Enantioselective enzymatic reactions in miniemulsions as efficient "nanoreactors".

The application of enzyme-catalyzed syntheses is of high industrial interest for the manufacture of enantiomerically pure compounds. The well-known high selectivity, in particular enantioselectivity, of enzymes represents a particular advantage; however, conducting enzymatic reactions at high substrate concentrations—a key criterion for technical applications—often proves difficult. A proven methodology for the accomplishment of (enzymatic) reactions at high substrate concentrations significantly exceeding the solubility limit and preferably in the range of > 100 gL 1 is the use of enzyme-compatible two-phase solvent systems, consisting of an aqueous phase and an organic solvent. This concept has been applied successfully for many enzymatic reactions. Besides high substrate concentrations and engineering advantages, in addition an increase of activity has been often observed when interface-active enzymes are used. Nevertheless, alternative concepts to the “classic” twophase solvent system are desirable that would result in increased homogeneity of the reaction mixture, increased overall interface area, and improvement of stirrability. These issues may be addressed by stable miniemulsions, homogenous mixtures in which the organic phase is dispersed in the form of very small “nanodroplets” with diameters in the range of 50–500 nm. The suitability of these liquid “nanoreactors”, the stable droplets in miniemulsions, for enzymatic transformations in general has been demonstrated recently by Landfester and co-workers for the polymerization of lactones. Here, we report the first enantioselective enzymatic reactions in minemulsions and the suitability of this concept for the preparation of optically active aand b-amino acids at very high substrate concentrations of 500 to > 800 gL . As a benchmark for comparison of the reaction characteristics of miniemulsions with those of the “classic” two-phase system, we chose the industrially established lipase-catalyzed hydrolysis of racemic b-amino acid n-propyl esters in methyl tert-butyl ether (MTBE)/water for the preparation of enantiomerically pure b-amino acids. When racemic b-phenylalanine n-propyl ester, rac-1, was used as a substrate under standard conditions in the two-phase solvent system, a conversion of 50% was obtained within 15 h at a high substrate concentration of 242 gL 1 [Scheme 1, Eq. (1)]. The reaction proceeds enantioselectively with an E value of > 100, and the resulting optically active b-amino acid (S)-bphenylalanine, (S)-2, was obtained with an enantiomeric excess of > 99.4% ee.