Imidazolium salts, ionic liquids

In our previous work [8], we rqjorted the synthesis of (2-oxo-l,3-dioxolan-4-yl)methacrylate (DOMA) finrn carbon dioxide and glycidyl methacrylate (GMA) using quaternary salt catalysts. In the present work, we studied the catalytic pra rmance of alkyhnethyl imidazolium salt ionic liquid in the synthesis of polycarbonate from the copolyraerization of CO2 with GMA. The influences of copolymerization variable like catalyst structure and reaction tenperature on the conversion of GMA and the yield of the polycarbonate have been discussed. 

Among these in situ protocols are those using ionic liquids as the solvent, or as both the solvent and the ligand. It was shown that the use of PdCOAc) in imidazolium-based ionic liquids forms in situ NHC-Pd(II) species [42], The use of methylene-bridged bis-imidazolium salt ionic liquids to form chelated complexes has also been reported [43], although better results have been obtained when Bu NBr is used as the solvent [44] and imidazolium salts were added together with PdCl in catalytic amounts [45]. Other related catalytic species such as bis-NHC complexes of silica-hybrid materials have been tested as recyclable catalysts [46,47]. 

Imidazolium-based ionic liquids (ILs) have been used extensively as media for the formation and stabilization of transition-metal nanoparticles [14—17]. These 1,3-dialkylimidazolium salts (Figure 15.3) possess very interesting properhes they have a very low vapor pressure, they are nonflammable, have high thermal and electrochemical stabilities, and display different solubilities in organic solvents [18-20]. 

The salts were investigated in the Diels-Alder reaction of crotonaldehyde with cyclopentadiene (Scheme 67). The yields obtained were between 35% and 40% with an endo. exo ratio of 90 10. The control reaction without the salt at -25 °C gave no product. The observed ee with the enantiopure salt 66 was less than 5%. Nevertheless, this was the first example which showed, that imidazolium-based ionic liquids can be used in substoichiometric amounts as Lewis acid catalysts. 

Blocking the C2 position with alkyl groups may afford C - H oxidative additions of the imidazolium salts yielding abnormal carbenes. This strategy was followed in the reaction a Pt(0) complex with C2-methylated imidazolium salts, which provided the oxidative addition of the C4,5 - H bond, as shown in Scheme 42 [158]. This behavior provides evidence that the substitution of imidazolium-based ionic liquids at the C2 may not be enough to prevent their involvement in reactions for which they are solvents. 

Certain quaternary salts of imidazoles have become important as ionic liquids, in particular salts of l-butyl-3-methylimidazolium (bmim). Preparation of imidazolium-containing ionic liquids, in the desired product ionic liquid as a solvent, is a virtually solvent-free, one-pot process proceeding in good to excellent yields <2003S2626>. There are also microwave-assisted, solvent-free processes for the preparation of imidazolium-based ionic liquids <2001CC643, 2003GC181>. 

Imidazolium-based ionic liquids, mostly the [BF4] and [PF6] salts, are commonly employed for hydrogenation reactions and it has been observed, that anions like triflate and acetate, which are reasonably good ligands, can deactivate the catalyst.[15]... 

Apart from the omnipresent imidazolium based ionic liquids, other classes of low melting salts have been successfully applied in carbon-carbon coupling reactions, notably tetraalkyl ammonium and phosphonium salts. The former include pyrrolidinium and piperidinium salts, but particularly tetrabutylammonium bromide, [(C4)4N]Br (mp. 103-105°C), which has been evaluated quite extensively in Heck reactions and a remarkable increase in reactions rates is frequently observed with this solvent. Those examples where [(C4)4N]Br acts merely as co-catalyst[2"61 rather than as reaction medium shall not be discussed here. 

Of course, the opposite has also been observed whereby an imidazolium salt oxidatively adds to a transition metal complex. [135,136]. This was first noticed in catalytic reactions involving transition metal phosphane complexes as catalysts and imidazolium based ionic liquids. Unexpected improvements in catalytic performance prompted investigations to find out whether the phosphane ligands had been replaced by more electron-rich NHC... 

A second source of amino functionalised imidazolium salts is the quest for functionalised ionic liquids [146,147]. Wan et al. used a piperidine functionaUsed imidazolium salt as a base in a palladium catalysed Heck reaction in an ionic liquid medium. The system was designed such that the phosphane Ugand, the base and the solvent were all imidazolium based ionic Uquids [148] (see Figure 3.51). Given the fact that imidazolium based ionic liquids can act as carbene precursors for coordination to transition metals [149] that coordinate better than phosphane Ugands (with cationic substituents in the present case), it is far from certain which is acmally the ligand in that carefully designed system. 

It should be mentioned that many imidazolium salts are liquids at room temperature and thus fall into the general category of ionic liquids. An excellent overview of the techniques to attach ionic liquids onto solid supports is available by Mehnert [251]. Here, the interested reader is directed to the concept that ionic liquids cannot only be attached covalently to the support, but also by simple deposition of the ionic liquid phase containing the catalyst onto the support surface. 

The palladium-catalysed arylation of olefins with aryl halides, the Heck reaction, is usually performed in polar solvents such as acetonitrile or dimethyl sulfoxide, in combination with a base and a Pd(ii) pre-catalyst that may or may not be associated with a phosphorus ligand. Given that quaternary ammonium or phosphonium salts are known to increase reaction rates, ILs emerged as promising solvents for this reaction.In the case of imidazolium-based ionic liquids, the solvent, beside providing an unusual coulombic environment. 

Miao and his coworkers developed an efficient and reusable catalytic system containing TEMPO functionalized imidazolium salt ([Imim-TEMPO]" X", Cat-la or Cat-lb), a carboxylic acid substituted imidazolium-based ionic liquid ([Imim-COOH]" X , Cat-2a or Cat-2b), and NaNO for the aerobic oxidation of alcohols with oxygen in water as solvent (Scheme 14.34) [31]. 

The interest in functionalized ionic liquids is growing because ionic liquids bearing ether, amino or alcohol functionalities have been shown to display special properties, including the ability to dissolve a larger amoimt of metal halide salts and to extract heavy metal ions from aqueous solutions. Imidazolium-based ionic liquids with ether and hydroxyl (see Section 2.2.1), thiourea, thioether and urea (see Section 2.2.8) " have been prepared following the standard quatemization procedure. A straightforward approach has been described for the preparation of imidazolium (as well as pyridinium) cations with ester, ketone or cyanide functionalities 1-methylimidazole reacts with methanesulfonic acid to provide the imidazolium salt 11, which undergoes a Michael-type reaction with methyl vinyl ketone as a ,j8-unsaturated compound to produce the ionic liquid 12 (Scheme 5). ... 

At the same time as new applications of ionic liquids are discovered on almost a daily basis, limitations of these reaction media are also uncovered. While studying the Morita-Baylis-Hillman reaction in ionic liquids, Aggarwal observed that [bmim][Cl] was deprotonated by the weak base present in the reaction mixture, leading, after reaction with benzaldehyde, to salt 111 (Scheme 49). Deprotonation of imidazolium salts with strong bases (KCyBu or NaH) is well known, providing, for example, an easy route to Pd-carbene complexes (Section 2.3.5.1). However, this observation limits the use of imidazolium-based ionic liquids even in weakly basic conditions, where they can react with electrophiles. It also explains previous works reporting low yields for reactions performed in these conditions, such as the Horner-Wadsworth-Emmons reaction in [emimlCPFe] or [emim][BF4]. ... 

---