Calcined Layered Double Hydroxides: Catalysts for Xanthene, 1,4-Dihydropyridine, and Polyhydroquinoline Derivative Synthesis.

This work is generally focused on the synthesis of NiFeTi-layered double hydroxides (LDHs) using a hydrothermal route, which were calcined at various temperatures (varying from 200 to 600 °C). The synthesized materials were physicochemically characterized. X-ray diffraction results revealed the loss of the layered structure on calcination resulting in the formation of layered double oxides (LDOs) or mixed metal oxides, which was also supported by Fourier transform infrared studies. Scanning electron microscopy results also show loss of the layered structure and the creation of LDOs on increasing the temperature. These LDOs were tested as the catalysts for the synthesis of biologically significant xanthene, 1,4-dihydropyridine, and polyhydroquinoline derivatives. Among all, NiFeTi LDH calcined at 600 °C proved to be the best catalyst for the synthesis of these derivative compounds under optimized conditions. The advantages obtained were excellent yields in a lesser reaction time. Stability and reusability were also assessed; the catalyst was stable even after five cycles. Furthermore, the memory effect of the obtained NiFeTi CLDH calcined at 600 °C confirms that the material so formed is a calcined state of LDH itself. High catalytic efficiency, easy fabrication, and recycling ability of NiFeTi CLDH calcined at 600 °C make it a potential catalyst for the synthesis of xanthene, 1,4-dihydropyridine, and polyhydroquinoline derivatives.

Introduction

Layered double hydroxides or LDHs (also named as hydrotalcite-like compounds) are anionic-clay materials. LDHs are made up of brucite-like layers comprising positive charge and intercalated anions (NO3–, CO32–, SO42–, Cl–, etc.) and H2O molecules. LDHs, so far, have been extensively employed in various fields, such as catalysis,[1] adsorption,[2] ion exchange,[3] drug carriers,[4] electrode materials,[5] and anticorrosion agents.[6] The general empirical formula of LDHs is represented by [M(1–IIMIII(OH)2](A)·mH2O, where MII represents divalent metallic cations (Mg2+, Cu2+, Fe2+, Ni2+, Ca2+, Co2+, Zn2+, etc.), MIII (Fe3+, Al3+, Cr3+, Ga3+, etc.) represents trivalent metallic cations, x denotes the molar ratio (MIII/(MII + MIII)), and A represents the interlayered anions with n valency. To date, many different types of stacking arrangements have been designed, such as sheets stacked in hexagonal symmetry with two layers/unit cell (manasseite) and rhombohedral symmetry with three layers/unit cell (hydrotalcite) and in minor symmetrical stacking arrangements. The interlayer bonding in LDHs is relatively very weak, which makes them very efficient for capturing inorganic and organic anions. The properties of LDHs, which make them multifunctional materials, are their great ability for anion exchange, a high surface area, and a high thermal stability.[7] Carbonate is the most common anion present in the interlayers of natural LDHs. However, a wide range of anions have been used for charge-balancing in various studies such as nitrates, oxyanions, halides, silicates, organic anions, complex anions, and polyoxometalate anions. Moreover, mixed metal oxides or layered double oxides (LDO) generated after thermally controlled calcination are efficient in showing large specific surface areas, high basic properties, greater adsorption capacity, elemental synergetic effects, thermal stability, and memory effects (ability to reconstruct their structure). Thus, nowadays, LDHs and their calcined form have attained significant attention.[8]

Xanthene and its derivatives have become an important class of organic chemistry since last few decades because of their wide range of biological, pharmaceutical, and medicinal applicability such as antibacterial,[9] antiviral,[10] anti-inflammatory,[11] antidepressants,[12] antiplasmodial,[13] and antimalarial agents.[14] Furthermore, they have also been used as local dyes (examples: fluorone, rosamine, fluorescein, etc.),[15] precursors for many useful organic compounds,[16] in laser technologies (example-RO67-4853),[17] pH-sensitive fluorescent materials for the visualization of biomolecules moieties,[18] peptide synthesis,[19] and antagonists for drug-resistant leukemia lines (Nguyen et al., 2009).[20] Structures of some known xanthenes are depicted in Figure with applications as fluorescent and dye materials (fluorone, rosamine, and fluorescein)[21] and showing pharmacological activity (RO67-4853, rhodamine, and amsacrine).[22] 1,4-Dihydropyridine (1,4-DHP) and polyhydroquinoline derivatives are also essential classes of organic chemistry significantly present in many synthetic drugs, biologically active natural products, and chemical intermediates.[14]

Figure 1

Structures of some xanthenes.

To date, many methods have been employed for preparing many of these organic derivatives.[14] Among all the methods, the simplest routes for synthesizing symmetrical and unsymmetrical xanthenes are the condensation of different aldehydes with cyclic 1,3-dicarbonyl compounds, β-naphthol, or with a 1:1 mixture of cyclic 1,3-dicarbonyl compounds and β-naphthol. So far, many catalysts are used for preparing various xanthenes, such as PVSA,[23] nano-TiO2,[24] [Et3NSO3H]Cl,[25] SbCl3/SiO2,[26] core/shell Fe3O4@GA@isinglass,[14] amberlyst-15,[27] boric acid,[28] and sulfamic acid.[29] However, such catalysts suffer from various disadvantages such as low yields, extensive workup, harsh reaction conditions, toxic solvents, and use of excessive and expensive catalysts. Furthermore, only limited catalysts are capable of synthesizing different kinds of xanthenes. Therefore, it has become important to find an alternative route for an environment-friendly preparation of xanthene derivatives. LDHs and calcined-LDHs (also named as LDOs) could be better alternatives for greener production of xanthenes, 1,4-DHP, and polyhydroquinoline derivatives because they can provide a large surface area as the catalytic support for catalyzing organic reactions. To the best of our knowledge, no such LDH or calcined LDH forms have been used so far for the preparation of these derivatives making this approach a novel one for such synthesis.

In this work, we have resynthesized a biocompatible NiFeTi LDH by using a hydrothermal method with urea as a template, which was previously used by our group for the water remediation.[7] Because of the property of a high surface area; LDHs and calcined LDHs might act as the catalytic support for various organic transformations enhancing their applications in the heterogeneous catalysis field apart from their excellent efficiency for the water purification. Thus, we have calcined ternary LDHs at multiple temperatures. The obtained materials were characterized using different techniques such as X-ray diffraction (XRD), Fourier transform infrared (FTIR), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The so-obtained materials were tested for the one-pot synthesis of xanthenes, 1,4-DHP, and polyhydroquinoline derivatives for the very first time, under greener conditions.

Results and Discussion

Characterization of NiFeTi CLDH6

The XRD patterns of regenerated NiFeTi LDH and its calcined forms are depicted in Figure . The XRD pattern of NiFeTi LDH was in a great correlation with that of the previously reported pure form of NiFeTi LDH.[7] The XRD pattern of NiFeTi LDH (Figure a) shows sharp peaks at 10.63, 22.09, and 34.08° corresponding to the (003), (006), and (009) diffraction planes, respectively, which confirms the formation of CO32–-intercalated NiFeTi LDH. The obtained d-spacing for (003), (006), and (009) diffraction planes is 0.79, 0.39, and 0.26 nm, respectively. When calcination was carried out at 200 °C, the layered structure of the material starts collapsing, which can be clearly understood from the illustration of the reduction of the peak intensity in the lower 2θ range (Figure a), because of the loss of water molecules from the material. When the calcination temperature was further increased (up to 600 °C), the complete destruction of the layered structure was observed, resulting in the formation of mixed metal oxides by the removal of all the intercalated carbonate ions and water molecules.[30]

Figure 2

XRD patterns of NiFeTi LDH (a), NiFeTi CLDH1 (b), NiFeTi CLDH2 (c), NiFeTi CLDH3 (d), NiFeTi CLDH4 (e), NiFeTi CLDH5 (f), and NiFeTi CLDH6 (g).

High-resolution (HRTEM) images of NiFeTi CLDH6 depicted in Figure show the existence of mixed metal oxide aggregates formed by the rupture of the layered structure of NiFeTi LDH.[30] The obtained fringe widths of NiFeTi CLDH6 are 0.29 and 0.22 nm (depicted in Figure d). SEM images of NiFeTi CLDH1 to NiFeTi CLDH6 are depicted in Figure , which clearly show the distortion of the layered structure of LDH with the increase of temperature.

Figure 3

HRTEM images (a–d) of NiFeTi CLDH6.

Figure 4

SEM images of (a) NiFeTi CLDH1, (b) NiFeTi CLDH2, (c) NiFeTi CLDH3, (d) NiFeTi CLDH4, (e) NiFeTi CLDH5, and (f) NiFeTi CLDH6.

The FTIR spectra of the NiFeTi LDH and its calcined forms are illustrated in Figure . The FTIR spectrum of the NiFeTi LDH could be easily correlated with the previously synthesized NiFeTi LDH.[7] The spectra of the NiFeTi LDH consist of a broad band at 3423 cm–1 because of the interlayered −OH group and H2O molecules. Shoulders observed at 3241 and 2991 cm–1 could be assigned to the interaction (H-bonding) between the interlayered carbonate ions and the water molecules. Also, the presence of carbonate ions could be confirmed by the existence of an asymmetric band at 1363 cm–1. On calcination, the bands of the carbonate ions started diminishing with every temperature increase and finally disappeared in all the calcined forms above 300 °C because of the complete removal of all the intercalated carbonate ions. Shoulders observed at 1632 cm–1 might be attributed to the vibrational absorptions of reversibly sorbed water molecules on the oxide surfaces (illustrated in Figure ).[30]

Figure 5

FTIR spectra of NiFeTi LDH (a), NiFeTi CLDH1 (b), NiFeTi CLDH2 (c), NiFeTi CLDH3 (d), NiFeTi CLDH4 (e), NiFeTi CLDH5 (f), and NiFeTi CLDH6 (g).

Catalytic Activity of NiFeTi CLDH6

The catalytic application of NiFeTi CLDH6 was investigated by synthesizing xanthene and 1,4-DHP derivatives. Preliminary, the best catalyst was selected from NiFeTi LDH and its calcined forms (NiFeTi CLDH1, NiFeTi CLDH2, NiFeTi CLDH3, NiFeTi CLDH4, NiFeTi CLDH5, and NiFeTi CLDH6) comparing the product yields (depicted in Table ) acquired from the catalytic reaction between 1 mmol of benzaldehyde and 2 mmol of dimedone in 10 mL ethanol at 80 °C. Among all the catalysts, NiFeTi CLDH6, because of higher available active sites with temperature increase, showed the highest catalytic ability and was preferred for synthesis. Moreover, the best reaction conditions (ideal solvent, catalyst amount, and best temperature) were optimized by monitoring a model reaction between benzaldehyde and dimedone.

Table 1

Optimization of Catalystsa

entry	catalyst	yield (%)b
1	NiFeTi LDH	60
2	NiFeTi CLDH1	72
3	NiFeTi CLDH2	78
4	NiFeTi CLDH3	87
5	NiFeTi CLDH4	89
6	NiFeTi CLDH5	92
7	NiFeTi CLDH6	96

Reaction conditions—benzaldehyde/dimedone = 1:2 (mmol), solvent (ethanol) = 10 mL, the amount of catalyst = 30 mg, temperature = 80 °C, and time = 1 h.

Isolated yields.

Different solvents might distract the environment by altering the catalyst stability, stability, and solubility of the reactants and reaction rates. Therefore, an appropriate solvent should be selected to avoid all the disturbances. To achieve the best catalytic conversion, various solvents were tested, and the corresponding xanthene yields are summarized in Table . After scrutiny of the results, ethanol is chosen as an ideal solution for the catalytic synthesis of xanthene, 1,4-DHP, and polyhydroquinoline derivatives. Furthermore, altering the catalyst amount up to 20 mg indicated that with every amount increment, the yield percentage also increased. Still, when the amount was further increased to 30 mg, no further change in % yield was detected. This might be because of the achievement of the maximum catalytic conversion efficiency of NiFeTi CLDH6.[31] Correspondingly, temperature also affects the reaction kinetics. Furthermore, to evaluate the effect of the temperature, the temperature was varied from 25 to 80 °C, and the results are summarized in Table . The refluxing temperature of ethanol, that is 80 °C, was obtained as the best temperature for achieving the maximum yield.

Table 2

Optimization of Solventsa

entry	catalyst	solvent	temperature (°C)	time (min)	yield (%)b
1	NiFeTi CLDH6	acetonitrile	85	60	65
2	NiFeTi CLDH6	methanol	70	60	76
3	NiFeTi CLDH6	water	100	60	30
4	NiFeTi CLDH6	ethanol	80	60	85

Reaction conditions—benzaldehyde/dimedone = 1:2 (mmol), solvent = 10 mL.

Isolated yields.

Table 3

Optimization of Synthesis Conditionsa

entry	catalyst	amount of catalyst (mg)	temperature (°C)	time (min)	yield (%)b
1	NiFeTi CLDH6	0	50	300	10
2	NiFeTi CLDH6	5	50	60	40
3	NiFeTi CLDH6	10	50	60	70
4	NiFeTi CLDH6	20	50	60	85
5	NiFeTi CLDH6	30	50	60	85
6	NiFeTi CLDH6	20	25	90	50
7	NiFeTi CLDH6	20	30	90	54
8	NiFeTi CLDH6	20	40	90	59
9	NiFeTi CLDH6	20	60	60	87
10	NiFeTi CLDH6	20	70	60	92
11	NiFeTi CLDH6	20	80	60	96

Reaction conditions—benzaldehyde/dimedone = 1:2 (mmol), solvent (ethanol) = 10 mL.

Isolated yields.

Using the above-optimized catalytic reaction conditions, a broad range of xanthene, 1,4-DHP, and polyhydroquinoline derivatives were synthesized by using benzaldehyde with electron-donating as well as electron-withdrawing groups (Tables and 5). The obtained products were characterized using NMR spectroscopy and were compared with the previously reported spectra. A few 1H NMR spectra of synthesized xanthene, 1,4-DHP, and polyhydroquinoline derivatives are depicted in the Supporting Information (Figures S1–S11).[31]

Table 4

NiFeTi CLDH6-Catalyzed Synthesis of Xanthene Derivativesa

productc	R1	aldehyde	time (min)	yield (%)b
3a	CH3	4-nitrobenzaldehyde	40	96
3b	CH3	2-nitrobenzaldehyde	60	90
3c	CH3	3-nitrobenzaldehyde	60	91
3d	CH3	2-bromobenzaldehyde	60	89
3e	CH3	3-methoxybenzaldehyde	80	88
3f	CH3	3-methylbenzaldehyde	70	90
3g	CH3	benzaldehyde	60	96
3h	CH3	4-methoxybenzaldehyde	80	84
3i	CH3	3-Bromobenzaldehyde	50	90
3j	CH3	4-methylbenzaldehyde	70	90
3k	CH3	4-cyanobenzaldehyde	60	94
3l	CH3	3-chlorobenzaldehyde	50	94
3m	CH3	4-chlorobenzaldehyde	60	93
3n	H	benzaldehyde	60	95
3o	H	3-Bromobenzaldehyde	50	94
3p	H	2-methylbenzaldehyde	90	90
3q	H	4-methylbenzaldehyde	70	89
3r	H	4-hydroxybenzaldehyde	80	90
3s	H	3-methoxybenzaldehyde	70	85
3t	H	3-methylbenzaldehyde	70	89
3u	H	4-nitrobenzaldehyde	40	93
3v	H	4-methoxybenzaldehyde	70	83

Reaction conditions—benzaldehyde/dimedone/1,3-cyclohexanedione = 1:2 (mmol), catalyst = 20 mg, solvent (ethanol) = 10 mL, and temperature = 80 °C.

Isolated yields.

Products characterized by NMR spectroscopy.

Table 5

NiFeTi CLDH6-Catalyzed Synthesis of 1,4-DHP and Polyhydroquinoline Derivativesa

product	aldehyde	1,3-dicarbonyl	time (min)	yield (%)
8a	benzaldehyde	5	70	85
8b	4-hydroxybenzaldehyde	5	60	80
8c	cinnamaldehyde	5	60	90
8d	propionaldehyde	5	60	92
8e	4-methylbenzaldehyde	5	60	80
8f	4-nitrobenzaldehyde	5	50	94
8g	4-chlorobenzaldehyde	5	50	92
8h	4-dimethylaminobenzaldehyde	6	60	94
8i	cinnamaldehyde	6	60	92
8j	4-hydroxybenzaldehyde	6	60	85
8k	propionaldehyde	6	70	88
8l	4-nitrobenzaldehyde	6	40	94
8m	2-nitrobenzaldehyde	6	40	92
8n	benzaldehyde	6	60	90
8o	4-methylbenzaldehyde	6	60	85

Reaction conditions—Aldehyde/(5)/(6) = 1:2 (mmol), catalyst = 20 mg, solvent (ethanol) = 10 mL, and temperature = 80 °C.

Isolated yields.

Products characterized by NMR spectroscopy.

The plausible mechanisms are depicted in Figures and 7. Calcined LDHs are supposed to show dual properties acidic as well as basic properties, acidic properties because of the positive charge on metal ions and basic properties because of the M–O– oxides formed from the calcination of LDH.[32] In the xanthene derivative synthesis (Figure ), the reaction starts with the activation of the carbonyl group (aldehyde) by NiFeTi CLDH6, which leads to the formation of the intermediate (A) by the nucleophilic attack of dimedone on the activated carbonyl group, following Michael addition by the second dimedone molecule resulting in the formation of intermediate B. Furthermore, the elimination of water after intramolecular cyclization results in the formation of the desired product.[31]Figure illustrates two pathways (A and B) of the possible mechanism for 1,4-DHP and polyhydroquinoline synthesis. Similar to the xanthene synthesis, in the catalytic synthesis of 1,4-DHP and polyhydroquinoline derivatives (Figure ), the reaction initiates by the activation of aldehyde and 1,3-dicarbonyl simultaneously by NiFeTi CLDH6. Then, these moieties react and result the Knoevenagel intermediate (I), which further reacts with a second 1,3-dicarbonyl molecule and produces the Michael addition intermediate (II). This intermediate further reacts with NH4OAc to give enamine, which on intramolecular cyclization results in the elimination of water, giving the required product.[14]

Figure 6

Plausible mechanism for xanthene synthesis using NiFeTi CLDH6.

Figure 7

Plausible mechanism for 1,4-DHP and polyhydroquinoline synthesis using NiFeTi CLDH6.

NiFeTi CLDH6 could be easily separated and recycled by using absolute ethanol. The recyclability and reusability of NiFeTi CLDH6 were evaluated by the model reaction. After the reaction completion, the catalyst was recovered using filtration, washed with ethanol and dried at 60 °C. The recovered catalyst was further used for five consecutive cycles, and the obtained yields (%) are illustrated in Figure . After every reusability stage, the FTIR spectrum of the recovered catalyst was recorded and summarized in Figure . From the results, it could be easily concluded that no appreciable change in the catalytic efficiency was recorded. Also, the FTIR spectra clearly illustrated that the recovered catalyst does not undergo any structural modifications. The elemental analysis of the recovered catalyst was also evaluated (reported in Table S1), and from the results, it could be inferred that no significant change in the elemental composition was reported. The SEM image of the recovered catalyst is illustrated in Figure S13.

Figure 8

Reusability of NiFeTi CLDH6 for xanthene.

Figure 9

FTIR spectra of (a) NiFeTi CLDH6, (b) NiFeTi CLDH6 after the 1st cycle, (c) NiFeTi CLDH6 after the 2nd cycle, (d) NiFeTi CLDH6 after the 3rd cycle, (e) NiFeTi CLDH6 after the 4th cycle, and (f) NiFeTi CLDH6 after the 5th cycle.

Apart from the reusability test, the memory effect was also studied, for which NiFeTi CLDH6 was stirred in 0.1 M Na2CO3 aqueous solution for 24 h, and the obtained material was tested by FTIR spectroscopy. From the results (depicted in Figure S12), it could be inferred that the re-occurrence of the band at 1363 cm–1 confirms the reformation of the layered structure because of the intercalation of carbonate ions. A comparative study of the catalytic ability of NiFeTi CLDH6 for catalytic synthesis of xanthene and 1,4-DHP derivatives with previously reported catalysts is illustrated in Table .

Table 6

Comparative Study of the Catalytic Efficiency of NiFeTi CLDH6a

entry	catalyst & catalyst amount	derivative	solvent/condition	time	yield (%)	refs
1	Fe3+-montmorillonite, 85 mg	xanthene	EtOH, 100 °C	6 h	94	(33)
2	[bmim]HSO4, 100 mg	xanthene	solvent free/80 °C	3 h	85	(34)
3	Zr(DP)2, 10 mol %	xanthene	EtOH/reflux	24 h	98	(35)
4	SO42–/ZrO2, 15 wt %	xanthene	EtOH/70 °C	8 h	95	(36)
5	NiFeTi CLDH6, 20 mg	xanthene	EtOH/reflux	1 h	96	this study
6	Al2(SO4)3, 10 mol %	1,4-DHP	EtOH/reflux	8 h	92	(37)
7	La2O3, 10 mol %	1,4-DHP	TFE	1–1.5 h	89	(38)
8	BiBr3, 2 mol %	1,4-DHP	EtOH	2 h	86	(39)
9	NiFeTi CLDH6, 20 mg	1,4-DHP	EtOH/reflux	70 min	85	this study

Abbreviations: bmim = 1-butyl-3-methylimidazolium, dp = dodecylphosphonate.

Conclusion

In summary, a novel and efficient NiFeTi CLDH6 was synthesized, characterized, and applied for the catalytic synthesis of xanthene, 1,4-DHP, and polyhydroquinoline derivatives in ethanol. This method is very much superior to previously known heterogeneous catalyses because of the various advantages such as no use of toxic solvents, great product yields, simple workup, less reaction time, and recyclable catalysts. NiFeTi CLDH6 showed significant recyclability up to 5 cycles. Therefore, it can be used as a potential catalyst for various sustainable syntheses.

Experimental Section

Synthesis of NiFeTi LDH and NiFeTi CLDH

NiFeTi LDH was resynthesized using the previously reported hydrothermal method.[7] For NiFeTi LDH synthesis, Ni(NO3)2·6H2O, Fe(NO3)3·9H2O, and TiCl4 were weighed and further added to 70 mL of decarbonated water. 1.5 g of urea was added to the mixture, and the mixture was vigorously stirred. The mixture was further transferred to an autoclave and hydrothermally aged for 48 h at 160 °C. The obtained product was filtered, washed with degassed water, and dried in an oven at 60 °C. The obtained NiFeTi LDH was calcined at various temperatures by heating 1 g of NiFeTi LDH for 4 h in a furnace (at different temperatures: 200, 300, 350, 400, 500, and 600 °C). The obtained mixed oxides were labeled as NiFeTi CLDH1, NiFeTi CLDH2, NiFeTi CLDH3, NiFeTi CLDH4, NiFeTi CLDH5, and NiFeTi CLDH6, respectively.

Catalytic Activity

Synthesis of 1,8-Dioxo-octahydroxanthene Derivatives

1 mmol of aldehyde, 2 mmol of 5,5-dimethylcyclohexane-1,3-dione/1,3-cyclohexanedione, and 20 mg of NiFeTi CLDH6 were stirred in 10 mL ethanol at 50 °C for a suitable time as depicted in Table . The reaction progress was monitored by thin-layer chromatography (TLC) by using ethyl acetate/hexane as the eluent. After the reaction completion, the reaction mixture was diluted by the addition of 20 mL of hot ethanol, and the catalyst was separated by the filtration method. The crude product was dissolved in ethanol and kept for recrystallization. The crystals so obtained of the pure product were washed with absolute ethanol and further air-dried. All the derived crystallized products were confirmed by NMR spectroscopy.

Synthesis of 1,4-DHP and Polyhydroquinoline Derivatives

1 mmol of aldehyde, 2 mmol of ethyl acetoacetate/dimedone, 1 mmol of ammonium acetate, and 20 mg of NiFeTi CLDH6 were stirred in 10 mL ethanol at 50 °C for a suitable time as depicted in Table . The reaction progress was detected by TLC by using ethyl acetate/hexane as the eluent. After the reaction completion, the reaction mixture was diluted by the addition of 20 mL of hot ethanol, and the catalyst was separated by the filtration method. The crude product was dissolved in ethanol and kept for recrystallization. The crystals so obtained of the pure products were washed with absolute ethanol and further air-dried. All the derived crystallized products were confirmed by NMR spectroscopy.