Chiral norbomadiene ligand

N-acyldehydrodipeptides were readily prepared either by the condensation of N -acyldehydro-a-amino acids with a-amino acid esters or by the reaction of the azlactones of dehydro-a-amino acid with a-amino acid esters (eq. 1). Asymmetric hydrogenation of the N-acyldehydrodipeptides thus obtained (eq. 2) was carried out by using rhodium complexes with a variety of chiral diphosphines such as -Br-Phenyl-CAPP (3), Ph-CAPP (3), (-)BPPM (4), (+)BPPM (4), (-)DIOP ( ), (+)DIOP ( ), diPAMP (6), Chiraphos (7), Prophos (S), BPPFA (9) and CBZ-Phe-PPM (Fig. 1)(10). The chiral catalysts were prepared in situ from chiral diphosphine ligand with [Rh(NBD)2l -CIO4 (NBD = norbomadiene). Typical results are summarized in Tables I-V. 

In essence, four types of Rh complexes have routinely been used as precursors for Rh-catalyzed hydrogenation reactions (see Fig. 3) neutral or cationic bis-diene (norbomadiene or 1,5-cyclooctadiene) Rh(l) complexes with CP or as anion, respectively, preformed diene-diphosphine complexes (see Fig. 1), mostly as anionic species and especially with air-sensitive ligands and/or P-chiral phosphines prone to racemization. Before the diene-diphosphine complexes become active catalysts, the diene has to be removed via hydrogenation. Depending on the diene and the ligand, this reaction can be rather slow, thereby leading to induction phases and/or decreased catalyst productivity for an overview, see Heller et al. [16]. The results presented show some clear trends as to which precursor type is most likely to be effective for a desired transformation ... 

Investigations aimed at finding new and easily accessible ligands for asymmetric transformations are an active area of research. It is widely known that chelating dienes such as 1-5-cycIooctadiene (cod) and norbomadiene (nbd) are stable ligands for late transition metal complexes, but use of their chiral versions has not been widely explored. Hayashi et and Defieber et have independently... 

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