Reactivity of 1,3-dipoles in aqueous solution. Part 4. Kinetics and mechanism of isomerisation of amidoximes in aqueous solution

Abstract

The isomerisation of Z-amidoximes (8) to the E-form (9) has been examined in aqueous solution as a function of pH and amidoxime structure. Marked acidic catalysis is observed and, in general, the protonated amidoxime reacts >10⁵-fold faster than the neutral form; at 25 °C, t_1/2 for the conversion of Z into E amidoximes is in the region of 0.01–1.0 s. Isomerisation is not catalysed by buffer species, ruling out an addition–elimination pathway. Since O-alkylamidoximes (26) isomerise at rates which are similar to those for the unalkylated oxime, it is concluded that the neutral oxime, rather than the nitrone tautomer, is the reactive species at high pH. The results are considered in terms of the mechanism of oxime isomerisation.