Polybrominated anthracenes: Selective synthesis of tetrabromoanthracenes as precursors for the corresponding tetracyanoanthracenes

Article abstract of DOI:10.1016/j.tetlet.2012.11.048

Selective and efficient methods for the preparation of both 2,7,9,10-tetrabromoanthracene 11 and 2,6,9,10-tetrabromoanthracene 12 are described. Photobromination of 2,9,10-tribromoanthracene 8 resulted in the formation of only one stereoisomeric heptabromide 10. Whilst thermal aromatization of the trans,cis,trans-1,2,3,4,6,9,10-heptabromo-1,2,3,4- tetrahydroanthracene 10 gave mainly 2,6,9,10-tetrabromoanthracene 12, the pyridine-induced elimination yielded 2,7,9,10-tetrabromoanthracene 11 as the only final product. Tetrabromide 11 was transformed into 2,7,9,10- tetracyanoanthracene 14, by copper-assisted nucleophilic substitution reaction, as a potential photoconductive product.

Full text of DOI:10.1016/j.tetlet.2012.11.048

Tetrahedron Letters 54 (2013) 312–314
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Polybrominated anthracenes: selective synthesis of tetrabromoanthracenes
as precursors for the corresponding tetracyanoanthracenes
a
b,
⇑
Kıymet Berkil Akar , Osman Çakmak
a
Department of Bioengineering, Faculty of Engineering and Natural Science, Gaziosmanpasa University, Tokat 60250, Turkey
Department of Chemistry, Faculty of Arts and Sciences, Gaziosmanpasa University, Tokat 60250, Turkey
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 July 2012
Revised 22 October 2012
Accepted 8 November 2012
Available online 23 November 2012
Selective and efficient methods for the preparation of both 2,7,9,10-tetrabromoanthracene 11 and
2,6,9,10-tetrabromoanthracene 12 are described. Photobromination of 2,9,10-tribromoanthracene 8
resulted in the formation of only one stereoisomeric heptabromide 10. Whilst thermal aromatization
of the trans,cis,trans-1,2,3,4,6,9,10-heptabromo-1,2,3,4-tetrahydroanthracene 10 gave mainly 2,6,9,
10-tetrabromoanthracene 12, the pyridine-induced elimination yielded 2,7,9,10-tetrabromoanthracene
1
1 as the only final product. Tetrabromide 11 was transformed into 2,7,9,10-tetracyanoanthracene 14,
Keywords:
by copper-assisted nucleophilic substitution reaction, as a potential photoconductive product.
Anthracene derivative
Bromoanthracene
Bromination
Ó 2012 Elsevier Ltd. All rights reserved.
Cyanoanthracene
Organic photoconductor
Anthracenes have been used in optical, electronic and magnetic
Tribromide 8 was prepared from anthracene 1 according to our
methods previously described (Scheme 1). The bromination of
1
,2
17
switches when combined with polymers, films and crystals and
several reviews have been published on this chemistry.3 Although
anthracenes can readily be substituted in positions 9 and 10, the
outer rings cannot be functionalized easily. In order to achieve
substitution at the 2-, 3-, 6- and 7-positions, considerable effort is
required.
–5
tribromide 8 was examined at 25 °C in CCl
4
by irradiation with a
projector lamp (150 W). The photobromination of 8 in CCl
4
resulted in the formation of only one stereoisomeric product,
heptabromide 10. Most of the product precipitated during the
reaction forming a solid product at the bottom of the reaction
vessel. A rapid and simple recrystallization resulted in the pure
heptabromide 10 in 94% yields (Scheme 2).
Publications involving brominated anthracenes6–10 mainly use
anthraquinones for bromination at an outer position, but a synthesis
of 2,7,9,10-tetrabromoanthracene 11 does not exist in the literature.
Recently, however, the synthesis of 2,3,6,7-tetrabromoanthracene
was reported via a twofold Bergman cyclization.11
Previously, we reported on the polyfunctionalization of
aromatic compounds by the selective preparation of polybromi-
1
2–17
nated aromatics.
romotetrahydroanthracenes 3 and 4 by direct bromination of 9,
0-dibromoanthracene 2. Whilst base-induced elimination of
hexabromide 3 with t-BuOK gave 2,3,9,10-tetrabromoanthracene
, the reaction of hexabromide 4 with DBU afforded 1,3,9,10-tet-
We described an efficient synthesis of hexab-
1
5
rabromoanthracene 6 as the sole product. Recently, we developed
a new, efficient and stereoselective method for the synthesis of
hexabromide 7 and opened up an efficient synthetic strategy for
the synthesis of the 2,9,10-trisubstituted anthracene derivatives
(9a, 9b).
⇑
Corresponding author. Tel.: +90 356 2521616; fax: +90 356 2521585.
E-mail address: Osman.cakmak@gop.edu.tr (O. Çakmak).
Scheme 1. Selective synthesis of polybrominated anthracenes.
0
040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.11.048

K. B. Akar, O. Çakmak / Tetrahedron Letters 54 (2013) 312–314
313
It is difficult to distinguish the two tetrabromides 11 and 12 by
H NMR. However, C NMR is very useful due to the two separate
1
13
signals for C-9 and C-10 in compound 11 (130.2, 129.5 ppm),
whilst these carbons in the other isomer 12 appear as one signal
(d 130.2) due to symmetry.
Having selectively obtained tetrabromide 11, we investigated
its value as a precursor of tetracyanide 14 which is a possible
organic photoconductor. For this purpose, tetrabromide 11 was
treated with CuCN in DMF at ca. 150 °C. The copper-assisted nucle-
ophilic substitution by cyanide gave compound 14 as the sole
product (Scheme 4).
Its IR spectrum exhibited a peak due to the nitriles (2231 cm
and the correct elemental analysis substantiated the structure of
Scheme 2. Selective preparation of the heptabromide 10 and the tetrabromoanth-
racenes 11 and 12.
À1
)
In their 1H NMR spectra, the four protons of the saturated
domain of compound 10 were very similar to those of hexabro-
mide 7, which provided evidence for the same stereochemistry of
+
14. The molecular ion at m/z 277 (base peak, M ÀH) accompanied
by fragment peaks suggested that the structure was the tetracya-
1
nide. The H NMR spectrum, with downfield shifts for all reso-
1
3
the bromine bearing carbons. The fourteen lines in the C NMR
spectra were consistent with those of structure 10. Elemental
and mass spectral studies revealed that compound 10 contained
seven bromine atoms.
nances, was also consistent with the cyanide structure. H1 and
H8 appeared as a singlet at d 8.98 while H4 and H5 appeared as a
doublet at d 8.71 (J = 9.2 Hz). H and H were a doublet of doublets
at d 8.06 (J = 9, 1.6 Hz). The C NMR spectrum also supported the
tetracyanide structure with three signals due to nitrile carbon
atoms at d 116.8, 115.0, 114.0.
In a patent disclosure, 2,6,9,10-tetrabromoanthracene (12)
was prepared under quite drastic conditions, in which high
temperature (between 100 and 200 °C) bromination of 9,10-dib-
romoanthracene 2 in nitrobenzene and the extraction of tribromo-
anthracene 8 from the insoluble tetrabromide 12 by digestion via
boiling the solvent (dichloromethane) were used. In order to im-
prove the yield, further bromination of tribromoanthracene 8
was required. Our method presented here is a more convenient
alternative method, which requires only mild reaction conditions,
3
6
1
3
Our results showed that the choice of base is crucial for
selective re-aromatization of the brominated aromatics. Pyridine-
promoted reactions of hexabromide 7 resulted in the elimination
1
8
2
of one mol each of HBr and Br . Thus, this aromatization afforded
17
tribromide 8 as the sole product (Scheme 1). Heptabromide 10
has the same stereochemistry as hexabromide 7. We thus obtained
the 2,7,9,10-tetrabromoanthracene (11) selectively by the reaction
of heptabromide 10 with pyridine in a relatively good yield (60%)
as expected (Scheme 2).
1
The H NMR of tribromide 8 was helpful for the assignment of
the structure of tetrabromide 11. AB systems (d 8.42, H
4 5
–H ; d
1
8
7
.68, H –H and a broad singlet d 8.75, H and H ) were observed.
3
6
1
8
compared to those previously described.
The B side of the AB signal system was a doublet of doublets due to
meta coupling (J = 1.75 Hz). Eight peaks in the 13C NMR spectrum of
this compound were also consistent with the 2,6,9,10-tetrabromo-
anthracene 11.
In this reaction, the selectivity is interesting because the forma-
tion of four tetrabromides and four pentabromides is possible
The synthesis of polybromosubstituted anthracenes is restricted
due to the reduced reactivity of the anthracenes towards bromine
1
9
after initial stages. However, here we have achieved efficient bro-
mination under mild conditions without any catalyst. In addition,
we showed that manipulation of reaction parameters is an impor-
tant procedure for selectivity. In general, both the bromination of
anthracene and the aromatization of the brominated anthracenes
led to various products.
In conclusion, we have described an efficient and convenient
synthesis of the heptabromide 10 and the tetrabromides 11, and
12. All of the four steps were highly selective and gave predomi-
nantly one product under mild conditions and simple reaction pro-
cedures. The polybrominated anthracenes 10, 11 and 12 are often
the starting points for polyfunctionalization leading to other
anthracene derivatives. These results are also important as there
are potential applications of these compounds as building blocks
in areas ranging from photonic devices to molecular machines. Poly-
cyanoanthracenes are electron acceptors, and thus are particularly
effective as sensitizers for organic photoconductors. 2,9,10-Tri- 9b
(
eight compounds in total) (Scheme 3). We also reacted heptabro-
mide 10 with a different base; thus, the treatment of heptabromide
with DBU led to a mixture of products that could not be separated
by chromatographic methods.
On the other hand, when heptabromide 10 was heated at
1
52–162 °C for 1 h without any solvent, tetrabromide 12 was
formed along with tribromide 8 in a ratio of 67:33. Tetrabromide
2 is insoluble in many solvents, and was isolated by the digesting
compound 8 with methylene chloride in a yield of 62%.
1
7
1
Tetrabromide 12 (mp 293–296 °C ) exhibits almost the same H
NMR spectrum as tetrabromide 11. It consists of three signal
systems, d 8.77 (s, 2H, H
1
and H
8
), d 8.46 (d, J34–56 = 9.2 Hz, 2H,
and H ).
H
4
and H ), d 7.71 (dd, J = 9.2, 1.6 Hz, H
5
3
6
Scheme 3. Possible bromoanthracenes from base-induced elimination of heptabr-
omide 10.
Scheme 4. Synthesis of tetracyanoanthracenes 13 and 14 from corresponding
tetrabromides.

3
14
K. B. Akar, O. Çakmak / Tetrahedron Letters 54 (2013) 312–314
3. Becker, H.-D. Chem. Rev. 1993, 93, 145–172.
and 2,6,9,10-tetracyanoanthracenes 13 are useful sensitizers.20
2
tizer for organic photoconductors was patented.
4
.
Bouas-Laurent, H.; Castellan, A.; Desvergne, J.-P.; Lapouyade, R. Chem. Soc. Rev.
000, 29, 43–55.
Bouas-Laurent, H.; Castellan, A.; Desvergne, J.-P.; Lapouyade, R. Chem. Soc. Rev.
001, 30, 248–263.
,6,9,10-Tetracyanoanthracene 13 which has been used as sensi-
2
18
5
.
2
6.
7.
8.
Kintzel, O.; Schlüter, A.-D. Acta Polym. 1997, 48, 212–214.
Hodge, P.; Power, G. A.; Rabjohns, M. A. Chem. Commun. 1997, 73–74.
Bloomer, J. L.; Zheng, W. Synth. Commun. 1998, 28, 2087–2095.
Acknowledgments
We thank Gaziosmanpasa University Research Foundation
Grant Nos: 2006/13 and 2009/09) and The Scientific and Techno-
logical Research Council of Turkey (TUBITAK, Grant No: 105T426).
9. Miller, G. P.; Mack, J. Org. Lett. 2000, 2, 3979–3982.
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