The first example of cis-truns isomerism in monoalkylhydroxamic acids, detected by 'H, I3C and "N NMR spectroscopy, is reported. The "N NMR spectrum of CH,CO"NHOH gave a clear assignment of the OH and NH protons of both 2 and E isomers. The assignment was confirmed by 'H and "C NMR spectroscopy. A rotational barrier of isomerization (AC,') of 17.8 kcal mol-' was calculated from the variabletemperature 'H NMR spectra using the method of Shanan-Atidi and Bar-Eli.

'H and "C NMR studies of N-methyl-substituted hydroxamic acids, RCON(CH,)OH (R = CH,, C,H, and C6Hs), show that the series exhibits &-trans isomerism about the C-N bond. The Z/E ratio increases in the series CH, < C,H, < n-CsH,, < n-C,H,, for a given solvent, indicating that steric interaction between R and the OH group is more important than that between R and the CH, group. In DMSO-d, the 2 isomer is preferentially stabilized by solvation, whereas in C,D6 and toluene-d,, the unsolvated E form is thermodynamically more stable, as predicted by ab initio molecular orbital calculations. (12) At high concentrations (in CDCI, , CD,CI, , C6D6 and toluend,,) the E isomer may also be stabilized by intermolecular association. In the 0-methyl-substituted series, RCONHOCH, (R = CH,, C6H,), although the Z isomer predominates in all solvents, at low temperatures there is evidence for enolic forms.

No abstract