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

Homologous series of dihydroxamic acids [HONHCO(CH2)(n)CONHOH with n = 0, 1 , 2, 3, 4 and 6] were prepared and trimethysilylated [1(n) and 2(n)]. The s olution NMR spectra (H-1,C-13, N-15) Of 1(n) show that the hydroxamic end g roups assume Z-Z and Z-E combinations of conformers. An exception is oxalod ihydroxamic acid, which assumes only one combination. C-13 cross-polarizati on magic angle spinning reproduces the solution chemical shift in this comp ound and indicates the Z-Z combination as determined earlier by x-ray diffr action. The trimethylsilylation produces compounds with a hydroximic struct ure on both ends, both groups being disilylated. Z-Z, Z-E and E-E isomer co mbinations are visible in the spectra and their ratio can be determined. Ag ain, oxalodihydroximic acid derivatives are an exception: only one silylate d product wall found and its geometry could not be determined. Selective de coupling experiments (N-15{H-1} and C-13{H-1}) are an inexpensive alternati ve to N-15 enrichment used to identify E and Z conformers. To differentiate hydroxamic and hydroximic structures, the most reliable parameter is the N -15 chemical shift, which differs in the two classes of compounds by about 120ppm. To differentiate E and Z hydroxamic conformers C-13 chemical shifts of C=O groups are preferable to N-15 chemical shifts but for distinguishin g E and Z isomers of the hydroximic structure both N-15 and C-13 NMR of the C=N group are useful. O-17 NMR data are of no practical value in this resp ect. Copyright (C) 2000 John Wiley & Sons, Ltd.