Ionic liquids and their binary systems with molecular solvents as catalyst and reaction medium for organic synthesis

Abstract

Protic and aprotic ionic liquids (ILs) derived from an organic base 1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU) were prepared and characterized.The structure and interactions between anions and cations of the AIL were studied computationally. Binary systems of these ILs with molecular solvents were prepared. Physicochemical properties of these ILs and their binary systems have been studied in detail. These systems have been evaluated as media and catalysts for a model reaction, Michael addition reaction through systematic kinetic studies. Efforts have been made to correlate physicochemical properties of these ILs and their binary systems withkinetic behavior. The protic ionic liquids (PILs) were prepared by neutralization reaction between a strong base, DBU and weak acids, water and acetic acid. The aprotic ionic liquid (AIL) was synthesized by alkylation of the base DBU followed by metathesis process. Thermal analysis by thermogravimetric and differential thermal analysis (TG-DTA) and spectral analysis by Fourier transform infrared (FTIR), 1H NMR, 13C NMRspectrometric analyses have been performed to characterize the prepared PILs and AIL.The H-bonding between anions and cations of the AIL was explained by computational study and the spectral results (FTIR and NMR) were correlatedwith calculated spectra.The binary systems with molecular solvents like water, acetic acid, and DBU were prepared at different molar ratio. Study of physicochemical properties of these ILs and their binary systems were carried out by performing the measurement of TG-DTA, density, viscosity, refractive index, and conductivity, and analysis of spectra and particle size. The interaction between ILs and solvent molecules has been explained from variation of the degradation temperature with change in composition of the ILs in binary systems. The arrangement of ions with molecular solvents and ionic mobility of the ions of ILs in the binary systems have been discussed using the conductivity results from impedance spectroscopy. The change in density and viscosity with mole fraction of the IL provided information regarding hydrogen bonding, ion-dipole interaction, dispersion forces between the ions and molecular solvents. The FTIR (MIR and NIR) spectra indicated the presence of trapped water in IL-rich region. Particle size analysis explained different microstructures in water-rich and IL-rich region of the binary systems of the AIL with water. The ILs and their binary systems were used as catalyst and medium for the study of the kinetics of Michael addition reaction. The products of the Michael addition reaction were characterized by chemical and spectral analyses. The mechanism of the reaction has been established from the kinetic resultsand theconstituents ions of ILs and the solvent molecules were found to play important role. Kinetic results of the Michael addition reaction have been correlated with physicochemical properties such as conductivity and degradation temperature of the binary systems of ILs. The mutual relationship between the physicochemical properties has also been discussed. The kinetic results of the Michael addition reaction using different ILs (PILs and AIL) and their binary systems have been compared and contrasted. The binary systems of ILs with molecular solvents showed enhancement in the rate compared to pure ILs and conventional organic bases.The AIL showed better catalytic performance than the PIL with the same anion due to the large size of the cation. When both the cations and anions of the PIL and AIL are varied, the better catalytic performance is exhibited by the AIL for basic nature of the anion and the bulky size of the cation. Therefore, the structural changes affect the catalytic performance of ILs on organic synthesis.Thus, ILs and their binary systems with tunable physicochemical properties can have a bright prospect in the field of organic synthetic chemistry as a better catalyst and reaction media.