Imidazolium salts carboxylic acid

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. 

BMIm]BF4 BMImCl [BMIm]PF6 CCS ecu [Choline] [Pro] 1 -butyl-3-methyl-imidazolium tetrafluoroborate 1 -butyl-3-methyl-imidazolium chloride 1 -butyl-3-methyl-imidazolium hexafluorophosphate CO2 capture and storage/sequestration CO2 capture and utilization (2-hydroxyethyl)-trimethyl-ammonium (iS )-2-pyrrolidine-carboxylic acid salt... 

A few years earlier, Herrmann et al. published a carboxylic ester functionalised imi-dazolium salt that was synthesised directly from imidazole and bromoacetic acid ethyl ester [216]. Owing to its method of synthesis the imidazolium salt is C -symmetric with two ester functional wingUp groups. Generation of the rhodium(I) and palladium(II) carbene complexes was realised by reaction of the imidazolium salt with a rhodium alkoxide precursor or with palladium(II) acetate in the presence of NaOEt and Nal (see Figure 3.76). The silver(I) oxide method had not been discussed in the literature at the time [11]. 

An example of a more standard, carboxyhc acid amide functionalised imidazolium salt and its apphcations come from the Ghosh group [120]. They reacted chloroacetic acid anilide with A-mesityl imidazole and obtained the respective carboxylic acid amide functionalised imidazolium salt which was reacted with sUver(I) oxide to form the corresponding silver(l) carbene complex featuring two trans-coordinated carbene ligands on the silver(I) ion (see Figure 4.39). 

Figure 4.39 A carboxylic acid amide functionalised imidazolium salt and its silver I) carbene...

Liao et al. also synthesised an imidazolium salt carrying two carboxylic acid amide functionalities and generated its silver carbene complex. It is distinguished by absolute linearity of the C-Ag-Cl vector and rather short Ag-C and Ag-Cl bond lengths. 

Carboxylic acid amide functionalised imidazolium salts can be reacted with group 10 halides directly in the presence of an inorganic base (e.g. K COj) to form homoleptic complexes (see Figure 4.41) [122]. Interestingly, the carbene units are cis to each other and not trans as would be expected (see Figures 4.30 and 4.38). The cis complexation is not aided by hydrogen bonding as the carboxylic acid amide functionality loses its proton upon complexation to nickel. 

A corresponding imidazolium salt carrying two carboxylic acid amide functionalised wingtip groups shows similar behaviour, but obviously without cis/trans isomers [123] (see Figure 4.43). 

Activation of the carboxylic acid amide proton occurs only after addition of potassium carbonate, similar to the reactivities observed with the monofunctionalised imidazolium salt in Figure 4.42. 

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. 

A very simple procedure for the preparation of amidinium salts is the condensation of carboxylic acids with excess amine in the presence of POCb. Using this method, the selfcondensation reaction of amide-POCb adducts is suppressed. The salts are usually isolated as perchlorates (132 Scheme 14). In a related reaction benzoic acid is transformed by means of iVA( -dimethylethylenediamine and TiCU to the imidazolium chloride, which was separated as the PFe salt (133). 4... 

Due to the attractivity of this method several groups have developed onium salt supported versions of classical reactions. For example, starting from hydroxyl derived imidazolium salts, formation of supported acrylates with acryloyl chloride followed by reaction with diene in refluxing toluene afforded Diels Alder adduct in good yields (>65%). After saponification, products are isolated without further purification [127], Alternatively, starting from carboxylic acid derived imidazolium salts, acyl chloride formation with thionyl chloride in acetonitrile, followed by reaction with 4-aminophenol led to supported N-arylamide. Williamson alkylation using NaOH as a base and subsequent cleavage from the onium salt support under acidic condition (HCI/I I2()/ AcOH) allowed for isolation of various alkoxy substituted anilines with >98% purity... 

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 alkylammonium nitrate salts and salts containing the l-alkyl-2-alkylimidazolium cation with chloride or bromide anions were mostly molten salts, in that their melting points were >100 °C. In contrast, the PILs that contained alkylammonium cations with small carboxylic acids and most of the salts with an imidazolium cation were predominately RT-PILs, or had low melting points. 

Lin, J.C.Y., Huang, C.-J., Lee, Y.-T., Lee, K.-M. and Lin, I.J.B., Carboxylic acid functionalized imidazolium salts Sequential formation of ionic, zwitterionic, acid-zwitterionic and lithium salt-zwitterionic liquid crystals, J. Mater. Chem. 21 (22), 8110-8121 (2011). 

Fig. 14. General scheme of one pot synthesis of carboxylic acid functionalized imidazolium salts by Michael addition...

Fig. 15. Preparation of carboxylic acid functionalized imidazolium salts using HBF4 and HPFe...

---