The kinetics and mechanism of thioamide hydrolysis promoted by gold(III) bromides in aqueous solution

Abstract

Kinetic and spectroscopic studies are reported of the reactions of N-cyclohexylthiobenzamide with the ions AuBr₄^– and trans-Au(CN)₂Br₂^– in aqueous solutions to give the corresponding O-benzamide. The effects of temperature, pH, ionic strength, and ambient bromide ion concentration are described and mechanisms for the reactions suggested. The reactions involve the rapid, stoicheiometric formation of S-amide–gold(III) ion adducts which decompose slowly to the O-amide. Comparison of the results with those of previous studies shows that, for square planar adducts, the rate of the desulphurisation process is increased by an increase in (i) the positive charge on the gold atom and (ii) the softness of the attached ligands. In the presence of added ambient bromide ions the gold ions and adducts are partially or completely converted into octahedral species. The desulphurisation process in the octahedral adducts is faster than in the square planar adducts.