Abstract
Ionic liquids (ILs) with 1,3-disubstituted imidazolium cations and the dimethyl phosphate (DMP) anion, as well as the chloride anion were prepared and characterized by 1H NMR spectra, chromatographic and titrimetric purity control, and determination of the moisture content and thermal stability. ILs with the DMP anion decompose only at temperatures above 240°C. These ILs were tested as both reaction media (solvents) and catalysts for the Knoevenagel condensation reaction. The impact of the most significant structure elements of ILs was evaluated for the rates and yields of the condensation reaction. IL anions have the greatest effect on the condensation reactions, and even the chloride anion has some catalytic effect on the Knoevenagel condensation. Side chains in the imidazolium cations influence the reaction course very little. The ability of the imidazolium cations to form hydrogen bonding with the transition state of the condensation reaction leads to a remarkable slowdown in the reaction rates.
Highlights
Condensation reactions have a major role in the transformations of organic substances
Ionic liquids (ILs) anions have the greatest effect on the condensation reactions, and even the chloride anion has some catalytic effect on the Knoevenagel condensation
ILs with the 1,3-disubstituted imidazolium cations (1, 2) and dimethyl phosphate and chloride anions were selected for the study because of their increased thermal and chemical stability [1] [2], the imidazolium salts with the chloride anion being used mainly for comparison
Summary
Condensation reactions have a major role in the transformations of organic substances. For eliminating these risks, in the last decades, ionic liquids (ILs), as substances that are environmentally friendly and harmless for workers, have been increasingly used instead of organic solvents. The role of hydrophilicity (or hydrophobicity) and/or specific solvation ability (hydrogen bond, etc.) has clearly been incompletely appreciated for the acceleration of organic transformations in IL media. On this account, an attempt is made in the present study to evaluate systematically the effect of most significant structure elements of ILs on the rate and yield of a commonly used condensation reaction
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