Biphasic disadvantage

The concept of this biphasic catalysis implies that the organometallic catalyst is soluble in only one phase whereas the reactants and products are confined almost entirely to the other phase. In most cases, the catalyst phase can be reused, and the products and reactants are simply removed from the reaction mixture by decantation. In successful processes involving biphasic catalysis, the advantages of homogeneous catalysis listed above may be realized without the disadvantages of expensive separation of catalysts from products. 

One method is to run the reaction in an aqueous buffer/organic solvent biphasic system. This makes it possible to work at high substrate and product concentrations and at the pH-optimum of the enzyme. In addition, in water-immiscible solvents the non-enzymahc addition of HCN to the carbonyl group is non-existent or extremely slow. Possible disadvantages are enzyme deactivation at the interface and the presence of organic solvent dissolved in the aqueous phase [15, 17, 18]. 

One of the major disadvantages of using these enzymes to reduce ketones is generally the very poor aqueous solubility of the substrates (usually <5-10 mM) and the fact that the co-factor regeneration system is sensitive to organic solvents. To address this limitation, biphasic and membrane bioreactors have been used that allow for moderate to high conversions with often >99% ee for selected products. This has been demonstrated with the (.S )-Adi I from Rhodococcus erythropolis,36-37... 

Shin and Kim developed various methods aimed at increasing the product concentrations of transaminase catalyzed amine resolutions, through the contin uous removal of product ketone from the reactions (Figure 14.21). The application of an aqueous/organic two phase system to the co transaminase catalyzed resolution of racemic ( methylbenzylamine 1 was found to be superior to an aqueous only system in product concentration obtained [26, 27]. A drawback of the biphasic system was an increased enzyme deactivation rate compared to the aqueous only system due to the aqueous/organic emulsion. Another disadvantage was the... 

Oxidation with oxygen gas can be carried out in aqueous biphasic systems. Primary and secondary alcohols have been oxidized to the corresponding aldehydes or ketones, respectively, using a palladium(I I) complex (Equation 4.11) [18]. No organic solvents were used, except the substrate is a solid, and the catalyst could be easily recycled and reused by simple phase separation, because the aqueous phase is the lower layer and so can be recycled. The only disadvantage of water as a solvent for oxidations with oxygen/air is the low solubility of oxygens in water. 

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