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. 2023 Sep 30;13:16469. doi: 10.1038/s41598-023-43746-y

Streamlining efficient and selective synthesis of benzoxanthenones and xanthenes with dual catalysts on a single support

Najmedin Azizi 1,✉, Fezzeh Farzaneh 1, Elham Farhadi 1
PMCID: PMC10542355  PMID: 37777606

Abstract

Using two catalysts on a single support can improve reaction efficiency, higher yields, improved selectivity, and simplified reaction conditions, making it a valuable approach for industrial transformation. Herein, we describe the development of a novel and effective heterogeneous catalyst, WCl6/CuCl2, supported on graphitic carbon nitride (W/Cu@g-C3N4), which was synthesized under hydrothermal conditions. The structure and morphology properties of the W/Cu@g-C3N4 were characterized using various spectroscopic techniques, including FTIR, XRD, TEM, TGA, EDX, and SEM. The W/Cu@g-C3N4 support material enabled the rapid and efficient synthesis of benzoxanthenones and xanthenes derivatives in high yields under mild reaction conditions and short reaction times. The W/Cu@g-C3N4 catalyst was also found to be easily recyclable, and its catalytic performance did not significantly decrease after five times use. The findings suggest that W/Cu@g-C3N4 is a promising chemical synthesis catalyst with significant implications for sustainable and cost-effective organic synthesis.

Subject terms: Catalysis, Green chemistry, Organic chemistry

Introduction

Recently, xanthenes and their derivatives have attracted considerable attention due to their potential applications in various fields, including biological and pharmaceutical sciences and laser technology1,2. These compounds have been found to exhibit a range of beneficial properties, such as anti-inflammatory, antimicrobial, antiviral, and antitumor activities, making them attractive targets for drug development3,4. Several methods for the preparation of biologically important 14-aryl-14H-dibenzo[a,j]xanthenes and 1,8-dioxo-octahydroxanthenes have been reported in the literature5–8, including the use of various catalysts or promoters9–11. However, most acid catalysts are highly corrosive and difficult to recover for subsequent reactions, which can limit their practical application12. To address this issue, there is a growing interest in developing green and reusable catalysts using simple and inexpensive starting materials for the environmentally safe synthesis of benzoxanthenone derivatives13–17. However, using acid catalysts in synthesizing benzoxanthenone products has several drawbacks, such as being highly corrosive and difficult to recover for subsequent reactions. Therefore, developing green and reusable catalysts has become increasingly crucial for the environmentally safe synthesis of benzoxanthenone derivatives. These catalysts should be made from simple, inexpensive starting materials and offer excellent catalytic activity, selectivity, and reusability. Developing such catalysts can provide a sustainable and cost-effective approach to chemical synthesis while minimizing the environmental impact.

Graphite carbon nitride (g-C3N4) is a polymeric material composed of carbon and nitrogen with a graphite-like structure18. It can be easily produced from inexpensive materials such as cyanamide, dicyanamide, melamine, urea, and thiourea via thermal condensation at around 550 °C19. In recent years, g-C3N4 has drawn attention for its widespread applications in catalysis, sensors, adsorption, drug delivery, gas separation, and solar cells20–23. As a strong support material, g-C3N4 has many acidic and basic Lewis sites, such as terminal and bridge NH– groups and N lone pairs in triazine/heptazine rings, potential sites for metal adsorption24–26. Additionally, g-C3N4 is highly stable against thermal and chemical attacks due to its tri-s-triazine ring structure and a high degree of condensation27–29. These structures resist high temperatures (up to 600 °C) and do not change their system30–35. Thus, g-C3N4 is a potential support material for various industrial applications36–39. Researchers have tried to design and fabricate nanostructures to enhance the photocatalytic activity of g-C3N4 by adjusting the size and surface properties. Furthermore, doping g-C3N4 with metal significantly improves its catalytic activity40–42.

Experimental

Materials and chemicals

All the chemicals utilized in this study were procured from commercially available sources and used without requiring additional modifications. Solvents were distilled before use. Fourier transform infrared (FT-IR) spectra were obtained using a Bruker Vector-22 infrared spectrometer with KBr pellets. Melting points were measured using a Buchi 535 melting point apparatus. The SEM and EDX analyses were performed using a TESCAN Vega Model scanning electron microscope. XRD patterns were obtained using a X'pert Pro model from Panalytical (Holland). TGA experiments were conducted using a TGA 209F1 thermoanalyzer instrument from Netzsch, (Germany). TEM of the samples were determined using a Zeiss EM10C Transmission Electron Microscope (Germany). NMR spectra were recorded Bruker Avance III HD spectrometer on a 500 or 300 MHz spectrometer for the 1H nucleus, and a 125.7 or 75 MHz spectrometer for the 13C nucleus, using CDCl3 or DMSO-d6 as solvent.

Composite preparation

Preparation of g-C3N4

The g-C3N4 nanoparticles were prepared using a previously reported method19. First, 10 g of melamine was placed in a 40 cm × 120 cm ceramic container without a lid and heated for 3 h at 550 °C with an increased rate of 5 °C/min. After the process was completed and the temperature reached an ambient level, a yellow powder was obtained. Heating the bulk sample at 550 °C for 2 h resulted in the formation of g-C3N4 nanosheets.

Preparation of W/Cu@g-C3N4

1 g of g-C3N4 was dispersed in 100 mL of methanol under stirring, and then the reaction mixture was subjected to ultrasound for 10 min at ambient temperature. In the next step, 200 mg of WCl6 and 200 mg of CuCl2 were dissolved in 10 mL of methanol in a flask using a stirrer. Then, the two solutions were mixed under stirring for 10 min. After that, the reaction mixture was heated at 60 °C for 2 h. The W/Cu@g-C3N4 catalyst was obtained after completion of the reaction, then filtered and washed with methanol and dried under vacuum at 60 °C.

General Procedure

Aldehyde (1 mmol) and either 2-naphthol or dimedone (2 mmol) were combined with W/Cu@g-C3N4 (40 mg) and stirred at 80 °C for 1–3 h without the use of a solvent. Thin-layer chromatography (TLC) was used to track the advancement of the reaction. Once the reaction was complete, ethyl acetate (30 mL) was added to the mixture, and the catalyst was separated from the reaction mixture by centrifugation. The solid product underwent recrystallization using either ethanol or ethyl acetate to achieve purification. Subsequently, all compounds were known, and their melting points were determined by comparing them with reference standards.

Results and discussion

Recently, there has been a growing emphasis on designing more environmentally friendly and sustainable organic reactions using cheap and non-toxic catalysts43–45. As part of our ongoing commitment to green chemistry, we have been exploring environmentally friendly reaction media, such as water and deep eutectic solvents, to prepare natural and biocompatible materials. This study presents an efficient, green, and rapid method for synthesizing benzoxanthenones and xanthenes derivatives using W/Cu@g-C3N4 as an eco-friendly and effective catalyst in solvent-free conditions. The W/Cu@g-C3N4 nanocomposite was prepared by supporting WCl6/CuCl2 on graphitic carbon nitride under hydrothermal conditions. The graphitic structure of carbon nitride served as a direct and rapid catalyst with potential support for double metal salts.

The FT-IR spectrum in Fig. 1 shows the presence of high-density triazine/heptazine rings in WCl6/CuCl2@g-C3N4. The absorption peak at 3000–3400 cm−1 is due to the –NH2 groups in the g-C3N4 or the NH stretching vibration and may also be attributed to water absorption from the air. The absorption peaks in the 1236–1639 cm−1 range correspond to the stretching vibrations of the carbon–nitrogen bonding groups that are efficiently incorporated in the g-C3N4 sample. Additionally, the strong absorption peak at 806 cm−1 indicates the bending vibration of the s-triazine rings. The FT-IR spectrum did not show any change in the prepared nanocomposite after WCl6 and CuCl2 doping on g-C3N4, indicating that the W/Cu@g-C3N4 remained stable during the synthesis of the nanoparticles.

Figure 1.

Figure 1

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FT-IR spectrum of W/Cu@g-C3N4.

Energy dispersive spectroscopic (EDS) analysis, as shown in Fig. 2, was used to determine the percentage and chemical composition of WCl6/CuCl2@g-C3N4. The EDS mapping confirms the presence of C, N, W, Cu, and Cl elements in the composite. The adoption of W and Cu onto the g-C3N4 the surface was successful.

Figure 2.

Figure 2

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EDS spectrum of W/Cu@g-C3N4.

To examine the surface morphology of the newly developed nanocomposite, SEM microscopy analysis was performed, and the results are presented in Fig. 3. The SEM spectrum of the W/Cu@g-C3N4 nanocomposite demonstrates that the small sheet-like layered structure has been maintained in the stacked state.

Figure 3.

Figure 3

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SEM images of W/Cu@g-C3N4.

The W/Cu@g-C3N4 XRD characterization is shown in Fig. 4. The characteristic bending peaks at 22.3° and 27.4° belonged to W/Cu@g-C3N4 .According to the XRD analysis. The nano catalyst’s crystalline structure has been preserved despite the reaction with the WCl6 and CuCl2.

Figure 4.

Figure 4

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XRD patterns of the W/Cu@g-C3N4.

Figure 5 of the study displays TEM (Transmission Electron Microscopy) images of the W/Cu@g-C3N4 nanocomposite. In the TEM images, the g-C3N4 component is observed to have a bulk morphology with a lamellar structure at the edges. It is described as having large sheets consisting of irregular micro-fragments. The g-C3N4 appears as a grey color in the images. On the other hand, the W and Cu components are represented by irregular spheres with some agglomeration. These particles can be visually distinguished in the images due to their dark color, which contrasts with the grey color of the g-C3N4. The TEM images provide evidence that the W and Cu particles are immobilized or supported on the surface of the g-C3N4 material. This observation aligns with the findings from the SEM images.

Figure 5.

Figure 5

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The TEM image of W/Cu@g-C3N4.

ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectroscopy) is a spectroscopic technique commonly used for the analysis of metal analytes and their concentrations in various samples, including nanocomposites. The presence and concentration of copper (Cu) and tungsten (W) in a nanocomposite were determined using a combination of EDX (Energy-Dispersive X-ray Spectroscopy) analysis and ICP-OES (Inductively Coupled Plasma-Optical Emission Spectroscopy) analysis. The EDX analysis indicated a Cu(II) content of 0.12% on the surface of the nanocomposite. To confirm the Cu(II) content obtained from the EDX analysis, ICP-OES analysis was performed. The ICP-OES test revealed a Cu(II) content of 0.10% in the nanocomposite. The close agreement between the values obtained from EDX (0.12%) and ICP-OES (0.10%) confirms the presence of Cu(II) in the nanocomposite at the given concentration. Similarly, the EDX analysis of the composite indicated a tungsten (W) content of 0.76 wt% on the nanocomposite surface. The value obtained from the ICP-OES analysis for the W loading was 0.74 wt%, which is in good agreement with the result obtained from the EDX analysis.

A TGA curve was employed to evaluate the thermal stability of W/Cu@g-C3N4, and the corresponding results are illustrated in Fig. 6. The TGA diagram indicates that the slight decrease in mass below 110 °C is related to the evaporation of water in the structure of W/Cu@g-C3N4, while the primary weight loss observed at temperatures above 400 °C is due to the decomposition of the structure. The subsequent mass loss observed above 600 °C is attributed to the decomposition of graphitic carbon nitride.

Figure 6.

Figure 6

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TGA diagram of the W/Cu@g-C3N4.

The synthesis of xanthenes and benzoxanthenones derivatives was achieved by reacting aldehyde (1 mmol) with dimedone and/or 2-naphthol (2 mmol) in the presence of W/Cu@g-C3N4 (40 mg) as the catalyst. The catalytic efficiency of W/Cu@g-C3N4 was evaluated through the reaction, and the optimal conditions are summarized in Table 1. Different parameters, such as solvent, temperature, and catalyst amount, were varied to optimize the reaction conditions. Ultimately, the highest efficiency in the shortest time was achieved with 40 mg of W/Cu@g-C3N4 at 80 °C (Table 1, entry 2). Various quantities of W/Cu@g-C3N4 were examined, and it was observed that an increased amount of catalyst (50 mg) did not result in a higher yield (Table 1, entry 7), while lower amounts (10 mg) resulted in decreased yield (Table 1, entry 10). Also, without the catalyst, only aldehyde and dimedone and Micheal addition products were recovered along with minor amount of products (12%) . Notably, the reaction was found to be unsuccessful when tested in organic solvents such as ethanol and ethyl acetate, chloroform, toluene, methanol, and water in the presence of 40 mg W/Cu@g-C3N4 (Table 1, entries 11–16). Additionally, composite elements, including g-C3N4 (Table 1, entry 17), and WCl6 and CuCl2 (Table 1, entries 18, 19) were tested independently, which led to a reduction in yield compared to the W/Cu@g-C3N4.

Table 1.

Optimization of the synthesis of xanthenes derivatives.

graphic file with name 41598_2023_43746_Figa_HTML.gif
Entry Catalyst Solvent (5 mL) Temp. (°C) Yields (%)a
1 – Neat 80 12
2 W/Cu@g-C3N4 (40 mg) Neat 80 92
3 W/Cu@g-C3N4 (40 mg) Neat 100 92
4 W/Cu@g-C3N4 (40 mg) Neat 60 64
5 W/Cu@g-C3N4 (40 mg) Neat 40 32
6 W/Cu@g-C3N4 (40 mg) Neat rt 26
7 W/Cu@g-C3N4 (50 mg) Neat 80 92
8 W/Cu@g-C3N4 (30 mg) Neat 80 81
9 W/Cu@g-C3N4 (20 mg) Neat 80 67
10 W/Cu@g-C3N4 (10 mg) Neat 80 58
11 W/Cu@g-C3N4 (40 mg) Ethyl acetate 80 51
12 W/Cu@g-C3N4 (40 mg) Ethanol 80 31
13 W/Cu@g-C3N4 (40 mg) Chloroform 80 48
14 W/Cu@g-C3N4 (40 mg) Toluene 80 54
15 W/Cu@g-C3N4 (40 mg) Water 80 23
16 W/Cu@g-C3N4 (40 mg) Methanol 80 28
17 g-C3N4 (50 mg) Neat 80 14
18 WCl6 (40 mg) Neat 80 54
19 CuCl2(40 mg) Neat 80 38
20 W@g-C3N4 Neat 80 76
21 Cu@g-C3N4 Neat 80 48
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aIsolated yields. Reaction time: 60 min. Reaction condition: Aldehyde (1 mmol) and dimedone (2 mmol) were combined with the listed catalyst and stirred at 80 °C for 60 min.

The synthesis of xanthenes and benzoxanthenones derivatives was carried out by reacting aldehyde (1 mmol) with dimedone (2 mmol) in the presence of W/Cu@g-C3N4 (40 mg) as the catalyst. Optimization of the reaction conditions was achieved by altering various parameters, including solvent, temperature, and the amount of catalyst. The optimal conditions were performed with 40 mg of W/Cu@g-C3N4 at 80 °C, which resulted in the highest efficiency in the shortest time. After obtaining the optimal conditions, we investigated the scope and limitations of the reaction by using different aromatic aldehydes with both electron-withdrawing and electron-donating groups. Upon obtaining the optimal conditions, we assessed the scope and limitations of the reaction by utilizing various aromatic aldehydes with both electron-withdrawing and electron-donating groups. By condensing various aromatic aldehydes with dimedone under the selected conditions, good to excellent yields were achieved within short reaction times, as summarized in Table 2. Aromatic aldehydes with electron-donating groups showed no significant difference in yield and reaction time compared to those with electron-withdrawing groups.

Table 2.

The synthesis of 1,8-dioxo-octahydroxanthenes in the presence of W/Cu@g-C3N4 as a catalyst.

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aIsolated yields. Aldehyde (1 mmol) and either dimedone (2 mmol) were combined with W/Cu@g-C3N4 (40 mg) and stirred at 80 °C for 1–2 h.

After the successful application of W/Cu@g-C3N4 in the preparation of 1,8-dioxo-octahydroxanthenes, the catalyst's application scope in the synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes was evaluated. Consistent with the prior reaction, the same reaction conditions were maintained, leading to favorable outcomes such as high yield, short reaction time, and excellent performance. The intended reaction was carried out by reacting aldehyde (1 mmol) with 2-naphthol (2 mmol) in the presence of W/Cu@g-C3N4 catalysts. 40 mg of catalyst was used for this reaction at a temperature of 110 °C, and 14-aryl-14H-dibenzo[a,j]xanthenes were obtained with a yield of 90% in 60 min.

To investigate the generality and domain of this method, different aromatic aldehydes with different structures were used in the model reaction. The results obtained from the reaction are presented in Table 2, where it can be observed that various aldehydes with both electron-rich and electron-deficient groups yielded good to excellent results (68–90%). Moreover, heteroaromatic aldehydes such as pyridine-4-carbaldehyde, pyridine-2-carbaldehyde, and thiophene-2-carbaldehyde also acted well as substrates in the reaction, leading to high efficiency in converting to dibenzo[a,j]xanthenes (Table 3).

Table 3.

The synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes in the presence of W/Cu@g-C3N4 as a catalyst.

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aIsolated yields. Aldehyde (1 mmol) and either 2-naphthol (2 mmol) were combined with W/Cu@g-C3N4 (40 mg) and stirred at 110 °C for 1–3 h.

To investigate the practical application of W/Cu@g-C3N4 in the synthesis of xanthenes derivatives, the efficiency and strength of the recycled catalyst were examined in a model reaction, as presented in Table 1. In order to minimize reaction errors, the reaction was conducted on a 5 mol scale, using 200 mg of catalyst for each run. Once the reaction was completed, the reaction mixture was cooled to room temperature, and ethyl acetate (30 mL) was added. The catalyst was then separated with the assistance of a centrifuge and washed with hot ethyl acetate. Afterward, it was dried. This procedure was repeated for four consecutive runs, and the results are depicted in Fig. 7. The findings demonstrated that W/Cu@g-C3N4 could be recycled and reused at least five times without experiencing a significant reduction in yield, as depicted in Fig. 7. This indicates that the catalyst exhibits good stability and can be effectively utilized in future reactions. Consequently, it represents a practical and cost-effective option for large-scale synthesis of xanthenes derivatives.

Figure 7.

Figure 7

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Recyclability of catalyst.

To provide additional confirmation of the stability of the W/Cu@g-C3N4 catalyst, several analytical techniques were employed, including TGA, EDX and TEM analysis. Both the fresh and recycled catalyst samples were subjected to these analyses. The TGA analysis revealed no significant changes in the weight loss behavior of the recycled W/Cu@g-C3N4 catalyst after five cycles. The FTIR spectra of the fresh and recycled catalyst samples displayed no appreciable differences in their chemical structures. TEM analysis provided visual evidence of the catalyst's structural integrity. The images of the fresh and recycled W/Cu@g-C3N4 catalyst showed no apparent changes in morphology or particle distribution. EDX analysis was conducted to evaluate the elemental composition of both the fresh and recycled catalyst samples. The results revealed a minor change in the composition of the catalyst after five cycles. Specifically, the element chlorine (Cl) was found to be absent in the recycled catalyst, indicating its removal or depletion during the recycling process.

The catalytic activity of W/Cu@g-C3N4 was evaluated and compared to previously reported reaction conditions for the production of benzoxanthenones. The results of this comparison are presented in Table 4. Based on the data in Table 4, it can be observed that the current protocol demonstrates higher efficiency and environmental friendliness compared to the previously reported methods. The new protocol offers several advantages, including a shorter reaction time, a reusable catalyst system, and a straightforward work-up procedure46–59.

Table 4.

A Table of comparison with reported methods in the literature.

Entry Catalyst Reaction conditions Reusability of catalyst Yield (%) References
1 Sulfated polyborate Neat (no solvent) at 100 °C; for 0.05 h 1 99 49
2 silica-bonded S-sulfonic acid Ethanol, 10 h; Reflux 1 98 50
3 manganese dihydrogen phosphate dihydrate Neat (no solvent) at 100 °C; 0.25 h 1 98 51
4 nano-zirconia-supported on sulfonic acid Neat (no solvent) at 100 °C; 2 h 3 94 52
5 [bmim]HSO4 [bmim]HSO4 100 °C; 0.416667 h 93 53
6 dimethyldodecylammonium)propanesulfonic acid hydrogen Water at 100 °C; for 1 h 1 93 54
7 1,3,5-trichloro-2,4,6-triazine;; Water, 120 °C; 0.833333 h 1 92 55
8 envirocat EPZ-10 water 70 °C; 2.5 h; 1 92 56
9 grafting of sulphamic acid on functionalized sawdust Ethanol 0.833333 h; Reflux 1 92 57
10 H3[PW7Mo5O40].12H2O on bentonite Neat (no solvent) 80 °C; 0.0833333 h 1 92 58
11 toluene-4-sulfonic acid Acetonitrile 80 °C; 1.16667 h 1 74 59
12 W/Cu@g-C3N4 Neat (no solvent) 80 °C; 1 h 5 92 This work
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The proposed reaction sequence in Fig. 8 provides a possible explanation for the formation of compound 3. According to the proposed reaction sequence, the catalytic system involving Cu and W on g-C3N4 synergistically activates carbonyl groups, acting as highly active Lewis acids. Initially, a Knoevenagel condensation reaction occurs between the activated aldehydes and dimedone 2, forming a Michael addition product. Subsequently, the nucleophilic attack of the 1,3-dicarbonyl compound on the activated Michael addition product takes place. This step is followed by an intramolecular cyclization process, leading to the formation of the benzoxanthenones compound, which corresponds to compound 3. In the absence of the W catalyst, the Michael addition products were found to be the major products formed in the reaction system. This suggests that the presence of the W catalyst plays a crucial role in shifting the reaction towards cyclization.

Figure 8.

Figure 8

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Proposed reaction mechanism.

Conclusions

The utilization of W/Cu@g-C3N4 as an environmentally friendly and efficient catalyst enabled the successful synthesis of benzoxanthenone and xanthene derivatives without the need for solvents, while the catalyst maintained its effectiveness upon reuse with good efficiency. The W/Cu@g-C3N4 nanocomposite was prepared under hydrothermal conditions by WCl6/CuCl2 supported on graphitic carbon nitride, which has several advantages, such as environmental safety of the catalyst and solvents, high catalytic efficiency, easy application, and high reaction speed. Using W/Cu@g-C3N4 as a catalyst also offers practical benefits, including its easy recyclability. After five times of use, no decrease in its catalytic performance was observed, indicating its high stability and cost-effectiveness for large-scale synthesis. Overall, the synthesis method using W/Cu@g-C3N4 provides an efficient, environmentally friendly, and practical approach for synthesizing benzoxanthenones and xanthenes derivatives (Supplementary Figures).

Supplementary Information

Supplementary Figures. (4.9MB, docx)

Acknowledgements

Financial support for this work by Chemistry and Chemical Engineering Research Center of Iran is gratefully appreciated. This work is based on research supported by the Iran National Science Foundation (INSF) under project No. 4013509.

Author contributions

F.F. performed material preparation, data collection, and analysis. E.F. wrote the first draft of the manuscript. N.A. was supervised and Wrote—the review and editing. All authors read and approved the final manuscript.

Funding

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Data availability

The data that support the findings of this study are available on request from the corresponding author.

Competing interests

The authors declare no competing interests.

Footnotes

Publisher's note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-023-43746-y.

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Data Availability Statement

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