The synthesis of polyhydroxycarboxylates Part III. Lanthanide(III) catalyzed addition of glycolate to —maleate a kinetic study

Abstract

The reaction mechanism of a Michael-like addition of glycolate (ga, 1) to maleate (male, 2) catalyzed by lanthanide(III) ions in aqueous solution giving carboxymethoxysuccinate (cmos, 3) is discussed with respect to its reaction kinetics. The reaction takes place in a ternary complex of Ln(male)(ga) with the addition of the ionized Ln(III) coordinated hydroxy group of 1 to the olefinic bond of 2 as the rate determining step. The first order rate constant in the ternary complex is around 1 min −1. Since exchange of the ligands is fast, the amount of the ternary complex Ln(male)(ga) formed in the mixed ligand system has to be calculated by its stability constant. The stability constants of Ln(ga) n , Ln(male) n , Ln(cmos)n ( n = 1, 2) and Ln(male)(ga) were determined potentiometrically. The p K a for the ionization of the Ln(III) coordinated hydroxyl group of 1 could be obtained indirectly from the reaction kinetics. The existence of the Ln(III) coordinated H −1ga ligand was supported by 13C NMR. The effective charge density of the cation plays an important role in decreasing the pKa of the hydroxyl group of 1 upon coordination. This effect is somewhat counteracted by a concomitant decrease of the nucleophilicity of the resulting alcoholate group in the addition reaction. The La(III) catalyzed reverse reaction ( 3 → 1 + 2) was found to be stereoselective, resembling enzyme catalysis.