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
Attempted Cyclizations of Diphenyl and Triphenylmethane
The rearrangement from the para to meta isomer of alkyl benzenes was
In 1893, Blitz synthesized a mixture of diphenyl and triphenylmethane
Rearrangement of Dibenzylbenzene
from benzene and aluminum chloride refluxed in chloroform.7 He
Ortho-,
reported that
synthesized.9
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
if the mixture was refluxed for over 4 hours, a third
o:m:p ratio was reported by Olah and co-workers who studied the
product, 9-phenylfluorene, was observed, presumably from cyclization
rearrangements of terphenyl isomers upon heating with AlCl3.3 The
of triphenylmethane.
Benzyl Group Migrations
meta-,
and
para-
isomers
of
dibenzylbenzene
were
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 paraIn our experiments, attempts to cyclize diphenyl or triphenylmethane
isomers of dibenzylbenzene interconvert or rearrange to form
with
anthracene with catalysis by triflic acid or aluminum chloride.
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 AlCl3OH2 as an acid. Work by Johnson and co-workers here at UNH has
shown that terphenyl isomers also rearrange with trifluoromethane
Computational Studies
Our attempts to repeat Blitz’s 1893 cyclization of triphenylmethane with
aluminum chloride were unsuccessful. Instead, anthracene was observed!
sulfonic acid (TfOH) in dichloroethane (DCE).4 These conditions are more
It was not possible to find a transition state for the benzyl group
migration. It is likely that the benzyl group completely dissociates.
Benzyl Group Fragmentation and Exchange
reliable than AlCl3 and yield a product distribution that correlates with the
Ipso-protonation leads to very facile fragmentation of benzyl groups. In
results previously published by Olah and co-workers.4
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
Scholl Reaction
In 1910, Scholl reported the formation of perylene when 1,1’-binaphthyl
involves interconversion of dibenzylbenzene isomers.
Acknowledgements
We are grateful for the generous support from the
National Science Foundation (CHE-0910826).
was heated with AlCl3.6 This type of reaction, featuring the coupling or
cyclization of arenes through formation of a C-C bond, is known as the
Scholl reaction.
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’s Thesis, 2012.