title: 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 [(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.
authors.affiliation: Departamento de Química, Facultad de Ciencias, Universidad de Burgos, Pza Misael Bañuelos s/n., 09001, Burgos, Spain. gespino@ubu.es
authors.forename: Gustavo
authors.initials: G
authors.lastname: Espino
authors.forename: Li
authors.initials: L
authors.lastname: Xiao
authors.forename: Michael
authors.initials: M
authors.lastname: Puchberger
authors.forename: Kurt
authors.initials: K
authors.lastname: Mereiter
authors.forename: Felix
authors.initials: F
authors.lastname: Spindler
authors.forename: Blanca R
authors.initials: BR
authors.lastname: Manzano
authors.forename: Félix A
authors.initials: FA
authors.lastname: Jalón
authors.forename: Walter
authors.initials: W
authors.lastname: Weissensteiner