Stereospecific Hydrogenations IV: Palladium-on-Poly-S-Valine and Palladium-on-Poly-S-Leucine

Studies concerning the influence of helical conformation on the enantioselective hydrogenations catalyzed by palladium-on-poly-S-amino acids are described. Poly-S-valine which does not normally form a helical conformation and poly-S-leucine which forms a right-handed helix were chosen as carriers for this study. Hydrogenations of α-methylcinnamic acid and α-acetamidocinnamic acid using palladium-on-poly-S-valine produced predominantly S(+)-dihydro-α-methylcinnamic acid and R( +)-phenyl-alanine (after hydrolysis), respectively. The hydrogenations catalyzed by palladium-on-poly-S-leucine produced predominantly R(−)-dihydro-α-methylcinnamic acid and S(−)-phenylalanine (after hydrolysis), respectively. These results indicate that the helical conformation of the polyamino acid carriers does influence the asymmetric induction observed. The asymmetric induction arising from the chirality of the screw sense of the polyamino acid appears to have a stronger influence on the configuration of the products than does the asymmetric induction arising from the configurations of the amino acid residues. The asymmetric induction arising from the helical conformation appears to mask any influence arising from the asymmetric carbon atoms of the amino acid residues.