Crowded piperidines with intramolecularly hydrogen-bonded nitrogen: synthesis and conformation study.
2,2,6,6-Tetramethyl substituted piperidines with a beta-branched N-alkyl substituent were synthesized by the photoreaction of N-Me precursors with ketones. The main conformation features of these sterically-hindered amines (established by NMR and IR spectroscopy) are a ring in the chair form, an eclipsed conformation for the N-substituent and an intramolecular OH...N bond. High barriers for the geminal substituent topomerization were measured for these piperidines at different temperatures by means of line-shape analysis of the temperature-dependent 13C and 1H NMR spectra. An MM3-derived conformation scheme indicated that, for one of the studied analogues, the rotation of the N-substituent determines a slow topomerization rate. A new mechanism of nitrogen inversion--a concerted hydrogen-bond dissociation/nitrogen inversion process--is considered for hydrogen-bonded amines.
Belostotskii
Anatoly M
AM
Chemistry Department, Bar-Ilan University, Ramat-Gan 52900, Israel. belostot@mail.biu.ac.il
Gottlieb
Hugo E
HE
Aped
Pinchas
P
D006860:Hydrogen Bonding; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D008968:Molecular Conformation; D010880:Piperidines / Q000737:chemistry*
D010880:Piperidines