Oxidative degradation of the antihypertensive drug losartan by alkaline copper(III) periodate complex in the presence and absence of ruthenium(III) catalyst: a kinetic and mechanistic study of losartan metabolite

Spectrophotometric kinetic technique has been used to study the oxidation of losartan by diperiodatocuprate(III) (DPC) in the presence and absence of ruthenium(III) catalyst in aqueous alkaline medium at constant ionic strength of 0.90 mol dm −3 . The stoichiometry of the reaction was found to be 1:2 in both catalyzed and uncatalyzed reactions, i.e., one mole of losartan requires two moles of DPC. The reaction products were identified and characterized by IR, GC–MS, and NMR spectral studies. The reaction shows first-order dependence on [DPC] and [ruthenium(III)] and apparently less than unit order in both losartan and alkali concentrations. The effects of added products, ionic strength, periodate, and dielectric constant on the rate of reaction have been studied. The rate of the reaction increased with increasing [alkali] and decreased with increasing [periodate] in both the cases. The reaction rates revealed that Ru(III) catalyzed reaction was about sevenfold faster than the uncatalyzed reaction. Based on experimental results suitable mechanisms are proposed. The proposed mechanisms and the derived rate law are in concurrence with the observed kinetics. The reaction constants involved in the different steps of the mechanism were calculated. The activation parameters with respect to slow step of the mechanism were computed and discussed, and thermodynamic quantities were also determined. Graphical abstract