Melamine trisulfonic acid catalyzed regioselective nitration of aromatic compounds with sodium nitrate under solvent-free conditions

1 Introduction

Nitro aromatic compounds are important chemicals and act as a wide range of useful materials such as solvents, dyes, pharmaceuticals, perfumes, explosives, and plastics in industry (Olah et al., 1989). They can also be employed as intermediates for the preparation of other compounds, particularly amines, by reduction (Tasneem et al., 2001). Nitration typically requires the use of a mixture of concentrated sulfuric acid with nitric acid, leading to an excessive waste stream. However, the majority of this method suffers from disadvantages such as low regioselectivity, over nitration, strongly acidic media, tedious work-up and oxidation of the reagents. Therefore, various methods have been reported in which regioselectivity, over nitration and the competitive oxidation of substrates have been the subject of investigation (Abdulla et al., 2011; Nowrouzi and Jonaghani, 2011; Manjo and Shriniwas, 2010; Sana et al., 2009; Wang and Lu, 2011; Kemdeo et al., 2010; Cheng et al., 2008; Koley et al., 2009). Although many useful procedures for the nitration of different aromatic compounds have been reported, the main disadvantage of these, is the relative difficulty in the preparation of the reagents and in the working-up of the reaction mixture. Moreover in the case of active aromatic compounds, such as anilines and phenols, nitration usually results in a complex mixture of mono and even di-nitrated products. It is especially difficult to perform selective mono-nitration of highly activated aromatic compounds, such as aniline and phenol derivatives. In the previous research, we have reported the preparation of melamine trisulfonic acid (MTSA), and its applications, as a new and efficient solid acid catalyst in the promotion of the protection of different types of functional groups and the synthesis of dihydropyrimidons and coumarins (Shirini et al., 2010a,b,c; Shirini and Albadi, 2010; Shirini et al., 2011). We performed the nitration reaction in the presence of this catalyst and hoped to obtain good results. Therefore, we have used sodium nitrate as the source of nitrate and melamine trisulfonic acid for the nitration of some aromatic compounds.

2 Experimental

All products were identified by comparison of their spectral, TLC and physical data with authentic samples. Chemicals were purchased from Fluka, Merck and Aldrich Chemical companies. Yields refer to isolated pure products.

2.1

2.1 General procedure

A mixture of aromatic compound (1 mmol), sodium nitrate (1 mmol) and MTSA (0.1 mmol), was pulverized in a mortar at room temperature for the appropriate time (Table 1). The reaction was monitored by TLC. On completion, CH₂Cl₂ (5.0 ml), was added to the mixture and filtered. Evaporation of the solvent followed by recrystallization or column chromatography on silica gel of the crude product gave the corresponding nitrated compounds in good to excellent yields.

Table 1 Nitration of aromatic compounds by sodium nitrate in the presence of MTSA.

Entry	Substrate	Product	Time (min)	Yield (%)a
1			5	87
2			12	88
3			30	86
4			55	87
5			5	89
6			30	87
7			22	89
8			18	87
9			30	88
10			60	82
11			15	88
12			60	75
13			45	82
14			32	85
15			30	87
16			5	86

a Isolated yield.

3 Results and discussion

In continuation of our research on the nitration of aromatic compounds (Tajik et al., 2007), we have found that aromatic rings can be nitrated by using sodium nitrate as the source of nitrate in the presence of melamine trisulfonic acid under solvent-free conditions (Scheme 1).

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Scheme 1

Nitration of aromatic compounds in the presence of MTSA.

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All the reactions were performed at room temperature for the appropriate times. A wide range of various aromatic compounds, were successfully reacted to afford the desired mono-nitrated products in excellent yields with complete regioselectivity. Among the various substrates studied, compounds containing electron donating atoms were found to be most reactive and converted to the corresponding mono-nitrated products in shorter reaction times. However, the rate of the reaction was slower when the ring contained an electron-withdrawing group (Table 1, entries 9, 13, 14). It has been observed that the chemoselective conversion of aromatic compounds into their para-substituted products was achieved in excellent yields. When the para position was blocked, mono-nitrated products were obtained over longer reaction times (Table 1, entry 3, 4, 13, 14). Phenol gave mainly the para isomer as the major product in short reaction time (Tables 1 entry 1), However 4-chloro phenol was reacted after 30 min to afford 2-nitro-4-choloro phenol (Table 1, entry 3). Also, it was observed that p-orientation is more favorable than o-orientation from the fact that 2,6-dimethyl phenol was more readily nitrated than 2,4-dimethyl phenol (Table 1, entries 5, 6). ortho-Chlorophenol and ortho-nitrophenol were quantitatively converted into the para-nitrated products with respect to the hydroxyl groups in good yields (Table 1, entries 2, 9). The substrate was nitrated ortho to the phenolic OH only when the para position was occupied (entries 3, 4, 6, 14, and 15). It is very important to note that the method can also be employed for the nitration of some activated bezaldehyde derivatives to the corresponding products in good yields (Table 1, entries 14–16).

A plausible mechanism of the reaction for this new system is the in situ generation of nitronium ion mechanism which is shown in Scheme 2.

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Scheme 2

Mechanism of nitration reaction of aromatic compounds.

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4 Conclusion

In conclusion, we have described an efficient method for the nitration of aromatic compounds using sodium nitrate and MTSA, under solvent-free conditions. Simple work-up, good chemoselectivity, high yields of the products and short reaction times, will make this procedure a useful addition to the available methods.