Synthetic Communicationsw, 37: 2225–2230, 2007 Copyright # Taylor & Francis Group, LLC ISSN 0039-7911 print/1532-2432 online DOI: 10.1080/00397910701397011
Mild and Selective Nitration of Phenols by Zeofen Mohammad A. Bigdeli Faculty of Chemistry, Teacher Training University, Tehran, Iran
Majid M. Heravi Department of Chemistry, School of Science, Azzahra University, Vanak, Tehran, Iran
Firouzeh Nemati Faculty of Chemistry, Teacher Training University, Tehran, Iran
Abstract: Certain phenols and naphthols were nitrated regioselectively with zeofen in dichloromethane as solvent at room temperature and in a short reaction time to give good yields. Keywords: nitration, phenols, zeofen
Among all electrophilic substitutions, nitration of aromatic compounds is one of the most important industrial processes[1 – 3] and is the subject of a large body of literature.[4,5] Nitrated phenolic compounds are very important chemicals that have applications as solvents, dyes, pharmaceuticals, agrochemicals, explosives, and plastics in industry. They are also useful intermediates for the preparation of other compounds, particularly amines, by reduction of nitro groups.[6] However, the majority of reported methods for nitration of aromatic compounds suffer from disadvantages such as low regioselectivity,[7]
Received in Poland October 23, 2006 Address correspondence to Majid M. Heravi, Department of Chemistry, School of Science, Azzahra University, Vanak, Tehran, Iran. E-mail:
[email protected] 2225
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overnitration,[8] strongly acidic media,[9] tedious workup,[10] and safety problems (storage, handling, using toxic transition-metal cations such as Hgþ2, Cuþ2, etc.).[11] These disadvantages have encouraged extensive efforts to develop alternative procedures such as using solid acids, other sources of NOþ 2 , organic nitrating agent, and so forth. Zeolite-based solid acid catalysts such as ZSM-5,[12] X,[6] Y,[6,12,13] and beta[12 – 14] have been studied for nitration of various activated as well as deactivated substrates. Besides zeolites, a variety of other solid acids such as sulfuric acid supported on silica,[15] clay-supported metal nitrates,[16 – 18] metalexchanged clays,[19] and metal-modified montmorillonite KSF,[20] have been studied for nitration of aromatic compounds. Furthermore, alternative nitrating agents, such as nitric acid in mixed acids;[7] nitronium salts in organic media;[21] metal nitrates of Na,[7] Ti, Zr, Fe,[22] Ni,[23] and Ce;[24] and organic nitrating agent such as acetyl nitrates[3] have been studied. Besides the organic solvents, ionic liquids have also been used as solvent.[25] Considering these reactions and environmental concerns, there is still a need for research on new reagents for regioselective nitration of phenols. Herein we report our results on the regioselective nitration of phenols using zeofen. We have introduced and investigated various performances of zeofen in different reactions.[26 – 28] Zeofen can be prepared easily by a combination of Fe(NO3)3 . 9H2O (0.8 g, 2 mmol) and a weight equivalent of HZSM-5 zeolite. This reagent can be stored in a dark brown bottle and be stable for at least 6 months. We believed that such reagent can also act as a nitrating agent, because ferric nitrate can act as a nitrating agent along with HZSM-5 zeolite, instead of Brønsted (H2SO4, HNO3) or Lewis acids (AlCl3, TiCl4). HZSM-5 zeolite was prepared by calcinations of NHAZSM-5 zeolite at 5008C for 8 h. The nitration reaction readily took place by adding the phenols and zeofen in the suitable solvent in a reaction vessel and efficiently stirring the resultant heterogeneous mixture at room temperature. The progress of reaction was monitored by thin-layer chromatography (TLC). Upon the completion of reaction, zeofen was separated by simple filtration. This method thus provides nitrated phenols directly, in a short reaction time and with good yields (Table 1). Although the nitration reaction also occurs in the absence of zeolite [only with Fe(NO3)3 . 9H2O], no selectivity has been observed in this reaction. Clearly zeolite (HZSM-5) helped to achieve high selectivity. The results are explained on the basis of diffusion and binding of phenols inside zeolite, which bring regioselectivity.[29] In conclusion, the mononitration of phenols with zeofen has been achieved with high regioselectivity and with excellent yields. In general, exclusive orthoselectivity was observed for all phenols subjected to this
Mild and Selective Nitration of Phenols by Zeofen
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Table 1. Nitration of phenols with zeofen Time (min)
Yield (%)a
1
30
75
2
10
55
3
10
80
4
40
65
5
10
75
6
10
90
7
15
90
8
20
90
Entry
a
Substrate
Products
Yields are based on isolated products.
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protocol.
The high regiospecifity, short reaction time, good isolated yield, and use of inexpensive and nontoxic materials make this methodology have wide synthetic and commercial utility. EXPERIMENTAL Chemicals were purchased from the Fluka, Merck, and Aldrich chemical companies. Melting points were determined on an Electrothermal 9100 and are not corrected. TLC on commercial aluminum-backed plates of silica gel 60 F254 was used to monitor the progress of reactions. Yields refer to isolated pure products. The nitration products are characterized by comparison of their spectral data (IR, 1H NMR), TLC, and melting points with authentic samples. Nitration of Phenols with Zeofen: Typical Procedure 4-Chloro phenol (0.12 g, 1 mmol) and zeofen (1 –1.5 eq.) in 3 mL of dichloromethane were magnetically stirred at room temperature. After the completion of the reaction, the reaction mixture was filtered (the progress of the reaction was monitored by TLC). The residue was washed with CH2Cl2 (2 5 mL) and dried over anhydrous sodium sulfate. The solvent was removed under vacuum. The crude product was purified by silica-gel dry flash chromatography using petroleum ether– ethyl acetate (98:2) as eluent. The yield was 0.15 g (88%), mp 89 –908C (lit.[30] mp 918C). ACKNOWLEDGMENTS The authors are thankful to the Teacher Training University Research Council for the partial financial support. We are also grateful to Dr. B. Mohajerani, Petrochemical Research Center of Iran, for a gift of zeolite.
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