Imidazolium systems

Palladium catalysts enable the dimerization of functional alkynes with selective head-to-tail coupling in the presence of bulky phosphine ligands, for both homocoupling of terminal alkynes or cross-coupling of mono and disubstituted alkynes [4, 6]. On the other hand, a palladium/imidazolium system gives linear (E)-enynes as the predominant products [7]. 

Lee HN, Singh NJ, Kim SK, et al. New imidazolium systems bearing two pyrene groups as fluorescent chemosensors for anions and anion induced logic gates. Tetrahedron Lett 2007 48 69-172. 

It is difficult to find neat ILs that work in Li-batteries as well as the ILs introduced in Sections 14.3.1 and 14.3.2. By neat it is meant that the IL contains only the supporting electrolyte and no additive. The non-imidazolium system, the quaternary asymmetric ammonium system, and the pyrazolium system can be regarded as neat examples of ILs that are well-suited to Li-batteries. 

Soluble supported catalysts of this class are also accessible. Thus, various thia-zolium, imidazolium and triazolium structures have been recently immobilized on PEG via dick chemistry [365], Thiazolium resins (151) were the most efficient catalysts (10mol.%) for the intramolecular Stetter reaction. Compound 154 (Scheme 10.23) was obtained in 81% with the supported catalyst whilst an 84% yield was obtained when the non-supported analog was employed. Imidazolium systems like 152 were also able to promote the carbene-catalyzed diastereoselective redox esterification. In this case, 155 was obtained with higher yields (77%) and stereoselectivities (18 1 E Z ratio) than when using the non-supported system (63% and 11 1). This can be associated with the positive influence of steric hindrance on the results of this reaction. 

Ionic liquids containing other classes of organic cations are known, and in addition to pyridinium and imidazolium systems, quaternary ammonium, phospho-nium, pyrrolidinium, and sulfonium cations combined with a variety of anions (Fig. 3.1-1 provides some common examples) have been reported and have been studied for applications in electrochemistry [7,8] and synthesis [9-11]. 

C11BTI-AICI3 system starts at X(AlCl3) = 0.45 and ends up to 0.55. Although the liquidus range of the C11BTI-AICI3 system is very low as compared to the imidazolium system, there is a eutectic point, which may be useful for different applications. 

Up to the present the principal interest in heteroaromatic tautomeric systems has been the determination of the position of equilibrium, although methods for studying fast proton-transfer reactions (e.g., fluorescence spectroscopy and proton resonance ) are now becoming available, and more interest is being shown in reactions of this type (see, e.g., references 21 and 22 and the references therein). Thus, the reactions of the imidazolium cation and imidazole with hydroxyl and hydrogen ions, respectively, have recently been demonstrated to be diffusion controlled. ... 

Another means of in situ metal-carbene complex formation in an ionic liquid is the direct oxidative addition of the imidazolium cation to a metal center in a low oxidation state (see Scheme 5.2-2, route b)). Cavell and co-workers have observed oxidative addition on heating 1,3-dimethylimidazolium tetrafluoroborate with Pt(PPli3)4 in refluxing THF [32]. The Pt-carbene complex formed can decompose by reductive elimination. Winterton et al. have also described the formation of a Pt-car-bene complex by oxidative addition of the [EMIM] cation to PtCl2 in a basic [EMIM]C1/A1C13 system (free CP ions present) under ethylene pressure [33]. The formation of a Pt-carbene complex by oxidative addition of the imidazolium cation is displayed in Scheme 5.2-4. 

In the light of these results, it becomes important to question whether a particular catalytic result obtained in a transition metal-catalyzed reaction in an imidazolium ionic liquid is caused by a metal carbene complex formed in situ. The following simple experiments can help to verify this in more detail a) variation of ligands in the catalytic system, b) application of independently prepared, defined metal carbene complexes, and c) investigation of the reaction in pyridinium-based ionic liquids. If the reaction shows significant sensitivity to the use of different ligands, if the application of the independently prepared, defined metal-carbene complex... 

In the author s group, much lower-melting benzenesulfonate, tosylate, or octyl-sulfate ionic liquids have recently been obtained in combination with imidazolium ions. These systems have been successfully applied as catalyst media for the biphasic, Rh-catalyzed hydroformylation of 1-octene [14]. The catalyst activities obtained with these systems were in all cases equal to or even higher than those found with the commonly used [BMIM][PF6]. Taking into account the much lower costs of the ionic medium, the better hydrolysis stability, and the wider disposal options relating to, for example, an octylsulfate ionic liquid in comparison to [BMIM][PF6], there is no real reason to center future hydroformylation research around hexafluorophosphate ionic liquids. 

As well as phosphorus ligands, heterocyclic carbenes ligands 10 have proven to be interesting donor ligands for stabilization of transition metal complexes (especially palladium) in ionic liquids. The imidazolium cation is usually presumed to be a simple inert component of the solvent system. However, the proton on the carbon atom at position 2 in the imidazolium is acidic and this carbon atom can be depro-tonated by, for example, basic ligands of the metal complex, to form carbenes (Scheme 5.3-2). 

We initially tested Candida antarctica lipase using imidazolium salt as solvent because CAL was found to be the best enzyme to resolve our model substrate 5-phenyl-l-penten-3-ol (la) the acylation rate was strongly dependent on the anionic part of the solvents. The best results were recorded when [bmim][BF4] was employed as the solvent, and the reaction rate was nearly equal to that of the reference reaction in diisopropyl ether. The second choice of solvent was [bmim][PFg]. On the contrary, a significant drop in the reaction rate was obtained when the reaction was carried out in TFA salt or OTf salt. From these results, we concluded that BF4 salt and PFg salt were suitable solvents for the present lipase-catalyzed reaction. Acylation of la was accomplished by these four enzymes Candida antarctica lipase, lipase QL from Alcaligenes, Lipase PS from Burkholderia cepacia and Candida rugosa lipase. In contrast, no reaction took place when PPL or PLE was used as catalyst in this solvent system. These results were established in March 2000 but we encountered a serious problem in that the results were significantly dependent on the lot of the ILs that we prepared ourselves. The problem was very serious because sometimes the reaction did not proceed at all. So we attempted to purify the ILs and established a very successful procedure (Fig. 3) the salt was first washed with a mixed solvent of hexane and ethyl acetate (2 1 or 4 1), treated with activated charcoal and passed into activated alumina neutral type I as an acetone solution. It was evaporated and dried under reduced... 

We investigated lipase-catalyzed acylation of 1-phenylethanol in the presence of various additives, in particular an E. additive using diisopropyl ether as solvent. Enhanced enantioselectivity was obtained when a BEG-hased novel IE, i.e., imidazolium polyoxyethylene(lO) cetyl sulfate, was added at 3-10 mol% vs. substrate in the Burkholderia cepacia lipase (hpase PS-C) catalyzed transesterification using vinyl acetate in diisopropyl ether or a hexane solvent system. ... 

Allylic substitutions catalysed by palladium NHC complexes have been studied and the activity and selectivity of the catalysts compared to analogous Pd phosphine complexes. A simple catalytic system involves the generation of a Pd(NHC) catalyst in situ in THF, from Pdj(dba)j, imidazolium salt and Cs COj. This system showed very good activities for the substitution of the allylic acetates by the soft nucleophilic sodium dimethyl malonate (2.5 mol% Pdj(dba)3, 5 mol% IPr HCl, 0.1 equiv. C (CO ), THF, 50°C) (Scheme 2.22). Generation of the malonate nncleophile can also be carried out in situ from the dimethyhnalonate pro-nucleo-phile, in which case excess (2.1 equivalents) of Cs COj was used. The nature of the catalytic species, especially the number of IPr ligands on the metal is not clear. 

In situ derived systems, in general, performed similarly to preformed complexes, in telomerisation of butadiene with MeOH, Tables 4.1 and 4.2 [68,70,71,77,78], In situ systans may be generated from free NHC or from imidazolium salt in combination with an appropriate Pd(0) or Pd(ll) source. Typically, 2-4 equivalents of imidazolium salt relative to Pd have been nsed [68,70,77], In situ catalysts derived from mono- and bis-Fc-snbstituted (Fc = ferrocenyl) imidazohnm and benzimidazolium salts (64-68) (Table 4.2) showed interesting telomerisation activities ascribed to the steric bulk of the Fc substituents [70]. Unsymmetrical salts 65 and 66 bearing A -Fc and A -Me... 

Related reactions, that have been catalysed by NHC/Pd systems, are the intramolecular Mizoroki-Heck using catalysts formed in situ from imidazolium salts and a Pd(0) source [69], and the arylation of allylic alcohols by a benzothiazole-Pd complex [70,71] (Scheme 6.14). 

The use of imidazolium salts for in situ catalyst formation was shown to be optimal for the coupling of TMS-protected alkynes even with sterically demanding aryl bromides and avoids the formation of homocoupling-derived products. For this reaction, Nolan reported that the activation of chlorobenzene by this catalytic system was possible in moderate yield [125] (Scheme 6.41). 

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