31P NMR and IR characterization of enantioselective olefin and arene hydrogenation catalysts containing a rhodium-chiral phosphine complex tethered on silica

Rhodium complexes of the chiral chelating phosphine (2S,4S)-4-(diphenylphosphino)-2-(diphenylphosphinomethyl)pyrrolidine (X-PPM) tethered on silica (SiO2), tethered on silica with supported palladium (Pd-SiO2), and in solution were characterized by P-31 NMR and IR spectroscopies. These studies show that the (X-PPM)Rh(COD)(+) complex, which is highly enantioselective for the hydrogenation of the prochiral olefin methyl-alpha-acetamidocinnamate (MAC), retains its composition in the tethered catalysts regardless of its mode of preparation or the presence of Pd on the silica surface. These investigations also show that the chiral diphosphine ligand remains coordinated to the rhodium during the reaction. Both the tethered and solution catalysts are moderately air sensitive prior to use, giving the free phosphine oxide of X-PPM, which is no longer coordinated to the rhodium. During and after use in catalytic reactions, the tethered rhodium complexes are extremely air-sensitive, but were characterized by P-31 NMR and IR spectra of their carbon monoxide derivatives. Finally, the catalysts were examined for their arene hydrogenation activity. It was established that I'd in the (X-PPM)Rh(COD)(+)/Pd-SiO2 catalyst causes the reduction of any uncomplexed Rh to metallic species during the hydrogenation reactions. It was these metallic Rh species that were responsible for the toluene hydrogenation activity of these tethered (X-PPM)Rh(COD)(+)/Pd-SiO2 catalysts. (C) 2002 Elsevier Science B.V. All rights reserved.