Buckybowls DOI: 10.1002/anie.201002515 The Bicorannulenyl Dianion: A Charged Overcrowded Ethylene** David Eisenberg, EdwardA. Jackson, Jennifer M. Quimby, Lawrence T. Scott, and Roy Shenhar* The interplay between structural strain and conjugation makes two families of polycyclic hydrocarbons—bucky- bowls [1] and overcrowded ethylenes (OEs) [2] —particularly appealing. This interplay gives rise to a rich stereochemistry in OEs, and allows buckybowls, which are polycyclic hydro- carbons that can be mapped onto fullerene surfaces, to be used as model compounds [1, 3] and possible precursors [4] for curved carbon-rich structures such as fullerenes and carbon nanotubes. [2] Buckybowls are also interesting because of their organometallic [5] and supramolecular [6] chemistry, photophys- ical properties, [7] and reduction capacity. [8, 9] Corannulene (C 20 H 10 ), the archetypal buckybowl, is the smallest curved subunit of C 60 . [1] Corannulene can accept several negative charges because of the double degeneracy of its lowest unoccupied molecular orbital (LUMO), which arises from its high symmetry. Reduction of corannulene and other buck- ybowls often leads to unusual structural and electronic phenomena, [8] such as anisotropic charge redistribution [10] and self-assembly through different binding modes. [11] The stereochemistry of OEs has fascinated chemists for over a century [12] and has promoted the return of OEs to the spotlight of material science as structural elements in molecular machines and switches. [2, 13] OEs can be intercon- verted between the principal conformations, that is, folded and twisted, by applying external stimuli, such as heat, [12, 14] pressure, [12, 15] photoexcitation, [14] and redox chemistry. [16] To the best of our knowledge, in every OE studied so far, reduction or oxidation relieves the steric hindrance present in the neutral molecule by decreasing the conjugation between the two units, thus effectively converting the OE into a twisted biaryl unit. [16, 17] An elegant exploitation of this property for switching was demonstrated by Feringa and co- workers, where a redox reaction of a bithiaxanthylidene derivative served as input in a three-state luminescent switch. [18] The opposite phenomenon, where reduction of a biaryl unit increases the double-bond character of the tether connecting the two subunits is extremely rare and has only been demonstrated so far in small molecules, such as biphenyl and 1,1’-binaphthyl. [19] Herein we describe the alkali-metal reduction of bicor- annulenyl (1), a large biaryl composed of two corannulene units, and demonstrate that the single bond tethering the two corannulene units acquires a substantial double-bond char- acter upon charging to a dianion, thus effectively transform- ing 1 into a charged OE (Scheme 1). The dynamic stereochemistry of neutral 1 has been recently elucidated by our research group. [20] Compound 1 contains two chiral elements: the asymmetry of a monosub- stituted corannulenyl bowl, and the helical chirality around the central bond. Together, these elements give rise to rich stereodynamics. The neutral molecule displays twelve stable conformations, but because of the low energy barrier for breaking the p conjugation between the units (1–2 kcal mol À1 , similar to biphenyl), [21] only three chiral diastereomers (three pairs of enantiomers) are observed in the 1 H NMR spectrum. The diastereomers interconvert through bowl-to-bowl inver- sions and rotations about the central bond. To facilitate the discussion, each conformation is annotated according to the following conventions : R/S for right or left twist of the tether ; P/M for the handedness of the bowls ; and a subscript number denoting the twist angle. Thus, enantiomers would have inverse annotations, for example, S.PP 139 and R.MM À139 . When bicorannulenyl is reduced by using potassium metal in [D 8 ]THF, [22] a diamagnetic species is observed in the 1 H NMR spectrum at low temperatures (Figure 1). The spectrum at 190 K consists of two sets of absorptions spread over a range of D = 6 ppm, each with one singlet and eight doublet signals, with a relative integration ratio of 3.8:1. As the temperature is increased, the spectrum disappears rever- sibly without change in chemical shifts, thus indicating rapid equilibrium with a thermally accessible triplet spin state. [23] 2D NMR experiments (COSY and NOESY) indicate that the two sets of signals represent two separate diastereomers that interconvert on the NMR timescale. All 1 H and 13 C NMR Scheme 1. Reduction of bicorannulenyl (1) with elemental potassium. [*] D. Eisenberg, Dr. R. Shenhar Institute of Chemistry and the Lise Meitner-Minerva Center for Computational Quantum Chemistry, The Hebrew University Jerusalem, 91904 (Israel) Fax: (+ 972) 2-658-5345 E-mail: roys@chem.ch.huji.ac.il Homepage: http://chem.ch.huji.ac.il/shenhar-group Dr. E. A. Jackson, J. M. Quimby, Prof. Dr. L. T. Scott Department of Chemistry, Merkert Chemistry Center Boston College Chestnut Hill, MA 02467 (USA) [**] Financial support from the National Science Foundation (NSF) and from The Lise Meitner-Minerva Center for Computational Quantum Chemistry is gratefully acknowledged. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201002515. Communications 7538  2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2010, 49, 7538 –7542