Flower-like mesoporous silica: a bifunctionalized catalyst for rhodium-catalyzed asymmetric transfer hydrogenation of aromatic ketones in aqueous medium

Functionalized flower-like mesoporous silica with a chiral organorhodium functionality incorporated within its silica framework is prepared through an assembly of chiral 4-((trimethoxysilyl)ethyl) phenylsulfonyl-1,2-diphenylethylenediamine and tetraethoxysilane under a cooperative dual-template approach followed by complexation with organorhodium complexes. Structural characterization discloses its mesostructure and well-defined single-site chiral organorhodium functionality, while electron microscopy analyses reveal the uniformly distributed three-dimensional spherical flowers constructed by the stacking of leaf-shaped nanoflakes. In particular, as a bifunctionalized heterogeneous catalyst, it shows excellent catalytic activity and high enantioselectivity in the asymmetric transfer hydrogenation of aromatic ketones in aqueous medium (more than 99% conversion and up to 97% ee). The superior catalytic performance is attributed to the synergistic effect of the salient cetyltrimethylammonium bromide phase-transfer function and confined chiral organorhodium catalytic nature. Furthermore, this heterogeneous catalyst could be recovered easily and reused repeatedly (ten times) without affecting its ee value, showing a practical application in asymmetric synthesis.