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1H and 13C NMR studies of isomerism in hydroxamic acids

✍ Scribed by David A. Brown; William K. Glass; Rajeswary Mageswaran; Salima Ali Mohammed

Publisher
John Wiley and Sons
Year
1991
Tongue
English
Weight
288 KB
Volume
29
Category
Article
ISSN
0749-1581

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✦ Synopsis

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. KEY WORDS Cis-trans isomerism K e t w n o l tautomerism 0-Methylhydroxamic acids

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