Introduction

Amines are widely used as antioxidants and intermediates in the production of many pharmaceuticals, polymers, dyestuffs, agricultural chemicals, photographic and rubber materials, chelating agents and other industrially important products [1–5]. So, the particular immense interest has been devoted to the preparation of amines by many ways [6–12]. Amines can be prepared by reductive amination of carbonyl compounds, alkylation of ammonia and reduction of azides, amides, nitriles, oximes as well as nitro compounds. Among these, reduction of nitro compounds is one of the most important and straightforward methods for the preparation of amines.

During the past decades, NaBH4 as a mild reducing agent has brought about revolutionary changes in the reduction of organofunctional groups in modern organic synthesis [9–12]. It is well known that the NaBH4 alone does not reduce nitro compounds under ordinary conditions. However, the reducing power of this reagent or its polymeric analog (BER: borohydride exchange resin) undergoes a drastic change towards the reduction of nitro groups by the combination with metal, metal halides or other promoters. NaBH4 in the presence of Pd/C [13], NiCl2 [14, 15], FeCl2 [16], CoCl2 [17, 18], TiCl4 [19], CuCl2 [20, 21], CuBr·Me2S [22], SbF3 [23], SbCl3 [24], BiCl3 [24–26], ZrCl4 [27], SnCl2 [28], CuSO4 [29], (NH4)2SO4 [30], Ni(OAc)2 [31, 32], Cu(acac)2 [33], Me3SiCl [34], Co(pyridyl)2 [35], Se [36], Sb [37], Raney nickel [38], nickel o-aminothiophenol Schiff base complexes [39] and bromoethanol-assisted phthalocyanatoiron [40] and BER in the presence of (CuCl, Cu(OAc)2, CoCl2 and PdCl2) [41] and Ni(OAc)2 [42] are some of the combination systems which have been reported for reduction of nitro compounds. In addition, PyZn(BH4)2 [43] and NaBH4/charcoal [44] have also been reported by our research group for the efficient reduction of various aliphatic and aromatic nitro compounds to the corresponding amines.

Along the outlined methodologies, herein, we wish to introduce an easy and practical protocol for rapid reduction of various aromatic and aliphatic nitro compounds to the corresponding amines with NaBH4/Ni2B (additive) system in H2O at room temperature or 75–80 °C (Scheme 1).

Scheme 1
scheme 1

Reduction of nitro compounds with NaBH4/additive Ni2B system

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Results and discussion

A literature review shows that though the application of NaBH4 in water or aqueous media has been reported for some reduction purposes [45–55], however, reduction of nitro compounds with NaBH4 in H2O as the sole solvent has not been reported yet. This subject and our ongoing attention to the development of modified hydroborate agents in organic synthesis [56–60] encouraged us to investigate the titled transformation in H2O as an easily available and ecologically safe solvent.

In looking for the efficient and stable catalysts in medium of H2O, we found that reduction of nitro compounds with NaBH4 in the presence of NiCl2 [14, 15] or Ni(OAc)2 [31, 32] in wet aprotic or alcoholic solvents has been carried out through the catalysis of precipitated fine black nickel boride (Ni2B). Moreover, the influence and catalytic activity of already prepared Ni2B and using it as an additive reagent to the reaction mixture have not been investigated yet. Prompted by this idea, we decided to study the reducing capability of NaBH4/Ni2B (additive) system in H2O as a green solvent for the reduction of nitro compounds.

The investigation was started by preliminary preparation of Ni2B through the reaction of aqueous Ni(OAc)2·4H2O and NaBH4 according to the reported procedure [61]. The prepared Ni2B was isolated, dried under air atmosphere and stored in a sealed bottle. The catalytic activity of this Ni2B was studied through the reduction of nitrobenzene with NaBH4 under different reaction conditions. The results of this investigation are summarized in Table 1.

Table 1 Optimization experiments for reduction of nitrobenzene to aniline with NaBH4/Ni2B (additive) system under different conditions
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Entries 1–7 show that reduction of PhNO2 with NaBH4 in the absence or presence of additive Ni2B in protic and aprotic solvents such as MeOH, EtOH, THF, CH3CN and CH3CN/H2O did not take place. However, using H2O as the sole solvent dramatically accelerated the rate of reduction (entries 8–12). These observations revealed that H2O was the best solvent choice and using a molar equivalent of 1:2.5:0.05 for PhNO2, NaBH4 and Ni2B, respectively, was the optimum for complete reduction of nitrobenzene. Subsequently, aniline was obtained in 95 % yield within 3 min at room temperature (Table 1, entry 11) (Scheme 2, path a).

Scheme 2
scheme 2

Reduction of nitrobenzene with NaBH4 in the presence of Ni2B as an additive reagent (Path a) or in situ precipitated one (Path b)

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To show the difference between catalytic activity of Ni2B as an additive reagent and the in situ precipitated one in the reaction mixture, we also performed reduction of nitrobenzene with 2.5:0.05 molar equivalents of NaBH4 and Ni(OAc)2·4H2O in H2O at room temperature. In this case, Ni2B as a fine black precipitate was also prepared, however, reduction of PhNO2 encountered with low efficiency and aniline was obtained in 55 % yield after 2 h (Scheme 2, path b). These results definitely exhibited that the catalytic activity of already prepared Ni2B as an additive reagent was superior to use it as the in situ precipitation in the reaction mixture.

Next, synthetic utility of NaBH4/additive Ni2B system in H2O was further studied by reduction of structurally different nitroarenes to amines. Table 2 summarizes the results of this investigation. As it is seen, all reactions were carried out successfully in H2O with 2.5:0.05 molar equivalents of NaBH4 and additive Ni2B at room temperature or 75–80 °C. The corresponding arylamines were obtained in high to excellent yields within 5–30 min.

Table 2 Reduction of aromatic and aliphatic nitro compounds with NaBH4/additive Ni2B system
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The study also showed that the chemoselective reduction of nitro group in the presence of carboxylic acid was achieved successfully through the reduction of 2-nitrobenzoic acid to anthranilic acid in 92 % yield (Table 2, entry 13). Molecules with the complexity of nitro and carbonyl groups did not show any selectivity, and both of the functional groups were reduced with the same reactivity. This fact was shown in the reduction of nitrobenzaldehydes and nitroacetophenones to their corresponding amino alcohols (Table 2, entries 15–19). By changing the solvent from H2O to a mixture of H2O-CH3CN (1:1 mL) and decreasing the quantity of NaBH4 to 0.5–1.5 molar equivalents, however, the selective reduction of carbonyl group versus nitro group was carried out successfully (Table 2, entries 14 and 20). Further examinations resulted that the present method was also efficient for the reduction of dinitroarenes using 3.5:0.1 molar equivalents of NaBH4 and additive Ni2B, respectively, in H2O at 75–80 °C (Table 2, entries 21–23).

Capability of NaBH4/additive Ni2B system in the reduction of aliphatic nitro compounds was also studied by the reduction of 1-nitrohexane, 2-nitroheptane and nitrocyclohexane with 2.5:0.05 molar equivalents of NaBH4 and Ni2B in H2O at room temperature. The results showed that as aromatic nitro compounds, this protocol was also efficient and the corresponding aliphatic amines were obtained successfully in 90–94 % yield within 10–15 min (Table 2, entries 24–26).

Brown in 1970 demonstrated that the combination system of NaBH4 and aqueous nickel salts (chloride, sulfate or acetate) released hydrogen gas and various alkenes could be hydrogenated through the catalysis of highly reactive Ni2B [62] (Eq. 1).

$$2 {\text{Ni(OAc)}}_{ 2} \;{ + }\; 4 {\text{NaBH}}_{ 4} \;{ + }\; 9 {\text{H}}_{ 2} {\text{O}}\; \to {\text{Ni}}_{ 2} {\text{B}}\;{ + }\; 4 {\text{Na(OAc)}}_{ 2} \;{ + }\; 3 {\text{B(OH)}}_{ 3} \;{ + }\; 1 2. 5 {\text{ H}}_{ 2} \; ( {\text{g)}}$$
(1)

The literature review also shows that reduction of nitro compounds to amines has been carried out by catalytic hydrogenation in the presence of homogeneous [63] or heterogeneous [64–73] catalysts. The exact mechanism of NaBH4/additive Ni2B system in H2O is not clear. However, we think that due to similar characteristics of the present protocol with Brown’s method and our observation to vigorous releasing of H2 gas on the surface of additive Ni2B, the reduction of nitro compounds with NaBH4/additive Ni2B system may take place through catalytic hydrogenation as well as hydride transferring from NaBH4.

In conclusion, we have shown that NaBH4/additive Ni2B system is an efficient protocol for rapid and green reduction of aliphatic and aromatic nitro compounds to the corresponding amines. Reduction reactions were carried out in H2O at room temperature or 75–80 °C. Reduction of dinitroarenes was also performed efficiently by this reducing system. Chemoselective reduction of nitro group in the presence of carboxylic acid was achieved successfully. By changing the solvent and decreasing the molar equivalents of NaBH4, the selective reduction of carbonyl group in the presence of nitro group was feasible. Simplicity of the method, availability of the reagents, mild reaction conditions, high yields and easy work-up procedure as well as using H2O as an ecologically safe solvent are the advantages which make this protocol a useful addition to the present methodologies.

Experimental

General

All reagents and substrates were purchased from commercial sources with high quality and they were used without further purification. IR and 1H/13C NMR spectra were recorded on Thermo Nicolet Nexus 670 FT-IR and 300 MHz Bruker spectrometers. The products were characterized by 1H/13C NMR and IR spectra followed by comparison of the obtained data with authentic ones [74–79]. All yields refer to isolated pure products. TLC was applied for the purity determination of substrates, products and reaction monitoring over silica gel 60 F254 aluminum sheet.

Preparation of Ni2B [61]

In a two-necked and round-bottomed flask (100 mL), a solution of Ni(OAc)2·4H2O (1.244 g, 5 mmol) in distilled water (50 mL) was prepared and the solution was stirred magnetically under N2 atmosphere. A solution of NaBH4 (10 mL, 1.0 M) in water was added to the prepared nickel acetate solution by a syringe over 30 s. When the gas evolution was ceased, a second solution of NaBH4 (5 mL, 1.0 M) was again added. The aqueous phase was decanted and the residue fine black precipitate was washed twice with ethanol (25 mL). Drying the precipitate under air atmosphere affords Ni2B in 85 % yield.

A typical procedure for reduction of nitrobenzene to aniline with NaBH4/additive Ni2B system

In a round-bottomed flask (10 mL) equipped with a magnetic stirrer, a mixture of nitrobenzene (0.123 g, 1 mmol) and H2O (2 mL) was prepared. Ni2B (0.006 g, 0.05 mmol) was then added and the mixture was stirred for 5 min. NaBH4 (0.095 g, 2.5 mmol) was also added and the resulting mixture was continued to stirring for 3 min at room temperature. TLC monitored the progress of the reaction (eluent, n-hexane/Et2O:5/3). After completion of the reaction, aqueous solution of KOH (2 %, 5 mL) was added and the mixture was stirred for 10 min. The mixture was extracted with EtOAc (3 × 8 mL) and then dried over anhydrous Na2SO4. Evaporation of the solvent affords the pure liquid aniline in 95 % yield (0.088 g, Table 2, entry 1).