Dialkylimidazolium synthesis

Room-temperature ionic liquids, salts with A,A-dialkylimidazolium cations in synthesis and catalysis 99CRV2071. 

The physical properties of ionic liquids have been extensively studied and some trends are beginning to emerge. In particular, ionic liquids based on 1,3-dialkylimidazolium cations have been investigated in detail, partly due the their wide use as solvents to conduct synthesis and catalysis. The attraction of the imidazolium cation in synthetic applications is because the two substituent groups can be varied to modify the properties of the solvent. For example, Table 4.1... 

Recent Applications of the Kolbe-Schmitt Carboxylation Synthesis of l,3-Dialkylimidazolium-2-Carboxylates... 

In comparison, the l,3-dialkylimidazolium-2-carboxylate isolated by Tommasi et al. [27, 40] was revealed to be a more versatile catalyst that allowed the synthesis of benzoylacetic acid from benzophenone and C02 in good yield and under mild conditions (isolated yield 81%). The presence of tetrafluoroborate- or tetraphenyl-borate sodium salts in the reaction was essential, as this allowed the formation of the related 1,3-dialkylimidazolium tetrafluoroborate or tetraphenylborate and the concomitant quantitative trans-carboxylation to sodium benzoylacetate. Likewise, compounds such as acetone, cyclohexanone, and phenylacetonitrile could also be converted with this system to afford the corresponding carboxylate salts (methyl a-cyanophenylacetate) (Scheme 5.8). Following the same general procedure, acetone was carboxylated, being simultaneously the cosolvent and reagent. 

Another method involves microwave irradiation. It has been described for the synthesis of 1,3-dialkylimidazolium tetrachloroaluminates [40]. This method precludes the use of volatile organic solvents and is faster, more efficient and also ecofriendly, affording high yields of the desired products. 

Synthesis of 1,3-dialkylimidazolium ionic liquids does not require very much effort. One can start from commercially available 1-methylimidazole for a one-step or imidazole for a two-step synthesis (Scheme 3). 

Quek S K, LyapkaloHuynh 1 M, Huynh H V. Synthesis and properties of NdV -dialkylimidazolium bis(nonafluorobutane-l-sulfonyl)imides A new subfamily of ionic liquids. Tetrahedron. 2006. 62, 3137-3145. 

Scheme 5 Synthesis of 1,3-dialkylimidazolium salts from the dialkylimidazolium carboxylate...

Holbrey JD, Reichert WM, Swatloski RP et al (2002) Efficient, halide free synthesis of new low cost ionic liquids 1,3-dialkylimidazolium salts containing methyl- and ethyl-sulfate anions. Green Chem 4 407- 13... 

The work from Sheldon s group [10] was the first to present the use of ionic liquids in the enzymatic synthesis of esters. Since then, there have been many reports on biosynthesis of esters in ionic liquids. De los Rios et al. [64,65] synthesised a wide range of aliphatic organic esters, commonly used in the perfumery, flavour and pharmaceutical industries, by transesteriflcation from vinyl esters and alcohols catalysed by free CaLB in different 1,3-dialkylimidazolium-based ILs (Fig. 7.2). They analysed the effects of the alkyl chain lengths of the acyl donor and the alcohol. The optimum (C6 for acyl donor and C4 for alcohol) chain lengths were found because the activity decreased with further increase in alkyl chain length. The authors attributed the enzyme behaviour to a substrate modulation mainly due to the different affinity of the lipase towards the different substrates and steric hindrance and denaturalisation by small alcohol molecules. 

The most common ionic liquids include alkylammonium, alkylphosphonium, A/-alkylpyridinium, and A/A/ -dialkylimidazolium cations. Two general methods for their preparation are acid-base neutralization reactions and metathesis of halide salts with a metal or ammonium salts. Alkylammonium, pyridinium, and imidazo-lium halides can be prepared by the reaction of the appropriate alkyl halide and amine. Preparation of l-ethyl-3-methylimidazolium chloride [emimjCl requires a sealed tube since it has a low boiling point. On the other hand, synthesis of [bmim] Cl can be achieved under conventional reflux conditions [33, 34]. 

The nature of both the cation and the anion determine the melting point of the ionic liquid. For example, if 1,3-dialkylimidazolium-based ionic liquids are to be liquids at room temperature the cation should be unsymmetrical [11]. Of all the ionic liquids, 1,3-dialkylimidazolium-based ionic liquids are most widely used in synthesis and catalysis. 

Not surprisingly, these materials are very popular and enjoy a plethora of applications in various domains of the physical sciences, and an impressive number of spedahzed reviews and books has appeared dealing with their synthesis, physicochemical properties and appHcations in synthesis, catalysis and separation processes [12-26]. This section does not intend to be comprehensive on the vast area of synthesis and appHcations of ILs rather it will attempt to provide a critical update of the basic principles and latest developments on the structure and properties of ILs (mainly those based on the 1,3-dialkylimidazolium cation), and their... 

As already pointed out, of the various known ILs, those derived from the combination of the 1,3-dialkylimidazolium cation with various anions are the most popular and investigated class (Scheme 3.5-9). This is most probably due to their facility of synthesis stability and the possibility of fine-tuning their physico-chemical properties by the simple choice of the N-alkyl substituents and/or anions (Table 3.5-3). 

The preparation of these salts is usually performed by a simple N-alkylation of 1-methylimidazole generating a l-alkyl-3-methylimidazolium cation followed by anion metathesis [27]. The synthesis can also start with imidazole, which is consecutively alkylated with alkyl hahdes (Scheme 3.5-9). The 1,3-dialkylimidazolium cations will be abbreviated throughout this section as [C Cylm], where Im stands for imidazolium and x and y are the number of the carbons of the alkyl chains. 

The use of quaternary phosphonium salts and 1,3-dialkylimidazolium hexafluorophosphates in green organic synthesis 07COC107. 

The real breakthrough in the use of ionic liquids as solvents came in 1992, when Wilkes and Zaworotko studied the formation of dialkylimidazolium salts associated with water-stable anions. This led to the discovery of the ionic liquids [emim][BF4] 5 (Figure 2.2) and [emim][N03], with melting points of 15°C and 38" C, respectively. These ionic liquids are hygroscopic, but they are not degraded in the presence of moisture. Shortly afterward, the synthesis of the corresponding hexafluorophosphate derivative 6 was described. Today, most ionic liquids are based on imidazolium derivatives associated with non-covalent anions. 

Synthesis of Biocidal Polycationic Polysiloxane with N,N -dialkylimidazolium Groups... 

Scheme 2 One-pot five-component synthesis of 1,3-dialkylimidazolium tetrafluoroborate ILs.

Wilkes, J. S., Levlsky, J. A, Wilson, R. A, and Hussey, C. L. [1982). Dialkylimidazolium chloroaluminate melts a new class of room-temperature ionic liquids for electrochemistry, spectroscopy and synthesis. Inorg. Chem., 21[3), pp. 1263-1264. 

For an example of continuous Radziszewski reaction in a microreactor setup, to prepare 1,3-dialkylimidazolium-based ionic liquids, see J. Zimmermann, B. Ondruschka, A. Stark, Org. Process Res. Dev. 2010,14,1102-1109. Efficient synthesis of 1,3-dialkylimidazolium-based ionic liquids the modified continuous Radziszewski reaction in a microreactor setup. 

Dialkylimidazolium tetrafluoroborates belong to the most well-known ionic liquids, and they have been studied with respect to their possible applications in organic synthesis [1,2], biocatalysis [3,4], extraction [5,6], as electrolytes in chemical power sources including electrochemical capacitors [7,8], as heat-transfer media [9,10], in chemical analysis [11,12] as lubricants [13,14], etc. 

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