Chiral (Iminophosphoranyl)ferrocenes: A New Class of Practical Ligands for Rhodium-Catalyzed Asymmetric Hydrogenation
Thanh Thien Co, Sang Chul Shim, Chan Sik Cho, and Tae-Jeong Kim
Organometallics, 24 (20), 4824 -4831, 2005
Abstract:
A series of new chiral (iminophosphoranyl)ferrocenes, {5-C5H4-(PPh2=N-2,6-R2-C6H3)}Fe{5-C5H3-1-PPh2-2-CH(Me)(Y)} (1a, Y = NMe2, R = Me; 1b, Y = NMe2, R = iPr; 1c, Y = OMe, R = iPr), {5-C5H4-(PPh2=N-2,6-R2-C6H3)}Fe{5-C5H3-1-(PPh2=N-2,6-R2-C6H3)-2-CH(Me)(Y)} (2a, R = Me, Y = NMe2; 2b, R = iPr, Y = NMe2,; 2c, R = Me, Y = OMe), and (5-C5H5)Fe{5-C5H4-1-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 (R3PCl2) 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 2(N,N) (2) or 2(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.
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