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

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.


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 1 H nucleus, and a 125.7 or 75 MHz spectrometer for the 13 C nucleus, using CDCl 3 or DMSO-d 6 as solvent.

Composite preparation
Preparation of g-C 3 N 4  The g-C 3 N 4 nanoparticles were prepared using a previously reported method 19 .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-C 3 N 4 nanosheets.
Preparation of W/Cu@g-C 3 N 4 1 g of g-C 3 N 4 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 WCl 6 and 200 mg of CuCl 2 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-C 3 N 4 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-C 3 N 4 (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 catalysts [43][44][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-C 3 N 4 as an eco-friendly and effective catalyst in solvent-free conditions.The W/Cu@g-C 3 N 4 nanocomposite was prepared by supporting WCl 6 /CuCl 2 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 WCl 6 /CuCl 2 @g-C 3 N 4 .The absorption peak at 3000-3400 cm −1 is due to the -NH 2 groups in the g-C 3 N 4 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-C 3 N 4 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 WCl 6 and CuCl 2 doping on g-C 3 N 4 , indicating that the W/Cu@g-C 3 N 4 remained stable during the synthesis of the nanoparticles.
Energy dispersive spectroscopic (EDS) analysis, as shown in Fig. 2, was used to determine the percentage and chemical composition of WCl 6 /CuCl 2 @g-C 3 N 4 .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-C 3 N 4 the surface was successful.
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-C 3 N 4 nanocomposite demonstrates that the small sheet-like layered structure has been maintained in the stacked state.
The W/Cu@g-C 3 N 4 XRD characterization is shown in Fig. 4. The characteristic bending peaks at 22.3° and 27.4° belonged to W/Cu@g-C 3 N 4 .According to the XRD analysis.The nano catalyst's crystalline structure has been preserved despite the reaction with the WCl 6 and CuCl 2 .In the TEM images, the g-C 3 N 4 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-C 3 N 4 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-C 3 N 4 .The TEM images provide evidence that the W and Cu particles are immobilized or supported on the surface of the g-C 3 N 4 material.This observation aligns with the findings from the SEM images.
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-C 3 N 4 , 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-C 3 N 4 , 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.
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-C 3 N 4 (40 mg) as the catalyst.The catalytic efficiency of W/Cu@g-C 3 N 4 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-C 3 N 4 at 80 °C (Table 1, entry 2).Various quantities of W/Cu@g-C 3 N 4 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-C 3 N 4 (    Vol:.( 1234567890) 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-C 3 N 4 (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-C 3 N 4 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.
After the successful application of W/Cu@g-C 3 N 4 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-C 3 N 4 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).To investigate the practical application of W/Cu@g-C 3 N 4 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-C 3 N 4 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 Table 2.The synthesis of 1,8-dioxo-octahydroxanthenes in the presence of W/Cu@g-C 3 N 4 as a catalyst.a Isolated yields.Aldehyde (1 mmol) and either dimedone (2 mmol) were combined with W/Cu@g-C 3 N 4 (40  mg) and stirred at 80 °C for 1-2 h.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.
To provide additional confirmation of the stability of the W/Cu@g-C 3 N 4 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-C 3 N 4 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-C 3 N 4 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.Table 3.The synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes in the presence of W/Cu@g-C 3 N 4 as a catalyst.a Isolated 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.The catalytic activity of W/Cu@g-C 3 N 4 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 procedure [46][47][48][49][50][51][52][53][54][55][56][57][58][59] .
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-C 3 N 4 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.

Conclusions
The utilization of W/Cu@g-C 3 N 4 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-C 3 N 4 nanocomposite was prepared under hydrothermal conditions by WCl 6 /CuCl 2 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-C 3 N 4 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-C 3 N 4 provides an efficient, environmentally friendly, and practical approach for synthesizing benzoxanthenones and xanthenes derivatives (Supplementary Figures).

Table 1 .
Optimization of the synthesis of xanthenes derivatives.a Isolated 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.

Table 4 .
A Table of comparison with reported methods in the literature.