Role of Anion and Cation in the 1‐Methyl‐3‐butyl Imidazolium Ionic Liquids BMImX: The Knoevenagel Condensation

Abstract

Abstract1‐Methyl‐3‐butylimidazolium ionic liquids BMImX (X anions: Ac−, HCO3−, Cl−, BF4−, PF6−, I−, CF3CO2−, (CF3SO2)2N−, CF3SO3−) were used as catalysts in the Knoevenagel condensation reaction. The catalytic activity was compared with that of the inorganic salts NaAc, NaHCO3, NaCl. Whereas it is quite important to select suitable cation and anion depending on the purpose, the basicity of the anion X− and the ability of the counter‐ion BMIm+ to avoid the ion pairing with anion X− was investigated. 1‐Methyl‐3‐butyl imidazolium acetate [BMImAc] and 1‐methyl‐3‐butyl imidazolium hydrogen carbonate [BMImHCO3] show the most significant catalytic power and their catalytic effect was studied on a wide range of aromatic, heteroaromatic or aliphatic aldehydes or ketones with active methylene compounds. The corresponding substituted alkenes were obtained in excellent yields (up to 98%) in the absence of any solvent at room temperature within short times. The interesting feature of this study includes explanation on role of the cation BMIm+ as proton donor (versus anions Ac− or HCO3−), in ion pair formation (with anion X−) and activator of the aldehyde structure.