Nano polypropylenimine dendrimer (DAB-PPI-G₁): as a novel nano basic-polymer catalyst for one-pot synthesis of 2-amino-2-chromene derivatives

Abstract

The nano polypropylenimine dendrimer (DAB-PPI-G₁) Lewis base catalyst was found to be a highly efficient and recoverable catalyst for the rapid and convenient synthesis of 2-amino-2-chromene derivatives through the three-component condensation of aromatic aldehydes, malononitrile or ethyl cyanoacetate and phenols under solvent-free conditions in excellent yields and short reaction times.

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Introduction

Dendrimers are monodisperse and usually highly symmetric, spherical compounds. Nano polypropylenimine is an important class of dendrimer. In fact, dendrimers are polymer nanostructures with unique capabilities. First generation polypropylenimine dendrimers consist of four free terminal NH₂ groups.¹ These groups make polypropylenimine (PPI) dendrimers polyvalent and also increase their basic properties. Polypropylenimine dendrimers are used as agents for targeted drug delivery and gene transfer.² The physical characteristics of dendrimers, including their monodispersity, water solubility, encapsulation ability, and large number of functionalizable peripheral groups, make these macromolecules appropriate candidates for evaluation as drug delivery vehicles (Fig. 1).

Fig. 1 Schematic representation of dendrimeric structures.

2-Amino-2-chromen derivatives have attracted extensive interest owing to their biological activity.³ There is a widespread interest in the synthesis of chromene and its derivatives owing to their diverse range of biological properties such as their antimicrobial,⁴ TNF-α inhibitory,⁵ antifungal,⁶ estrogenic,⁷ anticancer,⁸ anti-HIV,⁹ and anti-bacterial properites.¹⁰ Such compounds have also been applied in pigments,¹¹ pesticides and insecticides.¹² Among the several methods reported for the synthesis of 2-amino-2-chromen derivatives, the three-component reaction between aldehydes, malononitrile or ethyl cyanoacetate and phenols is particularly popular. Catalysts that have been used for this conversion include a basic ionic liquid [1-(n-butyl)-3-methylimidazolium hydroxide ([bmim]OH)],¹³ p-dimethylaminopyridine (DMAP),¹⁴ Triton B,¹⁵ K₂CO₃,¹⁶ MCM-41-NH₂,¹⁷ cetyltrimethylammonium chloride (CTACl),¹⁸ cetyltrimethylammonium bromide (CTABr),¹⁹ K₃PO₄·3H₂O,²⁰ piperazine,²¹ tetramethylguanidine,²² H₁₄[NaP₅W₃₀O₁₁₀],²³ CuSO₄·5H₂O,²⁴ methanesulfonic acid,²⁵ KF-Al₂O₃,²⁶ potassium phthalimide-N-oxyl,²⁷ the nanostructured diphosphate Na₂CaP₂O₇,²⁸ DBU,²⁹ and Ca(OH)₂.³⁰ However, most of the reported methods suffer from one or more of the following drawbacks: low yields, long reaction times, harsh reaction conditions, tedious work-up leading to the generation of large amounts of toxic waste, and the use of unrecyclable, hazardous, catalysts. Therefore, the use of eco-friendly and non-toxic catalysts with high efficiency in the synthesis of these compounds is highly regarded.

Results and discussion

During the course of our recent studies directed toward the development of practical, safe and environmentally friendly procedures for some important transformations,³¹ we hoped to report a simple and efficient procedure for the synthesis of 2-amino-chromenes using the nano polypropylenimine dendrimer (DAB-PPI-G₁) under solvent-free conditions (Scheme 1).

Scheme 1 One-pot synthesis of 2-amino-2-chromene derivatives.

The core of PPI is a diamine (commonly 1,4-diaminobutane). The reaction began with the addition of four molecules of acrylonitrile to the central 1,4-diaminobutane core, and produced the generation 0.5 polypropylenimine dendrimer with four terminal nitrile groups. In the next step, we reduced the nitrile groups using NaBH₄ in the presence of anhydrous cobalt chloride catalyst and produced the generation 1 nano polypropylenimine dendrimer with four terminal amine groups (Scheme 2).

Scheme 2 Synthesis of the nano polypropylenimine dendrimer (DAB-PPI-G₁).

Scanning electron microscopy images of the nano polypropylenimine dendrimer are shown in Fig. 2A. The morphology indicates agglomeration of the dendrimer particles and a porous structure. The images shown in Fig. 2B have been prepared by an optical microscope with nano focus, they show the approximate size of particles of PPI dendrimer that have been sprayed onto the glass through the use of a color scale (Fig. 2B).

Fig. 2 (A) Scanning electron microscopy images of dendrimer PPI G₁ (i and ii); (B) optical microscopy nano focus images of dendrimer PPI G₁: (i) dendrimer PPI sprayed on glass (a film of PPI G₁) and (ii) an image obtained from the particle sizes within the PPI G₁ film on glass; (C) particle size dispersion of PPI by a dynamic light scattering method (DLS).

The size dispersion of the PPI synthesized by the proposed method was investigated by dynamic light scattering (DLS) and shown in Fig. 2C. As it can be seen, the nanoparticle size is between 13–53 nm and the mean diameter is 24 nm.

Creation of molecular functionality and diversity³² from common starting materials while combining economic³³ and environmental³⁴ aspects constitutes a great challenge in modern organic chemistry.³⁵ In these contexts, multicomponent reactions (MCRs) under solvent-free conditions are valuable procedures in organic synthesis because a multistep reaction may produce considerable amounts of environmentally unfavorable wastes mainly due to a series of complex isolation procedures, which often need expensive, toxic, and hazardous solvents after each step. Malononitrile is one of the most versatile reagents to be used in MCRs because of the high reactivity of both the methylene and the cyano groups.³⁶ From this perspective, we used malononitrile (1.5 mmol) as the active reagent to react with benzaldehyde (1 mmol) and 1-naphthol (1 mmol) to optimize the reaction conditions at first. A summary of the optimization experiments is provided in Table 1.

Table 1 Optimization of reaction conditions

Entry	Catalyst	Conditions	Time (min)	Yield^a (%)
a Isolated yields.b No reaction.
1	PPI (15 mol%)	Solvent-free/110 °C	3	95
2	EDA (5 drops)	Solvent-free/110 °C	3	70
3	DAB (5 drops)	Solvent-free/110 °C	3	45
4	TEA (5 drops)	Solvent-free/110 °C	7	85
5	DEA (5 drops)	Solvent-free/110 °C	5	86
6	PAMAM (5 drops)	Solvent-free/110 °C	3	Trace
7	PPI (15 mol%)	Solvent-free/120 °C	3	90
8	PPI (15 mol%)	Solvent-free/100 °C	3	74
9	PPI (10 mol%)	Solvent-free/110 °C	3	86
10	PPI (20 mol%)	Solvent-free/110 °C	3	89
11	—	Solvent-free/110 °C	20	—^b

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Initially, we performed the three-component reaction using the nano polypropylenimine dendrimer (PPI) as the catalyst under solvent-free conditions at 110 °C, and exhilaratingly obtained the target product in 95% yield after 3 min (Table 1, entry 1). To further optimize the reaction conditions, a series of catalysts such as ethylenediamine (EDA), 1,4-diamino butane (DAB), triethanolamine (TEA), diethanolamine (DEA), and poly(amido amine) (PAMAM) dendrimers were investigated (entries 2–6). Compared with other catalysts, PPI was the best catalyst in terms of the yield and reaction time (entry 1). Next, we screened the proper temperature for the model reaction (entries 7–8). Afterwards, the loading of PPI was selected, and 15 mol% of PPI was established as the optimal amount (entries 9–10). To evaluate the effect of PPI, the reaction was examined in the absence of PPI under solvent-free conditions at 110 °C; no product was formed after 20 min at 110 °C (entry 11).

In recent years, various nano-materials have been used as suitable catalysts for the synthesis of various organic compounds.³⁷ They increase the efficiency of time consuming reactions compared to regular catalysts. One efficient approach to enhance the catalytic activity of catalysts is their nanostructure construction. Favorable properties of nano polypropylenimine dendrimers (PPI) including increased surface area, saving of material, and cost efficiency, make them more efficient catalysts than their counterparts. In our quest to prove the efficiency of nano polypropylenimine dendrimers (PPI), where nanostructures play an important role in the catalytic properties, the synthesis of 6a was investigated using similar non nanostructures (Table 1, entries 2–6). As it can be seen, better efficiency for the nano polypropylenimine dendrimer (PPI) is found in the synthesis of 6a.

Based on the optimized reaction conditions, the reactions of 1 or 2-naphthol or resorcinol, malonitrile, and various aromatic (heterocyclic) aldehydes were investigated. As shown in Table 2, aromatic aldehydes carrying either electron-withdrawing or electron-donating substituents afforded excellent yields of product with high purity at 110 °C under solvent-free conditions.

Table 2 One-pot synthesis of 2-amino-2-chromene derivatives

Entry	R	Phenols	R^1	Products	Time (min)	Yield^a (%) (ref.)
a Isolated yields.
1	C6H5		CN		3	95 (ref. 24)
				6a
2	2-ClC6H5		CN		4	95 (ref. 24)
				6b
3	3-NO2C6H5		CN		12	94 (ref. 24)
				6c
4	4-ClC6H5		CN		4	96 (ref. 24)
				6d
5	3-NO2C6H5		CO2Et		20	88 (ref. 25)
				6e
6	4-FC6H5		CN		15	78 (ref. 26)
				6f
7	4-ClC6H5		CO2Et		30	95 (ref. 26)
				7g
8	4-BrC6H5		CO2Et		30	89 (new)
				7h
9	4-OHC6H5		CO2Et		50	88 (new)
				7i
10	4-ClC6H5		CN		15	92 (ref. 27)
				7j
11	4-NO2C6H5		CO2Et		20	89 (new)
				7k
12	3-NO2C6H5		CO2Et		20	90 (ref. 15)
				7l
13	C6H5		CN		20	82 (ref. 27)
				8m
14	4-BrC6H5		CN		25	85 (ref. 27)
				8n
15	2-Furyl		CN		30	76 (ref. 27)
				8o
16	4-ClC6H5		CN		20	87 (ref. 27)
				8p
17	2-ClC6H5		CN		20	89 (ref. 27)
				8q

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A possible reaction mechanism is proposed in Scheme 3. We presume that the reaction proceeds via initial formation of I through a Knoevenagel condensation of the aryl aldehyde and malononitrile. Then, 1-naphthol loses a hydrogen atom under the action of PPI. Subsequently, Michael addition between II and I produces intermediate III, followed by intramolecular cyclization to form IV. The isomerization of IV gives the final product 6.

Scheme 3 Proposed mechanism.

Table 3 summarizes the results and compares them with results obtained by other groups. Based on this comparison, our method is simpler and more efficient for the synthesis of 2-amino-2-chromene derivatives. These results clearly demonstrate that the polypropylenimine dendrimer catalyst is better and in all cases much more efficient for this reaction than the other catalysts (including acidic and basic catalysts).

Table 3 Comparison of methods for the synthesis of 2-amino-2-chromene including acidic and basic catalysts

Entry	Compounds	Conditions	Time (min)	Yield (%)
1		Solvent-free/110 °C/PPI (present work)	3	95
		MCM-41-NH2/H2O/70 °C (ref. 17)	30	69
		CTABr/H2O/ultrasonic/rt (ref. 19)	150	92
		Piperazine/MWI/solvent-free (ref. 21)	7	88
		CuSO4·5H2O/water/reflux (ref. 24)	60	95
		Methanesulfonic acid/CH3CN/reflux (ref. 25)	180	90
	7a	Na2CaP2O7/water/reflux (ref. 28)	300	81
2		Solvent-free/110 °C/PPI (present work)	4	95
		DMAP/solvent-free/MWI (ref. 14)	60	95
		CTABr/H2O/ultrasonic/rt (ref. 19)	150	80
		CuSO4.5H2O/water/reflux (ref. 24)	30	85
	7b	KF-Al2O3/EtOH/80 °C/reflux (ref. 26)	300	86
3		Solvent-free/110 °C/PPI (present work)	12	94
		CTABr/H2O/ultrasonic/rt (ref. 19)	40	93
		K3PO4·3H2O/100 °C/solvent-free (ref. 20)	60	77
		Methanesulfonic acid/CH3CN/reflux (ref. 25)	240	90
	7c	Na2CaP2O7/water/reflux (ref. 28)	300	84

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Experimental

General

All the chemicals required were purchased from Merck or Aldrich. Melting points were determined using Electrothermal 9100 apparatus. FT-IR spectra were recorded on a Shimadzu FT IR-8400S instrument (with the samples as KBr disks for the range 400–4000 cm⁻¹). ¹H and ¹³C NMR spectra were determined on a Bruker DRX-300 Avance spectrometer in DMSO-d₆ or CDCl₃, and shifts are given in δ downfield from tetramethylsilane (TMS) as an internal standard. Optical microscopy was performed with a nano focus AG lindner strabe 98 D-46149 Oberhausen.

1. Synthesis of nano polypropylenimine dendrimer (PPI).
1.1. Synthesis of polypropylenimine dendrimer generation 0.5 (DAB-PPI-G_0.5). To a mixture of 1,4-diaminobutane (10 mL) was added dropwise acrylonitrile (35 mL) at 0 °C for 30 min. Then, the excess acrylonitrile was removed by rotary vacuum at 0.1 mm and a temperature of 45 °C. The polypropylenimine dendrimer generation 0.5 (DAB-PPI-G_0.5) was obtained as an oily viscous light yellow solution in 95% yield.
1.2. Synthesis of polypropylenimine dendrimer generation 1 (DAB-PPI-G₁). To a mixture of polypropylenimine dendrimer generation 0.5 (10 g) in methanol (35 mL) at 0 °C, was added NaBH₄ (6.23 g) and anhydrous cobalt chloride (0.0002 g). The reaction mixture was stirred for 60 min at this temperature. Then, the mixture was filtered, washed with methanol (30 mL), and then dried at room temperature to give polypropylenimine dendrimer generation 1 (DAB-PPI-G₁) as a white powder (92% yield).

2. General procedure for synthesis of 2-amino-2-chromene. PPI (15 mol%) was added to a mixture of the aromatic aldehyde (1 mmol), active methylene (malononitrile or ethyl cyanoacetate) (1 mmol), and phenol (α or β-naphthol or resorcinol) (1 mmol) in a 10 mL flask at room temperature. The temperature was then raised to 110 °C and maintained for the appropriate time (see Table 2) with stirring. After the completion of the reaction as indicated by TLC (hexane–ethyl acetate, 4 : 1), hot EtOH (96%, 5 mL) was added and the mixture was stirred for 2 min.

Next, the resulting crude product was poured onto a mixture of crushed ice and cold water to get the pure 2-amino-2-chromene derivative as a solid product, which was separated, filtered, and recrystallized from EtOH (3 mL).

3. Spectral data for new compounds.
3.1. Ethyl-3-amino-1-(4-bromo phenyl)-1H-benzo[f]chromene-2-carboxylate (7h). White solid, m.p. 204–206 °C; IR (KBr, cm⁻¹): 3328, 3463, 3082, 2977, 1670, 1635, 1504, 1222, 1072; ¹H NMR (300 MHz, CDCl₃) δ ppm: 1.19 (t, 3H, CH₃), 4.67 (m, 2H, CH₂), 5.35 (s, 1H, CH), 6.13 (br, 2H, NH₂, D₂O exchangeable), 7.19–7.76 (m, 10H); ¹³C NMR (75 MHz, CDCl₃) δ ppm: 14.6, 36.7, 59.7, 79.8, 116.7, 118.5, 119.8, 123.3, 124.9, 127.0, 128.6. 129.0, 130.0, 130.8, 131.2, 131.3, 145.5, 147.0, 159.9, 169.0; anal. calcd for C₂₂H₁₈BrNO₃: C, 62.25; H, 4.27; N, 3.30. Found: C, 62.13; H, 4.35; N, 3.18.
3.2. Ethyl-3-amino-1-(4-hydroxy phenyl)-1H-benzo[f]chromene-2-carboxylate (7i). White solid, m.p. 129–130 °C; IR (KBr, cm⁻¹): 3332, 3463, 3062, 2981, 1662, 1593, 1512, 1222, 1076; ¹H NMR (300 MHz, CDCl₃) δ ppm: 1.39 (t, 3H, CH₃), 4.24 (m, 2H, CH₂), 5.62 (s, 1H, CH), 6.33 (br, 2H, NH₂, D₂O exchangeable), 7.05–7.89 (m, 10H), 10.21 (s, 1H, OH); ¹³C NMR (75 MHz, CDCl₃) δ ppm: 14.6, 36.7, 59.7, 79.9, 116.7, 118.5, 123.3, 124.8, 127.0, 128.2, 128.6, 129.0, 129.5, 130.9, 131.3, 131.7, 145.0, 147.0, 159.9, 169.0; anal. calcd for C₂₂H₁₉NO₄: C, 73.10; H, 5.30; N, 3.87. Found: C, 72.95; H, 5.46; N, 3.79.
3.3. Ethyl-3-amino-1-(4-nitro phenyl)-1H-benzo[f]chromene-2-carboxylate (7k). Yellow solid, m.p. 180–182 °C. IR (KBr, cm⁻¹): 3301, 3448, 3070, 2990, 1678, 1639, 1515, 1390, 1224, 1072; ¹H NMR (300 MHz, CDCl₃) δ ppm: 1.39 (t, 3H, CH₃), 4.25 (m, 2H, CH₂), 5.71 (s, 1H, CH), 6.44 (br, 2H, NH₂, D₂O exchangeable); 7.08–8.06 (m, 10H); ¹³C NMR (75 MHz, CDCl₃) δ ppm: 14.6, 37.2, 59.9, 78.9, 116.7, 117.5, 123.0, 123.6, 125.1, 127.2, 128.8, 129.0, 129.5, 130.7, 131.3, 146.2, 147.1, 153.8, 160.0, 168.7; anal. calcd for C₂₂H₁₈N₂O₅: C, 67.66; H, 4.65; N, 7.18. Found: C, 67.57; H, 4.81; N, 7.02.
3.4. Polypropylenimine dendrimer generation 0.5. Yellow solution. IR (KBr, cm⁻¹): 2250 (C–N), 720 (CH₂–DAB), 1220, 1315 (C–N).
3.5. Polypropylenimine dendrimer generation 1. Yellow solid. IR (KBr, cm⁻¹): 3400 (N–H), 1668 (N–H), 1219, 1315 (C–N), 1130 (C–C).

Conclusion

In conclusion, we have reported a green and eco-friendly synthesis of 2-amino-2-chromene derivatives using a highly efficient polymeric nanomaterial PPI catalyst. PPI is classified as a dendrimer with special properties such as being antimicrobial, antibacterial, non-toxic, and environmentally friendly. The most important advantages of this reaction include a mild, green synthesis; avoidance of the use of toxic organic solvents, excellent yield, short reaction times and a simple work up procedure.

Acknowledgements

We are thankful to the University of Hakim Sabzevari Research Council for the partial support of this research.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra04458h

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