Izzotti Anthony R, Gleason James L
Department of Chemistry, McGill University 801 SherbrookeW. H3A 0B8 Montreal QC Canada
Chem Sci. 2024 Oct 8;15(44):18592-600. doi: 10.1039/d4sc05470a.
The presence of a small spirocyclic ring at an adjacent position alters the conformational preference for equatorial substitution in six-membered rings. DFT calculations and low-temperature H NMR experiments demonstrate that alkyl groups larger than methyl possess negative A-values when geminal to a spirocyclopropane, with larger groups such as isopropyl and -butyl being exclusively axial at -78 °C. Similar effects are found for heteroatoms, including halogens, and for a range of other electron-withdrawing substituents. Similar effects are observed for other strained rings (epoxide, cyclobutane, oxetane) and the concepts extend to acyclic models as well as heterocycles such as piperidines and piperazines. The origin of the effect is traced to an increase in torsional strain in combination with hyperconjugative effects in the case of electron-poor groups.
在相邻位置存在一个小的螺环会改变六元环中赤道取代的构象偏好。密度泛函理论计算和低温氢核磁共振实验表明，当与螺环丙烷偕位时，大于甲基的烷基具有负的A值，在-78°C时，较大的基团如异丙基和叔丁基完全处于轴向。对于杂原子（包括卤素）以及一系列其他吸电子取代基也发现了类似的效应。对于其他张力环（环氧化物、环丁烷、氧杂环丁烷）也观察到了类似的效应，并且这些概念也扩展到了无环模型以及杂环如哌啶和哌嗪。该效应的起源可追溯到在缺电子基团的情况下扭转应变的增加以及超共轭效应。
