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RSC Advances
Page 3 of 4
DOI: 10.1039/C5RA27528H
Journal Name
COMMUNICATION
Figure 2. 1H-NMR spectrum of (a)bromobenzene, (b)AlCl3- and (c)
AlBr3-catalyzed disproportionation of bromobenzene.
Figure 1. 1H- (top) and 13C-NMR (bottom) spectrum of 5c in CD2Cl2.
The observed isomerization of bromobenzene in the case of Scheme 4. The Plausible Mechanism of Disproportionation of
Friedel-Crafts alkylation showed significant agreement with the Bromobenzene
possibility of migration of the aromatically bound bromine atom in
halogenation by AlBr3,25 leading to protonation at a carbon atom
the presence of aluminum chloride or bromide.22 Kooyman23
and subsequent debromination to form benzene and Br2-AlBr3
investigated by the reaction of dibromobenzenes with AlBr3 at room
complex, which can then be able to rebrominated in any of the
temperature to form an equilibrium mixture of dibromo compounds
three available positions of bromobenzene. Whilst pathway [2]
(60% meta, 35% para, 5% ortho), bromobenzene and
reacts in a mechanism involving protonation of the ring to a σ
tribromobenzenes, indicating the disproortionations and
complex, from which the transbromination occurred by nucleophilic
transbrominations involving bromobenzenes in the presence of
replacement of a second molecule of bromobenzene. Subsequent
AlBr3.24
elimination gave the undesired dibromides. The disproportionation
In view of the above results, it seemed of interest to further
of bromobenzene may proceed, however, even in the absence of
investigate the information on the mechanism with respect to the
added water, as there is the possibility of traces of moisture
formation of the isomerization products under Friedel-Crafts
presented in chemicals and apparatus used.
conditions, which prompted us to attempt the reaction of
In summary, we have demonstrated a straightforward route from
bromobenzene with AlCl3 and AlBr3 at room temperature,
readily available starting materials to a valuable class of rigid cores,
respectively. The intriguing reaction was readily monitored by 1H-
which might be of general use as rigid building blocks for
NMR, indicating the evidence of isomers formation as shown in
applications in organic chemistry and material science. This simple
Figure 4.. Formation of these disproportionation products might
and efficient route should be of great potential for preparation of
suggest that isomerization of bromobenzene occured by an
other synthetic rigid derivatives of 1. Whilst the action of AlCl3 or
intermolecular rather than an intramolecular process,24 which is
AlBr3 on bromobenzene is definite and energetic, while FeCl3 has no
consistent with the formation of isomer mixtures in the
such effect on the same substrate under same conditions. Studies
hexasubstituted case.
along this methodology and further development of rigid cores
In the course of the present research set forth above, it is to be
using 4a-c as building block are currently underway in our
expected that bromobenzene under vigorous Friedel-Crafts
laboratory.
conditions is unstable and difficult to handle, and thus provides low
Acknowledgements
regioselectivity. The isomerization pathway [1] demonstrated in
Scheme 4 should be due to HBr formed from the possible aromatic
This work was financially supported by National Natural Science
Foundation of China (Grant no. 21476051, 21506038), Natural
Science Foundation of Guangdong Province (Grant no.
2014A030310307), Science and Technology Project of Guangdong
Province (Grant no. 2013B010403028) and the China Postdoctoral
Science Foundation (2014M562148
,2015T80897).
Notes and references
1
D. A. Tomalia and A. M. Naylor, Angew. Chem. Int. Ed. Engl.,
1990, 29, 138.
R. J. Dong, Y. F. Zhou and X. Y. Zhu, Accounts Chem. Res.,
2014, 47, 2006.
2
3
A. P. Cote, A. I. Benin, N. W. Ockwig, M. O'Keeffe, A. J.
Matzger and O. M. Yaghi, Science, 2005, 310, 1166.
4
T. Ben and S. L. Qiu, Cryst Eng Comm, 2013, 15, 17.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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