Ionic liquids medium/solvent effects

Recently, ionic liquids have been employed as solvents in many catalytic processes, because they provide effective media for reactions involving ionic intermediates. Easy separation of organic products from ionic Uquids is also advantageous. An efficient and rapid method for the oxidation of cydohexene in an ionic liquid medium has been devdoped using a microreactor (length of the channel of the microreactor 3 cm, width 200 pm, depth 50 pm) (Scheme 7.10) [96, 99]. The yield of product is higher than that with conventional batch reactors. The water-soluble ionic liquid 1-butyl-3-methylimidazoUum tetrafluoroborate is used to improve the solubility of cydohexene in the reaction buffer. 

The hydrogenation of C02 in the presence of amines to give dialkylformamides has been carried out directly in an IL/scC02 system. In this case, the ionic liquid was shown to play a dual role [74]. It is an effective solvent for the ruthenium phosphine catalyst and at the same time allows a distinct phase distribution of the polar carbamate intermediates and the less polar products formed during the conversion of C02. As a result, the selectivity of the reaction can be increased over conditions where scC02 is used as the sole reaction medium. 

Intermolecular hydroamination or hydroarylation reactions of norbornene and cyclo-hexadiene carried out with catalytic amounts of Brpnsted or Lewis acid in ionic liquids have been found to provide higher selectivity and yields than those performed in classical organic solvents. This effect was attributed to the increases of the acidity of the medium and stabilization of ionic intermediates through the formation of supramolec-ular aggregates with the ionic liquid.38... 

Few experiments allow one to bridge gas-phase electron transfer mechanism to liquid (or condensed)-phase electron transfer reactions. The major problem is to model the so-called solvent coordinates in the gas phase. Of course, clusters seems to be the ideal medium to build solvent effects in a stepwise manner. However, clusters are much colder than liquids, with the consequence that only a limited number of isomers are explored, as compared with the room temperature configurations involved in liquid processes. Discrepancies are observed in the case of cluster solvation of ionic molecules in clusters Nal remains at the surface of water clusters whereas it dissolves in bulk water [275]. Clusters thus do not allow one to explore in a single step all the aspects of a liquid-phase electron-transfer reaction. Their main advantage arises from this limitation since they allow one to study separate aspects of the solution processes. 

Ionic liquids are however more just than a bulk medium and the dielectric constant may be not the best parameter to define ILs polarity. They are constituted by positive and negative ions which can exert various effects. Recently, the microscopic properties of ILs, i.e. the ability of these media to interact with specific dissolved species (reagents, transition states, intermediates and products), have been measured and several polarity scales, previously developed for common molecular solvents, have been extended to ILs. At variance with molecular solvents, ILs are characterized by complex interaction forces between anion and cation and these interactions are competitive with the ability of both anion and cation to interact with dissolved species thus, multiparameters solvatochromic correlations, better than single point measurements, resulted useful to understand the solvent polarity. ... 

The cationic nickel complex [ /3-allylNi(PR3)]+, already described by Wilke etal. [21], as an efficient catalyst precursor for alkene dimerization when dissolved in chlorinated organic solvents. It proved to be very active in acidic chloroaluminate ionic liquids. In spite of the strong potential Lewis acidity of the medium, a similar phosphine effect is observed. Biphasic regioselective dimerization of propylene into 2,3-dimethylbutenes can then be achieved in chloroaluminates. However, there is a competition for the phosphine between the soft nickel complex and the hard aluminum chloride coming from the dissociation of polynuclear chloroaluminate anions. Aromatic hydrocarbons, when added to the system, can act as competitive bases thus preventing the de-coordination of phosphine ligand from the nickel complex [22 b]. Performed in a continuous way, in IFP pilot plant facilities, dimerization of propene and/or butenes with this biphasic system (Difasol process) compares... 

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