Dications of Fluorenylidenes. The Relationship between Redox Potentials and Antiaromaticity for Meta- and Para-Substituted Diphenylmethylidenefluorenes
Electrochemical oxidation of meta-substituted diphenylmethylidenefluorenes (3a−g) results in the formation of fluorenylidene dications that are shown to be antiaromatic through calculation of the nucleus independent chemical shift (NICS) for the 5- and 6-membered rings of the fluorenyl system. There...
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Published in | Journal of organic chemistry Vol. 70; no. 26; pp. 10709 - 10716 |
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Main Authors | , , , , , , |
Format | Journal Article |
Language | English |
Published |
WASHINGTON
American Chemical Society
23.12.2005
Amer Chemical Soc |
Subjects | |
Online Access | Get full text |
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Summary: | Electrochemical oxidation of meta-substituted diphenylmethylidenefluorenes (3a−g) results in the formation of fluorenylidene dications that are shown to be antiaromatic through calculation of the nucleus independent chemical shift (NICS) for the 5- and 6-membered rings of the fluorenyl system. There is a strong linear correlation between the redox potential for the dication and both the calculated NICS and σm. Redox potentials for formation of dications of analogously substituted tetraphenylethylenes shows that, with the exception of the p-methyl derivative, the redox potentials for these dications are less positive than for formation of the dications of 3a−g and for dications of p-subtituted diphenylmethylidenefluorenes, 2a−g. The greater instability of dications of 2a−g and 3a−g compared to the reference system implies their antiaromaticity, which is supported by the positive NICS values. The redox potentials for formation of the dications of meta-substituted diphenylmethylidenes (3a−g) are more positive than for the formation of dications of para-substituted diphenylmethylidenes (2a−g), indicating their greater thermodynamic instability. The NICS values for dications of 3a−g are more antiaromatic than for dications of 2a−g, which is consistent with their greater instability of the dications of 3a−g. Although the substituted diphenylmethyl systems are not able to interact with the fluorenyl system through resonance because of their geometry, they are able to moderate the antiaromaticity of the fluorenyl cationic system. Two models have been suggested for this interaction, σ to p donation and the ability of the charge on the substituted ring system to affect delocalization. Examination of bond lengths shows very limited variation, which argues against σ to p donation in these systems. A strong correlation between NICS and σ constants suggests that factors that affect the magnitude of the charge on the benzylic (α) carbon of the diphenylmethyl cation affect the antiaromaticity of the fluorenyl cation. Calculated atomic charges on carbons 1−8 and 10−13 show an increase in positive charge, and therefore greater delocalization of charge in the fluorenyl system, with increasing electronegativity of the substituent. The change in the amount of positive charge correlated strongly with NICS, supporting the model in which the amount of delocalization of charge is related to the antiaromaticity of the species. Thus, both aromatic and antiaromatic species are characterized by extensive delocalization of electron density. |
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Bibliography: | istex:5EC58B7B3F634F302C29CDC2C4D1B9107CFB97C9 ark:/67375/TPS-SN2FMGSN-8 ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23 |
ISSN: | 0022-3263 1520-6904 |
DOI: | 10.1021/jo051599u |