Hydrolysis of N-cyclohexyl methylthiobenzimidate in the presence of tetrachlorogold(III) ions and the effects of hydrogen, halide, and other ions

Abstract

In aqueous acid the hydrolysis of N-cyclohexyl methylthiobenzimidate is promoted by tetrachlorogold(III) ions, and ions derived thereform. A kinetic study shows that the reaction involves the rapid co-ordination of a small fraction of the free thiobenzimidate ester, or of its protonated form, to the species [AuCl₃OH]^–, [AuCl₃H₂O], [AuCl₄]–, and [AuCl₆]^3– by displacement of chloride. The slow step is the decomposition of the resulting gold-ester complex either unimolecularly (where possible) or via attack by water. The principal complexes involved under given conditions depend upon the values of [H₃O⁺] and [Cl^–]. All these effects are described. The purely organic product is always N-cyclohexylbenzamide. The other product(s) is a complex organogold sulphide whose exact composition probably depends upon the ionorganic content of the reaction mixture. Normally two equivalents of gold are consumed per equivalent of ester. In the presence of iodide ions catalysis of the gold-promoted hydrolysis is found. This arises from the relatively rapid formation of mixed chloroiodogold(III) complexes (e.g. [AuCl₃I]^–) which co-ordinate a greater fraction of the ester and thus accelerate its hydrolysis. The detailed effects of hydrogen and chloride ion concentrations on the iodide catalysed reaction are described. Of other soft bases tested as catalysts, bromide ion is moderately effective, but cyanide and thiocyanate ions, although they enter the gold(III) complexes, lead to species which provide no catalysis of thiobenzimidate ester hydrolysis.