Silylation of N,O-Diacylhydroxylamines:  NMR Spectra and Structure of the Products

Silylation of N,O-diacylhydroxylamines with N-(tert-butyldimethylsilyl)-N-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 Z isomer is formed almost exclusively from N-benzoylhydroxylamines, and a mixture of E and Z isomers is produced from N-acetyl derivatives. The relative stability of all isomers in various conformations was confirmed by calculations at the B3LYP/6-31G(d,p) level. The tert-butyldimethylsilyl derivatives have been fully characterized by their ¹H, ¹³C, ¹⁵N, and ²⁹Si NMR spectra.