Imidazohum salts, ionic liquids

The salts catalyzed the Diels-Alder reaction of crotonaldehyde with cyclopenta-diene (Scheme 16.42). The obtained yields were 35-40% 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 49 was less than 5%. However, this was the first example to show that imidazohum-based ionic liquids can be used in substoi-chiometric amounts as Lewis acid catalysts. 

Moreover Fuchigami s group found that a combination of Et NF /iHF ( = 4,5) and imidazohum ionic liquids was highly effective for anodic fluorination of phtha-lides [26]. As shown in Table 8.1, conventional electrolysis in organic solvents resulted in low yields. Since the oxidation potential of phthaHde is extremely high (2.81 V vs. SCE), the oxidation of solvents takes place preferentially or predominantly (in DME). Even under solvent-free conditions (in liquid fluoride salts solely), the yield is also low due to simultaneous oxidation of the fluoride salts during the electrolysis. In sharp contrast, when ionic liquid, l-ethyl-3-methylimidazo-lium trifluoromethanesulofonate ([EMIM][OTf]) is used, the yield increased markedly. The combination of [EMIM][OTf] and EtaN-SHF is the best choice to... 

Thermal Stability According to a literature search, it is known that ionic liquids are thermally stable (300°C to 400°C). The imidazohum salts are reported to be thermally stable up to 300°C. Above 300°C, there is a cleavage of the C-N bond of the imidazolium nitrogen and the carbon of the alkyl chain (57, 66-68). It was found that different anions also affect the thermal properties of the ionic hquids (66-68). Ionic liquids with less nucleophilic or coordinating anions show a higher thermal stability (66-69). 

Without special drying procedures and completely inert handling, water is omnipresent in ionic liquids. Even the apparently hydrophobic ionic liquid [BMIM][(CF3S02)2N] saturates with about 1.4 mass% of water [15], which is a significant molar amount. For more hydrophilic ionic hquids the water uptake from air can be much higher. Imidazohum hahde salts in particular are known to be extremely hygroscopic, which is one of the reasons why it is so difficult to make completely proton-free chloroaluminate ionic hquids. 

Ionic liquids (ILs) were introduced in catalysis by Yves Chauvin in the 1990s [33a] and have received considerable attention in this field [33b,cj. Yves Chauvin introduced the imidazolium salts that are the most frequently used ILs in catalysis. ILs are valuable media for catalysis with PdNPs because the substituted imidazolium cation is bulky, favoring the electrosteric stabilization of NPs, as do the t-Bu N salts in Fig. 1.1. The size of the cation (that can eventually be tuned by the choice of the N-alkyl substituents) also has an important influence on the stabilization, size and solubUity of the NPs, these factors playing a role in catalysis. ILs are also non-innocent, however, as they readily produce Pd-N-heterocyclic carbene complexes upon deprotonation of the imidazoHum salt at sufficiently high temperature. Thus, these carbene ligands can be bound to the NP surface or give mononuclear mono- or bis-carbene complexes subsequent to leaching of Pd atoms from the PdNP surface (vide infra) [33]. 

In this review, the use of N-heterocyclic carbene complexes derived from imidazoHum salts as a part of the different strategies involving ionic liquids has not been considered. Nevertheless, it can be important to take into account this reactivity, especially when 1,3-dialkylimidazolium ILs are involved. For more details about this aspect and applications in hydroformylation, one can refer to Herrmanrfs recent review [55] and the later publication of Poyatos et al. in 2003 [56]. 

Preliminary mechanistic studies by Fernandez, Peris, and Crudden provided evidence that, under smooth hydroformylation conditions, NHCs remain ligated in the coordination sphere of rhodium [55, 68]. A similar conclusion was drawn by Scholten and Dupont for hydroformylation in ionic liquids [69]. A serious problem sometimes faced is the reductive elimination of carbene ligand from the metal as imidazohum salt under the effect of H2, as found in hydrogenation reactions [70]. 

New concepts or reaction media may also result in serious effects toward catalysis. Thus, the use of ionic liquids on the basis of C -unsubstituted imidazohum salts as solvents for the palladium-catalyzed methoxycarbonylation of iodobenzene leads to protolysis of the Pd-phenyl intermediate and the undesired formation of benzene [41]. Chlorobenzene is also formed in the presence of imidazohum chlorides. This explains why phosphites added did not show modifying properties, because they are... 

Ionic liquids are salts that have, per definition, a melting point below 100 °C. If their melting point is below room temperature they are called room temperature ionic liquids (RTIL). The latter have attracted much interest in recent years as novel solvents for reactions and electrochemical processes [116]. Some of these liquids are considered to be green solvents [117]. The most commonly used liquids are based on imidazoHum cations like 1-butyl-3-methylimidazolium [bmim] with an appropriate counter anion like hexafluorophosphate [PFs]. Many ionic liquids are known to accelerate reactions. In most cases, achiral ionic liquids are applied and have been reviewed [116], Here, the few examples of chiral ionic liquids (CILs) as catalysts are discussed. 

Howarth et al. [123] published in 1997 a report on the preparation of ionic liquids 48 and 49. They showed that imidazoHum cations can be used as Lewis acid centers in catalytic amount rather than as solvent (Scheme 16.41). The bromide salts 48a and 49 were prepared by a literature procedure [124] from TMS protected imidazole 47 and ethyl bromide or (S)-l-bromo-2-methylbutane in refluxing toluene in 46% and 21% yield, respecHvely. Salt 48a was converted into salt 48b with AgCFsGOO in 89% yield. 

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