Biphasic aqueous—organic systems

Kinetic studies performed on model compounds were aimed at understanding the effect of different parameters on the selectivity. They showed that selectivity was achieved only when A336 was present. In fact, in the absence of A336 and of the base the hydrodehalogenation of p-chloroacetophenone proceeded aU the way to ethylcyclohexane in the biphasic aqueous-organic system. When A336 was added, selectivity was reversed—chloride was removed first—and the selective dehalogenated benzyl alcohol was obtained. ... 

The C-C couphng of aromahc hahdes usually requires the addition of a base in stoichiometric amounts, and the most common Bronsted bases are water soluble and can be employed conveniently in biphasic aqueous/organic systems. 

Ascon-Cabrera, M., Lebeault, J.-M. (1993). Selection ofxenobiotic-degrading microorganisms in a biphasic aqueous-organic system. Applied and Environmental Microbiology 59 1717-1724. 

P. Kuhl, A. Konnecke, G. Boring, H. Daumer, H.D. Jakubke, Enzyme catalyzed peptide synthesis in biphasic aqueous-organic systems. Tetrahedron Lett. 21 893-896 (1980)... 

Den Hollander et al. [ 14,16] investigated the enzymatic hydrolysis of penicillin G to phenylacetic acid and 6-aminopenicillanic acid in biphasic aqueous-organic systems without pH-control. In a preliminary study, the two phases were counter-currently contacted in a discrete manner, so that equilibriiun was reached in each stage. Sets of three and five shake flasks served to mimic equilibrium stages in the counter-current set-up. It was shown, that counter-current contact leads to significant improvement of the equilibrium conversion relative to the batch or co-current situation. When penicillin G was fed in an intermediate stage, either exit contained mainly one of the two products. This simplifies product recovery. 

Lowering the water-activity of the medium [1565] by using water-miscible organic cosolvents such as ethanol or methanol. Alternatively, the reaction can be carried out in a biphasic aqueous-organic system or in a monophasic organic solvent (e.g., ethyl acetate, di-/-propyl, or methyl t-butyl ether) which contains only traces of water to preserve the enz3mie s activity. 

In this chapter, latest advancements in solvent engineering in bioreductions and greener needs for bioreaction media have been discussed in depth with recent examples. Solvents for bioreductions may be categorized as (i) aqueous (ii) water/water-miscible (monophasic aqueous-organic system) (iii) water/ water-immiscible (biphasic aqueous-organic system) (iv) nonaqueous (mono-phasic organic system, including solvent-free system) and (v) nonconventional media (e.g., ionic liquids, supercritical fluids, gas-phase media, and reverse micelles). 

The use of biphasic aqueous-organic systems in biocatalytic reductions is of great interest because the enzyme and its cofactor are dissolved in the aqueous phase, where the reaction takes place, while the hydrophobic substrate and product are mostly located in organic solvent layer and partitioned into the aqueous phase. This distribution reduces the concentrations of toxic substrate and product around the enzyme in aqueous layer and relieves the enzyme from substrate and product inhibition. Other distinctive features of this biphasic system are simple separation, easy regeneration of the enzyme, and easy recovery of the products. However, in this system, the reaction rates are relatively low because of a low rate of mass-transfer across the interface. Although this hindrance can be eliminated by intensive agitation, the increased interface often results in faster denaturation and inactivation of the enzyme. 

Hereby, poly(p-phenylene) polymers containing ether and carbonyl linkages in the polymer backbone are accessible. By polymerization of the AB2 monomer 3,5-dibromobenzene boronic acid in a biphasic aqueous/organic medium, Kim and Webster obtained hyperbranched polyphenylenes [233]. Suzuki polycondensation in aqueous systems has proven to be a versatile method, which has been applied to the synthesis of various polymer types ]234]. 

The temperature of the oil bath is 130°C. The initially observed biphasic (aqueous-organic) reaction system gradually turns to a homogeneous solution as the reaction proceeds. 

In the case of benzylideneacetone, the conversion of 61% was significantly lower than with a biphasic aqueous-organic catalyst system (87% conversion) or the conventional homogeneous catalyst system (100% conversion). However, the observed selectivity in the case of catalyst 22 was 96%, i.e. almost selective reduction of the olefinic moiety took place whereas the carbonyl substructure... 

Multiply substituted and higher olefins could be successfully hydroformy-lated in PEG-containing organic-organic biphasic systems using the [Rh(PEG)J catalyst precursor. This compoiuid is also active in aqueous-organic systems, and its properties are discussed in the subsection on Hydroformylation under Aqueous-Organic Biphasic Catalysis. 

Preparative-scale Kinetic Resolution Using Aldolase Antibodies in a Biphasic Aqueous-Organic Solvent System... 

Depicted in Fig. 2, microemulsion-based liquid liquid extraction (LLE) of biomolecules consists of the contacting of a biomolecule-containing aqueous solution with a surfactant-containing lipophilic phase. Upon contact, some of the water and biomolecules will transfer to the organic phase, depending on the phase equilibrium position, resulting in a biphasic Winsor II system (w/o-ME phase in equilibrium with an excess aqueous phase). Besides serving as a means to solubilize biomolecules in w/o-MEs, LLE has been frequently used to isolate and separate amino acids, peptides and proteins [4, and references therein]. In addition, LLE has recently been employed to isolate vitamins, antibiotics, and nucleotides [6,19,40,77-79]. Industrially relevant applications of LLE are listed in Table 2 [14,15,20,80-90]. 

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