Multiphase Oxidation of Aniline to Nitrosobenzene with Hydrogen Peroxide Catalyzed by Heteropolyacids

Abstract

Keggin-type heteropolyacids have been used as catalyst for high-yielding oxidation reactions in multiphase conditions. This simple and efficient procedure promoted the conversion of anilines to the corresponding nitroso and nitro derivatives. In comparison with homogeneous system the oxidation from anilines to nitroso compounds and nitro compounds were more effective and the solvents used were not toxic.

Key words

multiphase system - Keggin polyoxometalate - hydrogen peroxide - oxidation - aniline

References and Notes

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17

Results not yet published.

18

The commercial catalyst, H₃PMo₁₂O₄₀ (M12P), was obtained from Aldrich. All the others were prepared according to conventional procedures described in ref. 16. Catalysts were characterized with a Bruker IFS 66 FT-IR equipment, employing KBr discs. The acidity was measured via potentiometric titration, adding little amounts of a solution of n-butylamine in MeCN (0.05 N) to the suspended solid (0.05 g) in MeCN (90 mL) stirred during the whole procedure. The difference of potential (mV) was recorded with a digital pHmeter (Instrumentalia S.R.L.), equipped with a combined electrode.

19

General Procedure for Oxidation in Homogeneous Conditions: Reactions were carried out in a 25-mL 3-necked round-bottomed flask, equipped with a refrigerator and thermostated at working temperature. In the reactor were charged aniline (1 mmol), a solution of n-decane in MeCN as internal standard (0.56 mL), MeCN to reach an overall volume of 5 mL, a solution of H₂O₂ (35% p/V, 5 mL) and heteropolyacid (2 mmol%). The reaction mixture was stirred with a magnetic bar at 700 rpm.
General Procedure for Oxidation in Multiphasic Conditions: The aforementioned procedure was employed for reactions in multiphasic conditions, with the addition of Aliquat 336 (2.32 mL) in isooctane (10% p/V) solution and employing isooctane instead of MeCN as organic solvent.
Samples Analysis: Samples were withdrawn from the organic phase at 20 min time intervals during a total reaction period of 420 min. Each sample volume was approximately 20 µL and it was diluted with 1-2 mL of isooctane or MeCN. The solution was stirred in the presence of silica gel and filtered to remove the Aliquat 336 before the injection in the GC. Concentrations were calculated with the internal standard method. Conversions were obtained with GC analysis performed with a Varian GC 3400 instrument, in the presence of n-decane as internal standard. The capillary column was a 30 m Chromopack CP Sil 8 CB, whose diameter was 0.32 mm. The identification of products was performed with GC-MS (detector HP 5971) for comparison with authentic samples.