Ring‐substituted benzohydroxamic acids: ¹H, ¹³C and ¹⁵N NMR spectra and NHOH proton exchange

Abstract

NMR spectra (1H, 13C, 15N) of para- and meta-substituted benzohydroxamic acids were studied in dry dimethyl sulfoxide solutions. The 13C chemical shifts were very close to those found by cross-polarization magic angle spinning in solids, the hydroxamic (not hydroximic) structure of which is unambiguous. The hydroxamic structure of these acids in DMSO solutions was proved independently by their 15N chemical shifts. The 15N and 1H chemical shifts of the NH-OH fragment showed excellent mutual dependences and dependences on the nature of the ring substituent. According to these dependences and ab initio energy calculations, all the acids assume the same Z conformation. Proton exchange between hydroxamic OH and NH groups in DMSO proceeded by both intra- and intermolecular exchange and the rates did not exhibit any simple relationship to the substituent constants.

15N NMR Spectroscopy in Structural Analysis: An Update (2001 - 2005)

“…They conluded that (Z)-tert-butyldimethylsilyl-N-tertbutyldimethylsilyloxybenzoimidate, (Z)-tert-butyldiphenylsilyl-N-tert-butyl-diphenylsilyloxybenzoimidate, and N-tertbutyldiphenylsilyloxybenzoimidic acid possess hydroximic structures. [213]. For a series of substituted benzohydroxamic acids, the 15 N chemical shifts ranged from 162.7 to 170.7 ppm, indicating that in DMSO solution all compounds were present in the hydroxamic form Fig.…”

“…In order to obtain aromatic derivatives with a proton transferred to the nitrogen atom, the acidity of the OH group must be increased by appropriate substitution. Isomers of hydroxamic acids [213]. Furthermore, comparison of the contribution of the NH form in solution and in the solid state reveals that the NH form is more abundant in the solid state for the aromatic derivatives, although the extent varies greatly [152].…”

“…Calculated 15 N CS have been used, for example, to study the tautomerism of pyrimidines [211,212], the conformational behavior and NH OH proton exchange of ring-substituted benzhydroxamic acids [213], the identity of the double-proton transfer in (3Z)-3-hydroxy-1,4-di(quinolin-2-yl)-but-3-en-2-one [214], the furoxan rearrangement of pyridofuroxan derivatives [215], and the conformation of amines [216]. Specific interactions (hydrogen bonding) between solute and solvent are not usually well described by using only the continuum solvent model and must be simulated by the explicit addition of solvent molecules.…”

Unusual Catalytic Process Involving OH and NH Exchange

“…To our knowledge, this is the first example observed for such type of exchange on a phenyl ring. However, the proton NH/OH exchange using 1 H, 13 C and 15 N NMR was previously observed [17]. Also, the NH/OH exchange as well as proton exchange is a familiar feature on metal centers [18][19][20].…”

Synthesis, structural characterisation and antibacterial activity of bis(1-phenyl-1,3-butanedionato)non-oxovanadium(IV) hydroxamates

“…The powerful biological activity of structurally heterogeneous hydroxamic acids is related to their ability to form stable chelates with a variety of metal ions [12,13]. Metal complexes of hydroxamic acids have also attracted considerable attention because of their tautomerisation [14] exhibiting hydroxamic or hydroximic acid forms. The theoretical and experimental studies have suggested that the hydroxamic form is prevalent in free acids [15] or metal hydroxamates [16].…”