Kinetics and stereochemistry of β-elimination of (S)-O-acylthreonine in Λ- and Δ-bis-[N-salicylidene-(S)-O-acylthreoninato]cobaltate(III) ions. Implications for vitamin B6-catalysed reactions

Abstract

Diastereoisomeric complexes of Λ- and Δ-bis-[N-3-R′-salicylidene-(S)-O-acylthreoninato]cobaltate(III) have been synthesized. Deacylation of the complexes at 25 °C and pH 9–11.0 yields a mixture of isomeric Λ- and Δ-bis-(N-3-R′-salicylidene-α-aminodehydrobutyrato)cobaltate(III) ions. Deacylation is subject to general base catalysis. A comparison of the rate constants for epimerization of Λ- and Δ-bis-[N-3-R′-salicylidene-(S)-threoninato]cobaltate(III) ions with those for deacylation of their acyl derivatives shows that deacylation follows the (E1 cB)_I mechanism. The ratio of the products formed via the syn- or anti-elimination mechanism depends on the nature of the base which catalyses the process. The neutral base DABCO catalyses syn-elimination predominantly whereas the negatively charged carbonate ion brings about anti-elimination. The stereochemistry of pyridoxal-dependent reactions of β-elimination has been discussed along the lines of the data obtained.