Kinetic Studies on Lipase-Catalyzed Acetylation of 2-Alkanol with Vinyl Acetate in Organic Solvent

Lipase-catalyzed acetylation of 2-alkanol with vinyl acetate has been studied kinetically using Burkholderia cepacia lipase (BCL), enantiomerically pure (R)- and (S)-2-alkanols and different organic solvents. The rate equation was derived by the steady state method for the simplified mechanism. The second order rate constants (kR and kS) for (R)- and (S)-2-alkanols were evaluated from the slopes of the double reciprocal plots, v-1 vs. [2-alkanol]-1, where v is the initial rate of the reaction. The log kR value increased with the solvent hydrophobicity log P, where P is a partition coefficient of a given solvent between octanol and water. The log kS value also increased with log P except the bulky solvents such as 1,4-dioxane and cyclohexane, in which the rates were faster than those expected from the log kS vs. log P plot. The slope of log kS vs. log P plot was larger than that for (R)-2-alkanol. Thus, log E (E=kR/kS: enantioselectivity) decreased with log P except the bulky solvents. The rate constants and the enantioselectivity were different depending on the structure (carbon number CN) of 2-alkanol. The log E vs. CN plot was minimized at CN=8 and 10 and the log kS vs. CN plot maximized at CN=8 and 10. In contrast the log kR vs. CN plot showed a different feature from the log E vs. CN plot. These facts suggest that dependence of E on CN is more strongly affected by the reactivity of (S)-2-alkanol than that of (R) isomer in this acetylation.