Effect of the Dihedral Angle of Biaryl-Bridged Bisphosphite Ligands on Enantioselectivity and Regioselectivity of Asymmetric Hydroformylation

Abstract

Eight new biaryl-bridged bisphosphite ligands have been synthesized and applied in rhodium-catalyzed asymmetric hydroformylation of styrene, allyl cyanide, and vinyl acetate. X-ray crystallographic studies of square planar LRh(acac) complexes of four of these bisphosphite ligands revealed that the dihedral angle of the bridging biaryl moiety depends on its identity and lies between 59.8° and 80.0°. A correlation between the dihedral angle in these Rh complexes and hydroformylation enantioselectivity and regioselectivity for both allyl cyanide and vinyl acetate is reported. Smaller dihedral angles were found to lead to increased regio- and enantioselectivity. Density functional theory calculations of a five-coordinate model complex (LRh(CO)2H) show that decreased dihedral angles lead to smaller P−Rh−P bite angles. Although large bite angles have previously been correlated with increased hydroformylation regioselectivity, these results provide the first demonstration of a bite angle effect on enantioselecti...