Facile and eco-friendly synthesis of chromeno[4,3-b]pyrrol-4(1H)-one derivatives applying magnetically recoverable nano crystalline CuFe₂O₄ involving a domino three-component reaction in aqueous media

Abstract

Nanocrystalline CuFe₂O₄ catalyzed one pot three component synthesis of chromeno[4,3-b]pyrrol-4(1H)-one derivatives has been achieved in aqueous media. The reaction between the essential building blocks (amine, glyoxal monohydrate and 4-aminocoumarin) has been performed at low catalyst loading, leading to a high yield of the desired heterocyclic scaffold. CuFe₂O₄ magnetic nanoparticles were prepared by a simple and effective citric acid complex method and characterized by using XRD, FT-IR, EDX and TEM and HRTEM images. The easy recovery and reusability of the catalyst and operational simplicity of the process makes the protocol attractive, sustainable, and economic.

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Introduction

Coumarin fused heterocycles are one of the most interesting classes of compounds due to their presence in many pharmaceutically active compounds¹ and many natural products with unique physical properties.² In addition, many of the coumarin fused heterocycles show antitumor,³ antibacterial,⁴ antifungal,⁵ anticoagulant,⁶ anti-inflammatory,⁷ antiviral⁸ and anti-HIV activities.⁹ On the other hand, among the various synthetic and naturally occurring heterocyclic structures, the pyrrole core containing molecular framework is the most prevalent. Compounds containing a pyrrole ring are the core structure of some natural products¹⁰ and pharmaceuticals including the highly efficient drug atorvastatin calcium,^11a anti-inflammatants,^11b antitumor agents,^11c immunosuppressants^11d etc. Pyrroles are also widely used in materials science,¹² bioorganic chemistry,¹³ supramolecular chemistry,¹⁴ as useful intermediates in organic synthesis¹⁵ and organic conducting materials.¹⁶ As both the coumarin and pyrrole systems show a broad range of biological properties, fusion of these two active scaffolds would generate a novel molecular templates that are expected to exhibit interesting biological properties. In fact, coumarin fused pyrrole derivatives are considered privileged structural motifs and comprise important classes of marine natural products; few of them display notable biological and pharmacological properties (Fig. 1). Lamellarins D (I), K (II) and M (III) have been found to be cytotoxic to a wide range of cancer cell lines.¹⁷ Lamellarin D (I) is also a potent inhibitor of human topoisomerase I¹⁸ and lamellarin H (IV) is a potent inhibitor of both molluscum contagiosum virus topoisomerase and HIV-1 integrase.¹⁹ Compound V were proven as a potent MDR reversal agent that hypersensitizes P-gp-resistant tumour cell lines to front-line conventional therapeutic agents.²⁰

Fig. 1 Biologically active coumarin fused pyrrole derivatives.

Considering the widespread application of pyrollo-coumarin structural motif and lack of proper economic and environmentally benign methods for the synthesis of this heterocyclic scaffold, it is essentially needful to further develop a versatile and more user-friendly protocol employing a multicomponent domino reaction. Functionalized nanoparticles have emerged as viable alternatives to conventional catalytic materials. Nano-catalysts mimic homogeneous (due to high surface area and easily accessible reaction centers of nano-catalyst) as well as heterogeneous (stable, easy to handle) catalytic systems. The main inconvenience, however, is that such small particles are almost impossible to separate by conventional means like simple filtration. To overcome this issue, the use of magnetic nanoparticles has emerged as a feasible solution; their insolubility and paramagnetic nature enables easy and efficient separation of the catalysts from the reaction mixture by using an external magnet. In recent years, the magnetic nanoparticles have been widely employed as novel recoverable catalysts instead of traditional metal catalysis, homogeneous Brønsted acid catalysis, organocatalysis and enzymatic catalysis. Among the several magnetic materials tested, copper ferrites have attracted wide interest as a catalyst due to synergetic catalytic effect between copper and iron sites.²¹ In continuation of our research program aimed toward the design and synthesis of novel heterocyclic systems,²² herein, we wish to disclose a rapid, high yielding, green synthetic protocol for a variety of chromeno[4,3-b]pyrrol-4(1H)-one derivatives by assembling the basic building blocks in aqueous medium using nano CuFe₂O₄ as the efficient magnetically recoverable catalyst.

Results and discussion

To the best of our knowledge, only a few references exist concerning the synthesis of coumarin fused pyrrole scaffold chromeno[4,3-b]pyrrol-4(1H)-one derivatives. However, most of these methods^23a–d suffer from one or more drawbacks such as multistep approach, use of harsh organic reagents and solvents, lower yields of the desired products etc. Peng et al.^23e reported one step synthesis of coumarin fused pyrrole but that involves palladium catalysis which makes the protocol expensive. Recently, a three component synthesis of chromeno[4,3-b]pyrrol-4(1H)-one derivatives was reported by Chen et al.,^23f but major drawbacks of the protocol are non-recoverability of the catalyst and use of high boiling solvent. So, we invented a synthetic protocol which is more efficient in all aspects.

At the onset of our investigation, for the synthesis of chromeno[4,3-b]pyrrol-4(1H)-one derivatives, the reaction of phenyl glyoxal monohydrate (1 mmol), aniline (1 mmol), and 4-aminocoumarin (1 mmol) was selected as the prototypical reaction to screen the experimental conditions (Scheme 1). At first, we have employed representative Brønsted and Lewis acid catalysts ZnO, Al₂O₃, SiO₂, I₂, p-toluenesulphonic acid, acetic acid, L-proline, Fe₂O₃ and CuO to judge their catalytic efficacy for the three-component reaction in aqueous medium at 70 °C. It was evident that in absence of any catalyst the reaction was unlikely to proceed to give the expected product even after heating the reaction mixture for about 20 h in aqueous medium at 70 °C (Table 1, entry 1). It is also noteworthy to mention that no product was detected when ZnO, Al₂O₃ and SiO₂ were applied for the three component coupling reaction (Table 1, entries 2–4). As shown in Table 1, influence of the I₂, p-toluenesulphonic acid, acetic acid, L-proline in the above reaction was not very much pronounced and very poor yield of the desired product was obtained (Table 1, entries 5–8). Entries 9 and 10 in Table 1 clearly showed that Fe₂O₃ and CuO were superior to other conventional Brønsted and Lewis acid catalysts applied for the desired synthesis of chromeno[4,3-b]pyrrol-4(1H)-one derivative. The above observations encouraged us to think about a catalyst having both metal ions, Fe³⁺ and Cu²⁺, to catalyze the domino reaction sequentially. For this, we have advocated CuFe₂O₄, a mixed metal-oxide in nano range size and as expected CuFe₂O₄ provided a satisfactory result (Table 1, entry 11).

Scheme 1 Synthesis of chromeno[4,3-b]pyrrol-4(1H)-one derivative.

Table 1 Optimization of reaction conditions for the synthesis^a of chromeno[4,3-b]pyrrol-4(1H)-one derivative 1a

Entry	Catalyst	Catalyst load (mol%)	Solvent	Time (h)	Yield^b (%)
a All reactions were carried out with 4-aminocoumarin (1 mmol), phenyl glyoxal monohydrate (1 mmol), and aniline (1 mmol) and specified catalyst in 5 ml solvent at 70 °C.b The yield of isolated products.
1	—	10	H2O	20	—
2	ZnO	10	H2O	10	—
3	Al2O3	10	H2O	10	—
4	SiO2	10	H2O	10	—
5	I2	10	H2O	8	22
6	p-Toluenesulphonic acid	10	H2O	8	26
7	CH3CO2H	10	H2O	8	32
8	L-Proline	10	H2O	8	30
9	Fe2O3	10	H2O	5	45
10	CuO	10	H2O	5	43
11	Nano CuFe2O4	10	H2O	2	92
12	Nano CuFe2O4	10	EtOH	2	81
13	Nano CuFe2O4	10	Toluene	2	64
14	Nano CuFe2O4	10	CH3CN	2	59
15	Nano CuFe2O4	10	DMF	2	53
16	Nano CuFe2O4	12	H2O	2	92
17	Nano CuFe2O4	8	H2O	2	88
18	Nano CuFe2O4	5	H2O	2	78

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Various solvents like water, ethanol, toluene, acetonitrile and DMF were also screened to test the efficiency of the catalysts in different reaction media and the results are summarized in Table 1. The reaction using ethanol and water as the solvents gave the corresponding product in high yields (Table 1, entries 11 and 12). From the economic and environmental point of view, water was chosen as the reaction medium for all further reactions. It is worth noting that the quantity of the catalyst plays a vital role in the formation of the desired product. It was found that 10 mol% of nano CuFe₂O₄ was sufficient to give the product up to 92% isolated yield (Table 1, entry 11). The yield remained unaffected when the catalyst loading was increased to 12 mol% (Table 1, entry 16). However, the yield was decreased significantly when the catalyst loading was reduced to 8 mol% and 5 mol% (Table 1, entries 17 and 18).

Nano crystalline CuFe₂O₄ was characterized by X-ray diffraction study, TEM images, FT-IR spectra and EDX analysis. The XRD patterns of CuFe₂O₄ calcined at 500 °C temperature is shown in Fig. 2. The crystalline nature of the CuFe₂O₄ appears in the XRD pattern. Six peaks at 18.3, 30.3, 35.6, 42.8, 57.1, and 62.98 can be assigned to the (101), (200), (211), (221), (303), and (224) diffraction peaks of CuFe₂O₄ spinel, respectively.²⁴ The morphology of CuFe₂O₄ was established by TEM shown in Fig. 3(a). The TEM image reveals that the nanoparticle catalyst has a spherical shape and the nanoparticles are almost uniform in size with a narrow distribution. The size of the CuFe₂O₄ particles is approximately 15–18 nm. The HRTEM image of the CuFe₂O₄ nano particles shown in Fig. 3(b) indicates high degree of crystallinity and 0.38 nm spacing between two adjacent lattice planes corresponding to the (101) lattice planes of CuFe₂O₄. As shown in Fig. 4, the FT-IR spectra of CuFe₂O₄ calcined at 500 °C temperature clearly indicates the presence of the peaks (559 cm⁻¹) for the Fe–O stretching vibration. Elemental analyses of the nano CuFe₂O₄ were performed at EDX equipped onto TEM. Quantitative EDX analysis showed Fe, Cu and O were the main elemental components (Fig. 5). The Cu-ferrite NPs analyzed as Fe = 29.28%, Cu = 13.99%, O = 56.69%. The analysis indicates that nanoparticles in the array are of initial formula CuFe₂O₄.

Fig. 2 XRD patterns of CuFe₂O₄ (a) before reaction and (b) after six run (c) crystal planes.

Fig. 3 (a) TEM image (b) HRTEM image of CuFe₂O₄.

Fig. 4 FT-IR spectra of CuFe₂O₄ before reaction and after six runs.

Fig. 5 EDX spectra of CuFe₂O₄ NPs.

Having identified the optimal conditions, the proposed catalytic system was employed to synthesize an array of chromeno[4,3-b]pyrrol-4(1H)-one derivatives and the results are summarized in Table 2. In order to explore the scope and the limitations of this catalytic domino reaction with the identified optimized reaction conditions in hand, we investigated a wide variety of commercially available amine, 4-amino coumarin and glyoxal derivatives under the optimized reaction conditions. The aromatic amine derivatives with electron withdrawing substituents in this three-component protocol showed superior reactivity compared to that of electron releasing substituents or unsubstituted aromatic amine derivatives. Furthermore, in the presence of a sensitive heterocyclic core containing amine derivatives (Table 2, entries 1n and 1o), the reaction proceeded successfully to provide the desired products in high yield. It was evident from Table 2 (entries 1p and 1q) that aliphatic amine derivatives offered slightly lower reactivity compared to aromatic amine derivatives. N-Substituted 4-aminocoumarin derivatives also produced the corresponding product in satisfactory yield (Table 2, entries 1r, 1s and 1t). To widen the scope of this method, different aromatic glyoxal and methyl glyoxal were also tested to synthesize the array of chromeno[4,3-b]pyrrol-4(1H)-one derivatives (Table 2, entries 1u–1x) and satisfactory outcome strengthens this protocol.

Table 2 Three component synthesis of chromeno[4,3-b]pyrrol-4(1H)-one derivatives

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With these outstanding results in hand, we are now in a position to propose the plausible mechanism for the reaction (Scheme 2). Initially, an imine intermediate A is formed via the condensation between amine and phenyl glyoxal monohydrate which is activated by the Fe³⁺ centre of CuFe₂O₄ NPs. The imine intermediate is immediately attacked by the Michael donor C-3 centre of 4-amino coumarin to produce intermediate B which upon intramolecular cyclization and dehydration affords the chromeno[4,3-b]pyrrol-4(1H)-one structural core. Both Fe³⁺ and Cu²⁺ centres participate to catalyse the reaction course. The difference in reactivity of various aromatic amine derivatives can also be explained by this mechanistic course. In comparison to the aromatic amines having electron releasing substituents or no substituent, aromatic amines with electron withdrawing substituents produce more reactive intermediate A which is attacked rapidly by Michael donor centre of 4-amino coumarin in the most crucial step of this domino process.

Scheme 2 Proposed mechanisms for the formation of chromeno[4,3-b]pyrrol-4(1H)-one.

The various products obtained (1a–1x) were characterized using IR, ¹H-NMR, ¹³C-NMR and ESI-MS analysis. Finally, the structure of the compound 1a was confirmed using single-crystal XRD analysis (Fig. 6).²⁵

Fig. 6 ORTEP diagram of compound 1a (CCDC no. 1457208).

Catalyst separation and product isolation is one of the most crucial steps of organic synthesis. A heterogeneous catalyst is more advantageous than homogeneous as the former can be easily recovered and reused. The recovery of nano catalyst by filtration is not an efficient process. Extractive isolation of products also requires excessive amounts of organic solvents. However, in the present protocol, after completion of the reaction, water was removed under reduced pressure from the reaction mixture and was stirred with 5 ml ethanol and within a few seconds after stirring was stopped, the catalyst was deposited on the magnetic bar and then easily removed using an external magnet, leaving a clear reaction mixture. The recovered catalyst was then washed successively with ethanol and distilled water and dried under vacuum. This recycled catalyst was used successfully for the synthesis of other chromeno[4,3-b]pyrrol-4(1H)-one derivatives applying the same developed protocol. The catalyst was found to be reusable for at least six cycles without any considerable loss of activity. We have also investigated the structural stability of CuFe₂O₄ catalyst by comparing its XRD and FT-IR spectra before and after sixth run in the one-pot synthesis of chromeno[4,3-b]pyrrol-4(1H)-one derivatives (1a). The results obtained are illustrated in Fig. 2 and 4 respectively. It is evident that the XRD and FT-IR spectra of the catalyst obtained before and after sixth run were almost same indicating the structural stability of magnetic nanoparticles under the applied reaction condition.

Overall this protocol for the synthesis of chromeno[4,3-b]pyrrol-4(1H)-one derivatives is beneficial over the previously reported methods in all aspect.

Conclusions

In summary, we have developed a sustainable, green, and economic protocol for the one-pot, three-component synthesis of a library of chromeno[4,3-b]pyrrol-4(1H)-one derivatives with the aid of a nano magnetic CuFe₂O₄ catalyst. Most of the products were isolated by simple filtration, avoiding the use of conventional silica gel column chromatography. The anchoring of CuFe₂O₄ magnetic nanoparticles as catalyst removes the basic inconvenience of catalyst separation. We believe that this convenient and efficient synthetic protocol would definitely grab the attention of the synthetic chemists, biologists as well as pharmaceutical industries.

Experimental

Preparation of catalyst

CuFe₂O₄ was prepared by using a simple modified literature method.²⁴ Fe(NO₃)₃·9H₂O (4 mmol), Cu(NO₃)₂·3H₂O (2 mmol) and citric acid (9 mmol) were dissolved completely in distilled water (50 ml). The solution was heated up to 90 °C to evaporate the water in an oil bath under continuous stirring and the citric acid was then decomposed at 300 °C. After the reaction, the resultant powder was calcined at 500 °C for 2 h to get the CuFe₂O₄ samples.

General procedure for the synthesis of chromeno[4,3-b]pyrrol-4(1H)-one derivatives

A mixture of 4-aminocoumarin (1 mmol), phenyl glyoxal monohydrate (1 mmol), and aniline (1 mmol) and CuFe₂O₄ (10 mol%) was stirred in water (5 ml) at 70 °C for a required period of time (TLC). On completion of the reaction, water was removed under reduced pressure from the reaction mixture and the resulting mass further stirred with 5 ml ethanol and within a few seconds after stirring was stopped, the catalyst was deposited on the magnetic bar and removed using an external magnet, leaving a clear solution. The solvent (ethanol) was then evaporated under vacuum to achieve the crude product which was crystallized from ethanol to get the pure products.

Acknowledgements

We gratefully acknowledge the financial support from U.G.C and Calcutta University. M. S. and K. P. thank U.G.C, New Delhi, India for the grant of Junior and Senior Research fellowships, respectively.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1457208. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra06979g

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