Static and Dynamic Stereochemistry of O-Substituted N-9-Triptycylhydroxylamines

The static and dynamic stereochemistry of N-9-triptycylhydroxylamine derivatives (TpNHOR) with O-benzyl (3), O-ethyl (4), and O-phenyl (5) groups were studied. X-ray crystallographic analysis revealed that these compounds reside in a chiral conformation, with the Tp–N–O–R dihedral angle of 140–160°. Dynamic NMR studies of 3 and 4 show the presence of two separate stereomutation processes: chirality reversal (enantiomer interconversion) and rotation of the Tp–N bond with retaining the chirality, the latter having a lower energy barrier. Compound 5 gave solely information on the Tp–N rotation because of the lack of a probe for chirality reversal.