Simultaneous synthesis of modified Binol-periodic mesoporous organosilica SBA-15 type material. Application as catalysts in asymmetric sulfoxidation reactions

Abstract

The synthesis of a chiral periodic mesoporous organosilica (chiral PMO) SBA-15 type, using a bis-silylated Binol precursor, has been studied through two different heterogenization routes. On the one hand, the immobilization of a (R)-(+)-1,1′-bi-2-naphthol (Binol) derivative was accomplished by following a standard multistep synthesis methodology. On the other hand, a new route consisting of an easy one-step synthesis was developed achieving a simultaneous formation of mesoporous structure and Binol chiral ligand immobilization in the walls. The addition of KCl “salting out” electrolytes favored the micellization, obtaining the well-ordered chiral PMO materials. The thioanisole asymmetric oxidation reaction was used to validate the enantio-catalysts activity. The materials synthesized by the multistep method reached yield and enantiomeric excess of 41 and 15 %, respectively, while the synthesized one-pot chiral PMO materials achieved up to 58 and 42 %, respectively. This difference could be attributed to a more homogenous distribution of the chiral moiety as well as the simultaneous micellization and formation of the siliceous mesostructure, in the one-pot procedure. This promotes higher reagents accessibility to the active center, and therefore an enhancement of the chiral induction. Thereby, materials with Binol ligand incorporated into the three-dimensional silica framework were successfully accomplished.