A highly active and recyclable catalyst for the synthesis of indole and phenyl ether

Abstract

A new simple catalytic system consisting of copper-aluminium and hydrotalcite (CuAl–HT) has been developed using a facile one-pot method without harm to the environment. The catalyst was characterized using TEM, XRD and XPS. It could be used as an efficient catalyst for the synthesis of both indole and phenyl ether. As expected, the catalyst afforded high catalytic activity for the selective synthesis of indole via intramolecular dehydrogenative N-heterocyclization of 2-(2-aminophenyl)ethanol. Meanwhile, it also exhibited superior catalytic properties for an Ullmann-type coupling reaction to synthesise phenyl ether from iodobenzene and phenol. The CuAl–HT catalyst showed higher activity than conventional catalysts based on copper and could be recycled several times with stable catalytic activity. This procedure has real economic advantages since no expensive materials were used.

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Introduction

Benzo-fused N-heterocyclic compounds, particularly indoles, are important precursors for the synthesis of fine chemicals, pharmaceuticals, dyes and agrochemicals.¹ A number of methods for the synthesis of indoles have been developed and reviewed.² To date, considerable effort has focused on the synthesis of indoles by transition-metal-catalyzed N-heterocyclization of non-amino alcohols.³ However, the synthesis of such substituted N-heterocycles with many traditional approaches suffers from requiring harsh conditions and delivering poor selectivity. Recently, an attractive route for the synthesis of N-heterocycles has been to utilize alcohols as substrates as these are readily available and easy to handle.⁴ A variety of transition metals catalysts such as ruthenium,⁵ iridium⁶, palladium,⁷ nickel and copper,⁸ have been reported for the intramolecular dehydrogenative N-heterocyclization of 2-(2-aminophenyl)ethanol and its derivatives. Although the majority of the reported catalytic systems are active for this reaction, they are significantly more expensive, non-recoverable, and even have long reaction times. From ecological and practical points of view, the development of a reusable and less expensive base-metal catalyst is highly desirable. Given that the oxidation of an alcohol to an aldehyde is the key step in this route, a Cu catalyst is an attractive alternative.⁹

On the other hand, aryl carbon–oxygen forming reactions are important transformations in synthetic chemistry.¹⁰ The formation of diaryl ethers via a C–O cross-coupling reaction represents a powerful and straightforward method in organic synthesis.¹¹ Diaryl ethers are not only important structures in biological systems, but are also common moieties in pharmaceutical and materials research.¹² Common synthesis methods of diaryl ethers usually require the coupling reaction of phenols with aryl halides in the presence of a catalyst containing a transition metal. It has been reported that Pd-catalyzed methods can function in this role, while organic–inorganic hybrid materials¹³ have also been used to catalyse Ullmann-type coupling reactions for the synthesis of phenyl ether, but their high costs and elaborate ligands are drawbacks when compared with copper-mediated reactions.¹⁴

Herein, we designed and prepared CuAl–HT catalysts with three different levels of copper and aluminum content, and tested their application for the synthesis of indole and phenyl ether. The catalyst can successfully promote the oxidant-free dehydrogenation of various alcohols under liquid-phase conditions. This feature would increase the scope of this oxidative intramolecular oxidative cyclization and allow us to prepare N-heterocycles using substrates containing an alcohol functionality. Meanwhile, the catalysts afforded excellent yields of diaryl ethers in the Ullmann-type coupling reaction to synthesise phenyl ether from iodobenzene and phenol. It is important to note that the CuAl–HT catalyst could be recycled through a simple filtration of the reaction solution and used for 6 consecutive trials without a significant loss of its reactivity. In addition, this material is easy to prepare, and is harmless to the environment.

Experimental

Materials

Copper(II) nitrate hydrate, aluminum nitrate nonahydrate, sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, calcium chloride, toluene, dimethylsulfoxide, dioxane, dimethyl formamide, o-xylene, acetonitrile, tetrahydrofuran, sodium bicarbonate, copper acetate and copper powder were purchased from Sinopharm Chemical Reagent Co., Ltd. Various reaction reagents, such as 2-(2-aminophenyl)ethanol, sodium isopropyl alcohol, phenol, iodobenzene, p-iodonitrobenzene, p-cresol, p-iodoanisole, p-methoxyphenol, p-chlorophenol and were purchased from Alfa Aesar. All chemicals were of analytical grade and used as received without further purification. Deionized water was used throughout the experiments.

Preparation of CuAl–HT catalysts

CuAl–HT3 (Cu : Al = 3 : 1) was prepared as follows: about 100 mL of deionized water was taken into a 250 mL three neck round bottom flask and stirred at 25 °C with an overhead mechanical stirrer. A mixture of solution of Cu(NO₃)₂·3H₂O (16.46 g, 0.0702 moles) and Al(NO₃)₃·9H₂O (8.44 g, 0.0225 moles) in deionised water were added simultaneously drop-wise from the respective burettes into the round bottomed flask. The pH of the reaction mixture was maintained constantly (8–9) through the continuous addition of a base solution (NaOH/Na₂CO₃). The resulting slurry was washed with distilled water thoroughly to give a pH of 7 and separated by filtration. The collected sample (Cu²⁺Al–HT3 (Cu : Al = 3 : 1)) was dried at 70 °C for 12 h, and then treated in a 20% H₂/N₂ flow (100 cm⁻³ g⁻¹ min⁻¹) at 300 °C (ramping rate, 5 °C min⁻¹) for 2 h and passivation (0.1% O₂/N₂, 40 cm⁻³ g⁻¹ min⁻¹) at ambient temperature for 4 h. CuAl–HT2 (Cu : Al = 2 : 1) and CuAl–HT3 (Cu : Al = 1 : 1) were prepared using the same procedure.

Characterization of catalysts

All of the reagents and solvents were commercially available and used without further purification. XRD measurements were performed on a Rigaku D/max-2400 diffractometer using Cu-Kα radiation as the X-ray source in the 2θ range of 10–85°. The conversion was estimated using GC (P.E. AutoSystem XL) or GC-MS (Agilent 6890N/5973N). The Cu–Al content of the catalyst was measured using inductively coupled plasma (ICP) on IRIS Advantage analyzer. The morphology of the catalyst was observed using a Tecnai G2 F30 transmission electron microscope and the samples were obtained by placing a drop of a colloidal solution onto a copper grid and evaporating the solvent in air at room temperature. Inductively coupled plasma (ICP) measurements indicated that CuAl–HT3 contained 66.58% Cu and 9.84% Al₂O₃, CuAl–HT2 contained 59.44% Cu and 12.80% Al₂O₃, and CuAl–HT1 contained 45.20% Cu and 19.86% Al₂O₃.

Catalytic tests

Dehydrogenative reaction. Under nitrogen atmosphere, a 10 mL round-bottomed flask was charged with 2-(2-aminophenyl)ethanol (0.5 mmol), KO-t-Bu (0.5 mmol), CaCl₂ (0.25 mmol), o-xylene (4.0 mL) and catalyst (0.1 g). The reaction was performed at 140 °C for 12 h and was monitored using GC analysis. For recycling, the recovered catalyst was washed with methanol several times, and then dried under vacuum at 60 °C.

Ullmann-type coupling reaction. Under nitrogen atmosphere, a 10 mL round-bottomed flask was charged with phenol (1.0 mmol), iodobenzene (1.0 mmol), KF (2.0 mmol), DMSO (4.0 mL) and catalyst (0.2 mmol). The reaction was performed at 135 °C for 16 h and was monitored using GC analysis. For recycling, the recovered catalyst was washed with methanol several times, and then dried under vacuum at 60 °C.

Results and discussion

Characterization of catalysts

The phase composition and structure of the catalysts were determined using XRD. Fig. 1 shows the XRD patterns of the catalysts between 10° and 85°. Bragg reflections at 2θ values of 43.5°, 50.6°, 74.4° represent the (111), (200), (220) planes of the fcc crystal structure of copper metal. The sharp and strong peaks revealed that the Cu nano-crystals were highly oriented.

Fig. 1 XRD patterns of the catalysts: (a) CuAl–HT1; (b) CuAl–HT2; (c) CuAl–HT3.

To further probe the morphology of the catalyst, we used TEM measurement to analysis the catalysts. Fig. 2 shows the TEM images of the catalysts, in which well-dispersed copper nanoparticles could be observed clearly. The differences in synthesis conditions and the Cu(NO₃)₂·3H₂O and Al(NO₃)₃·9H₂O content affected the particle size of the copper nanoparticles. The TEM images in Fig. 2 show that the copper particle size is in the range of 10–30 nm for all of the as-prepared catalysts. This indicated that the copper nanoparticles were uniformly distributed in the hydrotalcite, which was advantageous for their catalytic activity.

Fig. 2 TEM micrographs of the catalysts: (A) CuAl–HT1, (B) CuAl–HT2; (C) CuAl–HT3.

Fig. 3 presents XPS elemental survey scans of the surface of the CuAl–HT3 catalyst (CuAl–HT1 and CuAl–HT2 are similar to CuAl–HT3). Peaks corresponding to copper and aluminum oxide could be clearly observed. To ascertain the oxidation state of Cu and Al₂O₃, X-ray photoelectron spectroscopy (XPS) studies were carried out. The XPS analysis of the spent Cu(0) and Al₂O₃ are shown in the Fig. 3b and c. As expected, the spectrum of the Cu region confirmed the presence of Cu(0) with peak binding energies of 952.5 eV and 932.7 eV, while there was a peak binding energy of 74.4 eV for Al₂O₃.

Fig. 3 XPS of the catalysts (a) CuAl–HT3, (b) CuAl–HT3 showing the Cu 2p_1/2 and Cu 2p_3/2 binding energies, and (c) CuAl–HT3 showing the Al 2p binding energy.

Catalysts used for indole synthesis

Table 1 presents the influence of solvent on the N-heterocyclization reaction for the synthesis of indole over CuAl–HT2 at different temperatures and times. It was observed that the reaction provided a good yield with the use of DMSO and o-xylene as a solvent, producing the best results with o-xylene (100% yield) (Table 1, entries 1–7), which was then employed in further studies. According to the experimental data, the yield of indole at 140 °C was obviously higher than at 100 °C (Table 1, entries 7 and 8).

Table 1 Effects of solvent, temperature and reaction time^a

Entry	Solvent	T [°C]/t (h)	Yield^b [%]
a Reaction conditions: 2-(2-aminophenyl)ethanol (0.5 mmol), KO-t-Bu (0.5 mmol), CaCl2 (0.25 mmol) and CuAl–HT2 (0.1 g), under 1 atm of N2.b Determined using GC.
1	MeCN	60/24	39
2	Toluene	100/20	1
3	DMSO	140/12	84
4	Dioxane	140/12	32
5	DMF	140/12	4
6	THF	70/12	Trace
7	o-Xylene	140/12	100
8	o-Xylene	100/12	12

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The reaction was carried out using 0.5 molar amounts of 2-(2-aminophenyl)ethanol in o-xylene (4 mL) in the presence of CuAl–HT2 as the catalyst with different bases. The results are summarized in Table 2. The results of these reactions showed that KO-t-Bu was much more effective and led to better yields (Table 2, entry 8). Other bases, such as Na₂CO₃ and NaHCO₃, were also effective (Table 2, entries 4 and 6), while KOH, NaOH and K₂CO₃ were not (Table 2, entries 2, 3 and 5). In contrast, the yield of indole was increased by the addition of a Lewis acidic additive (CaCl₂) (Table 2, entries 1 and 8). The results indicated that CaCl₂ played a great role in the N-heterocyclization reaction.

Table 2 Effects of base and additive on the reaction^a

Entry	Base	Additive	Yield^b [%]
a Reaction conditions: 2-(2-aminophenyl)ethanol (0.5 mmol), KO-t-Bu (0.5 mmol), CaCl2 (0.25 mmol), o-xylene (4.0 mL) and CuAl–HT2 (0.1 g), 140 °C, 12 h, under 1 atm of N2.b Determined using GC.
1	KO-t-Bu	None	61
2	KOH	CaCl2	63
3	NaOH	CaCl2	15
4	Na2CO3	CaCl2	99
5	K2CO3	CaCl2	25
6	NaHCO3	CaCl2	96
7	None	CaCl2	96
8	KO-t-Bu	CaCl2	100

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Finally, the catalytic activity for the N-heterocyclization reaction with various catalysts was compared (Table 3). CuAl–HT2 showed the highest catalytic activity and gave the corresponding product in 100% yield under the present conditions (Table 3, entry 10). The reaction did not readily proceed in the absence of the catalyst, or in the presence of Cu(AcO)₂, CuCl₂, CuCl, Cu powder, Ni₄₅Cu₁₀, Cu/Al₂O₃ or Cu²⁺Al–HT2 (Table 3, entries 1–8). The activities of a series of Cu–Al catalysts with different Cu/Al mass ratios (Table 3, entries 9, 10 and 13) were tested for the reaction. Out of the CuAl–HT catalysts CuAl–HT1, CuAl–HT2 and CuAl–HT3, the CuAl–HT2 catalyst was found to be the most effective catalyst for the N-heterocyclization reaction. Meanwhile, the catalyst revealed a remarkable activity and was reused for up to four consecutive cycles without an appreciable loss of its high catalytic performance (Table 3, entry 11). This result might be due to the partial oxidation of the Cu nanoparticles on the surface of the catalyst. Furthermore, the reaction catalyzed by CuAl–HT2 proceeded even in air to give indole in a yield of 83% (Table 3, entry 12).

Table 3 N-heterocyclization of 2-(2-aminophenyl)ethanol using various catalysts^a

Entry	Catalyst	Yield^b [%]
a Reaction conditions: 2-(2-aminophenyl)ethanol (0.5 mmol), KO-t-Bu (0.5 mmol), CaCl2 (0.25 mmol), o-xylene (4.0 mL) and catalyst (0.1 g), 140 °C, 12 h, under 1 atm of N2.b Determined using GC.c Yield after fifth cycle.d Reaction in air.
1	None	63
2	Cu(AcO)2	26
3	CuCl2	14
4	CuCl	16
5	Cu powder	69
6	Ni45Cu10 (ref. 16)	66
7	Cu/Al2O3	67
8	Cu^2+Al–HT2	58
9	CuAl–HT1	98
10	CuAl–HT2	100
11	CuAl–HT2	91^c
12	CuAl–HT2	83^d
13	CuAl–HT3	97

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We investigated the reactions using a variety of alcohols as the substrates under the reaction conditions and the results are summarized in Table 4. It can be concluded that all of the electron-neutral, electron-rich and electron-poor alcohols could react very well to generate indoles, producing excellent yields under the standard reaction conditions.

Table 4 Substrate extension^a

Entry	Substrates	Yield^b [%]
a Reaction conditions: alcohol (0.5 mmol), KO-t-Bu (0.5 mmol), CaCl2 (0.25 mmol), o-xylene (4.0 mL) and CuAl–HT2 (0.1 g), 140 °C, 12 h, under 1 atm of N2.b Determined using GC.
1	5-H	100
2	5-Me	99
3	5-CO2Me	94
4	5-MeO	99
5	5,6-(MeO)	95
6	6-MeO	94

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The separation and recovery of the catalyst from the reaction system is one of the most important issues, therefore the reusability of CuAl–HT2 was further investigated. To make the synthetic protocol more economical, a recyclability study of the catalyst was conducted for the synthesis of indole via dehydrogenative N-heterocyclization (Fig. 4). We observed that the catalyst was highly active under the present reaction conditions and could be effectively reused for six consecutive cycles.

Fig. 4 Catalyst recyclability study.

From these essential observations and discussions on these Cu–Al based catalysts, we proposed a catalytic cycle shown in Scheme 1 for the selective direct synthesis of indole via intramolecular dehydrogenative N-heterocyclization of 2-(2-aminophenyl)ethanol using the CuAl–HT2 catalyst, and named it a “borrowing hydrogen” mechanism.^5,15 The first step of the reaction would involve the oxidation of an alcohol to the corresponding carbonyl compound and a hydrido Cu–Al species. KO-t-Bu slightly promoted the initial step of the reaction in the presence of the catalyst. Then the carbonyl intermediate would readily cyclize to afford indole via intramolecular nucleophilic attraction of the amino group to the carbonyl carbon followed by dehydration. A Lewis acid (CaCl₂) may promote the formation of indole. The results in Table 2 showed that CaCl₂, with higher Lewis acidity, played a great role in the N-heterocyclization reaction. From the consideration of the reaction equilibrium, the removal of water (by anhydrous CaCl₂) clearly helped to drive the reaction towards indole production. The release of hydrogen in the reaction of the hydrido Cu–Al could regenerate the catalytically active Cu–Al species.

Scheme 1 A possible “borrowing hydrogen” mechanism.

Catalysts used for phenyl ether synthesis

Table 5 presents the influence of solvent on the Ullmann-type coupling reaction for the synthesis of phenyl ether over CuAl–HT3 at different temperatures and times. It was observed that the reaction provided a good yield with DMSO as solvent, producing the best results with DMSO (95% yield) (Table 5, entries 5–3), which was then employed in further studies. According to the experimental data, the yield of phenyl ether at 135 °C is obviously higher.

Table 5 Effects of solvent, temperature and reaction time^a

Entry	Solvent	T [°C]/t (h)	Yield^b [%]
a Reaction conditions: phenol (1.00 mmol), iodobenzene (1.00 mmol), CuAl–HT2 (0.02 g contains 0.2 mmol of Cu), and KF (2.00 mmol) in DMSO (4 mL) at 135 °C and stirring for 16 h.b Determined using GC.
1	MeCN	70/24	Trace
2	Toluene	100/20	Trace
3	DMSO	135/16	95
4	Dioxane	90/16	10
5	DMF	130/16	55
6	THF	60/16	Trace
7	o-Xylene	140/16	Trace

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The reaction was carried out using 1 mole of phenol DMSO (4 mL) in the presence of CuAl–HT3 as the catalyst with different bases. The results are summarized in Table 6. The results of these reactions show that KF was much more effective and led to a better yield (Table 6, entry 1). Other bases, such as Na₂CO₃ and Cs₂CO₃, were also effective (Table 6, entries 4 and 8). The results indicated that KF played a great role in the Ullmann-type coupling reaction.

Table 6 Effect of base^a

Entry	Base	Yield^b [%]
a Reaction conditions: phenol (1.00 mmol), iodobenzene (1.00 mmol), CuAl–HT2 (0.02 g, contains 0.2 mmol of Cu) and DMSO (4 mL) at 135 °C and stirring for 16 h.b Determined using GC.
1	KF	95
2	KOH	46
3	NaOH	43
4	Na2CO3	51
5	K2CO3	83
6	NaHCO3	47
7	None	36
8	Cs2CO3	53

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Subsequently, the catalytic activities of various catalysts were compared for the Ullmann-type coupling reaction (Table 7). CuAl–HT3 showed the highest catalytic activity and gave the corresponding product in 94% yield under the present conditions (Table 7, entry 11). The reaction did not readily proceed in the absence of the catalyst, or in the presence of Cu(AcO)₂, CuCl₂, CuCl, Cu powder, Ni₄₅Cu₁₀, Cu/Al₂O₃ or Cu²⁺Al–HT3 (Table 7, entries 1–8). The activities of a series of Cu–Al catalysts with different Cu/Al mass ratios (Table 7, entries 9 and 10) were tested for the reaction. Out of the CuAl–HT catalysts CuAl–HT1, CuAl–HT2 and CuAl–HT3, the CuAl–HT3 catalyst was found to be the most effective catalyst for the Ullmann-type coupling reaction.

Table 7 Ullmann-type coupling reaction using various catalysts^a

Entry	Catalyst	Yield^b [%]
a Reaction conditions: phenol (1.00 mmol), iodobenzene (1.00 mmol), and KF (2.00 mmol) in DMSO (4 mL) at 130 °C and stirring for 16 h.b Determined using GC.
1	None	10
2	Cu(AcO)2	38
3	CuCl2	16
4	CuCl	13
5	Cu powder	60
6	Ni45Cu10	58
7	Cu/Al2O3	72
8	Cu^2+Al–HT3	58
9	CuAl–HT1	90
10	CuAl–HT2	91
11	CuAl–HT3	94

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We investigated the reactions using a variety of aryl iodides and phenols as the substrates under the reaction conditions and the results are summarized in Table 8. It can be concluded that all of the electron-neutral, electron-rich and electron-poor aryl iodides could react with phenol very well to generate the corresponding cross-coupling product in excellent yields under the standard reaction conditions.

Table 8 Substrate extension^a

Entry	Phenol	Aryl iodide	Yield^b [%]
a Reaction conditions: phenol (1.00 mmol), aryl iodide (1.00 mmol), CuAl–HT2 (0.02 g, contains 0.2 mmol of Cu), and KF (2.00 mmol) in DMSO (4 mL) at 135 °C and stirring for 16 h.b Determined using GC.
1	C6H5OH	p-CH3OC6H4I	88
2	C6H5OH	C6H5I	94
3	C6H5OH	p-NO2C6H4I	98
4	p-CH3C6H4OH	p-CH3OC6H4I	91
5	p-CH3C6H4OH	C6H5I	90
6	p-CH3C6H4OH	p-NO2C6H4I	98
7	p-ClC6H4OH	p-CH3OC6H4I	92
8	p-ClC6H4OH	C6H5I	88
9	p-ClC6H4OH	p-NO2C6H4I	98

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The separation and recovery of the catalyst from the reaction system was one of the most important issues, therefore the reusability of CuAl–HT3 was further investigated. To make the synthetic protocol more economical, a recyclability study of the catalyst was conducted for the synthesis of phenyl ether via an Ullmann-type coupling reaction (Fig. 5). We observed that the catalyst was highly active under the present reaction conditions and could be effectively reused for six consecutive cycles.

Fig. 5 Catalyst recyclability study.

Conclusions

In conclusion, we have demonstrated that the CuAl–HT2 catalyst acts as a highly efficient, recyclable heterogeneous catalyst for the dehydrogenative N-heterocyclization of 2-(2-aminophenyl)ethanol in the presence of base and a catalytic amount of Lewis acid. The present catalytic system would provide a new and useful method for the synthesis of various N-heterocyclic compounds. Also we have found an efficient and economic catalyst system for the Ullmann synthesis of phenyl ether by using CuAl–HT3 as the catalyst in DMSO. The cross-coupling reactions of phenols with iodobenzene generated the corresponding coupling products in excellent yields under the reaction conditions. Furthermore, CuAl–HT can be recovered and recycled by a simple filtration of the reaction solution and used for 6 consecutive cycles without a decrease in activity.

Acknowledgements

The authors are grateful to Projects in Gansu Province Science and Technology Pillar Program (1204GKCA047), and the Key Laboratory of Catalytic engineering of Gansu Province, China Gansu Province for financial support.

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