A1H NMR and molecular modelling investigation of diastereotopic methylene hydrogen atoms
作者:Cl�udio F. Tormena、Matheus P. Freitas、Roberto Rittner、Raymond J. Abraham
DOI:10.1002/mrc.1011
日期:2002.4
The 1H NMR spectra of methyl 3-bromo-2-methylpropionate (1a) and the corresponding chloro compound (2a) show no long-range coupling between the methyl and methylene protons. In contrast, in the analogous dihalocompounds, methyl 2,3-dibromo-2-methylpropionate (1b) and methyl 2,3-dichloro-2-methylpropionate (2b), one of the methylene protons exhibits a large 4JHH coupling (0.8 Hz) to the methyl group, but the other proton shows no observable splitting. This can be explained quantitatively by calculations of the conformational preferences in these compounds combined with the known orientation dependence of the 4JHHcouplings. One conformer predominates in the dihalo compounds 1b and 2b, and this is responsible for the 4JHH coupling. In 1a and 2a all three conformers are populated and the 4JHH couplings average to zero. The technique is a potentially general method of unambiguously assigning diastereotopic methylene protons. Copyright © 2002 John Wiley & Sons, Ltd.
3-溴-2-甲基丙酸甲酯 (1a) 和相应的氯化合物 (2a) 的 1 H NMR 谱显示甲基和亚甲基质子之间没有长程耦合。相比之下,在类似的二卤代化合物 2,3-二溴-2-甲基丙酸甲酯 (1b) 和 2,3-二氯-2-甲基丙酸甲酯 (2b) 中,其中一个亚甲基质子表现出较大的 4JHH 耦合 (0.8 Hz)到甲基,但另一个质子没有显示出可观察到的分裂。这可以通过计算这些化合物的构象偏好并结合已知的 4JHH 偶联方向依赖性来定量解释。在二卤代化合物 1b 和 2b 中,一种构象异构体占主导地位,这就是 4JHH 偶联的原因。在 1a 和 2a 中,所有三个构象异构体均已填充,并且 4JHH 耦合平均值为零。该技术是明确分配非对映亚甲基质子的潜在通用方法。版权所有 © 2002 约翰威利父子有限公司