Catalysts separation, fluorous

The use of thermomorphic systems has recently been studied as a way of achieving catalyst separation in homogeneous catalysis. For example, a biphasic hydroformylation catalyst system was developed to take advantage of the unusual solvent characteristics of perfluorocarbons combined with typical organic solvents (4). Fluorous/organic mixtures such as perfiuoromethylcyclohexane... 

Fluorous Catalysts and Fluorous Phase Catalyst Separation for Hydrogenation Catalysis... 

I. T. Horvath, J. R abai, Facile Catalyst Separation without Water Fluorous Biphase Hydroformylation of Olefins , Science 1994, 266, 72. 

If nonvolatile liquids are to be used to avoid the problems associated with volatile organic solvents, then it is very desirable that there is some convenient way of recovering the reaction products from the liquid. This approach is used in the biphasic systems described in Chapters 2-5. In the fluorous biphase (Chapter 3), reagents and catalysts are fine-tuned by adding perfluoroalkyl chains, known as ponytails , to ensure that only those chemicals will mix with the fluorous layer. Purification is simply a matter of separating the two phases. Transition metal catalysts with fluorous ligands will remain in the fluorous phase, and the whole catalyst-solvent mixture may be reused for another batch of reactions, as shown schematically in Figure 1.20b. 

Horvath, I.T. and Rabai, J. (1994) Facile catalyst separation without water fluorous biphase hydroformylation of olefins. Science, 266, 72. 

Horvath, I.T., Kiss, G., Cook, R.A., Bond, J.E., Stevens, P.A., Rabai, J. and Mozeleski, E.J. (1998) Molecular engineering in homogeneous catalysis one-phase catalysis coupled with biphase catalyst separation. The fluorous-soluble HRh(CO) P[CH2CH2(CF2)5CF3]3 3 hydroformylation system. J. Am. Chem. Soc., 120, 3133. 

Apart from its use for solvation and separation of catalysts, the fluorous phase can also be used advantageously for separation processes during workup. Strategic synthesis planning is facilitated by tagging with fluorous residues to overcome the frequently limiting recovery and purification difficulties [1], As in solid-phase syntheses, an excess of components can be used to drive the reactions to completion. Side products can easily be separated if, for example, only the product is tagged with fluorous alkyl residues and therefore precipitates from the reaction mixture or is extracted with the fluorous phase. 

Fluorous biphasic catalysis was pioneered by Horvath and Rabai [54, 55] who coined the term fluorous , by analogy with aqueous , to describe highly fluori-nated alkanes, ethers and tertiary amines. Such fluorous compounds differ markedly from the corresponding hydrocarbon molecules and are, consequently, immiscible with many common organic solvents at ambient temperature although they can become miscible at elevated temperatures. Hence, this provides a basis for performing biphasic catalysis or, alternatively, monophasic catalysis at elevated temperatures with biphasic product/catalyst separation at lower temperatures. A number of fluorous solvents are commercially available (see Fig. 7.16 for example), albeit rather expensive compared with common organic... 

Other use of the functionalized chiral BINOL includes the 5,5, 6,6, 7, 7, 8,8 -octahydro derivative developed by Chan and coworkers, the titanium complex of which is more effective than BINOL in the enantioselective addition of triethylaluminum and diethylzinc a 4,4, 6,6 -tetrakis(perfluorooctyl) BINOL ligand developed for easy separation of the product and catalyst using fluorous solvents for the same zinc reaction an aluminum complex of 6,6 -disubstituted-2,2 -biphenyldiols used by Harada and coworkers in the asymmetric Diels-Alder reaction a titanium complex of (5 )-5,5, 6,6, 7,7, 8,8 -octafluoro BINOL employed by Yudin and coworkers in the diethylzinc addition, in the presence of which the reaction of the enantiomeric (/f)-BINOL is promoted . 

Last but not least, the success of aqueous-phase catalysis has drawn the interest of the homogeneous-catalysis community to other biphasic possibilities such as or-ganic/organic separations, fluorous phases, nonaqueous ionic liquids, supercritical solvents, amphiphilic compounds, or water-soluble, polymer-bound catalysts. As in the field of aqueous-phase catalysis, the first textbooks on these developments have been published, not to mention Job s book on Aqueous Organometallic Catalysis which followed three years after our own publication and which put the spotlight on Job s special merits as one of the pioneers in aqueous biphasic catalysis. Up to now, most of the alternatives mentioned are only in a state of intensive development (except for one industrial realization that of Swan/Chematur for hydrogenations in scC02 [2]) but other pilot plant adaptations and even technical operations may be expected in the near future. 

Although the conventional solvents such as aliphatic and aromatic hydrocarbons and ethers does not influence the n/iso ratio in hydroformylation (30), in supercritical carbon dioxide higher n/iso ratio was found for propene (44,45) and for 1-octene (46,47). In perfluoroalkane-toluene mixture, practically the same product composition is obtained as in toluene alone, but the former solvent combination allows a one-phase catalysis coupled with a temperature-dependent effective biphase catalyst separation (48-51). For the review of fluorous biphasic hydroformylation, see Ref (52). 

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