Ionic halide-free

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]. 

Addition of hydrogen halide across fluoroalkenes and fluoroalkylalkenes is an important route to halogen-containing fluoroorganics Both ionic and free radical... 

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... 

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]... 

Ionic and free-radical reactions leading to the formation of ketonic products can occur simultaneously in the copper-catalyzed reaction of a Grignard reagent with a sterically hindered acid halide. These reactions have been studied by Dubois and co-workers 102-108, 194). Contrary to the report of Percival et al. 229), ferric chloride inhibits the catalytic role of copper and does not favor the formation of ketones 106, 108). The scheme depicted in Fig. 4 is proposed 103) to account for the products of the reaction. Similar radical reactions were suggested by Khar-asch et al. 169) to explain the role of cobalt chloride in like reactions. 

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. 

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... 

For synthetic purposes cyclized radicals are preferentially trapped by chlorine [16], bromine [60], or iodine atom donors [54] to provide y9-functionalized tetrahy-drofurans, for instance halides 35-37 (Scheme 9), which serve as building blocks for subsequent ionic or free-radical reactions. This radical version of the classical halogen cyclization (Bartlett reaction [61]) is particularly useful if functionalized tetrahydrofurans can be obtained from terminal alkyl- or aryl-substituted alkenols. If these compounds are reacted for example with iodine or with A -bromosuccin-imide, tetrahydropyrans are formed from ionic cyclofunctionalizations [62], If, however, the corresponding alkenols are converted into a thiohydroxamic acid... 

However, the formation of the metal-carbene complex was not observed in pure, halide-free [BMIM][Bp4], indicating that the formation of carbene depends on the nucleophilicity of the ionic liquid s anion. To avoid the formation of metal-carbene complexes by deprotonation of the imidazolium cation under basic conditions the use of 2-methyl-substituted imidazolium is frequently suggested. However, it should be mentioned here that strong bases can also abstract a proton to form the vinyl imidazolidene species which may also act as a strong ligand to electrophilic metal centers. 

Some ionic liquids have been found to release highly toxic and very corrosive degradation products when undergoing hydrolytic decomposition. This discovery was especially harmfiil for the green image of ionic liquids as it addressed, in particular, the main work horses of ionic liquid research of the time, namely the hexafluorophosphate and tetrafluoroborate systems which indeed release HF on contact with water [47]. Today, we know that many ionic liquids are very stable to hydrolysis. Moreover, we have developed many ionic liquids which are completely halide-free [48] so that the release of HQ or HF can certainly be avoided in both hydrolysis and combustion processes. 

Hydrogen halides also add to alkynes. The addition of HBr to alkjmes can be difficult to interpret because (as with alkenes) both ionic and free radical mechanisms may occur, and the free radical process can be difficult to suppress. Reaction of HBr with propjme (63) in the liquid phase at —78°C led to the formation of (Z)-l-bromopropene (64, equation 9.63), indicating stereoselective anti addition. When the reaction was carried out at room temperature, however, a mixture of Z (64) and (65) isomers was obtained (equation 9.64). The results suggested that addition of a bromine atom to propyne produces the vinyl radical 66, which abstracts a hydrogen from HBr to produce 64 at -78°C but which can isomerize (with Eg > 17kcal/mol) to the radical 67 at room temperature. 

Dubois and his associates have studied the copper-catalyzed reaction of Grignard reagents and acid halides (Dubois and Boussu, 1969, 1970, 1971 Dubois et al., 1967a,b, 1969, 1971 MacPhee and Dubois, 1972). The reaction with a sterically hindered acid halide results in a symmetrical ketonic product, suggesting the operation of both ionic and free radical mechanisms. 

Leclercq, L., Suisse, L, Nowogrocki, G. and Agbossou-Niedercorn, F., Halide-free highly-pure imidazolium triflate ionic liquids Preparation and use in palladium-catalysed allylic alkylation, Green Chem. 9 (10), 1097-1103 (2007). 

R. Yang, H. Zhang, D. Kirichenko, K Rogers, R. D., Ionic liquids via reaction of the zwitterionic l,3-dimethylimidazolium-2-carboxylate with protic adds. Overcoming synthetic limitations and establishing new halide free protocols for the formation of ILs. Green Chem, 2007, 9 (1), 90-98. 

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