An unexpected inversion of enantioselectivity in direct asymmetric aldol reactions on a unique L-proline/γ-Al2O3 catalyst

L-proline adsorbed on γ-Al2O3 unexpectedly switches the enantioselectivity of the direct asymmetric aldol reaction of acetone with p-nitrobenzaldehyde from 68% ee (R configuration for free L-proline catalyst) with 80% yield to 21% ee (S configuration) with 78% yield. The inversion of enantioselectivity was also observed in the direct asymmetric aldol reactions of acetone with several other aromatic aldehydes catalyzed by the L-proline adsorbed on γ-Al2O3. This inversion phenomenon is found to be general for different types of amino acids adsorbed on γ-Al2O3. The hydroxyl groups on γ-Al2O3 are found to be involved in the inversion induction of enantioselectivity in these direct asymmetric aldol reactions.