Introduction

Multi-component reactions (MCRs) facilitate the simultaneous occurrence of multiple chemical reactions, resulting in the formation of complex heterocyclic structures that are valuable for creating multi-functional heterocyclic compounds with drug-like properties1,2,3,4,5,6,7. These reactions have been extensively studied in the field of organic chemistry and have been applied in various synthetic approaches. Over the past several decades, there has been a significant focus on the synthesis of new heterocyclic compounds, particularly for the development of heterocyclic-based molecules8,9,10,11,12,13,14,15,16,17.

Pyran derivatives are a crucial category of heterocyclic compounds, notable for their significant biological and pharmacological effects, including anticoagulant, spasmolytic, diuretic, anticancer, and antianaphylactic properties, as well as their potential in treating Alzheimer’s disease, AIDS-related dementia, and myoclonus. These compounds have garnered substantial interest due to their diverse applications. Pyran rings, including structures like pyranopyrimidines, pyranopyridines, pyridine-2-ones, pyranopyrazoles, and polyazanaphthalenes, exhibit high reactivity, making them versatile precursors for synthesizing a broad spectrum of heterocyclic compounds. Recent literature has documented various synthetic methods for these compounds, with the simplest approach being a one-pot, three-component condensation reaction involving malononitrile, an aldehyde, and 1,3-dicarbonyl compounds under different conditions. 2-amino-4 H-pyran derivatives are widely applicable as photoactive materials, while substituted 4 H-pyrans feature in the structure of natural products18. Although various methods exist for preparing tetrahydro-4 H-benzopyran derivatives, some suffer from issues like toxic metals, volatile organic solvents, high costs, and low yields19,20,21.

Copolymers are a class of polymers composed of two or more different monomers that are chemically bonded together in a repeating pattern. These materials play a crucial role in a wide range of industries, including plastics, textiles, and pharmaceuticals, due to their unique properties and versatility in applications22,23,24. Copolymers exhibit several useful properties that make them valuable in various chemical reactions. One key advantage is their enhanced reactivity, allowing for tailored synthesis routes and the incorporation of specific functionalities. Additionally, copolymers offer tunable properties, enabling researchers to adjust characteristics such as flexibility, strength, and thermal stability to meet specific requirements. Moreover, copolymers contribute to improved mechanical strength in reaction products, making them ideal for applications where durability and structural integrity are essential25,26,27,28. Among copolymers, poly(methyl methacrylate–maleic anhydride) [P(MMA–MAH)] copolymer is a versatile material with a fascinating history, unique properties, and diverse applications. This article provides insights into the synthesis, properties, and applications of PMMAn copolymer, shedding light on its significance in various industries29,30,31,32.

The PMMAn copolymer exhibits a wide range of properties that make it suitable for various applications. For example, it has several applications in aviation, dentistry, optical devices, membrane modification and electronics. Its mechanical properties, including high tensile strength and impact resistance, contribute to its use in structural applications. Additionally, its thermal stability and chemical resistance make it a preferred choice in demanding environments33,34,35. The versatility of PMMAn copolymer is evident in its applications across different industries. In the biomedical field, PMMAn copolymer is utilized in medical devices and implants due to its biocompatibility and durability. In coating applications, PMMAn copolymer provides a protective layer that enhances surface longevity. Moreover, in adhesive applications, PMMAn copolymer offers strong bonding properties for reliable adhesion36,37,38. The synthesis of tetrahydrobenzo[b]pyran derivatives holds significance due to their diverse properties39,40, including anti-coagulant, diuretic, spasmolytic, anti-cancer, and anti-anaphylactic characteristics41.

Copolymers are defined as polymers derived from more than one species of monomer, offering a wide range of properties that can be tailored for specific applications. PMMA (polymethyl methacrylate) is a versatile copolymer that plays a significant role in organic synthesis due to its excellent optical clarity, weather resistance, and impact strength. In the current study, PMMA serves as a critical component, enhancing the understanding of copolymer behavior and its potential in advanced material development. Polymer-supported organic catalysts are largely insoluble in most reaction solvents, which allows for easy recovery and recycling of the catalysts. There are functional groups on the surface of PMMA, which can adsorb noble metal ions such as Fe3O4. Homogeneous size of micrometer is favorable to recycle of the catalyzer. Supported catalysts are essential in organic synthesis as they facilitate reactions with high efficiency and selectivity. These catalysts are immobilized on various supports, which not only stabilize the catalyst but also facilitate its recovery and reuse, making the process more sustainable. Natural asphalt, with its complex mixture of hydrocarbons, provides a robust support for catalysts, offering unique properties that can be tailored for specific reactions42. Magnetic nanoparticles have emerged as a promising support due to their ease of separation from reaction mixtures using an external magnetic field, simplifying catalyst recovery and reducing process time43,44,45.

Magnetic nanoparticles have emerged as a pivotal material in the synthesis of recyclable catalysts, offering a unique combination of properties that enhance catalytic efficiency and sustainability46,47,48,49,50. Their small size and high surface area-to-volume ratio provide ample active sites for chemical reactions, while their magnetic properties enable easy separation from reaction mixtures using external magnetic fields51,52,53,54,55. This separation capability is crucial for catalyst recycling, as it allows for the reuse of catalysts without the need for complex filtration or centrifugation processes56,57,58,59,60. Consequently, the use of magnetic nanoparticles in catalysis not only reduces waste but also lowers the overall cost of chemical processes by minimizing the need for fresh catalysts in each reaction cycle. Moreover, the ability to recover and reuse catalysts contributes to the development of greener and more environmentally friendly chemical synthesis methods60,61,62,63,64,65,66,67,68. Metal-organic frameworks (MOFs) are a class of porous materials composed of metal ions or clusters coordinated to organic ligands, creating well-defined pores that can accommodate various catalysts69,70. Their high surface area and tunable pore size make MOFs ideal for catalytic supports, allowing for the precise control of catalytic activity and selectivity23,71. Silicates, such as zeolites, are another class of materials used as catalytic supports. Their microporous structure provides a confined space for catalytic reactions, enhancing the selectivity and yield of the desired products. The use of these supports in conjunction with PMMA copolymers opens new avenues for the development of advanced materials with tailored properties for specific applications in organic synthesis72,73.

The aim of this study is to develop a novel magnetic nanocatalyst based on Fe3O4@Hydrol-PMMAn for the one-pot synthesis of tetrahydrobenzo[b]pyrans through multicomponent condensation reactions. The use of polymer in this nanocatalyst is crucial as it provides structural stability and effectively supports the Fe3O4 nanoparticles, enhancing reactivity and enabling catalyst recyclability. The hydrolyzed PMMAn polymer introduces active functional groups that facilitate the strong adsorption and retention of magnetic nanoparticles. Fe3O4@Hydrol-PMMAn enables a greener reaction pathway by minimizing the need for toxic solvents and high-energy conditions, aligning with principles of sustainable chemistry. This catalyst is designed for three different condensation reactions and operates under mild, solvent-free conditions. In this study, the catalyst’s performance in condensation reactions with aldehydes, malononitrile, and dimedone/ethyl acetoacetate/1,3-cyclohexanedione will be evaluated, with optimization of key parameters such as catalyst loading, temperature, and reaction time to achieve an efficient synthesis of the target compounds. Due to its rapid reaction times, high yield and recyclability, this nanocatalyst holds significant potential for applications in sustainable and green synthetic processes.

Experimental

Material and method

All chemicals were purchased from Merck and Sigma Aldrich and were used as received. Melting points were measured using an Electro thermal 9100 apparatus. The fourier-transform infrared spectroscopy (FT-IR) spectra were obtained with a Shimadzu IR-470 spectrophotometer. X-ray diffraction (XRD) patterns were recorded using a Philips PW-1830. Magnetic analysis curves were acquired using a vibrating sample magnetometer (VSM) model MDKB from Danesh Pajohan Kavir Co. Kashan, Iran. Scanning electron microscopy (SEM) images of the nanocatalyst were captured with a MIRA3TESCAN-XMU instrument. Elemental analysis (EDS) of the nanocatalyst was performed using a TESCAN4992 instrument. Thermogravimetric analysis (TGA) was conducted using an SDT Q600 V20.9 Build 20 instrument.

Preparation of the MMA–MAH copolymer (PMMAn)

The MMA–MAH copolymer was synthesized via in situ chemical polymerization. Initially, a homogeneous mixture of methyl methacrylate (MMA) and maleic anhydride (MAH) was prepared in ethyl acetate (EtOAc) as the solvent. The copolymerization process was initiated by introducing benzoyl peroxide (BPO) as initiator at a concentration of 1 wt % relative to the total weight of the monomers. The reaction was conducted at temperatures between 80 ºC and 90 ºC for 12 h with continuous stirring to ensure uniformity. Upon completion of the reaction, the mixture was precipitated by gradual addition to methanol, effectively isolating the product. The resulting precipitate was subsequently washed three times with toluene to remove any unreacted monomers and residual MMA homopolymers. The final undissolved precipitate was confirmed as the MMA–MAH copolymer. The collected product was then dried in an oven at 60 ºC, yielding a colored powder29.

Hydrolysis of PMMAn copolymer

After copolymerization, the copolymer was subjected to hydrolysis32,74. The hydrolysis of the PMMAn copolymer was carried out to enhance its functionality and improve solubility in aqueous environments. In this process, the PMMAn copolymer was treated with an aqueous alkaline solution, typically NaOH, at a controlled temperature. The reaction conditions were optimized to facilitate the cleavage of the anhydride linkages within the copolymer structure. During hydrolysis, the maleic anhydride segments underwent nucleophilic attack by hydroxide ions, leading to the formation of carboxylate groups. This transformation not only increased the hydrophilicity of the copolymer but also enabled the introduction of reactive sites that could be utilized in subsequent modifications. This process leads to the hydrolysis of the MMA-MAH copolymer, which leads to the formation of a copolymer with carboxylic acid groups.

Preparation of Fe3O4@Hydrol-PMMAn

Recent investigations have explored various methodologies for synthesizing iron magnetic nanoparticles75,76,77. The process began by sonicating 0.15 g of the copolymer in 15 mL of deionized water at room temperature for 1 h. Subsequently, 0.495 g of FeCl₂·4 H₂O and 1.365 g of FeCl₃·6 H₂O were introduced to the mixture, followed by an additional hour of ultrasonication to ensure thorough dispersion. Following this, the reaction mixture was heated at 80 °C for 15 min. Then, 5 mL of ammonium hydroxide was added, allowing the solution to stir at room temperature for 24 h, which resulted in the formation of a brown precipitate. To isolate the precipitate, a magnet was used to attract the iron magnetic nanoparticles from the solution. After magnetic separation, the nanoparticles were washed multiple times with a water-ethanol solution in a 1:1 ratio to remove any unreacted materials. The final product was then placed in an oven at 50 °C for drying78,79,80.

Typical method for the synthesis of tetrahydrobenzo[b]pyrans in the presence of Fe3O4@Hydrol-PMMAn

The synthesis of tetrahydrobenzo[b]pyrans was carried out by combining dimedone (1 mmol: 0.14 g), malononitrile (1 mmol: 0.066 g), benzaldehyde (1 mmol: 0.17 mL), and Fe3O4@Hydrol-PMMAn (0.01 g) in a 5 mL round-bottom flask. The reaction mixture was stirred in an oil bath at 80 °C until completion, as confirmed by thin-layer chromatography (TLC) (monitored by TLC; n-hexane/ethyl acetate, 4/1). Upon completion of the reaction, the mixture was extracted using warm ethanol (10 mL) to separate the liquid products from the catalyst. The crude products, which were soluble in warm ethanol, were carefully isolated from the catalyst. The remaining viscous liquid was washed with ethyl acetate (5 mL) and dried under reduced pressure to prepare for the next run. Finally, the solid residue (crude product) was purified through recrystallization using a mixture of ethanol and water (9:1) to obtain the desired tetrahydrobenzo[b]pyran product.

Result and discussion

Synthesis and characterization of Fe3O4@Hydrol-PMMAn nanocomposite

The Fe3O4@Hydrol-PMMAn nanocatalyst was synthesized in a three-step process. First, methyl methacrylate was polymerized onto maleic anhydride using benzoyl peroxide as an initiator, forming a PMMAn copolymer. Next, the copolymer underwent hydrolysis in an alkaline solution to introduce functional groups. Finally, Fe3O4 nanoparticles were deposited onto the hydrolyzed copolymer via in situ coprecipitation, resulting in the Fe3O4@Hydrol-PMMAn nanocomposite. The structure and properties of the synthesized composite were confirmed through various analytical techniques, including FTIR, XRD, SEM, EDX, VSM, and TGA, which indicated the successful incorporation of iron nanoparticles within the hydrolyzed PMMAn matrix (Scheme 1).

Scheme 1
Scheme 1The alternative text for this image may have been generated using AI.
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Preparation Procedure of the Fe3O4@Hydrol-PMMAn nanocomposite.

Investigation of the FT-IR spectra of the Fe3O4, PMMAn, Hydrol-PMMAn, and Fe3O4@Hydrol-PMMAn (Fig. 1a-d) ensured the successful construction of the targeting nanocomposite. As illustrated in Fig. 1a, the FT-IR spectrum of Fe3O4 displays two characteristic peaks at 3430 cm⁻¹ and 748 cm⁻¹, corresponding to the O–H and Fe–O stretching vibrations, confirming the formation of Fe3O4 (Fig. 1a). In the spectrum of PMMAn (Fig. 1b), the IR spectrum of the copolymer formed from PMMAn displays characteristic absorption bands that reflect the functional groups present in copolymer. A strong peak around 1730–1850 cm⁻¹ indicates the carbonyl (C = O) stretching vibrations from the ester group of MMA and the anhydride group of MAH suggesting successful incorporation of both components into the copolymer structure. Additionally, a peak near 1443–1665 cm⁻¹ corresponds to the C = C stretching vibrations from the double bonds in MAH, which may remain intact during copolymerization. The O–H stretching vibrations associated with the carboxylic acid groups can appear as broad bands in the range of 3350–3650 cm⁻¹, indicating potential hydrogen bonding within the copolymer network. The C–O stretching vibrations can be observed around 1140–1250 cm⁻¹, providing further evidence of the ester and anhydride functionalities. C–H stretching vibrations from the aliphatic chains of the copolymer are visible in the region of 2950–3000 cm⁻¹. Overall, the IR spectrum of the PMMAn copolymer demonstrates the successful integration of both monomers MMA and MAH81. In the IR spectrum of hydrolyzed PMMAn (Fig. 1c: Hydrol-PMMAn), the prominent peak around 1725 cm⁻¹, corresponding to the carbonyl (C = O) group of the ester, may shift or decrease in intensity due to the conversion to a carboxylic acid. The C–O stretching vibrations, previously observed around 1140–1250 cm⁻¹, will also be affected, potentially shifting to lower wavenumbers as the ester bond breaks (1140–1197 cm⁻¹). The region between 2950 and 3000 cm⁻¹ will still show C–H stretching from the remaining methyl and methylene groups. Overall, the spectrum of hydrolyzed PMMAn reflects the transformation from an ester to a carboxylic acid, highlighting the significant changes in functional groups associated with the hydrolysis process. The IR spectrum of magnetized hydrol-PMMAn reveals distinct features that indicate the presence of magnetic nanoparticles incorporated into the polymer matrix (Fig. 1d). The characteristic carbonyl (C = O) absorption band, typically found around 1720–1750 cm⁻¹, may remain but can show changes in intensity or slight shifts due to interactions with the magnetic particles (C = O : 1718 cm⁻¹). Additionally, the C–O stretching vibrations observed in the region of 1140–1250 cm⁻¹ also was be modified, reflecting the influence of the nanoparticles on the polymer structure. The spectrum retains the C–H stretching vibrations from the methyl and methylene groups in the range of 2920–3000 cm⁻¹, indicating that the overall polymer backbone is intact. However, the presence of magnetic nanoparticles can introduce new absorption features, particularly in the lower wavenumber region (650–700 cm⁻¹), which may correspond to metal-oxygen (M–O) bonds or other interactions related to the specific magnetic material used82,83. These changes collectively provide insights into the incorporation of magnetic properties into the hydrol-PMMAn matrix, highlighting the potential for enhanced functionalities in various applications.

Fig. 1
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FT-IR spectra of (a) Fe3O4, (b) PMMAn, (c) Hydrol-PMMAn, (d) Fe3O4@Hydrol-PMMAn.

The XRD pattern of the Fe3O4@Hydrol-PMMAn nanocomposite is displayed in Fig. 2. The pattern features several sharp peaks, indicative of the crystalline nature of the nanocomposite, suggesting an ordered arrangement of atoms within the structure. The most significant peaks appear at 2Ɵ positions of approximately 29.9º, 35.4º, 43.1º, 48º, 54.9º, 57.5º, and 62.7º, which are characteristic of Fe3O484. Specifically, the peaks correspond to the crystalline planes (220), (311), (400), (422), (511), and (440) of Fe3O4 nanoparticles, confirming the successful formation of magnetite within the nanocomposite. Overall, the results from the XRD analysis confirm the successful synthesis of the Fe3O4@Hydrol-PMMAn nanocomposite, providing valuable insights into its structural integrity and crystallinity, essential for its application as a catalyst.

Fig. 2
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XRD patterns of Fe3O4@Hydrol-PMMAn nanocomposite.

Figure 3 presents the thermogravimetric analysis (TGA) of the synthesized nanocomposite, which tracks changes in weight as the material is heated. The fluctuations in the derivative weight indicate the presence of multiple stages of decomposition or reactions occurring with increasing temperature. The initial sharp decline in weight, approximately 15%, observed in the temperature range of 50 to 280 °C (blue line), suggests a significant loss of mass, likely due to the evaporation of water and other volatile components present in the compound. As the temperature rises to 750 °C, the structural integrity of the sample is compromised, leading to further decomposition. The latter part of the graph reveals a plateau, indicating that the rate of weight change has stabilized. This stabilization implies that the material has largely decomposed or reached a stable state. The corresponding sharp peak in the derivative weight (yellow line) further signifies that this mass loss occurred rapidly. In summary, these results demonstrate that the Fe3O4@Hydrol-PMMAn nanocomposite exhibits good thermal stability, making it a promising candidate for various applications.

Fig. 3
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TGA of Fe3O4@ Hydrol-PMMAn.

To investigate the morphology, size, and distribution of the synthesized Fe3O4@Hydrol-PMMAn copolymer, scanning electron microscopy (SEM) analysis was conducted on the sample (Fig. 4). The images were captured at various magnifications, with scale bars of 50 μm, 1 μm, 500 nm, and 200 nm. Key parameters include the use of an “InBeam” detector, a working distance (WD) of 5.74 mm, and a brightness level (B.I.) of 7.00. The SEM images reveal a distinct rod-like morphology characteristic of the nanocomposite. This morphology suggests that the copolymer is encapsulated by Fe3O4 nanoparticles, which have formed nano-rods on the surface. The observed rod-like structure may result from the incorporation of Fe3O4, leading to the formation of a molecular network of the copolymer through interchain interactions, facilitated by the coordination of Fe atoms with carboxyl groups (–COOH) as illustrated in Scheme 1. Additionally, the images indicate clustering of the nanoparticles, which may suggest some degree of aggregation or sintering. This clustering behavior is notable and could have implications for the material’s properties and performance in various applications.

Fig. 4
Fig. 4The alternative text for this image may have been generated using AI.
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SEM of Fe3O4@Hydrol-PMMAn nanocomposite.

Figure 5 displays the results of the EDX analysis, showing distinct peaks at specific energy levels, labeled with the chemical symbols for the constituent elements: “Fe” for iron, “C” for carbon, and “O” for oxygen. The spectrum reveals three prominent peaks, with the highest peak corresponding to oxygen, followed by two significant peaks for iron at different energy levels. The presence of these peaks confirms the elemental composition of the sample. The intensity of the peaks indicates that oxygen is the most abundant element, followed by iron. In contrast, the carbon peak is relatively small, suggesting that carbon is present in much lower quantities compared to iron and oxygen. Overall, the EDX analysis provides valuable insights into the composition of the Fe3O4@Hydrol-PMMAn nanocomposite, highlighting the predominance of oxygen and iron in the material.

Fig. 5
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EDX spectrum of Fe3O4@Hydrol-PMMAn nanocomposite.

Superparamagnetic of polymer magnetic composites is a critical property to ensure their application in various applications such as biomedicine and catalysts for good separation. To study the influence of the PMMA shell on the magnetic property of the Fe3O4 nanoparticles, magnetization of the sample was evaluated by VSM. The Fig. 6 shows the vibrating sample magnetometer (VSM) analysis of the designed nanomaterial. As shown, this material has good magnetic property and supermagnetic character with saturation magnetization as high as around 35 emu/gr. This result indicates that the magnetic property of Fe3O4@Hydrol-PMMAn was not influenced by copolymerization of MMA-MA on the surface of Fe3O4. In fact, no hysteresis was observed.

Fig. 6
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VSM curve of the Fe3O4@Hydrol-PMMAn nanocomposite.

Tetrahydrobenzo[b]pyran reaction of benzaldehyde dimedone, and malononitrile catalyzed by Fe3O4@Hydrol-PMMAn nanocomposite

To establish an effective procedure for the synthesis of tetrahydrobenzo[b]pyran was achieved through a one-pot three-component condensation reaction involving dimedone, malononitrile, and benzaldehyde, utilizing Fe3O4@Hydrol-PMMAn as a catalyst. This reaction was selected as a model system. Various reaction conditions, including different catalyst loadings and solvents, were systematically screened to identify the optimal conditions for this three-component reaction (Table 1). The best results were achieved using 0.015 g of the Fe3O4@Hydrol-PMMAn catalyst under solvent-free conditions, completed in just 5 min at 80 °C (Table 1, entry 6). Control experiments demonstrated that the absence of the catalyst resulted in no observable cross-coupling reaction, confirming its essential role in facilitating the reaction (Table 1, entry 9).

Table 1 Optimization of reaction conditions 2-amino-7,7-dimethyl-5-oxo-4-phenyl-5,6,7,8-tetrahydro-4 H-chromene-3-carbonitrile by Fe3O4@ Hydrol-PMMAna.
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Following the establishment of optimal reaction conditions, a scope assessment was conducted to evaluate the versatility of these conditions in coupling various aldehydes, including those with substituents such as -Cl, -Br, -NO₂, -OH, and -OMe (Table 2). As shown in Table 2, the reaction successfully produced the desired tetrahydrobenzo[b]pyrans with good to high yields and a short reaction time. This demonstrates the effectiveness of the optimized conditions. Notably, the results indicate that aldehydes with electron-withdrawing groups exhibited higher reactivity compared to those with electron-donating groups, as evidenced by the variations in yields. This assessment highlights the influence of electronic effects on the reactivity of the aldehydes, providing valuable insights into the design of future reactions using Fe3O4@Hydrol-PMMAn as a catalyst.

Table 2 One-pot synthesis of aldehydes derivatives with dimedone, and malononitrile catalyzed by Fe3O4@Hydrol-PMMAn a.
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In light of the favorable reaction conditions established for the synthesis of tetrahydrobenzo[b]pyrans, further investigations were conducted to evaluate the efficacy of these conditions using alternative substrates (Table 3). Specifically, dimedone was replaced with 1,3-cyclohexanedione (Table 3: entries 1–5) and ethyl acetoacetate (Table 3: entries 6–11), in the presence of various aldehydes. This approach aimed to assess the versatility and general applicability of the optimized reaction conditions while maintaining high yields. Under the optimized parameters, the reactions were completed efficiently, yielding the desired products in high yields within a concise timeframe of 5 to 30 min (Table 3: entries 1–11). The successful incorporation of ethyl acetoacetate and 1,3-cyclohexanedione demonstrates the robustness of the Fe3O4@Hydrol-PMMAn catalytic system, highlighting its effectiveness not only for conventional substrates but also for a broader range of reactants. These findings underscore the adaptability of the reaction conditions and suggest significant potential for further applications in synthetic organic chemistry, paving the way for future studies involving diverse substrates within this efficient catalytic framework.

Table 3 Comparative analysis of yields using different substrates in tetrahydrobenzo[b]pyran synthesis a.
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In a separate study, we assessed the reusability of the catalyst during the condensation reaction involving benzaldehyde (1 mmol), dimedone (1 mmol), and malononitrile (1 mmol) (Fig. 7). This investigation is crucial for evaluating the practical applicability and efficiency of the catalyst in continuous processes. To isolate the catalyst from the reaction mixture after completion, we utilized warm ethanol for extraction. This method was chosen for its effectiveness in dissolving the organic components while allowing for the solid catalyst to be easily separated. Following extraction, the recovered catalyst was carefully dried and then applied to a new reaction under the same optimized conditions. The performance of the reused catalyst was monitored over four consecutive reaction cycles. Remarkably, the catalyst maintained its catalytic activity within the margin of experimental error, demonstrating consistent yields and reaction times similar to those observed in the initial cycle. These findings suggest that the catalyst exhibits excellent stability and reusability, which are critical attributes for practical applications in organic synthesis.

Fig. 7
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The reaction of 2-amino-7,7-dimethyl-5-oxo-4-phenyl-5,6,7,8-tetrahydro-4 H-chromene-3-carbonitrile as model reaction in the presence of Fe3O4@Hydrol-PMMAn at 80 °C under solvent free conditions.

Finally, to verify that the structure of the recovered catalyst was preserved, we examined it after the fourth run of the 2-amino-7,7-dimethyl-5-oxo-4-phenyl-5,6,7,8-tetrahydro-4 H-chromene-3-carbonitrile reaction using under optimized conditions (Fig. 8). FTIR and XRD analyses confirmed that the catalyst’s structure remained completely intact throughout the recycling process (Fig. 8).

Fig. 8
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FTIR image (left) and XRD pattern (right) of Fe3O4@Hydrol-PMMAn after 4 runs.

To indicate the heterogeneous nature of the catalyst, a hot filtration test was performed the synthesis of tetrahydrobenzo[b]pyran through a one-pot, three-component condensation reaction involving benzaldehyde, dimedone, and malononitrile (Fig. 9). The reaction mixture was stirred in the presence of ethanol solvent under reflux conditions. After 2.5 min, which represented the half-time of the reaction, the catalyst was quickly removed using a magnet from reaction vessel. The yield of the product was determined to be 45%. The filtered mixture was allowed to continue reacting for an additional 2.5 min, resulting in a yield of 46%. Since there was no substantial increase in the product yield, it can be inferred that a heterogeneous mechanism is at play during the catalyst’s recycling process.

Fig. 9
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Reaction kinetics, hot filtration studies for the tetrahydrobenzo[b]pyran reaction of benzaldehyde, dimedone, and malononitrile catalyzed by Fe3O4@Hydrol-PMMAn (0.015 g) in EtOH under reflux condition.

In a proposed mechanism (Scheme 2)21, firstly, the reaction begins with the nucleophilic attack of malononitrile on the carbonyl carbon of benzaldehyde (I). This forms a tetrahedral intermediate, which subsequently undergoes proton transfer to stabilize the structure. The tetrahedral intermediate collapses, leading to the formation of a β-hydroxy nitrile intermediate. This step involves the elimination of a hydroxide ion (II). Meanwhile, dimedone undergoes tautomerization to form a keto-enol (III). This keto-enol is nucleophilic and can participate in further reactions. The keto-enol formed from dimedone attacks the carbonyl carbon of the β-hydroxy nitrile intermediate, resulting in the formation of a new carbon-carbon bond and yielding a more complex intermediate (IV). The resulting intermediate undergoes an intramolecular cyclization, leading to the formation of the tetrahydrobenzo[b]pyran structure. This step may involve the loss of a small molecule, such as water, facilitating the ring closure. The final product, tetrahydrobenzo[b]pyran, is obtained after tautomerization or further rearrangements, which may help stabilize the structure. Similarly, when ethyl acetoacetate is employed in place of dimedone, the initial interaction remains the same: malononitrile reacts with benzaldehyde to form the β-hydroxy nitrile intermediate. The subsequent steps (pathway B) also largely mirror the previous pathway (pathway A). Ethyl acetoacetate undergoes deprotonation to form an enolate ion, which then attacks the carbonyl of the β-hydroxy nitrile, facilitating the cyclization step. While the overall mechanism remains consistent between the two pathways, a minor difference lies in the nature of the enolate formation. Ethyl acetoacetate may produce a slightly different stability profile for the enolate ion compared to dimedone, which could influence the reaction kinetics or yields. However, this difference does not significantly alter the fundamental pathway leading to the formation of tetrahydrobenzo[b]pyran. In summary, both pathways demonstrate the versatility of the reaction conditions, allowing for the successful synthesis of tetrahydrobenzo[b]pyran using either malononitrile with benzaldehyde followed by dimedone or ethyl acetoacetate, confirming the robustness of the catalytic system employed.

Scheme 2
Scheme 2The alternative text for this image may have been generated using AI.
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Proposed mechanism for tetrahydrobenzo[b]pyran reaction in the presence Fe3O4@Hydrol-PMMAn.

To evaluate the applicability and efficiency of the Fe3O4@Hydrol-PMMAn catalyst in the synthesis of tetrahydrobenzo[b]pyrans, we compared our catalyst’s performance with those of other catalysts reported in the literature. The key outcomes, including reaction yield and time, are summarized in Table 4, showcasing the advantages of Fe3O4@Hydrol-PMMAn in terms of both rapid reaction times and high yields. Our catalyst demonstrates several distinguishing features that contribute to its superior performance. First, Fe3O4@Hydrol-PMMAn enables a greener reaction pathway by minimizing the need for toxic solvents and high-energy conditions, aligning with principles of sustainable chemistry. Second, it is cost-effective, as its preparation relies on accessible, inexpensive materials, and it can be reused across multiple reaction cycles without significant loss of activity, as verified through recyclability tests. Additionally, the structural stability of Fe3O4@Hydrol-PMMAn under reaction conditions ensures consistent catalytic performance, enhancing its practicality for diverse applications in organic synthesis. Compared to conventional catalysts, Fe3O4@Hydrol-PMMAn combines operational simplicity with high efficiency, which not only accelerates the synthesis process but also reduces waste generation. This combination of factors-greener methodology, cost-effectiveness, stability, and recyclability-makes Fe3O4@Hydrol-PMMAn an ideal catalyst for broader applications, offering substantial advantages over traditional systems.

Table 4 Comparison of present method with other reported protocols for the synthesis of highly substituted pyran derivatives.
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Conclusion

In conclusion, this study successfully presents a novel Fe3O4@Hydrol-PMMAn catalyst synthesized through a straightforward, innovative approach that combines hydrophilic polymers with iron oxide nanoparticles. A comprehensive characterization using FT-IR, XRD, SEM, TGA, EDX, and VSM confirmed the catalyst’s structural and compositional integrity. The Fe3O4@Hydrol-PMMAn catalyst proved to be highly efficient in synthesizing tetrahydrobenzo[b]pyrans, achieving excellent yields in remarkably short reaction times, ranging from as little as 4 min to a maximum of 30 min. Furthermore, hot filtration tests confirmed the stability and robustness of the catalyst throughout the process. Notably, the catalyst demonstrated high reusability, maintaining its catalytic activity with minimal decline across four cycles, making it a practical and sustainable choice for organic synthesis. This efficient process requires no additional additives, minimal catalyst quantities, and eliminates long reaction times, showcasing a more environmentally friendly approach. To our knowledge, this work stands as a pioneering effort in utilizing a polymer-supported iron oxide catalyst for the synthesis of tetrahydrobenzo[b]pyrans through condensation reactions. These results underscore the potential of Fe3O4@Hydrol-PMMAn as a valuable tool for sustainable advancements in organic synthesis, providing an economical, rapid, and eco-friendly pathway for future applications in the field.