Structure of monosilylated benzhydroxamic acids in crystals and solutions

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.