Chiral (iminophosphoranyl)ferrocenes: A new class of practical ligands for rhodium-catalyzed asymmetric hydrogenation

A series of new chiral (iminophosphoranyl)ferrocenes, {η⁵- C₅H₄-(PPh₂=N-2,6-R₂-C ₆H₃)}-Fe{η⁵-C₅H ₃-1-PPh₂-2-CH(Me)(Y)} (1a, Y = NMe₂, R = Me; 1b, Y = NMe₂, R = ⁱPr; 1c, Y = OMe, R = ⁱPr), {η⁵-C₅H₄-(PPh₂=N-2,6-R ₂-C₆H₃)}Fe{η⁵-C ₅H₃-1-(PPh₂=N-2,6-R₂-C ₆H₃)-2-CH-(Me)(Y)} (2a, R = Me, Y = NMe₂; 2b, R = ⁱPr, Y = NMe₂,; 2c, R = Me, Y = OMe), and (η⁵-C₅H₅)Fe{η⁵-C ₅H₄-1-PR₂-2-CH(Me)N=PPh₃} (3a, R = Ph; 3b, R = C₆H₁₁), have been prepared from the reaction of 1,1′-bis(diphenylphosphino)-2-[(dimethylamino)ethyl]ferrocene with aryl azides (1 and 2) and the reaction of phosphine dichlorides (R ₃PCl₂) with 1,1′-bis-(diphenylphosphino)-2- (aminoethyl)ferrocene (3), respectively. They form rhodium complexes of the type [Rh(NBD)(L)]ClO₄ (4-6; L = 1-3), where the ligand (L) adopts an η²(N,N) (2) or η²(P,N) mode (3), as expected. In the case of 1, a potential tridentate species, however, a chelating bidentate mode through the -CH(Me)NMe₂ and -PPh₂ groups is realized with the exclusion of the -P=NAr group from the coordination sphere, as confirmed by the X-ray crystal structure of [Rh(NBD)(1b)]ClO₄ (4b). The new ligands (1-3) exhibit exceptionally high enantioselectivity (up to 99%) and catalytic activity in the Rh-catalyzed asymmetric hydrogenation of (E)-2-methylcinnamic acid, (Z)-2-acetamidocinnamate, and (Z)-2- acetamidoacrylate.