Synthesis of a self-assembling gold nanoparticle-supported organocatalyst for enamine-based asymmetric aldol reactions

The self-assembling gold nanoparticle (GNP)-supported l -proline derivative, which was readily synthesized in four steps from 4-hydroxy- l -proline and chloroauric acid, was used for enamine-based aldol reactions. The modified Brust-Schiffrin (BS) synthesis was favored over the ligand exchange reaction in order to develop a new and simple synthetic pathway for nanoparticle-supported catalyst. Use of immobilized organocatalyst on nanoscale solid carrier led to excellent selectivities (up to 94:6 dr and 94% ee) in asymmetric reactions of ketones and benzaldehydes. GNPs operate under pseudo-homogeneous conditions which are easily recycled and reused at least five times without significant loss of weight, activity, diastereo- and enantioselectivities.