Synthesis and applications of ionic liquids derived from natural sugars

Aiming to develop environmentally compatible chemical syntheses, the replacement of traditional organic solvents with ionic liquids (ILs) has attracted considerable attention. ILs are special molten salts with melting points below 100 degrees C that are typically constituted of organic cations (imidazolium, pyridinium, sulfonium, phosphonium, etc.) and inorganic anions. Due to their ionic nature, they are endowed with high chemical and thermal stability, good solvent properties, and non-measurable vapor pressure. Although the recovery of unaltered ILs and recycling partly compensate their rather high cost, it is important to develop new synthetic approaches to less expensive and environmentally sustainable ILs based on renewable raw materials. In fact, most of these alternative solvents are still prepared starting from fossil feedstocks. Until now, only a limited number of ILs have been prepared from renewable sources. Surprisingly, the most available and inexpensive raw material, i.e., carbohydrates, has been hardly exploited in the synthesis of ILs. In 2003 imidazolium-based ILs were prepared from o-fructose and used as solvents in Mizoroki-Heck and Diels-Alder reactions. Later on, the first chiral ILs derived from sugars were prepared from methyl D-glucopyranoside. In the same year, a family of new chiral ILs, obtained from commercial isosorbide (dianhydro-D-glucitol), was described. A closely related approach was followed by other researchers to synthesize mono- and bis-ammonium ILs from isomannide (dianhydro-D-mannitol). Finally, a few ILs bearing a pentofuranose unit as the chiral moiety were prepared using sugar phosphates as glycosyl donors and 1-methylimidazole as the acceptor.