One-step synthesis of a thioester chiral PMO and its use as a catalyst in asymmetric oxidation reactions

Abstract

The study of key aspects for the heterogenization of chiral homogeneous catalysts is addressed in this work. Two different heterogenization routes were evaluated. On the one hand, the immobilization of a tartaric acid derivative (Sharpless catalyst) was accomplished by following a two-step synthesis methodology. In the first step, the mesoporous support SBA-15 was functionalized with different percentages of mercaptopropyl-(trimethoxy)-silane (MPTMS) through a co-condensation method, obtaining very high degrees of MPTMS incorporation. In the second step, the chiral moiety is anchored onto the previously organically modified silica by means of a thiotransesterification reaction. On the other hand, the second route consisted of the formation of mesoporous structure and chiral ligand immobilization in the walls, simultaneously, in only one step. In this case, the chiral organic functionality was incorporated into the wall silica framework, since the built-organic complex is a bis-silane, obtaining a chiral PMO. Finally, once characterized, the synthesized materials were proved in an asymmetric catalytic test in order to study their enantioselective properties. The chosen reaction was the thioanisole enantioselective sulfoxidation, using cumene hydroperoxide as an oxidant. The materials synthesized by the multi-step method reached 40% enantiomeric excess (ee), while the synthesized one-pot chiral PMO materials reached up to 50% ee. Thereby, materials with the chiral Sharpless ligand incorporated into the three dimensional silica framework were successfully accomplished, as well as its ability to induce higher chirality in the enantiomeric tested reaction than the conventional methods of chiral ligand heterogenization onto a silica framework.