Highly efficient solvent free synthesis of α-aminophosphonates catalyzed by recyclable nano-magnetic sulfated zirconia (Fe₃O₄@ZrO₂/SO₄²⁻)

Abstract

In this project, nano-magnetic sulfated zirconia Fe₃O₄@ZrO₂/SO₄²⁻ was prepared and characterized using various instrumental methods. Sulfated zirconia supported on magnetic nanoparticles can act as a well-organized nanocatalyst and can be easily separated from the reaction mixture using an external magnetic field. Nano-Fe₃O₄@ZrO₂/SO₄²⁻ is a heterogeneous acidic catalyst and has particular advantages such as a facile synthesis procedure, high activity, easy separation and reusability. It was applied as an efficient nanocatalyst in the synthesis of α-aminophosphonate derivatives in the Kabachnik–Fields reaction. This synthetic method has several advantages including high yields, short reaction times, easy workup and environmentally benign reaction conditions.

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Introduction

α-Aminophosphonates are structurally similar to α-amino acids and they are also an important class of compounds in medicinal chemistry, as they are involved in the metabolism of α-amino acids.^1–3 α-Aminophosphonates have some biological applications, for example as anti-cancer drugs, strong antibiotics, an anti-clotting agent and a nervous system activator, and in antibody production.

For the first time, Arbuzov produced alkyl phosphonates from the reaction of trialkyl phosphates with alkyl halides. In 1982 Osipova understood that N-alkoxy-α-aminophosphonates can be produced by nucleophilic addition of trimethyl phosphates to oximes which were converted to α-aminophosphonates after a one step reduction. Some disadvantages, like low yields, could be mentioned.⁴ In 1992, Genet obtained α-aminophosphonates using trialkyl phosphite.⁵ Also, Kabachnik–Fields three component reaction, includes the one pot condensation of amine, aldehyde and di- or tri-alkyl phosphite and Pudovik reaction, the addition of dialkyl phosphites to imine in the presence of a Lewis acid or a base, has been done.^3,6–24

Most catalysts used in the synthesis of α-aminophosphonates have shortcomings such as high cost, the use of stoichiometric amounts of catalyst, sensitivity to moisture, and difficulties in separation and recycling. Using reagents on mineral substrates causes some advantages, such as smooth and easy process reaction conditions, pure products, increased selectivity, reduction of the by-product and waste generated, and the speed of response and recovery capabilities, so they are recommended as catalysts.^3,11,25 Antimony trichloride stabilized on an aluminum oxide bed is one of the examples of the catalysts which have been recently used in the Kabachnik–Fields reaction in acetonitrile and under reflux conditions.²⁶ Using microwave irradiation is the other method for the synthesis of α-aminophosphonate, which is done under totally different conditions.²⁷ Amberlite-IR 120 resin is used as a heterogeneous catalyst in many chemical reactions, including the preparation of α-aminophosphonates under microwave irradiation.²⁸

Using heteropolyacids as catalysts for efficient and green conditions has been reported in many heterogeneous reactions. These classes of compounds are eco-friendly, reusable, non-toxic and easily removed. Producing α-aminophosphonates in the presence of H₃PW₁₂O₄₀ as a catalyst using trimethyl phosphite in CH₂Cl₂ at room temperature has also been reported.^29,30 Tosyl chloride in catalytic amounts has been used in the preparation of α-aminophosphonates in CH₂Cl₂ in the presence of alcohols and at room temperature. One of the weaknesses of this catalyst is non-recyclability.³¹ Hydrous zirconium chloride at 35 °C and under solvent-free conditions or in temperatures above 80 °C and with a shorter duration, can catalyze the α-aminophosphonate synthesis.³²

The other new method that has been developed for the synthesis of these materials uses a catalytic amount of sulfonium bromide. In the presence of this catalyst the reaction is carried out with a short time, low temperature and high efficiency. However it is limited to aliphatic aldehydes.³³ Other work on the synthesis of α-aminophosphonate uses sulfamic acid as a heterogeneous and recyclable catalyst.³⁴ Another method for this synthesis is the usage of magnesium ferrite magnetic nanoparticles as the catalyst. This catalyst is reusable and can be easily separated from the reaction mixture.³⁵ The advantage of the recent approach of using a magnetic nanoparticle catalyst coated with dehydroascorbic acid as a heterogeneous catalyst with magnetic properties is the recyclability in the synthesis of α-aminophosphonates.³⁶ α-Aminophosphonates are also prepared using Mg(ClO₄)₂.³⁷

The problems in recycling Brønsted acids and Lewis acids causes them to be used only one time. The separation of them from the reaction environment is difficult and causes the remaining of impurities in the reaction. In continuation of our previous research on the development of new synthetic methods of nano-magnetic sulfated zirconia as a solid acid catalyst in organic synthesis,^38,39 herein we investigate the catalytic properties of Fe₃O₄@ZrO₂/SO₄²⁻ in the Kabachnik–Fields reaction, which in this case, in addition to the benefits of having a super acid catalyst and heterogeneous catalyst, has an easy recovery and can resolve the separation difficulties using the magnetic property. To assess the efficiency of the synthesized catalyst, it was applied in the Kabachnik–Fields reaction for the synthesis of α-aminophosphonates: the three component Kabachnik–Fields reaction between 2-chlorobenzaldehyde, aniline and dimethyl phosphite as a model reaction in the presence of nano-magnetic sulfated zirconia (Scheme 1).

Scheme 1 Kabachnik–Fields reaction using nano-magnetic sulfated zirconia Fe₃O₄@ZrO₂/SO₄²⁻.

Results and discussion

Catalyst characterization

Six characteristic peaks of Fe₃O₄ (2θ = 30.1, 35.4, 43.0, 53.5, 57.0 and 62.5) and three diffraction peaks of ZrO₂ (30.6, 51.1, and 60.1) can be seen in the XRD patterns in Fig. 1. In the XRD pattern of Fe₃O₄@ZrO₂/SO₄²⁻, the Fe₃O₄ peaks of synthesized Fe₃O₄@ZrO₂/SO₄²⁻ can be observed, which shows that no change in the peak’s position during the production of Fe₃O₄@ZrO₂/SO₄²⁻ has occurred, and this demonstrates the retention of the crystalline structure of the magnetite. Due to the XRD pattern of Fe₃O₄@ZrO₂/SO₄²⁻, it can be shown that the major form of this catalyst is in a tetragonal phase after calcination.

Fig. 1 The X-ray diffraction patterns of Fe₃O₄ and Fe₃O₄@ZrO₂/SO₄²⁻.

From the FTIR spectrum of Fe₃O₄@ZrO₂/SO₄²⁻ in Fig. 2, a broad peak around 3400 cm⁻¹ can be seen that can be attributed to the OH stretching vibration. This peak is related to the absorption of water molecules on the catalyst surface that cause a broadening of the hydrogen bond. A peak around 1630 cm⁻¹ is related to the bending mode of the H₂O bonds.

Fig. 2 FT-IR spectra of the Fe₃O₄, ZrO₂, Fe₃O₄@ZrO₂ and Fe₃O₄@ZrO₂/SO₄²⁻.

A strong peak for the stretching vibration of S=O appeared at 1126 cm⁻¹ and a peak at 1400 cm⁻¹ can be attributed to the asymmetric stretching vibration of the S=O covalent bond. Peaks around 1600 cm⁻¹ appeared for the Zr–O bond and the absorption at 500 cm⁻¹ is related to the Fe–O bond.

A scanning electron microscope (SEM) image can provide information about the size and morphology of the catalyst structure. The SEM in Fig. 3 has shown that the catalyst structure is uniform. Its constituent particles are roughly the same size and the average particle size is about 30 nm. A particle size distribution curve obtained using the scanning electron microscope image shows an average size of 40–30 nm for 200 particles. The graphs were plotted using Excel software (Fig. 4).

Fig. 3 SEM of Fe₃O₄@ZrO₂/SO₄²⁻.

Fig. 4 Particle size distribution curve for Fe₃O₄@ZrO₂/SO₄²⁻ using Excel software.

According to the results of the VSM analysis, the saturation magnetization curve depicted in Fig. 5, there is not any hysteresis in the magnetization sweep. Therefore, the magnetic coercivity of the nanostructure is zero, which suggests that the sample containing Fe₃O₄ is superparamagnetic.

Fig. 5 (A) VSM of Fe₃O₄@ZrO₂/SO₄²⁻, synthesized using 0.3 g Fe₃O₄ and calcinated in an air atmosphere. (B) VSM of Fe₃O₄@ZrO₂/SO₄²⁻, synthesized using 3 g Fe₃O₄ and calcinated in a nitrogen furnace.

The TEM image in Fig. 6 shows quantitative measures of particle and grain size, size distribution, and morphology.

Fig. 6 TEM image of Fe₃O₄@ZrO₂/SO₄²⁻.

Finally, to confirm the structure of Fe₃O₄@ZrO₂/SO₄²⁻, an EDX spectrum was obtained. As shown in Fig. 7, this confirms the existence of iron, oxygen, and sulfur zirconium in the sample of this catalyst.

Fig. 7 Energy dispersive X-ray spectroscopy of Fe₃O₄@ZrO₂/SO₄²⁻.

N₂ adsorption measurements were used to investigate the BET specific surface area of the Fe₃O₄@ZrO₂/SO₄²⁻. As can be seen from Fig. 8, the BET surface area of Fe₃O₄@ZrO₂/SO₄²⁻ is 121 m² g⁻¹. The high specific surface may endow the material with stronger catalytic properties. The corresponding pore-size distribution determined using the BJH method is also shown in Fig. 8 (3.77 nm).

Fig. 8 Nitrogen adsorption–desorption isotherm and pore-size distribution of Fe₃O₄@ZrO₂/SO₄²⁻.

Catalytic properties

The proposed mechanism of the Kabachnik–Fields reaction in the presence of a Fe₃O₄@ZrO₂/SO₄²⁻ catalyst. The proposed mechanism for the synthesis of α-aminophosphonate with the Fe₃O₄@ZrO₂/SO₄²⁻ catalyst is shown in Scheme 2. According to this mechanism, the catalyst can facilitate the formation of imine intermediates by activating the carbonyl group.³⁹

Scheme 2 The proposed mechanism of the Kabachnik–Fields reaction in the presence of a Fe₃O₄@ZrO₂/SO₄²⁻ catalyst.

In the presence of the catalyst, the carbon of imine is attacked by phosphite and the desired product is obtained.

The optimal conditions for the Kabachnik–Fields reaction in the presence of a Fe₃O₄@ZrO₂/SO₄²⁻ catalyst. At first, the reaction was done in ethanol, at 80 °C and in the presence of 80 mg of catalyst, and a yield of 65% was obtained. To improve the efficiency of the reaction, the effect of different solvents such as water, acetonitrile and solvent-free conditions were studied and the best yields were obtained under solvent-free conditions. The effect of temperature on the reaction efficiency was investigated. Therefore, the optimal conditions were obtained: 80 mg of the catalyst, and a temperature of 80 °C under solvent free conditions (Table 1).

Table 1 The various conditions for the Kabachnik–Fields reaction in the presence of a Fe₃O₄@ZrO₂/SO₄²⁻ catalyst^a

Entry	Temperature (°C)	Solvent	Time (min)	Catalyst (mg)	Yield^b (%)
a Reaction conditions: 2-chlorobenzaldehyde (1.0 mmol), aniline (1.0 mmol), dimethyl phosphite (1.2 eq.), catalyst (aldehyde or ketone was added with catalyst), solvent (2.0 mL).b Isolated yields.
1	80	EtOH	20	80	65
2	80	H2O	20	80	Low
3	80	CH3CN	20	80	65
4	80	—	20	80	93
5	RT	—	20	80	10
6	60	—	20	80	65
7	100	—	20	80	93
8	80	—	20	60	75
9	80	—	20	40	73
10	80	—	20	100	95

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After obtaining the optimized conditions, the reaction for the preparation of α-aminophosphonate was carried out under solvent free conditions and at 80 °C. In the first step, the reaction was studied without catalyst and then with the iron oxide magnetic nanoparticles, oxide nanoparticle zirconium and zirconium(IV) chloride as a catalyst of this reaction. The results are summarized in Table 2 (4–7).

Table 2 Various conditions for the preparation of α-aminophosphonate with and without catalysts^a

Entry	Catalyst	Catalyst (mol%)	Time (min)	Yield^b (%)
a Reaction conditions: 2-chlorobenzaldehyde (1.0 mmol), aniline (1.0 mmol), dimethyl phosphite (1.2 eq.), catalyst, solvent-free, 80 °C.b Isolated yields.
1	—	—	24 (h)	30
2	Nano-Fe3O4	10	20	76
3	Nano-ZrO2	10	20	82
4	ZrCl4	10	20	85
5	ZrO2/SO4^2−	80 (mg)	20	45
6	Fe3O4@ZrO2	80 (mg)	20	36
7	Fe3O4@ZrO2/SO4^2−	80 (mg)	20	93

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In the absence of a catalyst, the reaction did not progress. Nano-magnetic sulfated zirconia catalyst (Fe₃O₄@ZrO₂/SO₄²⁻), in comparison with other catalysts, has a higher yield. It was observed that the addition of Fe₃O₄ to sulfated zirconia dramatically affected the catalytic activity in the synthesis of α-aminophosphonates (entries 5 and 7, Table 2), and it was found that iron oxide (Fe₃O₄) modifies the sulfated zirconia surface by generating acidic sites of medium strength. Sufficiently doping the sulfated zirconia with iron oxide can increase the total number of acidic sites with lower strength. These changes, especially the simultaneous presence of two acidic type sites with different strengths, are interesting for catalytic activity. The ionic nature of S=O imparts a Brønsted acid site to the catalyst, which enhances the acidity of the material, thereby reinforcing the catalytic activity (entries 6 and 7, Table 2).

The optimal conditions for the development of other derivatives of α-aminophosphonates. This reaction, performed with simple and sensitive aldehydes such as thiophene carbaldehyde and also several amines, allowed the products to be obtained with good yields. In addition to aldehydes, ketones were also tested. For example, cyclohexanone was reacted with aniline and dimethyl phosphate and the product was obtained in good yield. All the derivatives have been shown in Table 4.

Recyclability of the Fe₃O₄@ZrO₂/SO₄²⁻ catalyst in the Kabachnik–Fields reaction. Reusability of the catalyst for the reaction between 2-chloro benzaldehyde, aniline and dimethyl phosphite was investigated. When the reaction was completed, the catalyst was separated with an external magnetic field and then was washed with ethanol and dried at room temperature, and subsequent reactions without further purification followed. The catalyst was recycled and used 5 times in a row without a significant change in the catalytic capability (Fig. 9).

Fig. 9 Recyclability of the Fe₃O₄@ZrO₂/SO₄²⁻ catalyst in the Kabachnik–Fields reaction.

Comparison of the results obtained in the synthesis of α-aminophosphonate in the presence of Fe₃O₄@ZrO₂/SO₄²⁻ catalyst and other catalysts which have been reported. Finally, a comparative study of the catalytic system with some others recently reported for a one-pot reaction between benzaldehyde, aniline and dimethyl phosphite indicates that the catalytic properties of Fe₃O₄@ZrO₂/SO₄²⁻ are comparable with those of other reported catalytic systems (Table 3).

Table 3 Comparison of the results obtained in the synthesis of α-aminophosphonate in the presence of Fe₃O₄@ZrO₂/SO₄²⁻ catalyst and other catalysts^a

Entry	Catalyst (mol%)	Solvent	Time (h)	Temp (°C)	Yield (%)	Ref.
a Catalysts: (1) – SbCl3/Al2O3, (2) – oxalic acid, (3) – MgFe2O4, (4) – DHAA@Fe3O4, (5) – Fe3O4@ZrO2/SO4^2−.
1	Cat 1 (5)	CH3CN	3	RT	90	26
2	Cat 2 (10)	—	2	50	98	39
3	Cat 3 (10)	—	2	35	94	35
4	Cat 4 (0.09)	—	2	40	93	36
5	Cat 5 (80 mg)	—	30 (min)	80	95	Present work

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Experimental

Chemicals and instruments

All chemicals used, such as salts of iron(III) chloride, iron(II) chloride, zirconium(IV) chloride, ammonium sulfate, aldehydes, ketones, amines, and dimethyl phosphite and solvents, were supplied by Merck and Aldrich chemical companies. FT-IR spectra were obtained over the region 400–4000 cm⁻¹ with a Nicolet IR 100 FT-IR spectrometer with spectroscopic grade KBr. The powder X-ray diffraction pattern was recorded using a X-PERT- PRO diffractometer with Cu Kα (λ = 1.54 Å) irradiation, in the range of 5 to 80 (2θ) with a scan step of 0.026. The morphology of the catalyst was studied with scanning electron microscopy using a SEM (KYKY, EM 3200) on gold coated samples. The magnetic properties of Fe₃O₄@ZrO₂/SO₄²⁻ nanoparticles were measured with a vibrating magnetometer/Alternating Gradient Force Magnetometer (VSM/, MDKFD). The BET surface area and pore volume of Fe₃O₄@ZrO₂/SO₄²⁻ was determined using micromeritics ASAP 2020 apparatus using nitrogen as the analysis gas.

Preparation of the magnetic Fe₃O₄ nanoparticles (MNPs)

100 ml of distilled water was used to dissolve FeCl₃·6H₂O (12.2 g, 0.04 mol) and FeCl₂·4H₂O (4.7 g, 0.02 mol) under vigorous stirring at room temperature. After 10 minutes of stirring, the solution was heated at 50 °C under nitrogen atmosphere. The ammonium hydroxide solution (25%) was added consequently to the above solution until the pH of solution reached 9. After cooling the mixture to room temperature, composed black solution was separated by external magnet. Finally, the obtained Fe₃O₄ nanoparticles were repeatedly washed with deionized water followed by drying at 60 °C in vacuo.

General procedure for the synthesis of Fe₃O₄@ZrO₂/SO₄²⁻

Zirconium chloride (30 g) was dissolved in 1000 ml EtOH : H₂O (1 : 1) to form a clear solution and aqueous ammonia was added dropwise to the solution with vigorous stirring until the pH reached 2. Afterward, Fe₃O₄ nanoparticles (3 g) were dispersed in the prepared solution by ultrasonication for 5 min and to reach pH 9, aqueous ammonia (10%) was added dropwise. The precipitate was obtained and washed with distilled water until the filtrate shows absence of chloride ions (confirmed by AgNO₃ test). The obtained solid was dipped in (NH₄)₂SO₄ (3 mol L⁻¹) aqueous solution at room temperature for 24 h. After filtration and drying in an oven, the solid materials were calcined for 6 h at 600 °C under N₂ atmosphere.

General procedure for the synthesis of α-aminophosphonates

A mixture of aldehydes or ketones (1 mmol), amine (1 mmol) and dimethylphosphite (1.2 mmol) in the presence of Fe₃O₄@ZrO₂/SO₄²⁻ (80.0 mg) was stirred at 80 °C for the appropriate reaction time (for the period of time listed in Table 4). The reaction was monitored using TLC (50 : 50 EtOAC/n-hexane), dichloromethane was added after completion of the reaction, and the catalyst was recovered with an external magnet (or filtration). A saturated aqueous NaHCO₃ solution (20 mL) and brine (20 mL) were added, the mixture was extracted with EtOAc (25 mL), and the organic layer was dried over anhydrous Na₂SO₄. The solvent was evaporated under reduced pressure, and the crude product was purified either by recrystallization or by preparative TLC (silica gel) (eluent: 50 : 50 EtOAC/n-hexane) if necessary. The products thus obtained (in Table 4) were characterized by MP, IR, and NMR spectroscopy; spectral data for selected products are presented in the ESI.†

Table 4 Synthesis derivatives of α-aminophosphonate in the presence of the catalyst Fe₃O₄@ZrO₂/SO₄^2−a

Entry	Time (min)	Carbonyls	Amines	Products	Yield^b (%)	Mp (°C) [Ref.]
a Reaction conditions: aldehyde or ketone (1.0 mmol), amine (1.0 mmol), dimethyl phosphite (1.2 eq.), catalyst (80 mg), under solvent-free conditions, 80 °C.b Isolated yield.c Reaction conditions: aldehyde (1.0 mmol), amine (2.0 mmol), dimethyl phosphite (2.4 eq.), catalyst (100 mg), solvent-free, 80 °C.
1	30	PhCHO	Aniline		95	90–92 [41]
2	20	2-(Cl)C6H4CHO	Aniline		93	128–129 [42]
3	15	4-(Cl)C6H4CHO	Aniline		95	139–140 [43]
4	30	2,4-(Cl)2C6H3CHO	Aniline		89	110–112 [42]
5	70	2,6-(Cl)2C6H3CHO	Aniline		85	98–100 [19]
6	25	4-(Me)C6H4CHO	Aniline		93	125–128 [45]
7	15	4-(OMe)C6H4CHO	Aniline		90	123–124 [41]
8	20	4-(NO2)C6H4CHO	Aniline		96	127–128 [41]
9	40	3-(NO2)C6H4CHO	Aniline		95	122–124 [44]
10	20	3-(OH)C6H4CHO	Aniline		96	130–132, present work
11	80	Thiophen-2-carbaldehyde	Aniline		80	83–86 [19]
12	30	1-Naphthaldehyde	Aniline		90	143–145 [45]
13	20	Benzaldehyde-4-(dimethyl amino)	Aniline		85	144 [13]
14^c	15	Terephthaldehyde	Aniline		90	170–173 [46]
15	10	4-(NO2)C6H4CHO	p-Toluidine		92	209–211 [47 and 52]
16	20	4-(OMe)C6H4CHO	p-Toluidine		88	96–99 [41]
17	15	4-(Cl)C6H4CHO	4-Nitroaniline		76	160–162 [13]
18	25	4-(OMe)C6H4CHO	4-Nitroaniline		82	150–152 [13]
19	15	4-(NO2)C6H4CHO	4-Nitroaniline		80	170–173 [13]
20	50	Cyclohexanone	Aniline		75	99–102 [13]
21	120	Acetophenone	Aniline		70	125–129 [13]

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Back titration in aqueous media of catalyst Fe₃O₄@ZrO₂/SO₄²⁻

The amount of [H⁺] released by the catalyst was determined using back titration. In a container which contained 35 mL of distilled water, 0.5 g of NaCl, 0.5 g catalyst and 10 mL NaOH (0.1 M) were added and stirred for 24 h with a magnetic stirrer until neutralized with [H⁺] which was produced from catalyst hydrolysis. Then three drops of phenolphthalein were added to the container and the solution was titrated with 0.1 M solution of hydrochloric acid. The end point was reached when the colour changed from pink to colourless.

Spectral data. Reported in the ESI.†

Conclusion

There are many routes to synthesized α-aminophosphonate derivatives.^40–53 In this work we introduced sulfated zirconia supported on magnetic nanoparticles as an effective nanocatalyst for the Kabachnik–Fields reaction for the synthesis of α-aminophosphonate derivatives. The best advantages of Fe₃O₄@ZrO₂/SO₄²⁻ as a nanocatalyst are easy separation by the external magnetic field and the recyclability of the catalyst without loss of activity, short reaction time, high yield, and an easy work up process. The thermal durability of Fe₃O₄@ZrO₂/SO₄²⁻ and good resistance of the functional groups in the washing process are two brilliant benefits of this compound as a catalyst in reactions.

Acknowledgements

The authors gratefully acknowledge the partial support from the Research Council of the Iran University of Science and Technology.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra13173a

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