SPONTANEOUS, METAL ION- AND PYRIDOXAMINE-CATALYZED DECARBOXYLATION OF 2-OXALOPROPIONIC ACID

摘要

The proton association constants for the carboxyl protons of 2-oxalopropionic acid (OPA), acetoacetic acid (AAA), and (alpha)-ketobutyric acid (AKBA) were determined potentiometrically, and the dissociation constant of the enolic proton of OPA, H(,2)L, was determined spectrophotometrically. The metal stability constants of AAA and AKBA were calculated for Zn(II), Al(III), and Cu(II) chelates. The stability constants of 1:1 OPA chelates of Zn(II), Al(III), and Cu(II) were determined in addition to stability constants of a deprotonated species, MH(,-1)L('-), for Zn(II) and a 2:1 complex for Al(III), ML(,2)('-). The Cu(II) investigation showed the existence of a deprotonated complex, MH(,-1)L('-), and of binuclear complexes.;The kinetics of the spontaneous decarboxylation of OPA to the enolate intermediate of AKBA with subsequent ketonization, and of (beta)-deuteration via enolization, have been studied by NMR in aqueous solution at 31(DEGREES)C. Both observed and species specific rate constants have been determined for these reactions. Under the reaction conditions employed the hydrate species exists in appreciable concentrations at low pH while the concentration of the enol species is not significant. Distributions of species formed by the coordination of 2-oxalopropionic acid by Zn(II), Al(III) and Cu(II) ions are reported and kinetics of decarboxylation are discussed in terms of the species present in solution. Studies of initial rates of metal ion-catalyzed decarboxylation of 2-oxalopropionic acid show dependence on the degree of formation of the metal chelate (ML). Rate constants for the decarboxylation of ZnL, CuL, AlL('+), and AlL(,2)('-) were determined. The equilibrium constants for ML(keto) (DBLHARR) ML(enol) were evaluated for the Zn(II) and Al(III) systems as was the keto-enol equilibrium constant for AlL(,2)('-)(enol) (DBLHARR) AlL(,2)('-)(keto).;The conditional Schiff base formation constants of pyridoxamine and OPA and the rates of the vitamin-B(,6) catalyzed decarboxylation of OPA were determined spectrophotometrically in basic aqueous solutions. The observed rate constant of decarboxylation exhibited pH dependence with the maximum occurring at approximately the pH corresponding to maximum ketimine formation. The pKa's and the specific rate constants of the various protonated ketimine forms have been estimated from the rate constant-pH profiles.