Synthesis of α- cyanostyrylbenzimidazoles under solvent-free conditions using L-proline as catalyst: A green approach

A simple and green methodology for the Knoevenagal condensation of 2-(1H-benzimidazol-2-yl)acetonitrile 3 with benzaldehydes 4(a-h) yielding 2-(α-cyanostyryl)benzimidazole 5(a-h) has been developed under solvent-free (physical grinding) conditions and also under solution phase using ethanol as solvent with L-proline as a catalyst. Compounds 5(a-h) were alternatively prepared by the Knoevenagal condensation of ethyl cyanoacetate 2 with 4(ah) by simple physical grinding using L-proline as catalyst, yielding ethyl 2-cyano-3-phenylacrylate 6(a-h), and subsequent treatment with o-phenylenediamine 1 by simple heating. N-Methylation of 5a was done under physical grinding conditions using tetrabutylammonium bromide (TBAB) as surface catalyst and K2CO3 as base yielding 2- (1-methyl-1H-benzimidazol-2-yl)-3-phenylacrylonitrile 8a in good yield. 8a was alternatively prepared by the Knoevenegal condensation of (1-methyl-1H-benzimidazol-2-yl) acetonitrile 7 with 4a under solid phase synthesis using L-proline as catalyst. 7 was prepared by the methylation of 3 under physical grinding. 3 in turn was obtained by the condensation of 1 and 2 under simple heating conditions. The entire sequence of reactions was done under green conditions

Author(s): P. N. Kishore Babu, B. Rama Devi and P. K. Dubey

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