Enantioselectivity, swelling and stability of 4-hydroxyprolinol containing acrylic polymer beads in the asymmetric reduction of ketones

Abstract

A new 4-hydroxy-a,a-diphenyl-L-prolinol containing polymethacrylate, prepared without chromatography by a large scale adaptable synthesis, has been evaluated as a catalyst in the asymmetric reduction of 1-arylethanones. Using 1-(4-bromophenyl)ethanone as the model substance, the reduction was tested with various borane sources and solvents. The best swellings of the polymer and reactivity were observed in THF using N,N-diethylaniline borane complex as the hydride source. The selectivity in the reduction of 1-(4-bromophenyl)ethanone was found to depend on the substrate concentration and catalyst loading. Using the best conditions identified, a series of 1-arylethanones was reduced to their corresponding enantioenriched secondary alcohols. High rates and ee-values were obtained in the reduction of acetophenones containing electron withdrawing groups in the aromatic ring, whereas a moderate selectivity was the result for products containing electron donating aromatic substituents. Upon recovery of the polymer beads, it was found that vacuum drying led to extensive rupturing, while the bead structure was intact if washed with methanol and air dried at atmospheric pressure. Repeated use of the polymer catalyst gave the product alcohol with a lower 90% ee. Elemental analysis showed this to be due to the loss of the chiral prolinol unit.