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Nitrile oxides (3) are rapidly generated in situ on dissolution of the corresponding hydroxamoyl chlorides (1) in aqueous solution a t pH > 4; reaction of (3) with the solvent is slow at pH < 10. Primary and secondary amines react stereospecifically t o give only theZ-amidoximes (1 1 ), in which the nucleophile and OH groups are cis. With primary amines and ammonia the Z-isomer is also thermodynamically favoured but with the more bulky secondary amines subsequent isomerisation to the more stable €-isomer (1 2) occurs. Several criteria (p +0.53 for substituent variation in the nitrile oxide ; Pnllc 0.48 for primary amine variation ; small enhancement shown b y ' a-effect ' amines ; small steric effect of ortho-substituents in nitrile oxide) are consistent with a transition state for amidoxime formation in which there is little bond formation between the amine and the carbon of the nitrile oxide.

The isomerisation of Z-amidoximes (8) to the €-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 >106-fold faster than the neutral form; a t 25 "C, r j for the conversion of Z into E amidoximes is in the region of 0.01 -1 .O s. Isomerisation is not catalysed by buffer species, ruling out an addition-elimination pathway.Since 0-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 a t high pH. The results are considered in terms of the mechanism of oxime isomerisation, SEVERAL mechanisms have been identified 3-12 for the geometric interconversion of imine derivatives (4) (Scheme 1). These include the uncatalysed direct interconversion involving overall nitrogen inversion, which is thought to occur generally by movement of the group YH through a transition state where it becomes colinear with the C=N group (the lateral shift mechan-

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NN-Disubstituted amidoximes, prepared under mild conditions as pure (2) isomers, undergo rapid and complete isomerisation about the carbon-nitrogen double bond in aqueous solution.