Transition metal-catalyzed oxidation of L-asparagine by platinum(IV) in acid medium: a kinetic and mechanistic study

Kinetic investigations of Pt(IV) oxidation of l-asparagine catalyzed by transition metals with different valencies, namely Ag(I), Pd(II), Cr(III) and Zr(IV), were studied spectrophotometrically in sulfuric acid medium at constant ionic strength and temperature. The reaction was first order in [Pt(IV)], while the orders with respect to the concentrations of Asn, sulfuric acid and transition metal were less than unity over the concentration range studied. The rate constants decreased with increasing ionic strength and dielectric constant, while increasing temperature enhanced the rate. The order of catalytic efficiency was: Ag(I) > Cr(III) > Pd(II) > Zr(IV). A mechanism involving the formation of a complex between the catalyst and substrate is proposed. Oxidation by Pt(IV) is suggested to take place by an inner-sphere mechanism in which Pt(IV) is reduced to Pt(II) on the catalyst-bound substrate in a one-step two-electron transfer process. The oxidation products of Asn were identified as alpha-formyl acetamide, ammonium ion and carbon dioxide. The rate law associated with the reaction mechanism was deduced. Activation parameters of the reactions were evaluated and discussed.