《Exploiting Nanospace for Asymmetric Catalysis# Confinement of Immobilized》.pdf

Exploiting Nanospace for Asymmetric Catalysis: Confinement of Immobilized, Single-Site Chiral Catalysts Enhances Enantioselectivity JOHN MEURIG THOMAS*,† AND ROBERT RAJA*,‡ †Department of Materials Science, University of Cambridge, Cambridge CB2 3QZ, U.K., ‡School of Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, U.K. RECEIVED ON SEPTEMBER 27, 2007 C O N S P E C T U S n the mid-1990s, it became possible to pre- Ipare high-area silicas having pore diame- ters controllably adjustable in the range ca. 20-200 Å. Moreover, the inner walls of these nanoporous solids could be functionalized to yield single-site, chiral, catalytically active orga- nometallic centers, the precise structures of which could be determined using in situ X-ray absorption and FTIR and multinuclear magic angle spinning (MAS) NMR spectroscopy. This approach opened up the prospect of performing heterogeneous enantiose- lective conversions in a novel manner, under the spatial restrictions imposed by the nanocavities within which the reac- tions occur. In particular, it suggested an alternative method for preparing pharmaceutically and agrochemically useful asymmetric products by capitalizing on the notion, initially tentatively perceived, that spatial confinement of prochiral reac- tants (and transition states formed at the chiral active center) would provide an altogether new method of boosting the enan- ti