Ionic functionalisation

J. H. Davis, Jr., Working Salts Syntheses and Uses of Ionic Liquids Containing Functionalised Ions,"... 

The second group of recently developed ionic liquids is often referred to as task specific ionic liquids in the literature [15]. These ionic liquids are designed and optimised for the best performance in high-value-added applications. Functionalised [16], fluorinated [17], deuterated [18] and chiral ionic liquids [19] are expected to play a future role as special solvents for sophisticated synthetic applications, analytical tools (stationary or mobile phases for chromatography, matrixes for MS etc.), sensors and special electrolytes. 

In 2002 Mehnert and co-workers were the first to apply SILP-catalysis to Rh-catalysed hydroformylation [74], They described in detail the preparation of a surface modified silica gel with a covalently anchored ionic liquid fragment (Scheme 7.7). The complex N-3-(3-triethoxysilylpropyl)-4,5-dihydroimidazole was reacted with 1-chlorobutane to give the complex l-butyl-3-(3-triethoxysilylpropyl)- 4,5-dihydroimidazolium chloride. The latter was further treated with either sodium tetrafluoroborate or sodium hexafluorophosphate in acetonitrile to introduce the desired anion. In the immobilisation step, pre-treated silica gel was refluxed with a chloroform solution of the functionalised ionic liquid to undergo a condensation reaction giving the modified support material. Treatment of the obtained monolayer of ionic liquid with additional ionic liquid resulted in a multiple layer of free ionic liquid on the support. 

The earliest recognised examples of synthetic supramolecular structures were the complexes formed from crown ethers and metal cations [19]. Since then numerous macrocycles have been synthesised. Representative examples are the cryptands [20], These differ from crown ethers in that the former contains a tridimensional cavity while the latter are characterised by a hole. Similarly, calix[4]arenes are compounds with a cup -like structure that through lower rim functionalisation gives rise to a hydrophilic and a hydrophobic cavity, thus allowing the reception of ionic species in the former and neutral species in the latter. Most of the above mentioned macrocycles are known for their capability to serve as cation receptors. 

Related to ionic liquids are substances known as deep eutectic solvents or mixtures. A series of these materials based on choline chloride (HOCH2CH2NMe3Cl) and either zinc chloride or urea have been reported (Abbott et al., 2002 2003). The urea/choline chloride material has many of the advantages of more well-known ionic liquids (e.g. low volatility), but can be sourced from renewable feedstocks, is non-toxic and is readily biodegradable. However, it is not an inert solvent and this has been exploited in the functionalisation of the surface of cellulose fibres in cotton wool (Abbott et al, 2006). Undoubtedly, this could be extended to other cellulose-based materials, biopolymers, synthetic polymers and possibly even small molecules. 

Abbott, A.P., T.J. Bell, S. Handa and B. Stoddart, Cationic Functionalisation of Cellulose Using a Choline Based Ionic Liquid Analogue, Green Chemistry, 8,784-786 (2006). 

In conclusion, chiral heterogeneous catalysts are prepared from chiral Rhodium diphosphine complexes and Al-MCM-41. The bonding supposedly occurs via an ionic interaction of the cationic complex with the host. Also a slight reduction of weak acidic sites of Al-MCM-41 has been observed. These catalysts are suitable for the hydrogenation of functionalised olefins. The organometallic complexes remain stable within the mesopores of the carrier at reaction conditions. The catalyst can be recycled by filtration or centrifugation. 

The use of functionalised ionic liquids in catalysis is still very much in its infancy. The presence of a functional group may serve any of the following purposes in order to improve the catalytic reaction (i) assist in the activation of the catalyst (ii) generate a novel catalytic species (iii) improve the stability of the catalyst (iv) optimise immobilisation and recyclability (v) facilitate product isolation and (vi) influence the selectivity of the reaction. In addition, the functionalised ionic liquid has to meet certain requirements to be of use in synthesis/catalysis. Most important is their inertness with respect to the reaction in question. Also, the right balance between catalyst stabilisation and activation has to be found and this might require careful tuning of the reaction conditions. 

Scheme 2.4 Preparation of carboxylic acid-functionalised ionic liquids via a zwitterionic intermediate...

Sulfonic acid functionalised ionic liquids may also be prepared via a zwitterionic intermediate from a Michael-type addition, as shown in Scheme 2.5.[84 85] In the first step 1-methylimidazole reacts with the sulfonic acid precusor 1,3-propane sultone to form a zwitterionic intermediate. Protonation with Bronsted acids affords ionic liquids with high purity that have proven to be highly efficient reaction media in, for example, esterification reactions.[86] Task-specific ionic liquids may also be prepared using semi-combinatorial methods.1871... 

Scheme 2.5 Preparation of sulfonic-acid functionalised ionic liquids...

It is also relatively easy to functionalise imidazolium cations at the 2-position.[88] For example, the phosphine derivatised salts shown in Figure 2.7 illustrate such a substitution pattern and they are easily prepare by virtue of the acidity of the 2-proton.[74] Substitution of the 2-proton tends to yield relatively high melting salts instead of ionic liquids. Despite this limitation the imidazolium-phosphine compounds are good ligands for catalysis improving the immobilisation potential of complexes dissolved in ionic liquids. 

Functionalised ionic liquids based on cations other than imidazolium have also been developed. For example, pyridinium cations functionalised with pentafluorosulfanyl[89] or alkyl-nitrile groups1901 have been prepared as cheaper alternatives to their imidazolium-based counterparts (see Figure 2.8). The latter have been evaluated in palladium catalysed C-C cross coupling reactions and improved catalyst retention and stability were observed in the nitrile-functionalised ionic liquid compared to the simple alkyl-analogue. Consequently, the nitrile-functionalised ionic liquid solution can be reused repeatedly without significant decrease in activity (see Chapter 6 for further information). 

Combining functionalised cations with functionalised anions may lead to highly task-specific ionic liquids, but very little has been undertaken in this area in the catalysis domain. However, a number of so-called dual-functionalised ionic liquids have been reported1271 and it has been shown... 

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... 

Figure 2.11 Surface modification using a functionalised ionic liquid...

Finally, there are a few examples where task-specific ionic liquids have been employed. These examples encompass an imidazolium ionic liquid with a chiral alkyl chain used to induce chirality,[13] an imidazole-imidazolium liquid which acts as both ligand and solvent[14] and a nitrile-functionalised pyridinium ionic liquid which also serve as ligand and solvent.[15] The application of these specific solvents is discussed in greater detail below. 

Figure 6.5 Nitrile-functionalised pyridinium ionic liquid and palladium derivatives...

The effect of nitrile-functionalised ionic liquids on the recycling potential was investigated in the coupling between iodobenzene and tributylphenyltin.[15] Several, closely related palladium-precursors, based on the ionic liquid cation [C3CNpy]+ and PdCP (Figure 6.5), were tested in the reaction, but the nature of the pre-catalyst did not have a significant influence on the rate. Independent from the ionic liquid used, rather low catalytic activity was observed and with 5 mol% catalyst, yields ranged between 43-65% after 12 hours at 80°C. 

Non-functionalised sp3 C in position to at least three F atoms sp2 C "semi-ionic", bound to intercalated F atom (very week contribution)... 

A second source of amino functionalised imidazolium salts is the quest for functionalised ionic liquids [146,147]. Wan et al. used a piperidine functionaUsed imidazolium salt as a base in a palladium catalysed Heck reaction in an ionic liquid medium. The system was designed such that the phosphane Ugand, the base and the solvent were all imidazolium based ionic Uquids [148] (see Figure 3.51). Given the fact that imidazolium based ionic liquids can act as carbene precursors for coordination to transition metals [149] that coordinate better than phosphane Ugands (with cationic substituents in the present case), it is far from certain which is acmally the ligand in that carefully designed system. 

From the three components, the 2-phosphino substituted imidazolium salt cannot serve as a normal carbene ligand, but both the amino functionalised imidazoUum salt (given the role as the base) and the nonfunctionalised imidazolium salt can act as carbene ligands and would enhance the catalytic activity of the system, if they did [149]. Furthermore, the amino functionalised imidazolium salt would stabilise the Pd(0) species due to its hemila-bile behaviour. One should use these ionic liquid systans with caution. 

A truly hemilabile amino functionalised NHC ligand was introduced by Jimdnez et al. who synthesised a series of rhodium(I) compounds using anunonium functionalised imidazolium salts as starting materials [150] (see Figure 3.52). Interestingly, initially an ionic rhodium(I) compound was obtained that did not contain a carbene-rhodium bond. The rhodium carbene complex could be obtained after further deprotonation and coordination of the amine sidearm to the metal occurred only after chloride abstraction with AgBF. ... 

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