Silylation of <i>N,O</i>-Diacylhydroxylamines:  NMR Spectra and Structure of the Products

Silylation of <i>N,O</i>-diacylhydroxylamines with <i>N</i>-(<i>tert</i>-butyldimethylsilyl)-<i>N</i>-methyltrifluoroacetamide produces only O-silylated products and no N-silylated derivative. Steric interactions of the substituents on the nitrogen atom with the O−N oxygen atom control the configuration on the CN bond in the product:  a <i>Z </i>isomer is formed almost exclusively from <i>N</i>-benzoylhydroxylamines, and a mixture of <i>E</i> and <i>Z</i> isomers is produced from <i>N</i>-acetyl derivatives. The relative stability of all isomers in various conformations was confirmed by calculations at the B3LYP/6-31G(d,p) level. The <i>tert</i>-butyldimethylsilyl derivatives have been fully characterized by their <sup>1</sup>H, <sup>13</sup>C, <sup>15</sup>N, and <sup>29</sup>Si NMR spectra.