Nitric acid in the presence of P2O5 supported on silica gel—a useful reagent for nitration of aromatic compounds under solvent-free conditions
Graphical abstract
Introduction
Nitroaromatic compounds are important chemicals that have application 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.1 Electrophilic aromatic nitration is one of the most important reactions in organic chemistry. However, the majority of the reported methods for nitration of aromatic compounds suffer from disadvantages such as low regioselectivity,2 over nitration,2(b), 2(d) strongly acidic media,3 tedious work-up,3 oxidation of the reagents,2(b), 4 and safety problems (storage, handling, using toxic transition metal cations such as Hg+2, Cu+2, etc.).5 These disadvantages have encouraged the researchers for extensive efforts to develop alternative procedures such as using solid acids1(b), 2(b), 4(b), 5(b), 6, other sources of NO2+ (nitronium salts,2(a), 7(a) N-nitropyridinium salts,7b nitrogen oxide,2(b), 2(c) and peroxynitrite8), organic nitrating agents (acetyl nitrate, benzoyl nitrate, and trimethylsilyl nitrate),9 etc.10
Reaction under solvent-free conditions has recently attracted attention.11, 12, 13 The advantage of these methods over conventional classical method is that they are cleaner reactions, decreased reaction time, and easier work-up.
Section snippets
Results and discussion
In continuation of our ongoing program to develop environmentally benign methods under solvent-free conditions,14, 15, 16 we wish to report an extremely convenient method for nitration of aromatic compounds with 65% nitric acid in the presence of P2O5 supported on silica gel under solvent-free conditions. The P2O5/silica gel was prepared by mixing a mixture of P2O5 and silica gel (0.063–0.2 mm) in a mortar and grinding with a pestle for 1 min to obtain a homogeneous mixture. This reagent is
General
Yields refer to isolated pure products after column chromatography. The products were characterized by comparison of their spectral (IR and 1H NMR) and physical data with those of authentic samples.18, 19, 20, 21, 22 All 1H NMR spectra were recorded at 300 and 500 MHz in CDCl3 relative to TMS (0.00 ppm) and IR spectra were recorded on Shimadzu 435 IR spectrometer. All reactions were carried out under solvent-free conditions at room temperature in a hood with strong ventilation.
Preparation of reagent [P2O5/silica gel (64%w/w)]
In a mortar, 4.5 g
Acknowledgements
We gratefully acknowledge the funding support received for this project from the Isfahan University of Technology (IUT), IR Iran (A.R.H.) and Grants GM 033138, MH 065503, NS 033650 (A.E.R.) from the National Institutes of Health, USA. Further financial support from Center of Excellency in Chemistry Research (IUT) is gratefully acknowledged.
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