A thermodynamic study of ketoreductase-catalyzed reactions 2. Reduction of cycloalkanones in non-aqueous solvents

The equilibrium constants for the ketoreductase-catalyzed reactions (cycloalkanone + 2-propanol = cycloalkanol + acetone) have been measured in n-hexane, n-pentane, and supercritical carbon dioxide SCCO2 (pressure = (8.0 to 12.0) MPa). The cycloalkanones included in this study were: cyclobutanone, cyclopentanone, cyclohexanone, cycloheptanone, and cyclooctanone. The equilibrium constants for the reactions involving cyclobutanone and cyclohexanone were measured in n-hexane over the range T = (288.35 to 308.05) K. The thermodynamic quantities at T = 298.15 K are: K = (0.763 ± 0.001); ΔrGm∘=(0.670±0.002)kJ·mol-1; ΔrHm∘=-(1.09±0.11)kJ·mol-1, and ΔrSm∘=-(5.9±0.4)J·K-1·mol-1 for the reaction involving cyclobutanone; and K = (15.7 ± 0.2); ΔrGm∘=-(6.82±0.02)kJ·mol-1; ΔrHm∘=-(4.6±1.0)kJ·mol-1, and ΔrSm∘=(7.4±3.3)J·K-1·mol-1 for the reaction involving cyclohexanone, respectively. An inspection of the equilibrium constants for these reactions in n-hexane, n-pentane, and SCCO2 shows that solvent dependence is not significant. The equilibrium constants of cycloalkanones decrease with increasing value of the number of carbons, NC with the exception of cyclohexanone. The cyclohexanol, which adopts a nearly strainless, idealized tetrahedral conformation around each carbon, is thermodynamically favored and more stable compared to other cycloalkanol rings, and this is reflected in the significantly higher value of the equilibrium constant obtained for this reaction. Comparisons with results obtained by using two independent thermochemical routes are also made.