Belostotskii Anatoly M, Gottlieb Hugo E, Aped Pinchas
Chemistry Department, Bar-Ilan University, Ramat-Gan 52900, Israel.
Chemistry. 2002 Jul 2;8(13):3016-26. doi: 10.1002/1521-3765(20020703)8:13<3016::AID-CHEM3016>3.0.CO;2-6.
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.
通过N-甲基前体与酮的光反应合成了具有β-支链N-烷基取代基的2,2,6,6-四甲基取代哌啶。这些空间位阻胺的主要构象特征（通过核磁共振和红外光谱确定）是椅式构象的环、N-取代基的重叠构象以及分子内的OH...N键。通过对温度依赖性的13C和1H核磁共振谱进行线形分析，在不同温度下测量了这些哌啶的偕取代基互变异构的高势垒。基于MM3的构象方案表明，对于其中一个研究的类似物，N-取代基的旋转决定了缓慢的互变异构速率。对于氢键胺，考虑了一种新的氮反转机制——协同氢键解离/氮反转过程。
