Introduction

Recently ionic liquids (ILs) are considered as dual reagents (catalysts and solvents) and a greener alternative for a range of various organic reactions, but the application of them as catalyst in the absence of solvent is vital and important in organic and inorganic chemistry [1]. As well as, ILs are an appropriate alternative instead of volatile and toxic organic solvents in the future due to high polarity, solubility with certain organic solvents and/or water, low vapor pressure, and excellent solubility in organic and inorganic materials [2]. On the other hand, ILs have been frequently used as solar cell, catalysts, biocatalysts, in nanomaterial synthesis, liquid–liquid separations, polymerization reactions, extraction, dissolution processes, electrochemistry and electrodeposition, and as supercapacitors [3–10].

Among organic compounds, the synthesis of xanthene and its derivatives has received specific attention among pharmacological activities and organic chemists due to having a wide range of biological and therapeutic properties such as antioxidant [11], cytotoxic [12], anti-proliferative [13], antifungal [14], anti-inflammatory, antiviral [15], and antibacterial properties [16]. Furthermore, xanthene and its derivatives would be used in laser technologies [17], as pH-sensitive fluorescent materials for visualization of biomolecules [18], sensitisers in dye-sensitized solar cells (DSSCs) [19], in photodynamic therapy [20], as hole-transporting materials in organic light-emitting devices (OLEDs) [21], and in the food industry as additives [22]. Xanthene and its derivatives are available in natural plants [23, 24]. For instance, examples of natural xanthenes are 3-isopropyl-9a-methyl-1,2,4a,9a-tetrahydroxanthene A, used as an antidote for all snake venoms, blumeaxanthene B and lumeaxanthene C, used to treat gynecological disorders (Scheme 1).

Scheme 1
scheme 1

Examples of natural xanthenes

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Different procedures have been employed for the preparation of xanthenes and benzoxanthenes [25–29]. In addition, one of the best, easiest, and cost-effective procedures for the synthesis of xanthene derivatives is condensation between aldehydes with 2-naphthols under different conditions in the presence of various Lewis and Brønsted acid catalysts such as Fe2(SO4)3.7H2O [30], polyvinylpolypyrrolidone-bound boron trifluoride [31], pentafluorophenyl ammonium triflate [32], silica sulfuric acid [33], γ-Fe2O3–HAp–Fe2+ NPs [34], H5PW10V2O40 [35], cellulose sulfuric acid [36], Yb(OTf)3 [37], 2,6-Pyridinedicarboxylic acid [38], silica-supported [2-(sulfooxy)ethyl]sulfamic acid [39], Selectfluor™ [40], carbon-based solid acid [41], I2 [42], ZnO NPs [43], p-toluenesulfonic acid/ionic liquid [44], trityl chloride [45], and polytungstozincate acid [46]. Although several of these procedures suffer from disadvantages such as the use of excess reagents/catalysts, tedious workup procedures, toxicity of the reagent, unsatisfactory yields, low yields of products, use of organic solvents, prolonged reaction time, use of the often expensive catalysts and non-recyclability, the focus on introducing efficient, economical, and solvent-free procedures with high activity, simple reaction workup, and reusability of the catalyst to overcome these problems is still in demand for the synthesis of xanthene derivatives.

It is necessary to introduce milder, simple reaction workup, solvent- and metal-free, faster, reusability of the catalyst, and generally a greener approach accompanied with higher yields for the synthesis of xanthene derivatives. Therefore, and according to the above-mentioned notes, it seems that silica-supported 1-(2-sulfooxy)ethyl)1H-pyridine-1-ium-chloride SiO2/[SEP]Cl as a catalyst can be an appropriate in the synthesis of these organic compounds.

In this research, we wish to report a rapid and cost-effective procedure for the synthesis of silica-supported 1-(2-sulfooxy)ethyl)1H-pyridine-1-ium-chloride SiO2/[SEP]Cl as a new, highly efficient, and Brønsted acid catalyst for the preparation of 14-aryl-14H-dibenzo[a,j]xanthene 3a–p derivatives under solvent-, metal-free and thermal conditions (Scheme 2).

Scheme 2
scheme 2

One-pot synthesis of xanthene derivatives in the presence of BAIL under solvent-free conditions

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Experimental

General

All of the reagents used in the current study were purchased from Merck, Aldrich, and Fluka and used without further purification. All the products are known compounds and were characterized by comparing the IR (KBr), 1H NMR, and 13C NMR spectroscopic data and their melting point to the literature values. Nuclear magnetic resonance (1H NMR and 13C NMR) was recorded in CDCl3 solvent on a Bruker DRX-300 and -400 spectrometer using tetramethylsilane (TMS) as an internal reference. IR spectra were recorded on a Frontier FT-IR (PerkinElmer) spectrometer using a KBr disk. The purities of the substrates and reaction monitoring were accomplished by TLC on silica gel PolyGram SILG/UV 254 plates. Melting points (MP) were determined on a Thermo Scientific 9300 melting point apparatus.

Preparation of silica-supported 1-(2-sulfooxy)ethyl)1H-pyridine-1-ium-chloride SiO2/[SEP]Cl

In this study, silica-supported 1-(2-sulfooxy)ethyl)1H-pyridine-1-ium-chloride SiO2/[SEP]Cl as a BAIL catalyst was prepared according to the literature procedure (Fig. 1) [47]. Spectral data of 1-(2-sulfooxy)ethyl)1H-pyridine-1-ium-chloride are as follows: viscous brown oil: IR (KBr): OH (3200–3600 cm−1), C=N (1658 cm−1), C=C (1426 cm−1), S=O (1232 cm−1), and S–O (614 cm−1). 1H NMR (400 MHz, DMSO-d6): δ 3.13–3.16 (t, J = 5.6 Hz, 2H), 3.93–3.970 (t, J = 8 Hz, 2H), 8.10–8.17 (t, J = 12 Hz, 2H), 8.61 (s, 1H), 8.92–8.99 (t, J = 6.4 Hz, 2H), 9.00–9.42 (d, 1H). 13C NMR (100 MHz, DMSO-d6): 60.7, 61.8, 128.2, 142.2, 146.0.

Fig. 1
figure 1

Pictures of reaction between pyridine and 2-chloroethanol (a), 1-(2-sulfooxy)ethyl)1H-pyridine-1-ium-chloride (b), and silica-supported 1-(2-sulfooxy)ethyl)1H-pyridine-1-ium-chloride (c)

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General procedure for the preparation of 14-aryl-14H-dibenzo[a,j]xanthenes

A mixture of 2-naphthol (1 mmol), various aldehydes (1 mmol) and SiO2/[SEP]Cl (20 mol%) in a 10-mL round-bottomed flask connected to a reflux condenser was stirred in an oil bath at 110 °C. Completion of the reaction was indicated by TLC (monitored by TLC, ethylacetate:n-hexane 1:3). After completion, the solvent was removed under reduced pressure and the crude product was recrystallized with ethanol to afford the pure product in 81–95% yield. All the products were known compounds and characterized by comparing the melting point, 1H NMR and 13C NMR spectra with those reported in the literature.

14-(4-Bromophenyl)-14H-dibenzo[a,j]xanthenes (Table 4, entry 3e)

1HNMR (300 MHz, CDCl3): δ 5.34 (1H, s, CH), 6.16–6.48 (10H, m, Ar–H), 6.68–6.75 (4H, m, Ar–H), 7.19–7.22 (2H, m, J = 7.8, Ar–H). 13CNMR (75 MHz, CDCl3): δ 37.46, 116.65, 118.01, 120.21, 122.39, 124.37, 126.92, 128.91, 129.11, 129.88, 131.04, 131.24, 131.58, 143.98, 148.67.

4-(4-Methoxyphenyl)-14H-dibenzo[a,j]xanthenes (Table 4, entry 3 g)

1H NMR (300 MHz, CDCl3): 2.49 (3H, s, OMe), 5.36 (1H, s, CH), 5.59–5.61 (2H, d, J = 8.66 Hz, Ar–H), 6.32–6.45 (6H, m, Ar–H), 6.49–6.55 (2H, m, Ar–H), 6.69–6.76 (4H, m, Ar–H), 7.31–7.33 (2H, d, J = 8.48, Ar–H); 13C NMR (75 MHz, CDCl3): 37.17, 55.05, 113.89, 117.58, 118.06, 122.77, 124.28, 126.81, 128.80, 128.87, 129.23, 131.13, 131.47, 137.45, 148.69, 157.89.

14-(2-Chlorophenyl)-14H-dibenzo[a,j]xanthenes (Table 4, entry 3 l)

1HNMR (300 MHz, CDCl3): δ 5.69 (1H, s, CH), 5.81–5.82 (2H, d, Ar–H), 6.27–6.40 (5H, m, Ar–H), 6.50 (2H, d, Ar–H), 6.55–6.67 (4H, m, Ar–H), 7.63–7.66 (2H, J = 8.5, d, Ar–H). 13CNMR (75 MHz, CDCl3): δ 34.63, 118.02, 118.11, 123.46, 124.44, 126.93, 127.87–127.94, 128.66, 129.08, 129.60, 130.13, 130.89, 131.76, 131.81, 143.57, 148.95.

Results and discussion

For this work, BAIL C as a green and reusable catalyst was prepared by condensation reaction between pyridine and 2-chloroethanol that led to ionic liquid B, following which chlorosulfonic acid was added dropwise and slowly to ionic liquid B for 45–60 min at 0 °C, which afforded BAIL C (Scheme 3) [47].

Scheme 3
scheme 3

The synthesis of 1-(2-sulfooxy)ethyl)1H-pyridine-1-ium-chloride [SEP]Cl

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The structure of 1-(2-sulfooxy)ethyl)1H-pyridine-1-ium-chloride [SEP]Cl was characterized on the basis of its IR (KBr), 1H and 13C NMR data, which were presented in the “Experimental” section. The characterization of BAIL C in comparison to compounds A and B was further conducted by FT-IR spectrum (Fig. 2). As it can be seen from Fig. 2, the broad peak is at around 3200–3600 cm−1 which is related to the OH group. On the other hand, the spectra show that the broad peak is at around 1658 cm−1 for C=N and 1426 cm−1 for C=C, which imply both are related to pyridine ring, and the strong peak at 1232 cm−1 is related to the stretching vibration of S=O bond.

Fig. 2
figure 2

Comparison of IR spectrum of compounds A and B with BAIL C

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1H and 13C NMR spectra of the BAIL C are presented in Figs. 3 and 4. The important peaks of 1H NMR spectrum of BAIL C were related to the acidic hydrogen (SO3H) which was observed in 8.61 ppm. The 13C NMR spectrum of the BAIL C exhibited five signals in agreement with the proposed structure.

Fig. 3
figure 3

1H NMR spectrum of [SEP]Cl

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Fig. 4
figure 4

13C NMR spectrum of [SEP]Cl

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The activity of the new BAIL C as a reusable and green catalyst was tested using a one-pot condensation reaction of various aromatic aldehydes (1 mmol) with 2-naphthol (2 mmol) under solvent-free conditions for the preparation of biologically active 14-aryl-14H-dibenzo[a,j]xanthene derivatives (Scheme 2).

For this purpose, to optimize the reaction conditions for the synthesis of compound 3a, the condensation reaction of benzaldehyde (1 mmol) and 2-naphthol (2 mmol) was chosen using different amounts of BAIL as a heterogeneous and Brønsted acid catalyst under solvent-free conditions at room temperature (Table 1). As it can be seen in Table 1, when reaction was carried out in the absence of the catalyst, after 120 min the reaction was without yield (Table 1, entry 1). The best results were achieved when 20 mol% of the BAIL was appropriate to promote the reaction efficiently and give the product in excellent yield and in short reaction time (Table 1, entry 6). Additionally, it is worth noting that when a higher percentage of loading of the BAIL was used, the yields did not improve (Table 2, entry 6).

Table 1 Result of the amount of the catalyst in the synthesis of compound 3a under solvent-free conditions
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Table 2 Optimization of temperature in the synthesis of compound 3a under solvent-free conditions
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To optimize the temperatures, the condensation of benzaldehyde (1 mmol) and 2-naphthol (2 mmol) in the presence of 20 mol% of BAIL was checked, and the results are tabulated in Table 2. As it can be seen in Table 2, temperature increase was appropriate for the synthesis of compound 3a. Therefore, the model reaction in solvent-free conditions at 110 °C leads to the highest yield (94%) and shortest reaction time (15 min) compared to the other temperatures.

In the next study, to compare the efficiency of solvent-free conditions versus solvent conditions, the condensation reaction between benzaldehyde (1 mmol) and 2-naphthol (2 mmol) using BAIL (20 mol%) was tested in various solvents such as ethanol, methanol, chloroform, dichloromethane, and ethyl acetate under reflux conditions, and the results are summarized in Table 3. As it can be seen in Table 3, low yields of the product and longer reaction time rather than solvent-free conditions were obtained. Hence, performing the reaction under solvent-free conditions and in the presence of 20 mol% of BAIL at 110 °C was determined as the optimal condition.

Table 3 Optimization of various solvents in the synthesis of compound 3a on the model reaction under reflux condition
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To assess the efficiency and the scope of BAIL for the synthesis of biologically active 14-aryl-14H-dibenzo[a,j]xanthene derivatives, a variety of aromatic aldehydes (containing electron-withdrawing, electron-donating groups, and halogens on their aromatic ring) were reacted with 2-naphthol in the optimal reaction conditions to produce the desired products 3a–p in excellent yields and in short reaction time (Table 4). As it can be seen in Table 4, in all cases, aromatic aldehydes 3a–p containing both electron-donating and electron-withdrawing groups (–NO2, –Me, –OMe, –Br, –Cl, –OH, –NR2) reacted with 2-naphthol under solvent-free conditions at 110 °C for an appropriate time (10–30 min) and good to excellent yields (81–95%) to generate 14-aryl-14H-dibenzo[a,j]xanthene derivatives.

Table 4 The solvent-free synthesis of biologically active 14-aryl-14H-dibenzo[a,j]xanthene derivatives using silica supported 1-(2-sulfooxy)ethyl)1H-pyridine-1-ium-chloride
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The recovery and reusability of the catalyst is a very important factor in industry and green chemistry, and also one of the advantages of heterogeneous catalyst. It is also worth noting that silica-supported 1-(2-sulfooxy)ethyl)1H-pyridine-1-ium-chloride SiO2/[SEP]Cl as a catalyst can be recovered at the end of the reaction and can be used six times without losing its activity (Fig. 5). The yields of 4-(4-nitrophenyl)-14H-dibenzo[a,j]xanthenes 3i obtained after 15 min include 95, 93, 93, 91, 91 and 88%, respectively.

Fig. 5
figure 5

Recyclability of the SiO2/[SEP]Cl for the synthesis of compound 3i after 15 min

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The advantages of BAIL were compared (solvent, catalyst loading, time, yield, and temperature) with some other catalysts for the synthesis of compound 3a from the reaction between benzaldehyde with 2-naphthol, and the results are represented in Table 5. In 2012, Rao et al. (Table 5, entry 3) reported the preparation of 14-aryl-14H-dibenzo[a,j]xanthenes using ZnO NPs; in this procedure, the yield of products are low (80%) and reaction time is long (60 min). On the other hand, this procedure needs high temperature (150 °C) to complete the reaction. In 2008, Khaksar et al. (Table 5, entry 5) reported a highly efficient method for the synthesis of 14-aryl(alkyl)-14H-dibenzo[a,j]xanthenes using pentafluorophenyl ammonium triflate (PFPAT); this method relies on the use of toxic solvent like toluene, and requires very long reaction time (4.5 h). In addition, one of the other problems is that the catalyst used in this method is non-recyclable. Recently, Kumara et al. in 2006 reported the use of Selectfluor™ (Table 5, entry 7) as a catalyst for the preparation of 14-aryl(alkyl)-14H-dibenzo[a,j]xanthene derivatives in which the reaction completion time was very long and tedious (8 h), and also requires high temperature (125 °C). Wang et al. in 2009 reported the use of iodine (Table 5, entry 9) as a catalyst for the synthesis of 14-aryl(alkyl)-14H-dibenzo[a,j]xanthene derivatives under neat conditions; one of the main problems of this method is the use of iodine, which is toxic and biodegradable, and the reaction time is long. As it can be seen in Table 5, the reaction in the presence of BAIL was simpler, solvent free, economical, eco-friendly, the catalyst used was recyclable, and the reaction time was shorter. On the whole, it seems that this catalytic system can be an appropriate alternative method for the synthesis.

Table 5 Comparison of various catalysts used for the synthesis of compound 3a
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Conclusion

In summary, silica-supported 1-(2-sulfooxy)ethyl)1H-pyridine-1-ium-chloride SiO2/[SEP]Cl as a new and recoverable heterogeneous catalyst was employed for the synthesis of 14-aryl-14H-dibenzo[a,j]xanthene derivatives by the one-pot condensation reaction of various aldehydes with 2-naphthol under thermal and solvent-free conditions. This methodology offers very attractive features such as economic viability of catalyst, short reaction time, high yield of products, operational simplicity, easier workup procedure, ease of recovering and reusing of the catalyst, and higher turn-over frequency of the catalyst in comparison to previously reported catalysts.