Ionic liquids halogenation

In contrast, if halide-based ionic liquids were used under similar conditions, halogenated products were isolated. If hydrochloric acid was used in a nitrate-based ionic liquid, halogenated products also resulted. In methanesulfonate-based ionic liquids, nitric acid acts as an oxidising agent rather than a nitrating agent, which is capable of oxidising toluene to benzoic acid. Likewise it was possible to use a nitrate-based ionic liquid with added methanesulfonic acid [127],... 

This table illustrates pretty well that the large-scale ionic liquid will probably not comprise a diallcylimidazolium cation and a [CE3S02)2N] anion. Over a medium-term timescale, we would expect a range of ionic liquids to become commercially available for 25-50 per liter on a ton scale. Halogen-free systems made from cheap anion sources are expected to meet this target first. 

In this context, the use of ionic liquids with halogen-free anions may become more and more popular. In 1998, Andersen et al. published a paper describing the use of some phosphonium tosylates (all with melting points >70 °C) in the rhodium-catalyzed hydroformylation of 1-hexene [13]. More recently, in our laboratories, we found that ionic liquids with halogen-free anions and with much lower melting points could be synthesized and used as solvents in transition metal catalysis. [BMIM][n-CgHi7S04] (mp = 35 °C), for example, could be used as catalyst solvent in the rhodium-catalyzed hydroformylation of 1-octene [14]. 

The author anticipates that the further development of transition metal catalysis in ionic liquids will, to a significant extent, be driven by the availability of new ionic liquids with different anion systems. In particular, cheap, halogen-free systems combining weak coordination to electrophilic metal centers and low viscosity with high stability to hydrolysis are highly desirable. 

Similar conclusions concerning the effect of the anion and impurities in the ionic liquid on the cyclopropanation reaction have been drawn in a recently published study [47]. Ionic liquids with the formula [bmim][X], where X = OTf, NTf2, PFe, and BF4, were used. The catalyst used in this study was 6b-Cu(OTf). In all cases, good enantioselectivities (89-97% ee) were obtained and these are similar to those obtained in chloroform. The influence of the presence of halogen anions was tested by the addition of 5% [bmim] [Cl] or [bmim][Br] to [bmim][BF4]. In both cases, a catalytically inactive solution was obtained, showing the detrimental effect of these anions on the reaction. 

Finally, research efforts to replace hexafluorophosphate (and other halogen-containing) ionic liquids by some cheap and halogen-free ionic liquids in the Rh-catalysed hydroformylation should be mentioned. The first attempts in this direction were made by Andersen et al. [10] These authors investigated the hydroformylation of... 

TABLE 7.4. Biphasic hydroformylation using halogen-free octylsulfate and tosylate ionic liquids compared with the same reaction in [BMIM][PF6]... 

A summary of the research activities of the last four years reveals three different important trends (a) The design of new ionic ligands for excellent catalyst immobilisation in ionic liquids and high regioselectivity (b) the successful application of cheap, halogen-free ionic liquids in the biphasic Rh-catalysed hydroformylation (c) the successful development of unusual multiphasic reaction concepts for Rh-catalysed hydroformylation, namely catalysis in ionic liquid/supercritical C02 and SILP-catalysts. 

In recent times, the development of new ionic liquids has made great progress. Important developments include a range of new halogen-free ionic liquids (e.g., benzenesulfonates [13], toluenesulfonates, alkylsulfates [14], hydro-gensulfate [15, 16], dicyanamides [17], thiocyanates [18], etc.), as well as functionalized (task-specific) [19-23], fluorinated [24], deuterated [25] and chiral ionic liquids [26-30]. 

Winterton, N. Seddon, K. R. Patell, Y. Halogenation of unsaturated hydrocarbons in ionic liquids, World Patent WO 0037400 (2000). 

Halogen content If halogens in the anion are not crucial for specific reactions performed in the ionic liquid, they should be avoided. Moisture sensitivity, halogenide transfers, alcoholysis and toxic effects are often connected with halogen atoms in the molecule [27]. In addition, the hydrolysis products HCl or HF act corrosively. Within the project reported by Wasserscheid and coworkers they successfully developed ionic liquids with alkylsulfate groups as anions to overcome the halogen content. These new solvents show very favorable properties. 

The stability of dialkylimidazolium cation-containing ionic liquids can be a problem even at moderate temperatures in the presence of some reagents or catalysts. For example, when CsF and KF were used in the ionic liquid [BMIM]PFg to perform a halogen exchange reaction in an attempt to replace Br from bromo-carbons with F , it was found that alkyl elimination from the [BMIM] cation took place, forming methyl imidazole, 1-butene, 1-fluorobutane, and other unidentified products at 150°C overnight 69). The fluoride ion acted as a base that promotes elimination or substitution processes. 

The product of the electrochemical reaction was extracted with cyclohexane. The yields observed in the reactions of PhBr and PhCH2Br were 35 and 75%, respectively. In the reaction of PhCH2Br, no toluene was formed, indicating that the process was highly selective and that the reduction of the halogenated substrate was avoided. It was further verified that, at the end of the electrolysis, the catalytic system completely regained its reversibility. The nickel(II) catalyst remained totally in the ionic liquid after the extraction of products, and the catalyst system was reusable. 

Room-temperature ionic liquids may be used as green recyclable alternatives to chlorinated solvents for stereoselective halogenation.577 The bromination of alkenes and alkynes in [bmim][Br] is a/m -stereospecific, whereas that of 1,3-dienes gives selectively the 1,4-addition products. The reactions of arylacetylenes, however, are not selective when carried out in [bmim][PF6]. Tetraethylammonium trichloride, a stable crystalline solid may be used in the chlorination of alkenes and alkynes to afford the products with exclusive anti stereoselectivity.578 It has... 

Their utility has been demonstrated in various Friedel-Crafts reactions,44-47 halogen addition,48 49 electrophilic nitration of aromatics,50 and various hydrogenation processes51-53 including the Ru-catalyzed hydrogenation of C02 to /V,/V-dipropyl formamide in supercritical C02 under biphasic conditions 54 The use of in situ IR spectroscopy allowed Horvath and coworkers to demonstrate that the same substrate-catalyst and product-catalyst complexes are produced in Friedel-Crafts acetylation of benzene in ionic liquids as in CH2C12 55... 

Although these reactions operate under solvent-free conditions (with an excess of the arene), many Friedel-Crafts acylations utilize volatile and hazardous halogenated solvents. Here, their replacement by ionic liquids can considerably lower the environmental risks and provide a greener chemistry . Ionic liquids with their unique miscibility properties, high thermal stability and miniscule vapor pressure are valuable alternatives for the wide range of traditional solvents available. 

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