Acid-functionalized ionic liquids

Recently, novel sulphonyl-functionalized ionic liquids with S03H and S02C1 groups have been designed, synthesized and characterized. First reported were Bronsted-acidic functionalized ionic liquids bearing an alkane sulphonic acid group in an imidazole or triphenylphosphine cation (Fig. 15a) [86] or in a pyridinium cation (Fig. 15b) [87],... 

Akbari J, Heydari A (2009) A sulfonic acid functionalized ionic liquid as a homogeneous and recyclable catalyst for the one-pot synthesis of a-aminophosphonates. Tetrahedron Lett 50 4236-4238... 

Also, in a closely related study, a similar sol-gd synthetic pathvray was used to prepare silica gel supported ionic liquid deoximation catalysts from carboxylic acid-functionalized ionic liquids [85]. Here various aryl and alkyl oximes were converted into the corresponding 0x0 compounds with high conversions (up to 94%) and excellent selectivity (>99%) in aqueous acetone at room-temperature by coproduction of 2-propanone oxime. TONs of up to 200 h obtained with the SILC were about four times higher than the TONs obtained using pure ionic liquids as catalysts. 

Scheme 1 Breasted acid functionalized ionic liquids.

P. Karthikeyan, S.S. Kumar, A.S. Arunrao, M.P. Narayan, PR. Bhagat, A novel amino acid functionalized ionic liquid promoted one-pot solvent-free synthesis of 3,4-dihydropyrimidin-2-(lH)-thiones, Res. Chem. Intermed. 39 (2013) 1335-1342. 

Kraus, G.A., Guney, T., 2012. A direct synthesis of 5-aIkoxymethylfurfural ethers from fructose via sulfonic acid-functionalized ionic liquids. Green Chemistry 14, 1593—1596. 

Figure 4.1-11 The EXAFS data and pseudo-radial distribution functions of Co(ll) in (a) basic and (b) acidic chloroaluminate ionic liquid. Reproduced from reference 46 with permission.

Abstract The term Lewis acid catalysts generally refers to metal salts like aluminium chloride, titanium chloride and zinc chloride. Their application in asymmetric catalysis can be achieved by the addition of enantiopure ligands to these salts. However, not only metal centers can function as Lewis acids. Compounds containing carbenium, silyl or phosphonium cations display Lewis acid catalytic activity. In addition, hypervalent compounds based on phosphorus and silicon, inherit Lewis acidity. Furthermore, ionic liquids, organic salts with a melting point below 100 °C, have revealed the ability to catalyze a range of reactions either in substoichiometric amount or, if used as the reaction medium, in stoichiometric or even larger quantities. The ionic liquids can often be efficiently recovered. The catalytic activity of the ionic liquid is explained by the Lewis acidic nature of then-cations. This review covers the survey of known classes of metal-free Lewis acids and their application in catalysis. 

The acylative cleavage of the cyclic ether tetrahydrofuran showed an excellent yield (95%) to 4-iodobutylbenzoate when fully acidic ionic liquid [EMIM]Al2Cl7 was used, but the yield to di-functionalized product (61%) suffered when the mildly acidic halogenoaluminate ionic liquid (which has a mole fraction of AICI3 (x) at a value of 0.52) was used as the solvent. In contrast, for 1,5-dimethyltetrahydrofuran and tetrahydropyran, a good yield to the cleavage products was obtained when the mildly acidic chloroaluminate was used. 

Ionic liquids share parts of their molecular architectures with molecules that had been parameterized by existing force-field frameworks (AMBER, OPLS, CHARMM, etc.) for example imidazolium rings are present in hystidine, an amino acid. However ionic liquids have one essential difference they are composed of ions, and although they may be built by the same functional groups, the distributions of electrostatic charge will be specific. In order to represent correctly the structure and interactions of these new compounds, the sets of parameters that determine charge distributions and conformations (at the least) had to be developed for this class of substances. 

Functionalized ionic liquids possessing two Bronsted acid sites with COOH, HS04 or H2P04 groups were reported by Dan [40] in 2007 ... 

Recently, ionic liquids with amino acids as anions were synthesized by neutralization between [C2mim][OH] and amino acids [88], Tetrabutylphosphonium amino acids [P(C4)4][AA] were synthesized by the reaction of tetrabutylphosphonium hydroxide [P(C4)4][OH] with amino acids, including glycine, L-alanine, l-/1-alanine, L-serine and L-lysine [89], The esters or amide derivatives of bromoacetic acid were either commercially available or formed in one step via the reaction of bromoacetyl bromide with the appropriate alcohol or amine [90-92], An advantage of this route is that a wide range of ester and amide side chains can be prepared easily. For ionic liquids with anions other than bromide, a metathesis reaction was employed to introduce the counter ion of choice. Additionally, functionalized ionic liquids with electrophilic alkene-type appendages were synthesized. 

A very efiicient alternative method for the synthesis of functionalized ionic liquids in a simple two-step process has been proposed by Wasserscheid et al. [29]. In this approach, the imidazole or the required nucleophile is protonated by the acid whose anion vnU be incorporated into the final ionic liquid. A Michael acceptor. 

It should also be noted that not all functionalized ionic liquids are necessarily very expensive e.g. readily available, functionalized compounds, such as amino acids, may be applied as starting materials for their synthesis. Therefore, it is not excluded that functionalized ionic liquids of this kind will also make the transition to bulk ionic liquids over time. An example that impressively illustrates this possibility is the successful introduction of choline-based ionic liquids by Abbott s group [24] and Scionix for the electrodeposition of Cr-layers (for more details see Chapter 9). 

The interest in functionalized ionic liquids is growing because ionic liquids bearing ether, amino or alcohol functionalities have been shown to display special properties, including the ability to dissolve a larger amoimt of metal halide salts and to extract heavy metal ions from aqueous solutions. Imidazolium-based ionic liquids with ether and hydroxyl (see Section 2.2.1), thiourea, thioether and urea (see Section 2.2.8) " have been prepared following the standard quatemization procedure. A straightforward approach has been described for the preparation of imidazolium (as well as pyridinium) cations with ester, ketone or cyanide functionalities 1-methylimidazole reacts with methanesulfonic acid to provide the imidazolium salt 11, which undergoes a Michael-type reaction with methyl vinyl ketone as a ,j8-unsaturated compound to produce the ionic liquid 12 (Scheme 5). ... 

Fukumoto K, Ohno H (2006) Design and synthesis of hydrophobic and chiral anions from amino acids as precursor for functional ionic liquids. Chem Commun 2006 3081-3083... 

Scheme 20.4 Design of amino acids for functional ionic liquids. Reproduced from Ohno and Fukumoto [86] with permission from the American Chemical Society.

In 2013, Fang et al. prepared and utilized a biodegradable SO3H-functionalized ionic liquid (IL), 3-(iV, N-dimethyldodecylammonium)pro-panesulfonic acid ([DDPA][HS04]), as an efficient catalyst for a one-pot, three-component synthesis of a-aminophosphonates (23) from aromatic aldehydes (21), aromatic amines (22), and triethylphosphite/diethyl-phosphite at room temperature through the Kabachnik-Fields reaction under solvent-free conditions or in aqueous media (Scheme 9). 

DMAP = N,N-dimethylaminopyiidine, DMF = N -dimethylformamide, DCM = dichloromethane, PTAT = phenyltrimethylammonium tribromide, EO = ethylene oxide, PO = propylene oxide, ECH = epichlrohydrin, SO = styreneoxide, AGH = allylicglycidil ether. BO = butylene oxide, HO = n-hexene oxide, DO = N-decene oxide, PPO = 2-phenylpropylene oxide, PS-DFILX = polymer supported diol functionalized ionic liquids, acac = acetyl acetonate, CIL= carboxylic-acid functionalized imidazolium-based ILs, note active site consideration for TOP calculation is not uniform. Co-valent triazine complexes... 

Three-component condensation of silicylaldehyde and two different CH acids to give 2-amino-4//-chromenes, catalysed by base-functionalized ionic liquids, has been investigated experimentally and theoretically. Mechanisms of formation and reaction of camphor-derived amino ketones have been discussed. ... 

Scheme 2. Schematic synthetic routes of functional ionic liquids by (a) N-alkylation reaction (Davis, 2004) and (b) Michael addition reaction (Wasserscheid et al, 2003) to form ionic liquids of functional cations, and (c) the addition of acidic group to form ionic liquids of functional anion. A blank sphere represents a Lewis base such as imidazole, amine, phosphine, and sulfide, while a sphere with a plus sign represents the corresponding cation.

As one would expect, in those cases in which the ionic liquid acts as a co-catalyst, the nature of the ionic liquid becomes very important for the reactivity of the transition metal complex. The opportunity to optimize the ionic medium used, by variation of the halide salt, the Lewis acid, and the ratio of the two components forming the ionic liquid, opens up enormous potential for optimization. However, the choice of these parameters may be restricted by some possible incompatibilities with the feedstock used. Undesired side reactions caused by the Lewis acidity of the ionic liquid or by strong interaction between the Lewis acidic ionic liquid and, for example, some oxygen functionalities in the substrate have to be considered. 

Moreover, these experiments reveal some unique properties of the chlorostan-nate ionic liquids. In contrast to other known ionic liquids, the chlorostannate system combine a certain Lewis acidity with high compatibility to functional groups. The first resulted, in the hydroformylation of 1-octene, in the activation of (PPli3)2PtCl2 by a Lewis acid-base reaction with the acidic ionic liquid medium. The high compatibility to functional groups was demonstrated by the catalytic reaction in the presence of CO and hydroformylation products. 

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