Preparation, properties, and racemization kinetics of copper(II)-Schiff base-amino acid complexes related to vitamin B6 catalysis

Article abstract of DOI:10.1021/ic50091a029

Series of tridentate aldimine complexes of types 1, 2, and 3 (X = H, 4-, 6-NO₂) of copper(II) and zinc(II) and protonated copper(II) complexes of type 2, obtained as bromide salts, have been prepared from L-amino acids. Synthetic methods and characterization data are given. Also prepared were the bis-chelate amino acid ester complexes of copper(II), 7 and 8. The inertness of these two complexes to H-D exchange at the asymmetric carbons under basic conditions contrasts with the ready exchange of 4 in the absence of base. This result shows that facile exchange and racemization properties of 4 derive principally from the direct attachment of the electron-withdrawing HC=NM and COOC₂H₅ groups to the asymmetric center. The base-catalyzed racemization rates of four copper(II)-aldimine complexes in 95% ethanol at 50° were found to increase in the order Cu(sal-L-val) (3, X = H) ? Cu(pyr-L-val) (1) ? Cu(3,2-hpy-L-val) (2) < Cu(4-NO₂sal-L-val) (3, X = 4-NO₂). This order is essentially the same as that of qualitative catalytic effectiveness of the constituent o-hydroxyarylcarbonyl compounds in nonenzymatic transamination and reinforces in semiquantitative fashion the prevailing model of ligand electronic features requisite to catalytic activity of these compounds. ORD and CD spectra are presented which further establish the correlation between a negative Cotton effect or CD feature associated with an ultraviolet absorption band and the L configuration of the condensed amino acid in the copper(II) complexes 1-3.