Functionalized imidazolium ionic liquids

Scheme 13. Functionalized imidazolium ionic liquids from natural compounds.

Olivier J-H, Camerel P, Selb J, Retailleau P, Ziessel R (2009) Terpyridine-functionalized imidazolium ionic liquids. Chem Commun 9 1133-1135... 

Cuan J, Yan B (2014) Photofunctional hybrid materials with polyoxometalates and benzoate modified mesoporous silica through double functional imidazolium ionic liquid linkage. Microporous Mesoporous Mater 183 9-16... 

Schiekker HS, SUva DO, Gelesky MA, Stracke MP, Schrekker CML, Gongalves RS, Dupont J (2008) Preparation, cation-anion interactions and physicochemical properties of ether-functionalized imidazolium ionic liquids. J Braz Chem Soc 19(3) 426 33... 

Weng W, Zhang Z, Schlueter JA, Amine K (2013) Synthesis and electrochemical property of sulfone-functionalized imidazolium ionic liquid electrolytes. Electrochim Acta 92 392-396... 

Phosphine oxide-functionalized imidazolium ionic liquids (ILs) were synthesized as tuneable ligands for lanthanide complexation. The task specific ILs were prepared in three steps, by reacting allq l bromide-functionalized 1,2-dimethyl imidazolium ILs with (TfzN) or (PFg) anions and potassium diphenylphosphide. The final step was oxidation of the phosphine to the P-oxide (Scheme 8). ... 

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... 

Most importantly, [bmim][Br] could be recycled and reused in further reactions without any significant loss of its activity. This method is compatible with different functional groups, and the oxidation reactions were carried out in neutral conditions. Use of imidazolium ionic liquids as solvents for organic transformations with... 

Similarly, Au NPs have also been supported on r-GO an N-doped r-GO and the materials have been tested as catalysts for the aerobic oxidation of benzyl alcohol [40-41]. It was found that the presence of nitrogen as dopant is beneficial to obtain Au NPs with small size, therefore exhibiting enhanced catalytic activity [41]. The use of Au NPs and Au-Pd NPs supported on r-GO has also been reported to promote the aerobic oxidation of alcohols and oxidation of methanol to methyl formate [42]. In one of these examples, r-GO has been functionalized with imidazolium ionic liquid covalently anchored to the r-GO in order to increase the affinity of the support for Au NPs. 

Kalkhambkar RG, Laali KK (2012) Pd(OAc)2 catalyzed synthesis of 2-aryl- and 2-hetero-aryl-benzoxazoles and benzothiazoles in imidazolium ionic liquids (ILs) without additives and with recycling/reuse of the IL. Tetrahedron Lett 53(32) 4212-4215 Zolfigol MA, Khakyzadeh V, Moosavi-Zare AR, Zare A, Azimi SB, Asgari Z, Hasanine-jad A (2012) Preparation of various xanthene derivatives over sulfonic acid functionalized imidazolium salts (SAFIS) as novel, highly effieient and reusable catalysts. CR Chim 15(8) 719-736... 

Holbrey, J.D., Visser, A.E., Spear, S.K., Reichert, W.M., Swatloski, R.P., Broker, G.A. and Rogers, R.D., Mercury(II) partitioning from aqueous solutions with a new, hydrophobic ethylene-glycol functionalized bis-imidazolium ionic liquid. Green Chem. 5 (2), 129-135 (2003). 

The ease of formation of the carbene depends on the nucleophilicity of the anion associated with the imidazolium. For example, when Pd(OAc)2 is heated in the presence of [BMIM][Br], the formation of a mixture of Pd imidazolylidene complexes occurs. Palladium complexes have been shown to be active and stable catalysts for Heck and other C-C coupling reactions [34]. The highest activity and stability of palladium is observed in the ionic liquid [BMIM][Brj. Carbene complexes can be formed not only by deprotonation of the imidazolium cation but also by direct oxidative addition to metal(O) (Scheme 5.3-3). These heterocyclic carbene ligands can be functionalized with polar groups in order to increase their affinity for ionic liquids. While their donor properties can be compared to those of donor phosphines, they have the advantage over phosphines of being stable toward oxidation. 

In an extension of the work to chiral chemistry 244), the imidazolium cation of the ionic liquid was modified to carry a chiral substituent. The high cost of the chiral cation could be justified as the chiral ionic liquid, [MBMIM ] , can be reused. In the ATRP of soluble methyl acrylate, a small effect of the chiral ionic liquid on the polymer tacticity was observed. The use of a chiral ionic liquid as a solvent could lead to applications in other areas of catalysis. The synthesis of imidazolium-containing ionic liquids functionalized with chiral natural amino acids has already been reported 245), as have less expensive chiral ionic liquids(2- (5). 

These supported cycloadducts were then treated with a base (LiOH, NaOH) in a mixture of water and alcohol to give the expected free acid derivatives. However, while the latter compounds were readily recovered, the same was not true for the ionic liquid 4b, which was obtained as a dark brown liquid impure by NMR analysis. Very likely, the basic hydrolysis of the ester function caused the deprotonation of the imidazolium ring leading to a series of undesired side-reactions. Therefore, milder reaction conditions were explored to cleave the Diels-Alder product from the ionic liquid support. Handy and Okello found that the best method was the cyanide-mediated transesterification that gave the corresponding methyl esters 9-11 and allowed recover of 4b in at least 90% yield. It was also demonstrated that the recovered 4b could be used for further supported syntheses. In fact, in two subsequent mns the yields of the final ester compound were similar, indicating that the ionic liquid 4b could be efficiently recycled. 

Figure 3.3-1 Incorporation of groups with high affinities for ILs (such as cobaltacenium (i), guanadinium (ii), sulfonate (iv), and pyridinium (v)) or even groups that are themselves ionic liquid moieties (such as imidazolium (iii)) as peripheral functionalities on coordinating ligands increases the solubility of transition metal complexes in ILs.

New 2-hydroxypropyl-functionalized imidazolium cation ionic liquids (containing an appended hydroxyl functionality) have been made [116] by use of an atom efficient one-pot reaction befween 1-mefhylimidazole and acid with propylene oxide. Unfortunately the systems were not studied... 

An area in which functionalised ionic liquids are already playing an important role in catalysis is heterogenisation on solid supports. The general concept involves the immobilisation of imidazolium and other cationic fragments onto solid supports using appropriate functional groups attached to the cation. An ionic catalyst then resides within the ionic matrix and several examples of such supported ionic liquid phase catalysts are provided in the subsequent chapters of this book. The concept is illustrated in Figure... 

Yang et al. used a similar protocol (an ether functionality supported on a primary alkyl halide carrier) to introduce an acetal on either side of the imidazole ring generating an ether functionalised ionic liquid (IL) imidazolium salt [183] (see Rguie 3.58). The anion could be varied without loss of the IL property (melting point below 1(X) °C) [184]. Synthesis of the transition metal carbene complexes (palladium) was done by carbene transfer ftom the corresponding silver(I) complexes or by reaction with the metal acetate (nickel) [162] (see Figure 3.64). 

An interesting aspect is the application of these arene functionalised imidazolium salts as organometallic ionic liquids [139,142,143] whereby the imidazolium salt functions as the ionic liquid itself [139,143] or as the ionic liquid compatible part of the compound using the transition metal arene complex as a catalyst in biphasic applications [142],... 

Schrekker H S, Stracke M P, Schrekker C M L, et al. Ether-functionalized imidazolium hexafluorophosphate ionic liquids for improved water miscibilities. Ind. Eng. Chem. Res. 2007. 46, 7389-7392. 

Likewise, a thermoregulated phase transfer process within the aqueous/organic two-phase system has been reported by Jin and co-workers (cf. Section 3.1.1.1) [290]. A water-soluble supramolecular Rh catalyst based on functionalized /1-cyclodextrin was also described [291]. In a two-phase system this catalyst may function as a carrier for the transfer of both the starting material and the product between the different phases. As an alternative to polar media for biphasic hydroformylation, Chauvin et al., used ionic liquids based on imidazolium salts which are well known for dimerization reactions (cf. Sections 2.3.1.4 and 3.1.1.2.2) [270, 271, 292]. For introduction into technical processes the currently availability and price of ionic liquids could be a drawback, especially for bulk chemicals such as 0x0 products. 

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