Recyclable Silica-Supported Proline Sulphonamide Organocatalysts for Asymmetric Direct Aldol Reaction.

Abstract

We describe the preparation of several organosilicas based on proline sulphonamide scaffolds from two chiral monosilylated precursors following sol-gel and grafting procedures. The materials were characterised by using 29Si solid-state NMR spectroscopy, N2 sorption measurements, powder X-ray diffractogram (p-XRD) and elemental analysis. The catalytic performances and reusability of these hybrid silica materials have been evaluated in direct intermolecular asymmetric aldol reactions between ketones and aromatic aldehydes and in a Robinson annulation using simple and green conditions (water, room temperature). Good conversions and recyclability, and from moderate to good diastereo- and enantioselectivities have been achieved. The characteristics of the matrix, and structural features such as the nature and length of the linker, and the derivatisation site on the catalytic organic moiety have an influence on the activity and selectivity of the silica-supported organocatalysts.