Bicyclo[6.3.0]undecapentaenyl Anion: The Next Higher Homolog of the Indenyl Anion with Exceptionally Large Ion‐Pairing Effects on its Tropicity |
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Authors: | Hiroaki Ozoe Yasutaka Uno Prof?Dr Chitoshi Kitamura Prof?Dr Hiroyuki Kurata Prof?Dr Masaji Oda Dr John W Jones Jr Prof?Dr Lawrence T Scott Prof?Dr Takeshi Kawase |
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Institution: | 1. Department of Materials Science, Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, 671‐2280 (Japan);2. Present address: Department of Materials Science, School of Engineering, University of Shiga Prefecture, 2500, Hassaka‐cho, Hikone, Shiga 522‐8533 (Japan);3. Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560‐0043 (Japan);4. Present address: Department of Environmental and Biological Chemistry, Faculty of Engineering, Fukui University of Technology, Gakuen 3‐6‐1, Fukui 910‐8505 (Japan);5. Department of Chemistry and Center for Advanced Study, University of Nevada, Reno, Nevada, 89557‐0020 (USA);6. Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, MA 02467‐3860 (USA) |
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Abstract: | The title anion 1 was generated as a fairly thermally stable species in tetrahydrofuran (THF) and dimethylsulfoxide (DMSO) by the action of several bases (sodium hydride, potassium hydride, lithium diisopropylamide, and lithium hexamethyldisilazide) with appropriate bicyclo6.3.0]undecapentaenes. Variable‐temperature 1H NMR spectra of 1? Li+ in D8]THF reveal that the anion exhibits exceptionally large ion‐pairing effects; proton chemical shifts vary by more than 1 ppm as a function of ion‐pairing conditions. Thus, anion 1 , in a contact ion pair (Li+ at ambient temperature in THF), behaves as an aromatic cyclopentadienyl anion that is perturbed only slightly by the electronic effects of a paramagnetic cyclooctatetraene (COT), whereas 1 in a separated ion pair (Li+ at low temperatures in THF or at ambient temperature in DMSO) behaves as an overall paratropic species with a 12 π‐electron periphery. 13C NMR spectroscopy indicates no major skeletal rearrangement and only small variations of the electron density. The variable tropicity of 1 can be ascribed to small conformational changes of the molecule. In addition to its unusual, tunable tropicity, anion 1 can also serve as a versatile building block for the synthesis of cyclopentanoid conjugated systems fused to a fully unsaturated eight‐membered ring. A theoretical calculation predicts that the 10‐position of 1 should have the highest electron density. In agreement with this prediction, the reactions of 1 with electrophiles occur predominantly at the 10‐position. The corresponding ferrocene, two fulvenes, two diazo derivatives, and a COT‐fused azulene were obtained by the reactions of 1 with appropriate electrophiles. |
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Keywords: | aromaticity carbanions fused‐ring systems ion pairs NMR spectroscopy |
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