The Stereochemistry and Reactivity of Metal-Schiff Base Complexes. II. High Stereoselectivity in (1S,2S)-N,N′-1,2-Cyclohexylenebis-(salicylideneaminato)(sal2-(S,S)-chxn) Cobalt(III) Complexes with Amino Acids and the Optical Resolution of Amino Acids with Cobalt(III)-sal2-(S,S)-chxn

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Abstract
A series of mixed ligand cobalt(III)–Schiff base complexes with the general formula of [Co(sal2-(S,S)-chxn)(aa)] (aa=gly−, L- and D-ala−, L- and D-val−, L- and D-leu−, L- and D-thr−, or L- and D-trp−) were newly prepared from [Co(sal2-(S,S)-chxn)] and amino acids by air oxidation. All the complexes were found to take, stereoselectively, the Λ-cis-β1(fac)-structure, irrespective of the configuration of the amino acids. On the other hand, the reactions of [Co(sal2-(S,S)-chxn)] with an excess of DL-amino acids in open air gave [Co-sal2-(S,S)-chxn) (aa)], in which L-amino acids were selectively coordinated except for proline. These stereospecificities were found from the opticalpurity measurement of the unreacted amino acids which had been separated from the reaction solutions by extracting the formed complex with chloroform. The configuration and the optical purities of the unreacted amino acids recovered from 1 : 2 reaction solutions (complex: DL-amino acid) were as follows: ala, (D, 6–10%); leu, (D, 6–8%); met, (D, 6–8%); ser, (D, 10–12%); thr, (D, 27–30%); asp, (D, 16–18%); glu, (D, 6–8%); phe, (D, 29–31%); trp, (D, 41–43%); pro, (L, 48–50%). These stereoselectivities and stereospecificities were explained in terms of the thermodynamic origin and the stereochemical requirement of the complexes.

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