Zirconium Oxide Nanoparticles as an Efficient Catalyst for three-component Synthesis of Pyrazolo [ 1 , 2-a ] [ 1 , 2 , 4 ] triazole-1 , 3-diones Derivatives

An efficient and green protocol for the synthesis of pyrazolo[1,2-a][1,2,4]triazole-1,3-diones derivatives by one pot, three component coupling reaction of aromatic aldehyde, malononitrile, and 4-phenylurazole has been developed using ZrO2 nanoparticles (NPs) as the catalyst. The procedure is formed in high yields, short reaction time and an environmentally friendly specificity. keywords: multicomponent reaction, ZrO2 nanoparticles (NPs), solvent-free, Malononitrile.


INTRODUCTION
Multi-component reactions have been attracting much interest from synthetic chemists because they provide simple one-pot routs for the synthesis of complex molecules from simple and easily available starting materials.These processes are single-step and don't require separation and purification of intermediates and so save time, energy and raw materials. 1cently, heterogeneous catalysts have attracted the attention of researchers due to their economic and industrial significance and published reports indicate that they scored over homogeneous catalysts.Among these, nanoscale heterogeneous catalysts are highly preferred as they offer high surface area and low-coordinated sites, which are responsible for the higher catalytic activity, [2][3][4] having the advantage of easy product purification and reusability of the catalyst.

RESULTS AND DISCUSSION
To choose optimum conditions, first, the effect of temperature on the rate of the reaction was studied for the preparation of 7-amino-5-(4-chlorophenyl)-1, 2,3, 5-tetrahydro-1, 3-dioxo-2-phenylpyrazolo [1,2-a][1,2,4]triazole-6carbonitrile from the three-component condensation reaction of 4-phenylurazole, malononitrile, and 4-chlorobenzaldehyde under solvent-free conditions (Table 1).At 80 o C, the reaction proceeded smoothly and almost complete conversion of product was observed.Further increase in temperature to, 100 and 120 o C increased the rate of reaction.Therefore, we kept the reaction temperature as 100 o C (giving short reaction time and high yield).Next, the study set out to determine optimal amount of nano-ZrO 2 , the reaction was carried out by varying amount of the catalyst (Table 1).Maximum yield was obtained with 0.02 g (0.2 mmol) of the catalyst.Further increase in amount of nano-ZrO 2 , in the mentioned reaction did not has any significant effect on the product yield.
To study the scope of the reaction, a series of aldehydes were employed.The results are shown in Table 2.In all cases, aromatic aldehydes substituted with either electron-donating or electronwithdrawing groups underwent the reaction smoothly and gave the products in good yields.When this reaction was carried out with aliphatic aldehyde such as butanal or pentanal, TLC and 1 H NMR spectra of the reaction mixture showed a combination of starting materials and numerous products, the yield of the expected product was very poor.
[17] Although it is not clear how ZrO 2 acts as a catalyst for the reaction, on the basis of the surface of metal oxides exhibit both Lewis acid and Lewis base character 18,19 and according to the literature survey [15][16][17] , the suggested mechanism for the formation of the products is shown in Scheme 2. First, via a knoevenagel condensation of malononitrile 1 and arylaldehydes 2 in the presence of ZrO 2 NPs as the catalyst was afforded benzylidenemalononitrile 5 Then, Michael addition of 4-phenylurazole 3 to benzylidenemalononitrile 5 formed intermediate 6 that underwent cyclization and tautomerism affords the corresponding products 4.
The reusability of the catalyst was tested in the synthesis of 7-amino-5 triazole-6-carbonitrile, as shown in Figure 1.The catalyst was recovered after each run, washed with ethanol, dried in an oven at 100 o C for 15 min.prior to use and tested for its activity in the subsequent run.The catalyst was tested for 4 runs.It was seen that the catalyst displayed very good reusability (Figure 1).

CONCLUSIONS
We have developed a simple, efficient, one-pot and green protocol for the synthesis of pyrazolo[1,2-a][1,2,4]triazole-1,3-diones derivatives using nano-ZrO 2 as a reusable heterogeneous catalyst under solvent-free conditions.The simplicity, easy workup, as well as safety and reusability of catalyst are advantages of this method.

ACkNOwLEDGEMENTS
We gratefully acknowledge financial support from the Research Council of Islamic Azad University, Mahshahr branch.