Chiral Imidazolium Salts Derived from Amino Acids: Synthesis Characterization and Structure

A series of chiral imidazolium salts derived from natural amino acids has been obtained. The synthetic route involves a multicomponent condensation reaction between the corresponding amino acid {(S)-methionine, (S)-threonine, (S)-phenylalanine, (S)- valine and (S)-alanine} in acidic medium and ammonium acetate, glyoxal and formaldehyde in aqueous solution at 95°C to form an imidazole ring in one pot. The imidazole carboxylic acids [{R1-CH-CO2H}Im] (R1 = CH2CH2SCH3, CH(OH)CH3, CH2Ph, CH(CH3)2, CH3) (1a-e) were converted into sodium carboxylate compounds to carry out the esterification and N-alkylation of imidazole with an excess of the suitable alkyl halide in acetonitrile to give rise to a series of chiral imidazolium salts [{R1-CH-CO2R2}ImR2][X] (R1 = CH2CH2SCH3, CH(OH)CH3, CH2Ph, CH(CH3)2, and CH3; R2 = CH3, CH2CH3, X = I, Br) (2a-e and 3a-e). Characterization of all of compounds by IR, HR-MS, 1H and 13C{1H} NMR confirmed the proposed structures. The molecular structure of the imidazole carboxylic acid 1c (spatial group P21) as a zwitterion, was obtained by single-crystal X-ray diffraction study, showing that the chiral center is preserved.