Synthesis, Structure, and Resolution of Exceptionally Twisted Pentacenes Jun Lu, † Douglas M. Ho, † Nancy J. Vogelaar, ‡ Christina M. Kraml, §,| Stefan Bernhard, † Neal Byrne, | Laura R. Kim, | and Robert A. Pascal, Jr.* ,† Contribution from the Department of Chemistry, Princeton UniVersity, Princeton, New Jersey 08544, Department of Biology, Virginia Tech, Blacksburg, Virginia 24061, Wyeth Research, CN 8000, Princeton, New Jersey 08543, AccelaPure Corporation, Princeton, New Jersey 08544 Received August 15, 2006; E-mail: snake@chemvax.princeton.edu Abstract: 9,10,11,20,21,22-hexaphenyltetrabenzo[a,c,l,n]pentacene (2) and a dimethyl derivative (2m) were prepared by the reaction of 1,3-diphenylphenanthro[9,10-c]furan with bisaryne equivalents generated from 1,2,4,5-tetrabromo-3,6-diarylbenzenes in the presence of n-butyllithium, followed by deoxygenation of the double adducts with low-valent titanium. Both are bright red solids with a strong orange fluorescence in solution. The X-ray structures of these compounds show them to be the most highly twisted polycyclic aromatic hydrocarbons known. Compound 2 has an end-to-end twist of 144°, and the two crystallographically independent molecules of 2m have twists of 138° and 143°. Both molecules were resolved by chromatography on chiral supports, and the pure enantiomers have extremely high specific rotations (for 2,[R] D ) 7400°; for 2m, 5600°), but the molecules racemize slowly at room temperature (ΔG q rac ) 24 kcal/mol). Both the experimental geometry and the observed racemization barrier for 2 are in good agreement with computational studies of the molecule at a variety of levels. Attempts to prepare compound 2 by reaction of tetraphenylbenzyne with 9,10,12,13-tetraphenyl-11-oxacyclopenta[b]triphenylene (3, a twisted isoben- zofuran) gave no adducts, and attempts to prepare tetradecaphenylpentacene by reaction of hexaphenyl- isobenzofuran (11) with bisaryne equivalents gave only monoadducts. Introduction The term “acene” calls to mind a relatively rigid, flat molecule. However, such compounds are certainly not rigid, inasmuch as only a few kilocalories/mole are required to impart substantial bending or twisting distortions to naphthalene, anthracene, or the higher acenes. 1 Furthermore, there are many examples of sterically encumbered acenes that display severe distortions from planarity yet are perfectly stable under ordinary conditions. 1 Most such compounds are derivatives of naphtha- lene or anthracene that bear several bulky substituents or multiple benzannulations, but a few highly twisted naph- thacenes, 2,3 pentacenes, 4,5 and even one heptacene 6 have been described. For quite a few years the most highly twisted acene derivative was 9,10,11,12,13,14,15,16-octaphenyldibenzo- [a,c]naphthacene (1), with an end-to-end twist of 105°, 3 but two years ago we reported the synthesis and structure of 9,10,11,20,21,22-hexaphenyltetrabenzo[a,c,l,n]pentacene (2) as a preliminary communication. 7 This molecule is by far the most highly twisted acene to have been prepared, and it contains the most twisted naphthalene (60°), anthracene (88°), naphthacene (116°), and pentacene (144°) substructures known. In this paper we report the full details of the successful (and some notable but unsuccessful) syntheses of 2 and related, twisted pentacenes, the X-ray structures of 2 and its dimethyl derivative 2m (Scheme 1), and the resolution of both compounds into pure enantiomers with exceptional optical rotations. Results and Discussion Attempted Synthesis of Compound 2 via Single Aryne Addition. The preparation of compound 1 was achieved by † Princeton University. ‡ Virginia Tech. § Wyeth Research. | AccelaPure. (1) Review: Pascal, R. A., Jr. Chem. ReV. published online September 21, 2006 http://dx.doi.org/10.1021/cr050550l. (2) Fagan, P. J.; Ward, M. D.; Caspar, J. V.; Calabrese, J. C.; Krusic, P. J. J. Am. Chem. Soc. 1988, 110, 2981-2983. (3) Qiao, X.; Ho, D. M.; Pascal, R. A., Jr. Angew. Chem., Int. Ed. Eng. 1997, 36, 1531-1532. (4) Zhang, J.; Ho, D. M.; Pascal, R. A., Jr. Tetrahedron Lett. 1999, 40, 3859- 3862. (5) Schuster, I. I.; Craciun, L.; Ho, D. M.; Pascal, R. A., Jr. Tetrahedron 2002, 58, 8875-8882. (6) Duong, H. M.; Bendikov, M.; Steiger, D.; Zhang, Q.; Sonmez, G.; Yamada, J.; Wudl, F. Org. Lett. 2003, 5, 4433-4436. Published on Web 12/06/2006 10.1021/ja065935f CCC: $33.50 © 2006 American Chemical Society J. AM. CHEM. SOC. 2006, 128, 17043-17050 9 17043