Syntheses and applications of ionic liquids as solvents and reactants : natural substances dissolution, esterification ionic tagging (2012)

The present thesis deals with the applications of ionic liquids (ILs), especially carboxylate-based ILs. The first part describes the syntheses and uses of ILs as solvents for natural compounds to dissolve cellulose and to extract betulin. The second part reveals their applications as reactants for esterification and ionic tagging. Dissolution of cellulose allows easier processing of this important biogenic feedstock. For this, ILs have been proposed. To foster understanding of the structure activity relationship of IL and cellulose and to provide mandatory data for computer-aided cellulose solvent design, a systematic approach towards the synthesis and experimental determination was carried out and is described in Chapter 2. Three homologous series of all 27 combinations of the three cations: 1,3-dimethylimidazolium ([DMIm]), 1-ethyl-3-methylimidazolium ([EMIm]), and N,N-diethyl-N,N-dimethylammonium ([DEDMN]) with nine carboxylate anions were synthesized and characterized. Cellulose solubilities in seventeen synthesized ILs were determined. Alternatives were found to the most widely studied cellulose solvent [EMIm]Ac. Generally, the cellulose solubilities for carboxylate-based ILs with imidazolium cations were found to be in the same range, whereas those with the quaternary ammonium cation were found to be poor solvents for cellulose. Anions with an internal hydrogen bond had no detectable cellulose solubility. Moreover, anions with high beta values exhibited good solubilities, which confirmed that anion basicity was an important factor for dissolving cellulose. After addition of water to the cellulose-IL solution, the cellulose precipitated and the IL was readily recovered. The pretreated cellulose was shown to be more degradable with organic acid. Furthermore, the regenerated IL showed no apparent change when compared to the fresh one, showing good thermal and chemical stability. Moreover, carboxylate-based ILs were shown to be good solvents for the extraction of betulin from birch bark. Based on the unexpected reaction of [EMIm]Ac with dichloromethane, ILs as reactants for esterification and ionic tagging were developed, detailed in Chapter 3. The esterification of carboxylate-based ILs and alkyl halides was performed with moderate to excellent yields (33-99%) for a large substrate scope under neat conditions. Its SN2 mechanism provided a potential application in the configuration inversion of chiral alcohols. Additionally, chiral carboxylate-based ILs were tested for stereoselective esterifications with racemic electrophiles by kinetic resolution. In the preliminary results, 2-octyl lactate was obtained with a diastereoselectivity (d.r. = 57/43). Due to the high polarity of ILs, weakly polar products were readily separated via decantation or extraction. Furthermore, it was demonstrated that the ILs could be recovered into the reactants by three processes (the anion exchange resin method, the recyclable base DBU process, and the anion metathesis method). The disclosed reaction of 1-ethyl-3-methylimidazolium cation and chloromethyl acetate, which was yielded from the esterification of acetate and dichloromethane, offered a possibility to functionalize imidazolium cation at the 2-position with formaldehyde or ROCH2X compounds. A series of N-alkyl-N'-alkyl-2-hydroxymethyl- imidazolium cations was synthesized including a polymer containing ionic liquid-type structures. When a zwitterion N-alkyl-N'-alkylimidazolium-2-carboxylate as the starting material was employed, 2-hydroxymethylimidazolium-based ILs with various anions were obtained easily and products were separated readily. Furthermore, the resulting N-alkyl-N'-alkyl-2-hydroxymethylimidazolium cations were successfully grafted on two phosphorus ligands as ionic tags to facilitate catalyst separation in transition metal catalysis in ILs. Their effectiveness to reduce metal leaching was demonstrated.