Synthesis of 4 , 4 ’-( Arylmethylene ) bis ( 1 H-pyrazol-5-ols ) Using Silica-bonded Ionic Liquid as Recyclable Catalyst

4,4’-(Arylmethylene)bis(1H-pyrazol-5-ols) were synthesized in the presence of N-(3-silicapropyl)-N-methyl imidazolium hydrogen sulfate ([Sipmim]HSO4) as a recyclable solid acid catalyst from the reaction between aldehydes and 3-methyl-l-phenyl-5-pyrazolone in refluxing ethanol. [Sipmim]HSO4 showed much the same efficiency when used in consecutive reaction runs.


Introducation
In the recent years, ionic liquids were used as solvents due to their particular properties, such as the ability to dissolve many organic and inorganic substances and undetectable vapor pressure (Wasserscheid & Welton, 2007).In addition, Brønsted acidic task-specific ionic liquids (BAILs), such as those possessing HSO 4 -as a counter anion find a broad application in organic synthesis, acting as both solvents and catalysts.Keim and co-workers reported the synthesis of 1-butyl-3-methylimidazolium hydrogensulfate ([bmim]HSO 4 ) (Keim et al., 2000).In addition, in the year of 2002 ([bmim]HSO 4 ) was used as a catalyst in the Friedel-Crafts alkylation (Wasserscheid et al., 2002).The other applications of these acidic ionic liquids such as acetalization and thioacetalization of carbonyl compounds (Gupta et al., 2007), Fischer indole synthesis (Xu et al., 2007), acetylation of alcohols and phenols (Wang et al., 2008), preparation of azides from alcohols (Hajipour et al., 2009), selective nitration of phenols (Tajik et al., 2009), synthesis of 1,8-dioxo-octahydroxanthenes (Niknam & Damya, 2009), formylation of alcohols (Niknam et al., 2009), synthesis of polysubstituted quinolines (Tajik et al., 2011), have been proceeded with very good yields and selectivities.Recently, immobilization of acidic ionic liquids on solid supports has been designed and it can offer important advantages in handling, separation and reuse procedures.Based on economic criteria, it is desirable to minimize the amount of ionic liquid utilized in a potential process.Immobilized acidic ionic liquids have been used as novel solid catalysts, e.g., for esterification, nitration reactions (Qiao et al., 2006), acetal formation (Sugimura et al., 2007), and Baeyer-Villiger reaction (Chrobok et al., 2009).
During vigorous stirring, concentrated H 2 SO 4 (97 %) (2.9 mmol) was added drop by drop at 0 o C. Then the mixture was warm up to the room temperature, and was refluxing for 48 h.When the formed HCl was completely distilled of the condenser the solution was cooled and the CH 2 Cl 2 was removed under vacuum.To remove any water from the reaction mixture 10 mL of benzene was added to the crude ionic liquid and stirred for 3 h with magnetic stirrer at 50 o C. Formed azeotrope was distilled of yielding [Sipmim]HSO 4 .Elemental analysis showed the S content to be 1.77 %.According to S content the number of H + sites of [Sipmim]HSO 4 is 0.55 mmol/g.

General Procedure for the Synthesis of 4,4'-(arylmethylene)bis(1H-pyrazol-5-ols)
A round bottom flask contains 6 mL of ethanol was heated in an oil bath.When the temperature was reached to above 70 o C, aromatic aldehyde (1 mmol), 5-methyl-2-phenyl-2,4-dihydro-3H-pyrazol-3-one (2 mmol) and [Sipmim]HSO 4 (0.15 g, equal to 0.083 mmol of H + ) were added to a flask and heated under reflux conditions.After completion of the reaction, as indicated by TLC, the reaction mixture was filtered.The remaining was washed with warm ethanol (3 × 30 mL) in order to separate heterogeneous catalyst.After cooling the crude products were precipitated.The crude products were purified by recrystallization from ethanol (95 %).The recovered catalyst was dried and reused for subsequent runs.

Ph Ph
The FT-IR spectra of [Sipmim]HSO 4 was shown in Figure 1.The IR spectrum shows the overlap asymmetric and symmetric stretching bands of SO 2 with Si-O-Si stretching bands in the silica functionalized alkyl-sulfuric acid.The spectrum also shows a broad OH stretching absorption around 3600 to 2600 cm -1 .The thermogravimetric analysis (TGA) curve of [Sipmim]HSO 4 shows the mass loss of organic materials as they decompose upon heating (Figure 2).The weight loss below 125 o C corresponds to desorption of physically adsorbed solvent and surface hydroxyl groups.The weight loss contered at higher temperature of about 9 % between 150 and 500 o C should be attributed to the thermal decomposition of organic groups.The BET surface area using nitrogen adsorption isotherms at the temperature of liquid nitrogen which gave the results of a s,BET 1.11 m 2 g -1 and the total pore volume 0.1737 cm 3 g -1 (see supplementary material).
[Sipmim]HSO 4 showed much the same efficiency when used in six consecutive reactions runs (Figure 3).