Static and dynamic stereochemistry of N-alkyl- and N,O-dialkyl-N-9-triptycylhydroxylamine derivatives

Stereochemistry of N-9-triptycylhydroxylamine and its N-alkyl and N,O-dialkyl derivatives was studied. X-ray crystallographic analysis of N,O-diethyl-N-9-triptycylhydroxylamine revealed (a) the chiral nature of the molecule, (b) an almost tetrahedral geometry of the nitrogen atom, and (c) a nearly eclipsing arrangement of the O-Et bond with the nitrogen lone-pair orbital. In solution, enantiomerization takes place rapidly at room temperature on the NMR time scale but slows down at low temperatures. The stereomutation proceeds by way of two processes, "R-passing," and "O-passing," in which the alkyl (R) and oxy (OH or OR) groups attached to the nitrogen atom, respectively, pass over a benzene ring, accompanied by nitrogen inversion and partial rotation of the N-O bond. The energy barriers to these processes were obtained by dynamic NMR spectroscopy.