Dialkylimidazolium basicity

Ionic liquids are generally regarded as highly stable, and the widely used dial-kylimidazolium ionic liquids are indeed thermostable up to 300 °C [4]. The propensity of the [BF4] and [PF6] anions to hydrolyze with liberation of HF [37], which deactivates many enzymes, has already been mentioned. The [TfO] and [ Tf2N] anions, in contrast, are hydrolytically stable. Dialkylimidazolium cations have a tendency to deprotonate at C-2, with ylide (heterocarbene) formation. Such ylides are strong nucleophiles and have been used as transesterification catalysts, for example [38]. These could cause enzyme deactivation as well as background transesterification when formed in small amounts from anhydrous ionic liquids and basic buffer salts, for example. 

Most ions constituting ionic liquids can be categorized according to their Lewis acid/base properties (i.e., their capability to accept or to donate an electron pair) nevertheless, some ions may be considered according to the Bronsted definition, i.e., on the basis of their ability to accept or donate a proton. Typical ionic liquids are those based on neutral or very weakly basic anions (BF4, PF, NOf, CHsSO, 4 f2N ) and neutral (tetraalkylammonium, dialkyl-pyrrolidium, trialkylsulfonium) or weakly acidic cations (1,3-dialkylimidazolium and 1,2,3-trialkylimidazolium) (Figure 4.1). 

Fig. 3.5-4 Basic skeleton of the nitrogen-containing cations that can generate ILs N-alkylpyridinium 1,3-dialkylimidazolium 1,1 -...

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... 

The second reaction pathway ((b) in Scheme 5.3-2) is based on the well-known, relatively high acidity of the H atom in the 2-position of an 1,3-dialkylimidazolium ion [48]. The latter can be deprotonated (e.g. by basic ligands of the metal complex or by basic reactants) to form a metal-carbene complex. Xiao and coworkers demonstrated that a Pd imidazolylidene complex is formed when Pd(OAc)2 was heated in the presence of [BMIMjBr [49]. The isolated Pd-carbene complex was found to be active and stable in Heck coupling reactions (for more details see Section 5.3.2.4). Welton et al. were later able to characterize an isolated Pd-carbene complex obtained in this way by X-ray spectroscopy [50]. The reaction pathway to form the complex is displayed in Scheme 5.3-3. 

Ionic liquid stability is known to be a function of temperature (for details see Section 3.1) but the presence of nucleophiles/bases and the water content also have to be considered. There is no doubt that, under the conditions of a catalytic reaction, temperature stability issues are more complicated than imder the conditions of a TGA experiment. The presence of the catalyst complex, the reactants and impurities in the system may well influence the thermal stability of the ionic liquid. Basic and nucleophilic counter-ions, reactants and metal complexes may not only lead to deprotonation of 1,3-dialkylimidazolium ions (to form carbene moieties that will undergo further consecutive reactions) but will also promote thermal dealkylation of the ionic liquid s cation. If basic reaction conditions are required for the catalysis only tetraalkylphosphonium ions can be recommended as the ionic liquid s cation at this point in time. Tetraalkylphosphonium cations have been recently shown to display reasonabe stability, even under strongly basic conditions [290]. In contrast, all nitrogen-based cations suffer to some extent from either carbene formation, Hofmann elimination or rapid dealkylation (with alkyl transfer onto the nucleophilic anion). 

The 1,3-azoles are quatemised easily at the imine nitrogen with alkyl halides the relative rates are 1-methylimidazole thiazole oxazole, 900 15 1." In the case of imidazoles which have an A -hydrogen, the immediate product is a pro-tonated N-alkylimidazole this can lose its proton to unreacted imidazole and react a second time, meaning that reactions with alkyl halides give a mixture of imidazolium, 1-alkylimidazolium and 1,3-dialkylimidazolium salts. Furthermore, an unsymmetrically-substituted imidazole can give two isomeric 1-alkyl derivatives. The use of a limited amount of the alkylating agent, or reaction in basic solution, when it is the imidazolyl anion (section 21.4.1) which is alkylated, can minimise these complications. 

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