Theoretical investigation on chiral cinchona alkaloid salts-catalyzed asymmetric epoxidation of cyclic enones

Abstract

Chiral cinchona alkaloid salts-catalyzed asymmetric epoxidation of 2-cyclohexen-1-one with hydrogen peroxide (H2O2) has been investigated using density functional theory (DFT). The ring-closure step is rate limiting in the catalytic reaction. The enantioselectivity-determining step is initial nucleophilic addition involving two orientations of axial and equatorial. In (S)-catalyst j-mediated process, axial pathway is favored over equatorial leading to the major epoxide [2S,3S]-3. An opposite enantiomer [2R,3R]-3 is primarily generated in (R)-catalyst k-assisted case preferring equatorial pathway. The results indicate that the enantioselectivity of epoxidation is dominated by central chirality of the bifunctional catalysts in the activation of enone by primary amine salt via iminium formation and of H2O2 by tertiary amine reacting as a general base. The substituent effect is also discussed to clarify a tendency existing in experiment. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011