An Efficient Approach for the Synthesis of Pyrazolo [ 1 , 2 , a ] [ 1 , 2 , 4 ] Triazole-1 , 3-Diones Using an Electrochemical Cell

A mild and efficient method was applied for the one-pot three-component synthesis of pyrazolo[1,2-a][1,2,4]triazole-1,3-diones from the condensation between arylaldehydes, malononitrile and 4-phenylurazole using electrolysis in an undivided cell in the presence of sodium bromide as an electrolyte. This procedure has many advantages such as : no need to catalyst, clean work-up, short reaction time and high yield. The products were obtained just with the simple filtration.

For preparing biologically active compounds in organic, combinatorial, and medicinal chemistry the electrosynthetically multicomponent reactions (EMCRs) have been used widely 27 .Due to the electron transfer between an electrode and the substrate molecules, the formation of highly reactive intermediates is achieved under mild conditions, avoiding reductive or oxidant agents as well as acids, bases and related waste by-products so that it can be one of the various fields in green chemistry 28.In continue of our research on multi-component reactions [29][30][31][32] , Herein, we report an efficient and green synthesis of pyrazolo[1,2-a][1,2,4]triazole-1,3-dione derivatives using an electrochemical cell (scheme 1).

MATERIALS AND METHOD
All reagents and solvents were obtained from Fluka and Merck and used without further purification.TLC was performed on Silica-gel Polygram SILG/UV 254 plates.Melting points and IR spectra were measured on an Electro thermal 9100 apparatus and a JASCO FT-IR-460 plus spectrometer.Controlled-current coulometer and preparative electrolysis were performed using a SAMA potentiostate/galvanostate (Zahedan, Iran), respectively.The 1 H NMR spectra were obtained on Bruker DRX-400 Advance instruments with DMSO.

RESULTS AND DISCUSSION
To optimized the reaction conditions, the condensation between benzaldehyde, malononitrile and 4-phenylurazole was chosen as model reaction.The reactive mixture was stirred at room temperature, and this progress was monitored by TLC.The reaction is performed in alcoholic solvents in the presence of sodium bromide as an electrolyte.Various current quantities were applied under the mentioned conditions.As can be seen in Table 1, excellent conversions of the starting materials were obtained under 10 mA/cm2 current densities after 0.1 F/mol of electricity had passed.The current density of 10 mA/cm2, I = 50 mA, electrode surface Scheme 1.
Using mentioned optimized reaction, the reaction were explored for the synthesis of a wide variety of pyrazolo[1,2-a][1,2,4]triazole-1,3-diones using aromatic aldehydes, malononitriles and 4-phenylurazol.The results are summarized in Table 2.As shown in Table 2, the products were obtained in excellent yields.We p r o p o s e d m e c h a n i s m fo r t h e preparation of pyrazolo[1,2-a][1,2,4]triazole-1,3dione derivatives.First, deprotonation of an alcohol at the cathode leads to the formation of the alkoxide anion 33 .It's subsequent reaction in solution with malononitrile gives rise to the malononitrile anion.Then, Knoevenagel condensation of aldehyde 1, with the malononitrile anion takes place in the solution with the elimination of water and the formation of the corresponding a-cyanocinnamo-nitrile derivatives A. The subsequent hydroxide-promoted Michael addition of 4-phenylurazole 3 to the electron-deficient Knoevenagel adduct A followed by intramolecular cyclization results in the corresponding products 4, with regeneration of the alkoxide anion as the last step, which continues the catalytic chain process by the interaction with the next molecule of malononitrile.

CONCLUSION
The pyrazolo[1,2-a][1,2,4]triazole-1,3-dione derivatives were synthesised in the presence of sodium bromide as an electrolyte under neutral and mild conditions.The main advantages of this method are the very short reaction time, high yields, simple work-up, use of non-hazardous organic solvent and catalyst.