Synthesis, coordination behaviour, structural features and use in asymmetric hydrogenations of bifep-type biferrocenes

Abstract

A protocol for the synthesis of C₂- and C₁-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″- R²₂P-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.