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G. Espino, L. Xiao, M. Puchberger, K. Mereiter, F. Spindler, B. R. Manzano, F. A. Jalon, W. Weissensteiner:
"Synthesis, coordination behaviour, structural features and use in asymmetric hydrogenations of bifep-type biferrocenes";
Dalton Transactions, 2009 (2009), 2751 - 2763.

A protocol for the synthesis of C_2- and C_1-symmetric 2,2 -diarylphosphino-substituted biferrocenes (bifep-type ligands) is presented and the preparation of four representatives is described [(S_p,S_p)-2-R¹₂P-2 - R¹₂P-1,1 -biferrocene; 1 (bifep): R¹ = R² = Ph; 2: R¹ = Ph, R² = Cy; 3: R¹ = R² = 3,5-Me₂C₆H₃; 4: R¹ = 3,5-Me₂-4-OMe-C₆H₂, R² = 3,5-(CF₃)₂C₆H₃]. In addition, the synthesis of three palladium(II) complexes ([PdX₂(L)], 10: L = 1, X = Cl; 11: L = 4, X = Cl; 12: L = 1, X = C₆F₅ and of four bifep ruthenium complexes ( 13: [RuCl(p-cymene)( 1)]PF₆; 14: [RuI(p-cymene)( 1)]PF₆; 15: [RuCl(benzene)( 1)]PF₆; 16: [RuI(p-cymene)( 1)]I) is reported. In the solid state the biferrocene unit of complexes 10, 11 and 15 adopt either a (P)-shaped ( 10) or an (M)-shaped ( 11, 15) conformation. In solution, palladium complexes 10 and 11 are present as equilibrium mixtures of rapidly interconverting (P)- and (M)-shaped conformers. Rhodium- and iridium-mediated asymmetric hydrogenations of a number of olefins and one imine give products with only low to moderate enantiomeric excess, while in the ruthenium-catalyzed hydrogenation of ketones a maximum e.e. of 82% is obtained. The low enantioselectivities are assumed to be related to the conformational flexibility of bifep-type ligands.

http://dx.doi.org/10.1039/b816544k