Hexaalkoxytriphenylenes synthesized from facile solvent-free oxidative coupling trimerization

Article abstract of DOI:10.1080/00397911003707105

By grinding 1,2-dialkoxybenzene with anhydrous FeCl₃ powder using a glass pestle in a mortar at ambient temperature under solvent-free conditions, hexaalkoxytriphenylenes were obtained in good yield (up to 90%) in 30min. The solvent-free oxidative coupling reaction is a facile, efficient, green-chemistry method.

Full text of DOI:10.1080/00397911003707105

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Hexaalkoxytriphenylenes Synthesized
from Facile Solvent-Free Oxidative
Coupling Trimerization
Weibin Bai a b & Jinhuo Lin a b
a College of Chemistry and Material Science, Fujian Normal
University , Fuzhou , China
b Key Laboratory of Polymer Materials , Fuzhou , China
Published online: 25 Feb 2011.
To cite this article: Weibin Bai & Jinhuo Lin (2011) Hexaalkoxytriphenylenes Synthesized
from Facile Solvent-Free Oxidative Coupling Trimerization, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 41:6, 903-906, DOI:
10.1080/00397911003707105
To link to this article: http://dx.doi.org/10.1080/00397911003707105
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Synthetic Communications1, 41: 903–906, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911003707105
HEXAALKOXYTRIPHENYLENES SYNTHESIZED
FROM FACILE SOLVENT-FREE OXIDATIVE
COUPLING TRIMERIZATION
Weibin Bai and Jinhuo Lin
1
College of Chemistry and Material Science, Fujian Normal University,
Fuzhou, China and Key Laboratory of Polymer Materials, Fuzhou, China
GRAPHICAL ABSTRACT
Abstract By grinding 1,2-dialkoxybenzene with anhydrous FeCl3 powder using a glass
pestle in a mortar at ambient temperature under solvent-free conditions, hexaalkoxytri-
phenylenes were obtained in good yield (up to 90%) in 30 min. The solvent-free oxidative
coupling reaction is a facile, efficient, ‘‘green-chemistry’’ method.
Keywords Green chemistry; hexaalkoxytriphenylene; oxidative coupling; solvent-free
Discotic liquid crystals are usually composed of molecules with a disc-like core and
aliphatic tails. In recent years, the discotic liquid crystals have attracted much atten-
tion as novel organic semiconductors. Triphenylene derivatives are the most widely
studied discotic mesogens because of their ability to form highly ordered columnar
structures with overlapping p-electron systems. Especially hexasubstituted tripheny-
lene discotic liquid crystals have been drawing attention as photoconductors,
molecular wires and fibers, light-emitting diodes, and photovoltaic cells.[1]
By three consecutive Scholl reactions of 1,2-dimethoxybenzene, the hexa-
methoxytriphenylene was synthesized previously in H2SO4 using chloranil or iron
chloride as oxdidant.[2] Because triphenylene derivatives show mesomorphism only
Received September 26, 2009.
Address correspondence to Weibin Bai, College of Chemistry and Material Science, Fujian Normal
University, China and Key Laboratory of Polymer Materials, Fuzhou 350007, China. E-mail: bai-wb@163.com
903

904
W. BAI AND J. LIN
when the six peripheral alkoxy chains have a minimum of four carbon atoms (i.e.,
butoxy chains),[3] the reaction is more or less limited to the production of hexa-
methoxytriphenylene. If the long-chain derivatives are prepared, it is necessary to
replace the alkoxy groups to give different hexaalkoxytriphenylene discotics. The
method of cyclic anodic trimerization could be extended to other dialkoxybenzenes
to form hexaalkoxytriphenylene discotics directly,[4] but this slow reaction could be
applied on a very small scale and is limited in its use.
With the oxidative coupling reaction researched further, the use of FeCl3 as
oxidant was reported to synthesize the hexahexyloxytriphenylenes with 20% yield
by the trimerization of 1,2-dihexyloxybenzene in 70% H2SO4 at 80 ꢀC.[5] The syn-
thetic route was improved further to obtain better yields.[6] Cooke et al. reported
good yields could be obtained using FeCl3 supported on alumina, which could also
avoid the application of mineral acids.[7] However, the preparation of alumina=
FeCl3 catalyst is not facile, and the hazardous organic solvents are still unavoidable.
Recently, oxidative coupling trimerization has developed into the most commonly
used method to prepare different triphenylene derivatives.[8]
In addition to FeCl3, VOCl3,[9] and MoCl5[3] also have been used as oxidants for
the trimerization. The novel reagents could increase the yields under very mild conditions
with or without acid catalyst in a short time at room temperature. However, the high
price and toxicity hazard limited the application of the reagent of VOCl3 and MoCl5.
Solvent-free oxidative coupling reaction cannot only reduce pollution of
organic solvents such as dichloromethane and nitromethane[10] but also can over-
come the problem of insolubility of the reagents in organic solvents. Furthermore,
because molecules in a crystal are arranged tightly and regularly, a solvent-free
organic reaction occurs more efficiently and more selectively.[11] Here, we report a
novel, solvent-free oxidative coupling trimerization to prepare hexaalkoxytripheny-
lenes with anhydrous iron chloride as the oxidant. By comparison with the previous
preparation methods, the solvent-free grinding method cannot only avoid the
poisonous organic solvents and mineral acids but also can obtain good yields, so
the whole process is simple, mild, cost-efficient, and environmentally friendly, which
will have some social and economic benefits. The reaction is shown in Scheme 1.
Table 1 shows the yields of different hexaalkoxytriphenylenes obtained
depending on the rection time and the molecular ratio of 1,2-dialkoxybenzenes
Scheme 1. Solvent-free oxidative coupling trimerization of 1,2-dialkoxybenzenes.

OXIDATIVE COUPLING TRIMERIZATION
905
Table 1. Preparation of triphenylene derivatives using FeCl3 by solvent-free
oxidative coupling trimerization
Entry
R
Ratioa
Time (min)
Yield (%)
1
2
C4H9
C4H9
C4H9
C4H9
C4H9
C4H9
C4H9
C4H9
C4H9
C4H9
C4H9
C6H13
C6H13
C6H13
C2H5
C2H5
1:2
1:2
1:3
1:3
1:3
1:4
1:4
1:4
1:5
1:5
1:5
1:3
1:4
1:4
1:3
1:4
30
40
20
30
40
20
30
40
20
30
40
30
30
40
30
30
56.0
49.0
71.0
77.3
82.5
84.3
93.0
95.4
79.0
82.0
82.0
66.6
75.9
80.0
63.9
63.7
3
4
5
6
7
8
9
10
11
12
13
14
15
16
aMolecular ratio of 1,2-dialkoxybenzenes and FeCl3.
and FeCl3. As shown in Table 1, anhydrous FeCl3 as oxidation could give yields of
about 50% at room temperature when the molecular ratio is 1:2 and the reaction
time is more than 30 min. The result indicates that reaction was initiated by grinding,
which generated cation radicals mechanically. In addition, the solvent-free oxidative
reaction offered good yields with theoretical amount of oxidant. The yields of 77.3,
66.6, and 63.9% were obtained for 1,2-dibutoxybenzene, 1,2-dihexyloxybenzene, and
1,2-diethoxybenzene, respectively (entries 4, 12, and 15). It also can be seen that the
ratio of the oxidation and the reaction time could effect the reaction obviously. As
the amount of FeCl3 is increased, the yields increase too. A yield of 90% is obtained
for 1,2-dibutoxybenzene with 4 molecular equivalents of FeCl3 (entries 7 and 8).
When the ratio is 1:5, the yield decreases, indicating that excess of oxidant holds
back the trimerization process. So does the time. The yields could not increase
significantly after 30 min.
In conclusion, we have described a novel, solvent-free oxidative coupling tri-
merization to synthesize hexaalkoxytriphenylenes by grinding 1,2-dialkoxybenzenes.
At room temperature, good yields can be obtained by anhydrous FeCl3 as the oxi-
dant without any acid catalyst. The solvent-free oxidative coupling trimerization is
facile, efficient, time-saving, and environmentally friendly.
The synthetic procedure is illustrated by the trimerization of 2,3,6,7,10,11-
hexabutoxytriphenylene as an example: 0.44 g (2 mmol) of 1,2-dibutoxybenzene
and 1.30 g (8 mmol) of anhydrous FeCl3 powder were added into a mortar. After
grinding for about 30 min with a glass pestle, the mixture was put into cold ethanol
containing 5% hydrochloric acid and then stirred for 5 min. The crude product was
collected by filtration, then washed several times with cold ethanol; the refined
product was filtered and recrystallizated in the ethanol, yielding 0.41 g (93%) of
1
hexabutoxytriphenylene. H NMR (400 MHz, CDCl3, ppm): d 7.81 (s, H1, 6H), d
4.23 (t, H2, 12H, J ¼ 6.4), d 1.94 (m, H3, 12H, J ¼ 4.8), d 1.58 (m, H4, 12H,

906
W. BAI AND J. LIN
J ¼ 1.6), d 1.02 (t, H5, 18H, J ¼ 2.8). Elemental analysis (calculated for C42H60O6), C:
calc. 76.33%, found 76.39%; H: calc. 9.15%, found 9.27%. The purity of the product
is 99.05% as measured by high-performance liquid chromatography.
ACKNOWLEDGMENT
This work was supported by the Science Foundation for Youths of Fujian
Province (2010J05108).
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