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.

本研究合成了8种新型联芳基桥联双亚磷酸酯配体，并将其应用于苯乙烯、烯丙基氰与乙酸乙烯酯的铑催化不对称氢甲酰化反应中。对其中4种双亚磷酸酯配体所形成的平面正方形LRh(acac)配合物开展X射线晶体学研究，结果显示桥联联芳基部分的二面角随配体结构不同而变化，取值范围为59.8°至80.0°。本研究还揭示了这类铑配合物中的二面角与烯丙基氰、乙酸乙烯酯的氢甲酰化反应对映选择性及区域选择性之间的相关性：较小的二面角可同时提升区域选择性与对映选择性。对五配位模型配合物(LRh(CO)2H)的密度泛函理论(Density Functional Theory)计算表明，二面角越小，P-Rh-P咬角也越小。尽管此前已有研究将较大的咬角与更高的氢甲酰化区域选择性相关联，但本研究首次证实了咬角效应对不对称氢甲酰化反应对映选择性的影响。