Tunable solvents for homogeneous catalyst recycle

A novel class of tunable solvents facilitates recycle of homogeneous catalysts for both economic and environmental advantages. In a mixed organic-aqueous tunable solvent (OATS), reactions between water-soluble catalysts and moderately hydrophobic substrates can be run homogeneously, with subsequent phase separation for product purification and catalyst reuse. One example demonstrated is the dimethyl ether (DME)-water system, which has been employed as a benign alternative to organic solvents for alcohol dehydrogenase (ADH)-catalyzed reduction of hydrophobic ketones coupled with regeneration of the cofactor NADH. We also show the feasibility of a biphasic DME -water separation scheme to couple with the biocatalytic reaction. The subsequent downstream processing offers the advantages of easily isolating water-insoluble products from the aqueous phase and recycling enzyme - cofactor. Other OATS systems are discussed where the preferential dissolution of modest pressures of CO2 causes phase separations, which result in very large distribution coefficients of target molecules in the biphasic organic-aqueous system, with substantial promise as tunable solvents for biocatalysis.