'Backdoor Induction' of Chirality in Asymmetric Hydrogenation with Rhodium(I) Complexes of Amino Acid Substituted Triphenylphosphane Ligands.

This paper describes the synthesis and characterization of 5-(diphenylphosphanyl)isophthalic acid bioconjugates (Lig-[R]₂). In addition to symmetrically disubstituted conjugates with amino acids, peptides or amines, a convenient one-pot, two-step procedure for the synthesis of conjugates bearing two different substituents is reported. The 28 prepared phosphanes were used as monodentate ligands in the rhodium(I)-catalyzed hydrogenation of 2-acetamidoacrylate and ( Z)-α-acetamidocinnamate. The ligand with the smallest side-chain substituents Lig-[Ala-OMe]₂ ( 1a) revealed the highest selectivity, with up to 84 % ee. The catalysts presented herein are models of artificial metalloenzymes in which the outer-coordination sphere controls the selectivity in catalysis. The chirality of distant hydrogen-bonded amino acids is transmitted by 'backdoor induction' to the prochiral Rh^I center. [ABSTRACT FROM AUTHOR]