The Biphasic System

Using such a catalytic system implies identifying a solvent that can achieve selective catalyst solubilization, with no impact on its activity. Two of these solvents are now used in industrial liquid-liquid biphasic catalytic processes butane-1,4-diol in Shell s SHOP oligomerization process and water in Ruhrchemie/Rhone-Poulenc s olefin hydroformylation process. 

Choosing a selective solvent for the catalyst is one of the most significant steps in the development of such a process. Due to moisture sensitivity and reactivity of the Dimersol alkylaluminum co-catalyst, protic media like butanediol or water are not suitable at all. At an early stage, Chauvin et al. anticipated that ionic liquids (ILs) could meet biphasic liquid-liquid solvent requirements [8]. 

Foreseeing the possibility of tuning the physico-chemical properties of these compounds by varying the nature of the anion and/or the cation, then taking into account the ionic state of the Dimersol catalyst, it was expected that ILs based on chloroalkyl aluminates would fit well. They do, indeed  

Association of alkyl chloroaluminate anions with N,N -dialkylimidazohum cations leads to salts that are liquid at low temperatures. 

These ILs are poorly miscible with octenes and longer olefins (this poor miscibility, actually, is the first prerequisite to form a biphasic system). Butene solubility is sufficient to stabilize the nickel active species and ensure high catalytic activity. 

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In the homogeneous Dimersol process, the olefin conversion is highly dependent on the initial concentration of monomers in the feedstock, which limits the applicability of the process. The biphasic system is able to overcome this limitation and promotes the dimerization of feedstock poorly concentrated in olefinic monomer. 

Recently, the use of ionic liquids instead of organic solvents has been published for the biphasic system. For PaHNL and SbHNL, the reaction rates are increased in comparison to organic solvents without a change of enantioselectivity. ... 

Scheme 8.6. Rhodium-catalysed hydrogenation in the biphasic system PEG/scCC>2...

Scheme 8.7. Continuous-flow hydroformylation of long chain olefins in the biphasic system IL/SCCO2...

Scheme 8.8. Continuous flow enantioselektive hydrovinylation using the biphasic system IL/scCC>2...

Details of the first stereoselective hydrogenation in ionic liquids were published by the group of Chauvin [68], who reported the enantioselective hydrogenation of the enamide a-acetamidocinnamic acid in the biphasic system [BMIM][SbF6]/ iPrOH (ratio 3 8) catalyzed by [Rh(cod) (-)-diop ][PF6]. The reaction afforded (S)-N-acetylphenylalanine in 64% enantiomeric excess (ee) (Fig. 41.4). The product was easily and quantitatively separated and the ionic hquid could be recovered, while the loss of rhodium was less than 0.02%. 

A special example for a regioselective hydrogenation in ionic liquids was reported by our group and by DrieRen-Holscher [96, 97]. Based on investigations in the biphasic system water/n-heptane, the ruthenium-catalyzed hydrogenation of sorbic acid to ds-3-hexenoic acid according to Scheme 41.3 in the system [BMIM][PF6]/MTBE was studied [98],... 

Scheme 41.3 Regioselective hydrogenation of sorbic acid in the biphasic system [BMIM][PF6]/MTBE.

Immobilization of this complex in the biphasic system [BMIM][SbF6]/iPrOH showed better results compared to the non-modified complex Me-BDPMI (Fig. 41.8, 3). The ionic catalyst solution was reused three times without loss of activity (Table 41.12). At the fourth run the conversion decreased, though high conversions could be still realized by increasing the reaction time. 

This polymeric phase simultaneously heterogenizes the transition-metal complex and the ionic liquid, so that the catalyst is fully recyclable. The SILP-cata-lyst was less active than the homogeneous reference system, but clearly more active than the biphasic system (Table 41.18). 

The biphasic system was transferred to a separatory funnel (250 mL) and extracted with ether (3 x 40 mL). The organic fractions were combined. The solvent was removed using a rotary evaporator, to produce a yellow oil and a white solid (polymerized trimethoxysilane). 

The biphasic systemic disease in BN rats was also shown after administration by the respiratory or digestive route [187, 188], Even mercury-containing pharmaceutical products have been shown to induce immune-type glomerulonephritis [189],... 

It is essential that all PSs are multiphase. The easiest case to handle is the biphase system consisting of a condensed phase (solid) and a void inside porous particles or between consolidated ensembles of nonporous or porous particles. The void occupies a part of the volume, s, which is referred to as porosity. The other part of a PS volume is equal to ri=(l -e), and is termed density of packing. It is filled with the condensed phase (see Section 9.4). Generally, PSs can include various condensed phases of different structure, including combinations of solid(s) and liquid(s). 

If nonvolatile liquids are to be used to avoid the problems associated with volatile organic solvents, then it is very desirable that there is some convenient way of recovering the reaction products from the liquid. This approach is used in the biphasic systems described in Chapters 2-5. In the fluorous biphase (Chapter 3), reagents and catalysts are fine-tuned by adding perfluoroalkyl chains, known as ponytails , to ensure that only those chemicals will mix with the fluorous layer. Purification is simply a matter of separating the two phases. Transition metal catalysts with fluorous ligands will remain in the fluorous phase, and the whole catalyst-solvent mixture may be reused for another batch of reactions, as shown schematically in Figure 1.20b. 

As for a single phase system, the rate of the reaction is still dependent on the probability of reactants meeting and therefore on the concentration of the reagents. However, in the biphasic system, the critical concentration of these components is no longer their total concentration in the whole system but the concentration where the reaction takes place. This concentration will be dependent on a number of factors, and the most influential are the rate of diffusion of the reactants to the catalyst and the relative solubility of the reagents in each phase. These two factors are interdependent, and will be considered in turn. 

The results of this analysis of the product and catalyst distribution show that only a limited range of systems may be apphcable for the telomeriza-tion of butadiene and carbon dioxide. The reaction was performed in the biphasic systems EC/2-octanol, EC/cyclohexane and EC/p-xylene. The yield of 5-lactone reached only 3% after a reaction time of 4 hours at 80 °C. hi the solvent system EC/2-octanol triphenylphosphine was used as the hgand. With the ligand bisadamantyl-n-butyl-phosphine even lower yields were achieved in a single-phase reaction in EC or in the biphasic system EC/cyclohexane. The use of tricyclohexylphosphine led to a similar result, only 1% of the desired product was obtained in the solvent system EC/p-xylene, which forms one homogeneous phase at the reaction temperature of 80 °C. Even at a higher temperature of 100 °C and a longer reaction time of 20 hours no improvement could be observed. Therefore, we turned our interest to another telomerization-type process. 

We showed that the application of PEG/CO2 biphasic catalysis is also possible in aerobic oxidations of alcohols [15]. With regard to environmental aspects it is important to develop sustainable catalytic technologies for oxidations with molecular oxygen in fine chemicals synthesis, as conventional reactions often generate large amoimts of heavy metal and solvent waste. In the biphasic system, palladium nanoparticles can be used as catalysts for oxidation reactions because the PEG phase both stabilises the catalyst particles and enables product extraction with SCCO2. 

Fig. 13 Mean bubble sizes and specific surface areas of the dispersed gas phase in the biphasic system H2/H2O as a function of the hydrogen volume rate...

The resulting complex remained dissolved in the biphasic catalytic system. The 4-vinyl-l-cyclohexene product, obtained with 100% selectivity in [BMIM]PF6, was continuously separated from the reaction mixture by decantation, allowing the reuse of the remaining catalyst solution. The 1,3-butadiene conversion in the biphasic system was higher than that observed in homogeneous systems. Because the unconjugated product has a lower solubility in the ionic liquids than the conjugated butadiene feed, continuous separation of product contributes to the increased reaction rate in the ionic liquid. 

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