Chiral metal complexes 31. Enhanced enantioselectivity in a Co(III)-mediated synthesis of alanine, including the crystal structure of the intermediate Λ-β₁-[(3R,4R-diphenyl-1,6-di(2-pyridyl)-2,5-diazahexane)(2-amino-2-methylpropandioicacidato)cobalt(III)] perchlorate dihydrate

Reaction of the 2-amino-2-methylpropandioicacidato anion (AMMA^2-) with the Λ-β-[Co(R,R-picstien)Cl₂]⁺ complex cation (R,R-picstien is 3R,4R-diphenyl-1,6-di(2-pyridyl)-2,5-diazahexane) results in displacement of the chloride ions with retention of absolute configuration of the metal atom and the bidentate AMMA^2- stereospecifically adopting the β₁ and stereoselectively the pro-S configurations. Decarboxylation of this complex in warm acidic solution produces a mixture comprising 93±3% Λ-β₁-[Co(R,R-picstien)(R-ala)]²⁺ and 7±3% Λ-β₁-[Co(R,R-picstien)(S-ala)]²⁺, as shown by NMR experiments. The chiral environment imposed on the prochiral AMMA^2- fragment by the coordinated R,R-picstien ligand thus results in a stereoselective asymmetric synthesis of R-alanine in high enantiomeric excess. The crystal structure of the intermediate Λ-β₁-[Co(R,R-picstien)(AMMA)]ClO₄·2H₂O complex has been determined at -135 °C. Crystal data: C₃₀H₃₅N₅O₁₀Cl₁Co₁ is orthorhombic, space group P2₁2₁2₁, with a=9.598(2), b=11.964(2), c=26.477(3) Å and Z=4. The structure was refined by block-matrix least-squares methods to R=0.039 for 2622 diffractometer data. In the complex cation the bidentate AMMA^2- fragment coordinates such that an internal hydrogen bond exists between the uncoordinated carboxyl group and one of the amine hydrogens of the central link, (NH···O 2.770(5) Å). This forces the coordinated amino acid intermediate to adopt the R (pro-S) configuration.