Recyclable Mesoporous Organosilica Nanoparticles Derived from Proline-Valinol Amides for Asymmetric Organocatalysis

Abstract

Going a step further from bulk organosilicas to nanosized materials, we describe herein the preparation of mesoporous organosilica nanoparticles derived from mono- and bis-silylated proline-valinol amides, both by grafting on preformed mesoporous silica nanoparticles (MSN) and by a co-condensation method in neutral medium using Brij-56/CTAB as templates. This is the first report on the obtention of functionalized MSN by a co-condensation procedure with a structurally complex chiral precursor. The functionalized MSN have been characterized by elemental analysis, 29Si and 13C CP MAS NMR, transmission electron microscopy, scanning electron microscopy, N2-sorption measurements, dynamic light scattering, ζ-potential, and powder X-ray diffraction. We have evaluated the activity of these materials as recyclable catalysts in the asymmetric aldol reaction. The best organocatalysts are those derived from the monosilylated precursor, observing good diastereo- and enantiomeric ratios with a simple and environmentally friendly optimized protocol (water, 0 °C, absence of cocatalyst). The nanocatalyst was easily recovered by centrifugation and recycled up to five runs without loss of activity and selectivity. The use of organosilica nanoparticles reduces the problems of diffusion and low reaction rates encountered with bulk organosilicas.