The Catalysed Decarboxylation of Oxaloacetic Acid

The nature of the chelate compounds formed by transition metal ions with oxaloacetic acid in aqueous solution, has been investigated spectrophotometrically and potentiometrically. The mechanism of the catalysed reaction has been clarified. Thermodynamic information on the ketonic chelate compounds, which are the catalytically active species in decarboxylation, has been obtained by measuring association constants for dimethyloxaloacetic acid (II) (which cannot enolise),and comparing these with the known association constants for oxaloacetic acid (I), HO2C.CO.CH2.CO2H (I); HO2C.CO.C(Me)2.CO2H (II). Spectrophotometric studies have demonstrated the presence of enolic chelate compounds which are not decarboxylated. Approximate values for the proportion of enolic complex for oxaloacetate chelates of Ca2+, Mn2+, Zn2+, Co2+, Ni2+ and Cu2+ have been obtained. Spectrophotometrie measurements on the chelate compounds of oxaloacetic acid (I) and its ethyl ester (III), HO2C.CO.CH2.CO2Et (III) which cannot decarboxylate, have shown that oxaloacetate chelate compounds are formed very rapidly. The rise of optical density (270 mmu) with time to a maximum; produced by addition of some metal ions to aqueous solutions of oxaloacetic acid, is due to the production of an enolic pyruvate intermediate. The mechanism of decarboxylation, may be represented by, (diagram redacted) The changes of optical density with time are consistent with the above reaction scheme. Inhibition of decarboxylation at high copper ion concentrations has been found to occur, and the results are related to previous potentiometric studies of the copper chelates. Inhibition at high pH (> 6) is due to the production of kinetically inactive enolic complexes. The aniline catalysed decarboxylation of oxaloacetic acid has been studied by manometric, spectrophotometric, and potentiometric methods. Experiments with the half ester of oxaloacetic acid (III),have shown that in aqueous solution, the intermediate is the ketimine hydrate (A). Kinetic measurements have demonstrated that the rate of the aniline catalysed decarboxylation passes through a maximum at around pH 4. The pH-Rate profile is consistent with a catalytically active species (B), the fall in rate at pH greater than being attributed to ionisation according to the equation (equation redacted) Kinetic measurements have shown that the ketimine hydrate is present only in small amounts, under the experimental conditions used, and that it loses CO2 in the rate-determining step. In aqueous solution the mechanism is of the type, (diagram redacted) In ethanol, experiments with esters (III) and (IV) EtO2C.CO.CH2.CO2Et (IV) have shown that the catalytically active species is the ketimine (C). This compound is formed in quantitative yield. The aniline salt of compound (a), and the diethyl ester derivative of (C) have been isolated. The formation of the ketimine has been studied spectrophotometrically and shown to be kinetically second order. The rate of formation of the ketimine is equal to the rate of decarboxylation, indicating that in ethanol, the formation of the ketimine is the rate-controlling step in decarboxylation. Metal ion and amine catalysis have been compared with the metal ion activated enzymatic decarboxylation of some biologically important keto acids