Halide-Free Ionic Liquids

All the halide exchange reactions mentioned above proceed more or less quantitatively, causing greater or lesser quantities of halide impurities in the final product. The choice of the best procedure to obtain complete exchange depends mainly on the nature of the ionic liquid that is being produced. Unfortunately, there is no general method to obtain a halide-free ionic liquid that can be used for all types of ionic liquid. This is explained in a little more detail for two defined examples the synthesis of [BMIM][(CF3S02)2N] and the synthesis of [EMIM][BF4]. 

C4C1im][Cg-S04] TPPD-Guanidine 1-Octene 862-892 (2.5-2.9) Halide-free ionic liquid monophasic system, with addition of cyclohexene biphasic high activity reflection of high octane solubility. [30]... 

Other direct syntheses of halide free ionic liquids can be categorized into three groups (1) synthesis via N-heterocyclic carbene intermediates, (2) phosphorus based direct reactions with imidazoles and (3) sulfur-based direct reactions with imidazoles as discussed further below. 

Without special drying procedures and completely inert handling, water is omnipresent in ionic liquids. Even the apparently hydrophobic ionic liquid [BMIM][(CF3S02)2N] saturates with about 1.4 mass% of water [15], a significant molar amount. For more hydrophilic ionic liquids, water uptake from air can be much greater. Imidazolium halide salts in particular are laiown to be extremely hygroscopic, one of the reasons why it is so difficult to make completely proton-free chloroaluminate ionic liquids. 

For all research carried out with commercial ionic liquids we recommend a serious quality check of the product prior to work. As already mentioned, a good commercial ionic liquid may be colored and may contain some traces of water. However, it should be free of organic volatiles, halides (if not an halide ionic liquid), and all ionic impurities. 

In the 1990s John Wilkes and coworkers introduced air- and water-stable ionic liquids (see Chapter 2.2) which have attractive electrochemical windows (up to 3 V vs. NHE) and extremely low vapor pressures. Furthermore, they are free from any aluminum species per se. Nevertheless, it took a while until the first electrodeposition experiments were published. The main reason might have been that purity was a concern in the beginning, making reproducible results a challenge. Water and halide were prominent impurities interfering with the dissolved metal salts and/or the deposits. Today about 300 different ionic liquids with different qualities are commercially available from several companies. Section 4.2 summarizes the state-of-the-art of electrodeposition in air- and water-stable ionic liquids. These liquids are for example well suited to the electrodeposition of reactive elements such as Ge, Si, Ta, Nb, Li and others. 

Ionic liquids have been a popular topic of interest in 2002 and a review of the applications of these solvents in organic synthesis has been published (02ACA75>. New, densely functionalized fluoroalkyl-substituted imidazolium ionic liquids have been reported <02TL9497>. An ultrasound-assisted preparation of a series of ambient-temperature ionic liquids, l-alkyl-3-methylimidazolium halides, which proceeds via efficient reactions of 1-methyl imidazole with alkyl halides/terminal dihalides under solvent-free conditions, has been described <02OL3161>. New hydrophilic poly(ethyleneglycol)-ionic liquids have been synthesized from... 

Indium-mediated Barbier-type coupling between carbonyl compounds and allyl halides has been revealed to proceed effectively in diverse reaction media. Even under solvent-free conditions, allylation works well, although no reaction is observed with benzyl bromide and a-halo carbonyl compounds.59 Various aldehydes react with allyl bromide mediated by indium in liquid carbon dioxide to give homoallylic alcohols (Scheme 1). In contrast to the corresponding neat allylation, the liquid C02-mediated reaction can allylate solid aldehydes successfully.60 Indium-mediated allylations of carbonyl compounds with allyl bromide proceed in room temperature ionic liquids. In [bmim][BF4] and [bmim][PF6] (bmin l-butyl-3-methylimidazolium), the desired homoallylic alcohols are formed with good levels of conversion.61 Homoallyllic alcohols are also prepared by the reaction of resin-bound aldehydes (Equation (l)).62... 

A relatively inexpensive route to halide-free tetrafluoroborate ionic liquids has been reported, shown in Scheme 2.3. In this one-pot reaction between glyoxal, methylamine, w-butylaminc, formaldehyde, and tetrafluoroboric acid a mixture of dialkylimidazolium tetrafluoroborate salts... 

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