Fluorous Liquids

Fluorous biphasic catalysis is another active area in multiphasic homogeneous catalysis. The term fluorous was introduced [90] as the analogue to the term aqueous, to emphasize the fact that one of the phases of a biphase system is richer in fluorocarbons than the other. Fluorous biphase systems can be used in catalytic chemical transformations by immobilizing catalysts in the fluorous phase. A fluorous catalyst system consists of a fluorous phase containing a preferentially fluorous-soluble catalyst and a second product phase, which may be any organic or inorganic solvent with limited solubility in the fluorous phase (Figure 2.8a).

Conventional homogeneous catalysts can be made fluorous-soluble by incorporating fluorocarbon moieties to their structure in appropriate size and number. The most effective fluorocarbon moieties are linear or branched perfluoroalkyl chains with high carbon numbers that may contain other heteroatoms. The perfluoroaryl groups offer dipole-dipole interactions, making perfluoroaryl-containing catalysts soluble in common organic solvents and therefore less compatible with fluorous biphase systems. The most effective fluorous solvents are perfluorinated alkanes, perfluorinated dialkyl ethers and perfluorinated trialkylamines. Their remarkable chemical inertness, thermal stability and nonflammability, coupled with their unusual physical properties, make them particularly attractive for catalyst immobilization. These materials have low toxicity by oral ingestion, inhalation or intraper-

Figure 2.8 (a) Examples offluorous solvents and ligands; (b) typical cations and anions used for the formation of ionic liquids. Source: adapted from Sheldon et al. [2] and Cole-Hamilton and Tooze [64].

itoneal injection, but their thermal degradation can produce toxic decomposition products.

A fluorous biphase reaction could proceed either in the fluorous phase or at the interface of the two phases, depending on the solubilities of the substrates in the fluorous phase. When the solubilities of the substrates in the fluorous phase are very low, the chemical reaction may still occur at the interface, or appropriate phasetransfer agents may be added to facilitate the reaction. A fluorous biphase system might become a one-phase system by increasing the temperature [90]. Thus, a fluorous catalyst could combine the advantages of one-phase catalysis with biphasic product separation by running the reaction at higher temperatures and separating the products at lower temperatures. Alternatively, the temperature-dependent solubilities ofsolid fluorous catalysts in liquid substrates or in conventional solvents containing the substrates could eliminate the need for fluorous solvents [91].

Fluorous catalysts are best suited for converting apolar substrates into products of higher polarity, as the partition coefficients of the substrates and products will be higher and lower, respectively, in the fluorous phase.

It is generally considered that the major obstacle to the commercialization of reactions employing the fluorous biphasic concept is the cost of the fluorous solvent and the cost of the ligand, which must contain very large amounts of fluorine to retain the catalyst within the fluorous phase. Cost may be reduced by a proper selection of the solvent and ligand, but often the unsolved problem of progressive ligand degradation still exists. In addition, notably, the environmental persistence and very high global warming potential of fluorinated compounds may make their licensing for wide scale use problematic.

One advantage of fluorous ligands, which also has implications for reducing the cost, is that such electron-withdrawing fluorous ligands (called ponytails) increase the stability constants for complex formation and favor the linear product more than less electron-withdrawing ligands. This means that high selectivity towards the linear product can be obtained at low ligand:metal ratio (low ligand loading).

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