SHORT COMMUNICATION DOI: 10.1002/ejoc.201101374 Cyclotrimerization of Corannulyne: Steric Hindrance Tunes the Inversion Barriers of Corannulene Bowls Michael Yanney, [a] Frank R. Fronczek, [b] WilliamP. Henry, [a] Debbie J. Beard, [a] and Andrzej Sygula* [a] Keywords: Arynes / Cyclotrimerization / Bowl-to-bowl inversion / Corannulenes Palladium-catalyzed cyclotrimerization of corannulyne gen- erated from 2-trimethylsilylcorannulenyl triflate produced a C 60 H 24 hydrocarbon that prefers a highly nonplanar “twist” conformation of C 1 symmetry, as demonstrated by X-ray crys- tal structure determination and HDFT calculations. Its three corannulene subunits, identical in the idealized D 3h sym- metry, exhibit dramatically different inversion barriers (from Introduction Palladium-catalyzed cyclotrimerization of acetylenes provides a synthetically useful route to substituted ben- zenes. [1] 1,2-Didehydrobenzenes (benzynes) have also been successfully cyclotrimerized to afford triphenylenes. [2] Re- cent reports of the preparation of sterically hindered poly- cyclic aromatic hydrocarbons (PAHs) such as hexabenzotri- phenylene (1) by using o-TMS triflates as benzyne precur- sors [3] prompted us to attempt the synthesis of hydrocarbon C 60 H 24 (2) by cyclotrimerization of 1,2-didehydrocor- annulene (corannulyne) 3. The resulting cyclotrimer exhib- its an interesting conformational behavior that is driven by dramatically different bowl-to-bowl inversion barriers of formally identical corannulene subunits. In addition, this synthetic approach opens the possibility for generation of discotic bowl-shaped molecular networks of nanometric size. [a] Department of Chemistry, Mississippi State University, 310 President’s Circle, Mississippi State, MS 37962, USA Fax: +1-662-325-1618 E-mail: asygula@chemistry.msstate.edu [b] Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803, USA Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejoc.201101374. © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Org. Chem. 2011, 6636–6639 6636 8.4 to 17.3 kcal mol –1 ), tuned by the steric congestion inside the highly dissymmetric structure. Bowl-to-bowl inversion of the corannulene unit with the lowest activation barrier re- sults in pseudorotation of the molecule and gives rise to a 1 H NMR spectrum exhibiting only 12 distinct proton signals. The line-shape analysis of selected NMR signals gave an estima- tion of the barrier at 8.5–8.6 kcal mol –1 . Results and Discussion Our synthesis was achieved with 40 % yield (Scheme 1) by using 2-trimethylsilylcorannulenyl triflate (4), previously introduced by us as an efficient precursor for aryne 3. [4] The identity of 2 was first confirmed by MALDI/TOF spec- trometry, which showed the expected pattern for the molec- ular radical ion of C 60 H 24 . [5] Room-temperature 1 H and 13 C NMR spectra of 2 exhibit very broad bands presumably due to slow (on the NMR time scale) conformational in- terconversions that did not allow immediate confirmation of the product’s structure. [5] The final confirmation of the structure of 2 came from X-ray crystallography (Figure 1), which showed two symmetry independent molecules of the highly nonplanar conformer with both DCM and toluene solvate molecules present in the unit cell. [6] Scheme 1. Cyclotrimerization of corannulyne. Cyclotrimer 2 belongs to the class of overcrowded PAHs with “ideal” D 3h symmetry that are forced to adopt confor- mations of lower symmetry due to steric congestion. The distortion gives rise to either propeller-type conformations (up–down, up–down, up–down) or twist structures (up–down, up–up, down–down; Figure 2). [8] Conformational analysis of 2 is complicated by the bowl- shape of the corannulene subunits. This feature multiplies the number of possible conformations in comparison to the simpler analogues possessing planar rim attachments. How-