Introduction and History

1. Benzyl Group Migration and Exchange in Arenium Ions Hilary E. Miller and Richard P. Johnson University of New Hampshire, Department of Chemistry, Durham, NH Introduction and History Benzyl Group Migrations Attempted Cyclizations of Diphenyl and Triphenylmethane In 1893, Blitz synthesized a mixture of diphenyl and triphenylmethane from benzene and aluminum chloride refluxed in chloroform.7 He reported that if the mixture was refluxed for over 4 hours, a third product, 9-phenylfluorene, was observed, presumably from cyclization of triphenylmethane. Rearrangement of Dibenzylbenzene Ortho-, meta-, and para- isomers of dibenzylbenzene were synthesized.9 The rearrangement from the para to meta isomer of alkyl benzenes was first reported by Baddeley in the 1930’s.1 Allen and Pingert were the first to study similar rearrangements in terphenyl isomers with AlCl3.2 A 0:64:36 o:m:p ratio was reported by Olahand co-workers who studied the rearrangements of terphenyl isomers upon heating with AlCl3.3 The preference for the meta-substituted isomer corresponds with the formation of the most stable carbocation.4 The rearrangement occurs through the ipso-arenium ion, as shown below.5 Work is currently in progress to see if the ortho-, meta-, and para- isomers of dibenzylbenzene interconvert or rearrange to form anthracene with catalysis by triflic acid or aluminum chloride. In our experiments, attempts to cyclizediphenyl or triphenylmethane with triflic acid in dichloroethane were unsuccessful. Instead, dibenzylbenzene isomers and triphenylmethanol were observed. Synthesis of Anthracene It is believed that the AlCl3 catalyst reacts with water to generate AlCl3-OH2as an acid. Work by Johnson and co-workers here at UNH has shown that terphenyl isomers also rearrange with trifluoromethane sulfonic acid (TfOH) in dichloroethane (DCE).4 These conditions are more reliable than AlCl3 and yield a product distribution that correlates with the results previously published by Olah and co-workers.4 Computational Studies It was not possible to find a transition state for the benzyl group migration. It is likely that the benzyl group completely dissociates. Our attempts to repeat Blitz’s 1893 cyclization of triphenylmethane with aluminum chloride were unsuccessful. Instead, anthracene was observed! Benzyl Group Fragmentation and Exchange Ipso-protonation leads to very facile fragmentation of benzyl groups. In benzene-d6, we find rapid exchange, as seen by 1H NMR analysis. This matches previous work reported by Royston Roberts, who also reported formation of anthracene from diphenyl and triphenyl methane with AlCl3.8 A possible mechanism for the reaction is shown below. This mechanism involves interconversion of dibenzylbenzene isomers. Acknowledgements Scholl Reaction In 1910, Scholl reported the formation of perylene when 1,1’-binaphthyl was heated with AlCl3.6This type of reaction, featuring the coupling or cyclization of arenes through formation of a C-C bond, is known as the Scholl reaction. We are grateful for the generous support from the National Science Foundation (CHE-0910826). References • (1) (a) Baddeley, G.; Kenner, J. Chem. Soc. 1935, 303-9. (b) Nightingale, D. Carton, B. J. Am. Chem. Soc. 1940, 62, 280-3. (c) Norris, J. F.; Vaali, G. T. J. Am. Chem. Soc.1939, 61, 2131. • (2) Allen, C. F. H.; Pingert, F. P. J. Am. Chem. Soc. 1942, 64, 1365. • (3) Olah, G. A.; Meyer, M. W., J. Org. Chem. 1962, 27, 3682-3. • (4) Ajaz, A.; McLaughlin, E. C.; Skraba, S. L.; Thamatam, R.; Johnson, R. P. J. Org. Chem.2012, 77, 9487-95. • (5) Olah, G. A.; Meyer, M. W.; Overchuk, N. A. J. Org. Chem.1964, 29, 2313. • (6)Scholl, R.; Seer, C.; Weitzenbock, R. Ber. Dtsch. Chem. Ges. 1910, 43, 2202. • (7) Blitz, H. Ber. Dtsch. Chem. Ges.1893, 26, 1960-2. • (8) Roberts, R. J. Org. Chem.1987, 52, 1591-1599 • (9) McLaughlin, E. Master’sThesis, 2012.