Positive charge delocalization and anti-aromaticity of cations generated by protonation of benzo[a]fluoranthenes in superacid

IF 1.9 4区 化学 Q2 CHEMISTRY, ORGANIC
Takao Okazaki, Haruki Yamashita, Toshikazu Kitagawa
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Abstract

Benzo[a]fluoranthene (4a) is one of non-alternate polycyclic aromatic hydrocarbons. A reaction of 4a in CF3SO3H yielded a dark yellow solution. Direct NMR observation indicated the exclusive formation of carbocation 4aH+ by protonation at the C(8) position. The most deshielded 1H and 13C signals were observed at 8.99 ppm for H(12) and 8.29 ppm for H(1), 182.6 ppm for C(12b), 155.6 ppm for C(8a), and 149.4 ppm for C(7a). The signals for H(4) and H(5) were significantly more shielded than those of 4a. A reaction of 3-tert-butylbenzo[a]fluoranthene (4b) in CF3SO3H afforded carbocation 4bH+ by the protonation at the C(8) position. 4bH+ was gradually converted to 4aH+. The changes in 13C NMR chemical shifts (Δδ13C) suggested that positive charge was delocalized into mainly seven carbons in 4aH+ and 4bH+. The observed cations were found to be the most stable cations among the possible protonation cations by the DFT method. The NICS(1)zz values for the five-membered rings were calculated to be 35.6 for 4aH+ and 34.4 for 4bH+ by GIAO-B3LYP/6-311+G(2d,p). The experimental NMR and the NICS(1)zz data indicated that the five-membered rings in 4aH+ and 4bH+ exhibited anti-aromaticity.

Abstract Image

苯并[a]荧蒽在超酸中质子化产生的阳离子的正电荷分散和反芳香性
苯并[a]荧蒽(4a)是一种非替代性多环芳烃。4a 在 CF3SO3H 中反应生成深黄色溶液。直接核磁共振观察表明,C(8)位上的质子化作用只形成了碳位 4aH+。在 8.99 ppm(H(12))和 8.29 ppm(H(1))、182.6 ppm(C(12b))、155.6 ppm(C(8a))和 149.4 ppm(C(7a))处观察到了最去屏蔽的 1H 和 13C 信号。H(4) 和 H(5) 信号的屏蔽程度明显高于 4a 信号。3- 叔丁基苯并[a]荧蒽(4b)在 CF3SO3H 中发生反应,C(8)位质子化产生了碳位 4bH+。4bH+ 逐渐转化为 4aH+。13C NMR 化学位移(Δδ13C)的变化表明,正电荷主要分散到 4aH+ 和 4bH+ 的七个碳中。通过 DFT 方法发现,所观察到的阳离子是可能的质子化阳离子中最稳定的阳离子。通过 GIAO-B3LYP/6-311+G(2d,p) 方法计算出的五元环的 NICS(1)zz 值为:4aH+ 为 35.6,4bH+ 为 34.4。实验核磁共振和 NICS(1)zz 数据表明,4aH+ 和 4bH+ 中的五元环具有反芳香性。
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来源期刊
CiteScore
3.60
自引率
11.10%
发文量
161
审稿时长
2.3 months
期刊介绍: The Journal of Physical Organic Chemistry is the foremost international journal devoted to the relationship between molecular structure and chemical reactivity in organic systems. It publishes Research Articles, Reviews and Mini Reviews based on research striving to understand the principles governing chemical structures in relation to activity and transformation with physical and mathematical rigor, using results derived from experimental and computational methods. Physical Organic Chemistry is a central and fundamental field with multiple applications in fields such as molecular recognition, supramolecular chemistry, catalysis, photochemistry, biological and material sciences, nanotechnology and surface science.
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