Silica-Grafted Ionic Liquids as Recyclable Catalysts for the Synthesis of 3 , 4-Dihydropyrano [ c ] chromenes and Pyrano [ 2 , 3c ] pyrazoles

Silica-grafted N-propyl-imidazolium hydrogen sulfate ([Sipim]HSO4) is employed as a recyclable heterogeneous ionic liquid catalyst for the synthesis of 3,4-dihydropyrano[c]-chromenes by the reaction of aromatic aldehydes, malononitrile and 4-hydroxycoumarin at 100 ̊C under solvent-free conditions. Also, heterogeneous ionic liquid catalyst was used for the synthesis of pyrano[2,3-c]-pyrazoles by the reaction of aromatic aldehydes, malononitrile and 3-methyl-l-phenyl5-pyrazolone at 110 ̊C under solvent-free conditions. The heterogeneous ionic liquid showed much the same efficiency when used in consecutive reaction runs.


Synthesis of Dihydropyrano[c]chromenes
To study the effect of catalyst loading on the synthesis of 2-amino-4-aryl-5-oxo-4H,5H-pyrano[3,2-c]chromene-3carbonitrile the condensation reaction of malononitrile, 4-chlorobenzaldehyde, and 4-hydroxycoumarin was chosen as a model reaction (Table 1).The results show clearly that ionic liquids and silica-grafted ionic liquids (SGILs) are effective catalysts for this condensation and the optimal amount of SGILs was 0.1 g per 1 mmol of aldehyde under solvent-free conditions at 100˚C.The best result was obtained in the presence of [Sipim]HSO 4 .This condensation was carried out with the lower amounts of [Sipim]HSO 4 0.05 g and 0.07 g and the corresponding product was obtained in 88% and 91% yield (Table 1, entries 4 and 5).
Thereafter, a series of different 3,4-dihydropyrano-[c]chromene derivatives were prepared successfully from different aromatic aldehydes bearing electron-withdrawing and electron donating groups, 4-hydroxycoumarin and malononitrile under solvent-free conditions.Electron-withdrawing groups such as 3-nitro, 4-nitro, and 2-nitro-benzaldehyde reacted under optimized conditions into corresponding 1f, 1g, and 1h in 93%, 90%, and 89% yield after 30 min (  The possibility of recycling the catalyst was examined using the reaction of malononitrle, 4-chlorobenzaldehyde and 4-hydroxycoumarin under the optimized conditions.Upon completion, the reaction mixture was washed with warm ethanol (3 × 30 mL).The recovered catalyst was dried and reused for subsequent runs.The recycled catalyst could be reused fourth times without any additional treatment.No observation of any appreciable loss in the catalytic activity of [Sipim]HSO 4 was made (Table 2, entry 2).
The scope of this methodology was extended to the synthesis of pyrano[2,3-c]pyrazols.Initially three-component condensation reaction of malononitrile, 4-chlorobenzaldehyde, and 3-methyl-1-phenyl-1H-pyrazol-5 (4H)one was chosen as a model reaction and the effect of different catalysts was investigated at 110˚C under solvent-free conditions (Table 3).
The possibility of recycling the catalyst was examined using the reaction of malononitrle, 4-chlorobenzaldehyde and 3-methyl-1-phenyl-1H-pyrazol-5(4H)-one under the optimized conditions.Upon completion, the reaction mixture was washed with warm ethanol (3 × 30 mL).The recovered catalyst was dried and reused for subsequent runs.The recycled catalyst could be reused fourth times without any additional treatment.No observation of any appreciable loss in the catalytic activity of [Sipim]HSO 4 was made (Table 4, entry 2).

General Procedure for the Synthesis of 3,4-Dihydropyrano[c]chromenes
To a mixture of aromatic aldehyde (1 mmol), malonitrile (1 mmol), and 4-hydroxycoumarin (1 mmol), catalyst [Sipim]HSO 4 (0.1 g, 0.08 mmol of H + ) was added and the mixture was heated at 100˚C under solvent-free conditions.After completion of the reaction, as indicated by TLC, ethanol (10 mL) was added and the reaction mixture was filtered.The remaining was washed with warm ethanol (3 × 5 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.

Table 1 . Investigation the effect of catalyst and solvent on the reaction of 4-chlorobenzaldehyde, malononitrile and 4- hydroxycoumarin.
a Isolated Yield.