Chiral vanadyl salen complex anchored on supports as recoverable catalysts for the enantioselective cyanosilylation of aldehydes.: Comparison among silica, single wall carbon nanotube, activated carbon and imidazolium ion as support

The activity and enantiomeric excess (ee) (in some cases > 85%) obtained for the asymmetric addition of trimethylsilyl cyanide to aldehydes using different heterogeneous chiral catalysts are compared. A library of recoverable catalysts was developed by immobilization of a chiral vanadyl salen complex having a terminal carbon-carbon double bond onto a series of scaffolds including silica, single-wall carbon nanotubes, activated carbon and room-temperature ionic liquids. The covalent linkage has been achieved by radical initiated addition of mercapto groups to C=C. The highest enantiomeric excesses, similar to those obtained in the homogeneous phase, were achieved using silica as support or with the homogeneous tetra-tert-butyl salen catalyst dissolved in an imidazolium ionic liquid. The use of silica as support permits an easier separation and reuse of the catalyst from the reaction media. (C) 2004 Elsevier Ltd. All rights reserved.