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

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 [(Sp,Sp)-2-R1(2)P-2''- R2(2)P-1,1''-biferrocene; (bifep): R1=R2=Ph; : R1=Ph, R2=Cy; : R1=R2=3,5-Me2C6H3; : R1=3,5-Me(2-)4-OMe-C6H2, R2=3,5-(CF3)2C6H3]. In addition, the synthesis of three palladium(II) complexes ([PdX2(L)], : L=, X=Cl; : L=, X=Cl; : L=, X=C6F5 and of four bifep ruthenium complexes (: [RuCl(p-cymene)()]PF6; : [RuI(p-cymene)()]PF6; : [RuCl(benzene)()]PF(6); : [RuI(p-cymene)()]I) is reported. In the solid state the biferrocene unit of complexes , and adopt either a (P)-shaped () or an (M)-shaped (, ) conformation. In solution, palladium complexes and 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.