Hydrolase-catalyzed kinetic resolutions of chiral alcohols: Mechanistic study on the origin of the enantioselectivity

Mechanistic studies on the enantioselectivity in the hydrolase-catalyzed kinetic resolutions of racemic alcohols are described. Based on kinetic measurements, molecular orbital calculations and computer modeling with X-ray crystal structures of several lipases, we proposed the transition-state model that is consistent with the experimental observations such as (i) high enantioselectivity, (ii) broad substrate specificity and (iii) an empirical rule (R-preference for secondary alcohols). A large secondary alcohol having a tetraphenylporphyrin as the substituent was successfully resolved by several lipases, demonstrating the validity of our transition-state model. The S-preference of subtilisins for secondary alcohols was rationalized by applying the protocol used in the transition-state model for lipases to subtilisins. We also found that the lipase-catalyzed transesterifications of chiral alcohols in organic solvents can proceed even at -40°C. Interestingly, the E value increased with decreasing temperature, and a linear relationship was observed between 1n E and 1/T, from which the ΔΔH^‡ and ΔΔS‡ values were calculated. These thermodynamic parameters were useful for investigating the mechanism of the enantioselectivity of the hydrolases toward chiral alcohols.