Homo- and Heteroannularly Bridged Ferrocenyl Diphosphines in Asymmetric Hydrogenations

Four new enantiopure homo- and heteroannularly bridged ferrocenyl diphosphine ligands have been synthesized, characterized, and tested in enantioselective hydrogenations of olefins [methyl-(Z)-(α)-(acetamido)cinnamate, (Z)-(α)-methylcinnamic acid and dimethyl itaconate], ketones (ethyl pyruvate, methyl phenylglyoxylate, and ketopantolactone), and the imine 2-ethyl-N-(2-methoxy-1-methylethylidene)-6-methylbenzeneamine (MEAI). Generally, the homoannularly bridged ligand (S_c,R_p)-[η⁵-cyclopentadienyl][η⁵-4-dicyclohexylphosphino-3-diphenylphosphino-4,5,6,7-tetrahydro-1H-indenyl]iron(II) performed best, giving up to 90.6% ee in the rhodium-catalyzed hydrogenation of dimethyl itaconate and 73% ee in the iridium-catalyzed hydrogenation of MEAI. The molecular structures of all new ligands in the solid state have been determined by X-ray diffraction. A structural comparison of the four new diphosphines with Josiphos-type ligands revealed that the stiffness of the ligand backbone strongly influences the enantioselectivity observed.