Racemic and achiral alkyl aryl nitroxides were examined at various temperatures. The restricted rotation of the aminoxyl substituents was indicated both by the β-proton coupling constants and by the inequivalence of the aromatic ortho protons. These results are interpreted by the presence of two different rotamers which may lead to two splitting constants for the β-proton. The capability for inter- and intra-molecular hydrogen bond formation in the nitroxides investigated was demonstrated. However, complexes consisting of chiral radicals and chiral auxiliaries do not show hyperfine structures which unambiguously indicate diastereomeric species. Therefore, hydrogen bonding between chiral molecules is not sufficient for chiral recognition if an alteration of the hyperconjugation angle does not occur. Simple steric interactions can result in the doubling of the β-proton coupling constant, independently of diastereomeric complex formation.
在不同温度下对外消旋和非手性烷基芳基
硝基化合物进行了研究。β-质子耦合常数和芳香族正质子的不等价性都表明,
氨基羟基取代基的旋转受到了限制。对这些结果的解释是,存在两种不同的旋转体,这可能会导致 β-质子有两个分裂常数。所研究的硝基氧化物都能在分子间和分子内形成氢键。然而,由手性基和手性辅助剂组成的复合物并没有显示出明确表示非对映异构体的超精细结构。因此,如果不改变超共轭角,手性分子之间的氢键不足以实现手性识别。简单的立体相互作用就能使β-质子耦合常数加倍,而与非对映异构体复合物的形成无关。