PROTON-EXCHANGE AND EPIMERIZATION OF CO(III) CHELATED AMINO-ACIDS VIA CARBANION INTERMEDIATES

An earlier investigation into proton exchange and epimerization of amino acids chelated to Co(III)1 has been improved and extended to include the amino acids (AA) Phe, Val, Ala, Gly, Glu, and Asp in the complex cations Δ,Ʌ-[Co(en)2(S,-AA)]2+,+. Equilibrium concentrations of the diastereomers measured in H2O (KC(Δ-S/Ʌ-S) = 1.15 (Phe), 2.0 (Val), 1.0 (Ala, Gly), 0.85 (Glu), 0.67 (Asp)) vary little with ionic strength and are the same in D2O. Rate constants for OD−-catalyzed proton exchange at the 2-CH centers differ for the Δ-S and Ʌ-S diastereomers and can be related to the rate constant for epimerization provided the concept of a common carbanion intermediate is used. There is no correlation between the rate data and the overall charge on the complex. Selectivity differences are demonstrated in the reprotonation process (Ʌ-R/Ʌ-S = 1.6 (Phe), 1.6 (Val), 0.9 (Ala), 0.8 (Gly), 0.75 (Glu), 0.5 (Asp)), and these are shown to be thermodynamically driven. This corrects previous investigations on the AA = Asp and Glu complexes. 3H rate studies show a kinetic isotope difference of ∼8 for reprotonation in favor of 1H, but no selectivity difference between 3H and 1H in forming the Ʌ-R, Ʌ-S epimers. © 1990, American Chemical Society. All rights reserved.