Ludmila Soukupová

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

STRUCTURE OF SILYLATED BENZOHYDROXAMIC ACIDS

NMR spectra (13C, 15N and 29Si) of the products of trimethylsilylation and tert‐butyldimethylsilylation of benzohydroxamic acid and model derivatives of benzohydroximic acid were studied in solutions. The products are shown to have the structure of (Z)(syn)‐O,O′‐bis(trimethylsilyl) and ‐O,O′‐di(tert‐butyldimethylsilyl) derivatives of benzohydroximic acid [i.e. (Z)‐trimethylsilyl ester of N‐(trimethylsiloxy)benzoimidic acid and (Z)‐tert‐butyldimethylsilyl ester of N‐(tert‐butyldimethylsilyl)benzoimidic acid, respectively]. Whereas 15N NMR chemical shifts are the most useful NMR parameter for differentiation between derivatives of hydroxamic and hydroximic tautomers, differentiation between Z and E stereoisomers of hydroximic acid derivatives is more reliable on the basis of 1J(13C,13C) coupling within the C—CN moiety. Copyright

Benzhydroximic acids — NMR study of trimethylsilyl derivatives

Abstract NMR spectra ( 1 H 13 C, 15 N, and 29 Si) of trimethylsilylated para and meta substituted benzhydroxamic acids were studied in chloroform solutions. The silylation products have the structure of Z-O,O ′-bis(trimethylsilyl) derivatives of benzhydroximic acid, independently of the ring substituent. According to aromatic proton chemical shifts, the geometry of the hydroximic group and its torsion angle with the ring plane are not affected by the para substituent. The chemical shifts of the nuclei in the hydroximic part of the molecule show surprisingly strong dependence on the remote ring substituent. The two 29 Si chemical shifts exhibit essentially the same sensitivity to substitution despite the fact that the Si(O 1 ) is one bond closer to the substituent than the Si(O 4 ) silicon. It is suggested that while electron donor substituents increase the shielding of the silicon atoms they also increase the basicity of the oxygen in the SiO moiety, thus leading to the stronger hydrogen bonding with the solvent. Association with chloroform partially compensates the direct substituent effect on the shielding in the case of Si(O 1 ) silicon. The influence of other factors not covered by substituent constants is demonstrated by excellent correlations with the chemical shifts in analogous tert -butyldimethylsilyl derivatives.

DOSY OF SILYLATED SACCHARIDES

With steadily improving NMR hardware and software, diffusion ordered spectroscopy (DOSY) is becoming increasingly popular for analysis of mixtures (for a review and leading references, see ref. 1). In principle, DOSY enables separation of NMR signals from molecules that differ in their diffusion rates (or diffusion coefficients, D). In contrast to LC-NMR, DOSY achieves signal separation without physical separation of compounds by a suitably chosen pulse sequence that employs pulsed (magnetic) field gradients (PFG). The gradients attenuate NMR signals from slowly moving molecules less than the signals from fast moving molecules. Using a series of different gradient strengths, the signal attenuation is measured and analysis yields a diffusion coefficient for each resolved line in the spectrum. Theoretically, signals coming from the same molecule should have the same diffusion coefficient; hence on an appropriate 2D plot the NMR signals from the same molecule should appear on a line parallel to the NMR chemical shift axis. DOSY is an attractive option for analysis of mixtures of oligosaccharides differing in molecular size. This straightforward analysis is based on the generally accepted idea that, the larger the molecule, the slower it diffuses. It has recently been reported that hydrogen bonding can substantially affect diffusion rates. Although this effect was observed in a study of phenol and cyclohexanol, variation in the number of hydroxyl groups present in different carbohydrate molecules will complicate the interpretation of DOSY spectra in terms of molecular size. An obvious remedy would be to block the hydroxyl groups by suitable substituents. In this communication we want to demonstrate that trimethylsilylation offers such a possibility and, in addition, that DOSY methodology can be applied to Si J. CARBOHYDRATE CHEMISTRY, 20(1), 87–91 (2001)

29Si13C spin–spin couplings over an SiOCsp3 link

29Si13C spin–spin couplings over one, two, and three bonds as well as other NMR parameters [δ(29Si), δ(13C), δ(1H), 1J(13C1H), and 2J(29SiC1H)] were calculated and measured for a series of trimethylsilylated alcohols of the types Me3SiO(CH2)nCH3 and Me3SiOCH3−nRn(n = 03; R = Me, Ph, or Vi). The signs of the coupling constants determined for selected compounds can likely be extended to all such compounds, as supported by theoretical calculations. Similar to couplings between other pairs of nuclei, the 2‐bond and 3‐bond 29SiO13C couplings are of opposite signs (2J > 0 and 3J < 0), and their relative magnitudes depend on the extent of branching at the α‐carbon. Copyright

Structure of monosilylated benzhydroxamic acids in crystals and solutions

Abstract O-tert-butyldimethylsilyl (1) and O-tert-butyldiphenylsilyl (2) benzhydroxamates were prepared and their structure in the solid state was determined by X-ray diffraction and NMR spectroscopy (15N, 13C and 29Si). The solution NMR spectra indicate some processes. In chloroform, both compounds isomerize to an equilibrium mixture of hydroxamic (A) and hydroximic (B or C) derivatives at room temperature. The exact structure of the hydroximic derivative could not be determined. The ratio between the isomers is affected by the solvent. In benzene solutions the hydroximic isomer is preferred whereas in acetonitrile the opposite is true. In dimethylsulfoxide the NMR lines are broad but slow heating to 80°C or above produces disilylated derivatives of the type Z-O1,O4-bis(tert-butyldimethylsilyl)-benzhydroximate. The changes in structure are only partially reversible, the nature of the process responsible for line broadening is not clear.

The effect of solvent accessible surface on Hammett-type dependencies of infinite dilution 29Si and 13C NMR shifts in ring substituted silylated phenols dissolved in chloroform and acetone

Infinite dilution 29Si and 13C NMR chemical shifts were determined from concentration dependencies of the shifts in dilute chloroform and acetone solutions of para substituted O‐silylated phenols, 4‐R‐C6H4‐O‐SiR′2R″ (R = Me, MeO, H, F, Cl, NMe2, NH2, and CF3), where the silyl part included groups of different sizes: dimethylsilyl (R′ = Me, R″ = H), trimethylsilyl (R′ = R″ = Me), tert‐butyldimethylsilyl (R′ = Me, R″ = CMe3), and tert‐butyldiphenylsilyl (R′ = C6H5, R″ = CMe3). Dependencies of silicon and C‐1 carbon chemical shifts on Hammett substituent constants are discussed. It is shown that the substituent sensitivity of these chemical shifts is reduced by association with chloroform, the reduction being proportional to the solvent accessible surface of the oxygen atom in the Si‐O‐C link. Copyright

Structure of Alkali Metal Hydroxamates and their Lossen Rearrangement

Neutral alkali metal salts of benzhydroxamic acids described in the literature are in fact acid salts; neutral salts rearrange to N,N′-diarylureas under mild conditions.