Synthesis, characterization, and solution behavior of optically active cis β organocobalt salen complexes with L-amino acids

The reaction of a cis beta folded organocobalt derivative with a salen-type ligand, 1, isolated as racemic compound of Delta and Lambda enantiomers, with enantiomerically pure alpha-L amino acids is reported. The reaction between racemic 1 and L-tyrosine afforded a mixture of the two diastereoisomers Delta-2 and Lambda-2, which could be separated by fractional crystallization owing to the lower solubility of Delta-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 Delta 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 Delta and to the Lambda 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, Delta-3 is much more stable than Lambda-3, and the isomerization reaction Lambda-3 --> Delta-3 goes practically to completion.