Unusual enantioselectivities in heterogeneous organocatalyzed reactions: Reversal of direction using proline di- versus tri-peptides in the aldol addition

• Asymmetric aldol reactions were studied over supported di- and tri-peptides.
• The ee reversed on tripeptide catalyst as compared to the dipeptide.
• Quantum chemical calculations showed the conformation of the intermediates.
• The stabilities of these intermediates were in agreement with the inversion found.

The heterogeneous asymmetric direct aldol reactions between aldehydes (2-nitrobenzaldehyde, 2-methylpropanal) and ketones (acetone, cyclohexanone) in the presence of polystyrene (PS) resin supported di- and tripeptides were studied under otherwise identical experimental conditions at room temperature in batch reactor. For all three asymmetric aldol reactions reversal of enantiomeric excess was observed on H-Pro-Pro-Glu(OH)-PS catalyst (ee 39–50%) as compared to the H-Pro-Glu(OH)-PS catalyst (ee 52–95%). The determinant role of the conformation of the intermediate adducts held responsible for chiral induction is interpreted by quantum chemical calculations.

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