Structure determination of dihydroxamic acids and their trimethylsilyl derivatives by NMR spectroscopy

Homologous series of dihydroxamic acids [HONHCO(CH2)nCONHOH with n = 0, 1, 2, 3, 4 and 6] were prepared and trimethysilylated [1(n) and 2(n)]. The solution NMR spectra (1H, 13C, 15N) of 1(n) show that the hydroxamic end groups assume Z–Z and Z–E combinations of conformers. An exception is oxalodihydroxamic acid, which assumes only one combination. 13C cross-polarization magic angle spinning reproduces the solution chemical shift in this compound and indicates the Z–Z combination as determined earlier by x-ray diffraction. The trimethylsilylation produces compounds with a hydroximic structure on both ends, both groups being disilylated. Z–Z, Z–E and E–E isomer combinations are visible in the spectra and their ratio can be determined. Again, oxalodihydroximic acid derivatives are an exception: only one silylated product was found and its geometry could not be determined. Selective decoupling experiments (15N{1H} and 13C{1H}) are an inexpensive alternative to 15N enrichment used to identify E and Z conformers. To differentiate hydroxamic and hydroximic structures, the most reliable parameter is the 15N chemical shift, which differs in the two classes of compounds by about 120 ppm. To differentiate E and Z hydroxamic conformers 13C chemical shifts of C O groups are preferable to 15N chemical shifts but for distinguishing E and Z isomers of the hydroximic structure both 15N and 13C NMR of the C N group are useful. 17O NMR data are of no practical value in this respect. Copyright © 2000 John Wiley & Sons, Ltd.