Imidazolium salts ethers

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... 

The hydrosilylation of carbonyl compounds by EtjSiH catalysed by the copper NHC complexes 65 and 66-67 constitutes a convenient method for the direct synthesis of silyl-protected alcohols (silyl ethers). The catalysts can be generated in situ from the corresponding imidazolium salts, base and CuCl or [Cu(MeCN) ]X", respectively. The catalytic reactions usually occur at room tanperature in THE with very good conversions and exhibit good functional group tolerance. Complex 66, which is more active than 65, allows the reactions to be run under lower silane loadings and is preferred for the hydrosilylation of hindered ketones. The wide scope of application of the copper catalyst [dialkyl-, arylalkyl-ketones, aldehydes (even enoUsable) and esters] is evident from some examples compiled in Table 2.3 [51-53],... 

Dixneuf used [RuCl2(/>-cymene)]2 as a catalyst for the reaction of enyne 72a in the presence of imidazolium salt and CS2CO3 and obtained the enyne metathesis product 73a in a high yield. The enyne silyl ether 72b is converted under similar reaction conditions into r/i7ra-compound 73b which after the Tamao oxidation gives diol 74 (Scheme 28). In this reaction, V-heterocyclic carbene should be generated to coordinate to the ruthenium metal, but the actual species for this reaction is not well documented. 

Yang et al. used a similar protocol (an ether functionality supported on a primary alkyl halide carrier) to introduce an acetal on either side of the imidazole ring generating an ether functionalised ionic liquid (IL) imidazolium salt [183] (see Rguie 3.58). The anion could be varied without loss of the IL property (melting point below 1(X) °C) [184]. Synthesis of the transition metal carbene complexes (palladium) was done by carbene transfer ftom the corresponding silver(I) complexes or by reaction with the metal acetate (nickel) [162] (see Figure 3.64). 

By far more interesting are polyether functionalised imidazolium salts, especially those that lead to metallacrown ether functionalised carbene ligands. Development of this particular ligand class follows the by now familiar pattern of adapting an existing functionalised phosphane to the corresponding carbene. Here, the favourable properties of metallacrown ether functionalised phosphanes in catalytic processes [202-205] leads to the development of similarly functionalised NHC ligands. 

Figure 3.71 Synthesis of a concave, ether functionalised imidazolium salt.

Figure 3.72 Formation of transition metal carbene complexes with the concave ether functionalised imidazolium salt shown in Figure 3.71.

Synthesis of an ether linked bis-imidazolium salt and its transition metal carbene... 

Naturally, it is possible to synthesise a similar ligand system without central chirality and in fact without the unnecessary methylene linker unit. A suitable synthesis starts with planar chiral ferrocenyl aldehyde acetal (see Figure 5.30). Hydrolysis and oxidation of the acetal yields the corresponding carboxylic acid that is transformed into the azide and subsequently turned into the respective primary amine functionalised planar chiral ferrocene. A rather complex reaction sequence involving 5-triazine, bromoacetal-dehyde diethylacetal and boron trifluoride etherate eventually yields the desired doubly ferrocenyl substituted imidazolium salt that can be deprotonated with the usual potassium tert-butylate to the free carbene. The ligand was used to form a variety of palladium(II) carbene complexes with pyridine or a phosphane as coligand. 

Carbene complexes have been synthesized by various CH activation routes. For example, amines and ethers can undergo double geminal CH activation to yield a Fischer-type (heteroatom substituted) carbene and an imidazolium salt can yield iV-heterocyclic carbenes directly in the same way. In the latter case, abnormal binding via C-4 can occur as well as the usual C-2 binding. 

The 1-alkylimidazole (10 mmol) in DMF, methanol, aeetonitrile or dichloromethane (4ml) and the alkylating agent (lOmmol) in a screw-cap sealed reaction vessel are kept at 20°C (72h) or heated at I00°C (3h). Removal of the solvent and washing with dry ether (3x5 ml) gives the imidazolium salt. 

Also in 2013, Kuriyama and co-workers developed a series of ether-imidazolium salts 94 for the construction of (poly)heterobiaryls3 > Pd(OAc)2/94 proved to be the most effective catalyst system, coupling various heteroaryl chlorides and (hetero)aryl-boronic acids in good yields (Scheme 26). 

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). ... 

These Diels-Alder reactions were performed in dichloromethane, but the efficiency of imidazolium salts as Lewis acid catalysts suggested that ionic liquids could improve the rate of such transformations. Indeed, Diels-Alder reactions result in greater stereoselectivity when performed in a 5M solution of LiClOq in diethyl ether. 

N,N -Thiocarbonyldiimidazole stirred and allowed to react with methoxylamine in anhydrous ether, the resulting imidazolium salt heated to reflux in acetone, and the product isolated after 5 min. N-methoxythiocarbamoylimidazole. Y ca. 100%. F. e. s. U. Anthoni, C. Larsen, and P. H. Nielsen, Acta Chem. Scand. 22,1050 (1968). 

Another approach, particularly valuable for the synthesis of imidazolium salts containing sterically hindered substituents, for example, multiple-substituted phenyl groups at the nitrogen atoms, consists of a two-step protocol (route D) [28]. Under mild conditions, lequiv of glyoxal is treated with 2equiv of arylamine in -propanol to form the corresponding diimine as an intermediate. Subsequent cyclization by condensation with chloromethylethyl ether leads to the desired diarylimidazolium salt in moderate yields (40 -47%) without the need of a purification step. 

By use of TV-(trifluoroacetyl)- or TV-(trichloroacetyl)-imidazoleare obtained symmetric aliphatic and aromatic anhydrides even from carboxylic acids that do not form insoluble salts in benzene, ether, or THF (Table 13—1). In this case the acid is treated with the imidazolide in a 2 1 molar ratio, and an insoluble imidazolium trifluoro- or tri-chloroacetate is formed. 

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