Kinetic study of the solvolysis reactions of 1-aryl-3-acetoxymethyl-3-alkyltriazenes: evidence for iminium ion intermediates and the SN1 mechanism

Abstract

The first-order rate constants of the reactions of acetoxymethyltriazenes with nucleophiles have been measured. Acetoxymethyltriazenes undergo hydrolysis in phosphate buffer to give the corresponding arylamines, presumably via the hydroxymethyl- and monomethyl-triazenes. The acetoxymethyltriazenes undergo solvolysis in alcohols and in mixtures of alcohols and other solvents; the rate of solvolysis has been correlated with the Grunwald–Winstein parameter (Y) for solvent ionising power, thus supporting the hypothesis of an S_N1 mechanism and the intermediate formation of iminium ions during the solvolysis. The hypothesis is further supported by the non-common-ion effect; the presence of lithium chloride in the solvent greatly increases the rate of reaction, whereas lithium acetate causes a slight decrease in rate, attributable to a common-ion effect. Reaction of the acetoxymethyltriazene with sodium azide in aqueous acetone affords the α-azidomethyltriazene (a new type of triazene not previously reported) and provides supporting evidence for the iminium ion hypothesis. On the other hand, the acetoxymethyltriazene did not react with neat ethanethiol, providing further evidence for an S_N1 mechanism; and S_N2 reaction would be expected to proceed more quickly in the thiol than in the alcohol. It is shown that hydroxymethyltriazenes do not react via iminium ions and that functionalisation to a derivative such as the acetate is necessary for iminium ion generation. The implications of these results for the metabolism of xenobiotic N-alkyl compounds are discussed.