Kinetics and mechanism of reserpine oxidation by nitrous acid

Abstract

The kinetics of oxidation of the Rauwolfia alkaloid reserpine to 3,4-dehydroreserpine by nitrous acid have been measured in methanol–water media at various temperatures. The reactions are found to obey the following rate law: rate =a[Reserpine]_tot[Nit]_tot[H⁺]//_1+b[Nit] where a= 1.056 dm⁶ mol^–2 s^–1 and b= 139 dm³ mol^–1 at 25 °C in 20% methanol–water media. The presence of variable amounts, up to 1 × 10^–2 mol dm^–3 of the nucleophiles Cl^–, Br^– and SCN^– in the reaction media has no effect on the reaction rates. This kinetic behaviour has been rationalized on the basis of a mechanism involving the formation, in a first equilibrium step, of the isolated intermediate N¹-nitrosoreserpine which rearranges in a further acid-catalysed step to give the final product.