Conformational studies by dynamic NMR, 77 - Stereomutation of the enantiomers of hindered O-substituted oximes

As anticipated by Molecular Mechanics calculations, the (E) and (Z) isomers of diaryl ketone oximes containing a bulky substituent (PhS or Ph2N) in th e ortho position of the phenyl ring, display different conformational prefe rences. Whereas the (E) isomers exhibit a plane of symmetry at any accessib le temperature, the (Z) isomers exist as a pair of stereolabile enantiomers that were detected by low-temperature NMR spectroscopy in a chiral environ ment. In a number of O-substituted oximes, the enantiomerisation barriers o f the (Z) isomers were determined by monitoring the line shape of the NMR s ignals of diastereotopic methylene hydrogen atoms as a function of temperat ure. The four stereoisomers, generated by the combination of the conformati onal axial chirality with the configurationally stable chirality of a stere ogenic centre, have been all detected in a specifically substituted oxime a nd monitored in an appropriate chiral environment. The NMR results in solut ion were confirmed by X-ray diffraction measurements in the solid state.