Biphasic catalysts aqueous organic solvents, recyclable systems

In many cases only one of the enantiomers displays the desired biological effect, the other is ineffective or even harmful. The development of enantioselective catalysis in non-aqueous solvents has been closely followed by the studies of similar aqueous systems - logically, attempts were made in order to solubilize the ligands and catalysts in aqueous media. Using aqueous/organic biphasic systems (often water/ethyl acetate) one may have a possibility of recovery and recycle of the often elaborate and expensive catalysts. However, with a few exceptions, up till now catalyst recovery has been rather a desire than a subject of intensive studies, obviously because of the lack of large-scale synthetic processes. 

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. 

The method in which palladium complexes with hydrophilic phosphines are used in a biphasic system of water-organic solvent can be considered complementary to the standard protocol. In this case, boronate and palladium catalyst reside in the aqueous phase, while halide substrate is in the organic phase. In order for the reaction to run, the latter should be partitioned into the aqueous phase. Alternatively, oxidative addition may occur at the interface. Due to the low efQcienc of both methods, high loads of palladium catalyst and phosphine are required. Recycling is possible but is hampered by the accumulation of inorganic salts (hahde, borate) in the aqueous layer. 

The reason for this is reaction 5.2.4.1 the ligand acquires ionic character due to protonation by carbonic acid (aqueous solution of CO ). By bubbling through the biphasic system, the dissolved CO can be driven off. This makes the catalyst soluble in organic solvent again, and the organic layer can then be recycled. 

Reactions of the same substrate with several nucleophiles were also catalyzed by the water-soluble Pd-complex of a phosphinite-oxazoline ligand which was prepared from natural D-glucosamine (Scheme 6.23) [53]. The catalyst dissolves weU both in water and in CH3CN but not in diethyl ether. Therefore the reactions could be ran either in water/toluene biphasic systems or in homogeneous water/CHaCN solutions. In the latter case, phase separation could be induced by addition of diethyl ether upon which the catalyst moved quantitatively to the aqueous phase. The product was obtained from the organic phase by evaporation of the solvent(s) and the aqueous solution of the Pd-complex was recycled. In aqueous systems the... 

Catalysis in liquid-liquid biphasic systems has developed recently into a subject of great practical interest because it provides an attractive solution to the problems of separation of catalysts from products and of catalyst recycle in homogeneous transition metal complex catalysis. Two-phase systems consist of two immiscible solvents, e.g., an aqueous phase or another polar phase containing the catalyst and an organic phase containing the products. The reaction is homogeneous, and the recovery of the catalyst is facilitated by simple phase separation. 

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