Heterogeneous asymmetric hydroformylation of olefins on chirally modified Rh/SiO₂ catalysts

Heterogeneous chiral catalysts were prepared by modifying silica-supported rhodium (Rh/SiO₂) with chiral phosphorus ligands. The chirally modified Rh/SiO₂ catalysts exhibited high activity, regioselectivity, and enantioselectivity for the asymmetric hydroformylation of styrene and vinyl acetate. Up to 72% ee and 100% selectivity of branched aldehyde for the hydroformylation of vinyl acetate were obtained for (R)-BINAP-Rh/SiO₂ catalysts. It is noteworthy that the modification of Rh/SiO₂ with (S,S)-DIOP resulted in increased activity for the hydroformylation of vinyl acetate and gave a TOF of 128 h^-1, even higher than that of the unmodified Rh/SiO₂ catalyst (90 h^-1). It is found that chiral modifiers with bidentate phosphines and an optimized modifier/rhodium molar ratio close to 1.0 were prerequisites for chiral induction on the chirally modified catalysts. ³¹P MAS NMR results and IR spectra of adsorbed CO indicated that the chiral modification via the coordination of phosphines to rhodium produces chirally active sites on the Rh/SiO₂ catalysts.