title: Synthesis, characterization, and solution behavior of optically active cis beta organocobalt salen complexes with L-amino acids.
abstract: 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.
authors.affiliation: Dipartimento di Scienze Chimiche, Università di Trieste, via Licio Giorgieri 1, 34127 Trieste, Italy. dreos@univ.trieste.it
authors.forename: Renata
authors.initials: R
authors.lastname: Dreos
authors.forename: Giorgio
authors.initials: G
authors.lastname: Nardin
authors.forename: Lucio
authors.initials: L
authors.lastname: Randaccio
authors.forename: Patrizia
authors.initials: P
authors.lastname: Siega
authors.forename: Giovanni
authors.initials: G
authors.lastname: Tauzher
mesh_terms: D000596:Amino Acids / Q000138:chemical synthesis / Q000737:chemistry*; D002942:Circular Dichroism; D003035:Cobalt / Q000737:chemistry*; D018360:Crystallography, X-Ray; D006909:Hydroxyproline / Q000737:chemistry; D000432:Methanol; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D012996:Solutions; D013237:Stereoisomerism; D014443:Tyrosine / Q000737:chemistry
chemical_list: D000596:Amino Acids; D009942:Organometallic Compounds; D012996:Solutions; D003035:Cobalt; D014443:Tyrosine; D006909:Hydroxyproline; D000432:Methanol