The Stereochemistry and Reactivity of Metal-Schiff Base Complexes. III. The Kinetic and Thermodynamic Stereoselectivities in the Formation of <i>N</i>,<i>N</i>′-Ethylenebis(α-methylsalicylideneaminato)cobalt(III)Complexes with <scp>l</scp>-Proline, Hydroxy-<scp>l</scp>-proline, and <i>allo</i>-Hydroxy-<scp>l</scp>-proline

Abstract

Abstract Under air-oxidation conditions, the reaction of [Co(α-Me-sal2en)] with l-aaH, where α-Me-sal2en represents the dianion of N,N′-ethylenebis(α-niethylsalicylideneaniine) and where l-aaH denotes l-proline, hydroxy-l-proline, or allo-hydroxy-l-proline, proceeded rapidly to yield Λ-cis-β2–[Co(α-Me-sal2en)(l-aa)] stereoselectively, followed by the slow isomerization of the Λ-cis-β2-isomer thus formed to give the corresponding Δ-cis-β2-isomer in a yield of almost 100% under equilibrium conditions. The complexes thus formed were isolated and characterized by the use of their absorption, circular dichroism, and 1H-NMR spectra. The preferential formation of the Λ-cis-β2-isomer in the initial reaction was found to be kinetic in origin. The kinetic stereoselectivity was determined to be 87% for l-proline, 56% for hydroxy-l-proline, and 23% for allo-hydroxy-l-proline by the measurement of the rotation at 435 nm of the reaction solutions. On the other hand, no kinetic differentiation was observed for the formation of the similar cis-β2-complexes with l-alanine, l-valine, l-methionine, l-phenylalanine, l-tryptophan, N-benzyl-l-alanine, and N-methyl-l-alanine. On the basis of these data, the mechanism of the initial complexation was discussed. The high thermodynamic stereoselectivity for Δ-cis-β2-isomer was explained in terms of the intramolecular steric interaction between the pyrrolidine ring of the coordinated l-aa and the distorted α-Me-sal2en ligand in the complex.