The products, kinetics, and mechanisms of metal ion-promoted hydrolyses of thioimidate esters and the effects of hydrogen ions on thioimidate ester hydrolyses

Abstract

A kinetic study shows that the hydrolysis of thiomidate esters in the presence of hyrogen ions at pH ≲ 2 is promoted by mercury(II) ions. Methyl thiobenzimidate leads to benzonitrile and methyl N-cyclohexyl thiobenzimidate to N-cyclohexylbenzamide. The N-protonated forms of the esters, like their analogue the 1-[α-(methylthio)-benzylidene]piperidinium ion, are unreactive towards mercury(II) ions: the rate of reaction therefore falls as [H₃O⁺] rises. Mechanisms of the mercury-promoted reactions are given which are compatible with the observed reaction orders. The hydrolysis of the imidate esters at pH ≲ 2, in the absence of heavy metal ions, which leads predominantly to the thiol ester, is also inhibited by an increase in [H₃O⁺] and the explanation of this effect is disscussed. Unlike mercury(II) ions, silver ions have no detectable effect on the rate of hydrolysis of thioimidate esters at pH ≲ 2, but do affect the isolable products since they rapidly convert the thiol ester to the carboxylic acid.