Highly Modular P-OP Ligands for Asymmetric Hydrogenation: Synthesis, Catalytic Activity, and Mechanism

A library of enantiomerically pure P-OP ligands (phosphine-phosphite), straightforwardly available in two synthetic steps from enantiopure Sharpless epoxy ethers is reported. Both the alkyloxy and phosphite groups can be optimized for maximum enantioselectivity and catalytic activity. Their excellent performance in the Rh-catalyzed asymmetric hydrogenation of a wide variety of functionalized alkenes (26 examples) and modular design makes them attractive for future applications. The lead catalyst incorporates an (S)-BINOL-derived (BINOL=1,1'-bi-2-naphthol) phosphite group with computational studies revealing that this moiety has a dual effect on the behavior of our P-OP ligands. On one hand, the electronic properties of phosphite hinder the binding and reaction of the substrate in two out of the four possible manifolds. On the other hand, the steric effects of the BINOL allow for discrimination between the two remaining manifolds, thereby elucidating the high efficiency of these catalysts.