Metal complexes as models for vitamin B₆ catalysis

Abstract

Chapter I. Historical introduction to Vitamin B₆ Complexes. Chapter II. The aldimine complexes N(Salicylidene)glycinato and valinatozinc(II), N,~pyridoxylidene)valinatocopper(II) monohydrate and N-(3-Hydroxypyridyl-2-methylene)valinatocopper( II) hemihydrate have been prepared from L̳-valine. Synthetic methods and characterization data are given. Also prepared were the bis-chelate amino acid ester complexes, Bis[N- (2-ethoxycarbonyl-l-propyl)salicylaldiminato]copper(II) and Bis[N-(3-ethoxycarbonyl-2-propyl)salicylaldiminato]copper(II). The inertness of these two complexes to H-D exchange contrasts with the ready exchange in the absence of base of the complexes derived from a-amino acids. This result shows that facile exchange and racemization properties of Bis[N-(alkoxycarbonylalkyl)salicylaldimino] metal(II) complexes 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 N-Salicylidene-L̳-valinatocopper( II), Cu(sal-L̳-val) << N-Pyridoxylidene-L̳-valinatocopper(II) </- N-3-Hydroxypyridyl-2-methylene-L̳-valinatocopper(II) <N-4-NO₂ - Salicylidene-L̳-valinatocopper(II). This order is essentially the same as that of qualitative catalytic effectiveness of the constituent o̲-hydroxyarylcarbonyl compounds in nonenzyrnatic transamination and reinforces in semiquantitative fashion the prevailing model of ligand electronic features requisite to catalytic activity of these compounds.