Hydrogen fluorous phase

The strategy of using two phases, one of which is an aqueous phase, has now been extended to fluorous . systems where perfluorinated solvents are used which are immiscible with many organic reactants nonaqueous ionic liquids have also been considered. Thus, toluene and fluorosolvents form two phases at room temperature but are soluble at 64 °C, and therefore,. solvent separation becomes easy (Klement et ai, 1997). For hydrogenation and oxo reactions, however, these systems are unlikely to compete with two-phase systems involving an aqueous pha.se. Recent work of Richier et al. (2000) refers to high rates of hydrogenation of alkenes with fluoro versions of Wilkinson s catalyst. De Wolf et al. (1999) have discussed the application and potential of fluorous phase separation techniques for soluble catalysts. 

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

A cationic complex, formed in situ from 5 and [Rh(COD)2]OTf, was also active in biphasic hydrogenation [14]. No preference for the fluorous phase was found for ligands containing only one perfluoroalkyl tail, but neutral and cationic complexes, containing mono- and bidentate 4a or 5, respectively, were selectively dissolved in the fluorous phase. No leaching and recycling studies were performed. 

In this section we describe two approaches for using dendrimer-encapsulated metal particles to perform biphasic catalysis. The first is hydrogenation catalysis using dendrimers rendered soluble in the fluorous phase by electrostatic attachment of perfluoroether groups [103]. The second demonstrates the use of perfluoroether groups covalently hnked to the dendrimer exterior to carry out a Heck reaction [100]. 

Homogeneous hydrogenation in the fluorous phase has been so far reported only for a limited set of simple olefins (Richter et al., 1999, Rutherford et al., 1998), as exemplified with the neutral rhodium phosphine complex 18 as catalyst precursor (eq. 5.7). Isomerization of the substrate 1-dodecene (17a) was observed as a competing side reaction under the reaction conditions. The catalyst formed from 18 could be recycled using a typical FBS protocol, but deactivation under formation of metal deposits limited the catalyst lifetime. 

With < 1 mol% MTO cyclobutanones are fully converted within one hour. Another approach consists of the use of a fluorous Sn-catalyst under biphasic conditions [245]. A perfluorinated tin(IV) compound, Sn[NS02C8F17]4, was recently shown to be a highly effective catalyst for BV oxidations of cyclic ketones with 35% hydrogen peroxide in a fluorous biphasic system (Fig. 4.83). The catalyst, which resides in the fluorous phase, could be easily recycled without loss of activity. 

Tuning to increase solubility of some non-fluorous compounds in fluorous media has recently been reported. Krytox, a commercially available poly(per-fluoroether) lubricant, has a carboxylic acid end group and when this is added to an FC-72 phase, extraction of substituted pyridines is enhanced.This has been attributed to the formation of a hydrogen bond complex. This study shows that non-covalent interactions can be used in modifying the fluorous phase and tailoring its properties for a particular separation. 

Gases are often thought to be much more soluble in a fluorous phase than in organic solvents. This misapprehension has perhaps come about as a result of the extensive research into artificial bloods, which contain perfluorinated compounds. In terms of mole ratios, oxygen is five times more soluble in perfluoromethylcyclohexane than in THF, and hydrogen 4.5 times more... 

Figure 3.2 Principle of the Union Carbide hydroformylation process [7], The reactands, precatalyst, and fluorous ligand in a fluorocarbon-hydrocarbon system are heated under carbon monoxide and hydrogen pressure to enable the catalyzed reaction in a homogeneous phase. On cooling the system separates and the expensive catalyst can be separated and re-used with the fluorous phase.

Fluorous scavengers do not necessarily need to form a covalent bond with the species they have to remove into the fluorous phase. For example, lightly fluorous N,N -dialkyl ureas can bind to perfluorocarboxyl ic acids by hydrogen... 

Scheme 3.26 Lightly fluorous N.N -dialkyl urea (Rp = QFn) has a relatively low partition coefficient of 30 70 in a C-Fp/CHjCIj biphasic system. After addition of perfluoroheptanoic acid, the partition coefficient of the resulting hydrogen bonded complex is 99 1, and the urea is completely removed from the organic into the fluorous phase [29].

Sharpless has shown that phenylselenic acid catalyzes the epoxidation of olefins with hydrogen peroxide or f-BuOOH [41]. However, the toxicity of selenium compounds precludes many applications of this catalytic epoxidation. To avoid contamination of the reaction products with selenium compounds polystyrene-bound phenylselenic acid has been used [42]. An alternative to the solid phase chemistry is the immobilisation of the selenium catalyst in the fluorous phase... 

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. 

Fluorous-phase soluble hydrogenation catalysts are obtained by conversion of N-acryloxysuccinimide-containing fluoroacrylate polymers into phosphine ligands and hence Wilkinson catalyst analogues." ... 

Bergbreiter, D. E. Franchina, J. G. Case, B. L. Fluoroaciylate-Bound Fluorous-Phase Soluble Hydrogenation Catalysis. Org. Lett., 2000, 2, 393-395. 

Another possibility, taking advantage of the biphasic environment, is to use fluorous organic solvents as the catalyst phase instead of water [155]. Crooks and coworkers prepared dendrimer-stabilized colloid catalysts soluble in the fluorous phase and used the catalysts in hydrogenation [156] and in a Heck reaction [157]. In both cases the colloidal catalyst in the fluorous phase was recyclable and showed some interesting selectivities and products unique to the nanoenvironment in the dendrimer interior. 

Although hydrogenation reactions in ILs are stiU in many ways in their infancy, as are those in other alternative solvents such as fluorous phases and supercritical fluids, some comparisons can be made. Water has now been evaluated extensively as an immobilization solvent for biphasic hydrogenation reactions and while there are many excellent systems, water is limited by substrate solubility and the fact... 

These points are further illustrated by some of the fluorous triarylphosphines in Section X of Table 3.6. Entry X-9 shows that with one ponytail of formula Rf6(CH2)3 per ring, the toluene phase affinity is higher (80.5 19.5). With one ponytail of formula Rf8(CH2)3 per ring, the fluorous phase affinity becomes higher (33.4 66.6 enfly X-10). It has proved problematic to replace additional aryl hydrogen atoms with Rf8(CH2)3 ponytails. However, a clever way around this problem has been developed " " namely, silicon-based ponytails of the formula (Rf (CH2)2)jcSi(CH3)3 have been used as anchors for as many as three Rf6... 

---