Introduction

In the last few years, heterogeneous reagents and catalysts have been used increasingly in organic synthesis [1–4]. The development of efficient and new catalytic systems for various organic transformations is an active research area with the aim of developing milder reaction conditions [5].

4-Substituted-1,2,4-triazole-3,5-diones (TADs) have been used both as substrates and reagents in various organic reactions such as electrocyclic and oxidation reactions [6–11]. However, the unusual reactivity which makes 1,2,4-triazoline-3,5-diones (2, 4) of interest to organic chemists also makes them difficult to prepare and purify [12].

Even though urazole derivatives can be easily oxidized by a wide variety of oxidizing agents [13–23], this transformation is not easy because these compounds are very sensitive to the nature of the oxidizing agent and to the reaction conditions. In addition, most of the reported reagents produce by-products, which either destroy the product or which are difficult to remove. Many methods also suffer from other drawbacks, for example low yield, tedious work-up, toxicity, or expensive reagents.

Results and discussion

In order to overcome the above-mentioned limitations and in continuation of our ongoing program on the functionalization of organic compounds [24–29] we became interested in the use of new catalytic media, based on in-situ generation of nitric acid for selective oxidation of urazoles and bis-urazoles to triazolinediones.

Silica sulfuric acid is an efficient solid acidic source which has been used for different organic transformations [30]. It can be easily prepared by reaction of chlorosulfonic acid with silica gel (Scheme 1) [31]. Therefore, we decided to use silica sulfuric acid as an efficient acidic catalyst in the oxidation of urazoles and bis-urazoles with aluminium nitrate.

Scheme 1
scheme 1

 

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Consequently, we wish to report here selective oxidation of different types of urazole 1 or bis-urazole 3a to the corresponding triazolinediones 2 or 4a using Al(NO3)3.9H2O (I) and a catalytic amount of SiO2–OSO3H (II) in the presence of wet SiO2 (50% w/w) (Scheme 2).

Scheme 2
scheme 2

 

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The oxidation reactions were carried out under heterogeneous conditions in dichloromethane at room temperature giving excellent yields (Table 1).

Table 1 Oxidation of urazoles and bis-urazole to their corresponding triazolinediones using Al(NO3)3.9H2O (I) and catalytic amounts of SiO2–OSO3H (II) in the presence of wet SiO2 (50% w/w) in dichloromethane at room temperature
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The catalytic oxidation procedure is very simple. All reaction components except triazolinediones are insoluble in dichloromethane; therefore products can be easily obtained by simple filtration and evaporation of CH2Cl2.

To show the efficiency of the described system in comparison with previously reported procedures, we compared our results obtained for oxidation of 4-phenylurazole (as a typical example) with the best of the well-known data from the literature, as shown in Table 2.

Table 2 Comparison of the different methods used for the oxidation of 4-phenylurazole
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A plausible mechanism of this oxidation is outlined in Scheme 3 which proposes that the oxidation reaction occurs at the surface of wet SiO2 via in-situ generation of HNO3.

Scheme 3
scheme 3

 

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In summary, herein we report an efficient catalytic method for selective oxidation of urazoles and one bis-urazole under mild heterogeneous conditions with good to high yields. This method offers the advantage of shorter reaction times, high selectivity, cost effective reagent or catalyst, and easy workup.

Experimental

Chemicals were purchased from Fluka, Merck, and Aldrich chemical companies. Silica sulfuric acid was prepared as described elsewhere [31]. The spectral data (IR, 1H NMR, and 13C NMR) and melting points of products 2a–2i agreed well with literature data (2a–2g [13, 15], 2h–2i [20]).

Oxidation of 4-cyclohexylurazole (1d) to 4-cyclohexyl-1,2,4-triazoline-3,5-dione (2d) with Al(NO3)3.9H2O/SiO2–OSO3H, a typical experiment

A suspension of 0.183 g 1d (1 mmol), 0.344 g Al(NO3)3.9H2O (1.2 mmol), 3.03 g SiO2–OSO3H, and 0.3 g wet SiO2 (50% w/w) in 5 cm3 dichloromethane was stirred at room temperature for 150 min and then filtered. The residue was washed with 20 cm3 CH2Cl2. Anhydrous Na2SO4 (1.5 g) was added to the filtrate and then removed by filtration after 20 min. Finally, CH2Cl2 was removed by evaporation and 0.179 g 2d (99%) was obtained. The product was identical with that described elsewhere [13, 15].