Molecular clips and tweezers with corannulene pincers Lesya Kobryn a , William P. Henry a , Frank R. Fronczek b , Renata Sygula a , Andrzej Sygula a, * a Department of Chemistry, Mississippi State University, Mississippi State, MS 39762, USA b Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803, USA article info Article history: Received 3 September 2009 Revised 26 September 2009 Accepted 30 September 2009 Available online 4 October 2009 abstract Application of the synthetic methodology developed in our laboratory yields molecular clips and twee- zers with corannulene pincers and cyclooctadiene or cyclooctatetraene tethers. These highly nonplanar systems possess a potential for binding guest molecules of various sizes. One of the clips, bis(benzocor- annulene)[a,e]cyclooctadiene, exists in a ‘open’ chair conformation encapsulating solvating nitrobenzene in the solid state, while it is expected to prefer an internally p–p stacked twist-boat conformation in the gas phase. Ó 2009 Elsevier Ltd. All rights reserved. Molecular recognition of guest molecules by molecular recep- tors is of premium importance in self-assembly and self-organiza- tion of matter. The design and synthesis of molecular assemblies capable of such recognition is therefore a major objective of supra- molecular chemistry. 1 Molecular recognition processes strongly depend on weak, noncovalent interactions, among which the p–p stacking of conjugated surfaces of arenes play an important role. A novel type of stacking of curved conjugated carbon networks has been introduced by the discovery of fullerenes and related carbon-rich materials. In this context, buckybowls, bowl-shaped polycyclic aromatic hydrocarbons with accessible concave sur- faces 2 appear to be good candidates for the formation of p–p stacked supramolecular assemblies with fullerenes. Although this potential has long been recognized, 3 we only recently provided the first experimental evidence for the formation of a stable inclu- sion complex of C 60 with the ‘buckycatcher’ (1), a molecular clip with two corannulene (2) pincers and a tetrabenzocyclooctatetrae- ne tether. 4 1 2 C 60 @1 In the present Letter, we report the synthesis of a series of molecular clips 5 combining corannulene pincers and cyclooctadi- ene (COD) or cyclooctatetraene (COT) tethers, most notably bis(benzocorannuleno)[a,e]cyclooctadiene (3). These highly non- planar systems exhibit clefts of various sizes and shapes with potential to bind guest molecules with geometric selectivity. In addition, conformational flexibility of their tethers allows for an adoption of conformations unavailable for the more rigid tetra- benzoCOT core of the buckycatcher 1. O TMS OTf 3 4 5 Our syntheses utilize two synthons recently introduced by us, that is, isocorannulenofuran (diene) (4) and 2-trimethylsilylcor- annulenyl trifluoromethanesulfonate (5, a precursor for 1,2-dide- hydrocorannulene). 6 Both 4 and 5 allow for an introduction of corannulene subunits into larger molecular architectures by Diels–Alder reactions with appropriate dienophiles or dienes. Scheme 1 presents the synthesis of molecular clip 6 with a COT tether and corannulene and benzene pincers. The synthesis starts with a Diels–Alder reaction of 4 with benzocyclobutadiene gener- ated in situ by a standard procedure 7 which leads to the formation of two isomeric adducts, endo-7 and exo-7 (ca. 9:1) in a modest 55% yield based on consumed starting material. The isomers are sepa- rated by chromatography on silica gel and are characterized by NMR spectroscopy. In addition, endo-7 produced X-ray quality crystals and its crystal structure is shown in Figure 1. This highly nonplanar system exhibits the benzene ring of the benzocyclobu- tane moiety located over the convex side of the corannulene unit. The location results in a significant shielding of the four hydrogen atoms of the AA 0 BB 0 system of the benzene ring appearing in the 1 H NMR spectrum as broad multiplets centered at 5.4 and 5.9 ppm, upfield from the usual chemical shift of aromatic protons. In con- 0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2009.09.177 * Corresponding author. Tel.: +1 662 325 7612; fax: +1 662 325 1816. E-mail address: asygula@chemistry.msstate.edu (A. Sygula). Tetrahedron Letters 50 (2009) 7124–7127 Contents lists available at ScienceDirect Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet