonitrile, ethyl acetate, acetone, 50 : 50 water : ethanol mixture, solvent-free conditions and n-hexane using 0.04 g of catalyst under similar conditions. The better performance of water can be ascribed to the high polarity, dielectric constant, and hydrogen-bonding ability of water which facilitate the stabilization of polar intermediates and promote efficient interactions between the reactants and the catalyst surface. Solvents with a lower polarity, or with different acceptor properties were less efficient in promoting the multicomponent condensation reaction. For the optimization studies the aromatic aldehyde selected as representative of the electron withdrawing character was 4-chlorobenzaldehyde. The model reaction carried out under the optimized conditions, afforded 2-amino-4-(4-chlorophenyl)-7-hydroxy-4H-chromene-3-carbonitrile, which was isolated and characterized by melting point determination. The melting point obtained was in good agreement with the literature data. Therefore, the optimal conditions for 2-amino-4H-chromene synthesis involve using 0.04 g of biochar-NH2 in water at room temperature (Table 2, entry 7). Increasing the amount of 4-chlorobenzaldehyde and malononitrile from two to five equivalents did not lead to the formation of the bicyclic product (Table 2, entries 16–27). This observation can be explained by the electronic deactivation of the remaining reactive site after the first cyclization. In resorcinol, both hydroxyl groups contribute electron density to the aromatic ring and enhance the first Michael addition. Formation of the first chromene ring requires an adjacent aromatic carbon and a hydroxyl group. This loss of the cooperative activation effect of the two hydroxyl groups makes the carbon adjacent to the second hydroxyl group less nucleophilic. Hence, even in the presence of excess aldehyde and malononitrile, the second Michael addition and subsequent cyclization do not occur (Scheme 3).

Scheme 3. General reaction for the synthesis of 2-amino-4H-chromene in the presence of biochar-NH2 as a catalyst.

Scheme 3

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Table 2. Effect of different amounts of catalysts, temperature and solvent in the synthesis of 2-amino-4-(4-chlorophenyl)-7-hydroxy-4H-chromene-3-carbonitrile.

Entry Catalyst Catalyst amount (g) Solvent Temperature (°C) Time (min) Yield (%)
1 Biochar 0.04 H2O 25 120 48
2 Biochar-NO2 0.04 H2O 25 120 52
3 Biochar-NH2 0.005 H2O 25 120 55
4 Biochar-NH2 0.01 H2O 25 120 62
5 Biochar-NH2 0.02 H2O 25 30 72
6 Biochar-NH2 0.03 H2O 25 20 80
7 Biochar-NH2 0.04 H2O 25 7 97
8 No catalyst — H2O 25 120 48
9 Biochar-NH2 0.04 EtOH 25 120 67
10 Biochar-NH2 0.04 EtOH : H2O 25 120 94
11 Biochar-NH2 0.04 Acetonitrile 25 120 53
12 Biochar-NH2 0.04 Ethyl acetate 25 120 52
13 Biochar-NH2 0.04 n-hexane 25 120 48
14 Biochar-NH2 0.04 Acetone 25 120 43
15 Biochar-NH2 0.04 Solvent-free 25 120 58
a16 Biochar-NH2 0.04 H2O 25 24 h —
a17 Biochar-NH2 0.04 H2O Reflux 24 h —
a18 Biochar-NH2 0.04 Solvent-free 100 24 h —
b19 Biochar-NH2 0.04 H2O 25 24 h —
b20 Biochar-NH2 0.04 H2O Reflux 24 h —
b21 Biochar-NH2 0.04 Solvent-free 100 24 h —
c22 Biochar-NH2 0.04 H2O 25 24 h —
c23 Biochar-NH2 0.04 H2O Reflux 24 h —
c24 Biochar-NH2 0.04 Solvent-free 100 24 h —
d25 Biochar-NH2 0.04 H2O 25 24 h —
d26 Biochar-NH2 0.04 H2O Reflux 24 h —
d27 Biochar-NH2 0.04 Solvent-free 100 24 h —
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a

A mixture of 1 mmol resorcinol, 2 mmol malononitrile, 2 mmol 4-chlorobenzaldehyde.

b

A mixture of 1 mmol resorcinol, 3 mmol malononitrile, 3 mmol 4-chlorobenzaldehyde.

c

A mixture of 1 mmol resorcinol, 4 mmol malononitrile, 4 mmol 4-chlorobenzaldehyde.

d

A mixture of 1 mmol resorcinol, 5 mmol malononitrile, 5 mmol 4-chlorobenzaldehyde.

The biochar-NH2 composite is an effective catalyst in the three-component condensation of various aldehydes, malononitrile and resorcinol in water at room temperature, affording good to excellent results (Fig. 10). Aromatic aldehydes with electron withdrawing groups were more reactive than those with electron donating groups. The melting points of the synthesized products were in good agreement with the literature data.70–72 The reaction gave 2-amino-4H-chromene derivatives in good to excellent yields in all cases proving the wide scope of the catalyst and its effectiveness for the preparation of the target heterocyclic compounds. Such a catalytic protocol is more practical and attractive because of the mild reaction conditions, generally room temperature and water as solvent. The catalytic activity of the catalyst is attributed to the synergistic effect of biochar support and amine functional groups, which activate the reactants and increase the reaction rate.

Fig. 10. Synthesis of 2-amino-4H-chromene using biochar-NH2. Reaction conditions: resorcinol (1.0 mmol), malononitrile (1.0 mmol), aromatic aldehyde (1.0 mmol), biochar-NH2 (0.04 g), and H2O (5.0 mL) at room temperature.

Fig. 10

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3.10. Proposed mechanism for the synthesis of 2-amino-4H-chromene

Fig. 11 Proposed mechanism for the biochar-NH2-catalyzed one-pot synthesis of 2-amino-4H-chromene derivatives through a 3-component domino reaction. Activation of malononitrile by biochar-NH2 for Knoevenagel condensation of aldehyde with malononitrile to give intermediate A. Intermediate B is prepared by Michael addition of resorcinol to intermediate A. Finally, proton transfer and intramolecular nucleophilic addition of intermediate B give intermediate C, which tautomerizes and proton transfers to give the 2-amino-4H-chromene derivatives. The biochar-NH2 catalyst provides a basic environment to facilitate Knoevenagel condensation and Michael addition.73

Fig. 11. Proposed mechanism for the synthesis of 2-amino-4H-chromene derivatives using biochar-NH2 as a nanocatalyst.

Fig. 11

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3.11. Recyclability of the biochar-NH2 catalyst

One of the important advantages of this catalyst is its ability to be recovered and reused. After the reaction was finished, the biochar-NH2 catalyst was readily removed from the reaction media and reused following washing and drying. The reusability study indicated that the catalyst could be reused for five successive cycles without any significant decrease in the catalytic activity or product yield which indicates the good structural and chemical stability of the catalyst. Fig. 12 shows the recovery diagram of the biochar-NH2 catalyst. The blue diagram is related to the recovery and reuse of the catalyst in the multicomponent reaction for the synthesis of 1,8-naphthyridine derivatives, in which 4-chlorobenzaldehyde (1 mmol), malononitrile (1 mmol), and 2,6-diaminopyridine (1 mmol) were used as starting materials. In contrast, the green graph shows the performance of the recovered catalyst in the synthesis reaction of 2-amino-4H-chromene derivatives using 4-chlorobenzaldehyde (1 mmol), malononitrile (1 mmol), and resorcinol (1 mmol). The results of these two graphs are compared and it is observed that the catalyst has good activity in both reactions after several recycling cycles without a significant reduction in reaction efficiency.

Fig. 12. Recyclability of biochar-NH2 catalysis in the synthesis of 1,8-naphthyridine (red column) and 2-amino-4H-chromene (blue column).

Fig. 12

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To assess the structural stability of the biochar-NH2 catalyst after repeated use, the recovered catalyst after the fifth catalytic cycle was characterized by FT-IR spectroscopy and the spectrum was compared with that of the fresh catalyst (Fig. 13). It can be observed that the characteristic absorption bands of the fresh catalyst were largely retained after five consecutive reaction cycles with only slight shifts in the band positions. No significant changes in the overall spectral profile and disappearance of the characteristic bands were observed confirming that the chemical structure of the biochar-NH2 catalyst did not change during the recycling process. The small drop in catalytic activity observed upon repeated use is therefore mainly due to partial blockage of the active sites by adsorbed reaction species and/or minor catalyst loss during the recovery and washing steps rather than to structural degradation of the catalyst.

Fig. 13. Comparison of the FTIR spectra of fresh and recovered catalyst after five runs.

Fig. 13

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To evaluate the efficiency of the synthesized biochar-NH2 nanocatalyst, the catalytic performance was compared with several previously reported catalytic systems for the synthesis of 1,8-naphthyridine and 2-amino-4H-chromene derivatives (Table 3). The present catalyst for the synthesis of 1,8-naphthyridine derivatives (entry 2) gave an excellent yield of 98% in only 20 min in water at room temperature with low catalyst loading (0.04 g). In comparison, the previously reported Basu-HDI MOF catalyst was only able to afford a lower yield (85%) with reflux conditions in ethanol for a longer time (60 min) (entry 1). The results obtained clearly demonstrate the higher catalytic activity of biochar-NH2 under much milder and more environmentally benign conditions. A tendency to such a similar synthesis of 2-amino-4H-chromene derivatives was observed. The biochar-NH2 nanocatalyst (entry 7) produced the desired products in 97% yield within only 4 min in aqueous medium at room temperature, outperforming or matching other reported catalytic systems, including Na2CO3 under grinding conditions (30 min, 88%), Mg/Al hydrotalcite in water at 80 °C (5 h, 90%), methylene blue under visible-light irradiation (5 min, 83%), and Fe3O4@SiO2-BenzIm-FeCl/BiOCl magnetic photocatalyst under ultrasound irradiation (8 min, 98%) (entries 3–6). Overall, the comparative study reveals that the developed biochar-NH2 catalyst is a highly efficient, rapid and environmentally friendly catalytic system for the synthesis of both 1,8-naphthyridine and 2-amino-4H-chromene derivatives. The developed catalytic system provides a favorable balance between reaction efficiency, operational simplicity and green chemistry metrics in comparison to the previously reported methods.

Table 3. Comparison of the catalytic performance of the present biochar-NH2 nanocatalyst with representative literature reports for the synthesis of 1,8-naphthyridine and 2-amino-4H-chromene derivatives.

Entry Condition Time Yield (%) Ref.
1a Basu-HDI MOF (0.02 g) was used as a heterogeneous catalyst in EtOH under reflux conditions 60 min 85 30
2a Biochar-NH2 (0.04 g) was used as a recyclable nanocatalyst in H2O at room temperature 20 min 98 This work
3b Na2CO3 (0.021 g) was used as the catalyst under grinding conditions at 50 °C 30 min 88 74
4b Mg/Al hydrotalcite (HT) (0.0495 g) was used as the catalyst in H2O at 80 °C 5 h 90 75
5b Methylene blue (MB+) (1 mol%) was used as a photo-redox catalyst in H2O under white LED irradiation (18 W) at room temperature in an air atmosphere 5 min 83 71
6b Fe3O4@SiO2-BenzIm-Fc[Cl]/BiOCl (0.01 g) was used as a magnetic nanocatalyst in EtOH/H2O (3 : 2) under ultrasound irradiation (50 W) at room temperature 8 min 98 76
7b Biochar-NH2 (0.04 g) was used as a recyclable nanocatalyst in H2O at room temperature 4 min 97 This work
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a

Different catalyst for synthesis 1,8-naphthyridine using reaction of 4-chlorobenzaldehyde, malononitrile, and 2,6-diaminopyridine.

b

Different catalysts for the synthesis of 2-amino-4H-chromene using the reaction of 4-chlorobenzaldehyde, resorcinol and malononitrile.

4. Conclusion

In this study, an amine-functionalized biochar (biochar-NH2), synthesized from walnut shell biomass through nitration/reduction methodology. This nanocatalyst is economically viable and completely environmentally friendly and was used for the synthesis of 1,8-naphthyridine and 2-amino-4H-chromene derivatives. Also, surface functionalization and the presence of active amine groups on the nanocatalyst were confirmed by various analyses. Under mild, aqueous, and room temperature conditions, biochar-NH2 enabled rapid, high-yield multicomponent reactions, confirming its suitability for green heterocycle synthesis. These results indicate that engineered biochar can be a sustainable, cheaper and appropriate alternative to conventional metal-based or expensive nanocatalysts. It is highlighted in industrial applications due to its recyclability and reusability. The study was limited to a few reactions and starting materials, although the results are promising. Future studies should utilize more starting materials and a larger scale could be used to characterize the performance and industrial applications of NH2-biochar. Overall, this catalyst is a good candidate for sustainable applications.

Author contributions

Reza Hazbavi: methodology, validation, investigation, writing – original draft. Maryam Hajjami: funding acquisition, supervision, project administration, conceptualization, resources. Mohammad Ali Zolfigol: funding acquisition, validation, conceptualization, writing – review and editing. Zahra Siahpour: methodology, validation, investigation.

Conflicts of interest

The authors declare no competing financial interests.

Supplementary Material

RA-OLF-D6RA05766G-s001

Acknowledgments

The authors thank Bu-Ali Sina University for the financial support for this research.

Data availability

The datasets used and/or analyzed during the present study are available from the corresponding author upon reasonable request. All data generated or analysed during this study are included in this published article and its supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6ra05766g.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

RA-OLF-D6RA05766G-s001
RA-OLF-D6RA05766G-s001.pdf (963.9KB, pdf)

Data Availability Statement

The datasets used and/or analyzed during the present study are available from the corresponding author upon reasonable request. All data generated or analysed during this study are included in this published article and its supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6ra05766g.

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