A convenient and economic method for the synthesis of monohydroxy- pentaalkoxy- and hexaalkoxytriphenylene discotics

Article abstract of DOI:10.1016/j.tetlet.2005.02.099

A one-step process for the preparation of mono-functionalized triphenylene discotics is presented. Oxidative trimerization of ortho-dialkoxybenzenes using FeCl₃ in nitromethane and a catalytic amount of various acids, furnished monohydroxy-pentaalkoxytriphenylene in addition to hexaalkoxytriphenylene. These products can be easily separated by column chromatography over aluminium oxide.

Full text of DOI:10.1016/j.tetlet.2005.02.099

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 2603–2605
A convenient and economic method for
the synthesis of monohydroxy-pentaalkoxy- and
hexaalkoxytriphenylene discotics
Sandeep Kumar* and B. Lakshmi
Raman Research Institute, C. V. Raman Avenue, Bangalore 560 080, India
Received 16 December 2004; revised 10 February 2005; accepted 15 February 2005
Abstract—A one-step process for the preparation of mono-functionalized triphenylene discotics is presented. Oxidative trimeriza-
tion of ortho-dialkoxybenzenes using FeCl3 in nitromethane and a catalytic amount of various acids, furnished monohydroxy-pen-
taalkoxytriphenylene in addition to hexaalkoxytriphenylene. These products can be easily separated by column chromatography
over aluminium oxide.
Ó 2005 Elsevier Ltd. All rights reserved.
Triphenylene derivatives are the most widely synthesized
and well-characterized materials in the family of discotic
liquid crystals (DLCs). Triphenylene as a novel core for
DLCs was recognized in 1978 and since then more than
500 discotic liquid crystals based on a triphenylene core
have been prepared.1,2 These materials have been exten-
sively studied for various physical properties such as
one-dimensional charge and energy migration, electrolu-
minescence, ferroelectric switching, alignment and self-
assembling behaviour on surfaces and other properties.3
proved synthesis of triphenylene discotics using a novel
reagent, MoCl5.7
All these three reagents, chloranil, iron(III) chloride and
molybdenum(V) chloride, are insoluble in common or-
ganic solvents and, therefore, addition of these materi-
als, particularly on a large scale, is inconvenient. To
overcome this problem, we have discovered a liquid oxi-
dizing reagent, VOCl3 for performing the oxidative tri-
merization.8 Because of its miscibility in the solvent, it
brings about almost spontaneous trimerization of
ortho-dialkoxybenzenes into 2,3,6,7,10,11-hexaalkoxy-
triphenylenes. Though it allows milder reaction condi-
tions, fewer purification problems and affords a better
yield, it is expensive and moisture sensitive compared
to FeCl3. Therefore, we have examined the possibility
of using FeCl3 dissolved in nitromethane for this oxida-
tive trimerization reaction. It has been reported that the
trimerization reaction works only in dichloromethane, a
hazardous chlorinated solvent,8 so replacing it with
nitromethane would have an additional advantage.
Hexamethoxytriphenylene (HMTP) was first prepared
by the oxidative trimerization of 1,2-dimethoxybenzene
(veratrol) in H2SO4 using chloranil or iron(III) chloride
as oxidant.4 The reaction represents an unusual case of
the Scholl reaction where more than one aryl–aryl bond
is formed.5 Long alkoxy chain-substituted triphenylene
derivatives, necessary to show mesomorphism, were pre-
pared traditionally by demethylating HMTP and then
realkylating the resultant hexaphenol with an appropri-
ate alkyl halide. Later, the Boden group reported a di-
rect synthesis of long alkyl chain-substituted hexa-
alkoxytriphenylene DLCs using only a catalytic amount
of H2SO4 and iron(III) chloride as oxidant in a much
improved yield.6 We have recently reported a highly im-
A small amount of monohydroxy-pentaalkoxytriphenyl-
ene (3,6,7,10,11-pentaalkoxy-2-triphenylenol) often forms
in these reactions due to aryl-ether cleavage,9 but its
isolation is usually difficult. The mono-functionalized
triphenylenes are highly valuable precursors for the syn-
thesis of dimers, oligomers, side-chain polymers and net-
works. The physical properties of these non-
conventional liquid crystals are significantly different
Keywords: Triphenylene; Discotic liquid crystals; Monohydroxy-penta-
alkoxytriphenylene; Oxidative trimerization; Iron(III) chloride.
*
Corresponding author. Tel.: +91 8023610122; fax: +91
8023610492; e-mail: skumar@rri.res.in
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.02.099

2604
S. Kumar, B. Lakshmi / Tetrahedron Letters 46 (2005) 2603–2605
to those of conventional low molar mass liquid crystals.
However, because of synthetic problems in obtaining the
mono-functionalized triphenylenes, the potential utility
of their discotic dimers, oligomers and polymers has
not yet been fully explored. Recently, a few methods
have been developed for the synthesis of mono-function-
alized triphenylenes,2 but these all involve multi-step
synthesis. In this communication we report a one-step
method for the preparation of a mixture of hexaalkoxy-
triphenylene and monohydroxy-pentaalkoxytriphenyl-
ene and a convenient process for the isolation of the
latter.
As can be seen from the Table 1, the best yields of
2,3,6,7,10,11-hexakis(butyloxy)triphenylene 2a, 2,3,6,7,
10,11-hexakis(pentyloxy)triphenylene 2b and 2,3,6,7,10,
11-hexakis(hexyloxy)triphenylene 2c as well as of 3,6,
7,10,11-pentakis(butyloxy)-2-triphenylenol 3a, 3,6,7,10,
11-pentakis(pentyloxy)-2-triphenylenol 3b and 3,6,7,
10,11-pentakis(hexyloxy)-2-triphenylenol 3c were real-
ized using 0.3% trifluoroacetic acid (TFA) in nitrometh-
ane. The most commonly used acid, sulfuric acid
furnished smaller amounts of both products. While al-
most the same amount of hexaalkoxytriphenylenes 2
were obtained using HCl, a lower yield of the mono-
functionalized triphenylenes 3 was obtained with this
acid. Increasing the amount of acid, lowers the yield
of both the products. Reactions conducted in the
absence of acid gave lower yields (about 35%) of hexa-
alkoxytriphenylenes 2 and only traces of monohydr-
oxytriphenylenes 3. Longer chains prevent aryl-ether
cleavage and thus, trimerization of longer chain substi-
tuted ortho-dialkoxybenzenes 1b and 1c results in lower
yields of monohydroxy-pentaalkoxytriphenylenes 3b
and 3c.
The one-step synthesis of hexaalkoxytriphenylenes and
monohydroxy-pentaalkoxytriphenylenes is shown in
Scheme 1. In a typical reaction, the ortho-dialkoxybenz-
ene 1 (1 mmol) was taken in nitromethane (5 mL) with
the appropriate amount of acid catalyst.10 To this was
added a solution of FeCl3 (3 mmol) in nitromethane
(4 mL) and the reaction mixture was stirred at room
temperature for 30 min under anhydrous conditions. It
was then poured into cold methanol and the resultant
precipitate was filtered off. The crude product was puri-
fied by column chromatography over neutral aluminium
oxide. Elution of the column with 2–5% ethyl acetate in
petroleum ether afforded the hexaalkoxytriphenylene 2
while elution with 10% ethyl acetate in petroleum ether
furnished the pure monohydroxy-pentaalkoxytriphenyl-
ene 3. The products were characterized from their spec-
tral data, phase behaviour and by direct comparison
with authentic samples. The amount of various acids
used and yields of products are given in Table 1.
In conclusion, 15–20% yields of the valuable mono-
functionalized triphenylenes 3a–c can be isolated as
by-products of the one-step oxidative trimerization of
ortho-dialkoxybenzenes. The main product, hexaalkoxy-
triphenylenes, can be further converted into various
functionalized triphenylenes using our previously re-
ported method.9b The process reported here is a conve-
nient, economic and high yielding method for the
synthesis of these materials.
OR
OH
OR
OR
OR
RO
RO
RO
RO
RO
RO
FeCl3 in nitromethane
acid catalyst
+
OR
a: R = C4H9
b: R = C5H11
c: R = C6H13
1
2
3
OR
OR
Scheme 1. Synthesis of hexaalkoxy- and monohydroxy-pentaalkoxytriphenylenes.
Table 1. Preparation of triphenylene derivatives using FeCl3 in CH3NO2 solution at rt
Starting material
Acid (%)
Product (yield %)
1a, 1b, 1c
1a, 1b, 1c
1a, 1b, 1c
1a, 1b, 1c
1a, 1b, 1c
1a, 1b, 1c
1a, 1b, 1c
1a, 1b, 1c
1a, 1b, 1c
1a, 1b, 1c
1a, 1b, 1c
1a, 1b, 1c
TFA (0.3)
TFA (1.0)
TFA (5.0)
TFA (10.0)
H2SO4 (0.3)
H2SO4 (1.0)
H2SO4 (5.0)
H2SO4 (10.0)
HCl (0.3)
2a (65), 2b (45), 2c (53)
2a (64), 2b (40), 2c (48)
2a (45), 2b (39), 2c (30)
2a (42), 2b (8), 2c (28)
2a (58), 2b (43), 2c (50)
2a (47), 2b (40), 2c (49)
2a (40), 2b (24), 2c (36)
2a (17), 2b (13), 2c (9)
2a (64), 2b (36), 2c (49)
2a (48), 2b (35), 2c (45)
2a (31), 2b (33), 2c (28)
2a (12), 2b (25), 2c (22)
3a (20), 3b (17), 3c (15)
3a (18), 3b (12), 3c (14)
3a (6), 3b (9), 3c (1)
3a (5), 3b (6), 3c (0.5)
3a (19), 3b (13), 3c (12)
3a (7), 3b (10), 3c (11)
3a (5), 3b (1), 3c (9)
3a (1), 3b (0), 3c (0.5)
3a (14), 3b (10), 3c (9)
3a (7), 3b (8), 3c (5)
3a (1), 3b (1), 3c (0)
3a (0), 3b (0), 3c (0)
HCl (1.0)
HCl (5.0)
HCl (10.0)
Yield represents the isolated pure product after column chromatography over aluminium oxide.

S. Kumar, B. Lakshmi / Tetrahedron Letters 46 (2005) 2603–2605
2605
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difficult to cover all these and, therefore, only a few recent
references are given here. Bayer, A.; Zimmermann, S.;
Wendorff, J. H. Mol. Cryst. Liq. Cryst. 2003, 396, 1–22;
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10. Stock solutions of acid-containing nitromethane were
prepared by mixing appropriate amounts (by volume) of
AR grade concentrated acids (TFA, H2SO4, HCl) in
nitromethane.