Department of Chemistry, Annamalai University, Annamalainagar, Chidambaram, Tamilnadu 608 002, India.
Spectrochim Acta A Mol Biomol Spectrosc. 2012 Nov;97:942-7. doi: 10.1016/j.saa.2012.07.089. Epub 2012 Aug 3.
The high resolution (1)H NMR spectra of four 3-ethyl-4-hydroxy-4-phenylpiperidines 1-4 have been recorded in CDCl(3), C(6)D(6), and (CD(3))(2)CO and analysed. These compounds exist in chair conformation with axial orientation of hydroxyl group at C(4) and equatorial orientation of the remaining substituents. The chemical shifts of methyl protons of ethyl group are quiet novel and surprising and are closer to TMS in CDCl(3) and somewhat higher in C(6)D(6). Changing the solvent from CDCl(3) to (CD(3))(2)CO alters the chemical shifts of these protons and they are very closer to TMS (0.01 ppm) in 3-4 and negative in 2 i.e., less than zero. The results are interpreted in terms of the magnetic anisotropic effect of the phenyl rings at C(2) and C(4) which in turn depends on the conformations of the ethyl group at C(3) and hydroxyl group at C(4). DFT studies were also performed to predict the favoured conformations of ethyl group at C(3) and hydroxyl group at C(4). Chemical shifts were also computed theoretically in the favoured conformation and they also support negative chemical shifts of 2 in acetone-d(6).
已记录并分析了四种 3-乙基-4-羟基-4-苯基哌啶 1-4 在 CDCl3、C6D6和（CD3）2CO 中的高分辨率（1）H NMR 谱。这些化合物均以椅式构象存在，羟基处于 C4的轴向，其余取代基处于平伏键。乙基上甲基质子的化学位移非常新颖和令人惊讶，在 CDCl3中更接近 TMS，而在 C6D6中略高。将溶剂从 CDCl3变为（CD3）2CO 会改变这些质子的化学位移，在 3-4 中它们非常接近 TMS（0.01 ppm），而在 2 中为负，即小于零。结果根据 C2和 C4上的苯基环的磁各向异性效应进行解释，而这又取决于 C3上的乙基和 C4上的羟基的构象。还进行了 DFT 研究以预测 C3上的乙基和 C4上的羟基的优势构象。在优势构象中也进行了理论化学位移计算，它们也支持 2 在丙酮-d6中的负化学位移。
