Poly(4-vinylpyridinium) hydrogen sulfate: an efficient catalyst for the synthesis of xanthene derivatives under solvent-free conditions

Abstract

A simple and convenient procedure for the synthesis of xanthene derivatives is described through a one-pot condensation of 2-naphthol {2-hydroxynaphthalene-1,4-dione or the mixture of 2-naphthol and 2-hydroxynaphthalene-1,4-dione} with aryl aldehydes in the presence of poly(4-vinylpyridinium)hydrogen sulfate as an efficient, cheap, easily synthesised and eco-friendly catalyst under solvent-free conditions. Further, the catalyst can be reused and recovered several times without any variations in the yield of the product.

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In recent years, the development of heterogeneous catalysts has become a major area of research in synthetic organic chemistry due to the potential advantages of these materials over homogeneous systems such as simplified recovery, reusability, enhanced selectivity and reactivity, easy product isolation and incorporation in continuous reactors and microreactors.^1,2 This can lead to novel and environmentally benign chemical procedures for academia and industry.³ Application of solid acids in organic transformations is important because these reagents do less harm to the environment and have no corrosion or disposal problems. As economically and ecologically benign catalysts, their research and application have attracted much attention in chemistry and industry.^4–9 There are more than 100 industrial processes using over 103 solid acids as catalysts.¹⁰

It is clear that green chemistry not only requires the use of environmentally benign reagents and solvents, but also the recovery and reuse of the catalyst. One way to overcome the problem of recyclability of the traditional acid catalyst is to chemically anchor the reactive center onto a large surface area solid carrier.¹¹ In these types of solids, the reactive centers are highly mobile similar to homogeneous catalysts and at the same time these species have the advantage of being recyclable in the same fashion as heterogeneous catalysts. Functional polymers have the potential advantages of small molecules with the same functional groups.¹² Poly(4-vinylpyridine) is an interesting material because of its stable pyridyl group and ability to form charge transfer complexes with acidic dopants.¹³ Poly(4-vinylpyridine) seems to be an attractive support to immobilize mineral acids because of the basic nature of the pyridyl group.

The synthesis of xanthene derivatives, especially benzoxanthenes, has emerged as a powerful tool in organic synthesis due to their wide range of biological and therapeutic properties such as antibacterial,¹⁴ antiviral¹⁵ and anti-inflammatory activities¹⁶ as well as in photodynamic therapy¹⁷ and for antagonism of the paralyzing action of zoxazolamine.¹⁸ Furthermore, due to their useful spectroscopic properties, they are have been used as dyes,¹⁹ in laser technologies,²⁰ and in fluorescent materials for visualization of biomolecules.²¹ Many procedures describe the synthesis of xanthenes and benzoxanthenes including cyclodehydrations,²² alkylations γ to the heteroatoms,²³ trapping of benzynes by phenols,²⁴ cyclocondensation between 2-hydroxyaromatic aldehydes and 2-tetralone,²⁵ the reaction of 2-naphthol with aldehydes or acetals under acidic conditions and intramolecular phenyl carbonyl coupling reactions of benzaldehydes and acetophenones.²⁶ In addition, 14H-dibenzo[a.j]xanthenes and related products were prepared by reaction of 2-naphthol with formamide,²⁷ 2-naphthol-1-methanol,²⁸ carbon monoxide²⁹ and sulfonic acid.³⁰ Even though various procedures were reported, disadvantages including low yields, prolonged reaction times, use of an excess of reagents/catalysts and use of toxic organic solvents necessitate the development of an alternative route for the synthesis of xanthene derivatives.

In this paper, we investigate the synthesis of xanthene derivatives in the presence of poly(4-vinylpyridinium) hydrogen sulfate [P(4-VPH)HSO₄] under solvent-free conditions. The reported route is an efficient, convenient and novel method for condensation of aldehydes with 2-naphthol and 2-hydroxynaphthalene-1,4-dione or the mixture of 2-naphthol and 2-hydroxynaphthalene-1,4-dione in the presence of P(4-VPH)HSO₄ as solid acid (Scheme 1).

Scheme 1 The synthesis of xanthene derivatives in presence P(4-VPH)HSO₄ under solvent-free conditions.

In continuation of our ongoing research program on the development of new catalysts and methods for organic transformations,^31–34 an earlier report from our laboratory described the environmentally benign synthesis of poly(4-vinylpyridinium) hydrogen sulfate P(4-VPH)HSO₄ and the application in 1,1-diacetylation of aldehydes.³³ The structure of P(4-VPH)HSO₄ assured us to accept that this reagent can act as an efficient catalyst in reactions that need mild acidic reagents to speed up.

In an initial study, in order to examine the catalytic activity of catalyst, we examined the reaction benzaldehyde (1 mmol) with 2-naphthol (A, 2 mmol) under different conditions including refluxing in various solvents (MeOH, EtOH, THF, MeCN, EtOAc, and toluene) and also under solvent-free classical heating conditions. In refluxing solvents, after 6 h, the yields of products were low (< 45%). We found that the best results were obtained under solvent-free conditions in the presence of P(4-VPH)HSO₄ 2 mol% at 100 °C (Table 1). This may be explained due to the decreased diffusion of the reactant molecules in the presence of the solvent and the interference of solvent molecules with the active sites of P(4-VPH)HSO₄. When the reaction was carried out at higher temperature, TLC and ¹H NMR spectra of the reaction mixture showed a combination of numerous products, the yield of the expected product was slightly low.

Table 1 Comparison of the amount of catalyst at different temperatures and yields using conventional heating for the synthesis of 14-(phenyl)-14H-dibenzo[a,j]xanthene

Entry	Amount of catalyst/mg	T/°C	Yield^a (%)	Time/min
1	0	100	—	360
2	5	100	65	120
3	10	100	94	60
4	10	110	94	60
5	10	120	88	60
6	15	80	78	60
7	15	100	94	60
8	20	100	92	60

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The condensation of 2-naphthol and aryl aldehydes (carrying both electron-withdrawing and electron-donating groups), in the presence of P(4-VPH)HSO₄ 10 mg as a solid acid catalyst under solvent-free conditions using conventional heating at 100 °C, yielded desired 14H-dibenzo[a.j]xanthenes in high purity with excellent yields. The reactions required 52–78 min with conventional heating at 100 °C (Table 2, enteries 1–13). The nature of the functional group on the aromatic ring of the aldehyde exerted a slightly influence on the reaction time and the yield of dibenzo[a,j]xanthene. A decrease of the reaction rate was observed with arylaldehyde bearing electron-withdrawing (such as –NO₂) and electron-donating (such as –OCH₃) group in the para-position (Table 2, entry 7,10), in comparison to the unsubstituted arylaldehyde. The presence of a halogen (such as –Br and –Cl) group in the same position (Table 2, entries 4,13) enhanced both the rate and the yield of product. This slight difference is also seen in ortho-substituted benzaldehydes (Table 2, entries 5,8,11).

Table 2 P(4-VPH)HSO₄ catalyzed synthesis of xanthene derivatives^a

Entry	R	Time/min	Yield (%)^b	Mp °C
				Found	Reported [Ref.]
a Products were characterized by ^1H NMR, IR and melting point and also by comparison with the reported in literature data. Due to the very low solubilities of products 17 and 18, ^13C NMR spectra for these products were not reported. b Isolated yields.
1	C6H5–	55	94	184–185	185–187^35
2	4-CH3–C6H4–	72	93	230–231	228–229^35
3	3-Br–C6H4–	78	94	196–198	—
4	4-Br–C6H4–	65	96	297–298	296–298^36
5	2-NO2–C6H4–	72	94	295–260	293^30
6	3-NO2–C6H4–	72	92	213–215	210–211^35
7	4-NO2–C6H4–	70	96	312–314	312–313^35
8	2-CH3O–C6H4–	78	94	259–260	260^22c
9	3-CH3O–C6H4–	76	92	179–180	179–180^37
10	4-CH3O–C6H4–	72	90	204–205	203–206^35
11	2-Cl–C6H4–	57	92	214–216	216–218^35
12	3-Cl–C6H4–	54	93	210–213	184–185^37
13	4-Cl–C6H4–	52	96	290–292	290–292^35
14	C6H5–	70	94	306–308	305–307^38
15	4-CH3–C6H4–	68	90	305–306	304–307^38
16	4-Cl–C6H4–	62	96	> 320	330–332^38
17	Isatin	132	82	> 320	350 dec^39
18	Ninhydrin	140	78	230 dec	230 dec^39
19	C6H5–	40	90	320	319–320^40
20	4-CH3–C6H4–	25	91	256–258	255–256^40
21	4-Cl–C6H4–	40	94	300–311	305–306^40
22	4-CH3O–C6H4–	26	88	280–282	279–280^40
23	4-NO2–C6H4–	28	94	> 320	332–333^40

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After successfully synthesizing a series of 14H-dibenzo[a,j]xanthenes in good yields, we turned our attention toward the synthesis of 5H-dibenzo[b,i]xanthene-tetraones under similar conditions. We replaced the 2-hydroxynaphthalene-1,4-dione compound instead of 2-naphthol in same conditions (Scheme 1). We next examined the reaction with 2 equivalents of 2-hydroxynaphthalene-1,4-dione compound with various aldehydes. As expected, these substrates underwent smooth, one-pot conversion to give the corresponding 5H-dibenzo[b,i]xanthene-tetraones in good yields (Table 2, entries 14–18). From the results in Table 1, the slight variations in the yield of dibenzo[b,i]xanthene-tetraones was likely due to the presence of the different aromatic substituents on the aldehydes, although it must be noted that in general, the yield of most xanthene products was greater than 90%. Thus, 4-chlorobenzaldehyde (Table 1, entry 16), afforded slightly better xanthene yields over benzaldehyde and 4-methyl benzaldehyde (Table 2, entries 14,15). It was also observed that a more significant decrease in the yield of product with longer reaction times occurred when simple or substituted benzaldehydes were replaced with polycyclic aromatic precursors such as isatin and ninhydrin (Table 2, entries 17,18).

Finally, we have developed this synthetic method for one pot efficient synthesis of 14-aryl-14H-dibenzo[a,i]xanthene-8,13-diones by condensation of aldehydes with 2-naphthol and 2-hydroxynaphthalene-1,4-dione (Table 2, entries 19–23). The aromatic aldehydes containing electron-donating as well as electron-withdrawing groups underwent the conversion. However, the reaction conducted with benzaldehyde and 4-chlorobenzaldehyde showed longer reaction times.

Table 3, compared our results (amount of catalyst, reaction conditions, time, yield) with results obtained by other groups. As can be seen, our method is simpler, more efficient, and the reaction completed within a short period of time.

Table 3 Comparison of our results with results obtained by other groups for the synthesis of 14-(phenyl)-14H-dibenzo[a,j]xanthene

Entry	Catalyst	Amount of catalyst/mol%	Conditions	Time/h	Yield (%)^a	Ref.
a Isolated yields.
1	p-TsOH	10	Neat/125 °C	4	89	22c
2	Sulfamic acid	10	Neat/125 °C	8	93	30
3	K5CoW12O40·3H2O	10	Neat/125 °C	2	91	41
4	I2	20	Neat/90 °C	2.5	90	42
5	LiBr	15	Neat/130 °C	65 min	82	43
6	Montmorillonite K10	300 mg	Neat/120 °C	3	75	44
7	Amberlyst-15	10 mg	Neat/125 °C	2	94	45
8	Cellulose sulfuric acid	80 mg	Neat/110–115 °C	1.5	81	46
9	Silica sulfuric acid	80 mg	Neat/110–115 °C	1.5	89	46
10	Sulfuric acid	10	AcOH/110–115 °C	1.5	91	46
11	P(4-VPH)HSO4	2	Neat/100 °C	55 min	94	This work

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The recovery and reuse cycle of P(4-VPH)HSO₄ was also studied. Hence, we decided to study the catalytic activity of recycled solid acid P(4-VPH)HSO₄ for the synthesis of 14-(Phenyl)-14H-dibenzo[a,j]xanthene (Fig. 1). As shown in Fig. 1, the solid acid P(4-VPH)HSO₄ can be recycled at least five times without significant decrease in catalytic activity, the yields ranged from 94% to 93%.

Fig. 1 The recycling of P(4-VPH)HSO₄ in synthesis of 14-(phenyl)-14H-dibenzo[a,j]xanthene under solvent-free conditions.

Although the detailed mechanism of the above reaction was not fully established, the formation of dibenzoxanthenes could be explained by a reaction sequence similar to the literature reports^40,45,47,48 (Scheme 2). We proposed that the reaction proceeded via a reaction sequence of condensation, addition, cyclization and dehydration. First, the condensation of aldehyde and 2-naphthol (A) or 2-hydroxynaphthalene-1,4-dione (B) gave the intermediate (C and D). The addition of 2-naphthol (A) or 2-hydroxynaphthalene-1,4-dione (B) to (C and D) leading to the formation of (E,F and G), which on intermolecular cyclization and dehydration gave rise to the desired xanthene derivatives.

Scheme 2 The plausible mechanism of the reaction.

We have described a solid acid poly(4-VPH)HSO₄ catalyzed, highly efficient, one-pot protocol for the synthesis of xanthene derivatives by the condensation of an aldehyde and 2-naphthol or 2-hydroxynaphthalene-1,4-dione or the mixture of 2-naphthol and 2-hydroxynaphthalene-1,4-dione under solvent-free conditions in excellent yields. The present methodology also has several other advantages such as: high reaction rates and excellent yields; no side reactions; ease of preparation and handling; cost efficiency and effective recovery; reusability of the catalyst; use of an inexpensive catalyst with lower loading; and a simple experimental procedure. Further work to explore this novel catalyst in other organic transformations is in progress.

Experimental section

General procedure for the synthesis of xanthene derivatives

A mixture of 2-naphthol (2 mmol) {2-hydroxynaphthalene-1,4-dione (2 mmol) or the mixture of 2-naphthol (1 mmol) and 2-hydroxynaphthalene-1,4-dione (1 mmol)}, aldehyde (1 mmol) and P(4-VPH)HSO₄ (10 mg, ∼0.02 mmol) was heated at 100 °C under solvent-free conditions. The progress of the reaction was followed by TLC. After completion of the reaction, the reaction mixture was diluted with ethanol (10 mL) and stirred for 10 min in 80 °C. The solid (catalyst) were collected by filtration and the residue was kept at room temperature and the resulting crystalline product was collected by filtration. The product was found to be pure and no further purification was necessary.

Representative spectral data

14-(Phenyl)-14H-dibenzo[a,j]xanthene (Table 2, entry 1). Colorless crystals; m.p. 184–185 °C; IR (KBr): ν = 3070, 3020, 1620, 1590, 1430, 1400, 1250, 1150, 1075, 825, 740 cm⁻¹; ¹H NMR (300 MHz, CDCl₃): δ = 6.46 (s, 1H, CH), 6.96 (t, J = 7.2 Hz, 1H, ArH), 7.12 (t, J = 7.2 Hz, 2H, ArH), 7.36–7.58 (m, 8H, ArH), 7.74–7.81 (m, 4H, ArH), 8.37 (d, J = 8.4 Hz, 2H, ArH) ppm; ¹³C NMR (75 MHz, CDCl₃): δ = 148.7, 145.1, 131.4, 131.0, 128.7, 128.5, 128.2, 126.6, 126.4, 126.2, 124.0, 122.6, 118.0, 117.2, 38.0 ppm; Anal. Calcd for C₂₇H₁₈O: C, 90.47; H, 5.06. Found: C, 90.52; H, 5.09.

Spiro[dibenzo[b,i]xanthene-13,30-indoline]-pentaone (Table 2, entry 17). Orange powder; m.p. > 320 °C; IR (KBr): ν = 3411, 3091, 1722, 1666, 1605 cm⁻¹; ¹H NMR (300 MHz, DMSO-d6): δ = 6.77-8.12 (m, 12H, ArH), 10.82 (1H, s, NH) ppm; Anal. Calcd for C₂₈H₁₃NO₆: C, 73.20; H, 2.85; N, 3.05. Found: C, 73.15; H, 2.74; N, 3.01.

14-(4-Methoxylphenyl)-14H-dibenzo[a,i]xanthene-8,13-dione (Table 2, entry 22). Yellow powder, m.p. 280–282 °C; IR (KBr): ν = 2920, 1661, 1633, 1590, 1577, 1365, 1282, 1250, 1237, 1210 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ = 3.66 (s, 3H, CH₃), 5.92 (s, 1H, CH), 7.31–7.98 (m, 12H, ArH), 8.12 (d, J = 7.6 Hz, 1H, ArH), 8.16 (d, J = 8.0 Hz, 1H, ArH) ppm; ¹³C NMR (100 MHz, CDCl₃) δ = 178.2, 158.1, 156.9, 147.1, 135.3, 134.9, 131.8, 131.1, 131.0, 129.9, 129.5, 129.2, 128.3, 127.3, 125.4, 124.3, 123.6, 116.9, 116.5, 113.4, 54.9 ppm; Anal. Calc. for C₂₈H₁₈O₄: C, 80.37; H, 4.34. Found: C, 80.48; H, 4.25.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c2cy20276j

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