Synthesis, Characterization, and Solution Behavior of Optically Active <i>cis</i> β Organocobalt Salen Complexes with l-Amino Acids

The reaction of a cis β folded organocobalt derivative with a salen-type ligand, 1, isolated as racemic compound of Δ and Λ enantiomers, with enantiomerically pure α-l amino acids is reported. The reaction between racemic 1 and l-tyrosine afforded a mixture of the two diastereoisomers Δ-2 and Λ-2, which could be separated by fractional crystallization owing to the lower solubility of Δ-2. The absolute configuration of the two diastereomers was unequivocally assigned from the X-ray structure, using the known absolute configuration of the asymmetric carbon of the amino acid as internal reference. The reaction of racemic 1 with trans-4-hydroxy-l-proline afforded only the diastereoisomer with a Δ configuration of the tetradentate ligand, as proved by X-ray diffractometric analysis. For both l-tyrosine and trans-4-hydroxy-l-proline, the amino acid initially coordinates both to the Δ and to the Λ enantiomers of 1, leading to an about equimolar mixture of diastereoisomers. In the case of l-tyrosine the diastereoisomers have about the same energy, so that the successive isomerization is negligible. In the case of trans-4-hydroxy-l-proline, Δ-3 is much more stable than Λ-3, and the isomerization reaction Λ-3 → Δ-3 goes practically to completion.