Ionic liquids electrophilic aromatic

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... 

A review of solvent properties of, and organic reactivity in, ionic liquids demonstrates the relatively small number of quantitative studies of electrophilic aromatic substitution in these media.3 Studies mentioned in the review indicate conventional polar mechanisms. 1-Methylpyrrole reacts with acyl chlorides in the ionic liquid 1-butylpyridinium tetrafluoroborate to form the corresponding 2-acylpyrrole in the presence of a catalytic amount of ytterbium(III) trifluoromethanesulfonate.4 The ionic liquid-catalyst system is recyclable. Chloroindate(III) ionic liquids5 are catalytic media for the acylation, using acid chlorides and anhydrides, of naphthalene, benzene, and various substituted benzenes at 80-120 °C. Again the ionic liquid is recyclable. 

It is not surprising that electrophilic aromatic substitutions were the first organic reactions investigated using acidic room-temperature chloro-almninate(III) ionic liquids. Indeed, chloroaluminate(ni) species combine their properties of good solvents for simple arenes to their role as Lewis acid catalysts. In Friedel-Craft alkylations, polyalkylation is common as well as the isomerisation of primary halides to secondary carbonium ions. 

Borodkin GI, Shubin VG (2006) Electrophilic reactions of aromatic and heteroaromatic compounds in ionic liquids. Russian J Org Chem 42 1745—1770... 

Ionic liquids can provide an ideal medium for reactions that involve reactive ionic intermediates due to their ability to stabilize charged intermediates such as acyl cations in Friedel-Crafts acylation. As shown in the previous chapter, examples of electrophilic aromatic acylation are reported utilizing ionic liquid-catalysts based on classic Lewis acids, namely, [emim] Cl-aluminum chloride and [emim]Cl-iron trichloride, which can give acylation of both activated and deactivated aromatic compounds. ... 

Electrophilic aromatic substitutions are reactions of great importance for industry and ionic liquids have been used in these reactions as alternative reaction media, mainly with the aim of reducing the environmental impact of these processes. Only few reactivity data, which might be used to quantitatively estimate the effect of the ionic reaction medium on these reactions, have been collected. 

The iodination of aromatic compounds using the electrophilic fluorinating agent l-(chloromethyl)-l,4-diazabicyclo[2,2,2]octane tetrafluoroborate and iodine was carried out in a range of [BF4] and [PFe]" ionic liquids and generally gave high yidds [106]. Trihalide-based ionic liquids have been synthesised and the structure of the... 

Electrophilic reactions of aromatic and heteroaromatic compounds in ionic liquids 06ZOR1761. 

The Friedel-Crafts reaction was one of the first to be attempted in ionic liquids (for typical examples, see Scheme 20. Friedel-Crafts acylation, which allows easy functionalization of aromatic compounds to ketones, is of significant commercial importance. The electrophilic substitution with an acylating agent is catalyzed by an acid, typically AICI3. Since this catalyst can form a stable adduct with the carbonyl of the product, an excess of AICI3 is required, which gives rise to a copious amount of inorganic waste. 

The low volatility of ionic liquids and the easy separation of catalysts (which usually remain in these polar media) have made ionic liquids an interesting alternative to typically used organic solvents. Rather unsatisfactory results have been obtained in both copper-mediated [36] and copper-free [37] Sonogashira reactions, with aryl iodides being the only aromatic electrophiles coupled at reaction temperatures between 60 and 80 °C. It should further be noted that imidazolium-based ionic liquids are not necessarily innocent solvents, but can be deprotonated in the presence of bases to generate N-heterocycUc carbenes (NHCs). 

Knoevenagel condensation between aromatic aldehydes and active methylene compounds has also been induced by Lewis acid ionic liquids leading to the generation of electrophilic alkenes as major products. Extension of this methodology to o-hydroxybenzaldehydes resulted in the formation of coumarins (Harjani et al. 2002). 

Friedel—Crafts Reactions. The chloroaluminate(III) ionic liquids are ideal solvents for electrophilic substitutions. The Lewis acidity of such liquids can be finely tuned by adjusting the proportion of AICI3 in the melt, and this, in turn, affects the selectivity of the reactions. It is to be noted that aromatics dissolve fairly well in the chloroaluminate(III) ionic liquids which leads to high reaction rates. Since Friedel-Crafts reactions oft en make key steps in synthesis, the use of ionic liquids as both solvent and Lewis acid source opens new ways in process design (37). 

Copper(I)-catalyzed tandem reaction of isothiocyanates and iodophenols in ionic liquids produced 95 (13JOM(723)137) iodine-mediated electrophilic cychzation of osmabenzene resulted in the isolation and characterization the osmabenzene-fused oxathiole 96 (13AGE9251) and nickel chloride-catalyzed reaaion of aromatic aldehydes with 2-mercaptoethanol produced 97 only as a minor product together with the bis(2-hydroxyethyl)dithioac-etals (13TL5839). Lamivudine 98 and its enantiomer 99 have been efficiently asymmetrically synthesized via enzymatic dynamic kinetic resolution (13CC10376). The deprotection of 1,3-oxathiolanes 100 promoted by a range of bases has been examined (13TL2217). 

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