Reactions of dl-homocystine and 3,3′-dithiodipropionic acid with Pd(II) in aqueous hydrochloric solutions. Part II: Kinetics and mechanistic investigations

Abstract

Kinetics and mechanisms of a disulfide bond disproportionation in dl-homocystine (H2HCysS) and 3,3′-dithiodipropionic acid (H2DTDPA) induced by Pd(II) species were investigated spectrophotometrically in strong hydrochloric acidic media. The products of the disproportionation are thiolate and sulfinate complexes. The reaction of Pd(II) with H2DTDPA is autocatalytic with contributions from both catalytic and non-catalytic pathways. The reaction of Pd(II) with H2HCysS is irreversible and of the first order. For both ligands, a rate-determining step is the formation of a S,S′-binuclear complex species prior to the disproportionation. A subsequent electrophile-assisted disproportionation of a disulfide bond is caused by a nucleophilic attack of a water molecule. Both reactions proceed through an associative mechanism by a solvolytic pathway. The difference in the mechanisms for two ligands is explained in terms of steric availability of a disulfide bond.