Unraveling aromaticity: the dual worlds of pyrazole, pyrazoline, and 3D carborane
A new series of o-carborane-fused pyrazoles has been recently successfully synthesized. This fusion was expected to create a hybrid 3D/2D aromatic system, combining the 3D aromaticity of o-carborane with the 2D aromaticity of pyrazole. However, while the boron cage retains its aromatic character, th...
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2025-02-01
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| Series: | Beilstein Journal of Organic Chemistry |
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| Online Access: | https://doi.org/10.3762/bjoc.21.29 |
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| author | Zahra Noori Miquel Solà Clara Viñas Francesc Teixidor Jordi Poater |
| author_facet | Zahra Noori Miquel Solà Clara Viñas Francesc Teixidor Jordi Poater |
| author_sort | Zahra Noori |
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| description | A new series of o-carborane-fused pyrazoles has been recently successfully synthesized. This fusion was expected to create a hybrid 3D/2D aromatic system, combining the 3D aromaticity of o-carborane with the 2D aromaticity of pyrazole. However, while the boron cage retains its aromatic character, the pyrazole’s aromaticity is lost. As a result, rather than forming o-carborane-fused pyrazoles, the synthesis yielded o-carborane-fused pyrazolines, which are non-aromatic. The limited overlap between the π molecular orbitals (MOs) of the planar heterocycle and the n + 1 MOs of the carborane prevents significant electronic delocalization between the two fused components. This contrasts with the fusion of pyrazole and benzene to form indazole, where both rings maintain their 2D aromaticity. Our findings demonstrate that the peripheral σ-aromaticity of carborane and the π-aromaticity of the heterocycle are orthogonal, making a true 3D/2D aromatic system unachievable. The carborane is highly aromatic, generating highly negative NICS values (−25 to −30 ppm). We have observed that these high NICS values extend to fused rings, leading to incorrect estimations of aromaticity. Therefore, relying solely on NICS can be misleading, and other computational indicators, along with experimental or structural data, should be used to accurately assess aromaticity. |
| format | Article |
| id | doaj-art-ba33fd79e2d84a34aaf6402fbc3b15c0 |
| institution | DOAJ |
| issn | 1860-5397 |
| language | English |
| publishDate | 2025-02-01 |
| publisher | Beilstein-Institut |
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| series | Beilstein Journal of Organic Chemistry |
| spelling | doaj-art-ba33fd79e2d84a34aaf6402fbc3b15c02025-08-20T03:15:50ZengBeilstein-InstitutBeilstein Journal of Organic Chemistry1860-53972025-02-0121141242010.3762/bjoc.21.291860-5397-21-29Unraveling aromaticity: the dual worlds of pyrazole, pyrazoline, and 3D carboraneZahra Noori0Miquel Solà1Clara Viñas2Francesc Teixidor3Jordi Poater4Departament de Química Inorgànica i Orgànica & Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona, Martí i Franquès 1-11, 08028 Barcelona, Spain Institut de Química Computacional i Catàlisi and Departament de Química, Universitat de Girona, Maria Aurèlia Capmany 69, 17003 Girona, Catalonia, Spain Institut de Ciència de Materials de Barcelona, Consejo Superior de Investigaciones Científicas, Campus Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain Institut de Ciència de Materials de Barcelona, Consejo Superior de Investigaciones Científicas, Campus Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain Departament de Química Inorgànica i Orgànica & Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona, Martí i Franquès 1-11, 08028 Barcelona, Spain A new series of o-carborane-fused pyrazoles has been recently successfully synthesized. This fusion was expected to create a hybrid 3D/2D aromatic system, combining the 3D aromaticity of o-carborane with the 2D aromaticity of pyrazole. However, while the boron cage retains its aromatic character, the pyrazole’s aromaticity is lost. As a result, rather than forming o-carborane-fused pyrazoles, the synthesis yielded o-carborane-fused pyrazolines, which are non-aromatic. The limited overlap between the π molecular orbitals (MOs) of the planar heterocycle and the n + 1 MOs of the carborane prevents significant electronic delocalization between the two fused components. This contrasts with the fusion of pyrazole and benzene to form indazole, where both rings maintain their 2D aromaticity. Our findings demonstrate that the peripheral σ-aromaticity of carborane and the π-aromaticity of the heterocycle are orthogonal, making a true 3D/2D aromatic system unachievable. The carborane is highly aromatic, generating highly negative NICS values (−25 to −30 ppm). We have observed that these high NICS values extend to fused rings, leading to incorrect estimations of aromaticity. Therefore, relying solely on NICS can be misleading, and other computational indicators, along with experimental or structural data, should be used to accurately assess aromaticity.https://doi.org/10.3762/bjoc.21.293d aromaticitycarboranedensity functional theorynics ambiguitypyrazole |
| spellingShingle | Zahra Noori Miquel Solà Clara Viñas Francesc Teixidor Jordi Poater Unraveling aromaticity: the dual worlds of pyrazole, pyrazoline, and 3D carborane Beilstein Journal of Organic Chemistry 3d aromaticity carborane density functional theory nics ambiguity pyrazole |
| title | Unraveling aromaticity: the dual worlds of pyrazole, pyrazoline, and 3D carborane |
| title_full | Unraveling aromaticity: the dual worlds of pyrazole, pyrazoline, and 3D carborane |
| title_fullStr | Unraveling aromaticity: the dual worlds of pyrazole, pyrazoline, and 3D carborane |
| title_full_unstemmed | Unraveling aromaticity: the dual worlds of pyrazole, pyrazoline, and 3D carborane |
| title_short | Unraveling aromaticity: the dual worlds of pyrazole, pyrazoline, and 3D carborane |
| title_sort | unraveling aromaticity the dual worlds of pyrazole pyrazoline and 3d carborane |
| topic | 3d aromaticity carborane density functional theory nics ambiguity pyrazole |
| url | https://doi.org/10.3762/bjoc.21.29 |
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