title: Allosteric effects in asymmetric hydrogenation catalysis? Asymmetric induction as a function of the substrate and the backbone flexibility of C1-symmetric diphosphines in rhodium-catalysed hydrogenations.
abstract: The new unsymmetrical, optically active ligands 1,2-C(2)H(4)(PPh(2))(2'R,5'R-2',5'-dimethylphospholanyl) (L(a)) and 1,3-C(3)H(6)(PPh(2))(2'R,5'R-2',5'-dimethylphospholanyl) (L(b)) form complexes of the type [Rh(L)(cyclooctadiene)][BF(4)] where L = L(a) (1a) or L(b) (1b), [PtCl(2)(L)] where L = L(a) (2a) or L(b) (2b) and [PdCl(2)(L)] where L = L(a) (3a) or Lb (3b). The crystal structures of 2a and 2b show the chelate ligand backbones adopt delta-twist and flattened chair conformations respectively. Asymmetric hydrogenation of enamides and dehydroaminoesters using 1a and 1b as catalysts show that the ethylene-backboned diphosphine L(a) gives a more efficient catalyst in terms of asymmetric induction than the propylene-backboned analogue L(b). The greatest enantioselectivities were obtained with 1a and enamide substrates with ees up to 91%. Substrate-induced conformational changes in the Rh-diphosphine chelates are proposed to explain some of the ees observed in the hydrogenation of enamides.
authors.affiliation: School of Chemistry, University of Bristol, Cantock's Close, Bristol, UK.
authors.forename: Angharad
authors.initials: A
authors.lastname: Baber
authors.forename: Johannes G
authors.initials: JG
authors.lastname: de Vries
authors.forename: A Guy
authors.initials: AG
authors.lastname: Orpen
authors.forename: Paul G
authors.initials: PG
authors.lastname: Pringle
authors.forename: Karl
authors.initials: K
authors.lastname: von der Luehe