Novel poly(2-oxazoline)s with pendant l-prolinamide moieties as efficient organocatalysts for direct asymmetric aldol reaction

Abstract

Poly(2-oxazoline)-supported bifunctional organocatalysts have been prepared through a bottom-up protocol, which involves synthesis of well-defined poly(2-oxazoline) precursors bearing amino groups in the side-chains followed by amide coupling with N-Boc-L-proline then deprotection. The resultant L-prolinamido-functionalized polymers have proven to be significantly more active than their monomeric counterparts for the aldolisation of cyclic ketones with several substituted benzaldehydes under neat conditions. By using 15 mol% of the polymer as a catalyst, the direct aldol reaction products were isolated in high yields and with good diastereo- and enantioselectivity. Based on circular dichroism spectrum analysis, the enhancement in catalytic activity is probably related to the conformational changes of the pseudo-peptide scaffold of poly(2-oxazoline)s. In addition, these soluble polymeric catalysts can be recovered and reused by precipitation in ether for five catalytic cycles without significantly diminishing their efficiency.