Metathesis anion

The most common approach for water-insoluble ionic liquids is the addition of a lithium, sodium or potassium salt of the desired anion to an aqueous solution of the halide salt. The newly formed ionic liquid separates from the aqueous layer as a hydrophobic phase, allowing its clean isolation. For this procedure, an extremely thorough aqueous wash is required. Other hydrophilic derivatives such as mesylate of triflate can be used as a starting material instead of halide salts. 

The preparative methods employed generaUy follow similar Hnes, however, and representative examples are therefore reviewed below. The main goal of all anion 

The anion-exchange reactions of ionic liquids can be divided into two distinct categories direct reaction ofhalide salts with Lewis acids, and the formation of ionic liquids via anion metathesis. These two approaches will be dealt with separately as quite different ejqjerimental methods are required for each. 

The formation of ionic liquids by the reaction of halide salts with Lewis acids (most notably AICI3) dominated the early years of this area of chemistry. The great breakthrough came in 1951 with the report by Hurley and Weir on the formation of a salt that was liquid at room temperature based on the combination of 1-butylpyridinium with AICI3 in the relative molar proportions 1 2 (X = 0.66) [26].  

More recently, the groups of Osteryoung and Wilkes developed the technology of room temperature chloroaluminate melts based on 1-alkylpyridinium [27] and [RMIM]+ cations [6]. In general terms, the reaction of a quaternary halide salt Q+X with a Lewis acid MX results in the formation of more than one anion species, depending on the relative proportions of Q X and MX . Such behavior 

When [EMIM]C1 is present in a molar excess over AIQ3, only equilibrium (2.1-1) needs to be considered, and the ionic liquid is basic. When, a molar excess of AIQ3 over [EMIM]C1 is present on the other hand, an acidic ionic liquid is formed, and equilibria (2.1-2) and (2.1-3) predominate. Further details of the anion species present may be found elsewhere [28]. The chloroaluminates are not the only ionic liquids prepared in this maimer. Other Lewis acids employed include AlEtCl2 [29], BCI3 [30], CuCl [31], SnCl2 [32], and FeCb [33]. In general, the preparative methods employed for aU of these salts are similar to those indicated for AlQ3-based ionic liquids, as outlined below. 

Finally in this section, it is worth noting that some ionic liquids have been prepared by the reaction of halide salts with metal halides that are not usually thought of as strong Lewis acids. In this case only equilibrium (2.1.1) wiU apply, and the salts formed are neutral in character. Examples of these include salts of the type [RMIM]2[MCl4] (R = alkyl, M = Co, Ni) [34], and [EMIM]2[VOa4] [35]. These are formed by the reaction of two equivalents of [EMIM]C1 with one equivalent of MCI2 and VOQ2 respectively. 

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Thermal arene exchange of tetramethylthiophene with [(/ -cymene)RuCl2]2 affords 130 (89JA8828), which on reaction with AgBE4 and excess tetramethylthiophene yields 131. The Ru—S thiophenic cluster, 132, was synthesized by reaction of 130 with (Mc3Si)2S followed by anionic metathesis and formation of the PFg salt. The coordination geometry around each ruthenium atom is pseudooctahedral. 

Bis(phosphonio)isophosphindolium cations were expected to act as attractive ambidentate ligands toward the transition-metal-containing moieties. These expectations did not materialize with respect to CpM(CO)3 (M = Mo, W), since the anionic metathesis occurred to yield the ionic products 178 (96PSS125). 

Table 2.1-1 Examples of ionic liquids prepared by anion metathesis.

The choice of the anion ultimately intended to be an element of the ionic liquid is of particular importance. Perhaps more than any other single factor, it appears that the anion of the ionic liquid exercises a significant degree of control over the molecular solvents (water, ether, etc.) with which the IL will form two-phase systems. Nitrate salts, for example, are typically water-miscible while those of hexaflu-orophosphate are not those of tetrafluoroborate may or may not be, depending on the nature of the cation. Certain anions such as hexafluorophosphate are subject to hydrolysis at higher temperatures, while those such as bis(trifluoromethane)sulfonamide are not, but are extremely expensive. Additionally, the cation of the salt used to perform any anion metathesis is important. While salts of potassium, sodium, and silver are routinely used for this purpose, the use of ammonium salts in acetone is frequently the most convenient and least expensive approach. 

Diastereomerically pure (2W,5.V)-l,3-diaza-2-(2-methylphenyl)-3-phenyl-2-phosphabicyclo[3.3.0]octane (.S )-202] was formed in the thermal reaction of (S)-2-(anilinomethyl)pyrrolidines (197a) with o-TolP(NMe2)2 and isolated in 27% yield after recrystallization (Scheme 56) [87], Its cyclopalladation with palladium diacetate followed by anion metathesis gave dimer (5,5)-203 (Scheme 56) [87],... 

Similar observations were made with bis[alkoxy(alkylamino)carbene]gold complexes, which are prepared by ring opening of trinuclear precursors with trifluoromethanesulfonic (triflic) acid. Through anion metathesis, several crystalline forms were obtained, the structures of which were determined. The triflate salt has a chain of cations in the crystal, while the chloroform solvate of a />-benzoquinolate contains only monomeric cations. In the solvent-free crystals, dimers are present (Scheme 59).251... 

The primary advantage in the first step of the method described here (using 1-chlorobutane diluted in MeCN) is that it eliminates long reaction periods and allows the use of secondary alkyl halides without competitive elimination reactions. For example, the reaction of sec-butyl bromide with N-methylimidazole using the classical method (in neat alkyl halide) produces, along with the desired product, 20-30% of butenes and 1-methylimidazole hydrobromide. In the second step, the use of water as solvent allows the anion metathesis reaction to be quantitative in a very short time and allows the easy purification of the ionic liquids. Moreover, employing the potassium salt avoids the use of corrosive and difficult to handle hexafluorophosphoric add and the expensive silver tetrafluoroborate. ... 

Oxidation of iodoarenes was also effective with NaB03 H20 in Ac20/H2S04. The bromides were converted into the more useful ionic nitrates, trifluoroac-etates or tetrafluoroborates by oxidative anion metathesis, using the corresponding acid and 30% hydrogen peroxide, and refluxing the mixtures in methanol any liberated bromine was trapped by cyclohexene [20]. 

Bernhard and coworkers [58] have addressed the discovery of ionic iridium(III) and ruthenium(II) complexes by combinatorial luminophore synthesis and screening (Scheme 5.9). Starting from iridium trichloride, cyclometalation with (hetero)arylpyridyl ligands 45 (Fig. 5.17) gives rise to the formation of binuclear iridium complexes 46. Upon complexation with bidentate N,N- or P,P-ligands 47 (Fig. 5.17), the cationic complex 48 is formed, which upon anion metathesis with hexafluorophosphate is transformed into the target complex 49. In this sequence, the step from 46 to 48 was performed in a traditional and a parallel manner, the latter leading to a library of 100 iridium and 10 ruthenium complexes. 

Although silver chemistry has a long history in organic chemistry, it has typically been used in stoichiometric amounts and is developed mostly for anion metathesis... 

Heterocyclization reactions with saturated moieties (alcohols, amines, thiols, etc.) or acids on unsaturated counterparts (alkenes, allenes, alkynes, etc.) are not covered in this chapter since they are addition, and not isomerization, reactions. Silver is also widely used as an activating agent for producing highly reactive metallic cations (anion metathesis), which, in turn, may catalyze cycloisomerization reactions. This aspect is covered only when the silver control experiments give substantial positive results. 

The preparation of vinyliodonium salts from vinyl(trichloro)stannanes with (dichloroiodo)arenes was also explored during this period (equations 156 and 157)119 122. By this approach, the vinyliodonium compounds are available in unexceptional yields as the hexachlorostannate salts other counterions (X , BF4 ) can be introduced by anion metathesis. 

Many other triphenylphosphine complexes of this stoichiometry can be prepared by anion metathesis of [RhCl(PPh3)3]. Some of these reactions are illustrated in Scheme 5. 

They are more usually prepared by the action of nitrosyl compounds on rhodium(I) complexes containing labile ligands.1336"1339 It has also proved possible in one instance to prepare this type of complex by anion metathesis (equations 318 to 320).1340... 

The complexes are usually prepared by displacing ligands from other rhodium(I) complexes. Complexes containing two bidentate ligands may be prepared similarly. Additionally, simultaneous anion metathesis and ligand displacement may... 

The reactions of [Rh(dppe)2]Cl (Scheme 8) show that the major reaction is anion metathesis. 

Solvent free methods have also impacted on the preparation of other alternative reaction media. Namely, a range of ionic liquids (ILs) was prepared (including imidazolium, pyridinium and phosphonium salts) through halidetrapping anion metathesis reactions (Figure 2.17). The alkyl halide by-product was easily removed by vacuum or distillation and the products were obtained quantitatively in high purity. In addition to being solvent free, this route is more atom economic than the usual route to room temperature ionic liquids (RTILs) as it does not use silver(i), alkali metal or ammonium salts which are normally used in an anion metathesis reaction. 

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