{"title":"Positive charge delocalization and anti-aromaticity of cations generated by protonation of benzo[a]fluoranthenes in superacid","authors":"Takao Okazaki, Haruki Yamashita, Toshikazu Kitagawa","doi":"10.1002/poc.4647","DOIUrl":null,"url":null,"abstract":"<p>Benzo[<i>a</i>]fluoranthene (<b>4a</b>) is one of non-alternate polycyclic aromatic hydrocarbons. A reaction of <b>4a</b> in CF<sub>3</sub>SO<sub>3</sub>H yielded a dark yellow solution. Direct NMR observation indicated the exclusive formation of carbocation <b>4aH</b><sup><b>+</b></sup> by protonation at the C(8) position. The most deshielded <sup>1</sup>H and <sup>13</sup>C 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 <b>4a</b>. A reaction of 3-<i>tert</i>-butylbenzo[<i>a</i>]fluoranthene (<b>4b</b>) in CF<sub>3</sub>SO<sub>3</sub>H afforded carbocation <b>4bH</b><sup><b>+</b></sup> by the protonation at the C(8) position. <b>4bH</b><sup><b>+</b></sup> was gradually converted to <b>4aH</b><sup><b>+</b></sup>. The changes in <sup>13</sup>C NMR chemical shifts (Δδ<sup>13</sup>C) suggested that positive charge was delocalized into mainly seven carbons in <b>4aH</b><sup><b>+</b></sup> and <b>4bH</b><sup><b>+</b></sup>. The observed cations were found to be the most stable cations among the possible protonation cations by the DFT method. The NICS(1)<sub><i>zz</i></sub> values for the five-membered rings were calculated to be 35.6 for <b>4aH</b><sup><b>+</b></sup> and 34.4 for <b>4bH</b><sup><b>+</b></sup> by GIAO-B3LYP/6-311+G(2d,p). The experimental NMR and the NICS(1)<sub><i>zz</i></sub> data indicated that the five-membered rings in <b>4aH</b><sup><b>+</b></sup> and <b>4bH</b><sup><b>+</b></sup> exhibited anti-aromaticity.</p>","PeriodicalId":16829,"journal":{"name":"Journal of Physical Organic Chemistry","volume":null,"pages":null},"PeriodicalIF":1.9000,"publicationDate":"2024-07-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Journal of Physical Organic Chemistry","FirstCategoryId":"92","ListUrlMain":"https://onlinelibrary.wiley.com/doi/10.1002/poc.4647","RegionNum":4,"RegionCategory":"化学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q2","JCRName":"CHEMISTRY, ORGANIC","Score":null,"Total":0}
引用次数: 0
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.
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.