Comparison of supports for the electrostatic immobilisation of asymmetric homogeneous catalysts

To determine the influence of the support on the asymmetric hydrogenation of dehydroamino acids using noncovalently immobilised catalysts, Rhodium-MonoPhos was successfully immobilised on four different anionic carrier materials: a mesoporous aluminosilicate (AlTUD-1), phosphotungstic acid on alumina (PWTUD), Nafion, and a Nafion silica composite (SAC-13). These supported catalysts were evaluated in the asymmetric reduction of methyl-2-acetamidoacrylate. Although most of the catalysts were highly selective, the activity and the loss of rhodium were strongly dependent on the type of support. PWTUD appeared to be the best support for this application, because it gave the catalyst with the highest activity and virtually no leaching in any solvent. Its superior anchoring ability derives from the additional bonding between the Rh and the phosphotungstic acid. Nafion, on the other hand, was by far the poorest support, giving very low activity. Immobilisation with this support relies on encapsulation rather than on ionic interactions. Its encapsulating properties not only prevent the complex from going into solution, but also prevent the substrate from reaching the catalytic site.