CONFORMATIONAL-ANALYSIS .40. CONFORMATION OF 1-METHYL-1-PHENYLCYCLOHEXANE AND THE CONFORMATIONAL ENERGIES OF THE PHENYL AND VINYL GROUPS
CONFORMATIONAL-ANALYSIS .40. CONFORMATION OF 1-METHYL-1-PHENYLCYCLOHEXANE AND THE CONFORMATIONAL ENERGIES OF THE PHENYL AND VINYL GROUPS
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DOI：
10.1021/jo00322a056
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发表时间：
1981-01-01
影响因子：
3.6
通讯作者：
MANOHARAN, M
中科院分区：
文献类型：
--
作者：
ELIEL, EL;MANOHARAN, M
1971年，Allinger和Tribble 1 2使用力场方法计算出，在1-甲基-1-苯基环己烷（1，方案I）中，甲基（1.7 kcal/mol）和苯基（3.0 kcal/mol）的构象能3不应相加，在平衡时，轴向位置上具有较大苯基的构象优先0.9 kcal/mol。这是因为轴向苯基（垂直于环己烷椅的二等分面的苯基）的最稳定构象不受引入偕甲基取代基的扰动，而当引入偕甲基到赤道苯基环己烷中时发生相当大的扰动，赤道苯基环己烷的苯基取代基（在没有偕甲基的情况下）在环己烷椅的二等分面中最稳定。Allinger-Tribble预测与当代NMR数据4定性一致，但随后用1-甲基-1-苯基-4，4-二甲氧基环己烷进行的低温质子NMR工作并未定量证实。5这项工作5表明，轴向苯基的偏好仅为0.34 kcal/mol。然而，实验者说：5“这种差异是由于偕二甲氧基的存在还是由于某些计算不足仍然是一个有争议的问题。”鉴于我们目前正在研究的一些中生科和苦杏仁科的生物碱中轴向苯基多于偕亚甲基的重要性，我们认为用低温13 C
In 1971 Allinger and Tribble1 2 calculated, using a force-field approach, that in1-methyl-l-phenylcyclohexane (1, Scheme I) the conformational energies3 of methyl (1.7 kcal/mol) and phenyl (3.0 kcal/mol) should not be additive, the conformation with the larger phenyl group in the axial position being preferred at equilibrium by 0.9 kcal/mol. This comes about because the most stable conformation of the axial phenyl group (phenyl perpen-dicular to the bisector plane of the cyclohexane chair) is not perturbed by introduction of a geminal methyl sub-stituent, whereas considerable perturbation occurs upon introduction of geminal methyl into equatorial phenyl-cyclohexane whose phenyl substituent is (in the absence of the geminal methyl) most stable in the bisector plane of the cyclohexane chair. The Allinger-Tribble prediction was in qualitative agreement with contemporary NMR data4 but was not quantitatively borne out by low-temperature proton NMR work subsequently carried out with 1-methyl-l-phenyl-4, 4-dimethoxycyclohexane. 5 This work5 indicated a preference for the axial phenyl group of only 0.34 kcal/mol. However, the experimenters stated: 5 “Whether thisdiscrepancy is due to the presence of the gem-dimethoxy group or to some computational insuffi-ciency remains a moot point”. In view of the importance of the prevalence of axial phenyl over geminal methylene in a number of alkaloids of the mesembrine6 and amarillidacea7 families, which we are presently investigating, we felt that confirmation of the result of Anteunis’ group5 by low-temperature 13C