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

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 enantioselectivity in asymmetric
hydroformylation.