Enzymatic ketone reductions with co-factor recycling: Improved reactions with ionic liquid co-solvents

Ionic liquids have been applied to two pharmaceutically relevant ketone reductions mediated by isolated enzymes. Alcohol dehydrogenase isolated from Rhodococcus erythropolis (ADH RE) was found to mediate the asymmetric reductions of 4′-Br-2,2,2-trifluoroacetophenone to (R)-4′-Br-2,2,2-trifluoroacetophenyl alcohol and 6-Br-β-tetralone to its corresponding alcohol (S)-6-Br-β-tetralol. Both of these reactions employed a second enzyme, glucose dehydrogenase 103 (GDH 103) to recycle the co-factor NAD through the oxidation of glucose to gluconic acid. In the case of 4′-Br-2,2,2-trifluoroacetophenone the traditional organic co-solvent approaches were limited to a maximum product concentration of 10 g L−1 due to substrate deactivation of the biocatalyst. Employing 10% (v/v) [BMP][NTf2], a water immiscible ionic liquid, facilitated conversion of 50 g L−1 ketone to the chiral alcohol in less than 24 h. The initial rate of reaction was improved more than four times in the presence of 10% (v/v) ionic liquid compared to no co-solvent and the product could be readily isolated in 85% (w/w) overall yield with an ee of 99%. In the case of 6-Br-β-tetralone the reaction was found to proceed more favorably with both miscible and immiscible ionic liquid co-solvents compared to a number of organic solvents. Again, the product could be readily isolated in 88% (w/w) overall yield with an ee of >99%. For both bioconversions the stability of both the ADH RE and the GDH 103 co-enzyme was found to be enhanced by the presence of certain ionic liquids compared to both organic solvents and aqueous buffer. In the case of the immiscible ionic liquid [BMP][NTf2] present at a volume fraction of 10%, the measured enzyme half lives were 266 and >300 h, respectively. These promising results were obtained after screening only a limited range (11) of representative, commercially available ionic liquids. Critical factors in the screening of ionic liquids for bioconversion applications appear to be the solubility of the substrate, biocatalyst stability in the presence of the ionic liquid co-solvent and mass transfer rates from the ionic liquid into the aqueous phase.