Innocent solvent

Depending on the coordinative properties of the anion and on the degree of the cation s reactivity, the ionic liquid can be regarded as an innocent solvent, as a ligand (or ligand precursor), as a co-catalyst, or as the catalyst itself... 

Ionic liquids with wealdy coordinating, inert anions (such as [(CF3S02)2N] , [BFJ , or [PFg] under anhydrous conditions) and inert cations (cations that do not coordinate to the catalyst themselves, nor form species that coordinate to the catalyst under the reaction conditions used) can be looked on as innocent solvents in transition metal catalysis. In these cases, the role of the ionic liquid is solely to provide a more or less polar, more or less weakly coordinating medium for the transition metal catalyst, but which additionally offers special solubility for feedstock and products. 

Is there a "universal ionic liquid at the present state of development The answer is clearly no. Many of the ionic liquids commonly in use have very different physical and chemical properties (see Chapter 3) and it is absolutely impossible that one type of ionic liquid could be used for all synthetic applications described in Chapters 5-8. In view of the different possible roles of the ionic liquid in a given synthetic application (e.g., as catalyst, co-catalyst, or innocent solvent) this point is quite obvious. However, some properties, such as nonvolatility, are universal for all ionic liquids. So the answer becomes, if the property that you want is common to all ionic liquids, then any one will do. If not, you will require the ionic liquid that meets your needs. 

For Rh-catalysed hydroformylation the role of the ionic liquid as an innocent solvent is by far the most important. To our knowledge, none of the published research in this area claims special chemistry. The selectivity found with the different Rh-ligand complexes corresponds in most cases to the values obtained in traditional organic solvent or water (with the surprisingly low selectivity of TPPTS ligands in ionic liquids being a remarkable exception). Overall activities were found to be very comparable if mass transfer effects between the gas phase and the two immiscible liquid phases were overcome by proper stirring. 

Aware of the enhanced reactivity of trivalent [( ArO)3tacn)U] towards non-innocent solvents, such as ethers and chlorinated solvents, the reactivity of this molecule was challenged by exposure to more inert solvents like alkanes. Remarkably, recrystallization of 4 -bu u pentane solutions containing various cycloalkanes, i.e., methylcyclohexane, afforded the coordination of one cycloalkane molecule to the electron-rich U center (Scheme 4) (37). 

It has been reported that dichloromethane, frequently used in the reaction of poly(methimazolyl)borates, is not an innocent solvent, but is able to form slowly heterocyclic salts (Fig. 5.26) [H2C(mt)2BR2]Cl, (BR2 = BH2, BH(mt), 9-borabicyclononyl) which have been characterized by X-ray single-crystal studies.103... 

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). 

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