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

Abstract 1‐Methyl‐3‐butylimidazolium ionic liquids BMImX (X anions: Ac − , HCO 3 − , Cl − , BF 4 − , PF 6 − , I − , CF 3 CO 2 − , (CF 3 SO 2 ) 2 N − , CF 3 SO 3 − ) were used as catalysts in the Knoevenagel condensation reaction. The catalytic activity was compared with that of the inorganic salts NaAc, NaHCO 3 , 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 [BMImHCO 3 ] 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 HCO 3 − ), in ion pair formation (with anion X − ) and activator of the aldehyde structure.