Bicyclo[6.3.0]undecapentaenyl Anion: The Next Higher Homolog of the Indenyl Anion with Exceptionally Large Ion‐Pairing Effects on its Tropicity

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 bicyclo[6.3.0]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.

Bicyclo[6.3.0]undeca‐1(11),2,4,6,8‐pentaen‐10‐ylidene: An Aromatic Carbene with Ambiphilic Properties

“…10‐Diazobicyclo[6.3.0]undeca‐1(11),2,4,6,8‐pentaene ( 5 ) was prepared by following the reported procedure …”

“…Recently, we reported the synthesis, reactivity, and magnetic properties of bicyclo[6.3.0]undecapentaenyl anion 6 (Scheme ) . The anion shows large chemical‐shift changes in its 1 H NMR spectra that can be ascribed to ion‐pairing effects.…”

Arthur C. Cope Scholar Awards / Inorganic Nanoscience Award der ACS: R. E. Schaak / Priestley‐Medaille: T. J. Marks

“…Er interessiert sich für die Synthese von isomerenreinen Fullerenen und Kohlenstoffnanoröhren mit einheitlichen Durchmessern sowie von Strukturen wie geodätischen molekularen Schalen und Körben, aromatischen Gürteln und Graphennanobändern. Seine Veröffentlichung über das Elektronentransferverhalten von Fullerenfragmenten wurde auf dem Titelbild der Angewandten Chemie vorgestellt, und in Chemistry—An Asian Journal hat er über ein Bicyclo[6.3.0]‐Anion geschrieben …”

Arthur C. Cope Scholar Awards / ACS Inorganic Nanoscience Award: R. E. Schaak / Priestley Medal: T. J. Marks

“…Scott's research interests include the synthesis of isomerically pure fullerenes and uniform‐diameter carbon nanotubes as well as structures such as geodesic molecular bowls and baskets, aromatic belts, and graphene nanoribbons. His report on electron‐transfer behavior of fullerene fragments was featured on the cover of Angewandte Chemie , and he has also reported in Chemistry—An Asian Journal on the bicyclo[6.3.0] anion …”