Phenylalanine-based ligands

Brunner et al. [26] synthesized and applied so-called dendrizymes in enan-tioselective catalysis. These catalysts are based on dendrimers which have a functionalized periphery that carries chiral subunits, (e.g. dendrons functionalized with chiral menthol or borneol ligands). The core phosphine donor atoms can be complexed to (transition) metal salts. The resultant dendron-enlarged 1,2-diphosphino-ethane (e.g. 16, see Scheme 17) Rh1 complexes were used as catalysts in the hydrogenation of acetamidocinnamic acid to yield iV-acetyl-phenylalanine (Scheme 17) [26]. A small retardation of the hydrogenation of the substrate was encountered, pointing to an effect of the meta-positioned dendron substituents. No significantly enantiomerically enriched products were isolated. However, a somewhat improved enantioselectivity (up to 10-11% e.e.) was... 

An interesting asymmetric transformation is the asymmetric conjugate addition to a-acetamidoacryhc ester 30 giving phenylalanine derivative 31, which has been reported by Reetz (Scheme 3.10) [10]. The addition of phenylboronic acid 2m in the presence of a rhodium complex of l,T-binaphthol-based diphosphinite ligand 32 gave a quantitative yield of 31 with up to 11% enantiomeric excess. In this asymmetric reaction the stereochemical outcome is determined at the hydrolysis step of an oxa-7r-aUylrhodium intermediate, not at the insertion step (compare Scheme 3.7). 

The IR spectra of Co(II), Ni(II) and Cu(II) complexes of Schiff bases derived from condensation of 2-pyridine-carboxaldehyde with DL-alanine, DL-valine and DL-phenylalanine, show that all act as uninegative, bidentate ligands.507 Similar data for complexes of the heterocyclic Schiff base LH2 derived from 1 -amino-5-benzoyl-4-phenyl-li/-pyrimidin-2-one and 3-hydroxy-salicylalde-hyde show that the ligand is tridentate (0,N,0) in M(LH)2 (M = Co, Cu, Zn), but bidentate (N,0) in Ni(LH2)2Cl2.508... 

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