Lanthanide(III)-catalysed addition of glycolate to maleate. Investigation of intermediates using multinuclear magnetic resonance spectroscopy

Abstract

The lanthanum(III)-catalysed addition of glycolate to maleate to yield carboxymethoxysuccinate(3–)(cmos) is described. It is shown that the reaction only proceeds above pH 6, indicating the formation of di-ionised glycolate as a pre-equilibrium for the rate-limiting step, i.e. the addition of the CH₂–O^– to the olefinic bond. Due to strong complexation of La^III by the product cmos, the reaction requires one La^III ion per two cmos formed. The inhibitory effects of non-reacting strong chelators such as ethylenediaminetetra-acetate, nitrilotriacetate, and 2,6-pyridinedicarboxylate indicate the formation of mixed-ligand complexes leading to the addition reaction. This has been confirmed by Gd^III-induced ¹³C relaxation rate enhancements, and Dy^III-induced ¹⁷O shift measurements.