Reaction of Acetaldehyde with Some Optically Active Cobalt (III) Complexes Containing Coordinated Glycine

The resolution and assignment of absolute configuration of the isomers of [Co(en)(gly)₂]⁺ is reported. The optically active cations (+)-[Co(en)₂(gly)]²⁺, (+)-trans(O)-, (+)-β-cis(O)-, and (+)-α-cis(0)-[Co(en)(gly)₂]⁺ react with acetaldehyde in an aldol-type condensation reaction to give optically active threonine and allothreonine. Using kinetically controlled conditions, the reaction yields products with relatively high optical purities (16-35%). The isolated threonine and allothreonine contain an excess of the 5 isomer indicating that the aldehyde prefers to attack the S “side” of the coordinated glycine in these cations. However, analysis of the amino acid distribution suggests that the diversity of geometric environments exhibited by the cations has little effect on the stereochemical course of the reaction. Finally, the product distribution found reinforces the assignment of the absolute configuration of the cations based on circular dichroism arguments.