Vitamin B-6 Model Reactions (V): Synthesis,Conformation and Differential Reactivity of the A-(S,S) and A-(S,S) Diastereoisomers of Bis(Pyridoxylidene-L-Valinato)Cobalt(III) Complexes

Abstract

The A(S,S) and A(S,S) diastereoisomers of the bis(pyridoxylidene-L-valinato)cobalt(III) complex have been synthesized and separated from one another. Nuclear magnetic resonance investigations show that the Schiff base chelate ring conformations are different for the two isomers. These measurements also determine the absolute configuration. The rate of carbon-hydrogen bond breaking has been studied by monitoring the intial rate of loss of the amino acid α-proton resonance for each complex. In acidic solution, the A isomer reacts about five times more rapidly than the A isomer. Four-bond proton-proton spin coupling constants show that the most rapidly reacting complex is the one where the bond to be broken is most nearly dihedrally perpendicular to the plane of the aromatic ring. However, conductivity studies show substantial differences in solvation which could also explain differences in reactivity.