Rhodium-Catalyzed Asymmetric Hydrogenation of Functionalized Olefins Using Monodentate Spiro Phosphoramidite Ligands

Novel chiral monodentate phosphorus ligands, SIPHOS, were conveniently synthesized from 1,1‘-spirobiindane-7,7‘-diol. The Rh complexes of SIPHOS can catalyze the hydrogenation of α-dehydroamino esters in mild conditions, providing α-amino acid derivatives in up to 99% ee. Enamides
and β-dehydroamino esters can also be hydrogenated in good to excellent enantioselectivities (up
to 99% and 94% ee, respectively). The SIPHOS ligand with smaller alkyl groups on the N-atom
afforded higher enantioselectivity. The X-ray analysis of single crystal showed that the structure
of Rh/SIPHOS catalyst is [Rh(COD)((S)-SIPHOS-Me)2]+, which clarified the configuration of the
catalyst with the monodentate chiral phosphorus ligand in Rh-catalyzed asymmetric hydrogenation.
A positive nonlinear effect in the relationship of the optical purities of ligand and product was
observed in the hydrogenation of dehydroamino acid derivatives. The kinetic study of hydrogenation
showed that the reaction is zero order in the concentration of substrate and first order in the
concentration of Rh catalyst and the hydrogenation pressure. The rate of hydrogenation decreased
when the Rh/L ratio changed from 1:1 to 1:4.