Structural effects on valence tautomerism. Part 5. A carbon-13 nuclear magnetic resonance. Study on the effects of the π-acceptor ability of aryl substituents on the valence tautomerism of 7-aryl-2,5-di-t-butyl-cyclohepta-1,3,5-trienes
Structural effects on valence tautomerism. Part 5. A carbon-13 nuclear magnetic resonance. Study on the effects of the π-acceptor ability of aryl substituents on the valence tautomerism of 7-aryl-2,5-di-t-butyl-cyclohepta-1,3,5-trienes
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对价互变异构的结构影响第 5 部分：碳 13 核磁共振研究芳基取代基的 π 受体能力对 7-芳基-2,5-二叔丁基-环庚基的价互变异构的影响。 -1,3,5-三烯
DOI：
10.1039/p29840000461
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发表时间：
1984
期刊：
影响因子：
--
通讯作者：
K. Okamoto
中科院分区：
文献类型：
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作者：
K. Takeuchi;H. Fujimoto;T. Kitagawa;H. Fujii;K. Okamoto
使用可变温度 13C n.m.r. 研究了 CS2-CD2Cl2 中 8 个 7-芳基-2,5-二叔丁基环庚-1,3,5-三烯的环庚三烯 (CHT)-去甲二烯 (NCD) 平衡。根据信号强度计算低于聚结温度的平衡常数 (K=[NCD]/[CHT])；较高温度下的值通过方程 K=(δCHT–δ)/(δ–δNCD) 获得。 CHT 转化为 NCD 互变异构体的焓 (ΔHo) 按取代基顺序降低：p-MeO > p-Me > H > p-Br ∼p-Cl > m-Cl > p-CF3 > m,m'-Cl2，其中 p-MeO 和 m,m'-Cl2 的 ΔHo 值为 2.64 ± 0.11 和 –0.81 ± 0.13 kJ分别为 mol–1。 ΔHo 值不仅与哈米特 σ 常数线性相关，而且与通过 INDO 计算估计的与 C(7) 相邻的芳基碳原子的 π 受体能力（D 值）线性相关；这支持了霍夫曼和冈瑟的供体-受体理论。 NCD和CHT互变异构体中C(3)和C(4)的13C信号随着芳基D值的增加而发生低场偏移；根据哈米特图评估，前者的偏移比后者的敏感度高 4.3 倍。此外，NCD和CHT互变异构体之间对位碳原子的13C位移差异在7-(m,m'-二氯苯基)中比在7-苯基系统中更大。这些结果表明，NCD结构中的环丙烷环与芳基之间的共轭相互作用随着芳基的π受体能力的增加而增强。
The cycloheptatriene (CHT)–norcaradiene (NCD) equilibria of eight 7-aryl-2,5-di-t-butylcyclohepta-1,3,5-trienes in CS2–CD2Cl2 have been studied with the use of variable-temperature 13C n.m.r. The equilibrium constants (K=[NCD]/[CHT]) below the coalescence temperatures were calculated from signal intensities; those at higher temperatures were obtained by use of the equation K=(δCHT–δ)/(δ–δNCD). The enthalpy (ΔHo) for the conversion of the CHT into the NCD tautomer decreases in the substituent order p-MeO > p-Me > H > p-Br ∼p-Cl > m-Cl > p-CF3 > m,m′-Cl2, with the ΔHo values for p-MeO and m,m′-Cl2 2.64 ± 0.11 and –0.81 ± 0.13 kJ mol–1, respectively. The ΔHo values are linearly correlated not only with the Hammett σ-constants, but also with the π-acceptor ability (the D value) of the aryl carbon atom adjacent to C(7) as estimated by INDO calculations; this supports Hoffmann and Gunther's donor–acceptor theory. The 13C signals of C(3) and C(4) in both the NCD and the CHT tautomer undergo downfield shifts as the D value of the aryl group increases; the shift of the former is 4.3 times more sensitive than that of the latter as evaluated by the Hammett plot. In addition, the difference in the 13C shift of the para-carbon atom between the NCD and the CHT tautomer is larger in the 7-(m,m′-dichlorophenyl) than in the 7-phenyl system. These results suggest that conjugative interaction between the cyclopropane ring in the NCD structure and the aryl group increases with increase in the π-acceptor ability of the aryl group.