Oxidative C-C Bond Formation (Scholl Reaction) with DDQ as an Efficient and Easily Recyclable Oxidant Linyi Zhai, Ruchi Shukla, and Rajendra Rathore* Department of Chemistry, Marquette UniVersity, P.O. Box 1881, Milwaukee, Wisconsin 53201 rajendra.rathore@marquette.edu Received June 13, 2009 ABSTRACT DDQ in the presence of an acid is known to oxidize a variety of aromatic donors to the corresponding cation radicals. Herein, we now demonstrate that the DDQ/H+ system can be effectively utilized for the oxidative C-C bond formations or biaryl synthesis. The efficient preparation of a variety of polyaromatic hydrocarbons including graphitic hexa-peri-hexabenzocoronenes, ease of isolation of the clean products, and ready regeneration of DDQ from easily recovered reduced DDQ-H2 advances the use of DDQ/H+ for Scholl reactions. The Scholl reaction1 is one of the oldest C-C bond forming reactions and has been extensively utilized for intramolecular oxidative cyclodehydrogenation of various polybenzenoid hydrocarbons, e.g., eq 1. The reaction in eq 1 essentially represents an oxidative C-C bond formation (or biaryl synthesis) that has been especially efficacious for the oxidative cyclodehydrogenation of a variety of (substituted) hexaarylbenzenes and o- terphenyls to produce the corresponding planar polyaromatic hydrocarbons (PAH���s), i.e., hexa-peri-hexabenzocoronenes (HBC���s) and triphenylenes, respectively.2 The Scholl reaction can be accomplished by using a variety of oxidants such as FeCl3,3 CuCl2 or Cu(OTf)2 and AlCl3,4 Tl(O2CCF3)3 in CF3CO2H or BF3-OEt2,5 Pb(OAc)4/BF3-Et2O in MeCN,5,6 triethyloxonium hexchloroantimonate (Et3O+ SbCl6-),7 SbCl5,8 MoCl5,9 etc. Moreover, the Scholl reaction can also be effected by electrochemical oxidation.10 The involvement of cation radical intermediates (formed by 1-e- oxidation) in oxidative C-C bond formations or biaryl syntheses has (1) (a) Scholl, R. Mansfeld, J. Ber. Dtsch. Chem. Ges. 1910, 43, 1734��� 1746. (b) Kovacic, P. Jones, M. B. Chem. ReV. 1987, 87, 357���379. (2) (a) Berresheim, A. J. Mu��ller, M. Mu��llen, K. Chem. ReV. 1999, 99, 1747���1785. (b) Watson, M. D. Fechtenkotter, A. Mu��llen, K. Chem. ReV. 2001, 101, 1267���1300. (3) (a) Boden, N. Bushby, R. J. Headdock, G. Lozman, O. R. Wood, A. Liq. Cryst. 2001, 28, 139���144. (b) Boden, N. Bushby, R. J. Cammidge, A. N. Duckworth, S. Headdock, G. J. Mater. Chem. 1997, 7, 601���605. (4) (a) Simpson, C. D. Mattersteig, G. Martin, K. Gherghel, L. Bauer, R. E. Ra��der, H. J. Muellen, K. J. Am. Chem. Soc. 2004, 126, 3139���3147. (b) Ku��bel, C. Eckhardt, K. Enkelmann, V. Wegner, G. Mu��llen, K. J. Mater. Chem. 2000, 10, 879���886. (5) McKillop, A. Turrell, A. G. Young, D. W. Taylor, E. C. J. Am. Chem. Soc. 1980, 102, 6504���6512. (6) Aylward, J. B. J. Chem. Soc. B 1967, 1268���1270. (7) Rathore, R. Kumar, A. S. Lindeman, S. V. Kochi, J. K. J. Org. Chem. 1998, 63, 5847���5856. (8) (a) Yang, J.-S. Swager, T. M. J. Am. Chem. Soc. 1998, 120, 5321��� 5322. (b) Yamaguchi, S. Swager, T. M. J. Am. Chem. Soc. 2001, 123, 12087���12088. (c) Rose, A. Tovar, J. D. Yamaguchi, S. Nesterov, E. E. Zhu, Z. Swager, T. M. Philos. Trans. R. Soc. London, Ser. A 2007, 365, 1589���1606, and references cited therein. ORGANIC LETTERS 2009 Vol. 11, No. 15 3474-3477 10.1021/ol901331p CCC: $40.75 ��� 2009 American Chemical Society Published on Web 07/13/2009

been carefully probed by Parker and co-workers.11 We12 and others13 have recently shown that dichlorodi- cyano-p-benzoquinone (DDQ, Ered ) +0.60 V vs. SCE), in the presence of an acid, readily oxidizes a variety of aromatic donors (D), such as naphthalene, anthracene, hexaalkylbenzenes, 1,4-dialkoxybenzenes, biphenyl, etc., with oxidation potentials as high as ���1.7 V vs. SCE, to the corresponding cation radicals according to the stoi- chiometry shown in eq 2. It is important to emphasize that in the absence of an added acid, DDQ forms vividly colored electron donor-acceptor (EDA) complexes with various aromatic donors,14 and the highly endothermic electron transfer reaction in eq 2 is only possible in the presence of an acid.12,13 The electron-transfer stoichiometry in eq 2 was further verified by a quantitative isolation of a stable hydroquinone ether cation radical according to eq 3.12a,15 Herein, we will show that the DDQ/acid system (which readily oxidizes aromatic donors to the corresponding cation radicals) can be employed for Scholl reactions (or biaryl synthesis) as demonstrated by the preparation of a number of substituted triphenylenes and hexa-peri-hexabenzocoro- nenes under mild conditions. The usage of DDQ, instead of commonly utilized ferric chloride (FeCl3), as an oxidant in Scholl reactions is advanced owing to the following reasons: (i) Generally a large excess of FeCl3 is needed for the completion of Scholl reactions and in many cases the resulting products are contaminated with chlorinated com- pounds.16,17 A need for a large excess of FeCl3 can be avoided by usage of only 1 equiv of DDQ per C-C bond formation for the completion of Scholl reactions. (ii) The isolation of cyclized products and the recovery of the reduced DDQ-H2, which can be readily recycled into DDQ by oxidation with nitric acid, are easy.18 A variety of Scholl precursors (Figure 1) employed in this study were synthesized by using standard synthetic proce- dures, and the experimental details and the spectral data for 1a-n are compiled in the Supporting Information. Thus, when a 0.01 to 0.1 M solution of o-terphenyl 1a in a mixture of dichloromethane and methanesulfonic acid (10%, v/v) was treated with 1 equiv of DDQ under an argon atmosphere at ���0 ��C, the solution immediately turned dark green.19 Upon stirring for 5 min at 0 ��C, the reaction mixture took on a brown coloration. The reaction was then quenched by an addition of a saturated aqueous solution of sodium bicarbonate (20 mL). The dichloromethane layer was sepa- rated and washed with aqueous sodium bicarbonate (2 �� 10 mL) and dried over anhydrous MgSO4, then the solvent was evaporated to afford the corresponding triphenylene 2a in quantitative yield (i.e., eq 4). It is important to note that the reduced hydroquinone (DDQ-H2) in eq 4 readily dissolves into the aqueous sodium (9) (a) Kramer, B. Fro��hlich, R. Waldvogel, S. R. Eur. J. Org. Chem. 2003, 354, 9���3554. (b) Waldvogel, S. R. Aits, E. Holst, C. Fro��hlich, R. Chem. Commun. 2002, 1278���1279. (c) Kovacic, P. Lange, R. M. J. Org. Chem. 1963, 28, 968���972. (10) (a) Ronlan, A. Hammerich, O. Parker, V. D. J. Am. Chem. Soc. 1973, 95, 7132���7138. (b) Ronlan, A. Parker, V. D. J. Org. Chem. 1974, 39, 1014���1016. (c) Rathore, R. Kochi, J. K. J. Org. Chem. 1995, 60, 7479��� 7490. (11) Hammerich, O. Parker, V. D. AdV. Phys. Org. Chem. 1984, 20, 55���189, and references cited therein. (12) (a) Rathore, R. Kochi, J. K. Acta Chem. Scand. 1998, 52, 114��� 130. (b) Rathore, R. Zhu, C.-J. Lindeman, S. V. Kochi, J. K. J. Chem. Soc., Perkin Trans. 2 2000, 1837���1840. (13) (a) Handoo, K. L. Gadru, K. Curr. Sci. 1986, 55, 920���922, and references cited therein. (b) Also see: Eberson, L. Hartshorn, M. P. Persson, O. J. Chem. Soc., Perkin Trans. 2 1997, 195���201. (14) Rathore, R. Lindeman, S. V. Kochi, J. K. J. Am. Chem. Soc. 1997, 119, 9393���9404, and references cited therein. (15) Rathore, R. Burns, C. L. Deselnicu, M. I. Org. Synth. 2005, 82, 1���6. (16) Rempala, P. Kroulik, J. King, B. T. J. Am. Chem. Soc. 2004, 126, 15002���15003, and references cited therein. (17) Zhou, Y. Liu, W.-J. Zhang, W. Cao, X.-Y. Zhou, Q.-F. Ma, Y. Pei, J. J. Org. Chem. 2006, 71, 6822���6828, and references cited therein. (18) (a) Brook, A. G. J. Chem. Soc. 1952, 5040���5041. (b) Newman, M. S. Khanna, V. K. Org. Prep. Proced. Int. 1985, 17, 422���423. (c) Scott, J. W. Parrish, D. R. Bizzarro, F. T. Org. Prep. Proced. Int. 1977, 9, 91��� 94. Figure 1. Structure and numbering scheme for various Scholl precursors used in this study. Org. Lett., Vol. 11, No. 15, 2009 3475