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

✍ Scribed by Jan Schraml; Magdalena Kvíčalová; Vratislav Blechta; Ludmila Soukupová; Otto Exner; Hans-Michael Boldhaus; Frank Erdt; Claus Bliefert

• Publisher: John Wiley and Sons
• Year: 2000
• Tongue: English
• Weight: 96 KB
• Volume: 38
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/1097-458x(200009)38:9<795::aid-mrc732>3.0.co;2-a

✦ Synopsis

Homologous series of dihydroxamic acids [HONHCO(CH 2 n CONHOH with n D 0, 1, 2, 3, 4 and 6] were prepared and trimethysilylated [1(n) and 2(n)]. The solution NMR spectra ( 1 H, 13 C, 15 N) 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. 13 C 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 ( 15 Nf 1 Hg and 13 Cf 1 Hg) are an inexpensive alternative to 15 N enrichment used to identify E and Z conformers. To differentiate hydroxamic and hydroximic structures, the most reliable parameter is the 15 N chemical shift, which differs in the two classes of compounds by about 120 ppm. To differentiate E and Z hydroxamic conformers 13 C chemical shifts of C O groups are preferable to 15 N chemical shifts but for distinguishing E and Z isomers of the hydroximic structure both 15 N and 13 C NMR of the C N group are useful. 17 O NMR data are of no practical value in this respect.