Fe₃O₄@SiO₂/Schiff base/Pd complex as an efficient heterogeneous and recyclable nanocatalyst for chemoselective N-arylation of O-alkyl primary carbamates

Abstract

An efficient, heterogeneous and cost effective method has been developed using an Fe₃O₄@SiO₂/Schiff base/Pd complex as a magnetic and easily recyclable nanocatalyst for rapid and effective N-arylation of carbamates in good to excellent yield. The catalyst can be easily recovered and reused over six runs without significant decrease in the activity. Further highlights of this protocol are operational simplicity, versatility and relatively short reaction times.

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Introduction

Due to the biological activity of compounds which have a nitrogen atom in their structures and the importance of finding new ways to effectively synthesise such molecules, C–N coupling reactions have been a topic of interest in the last two decades.^1–5 Although in most of these methods almost all nitrogen sources such as amines,⁶ amides,⁷ indoles,⁸ azoles⁹ and amino acids¹⁰ have been used for the creation of C–N bonds, only a few examples of the application of carbamates^11–13 as another nitrogen source have been reported in the literature.

Carbamates are an important class of compounds in organic chemistry because of their considerable biological activities,^14–16 their use in the synthesis of polymers¹⁷ and as protecting groups in the synthesis of organic compounds.¹⁸

Carbamate skeletons are frequently found in a variety of biologically active molecules, especially in medicinal chemistry and pharmaceuticals. The most important group of carbamate derivatives are unsymmetrical compounds (Fig. 1), known to be widely used in modern drug design.

Fig. 1 The chemical structure of some important and biological active carbamates.

As the result, there are several reports for synthesis of secondary and tertiary carbamates including the commercial procedure which uses the phosgenation of amines,¹⁹ the reaction of dimethylcarbonates with amines,²⁰ the reductive carbonylation of nitro compounds^21,22 and by the reaction of CO₂ with amines and alcohols.^23,24

Aryl carbamates can be produced by the reductive carbonylation of nitroarenes.²⁵ They are also prepared by the reaction of isocyanate with aryl halides^26,27 and aryl boronic acids²⁸ in the presence of transition metal complexes of Pd and Cu. In one report they have been produced by using C–H functionalization method.²⁹

Magnetic nanocatalysts have gained great attention in the recent years. They can be easily removed from the reaction mixture by an external magnetic field and reused several times. This operational simplicity made them very useful and efficient catalysts in a wide range of organic reactions.^30–33

To the best of our knowledge there are only few reports which carry out the C–N coupling reaction of carbamates directly to afford aryl carbamates.^12,13,34 These protocols suffer from some major drawbacks, such as limitation in the scope of reaction,^13,28 using nonrecyclable homogeneous catalyst,^13,28 long reaction times and using expensive ligands for making the catalyst.^13,27

To overcome these shortcomings, herein we introduce a magnetic Pd nanocatalyst which is able to catalyze efficiently the C–N coupling reaction among primary O-alkyl carbamates and aryl halides and can be easily removed from the reaction by an external magnetic field and reused.

Results and discussion

The magnetic nanocatalyst was synthesized through the protocol which was reported in 2014 by Esmaeilpour et al.³⁵ Scheme 1 concisely describes the synthetic procedure of the catalyst.

Scheme 1 Synthetic procedure of Fe₃O₄@SiO₂/Pd(II) complex.

IR spectrum, XRD pattern and DLS image of the catalyst are shown in Fig. 3. According to IR spectrum, peaks at 1100, 1618, 2973 and 3414 are related to Si–O–Si, C=N, C–H and O–H groups respectively. In addition, the characteristic peaks of Fe₃O₄ are present in the XRD pattern of catalyst at 2θ = 30.1°, 35.4°, 43.1°, 53.4°, 57° and 62.6°, which were marked respectively by their indices (220), (311), (400), (422), (511) and (440). Moreover, characteristic peaks of Pd(0) cannot be observed in XRD pattern, which indicates that the Pd particles on the core–shell catalyst are Pd(II). DLS image of the catalyst shows the distribution of particles regarding size in which the most frequent size of particles was 32 nm. Also, the ICP analysis demonstrated that the Pd loading on the catalyst was 0.26 mmol g⁻¹. These results are consistent with the reported article and confirm the structure of the catalyst.

The magnetic property of the catalyst is demonstrated in Fig. 2. According to this figure, the catalyst has good dispersity in the reaction media and it can be easily separated from the mixture by an external magnetic field.

Fig. 2 (a) Good dispersity and (b) easy separation of the catalyst by an external magnetic field.

Fig. 3 (a) FT-IR spectrum, (b) XRD pattern and (c) DLS image of the catalyst.

Fig. 4 illustrates the FE-SEM image of the Pd nanocatalyst. The spherical morphology of the catalyst is clearly shown in this figure.

Fig. 4 FE-SEM image of the fresh catalyst.

Among the last reports for arylation of carbamates, the use of homogenous catalysts seems to be the only practical method, which suffers from the above mentioned drawbacks. In this work, for the first time, we report an efficient magnetic heterogeneous Pd nanocatalyst for N-arylation of primary O-alkyl carbamates, which can be easily recovered by an external magnetic field and reused, with aryl halides.

Initially, propyl carbamate and iodobenzene were selected as the model substrates and then their reaction in the presence of Fe₃O₄@SiO₂/Schiff base/Pd complex (0.14 wt%), as a magnetic nanocatalyst in the common solvents, such as DMF, DMSO, dioxane, toluene, dichloromethane, acetonitrile and ethanol, and bases, such as CsCO₃, K₂CO₃, NaOBu^t and triethylamine at different temperatures were explored. The corresponding results were summarized in Table 1. According to these data, the reaction showed the best efficiency when the model reaction was run in toluene and in the presence of 1.5 mmol NaOBu^t and 0.055 wt% (0.02 g) the Pd nanocatalyst at 100 °C for 2.5 hours (Table 1, entry 14).

Table 1 Optimization of the reaction parameters for the preparation of propyl phenyl carbamates^a

Entry	Solvent	Base	Catalyst (wt%)	Temp. (°C)	Yield (%)
a Reaction conditions: propyl carbamate (1 mmol), iodobenzene (1 mmol), base (1.5 mmol) and catalyst in 2 mL solvent for 12 h.
1	Toluene	NaOBu^t	0.14	100	92
2	DMF	NaOBu^t	0.14	100	Trace
3	CH3CN	NaOBu^t	0.14	Reflux	0
4	CH2Cl2	NaOBu^t	0.14	Reflux	0
5	Ethanol	NaOBu^t	0.14	Reflux	0
6	Dioxane	NaOBu^t	0.14	100	78
7	DMSO	NaOBu^t	0.14	100	20
8	Toluene	K2CO3	0.14	100	10
9	Toluene	CsCO3	0.14	100	53
10	Toluene	Et3N	0.14	100	0
11	Toluene	NaOBu^t	0.14	25	Trace
12	Toluene	NaOBu^t	0.14	50	38
13	Toluene	NaOBu^t	0.14	120	88
14	Toluene	NaOBu^t	0.055	100	94
15	Toluene	NaOBu^t	0.19	100	87

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With the optimized reaction parameters in hand, initially a variety of O-alkyl primary carbamates were treated with aryl iodide in toluene and in the presence of NaOBu^t and the catalyst at 100 °C to afford the corresponding N-aryl carbamates in good to excellent yields. The results were summarized in Table 2. A variety of carbamates were coupled with iodobenzene to afford the corresponding N-aryl carbamates in good to excellent yields. As it is illustrated in Table 2, in addition of the simple O-alkyl primary carbamates, this procedure was effective also for tert-butyl carbamate (Table 2, entry 2g) and benzyl carbamate (Table 2, entry 2i) which are widely used as the protecting groups in organic synthesis. Also, sterically hindered carbamates carried out the reaction very well (Table 2, entry 2j). In the case of O-aryl carbamates, although they underwent the reaction, as it is reported by Buchwald et al. they are unstable and cannot isolated and purified by chromatographic procedures.²⁷

Table 2 Coupling of O-alkyl primary carbamates with phenyl iodide^a

a Reaction conditions: carbamate (1 mmol), iodobenzene (1 mmol), sodium tert-butoxide (1.5 mmol), catalyst (0.055 wt%) in 2 mL toluene at 100 °C for 2.5 h.

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Also the coupling reaction of O-propyl carbamate with the other aryl iodides was explored and the results were represented in Table 3. According to these data, aryl iodides with electron donating and electron withdrawing substituents carried out the C–N coupling reaction efficiently and gave the corresponding secondary carbamates in excellent yields, however, aryl halides bearing electron withdrawing groups gave slightly higher yields (Table 3, entries 3c, 3d and 3e). Moreover, 3-iodothiophen, as a heterocyclic aromatic iodide, reacted with O-propyl carbamate in the presence of catalyst under optimized condition and produced the desired product in high yield (Table 3, entry 3f).

Table 3 Pd catalyzed C–N cross coupling of O-propyl carbamate with aryl halides^a

a Reaction conditions: O-Propyl carbamate (1 mmol), aryl halide (1 mmol), sodium tert-butoxide (1.5 mmol), catalyst (0.055 wt%) in 2 mL toluene at 100 °C.

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These interesting results encouraged us to study the activity of aryl bromides and aryl chlorides under the optimized condition of C–N coupling (toluene as solvent NaOBu^t as base in 100 °C) in the presence of Fe₃O₄@SiO₂/Schiff base/Pd complex. The data obtained from this study were also exhibited in Table 3. These results showed that aryl bromides, as the same as aryl iodides, carried out the C–N coupling reaction with O-propyl carbamate efficiently however with slightly lower yields than aryl iodides. The reaction between O-propyl carbamate and inactive aryl chlorides were occurred in longer reaction times and lower yields compared to aryl iodides and aryl bromides and the starting carbamate mainly remained (Table 3, entries 3a and 3g). While the yields are low for aryl chlorides in this protocol, these substrates are completely inactive in the majority of the methods reported in the literature.

To check chemoselectivity of the protocol, the reaction between 1-bromo-4-iodobenzene with O-propyl carbamate was investigated under optimized condition. The result of this study indicated that the C–N coupling reaction has been taken place only on the carbon atom bearing iodine and only propyl 4-bromophenylcarbamate was prepared (Table 4, entry 1). Then the same reaction was performed between 1-chloro-4-iodobenzene and O-propyl carbamate. In this case only propyl 4-chlorophenylcarbamate was obtained (Table 4, entry 2). Finally, the reaction of O-propyl carbamate with 1-bromo-4-chlorobenzene was examined and as it is mentioned in Table 4 only the propyl 4-chlorophenylcarbamate was observed in 88% yield (Table 4, entry 3). These results indicate excellent chemoselectivity and the C–N coupling reaction is occurred on the carbon atom bears less electronegative halogen atom.

Table 4 Chemoselectivity of the C–N coupling reaction of aryl halides

Entry	Substrate	Product 1 (yield%)	Product 2 (yield%)	Product 3 (yield%)
1		91	—	0
2		—	93	0
3		0	88	—

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In order to compare the reactivity among carbamates and amides, the C–N coupling reaction between O-propyl primary carbamate and propanamide with iodobenzene was examined. For this purpose, 1 mmol of iodobenzene was reacted with 1 mmol of O-propyl carbamate and 1 mmol of propanamide in the presence of catalyst under the optimized conditions. As it is depicted in Scheme 2, analysis of the reaction mixture appeared that propanamide remained unchanged and only O-propyl carbamate underwent the C–N coupling selectively. This could be due to the stronger nucleophilicity of nitrogen atom in carbamates in comparison to amides, which is important in Pd catalyzed C–N coupling reactions.³⁶

Scheme 2 Comparing the reactivity between O-propyl carbamate and propanamide toward C–N coupling reaction.

The recyclability of the catalyst was explored on the reaction between iodobenzene and O-propyl carbamates. After completion of the reaction, the catalyst was removed from the mixture by an external magnetic field and reused in the next same reaction. The data presented in Fig. 5 show that the Pd nanocatalyst is reusable after six runs without significant losing in activity.

Fig. 5 Recyclability of the catalyst.

Fig. 6 represents the XRD pattern and FE-SEM image of the recycled catalyst after sixth run. This figure shows that characteristic peaks of Fe₃O₄ are present well and also there is no observation of Pd(0) species in the XRD pattern of the catalyst after recycling. Also the FE-SEM image demonstrates that Pd nanocatalyst keeps its spherical shape but with little agglomeration, which is believed to be the main cause of yield decreasing.

Fig. 6 (a) XRD pattern and (b) FE-SEM image of the catalyst after sixth run.

To find responsible Pd species in the C–N coupling reaction, hot filtration test was performed on the reaction of iodobenzene and O-propyl carbamates after six runs. When the reaction was half done the Pd nanocatalyst were removed from the reaction mixture by an external magnetic field. Then the reaction mixture was allowed to proceed at the reaction temperature. Only a very small conversion occurred after the removal of catalyst from the reaction moiety (about 2%). These results showed that only a few species of Pd exist in the solution phase and the main responsibility of the reaction progress is due to the heterogeneous catalyst. Inductively coupled plasma (ICP) analysis was also performed for the fresh catalyst and the catalyst after one and after six runs to determine the amount of palladium leaching. According to this test the amount of palladium on the fresh catalyst was obtained to be 0.26 mmol g⁻¹. ICP analysis after one run showed only 0.3% for palladium leaching and after six runs it was determined to be 3.6%. These data along with those for hot filtration test, confirm the high heterogeneity and stability of the catalyst.

Experimental

General

All chemicals were purchased from the Merck, Flucka and Aldrich Chemical Companies in high purity. The products were characterized by comparison of their spectral and physical data such as NMR, FT-IR, MS, CHNS and melting point with available literature data. ¹H and ¹³C-NMR spectra were recorded with Bruker Avance DPX 250 MHz instruments with Me₄Si or solvent resonance as the internal standard. ¹H NMR spectroscopic data are reported as follows: chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, quint = quintet, sext = sextet, sept = septet, br = broad, m = multiplet), coupling constants (Hz), and integration. Fourier transform infrared (FTIR) spectra were obtained using a Shimadzu FT-IR 8300 spectrophotometer. The hydrodynamic size of the particles was measured by dynamic light scattering (DLS) techniques, using a HORIBA-LB550 particle size analyzer. Pd loading and leaching test was carried out with an inductively coupled plasma (ICP) analyzer (Varian, vista-pro). Determination of the purity of the substrate and monitoring of the reaction were accomplished by thin-layer chromatography (TLC) on a silica-gel polygram SILG/UV 254 plates.

General procedure for the preparation of Fe₃O₄ nanoparticles. To the solution of FeCl₃·6H₂O (4.8 mmol) in water (15 mL), added a mixture of polyvinyl alcohol (PVA 15000) (1.0 g), as surfactant, and FeCl₂·4H₂O (4.5 mmol) in water (15 mL). The resultant solution was stirred for 30 min at 80 °C. In the next step, hexamethylenetetramine (HMTA) (1.0 mol L⁻¹) was added dropwise with vigorous stirring to produce a black solid product and also the reaction media reaches pH 10. Next the mixture was heated for 2 hours at 60 °C. Finally, the black magnetite solid product was filtered and washed with ethanol three times and dried at 80 °C for 10 hours.

General procedure for the preparation of Fe₃O₄@SiO₂ core–shell. First, Fe₃O₄ (0.5 g, 2.1 mmol) was dispersed in a solution of ethanol (50 mL), deionized water (5.0 mL) and tetraethoxysilane (TEOS) (0.2 mL), followed by the addition of 5.0 mL of NaOH (10 wt%). This mixture was left to be stirred for 30 min at room temperature. Then the product, Fe₃O₄@SiO₂, was separated by an external magnet, washed with a solution of deionized water and ethanol three times and dried at 80 °C for 10 hours.

General procedure for preparation of the ligand. A solution of the stoichiometric amount of salicylaldehyde (1.0 mmol, 0.122 g) in ethanol (25 mL) was added dropwise to the 3-aminopropyl (triethoxy) silane (1.0 mmol, 0.176 g) in 25 mL ethanol then the mixture was stirred at room temperature for 6 hours. The resulting salen ligand, as the bright yellow precipitate, was separated by filtration and washed with ethanol (5.0 mL) and then dried in vacuum. The crude product was recrystallized from ethanol to obtain the pure product in 98% yield (0.271 g).

General procedure for preparation of the Pd(II) complex. Pd(OAc)₂ (0.224 g, 1.0 mmol) was added to the solution of the synthesized ligand (0.651 g, 2.0 mmol) in ethanol (25 mL) then the mixture was allowed to proceed in reflux conditions to complete the reaction. After the completion of complex formation, the resulting product was filtered and washed with ethanol. Then the Pd(II) complex was purified by recrystallization from ethanol.

General procedure for preparation of the Pd(II) complex supported on superparamagnetic Fe₃O₄@SiO₂ nanoparticles. Fe₃O₄@SiO₂ (2.0 g) was added to the solution of Pd(II) complex (1.0 mmol) in ethanol (10 mL) and the resultant mixture was refluxed for 12 hours. Next, hot ethanol and water was added to the mixture and then nanocatalyst, Fe₃O₄@SiO₂/Pd(II) complex, was separated by an external magnet and dried at 80 °C for 6 hours.

General procedure for synthesis of starting carbamates. Carbamates were prepared by the procedure reported in 2015 by Sardarian et al.³⁷

To a mixture of alcohol (1.0 mmol) and potassium cyanate (1.5 mmol) in a mortar, DBSA (1.5 mmol) was added and the mixture was pulverized until a uniform mixture was formed. Then the mixture was kept in an oven at 60 °C for 1 hour. After completion of the reaction, CH₂Cl₂ (15 mL) and saturated aqueous solution of NaHCO₃ (15 mL) were added to the resulting powder and organic layer was extracted, washed with water (3 × 15 mL), dried over anhydrous Na₂SO₄ and concentrated to afford the final product. Finally, by recrystallization from diethyl ether pure product was obtained.

General procedure for the arylation reaction of carbamates. To a mixture of carbamate (1.0 mmol), aryl halide (1.0 mmol) and sodium tert-butoxide (1.5 mmol) in toluene catalyst (0.055 wt%) was added. The mixture was heated to 100 °C and stirred for appropriate time. The progress of the reaction was monitored by TLC. After completion of the reaction and separation of catalyst by a magnetic field, 20 mL H₂O was added and the mixture was extracted with CHCl₃. The organic phase was washed with water (2 × 10 mL) and dried over anhydrous Na₂SO₄. Then the solvent was removed under reduced pressure. The resulting crude product was purified by flash chromatography to give the desired cross-coupling products in good to excellent isolated yields.

Ethyl phenylcarbamate. Reaction afforded white crystals (95% yield), mp = 49–50 °C (lit.³⁸ 50–51 °C). ¹H-NMR (250 MHz, 298 K, CDCl₃),δ ppm; 1.23 (t, J = 7.0 Hz, 3H), 4.15 (q, J = 7.0 Hz, 2H), 6.64 (s,br, 1H, NH), 6.97 (m, 1H), 7.25 (m, 4H). ¹³C-NMR (63 MHz, 298 K, CDCl₃), δ ppm; 14.55, 61.19, 118.66, 123.32, 129.01, 137.98, 158.40.

Propyl phenylcarbamate. Reaction afforded white crystals (94% yield), mp = 50–51 °C (lit.³⁹ 52–54 °C). ¹H-NMR (250 MHz, 298 K, CDCl₃) δ ppm; 0.90 (t, J = 7.5 Hz, 3H), 1.62 (sext, J = 7.5 Hz, 2H), 4.05 (t, J = 7.5 Hz, 2H), 6.64 (s, br, 1H, NH), 6.97 (m, 1H), 7.26 (m, 4H). ¹³C-NMR (63 MHz, 298 K, CDCl₃), δ ppm; 10.34, 22.26, 66.83, 118.63, 123.30, 129.01, 137.98, 153.74.

2-Propyl phenylcarbamate. Reaction afforded white crystals (94% yield), mp = 83–85 °C (lit.³⁸ 85–86 °C). ¹H-NMR (250 MHz, 298 K, CDCl₃) δ ppm; 1.22 (d, J = 7.5 Hz, 6H), 4.96 (q, J = 7.5 Hz, 1H), 6.55 (s, br, 1H, NH), 6.97 (m, 1H), 7.20 (m, 4H). ¹³C-NMR (63 MHz, 298 K, CDCl₃), δ ppm; 22.01, 68.70, 118.55, 123.20, 129.00, 138.09, 153.19.

1-Butyl phenylcarbamate. Reaction afforded white crystals (92% yield), mp = 60–62 °C (lit.³⁸ 61–63 °C). ¹H-NMR (250 MHz, 298 K, CDCl₃), δ ppm; 0.88 (t, J = 7.5 Hz, 3H), 1.30 (m, 2H), 1.59 (m, 2H), 4.01 (t, J = 7.5 Hz, 2H), 6.58 (s, br, 1H, NH), 6.98 (m, 1H), 7.25 (m, 4H). ¹³C-NMR (63 MHz, 298 K, CDCl₃), δ ppm; 13.72, 19.07, 30.96, 65.10, 118.66, 123.28, 129.00, 138.03, 153.82.

2-Butyl phenylcarbamate. Reaction afforded white crystals (90% yield), mp = 64–66 °C (lit.³⁹ 65 °C). ¹H-NMR (250 MHz, 298 K, CDCl₃), δ ppm; 1.32 (t, J = 7.5 Hz, 3H), 1.67 (d, J = 7.5 Hz, 3H), 1.84 (m, 2H), 4.66 (sext, J = 3.75 Hz, 1H), 6.62 (s, br, 1H, NH), 6.95 (m, 1H), 7.23 (m, 4H). ¹³C-NMR (63 MHz, 298 K, CDCl₃), δ ppm; 23.78, 25.38, 31.92, 73.60, 118.57, 123.16, 129.00, 138.17, 153.25.

Iso-butyl phenylcarbamate. Reaction afforded white crystals (93% yield), mp = 84–86 °C (lit.⁴⁰ 85–87 °C). ¹H-NMR (250 MHz, 298 K, CDCl₃), δ ppm; 0.88 (d, J = 7.5 Hz, 6H), 1.89 (m, 1H), 3.87 (d, J = 7.5 Hz, 2H), 6.68 (s, br, 1H, NH), 6.98 (m, 1H), 7.29 (m, 4H). ¹³C-NMR (63 MHz, 298 K, CDCl₃), δ ppm; 19.06, 27.96, 71.35, 118.69, 123.30, 129.00, 138.02, 153.83.

Tert-butyl phenylcarbamate. Reaction afforded white crystals (89% yield), mp = 134–135 °C (lit.⁴¹ 133–137 °C). ¹H-NMR (250 MHz, 298 K, CDCl₃), δ ppm; 1.45 (s, 9H), 6.42 (s, br, 1H, NH), 6.95 (m, 1H), 7.26 (m, 4H). ¹³C-NMR (63 MHz, 298 K, CDCl₃), δ ppm; 28.33, 68.98, 118.50, 123.00, 128.96, 134.38, 156.00.

Cyclohexyl phenylcarbamate. Reaction afforded white crystals (90% yield), mp = 79–80 °C (lit.³⁹ 80–81 °C). ¹H-NMR (250 MHz, 298 K, CDCl₃), δ ppm; 0.86 (m, 4H), 1.19 (m, 4H), 1.41–1.66 (m, 2H), 4.77 (quint, J = 5.5 Hz, 1H), 6.62 (s, br, 1H, NH), 6.96 (m, 1H), 7.24 (m, 4H). ¹³C-NMR (63 MHz, 298 K, CDCl₃), δ ppm; 9.70, 19.71, 29.01, 73.25, 118.57, 123.17, 129.00, 138.16, 153.49.

(–)Menthyl phenylcarbamate. Reaction afforded white crystals (84% yield), mp = 105–107 °C [α]¹⁷_D −120 °C (0.05, CDCl₃). ¹H-NMR (250 MHz, 298 K, CDCl₃) δ ppm; 0.72–1.00 (m, 12H), 1.33 (m, 2H), 1.62 (m, 2H), 1.99 (m, 2H), 4.59 (m, 1H), 6.55 (s, br, 1H, NH), 6.96 (m, 1H), 7.28 (m, 4H). ¹³C-NMR (63 MHz, 298 K, CDCl₃), δ ppm; 16.42, 20.81, 22.04, 23.46, 26.24, 31.39, 34.24, 41.34, 47.32, 75.08, 118.43, 123.14, 129.01, 138.16, 153.13.

Benzyl phenylcarbamate. The reaction afforded white crystals (90% yield), mp = 70–72 °C (lit.⁴² 71–73 °C). ¹H-NMR (250 MHz, 298 K, CDCl₃), δ ppm; 5.16 (s, 2H), 6.84 (s, br, 1H, NH), 7.02 (m, 1H), 7.09–7.38 (m, 9H). ¹³C-NMR (63 MHz, 298 K, CDCl₃), δ ppm; 66.91, 118.77, 123.43, 126.94, 128.24, 128.55, 128.97, 136.05, 137.82, 153.43.

Propyl(4-methylphenyl)carbamate. Reaction afforded white crystals (91% yield), mp = 52–54 °C (lit.⁴³ 53–54 °C). ¹H-NMR (250 MHz, 298 K, CDCl₃) δ ppm; 0.88 (t, J = 7.5 Hz, 3H), 1.61 (sext, J = 7.5 Hz, 2H), 2.21 (s, 3H), 4.03 (t, J = 7.5 Hz, 2H), 6.62 (s,br, 1H, NH), 7.02 (d, J = 7.5 Hz, 2H), 7.18 (d, J = 7.5 Hz, 2H). ¹³C-NMR (63 MHz, 298 K, CDCl₃), δ ppm; 10.36, 20.73, 22.30, 66.74, 118.83, 126.69, 129.49, 135.44, 153.94.

Propyl(4-cyanophenyl)carbamate. Reaction afforded white crystals (95% yield), mp = 88–90 °C. ¹H-NMR (250 MHz, 298 K, CDCl₃) δ ppm; 0.98 (t, J = 7.5 Hz, 3H), 1.71 (sext, J = 7.5 Hz, 2H), 4.15 (t, J = 7.5 Hz, 2H), 6.83 (s, br, 1H, NH), 7.50 (d, J = 10 Hz, 2H), 7.59 (d, J = 10 Hz, 2H). ¹³C-NMR (63 MHz, 298 K, CDCl₃), δ ppm; 10.28, 22.13, 67.44, 106.17, 115.52, 118.18, 133.36, 146.33, 158.66.

Propyl(4-nitrophenyl)carbamate. Reaction afforded white crystals (96% yield), mp = 114–115 °C (lit.⁴⁴ 115 °C). ¹H-NMR (250 MHz, 298 K, CDCl₃) δ ppm; 0.90 (t, J = 7.5 Hz, 3H), 1.64 (sext, J = 7.5 Hz, 2H), 4.09 (t, J = 7.5 Hz, 2H), 7.07 (s, br, 1H, NH), 7.50 (d, J = 7.5 Hz, 2H), 8.12 (d, J = 7.5 Hz, 2H). ¹³C-NMR (63 MHz, 298 K, CDCl₃), δ ppm; 10.26, 22.12, 67.55, 117.66, 125.19, 130.94, 144.13, 153.02.

Propyl(4-acetylphenyl)carbamate. Reaction afforded white crystals (90% yield), mp = 112–114 °C.

¹H-NMR (250 MHz, 298 K, CDCl₃) δ ppm; 0.91 (t, J = 7.5 Hz, 3H), 1.63 (sext, J = 7.5 Hz, 2H), 2.50 (s, 3H), 4.08 (t, J = 7.5 Hz, 2H), 6.95 (s,br, 1H, NH), 7.43 (d, J = 7.5 Hz, 2H), 7.85 (d, J = 7.5 Hz, 2H). ¹³C-NMR (63 MHz, 298 K, CDCl₃), δ ppm; 10.31, 22.18, 26.38, 67.22, 117.54, 129.87, 130.88, 142.51, 153.21, 200.49.

Propyl(naphthalen-1-yl)carbamate. Reaction afforded white crystals (90% yield), mp = 73–75 °C. ¹H-NMR (250 MHz, 298 K, CDCl₃) δ ppm; 0.90 (t, J = 7.5 Hz, 3H), 1.63 (sext, J = 7.5 Hz, 2H), 4.09 (t, J = 7.5 Hz, 2H), 6.90 (s, br, 1H, NH), 7.34–7.81 (m, 7H). ¹³C-NMR (63 MHz, 298 K, CDCl₃), δ ppm; 10.37, 22.31, 67.11, 120.50, 124.92, 125.80, 125.95, 126.16, 128.70, 128.79, 130.88, 132.60, 134.06, 154.64.

Propyl (benzo[d][1,3]dioxol-5-yl)carbamate. Reaction afforded white crystals (88% yield), mp = 78–80 °C. ¹H-NMR (250 MHz, 298 K, CDCl₃) δ ppm; 0.96 (t, J = 7.5 Hz, 3H), 1.67 (sext, J = 7.5 Hz, 2H), 4.10 (t, J = 7.5 Hz, 2H), 5.92 (s, 2H), 6.70 (m, 3H), 7.08 (s, br, 1H, NH). ¹³C-NMR (63 MHz, 298 K, CDCl₃), δ ppm; 10.34, 22.29, 66.83, 101.18, 108.05, 111.98, 114.89, 128.81, 143.67, 147.89, 154.11.

Propyl(thiophen-3-yl)carbamate. Reaction afforded white crystals (85% yield), mp = 70–72 °C. ¹H-NMR (250 MHz, 298 K, CDCl₃) δ ppm; 0.89 (t, J = 7.5 Hz, 3H), 1.62 (sext, J = 7.5 Hz, 2H), 4.06 (t, J = 7.5 Hz, 2H), 6.53 (s, br, 1H, NH), 6.87 (d, J = 5.5 Hz, 2H), 7.14 (d, J = 5.5 Hz, 1H). ¹³C-NMR (63 MHz, 298 K, CDCl₃), δ ppm; 10.31, 22.25, 66.98, 107.78, 120.70, 124.73, 153.26.

Propyl(4-boromophenyl)carbamate. Reaction afforded white crystals (91% yield), mp = 76–78 °C (lit.⁴⁵ 77–78 °C). ¹H-NMR (250 MHz, 298 K, CDCl₃) δ ppm; 0.96 (t, J = 7.5 Hz, 3H), 1.69 (sext, J = 7.5 Hz, 2H), 4.12 (t, J = 7.5 Hz, 2H), 6.76 (s, br, 1H, NH), 7.28 (d, J = 7.5 Hz, 2H), 7.39 (d, J = 7.5 Hz, 2H). ¹³C-NMR (63 MHz, 298 K, CDCl₃), δ ppm; 10.34, 22.23, 67.03, 120.19, 128.41, 131.92, 132.10, 153.59.

Propyl(4-chlorophenyl)carbamate. Reaction afforded white crystals (93% yield), mp = 82–84 °C. ¹H-NMR (250 MHz, 298 K, CDCl₃) δ ppm; 0.90 (t, J = 7.5 Hz, 3H), 1.62 (sext, J = 7.5 Hz, 2H), 4.05 (t, J = 7.5 Hz, 2H), 6.63 (s, br, 1H, NH), 7.18 (d, J = 9 Hz, 2H), 7.26 (d, J = 9 Hz, 2H). ¹³C-NMR (63 MHz, 298 K, CDCl₃), δ ppm; 10.32, 22.22, 67.03, 119.82, 128.28, 129.00, 136.58, 153.60.

Conclusion

In conclusion, we report here a new heterogeneous method for the arylation of various carbamates with aryl halides. The procedure is interesting due to some advantageous such as high yields, using a heterogeneous catalyst, easy separation and recyclability of catalyst and relatively short reaction times. The catalyst can be easily removed from the reaction mixture by an external magnetic field and reused for at least 6 runs.

Acknowledgements

Authors gratefully acknowledge the financial support of this work by the research council of Shiraz University.

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Footnote

† Electronic supplementary information (ESI) available: Copies of FT-IR, ¹H and ¹³C-NMR and MS spectra for all compounds. See DOI: 10.1039/c6ra17268g

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