Protic imidazolium ionic liquid

The dimerisation of methyl acrylate is an important reaction since the product, dimethyl dihydromuconate, represents a useful precursor for speciality chemicals such as cyclopentenones, as well as for the production of nylon-6,6. The synthetic challenge is to avoid formation of head-to-tail dimerisation products. The tail-to-tail dimerisation of methyl acrylate with palladium catalysts has been evaluated in protic and common imidazolium ionic liquids.17 21"231... 

Tail-to-tail dimerisation of methyl acrylate has been investigated with a rhodium catalyst in protic and neutral imidazolium ionic liquids.171 While acidic ionic liquids showed some advantage with palladium catalysts (vide supra), reaction rates with Rh(Cp )(C2H4)2 as catalyst were much lower in [HC4im][BF4] and [HCiim][BF4] relative to those in [C4Ciim][BF4], It was suggested that this was due to coordination of imidazole to the rhodium centre. 

Protic imidazolium and alkoxyimidazolium based ionic liquids as both solvents and Bronsted catalysts for catalytic reactions have been synthesized (Fig. 9) [58],... 

In general, for the proposed new solvents , it can be observed that the dipolarity/ polarisability and basicity are similar to those of AN and DMF. The acidity is quite different being highly increased, particularly in the case of AN. In relation to the protic solvent + imidazolium-based ILs binary solvent mixtures, the compositions of the mixtures are dependent on both the nature of the protic solvent as well as on the anion type of the ionic liquid. With ethanol and 2-ethoxyethanol as molecular solvents, the IL displays large solubility, particularly when the anions are [BF ] or [Cl]. The ILs [bmim][PF ] and [bmim][Br] exhibit the lowest solubility, so their mixtures were explored at the smallest range of compositions. 

Where solubility alone is the issue, simply changing solvent to permit all species to be dissolved allows the chemistry to proceed essentially as it would in aqueous solution were species soluble. Typical molecular organic solvents used in place of water include other protic solvents such as alcohols (e.g. ethanol), and aprotic solvents such as ketones (e.g. acetone), amides (e.g. dimethylformamide), nitriles (e.g. acetonitrile) and sulfoxides (e.g. dimethylsulfoxide). Recently, solvents termed ionic liquids, which are purely ionic material that are liquid at or near room temperature, have been employed for synthesis typically, they consist of a large organic cation and an inorganic anion (e.g. lV, lV,-butyl(methyl)-imidazolium nitrate) and their ionic nature supports dissolution of, particularly, ionic complexes. 

To show their suitability as replacements for conventional imidazolium-based ionic liquids the authors demonstrated that these protic ionic liquids performed well as solvents for the Heck reaction. The alcohol functionality also has the benefit that it can be used to tether substrates, allowing these ionic liquids to be used as recyclable homogenous supports, an alternative to traditional polymer resins [61]. 

The conjugate addition of indoles to electrophilic alkenes has been known for many years with early examples including methyl vinyl ketone [203] and nitroethylene [204]. A range of new catalysts have been explored. These include a variety of protic and Lewis acids. There has also been exploration of various supported catalysts. Both protic and Lewis acids have also been used in conjunction with ionic liquids, usually imidazolium salts. Many of these studies have been carried out with 1,3-diarylpropen-1-ones as the reactants, but some also include enones such as methyl vinyl ketone and cyclic enones. Chiral catalysts can give enantio-selective additions. Much of the work with chiral catalysts has been summarized by Bandini, MeUoni, Tommasi and Umani-Ronchi [205]. For the most part, the successful reactions have used either enones or nitroalkenes. There are few reports, for example, of addition to acrylate esters. 

Since the rate enhancement of MBH reaction in protic solvents or by use of LBBA bifunctional catalysts and Lewis acids as co-catalysts is attributed to the involvement of hydrogen bonding, it seemed that imidazolium-based ionic liquids would be a good choice of solvent for the MBH reaction because the acidic proton at C(2) is known to act as a donor to hydrogen bond acceptors. ... 

Influence of Protic Additives on the ORR in Imidazolium-Based Ionic Liquids... 

The role of additives with different proton activities in the mechanisms of the ORR have been reported in the literature for several imidazolium-based ionic liquids, and they are in concordance with those of previously reported on aprotic solvents (e.g. DMF, DMSO, ACN) upon addition of protic additives [13]. As an example. 

As mentioned before, the first ORR mechanism for ionic liquids reported in the literature was extrapolated from the literature on aprotic solvents. However, the role of protic additives in the ORR in the case of ILs was not fully understood leading to some discrepancies in the mechanisms depicted in Schemes 18.3 and 18.4 for similar imidazolium-based ILs. 

The influence of protic additives in the ORR has been extensively studied in numerous ionic liquids and it is normally detrimental to the stability of the superoxide anion under such conditions. In fact, early research work in the literature reported the instability of superoxide in imidazolium-based ionic liquid which was in fact due to the acidic moieties present in such ILs. 

Many reviews had shown the importance of IL such as in supercritical fluid applications by Seda Keskin et al. [54]. Measurements of thermochemical properties of imidazolium-based ionic liquid (ILs) carried out has been reported [55]. Tamar L. Greaves et al. [56] explained the properties and applications of protic ILs. Revisiting characteristics of ILs are explained by Rusen Feng [57]. Recently, room-temperature ILs are being used as lubricants [58], i.e., tribology, because ILs possess excellent properties such as non-volatihty, nonflammabiUty, and thermo-oxidative stability. 

ILs used in self-assembly systems can be divided in two categories protic ionic liquids (PILs, solvent properties are similar to water) and aprotic ionic liquids (AILs, largest group most of them derived from disubstituted imidazolium cations). Many surfactant ILs have thermotropic properties, forming liquid crystal phases with no additional solvent present. The solvophobic interaction between PILs and... 

Picquet M, Tkatchenko I, Tommasi I, Wasserscheid P, Zimmermann J (2003) Ionic liquids, 3. Synthesis and utilisation of protic imidazolium salts in homogeneous catalysis. Adv Synth Catal 345 959-962... 

Aggregation behavior in water of new imidazolium and pyridinium alkylcarboxylates protic ionic liquids. /, Coll. Inter. Scie., 340, 104-111, ISSN 0021-9797... 

---