Fluorous solvents, phase separation, effect

Although the biphasic properties of fluorous-organic systems are desirable for separations, monophasic conditions would favour enhanced reaction rates. Therefore, it is important to know the general miscibilities of fluorous solvents and the effect of temperature (Tables 7.2 and 7.3). In Table 7.2, the temperature given for the phase separation is a consulate or upper critical solution temperature. However, these temperatures should only be taken as a guide, as... 

To effect the reaction, one heats a biphasic mixture of an alkene, toluene (the organic solvent), a fluorous phosphine, a standard Rh salt, perflu-oromethylcyclohexane (the fluorous solvent), CO, and H2. When the hydroformylation is complete, the mixture is cooled so as to induce the phases to separate. After cooling, the toluene phase containing the organic products is separated from the fluorous... 

These fluorous Rh catalysts are also effective in the hydrosilylation of enones, which employs a similar binary solvent system (Scheme 6). Hexanes and ether can be used in place of toluene. Since these organic solvents are miscible with PFMC at room temperature, effective phase separation requires cooling to -30°C. 

Following the publication of the first example of fluorous biphase catalysis by Horvath and Rabai in 1994 [1], the immediate focus was to develop catalysts that would exhibit very biased partition coefficients with respect to fluorous and organic solvents. Such liquids are normally immiscible at room temperature. This was done by attaching ponytails of the formula (CH2)m(CF2) -iCF3 (abbreviated (CH2)mRf )> including arrays emanating from silicon atoms [2]. Catalysis was then effected at elevated temperatures, where fluorous and organic solvents are commonly miscible, with prod-uct/catalysis separation at the low-temperature two-phase limit. 

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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