ABSTRACT

The development of rhodium-catalyzed asymmetric olefin hydrogenation historically started with the work of Knowles

and Horner

who used

P

-chiral monodentate phosphines as ligands. Initial enantioselectivities were low, which was attributed to the many degrees of freedom of the rhodium complexes, in particular the free rotation around the Rh-P bond. Thus, it was a logical step when Dang and Kagan synthesized the first chiral bisphosphine ligand (DIOP) (

), which in principal should form a much more rigid complex. Their analysis was rewarded by the remarkably high ee at that time of 70% in the hydrogenation of 2-acetamidocinnamic acid.