Mass transfer limitation as a tool to enhance the enantiomeric excess in the enzymatic synthesis of chiral cyanohydrins

The enantioselective synthesis of cyanohydrins catalyzed by R-hydroxynitrile lyase in an aqueous-organic liquid two-phase system using, mass transfer limitation to enhance enantiomeric excess at 5°C and pH 5.5 is described. Benzaldehyde, a good substrate, and cinnamaldehyde, a notoriously difficult substrate, were used as model substrates and compared in order to establish the mass transfer limitation concept in a two-liquid phase system, where the non-enzymatic-racemic reaction competes. Enzyme concentration and phase volume ratio between organic and buffer phase were geared to one another to enhance the enantiomeric excess for each substrate. In both cases, after optimization, excellent chemical conversion (>99% on a 60 mmol scale), high throughput and high enantiomeric excess (benzaldehyde >99% and cinnamaldehyde >96%) were achieved.