Functionalised diimidazolium salts: the anion effect on the catalytic ability

Abstract

aThe catalytic ability of some functionalised diimidazolium ionic liquids was tested using the Michael addition of malononitrile to t-chalcone as probe reaction. Diimidazolium salts characterized by the presence of 1-(1imidazolylmethyl)-3,5-di-[1-(3-octylimidazolylmethyl)]benzene cation and differing in the anion structure were used. Both mono-and dianions were employed and among these some chiral anions generally used as organocatalysts were taken into account. Data collected were analysed both as function of ionic liquids structure and basicity, evaluated using the Hammett basicity function. Although the use of chiral anions did not allow performing a stereochemical control of the reaction, data collected demonstrate the high catalytic ability of the functionalised salts. Indeed, high conversions and yields were obtained under mild conditions and in significantly shorter reaction times with respect to the ones so far reported for this reaction.

Basic ionic liquids for catalysis: the road to greater stability

“…This is possible with water soluble ionic liquids 45,46 but is not a universal method as many ionic liquids are insoluble or unstable in water. The Hammett function (H 0 ) 47 has been used to measure the acidity of non-aqueous systems and has been further applied to acidic 48 and basic 49,50 ionic liquids. The Hammett function relies on the relative dissociation of acids in solution.…”

“…51 The extent of dissociation of acidic indicator solutions allows the Hammett basicity (H_, also called the reverse Hammett function) of an ionic liquid to be measured via UV spectrophotometry. 48,50 This method is limited by the assumption that the indicator is fully dissociated, and by the need to dissolve the ionic liquid, making the values solvent dependant. 52 A less quantitative method is to simply add ionic liquid into indicator solutions of different pK a values and visually note any colour change.…”

“…McNeice et al stabilized ionic liquids containing a strongly basic anion by preparing binary ionic liquids, combining a relatively base stable pyrrolidinium cation with iso-propoxide and the neutral anion Figure 8). 69 The ionic liquid basicity was measured using the inverse Hammett method, 50,57 where the ability of the ionic liquid to abstract a proton from an acidic indicator is assessed, as discussed in the section 2.3. The ionic liquids showed higher Hammett basicity (H_) than pyridine and triethylamine, and similar basicity to DBU, KOH and NaOMe.…”

Quinine based ionic liquids: A tonic for base instability

“…The determination of the pH of an ionic liquid is not straightforward when it is insoluble or unstable in water. The Hammett function (H 0 ) has been used as a measure of the acidity of non-aqueous Brønsted acid systems [27,28], and has more recently been used to determine basicity in non-aqueous systems [29,30]. The Hammett basicity (H_) of the quinine ionic liquids was measured by monitoring the UV absorbance of methanolic bromophenol blue at a fixed concentration of ionic liquid.…”

Task-Specific Organic Salts and Ionic Liquids Binary Mixtures: A Combination to Obtain 5-Hydroxymethylfurfural From Carbohydrates

“…Endowing imidazolium or other ILs with catalytic moieties gives rise to the so-called Task Specific Ionic Liquids (TSILs) (Giernoth, 2010; Yue et al, 2011). A wide range of functionalities can be easily incorporated in ILs, (Chiappe and Pomelli, 2014) and in this context, we have successfully applied this approach for the obtainment of TSILs able to promote different reactions from the synthesis of aryl azides (D'Anna et al, 2008) to Michael addition (Rizzo et al, 2016a) and heterocyclic rearrangements (Rizzo et al, 2014). In particular, using acid-functionalized TSILs for the conversion of biomass can be a viable route to obtain value added chemicals under mild homogeneous conditions.…”