Thermomorphic behavior

Each catalyst was tested in three successive catalytic runs of 2 h at a reaction temperature of 55 °C in order to take advantage of the thermomorphic behavior of the solvent mixture. This allowed the actual catalytic conversion to take place in a homogeneous reaction medium under these conditions, with the phases separating again upon cooling to ambient temperature at the end of the reaction. The catalysts tethered to the hyperbranched PEIs were also tested under similar reaction conditions, and the results obtained are represented in Figure 19.3. 

A method has been developed for the continuous removal and reuse of a homogeneous rhodium hydroformylation catalyst. This is done using solvent mixtures that become miscible at reaction temperature and phase separate at lower temperatures. Such behavior is referred to as thermomorphic, and it can be used separate the expensive rhodium catalysts from the aldehydes before they are distilled. In this process, the reaction mixture phase separates into an organic phase that contains the aldehyde product and an aqueous phase that contains the rhodium catalyst. The organic phase is separated and sent to purification, and the aqueous rhodium catalyst phase is simply recycled. 

Another approach to isolate the catalyst from the products is the application of perfluorinated catalytic systems, dissolved in fluorinated media [63], which are not non-miscible with the products and some commonly used solvents for catalysis like THE or toluene at ambient temperature. Typical fluorinated media include perfluorinated alkanes, trialkylamines and dialkylethers. These systems are able to switch their solubility properties for organic and organometallic compounds based on changes of the solvation ability of the solvent by moving to higher temperatures. This behavior is similar to the above-mentioned thermomorphic multiphasic PEG-modified systems [65-67]. 

There are several different methods to separate PNIPAM-supported catalysts from the reaction mixtures. Both liquid-solid separations and liquid-liquid separations can be used. The most frequently used liquid-solid separation method takes advantage of the varying solubility of polymers in different solvents. For example, PNIP AM can be precipitated from THF into hexanes. PNI-PAM copolymers also exhibit lower critical solution temperature (LCST) behavior. Specifically, PNIPAM and its copolymers can be prepared such that these polymers are soluble in water at low temperature but precipitate when heated up. This property may be used as either a purification method or a separation tech-nique.[l 1] A thermomorphic system is a liquid-liquid biphasic system developed in our group. It uses various solvent mixtures with temperature-dependent miscibility to effect separation of catalysts from substrates and products, as shown in Figure 2. 

In allylic substitutions, a similar behavior was observed in thermomorphic solvent mixtures to that in the aforementioned reaction in toluene (100% conversion in five consecutive reaction cycles, each 11 h at 70 °C in heptane/Et0H/H20, 10 9 1) [4]. 

In a related example, Bergbreiter and Bazzi [37] also prepared catalysts 30 and 31, which were functionalized with oligomeric polyethylene (PE MW = 500) groups. These species were found to exhibit thermomorphic solubiUty behavior they were soluble in hot, nonpolar solvents, but were essentially insoluble in any... 

---