Catalysts temperature dependent solubility

Thermoregulated phase-transfer and phase-separable catalysis are attractive catalyst recycUng techniques complementing other approaches of multiphase catalysis. They utilize temperature-dependent solubility or miscibiUty phenomena to switch between homogeneous reaction and heterogeneous separation stages. 

A compound with a highly temperatine-dependent property, such as solubility, is said to be thermomorphic. There are a variety of other catalyst systems that can be recovered on the basis of temperature-dependent solubilities, usually involving polymers [13-18]. Attention has also been given to nonthermal solubility switches . These include photochemically and chemically triggered precipitons [19-25], and the use of CO2 pressure to regulate solubilities [26-28]. The latter protocol is particularly suited to liuorous compounds. 

To our knowledge, there have been no previous attempts to develop a broad class of molecular catalysts that have temperature-dependent solubilities. When molecular catalysts are covalently bound to polymeric supports, they generally assume the solubihty properties of the host polymer. In the above fluorous catalysts, we Hke to think that a short segment of polymer is being grafted onto a molecular catalyst, hi other words, the ponytails can be viewed as pieces of Teflon , which impart more and more of the solubility characteristics of the polymer as they are lengthened. 

Gladysz JA, Tesevic V (2008) Temperature-Controlled Catalyst Recycling New Protocols Based upon Temperature-Dependent Solubilities of Fluorous Compounds and Solid/Liquid Phase Separations. 23 67-89... 

In 1993 Bergbreiter prepared two soluble polymer-supported phosphines that exhibited an inverse temperature-dependent solubility in water [52]. Although PEG-supported phosphine undergoes a phase-separation from water at 95-100 °C, the PEO-poly(propylene oxide)-PEO supported catalyst was superior as it is soluble at low temperatures and phase-separates at a more practical 40-50 °C. Treatment of a diphenylphosphinoethyl-terminated PEO-PPO-PEO triblock copolymer... 

In an extension of this work, the reuse of the polymeric catalyst was addressed and several new PE-poly(alkene) glycol copolymers were prepared [68]. Commercially available oxidized polyethylene (CO2H terminated, both high and low molecular weight) was converted to the acid chloride and reacted with Jeffamine D or Jeffamine EDR, and subsequently converted to the tributylammonium bromide salt with butyl bromide. These new quaternary salts were shown to catalyze the nucleophihc substitution of 1,6-dibromohexane with sodium cyanide or sodium iodide. While none of the polymeric quaternary salts catalyzed the reaction as well as tetrabutylammonium bromide, the temperature-dependent solubility of the polymers allowed removal of the polymer by simple filtration. 

In Chapter 7 we have already discussed the use of fluorous biphasic systems to facilitate recovery of catalysts that have been derivatized with fluorous ponytails . The relatively high costs of perfluoroalkane solvents coupled with their persistent properties pose serious limitations for their industrial application. Consequently, second generation methods have been directed towards the elimination of the need for perfluoro solvents by exploiting the temperature-dependent solubilities of fluorous catalysts in common organic solvents [42]. Thus, appropriately designed fluorous catalysts are soluble at elevated temperatures and essentially insoluble at lower temperatures, allowing for catalyst recovery by simple filtration. 

In most cases the catalytically active metal complex moiety is attached to a polymer carrying tertiary phosphine units. Such phosphinated polymers can be prepared from well-known water soluble polymers such as poly(ethyleneimine), poly(acrylic acid) [90,91] or polyethers [92] (see also Chapter 2). The solubility of these catalysts is often pH-dependent [90,91,93] so they can be separated from the reaction mixture by proper manipulation of the pH. Some polymers, such as the polyethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers, have inverse temperature dependent solubility in water and retain this property after functionalization with PPh2 and subsequent complexation with rhodium(I). The effect of temperature was demonstrated in the hydrogenation of aqueous allyl alcohol, which proceeded rapidly at 0 °C but stopped completely at 40 °C at which temperature the catalyst precipitated hydrogenation resumed by cooling the solution to 0 °C [92], Such smart catalysts may have special value in regulating the rate of strongly exothermic catalytic reactions. 

The inverse temperature-dependent solubility in aqueous media of polymer-bound palladium(0)-phosphine catalysts, based on the water-soluble polymer poly(Wisopropyl)acrylamide (PNIPAM) 28, was also used to recycle and reuse these catalysts in nucleophilic allylic substitutions (Equation (8)) and cross-coupling reactions between aryl iodides and terminal alkynes (Equation (9)). The catalyst was highly active in both reactions, and it was recycled 10 times with an average yield of 93% in the allylic nucleophilic substitution by precipitation with hexane. ... 

While polyethylene oligomers complete insolubility cold and solubility hot as a function of temperature provides a thermomorphic way to separate a catalyst and product, it should be noted that polymers are not the only vehicle for thermomorphic separations that involve a quantitative temperature-dependent solid/liquid separation. This is most evident in fluorous systems. For example, Gladysz has described several examples of fluorinated catalysts that are insoluble in organic solvents cold but soluble hot [74]. Qualitatively, these catalysts behave as if they were attached to a piece of Teflon that had temperature-vari-able solubility like the polyethylene oligomers above. Similar temperature-dependent solubility has been noted with other fluorous catalysts too [75-77]. 

In this respect, fluorous-phase operation is similar to temperature-regulated phase transfer catalysis (see Section 2.3.5) and to special versions of soluble polymer-bound catalysis (see Chapter 7). Alternatively, the temperature-dependent solubilities of solid fluorous catalysts in liquid substrates or in conventional solvents containing the substrates could eliminate the need for fluorous solvents. 

Gladysz s group has also reported the temperature-dependent solubility of the solid phosphine catalyst 5 in octane [3]. Between 20-80 and 20-100 °C, 5 exhibits ca. 60- and 150-fold increases of solubility in octane. Although octane is one of the best organic solvents for dissolving nonpolar fluorous compoimds, little 5 can be detected at 0 °C by GC (0.31 mM) or NMR. At 20 °C, millimolar concentration... 

Mikami s group has also demonstrated the advantage of the fluorous super-Lewis acids such as lanthanide tris(perfluorooctanesulfonyl)methide and perfluorooctane-sulfonimide complexes with respect to temperature-dependent solubility [13bj. For example, these complexes can be re-used for the Friedel-Crafts acylation reaction without fluorous solvents [Eq. (11)]. After the reaction mixture of anisole has been heated with acetic anhydride in 1,2-dichloroefhane in the presence of ytterbium perfluorooctanesulfonimide (10 mol%) at 80 °C for 6 h, the mixture is allowed to stand at -20 °C for 30 min to precipitate the ytterbium complex. The liquid phase is decanted and the residual lanthanide complex is re-used without isolation. No loss of activity is observed for the catalyst recovered. The total isolated yield of the product, which is combined from the three runs, is 78%. 

Figure 1 Catalyst recovery and product separation by temperature-dependent solubility of polyethylene oligomers. (Reprinted with permission of ACS from [la], 2002).

Temperature-dependent phase behavior was first applied to separate products from an ionic liquid/catalyst solution by de Souza and Dupont in the telomerization of butadiene and water [34]. This concept is especially attractive if one of the substrates shows limited solubility in the ionic liquid solvent. 

One possibility to raise the solubility of the polar catalyst in the solvent mixtures is to use a higher water content. In the TMS system si toluene DMF a larger amount of the semi-polar solvent is required to obtain a homogeneous solution, if more water is added. If the amount of water is doubled the amount of s3 increases from 1 5 4.4 to 1.35 5 6.1 and a ratio of 2 5 8.9 is needed, if the water content is four times higher. The same tendency is observed if different non-polar solvents s2 or different mediators s3 are used the higher the water content the more of the mediator s3 is required. The temperature dependency is almost not affected when more water is added to the solvent systems. 

TBHP and the molybdenum catalysts are soluble in imidazohiun-based RTlLs. The system becomes biphasic when the olefinic substrate is added. In all cases, the TOFs of the catalytic reactions are considerably lower with the ionic solvent than when performed without the ionic solvent (data reported in Table 9). This slower catalytic reaction may be due to dilution effects and phase transfer problems, especially with the olefin, which is quite insoluble in the RTIL. The conversion appears to be strongly temperature-dependent, as decreasing the temperature from 55 °C to 35 °C reduces the conversion by ca. 50% (entries 7 and 8, Table 9). With the dioxomolybdenum complexes 1 and 2, the epoxidation reaction proceeds with 100% selectivity (Table 9), whereas some diol is formed with the catalyst 3. 

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