Aqueous-hexane biphasic system

Figure 9.3 Asymmetric reduction of citral to both (R)- and (S)-citronellal in an aqueous-hexane biphasic system.

Converting olive oil to an interesterified fat (1- and 3-positionaI specific interesterification) Immobilized lipase from Rhizopus delemar Water saturated n-hexane No comparison of yields in aqueous and biphasic system available Yokozeki et al. (1982)... 

When the products are partially or totally miscible in the ionic phase, separation is much more complicated (Table 5.3-2, cases c-e). One advantageous option can be to perform the reaction in one single phase, thus avoiding diffusional limitation, and to separate the products in a further step by extraction. Such technology has already been demonstrated for aqueous biphasic systems. This is the case for the palladium-catalyzed telomerization of butadiene with water, developed by Kuraray, which uses a sulfolane/water mixture as the solvent [17]. The products are soluble in water, which is also the nucleophile. The high-boiling by-products are extracted with a solvent (such as hexane) that is immiscible in the polar phase. This method... 

Ionic liquids can be used in an ingenious way to overcome these problems. The palladium catalyst is dissolved in a water-immiscible IL solvent, for example, [bmim][PF6], followed by addition of the organic reactants and triethylamine either neat or in a hydrocarbon solvent (eg, hexane). In a triphasic process, water makes the third phase. The overall result in such a reaction is that the reactants and products will reside in the organic phase the catalyst in the ionic liquid phase and the [(C2H5)3NH]X by-product moves to the aqueous phase. In such a case, the catalyst can be directly reused upon phase separation. Alternatively, the reaction can be run in a biphasic system with no aqueous phase, and the catalyst-containing IL phase is washed free of the water-soluble by-products before recycling. 

Example 1. Kinetic Resolution in a Biphasic System [20]. A solution of antibody 84G3 (500 mg, 6.67 pmol active site, 0.0086 mol%) in phosphate buffered saline (PBS pH 7.4, 87.5 mL) at room temperature vas added to a solution of ( )-45 (20 g, 77.4 mmol) in toluene (600 mL) in a Teflon tube. The mixture vas shaken at 250 rpm. under argon at the same temperature and the ee vas monitored by chiral-phase HP LG. When ee > 99% vas reached (65 h) the mixture vas cooled to —20 °G for several hours to enable the aqueous phase to freeze. The organic phase vas decanted. The frozen aqueous phase vas left to thaw and extracted vith toluene (3x3 vol. equiv.) and 1 1 EtOAc-toluene (3x3 vol. equiv.). The combined organic phase vas dried over Na2S04, concentrated in vacuo, and purifled by silica gel column chromatography (1 3 EtOAc-hexane) to afford (S)-45 (10 g, 50%). 

Studies indicated that alkane compounds (e.g., hexane and heptane), which will not distort the water coat of enzyme molecules, are also good candidates. For example, an aqueous-hexane (1 1) biphasic system was identified to have an... 

To overcome the product inhibition caused by benzaldehyde, an extractive biphasic reaction system was applied with hexane used as an extractor. Thus, considerable enhancements of reaction yield, up to 96%, were achieved compared to the aqueous system with conversion of about 39%. 

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