Catalysis of Organic..

396 Recycling Homogeneous Catalystsmethods offers advantages and disadvantages that must be balanced cautiously.Homogeneously catalyzed reactions are highly efficient in terms of selectivity (i.e.regioselectivity, enantiomeric excesses) and reaction rates, due to theirmonomolecular nature. Unfortunately, catalyst recovery can be very difficult (due tothe homogeneous nature of the solution) and product contamination by residualcatalyst or metal species is a problem. In contrast, heterogeneously catalyzedreactions allow easy and efficient separation of high value products from the catalystand metal derivatives. However, selectivity and rates are often limited by themultiphasic nature of this system and/or variations in active site distribution from thecatalyst preparation.Catalyst separation is crucial for industrial processes – to minimize the wastestreams and to develop potential catalyst recycling strategies. Therefore, efforts havebeen made to improve the recovery of highly selective homogeneous catalysts bydeveloping new multiphasic solvent systems. We have developed several techniquesusing CO 2 -expanded liquids and supercritical fluids to create a medium forperforming homogeneous reactions while maintaining the facile separation ofheterogeneous systems.Results and DiscussionOne example of a recoverable homogeneous catalytic system involves the addition ofCO 2 to fluorous biphasic systems (1,2). In fluorous biphasic systems, a fluoroussolvent (perfluoroalkane, perfluoroether or perfluoroamine) is employed as anorthogonal phase, immiscible with most common organic solvents and water. Anorganometallic catalyst can be made preferentially soluble in a fluorous solvent byintroduction of one or more fluorous side chains, or “ponytails” (3) with hydrocarbonspacers (4) to mitigate the electron-withdrawing effects of the fluorines. Usually,multiple ponytails are required to impart preferential solubility to mostorganometallic complexes (5). The mutual immiscibility of fluorous and organicsolvents (6) provides an opportunity for facile separation of reaction components andthe recycle of the expensive fluorous-derivatized homogeneous catalyst. However,mass transfer limitations in biphasic systems can limit overall reaction rate. Insystems containing nonpolar solvents, such as toluene, heating the biphasic reactionmixture to around 90°C will induce miscibility (3). However for more polar orthermally labile substrates this is not a viable option as the consulate point is muchhigher than 100 °C (7,8). Thus, any polar reactants must be diluted into a nonpolarsolvent, introducing an extra volatile organic compound into the process. Instead ofheating, a homogenizing agent such as benzotrifluoride (BTF, 9) can be added tomixture. However, BTF is expensive and its recovery is not trivial.Alternatively, we have shown that CO 2 can be used to induce miscibility offluorocarbon-hydrocarbon mixtures (see Figure 1), even those involving polarcompounds such as methanol (2). Fluorinated organometallic complexes have beenwell established to have significant solubility in supercritical CO 2 , and their use ascatalysts in this medium is well developed (10). This allows the homogeneouslycatalyzed reaction to be carried out in the CO 2 -expanded homogeneous solution.