Oxidative trimerization of o-dialkoxybenzenes to hexaalkoxytriphenylenes: Molybdenum(v) chloride as a novel reagent

Article abstract of DOI:10.1039/a704130f

Symmetrically substituted hexaalkoxytriphenylenes are obtained from o-dialkoxybenzenes by oxidative trimerization with MoCl₅ in high yields; oxidative coupling of a 3,3′,4,4′-tetraalkoxybiphenyl and 1,2-dialkoxybenzenes yields unsymmetrically substituted derivatives of triphenylene and a direct coupling of a 3,3′,4,4′-tetraalkoxybiphenyl with alkoxyphenol produces the monofunctionalized triphenylenes.

Full text of DOI:10.1039/a704130f

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Oxidative trimerization of o-dialkoxybenzenes to hexaalkoxytriphenylenes:
molybdenum(v) chloride as a novel reagent
Sandeep Kumar* and M. Manickam
Centre for Liquid Crystal Research, P.O. Box 1329, Jalahalli, Bangalore 560 013, India
Symmetrically substituted hexaalkoxytriphenylenes are ob-
tained from o-dialkoxybenzenes by oxidative trimerization
with MoCl5 in high yields; oxidative coupling of a 3,3A,4,4A-
tetraalkoxybiphenyl and 1,2-dialkoxybenzenes yields un-
symmetrically substituted derivatives of triphenylene and a
direct coupling of a 3,3A,4,4A-tetraalkoxybiphenyl with
alkoxyphenol produces the monofunctionalized tripheny-
lenes.
triphenylenes but difficult to apply for a large scale produc-
tion.
We have discovered that molybdenum(v) chloride is a novel
reagent for the oxidative trimerization of 1,2-dialkoxybenzenes
to hexaalkoxytriphenylenes in high yield. The reaction occurs
under very mild conditions with or without acid catalyst and in
a very short time at room temperature (Scheme 1). In a typical
reaction, MoCl5 (1.4 g, 0.0051 mol) was added to a solution of
1,2-dibutoxybenzene (1.1 g, 0.005 mol) in 10 ml of dry CH2Cl2.
The reaction mixture was stirred at room temperature for 20 min
under anhydrous conditions. It was then poured over cold
MeOH (15 ml), diluted with water (20 ml) and extracted with
hexane (4 3 20 ml). The combined extracts were washed with
water and brine, dried over anhydrous sodium sulfate, the
solvent was removed under vacuum and the crude product was
purified by column chromatography over silica gel, yielding
1.04 g (95%) of hexabutoxytriphenylene. The product was
characterized by its spectral data, phase behaviour and a direct
comparison with an authentic sample. Yields of different
hexaalkoxytriphenylenes are given in Table 1. As MoCl5 is a 2
e2 oxidant, only ca. 1 equiv. of the reagent is required to
Hexasubstituted triphenylenes are the most widely synthesized
and studied discotic mesogens.1 Recently, these materials have
been discussed as potential candidates for opto-electronic
devices.2,3 Several research groups are currently working on the
synthesis of symmetrical, unsymmetrical and functionalized
triphenylene discotic liquid crystals.4–9 Here we report the
synthesis of a variety of triphenylene derivatives using a novel
reagent, molybdenum(v) chloride.
Synthesis of hexamethoxytriphenylene has been achieved
previously by oxidative trimerisation of 1,2-dimethoxybenzene
(veratrol) in H2SO4 using chloranil or iron chloride as oxidant.10
The trimerization involves three consecutive Scholl reactions
but surprisingly under normal Scholl conditions using alumin-
ium chloride and nitrobenzene this does not work well.10e
Triphenylene derivatives show mesomorphism only when the
six peripheral alkoxy chains have a minimum of four carbon
atoms, i.e. butoxy chains. The trimerization using chloranil is
limited to the preparation of hexamethoxytriphenylene only and
with higher homologues like 1,2-dihexyloxybenzene it gives
only a poor yield of hexahexyloxytriphenylene with many side
products.4g To prepare these long chain derivatives, the methyl
groups of hexamethoxytriphenylene are dealkylated by BBr3 or
HBr and the resultant hexaphenol is realkylated with the
appropriate alkyl halide to give different hexaalkoxytripheny-
lene discotics.
OR
OR
OR
OR
RO
RO
MoCl5, CH2Cl2
room temp., 20 min, 74–95%
OR
OR
1a–d
2a–d
a R = Bu
b R = C6H13
c R = C10H21
d R = Me
Hexamethoxytriphenylene can also be prepared by the cyclic
anodic trimerization of dimethoxybenzene and, using a chem-
ical or electrochemical reductive work-up, a moderate yield of
the product can be obtained.11 The method is not limited to
dimethoxybenzene but can be extended to other dialkoxy-
benzenes to form directly hexaalkoxytriphenylene discotics, but
this slow reaction is only applicable on a very small scale and,
thus, limited in its use.
Scheme 1
Table 1 Preparation of triphenylene derivatives using MoCl5 in CH2Cl2
solution
Starting
Material
H2SO4 (%)
Product
Yield (%)
The use of FeCl3 as oxidant was further explored by the
Ringsdorf group, which reported the synthesis of hexahexyl-
oxytriphenylene by the trimerization of 1,2-dihexyloxybenzene
1a
1a
1a
1a
1b
1b
1c
1c
1d
1d
3 + 4
3 + 4
3 + 6
3 + 6
0.0
0.2
0.5
1.0
0.0
0.2
0.0
0.2
0.0
0.2
0.0
0.2
0.0
0.2
2a
2a
2a + 7
2a + 7
2b
2b
2c
95
92
78 + 5
41 + 25
94
90
80
78
74
74
90
75
44
in 70% H2SO4 at 80 °C using iron chloride in 20% yield.12
A
further modification to this process was implemented by the
Boden group, who used only a catalytic amount of H2SO4
(0.3%) in CH2Cl2 and reported yields of 55–86% for different
alkoxytriphenylenes.4g Coupling of a tetraalkoxybiphenyl with
the appropriate dialkoxybenzene under the same conditions
gives unsymmetrical hexaalkoxytriphenylenes in 45–77%
yield.4f
Well-defined synthesis of unsymmetrical and low degree
substituted triphenylenes has been reported recently using
organometallic chemistry and FeCl3.7,8 These sophisticated
methods are useful for the preparation of low degree substituted
2c
2d
2d
5
5
7
7
47
Chem. Commun., 1997
1615

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perform the reaction. The best yields of hexaalkoxytripheny-
lenes were obtained without using any acid and gave better
yields than any of the known methods. We have also observed
that on increasing the amount of H2SO4, partial dealkylation of
hexaalkoxytriphenylenes occurs and variable amounts of
monohydroxypentaalkoxytriphenylene can be isolated (Table
1). The work-up of the reaction is very simple; in most cases, on
pouring the reaction mixture over MeOH, a white precipitate
forms which can be filtered and recrystallized. Otherwise,
addition of water followed by extraction with hexane gives a
colourless product.
The potential of this new reagent is currently under
investigation for the synthesis of different symmetrical, un-
symmetrical and functionalized triphenylenes. The effects of
different solvents, acids and concentrations are also under
investigation and the results will be published in due course.
We are very grateful to Professor S. Chandrasekhar and
Professor H. Ringsdorf for many helpful discussions.
Footnote and References
* E-mail: uclcr@giasbg01.vsnl.net.in
Synthesis of unsymmetrical and monofunctionalized tri-
phenylenes was achieved by coupling a 3,3A,4,4A-tetraalkoxy-
biphenyl and appropriate 1,2-dialkoxybenzenes (Scheme 2)
under similar reaction conditions. Thus, when 3,3A,4,4A-tet-
rabutoxybiphenyl 3, prepared by classical Ullman coupling of
dialkoxymonoiodobenzene, was coupled with 2-butoxyanisole
4, 2-methoxy-3,6,7,10,11-butoxytriphenylene 5 was formed in
90% yield. Symmetrical and nonsymmetrical trimers of 1,2-dia-
lkoxybenzenes were the two side products in this reaction.
1 For a brief review of discotics, see S. Chandrasekhar, Liq. Cryst., 1993,
14, 3. See also S. Chandrasekhar and S. Kumar, Science Spectra, 1997,
8, 66.
2 N. Boden, R. Bissell, J. Clements and B. Movaghar, Liquid Crystals
Today, 1996, 6, 1.
3 S. Kumar, P. Schuhmacher, P. Henderson, J. Rego and H. Ringsdorf,
Mol. Cryst. Liq. Cryst., 1996, 288, 211.
4 (a) N. Boden, R. J. Bushby, A. N. Cammidge, S. Duckworth and
G. Headdoc J. Mater. Chem., 1997, 7, 601; (b) N. Boden, R. J. Bushby
and A. N. Cammidge, Mol. Cryst. Liq. Cryst., 1995, 260, 307; (c)
N. Boden, R. J. Bushby and A. N. Cammidge, Liq. Cryst., 1995, 18, 673;
(d) N. Boden, R. J. Bushby, A. N. Cammidge and G. Headdock,
J. Mater. Chem., 1995, 5, 2275; (e) N. Boden, R. J. Bushby,
A. N. Cammidge and G. Headdock, Synthesis, 1995, 31; (f) N. Boden,
R. J. Bushby and A. N. Cammidge, J. Chem. Soc., Chem. Commun.,
1994, 465; (g) N. Boden, R. C. Borner, R. J. Bushby, A. N. Cammidge
and M. V. Jesudason, Liq. Cryst., 1993, 15, 851.
OR
OR′
RO
OR
OR′
RO
RO
MoCl5, CH2Cl2
+
room temp.,
30 min, 90%
OR
5 S. Kumar, Mol. Cryst. Liq. Cryst., 1996, 289, 247; S. Kumar and
M. Manickam, Mol. Cryst. Liq. Cryst., 1997, in the press.
6 J. A. Rego, S. Kumar and H. Ringsdorf, Chem. Mater., 1996, 8, 1402;
J. A. Rego, S. Kumar, I. J. Dmochowski and H. Ringsdorf, Chem.
Commun., 1996, 1031; P. Henderson, S. Kumar, J. A. Rego,
H. Ringsdorf and P. Schuhmacher, J. Chem. Soc., Chem. Commun.,
1995, 1059; F. Closs, L. Ha¨ussling, P. Henderson, H. Ringsdorf and
P. Schuhmacher, J. Chem. Soc., Perkin Trans. 1, 1995, 829; P.
Henderson, H. Ringsdorf and P. Schuhmacher, Liq. Cryst., 1995, 18,
191.
4
RO
OR
OR
3
OR
R = Bu, R′ = Me
5
Scheme 2
Monofunctionalized triphenylene, an extremely important
precursor for the synthesis of discotic dimers, oligomers and
polymers, can be prepared by direct coupling of a 3,3A,4,4A-
tetraalkoxybiphenyl and monoalkoxyphenol. Thus, oxidative
coupling of 3,3A,4,4A-tetrabutoxybiphenyl 3 and 2-butoxyphenol
6 using MoCl5 under similar reaction conditions yielded
2-hydroxy-3,6,7,10,11-butoxytriphenylene 7 in 47% yield
(Scheme 3).
7 R. C. Borner and R. F. W. Jackson, J. Chem. Soc., Chem. Commun.,
1994, 845.
8 J. W. Goodby, M. Hird, K. J. Toyne and T. Watson, J. Chem. Soc.,
Chem. Commun., 1994, 1701.
9 H. Naarmann, M. Hanack and R. Mattmer, Synthesis, 1994, 477.
10 (a) I. M. Matheson, O. C. Musgrave and C. J. Webster, Chem. Commun.,
1965, 278; (b) O. C. Musgrave and C. J. Webster, J. Chem. Soc., 1971,
139; (c) M. Piatelli, E. Fattorusso, R. A. Nicolaus and S. Magno,
Tetrahedron, 1965, 21, 3229; (d) C. Destrade, M. C. Mondon and
J. Malthete, J. Phys. Colloq., 1979, 40, C3; (e) O. C. Musgrave, Chem.
Rev., 1969, 69, 499.
11 K. Bechgaard and V. D. Parker, J. Am. Chem. Soc., 1972, 94, 4749;
V. Le Berre, L. Angely, N. Simonet-Gueguen and J. Simonet, J. Chem.
Soc., Chem. Commun., 1987, 984; V. Le Berre, J. Simonet and P. Batail,
J. Electroanal. Chem., 1984, 169, 325; J. Chapuzet and J. Simonet,
Tetrahedron, 1991, 47, 791.
OR
OH
RO
OR
OH
OR
RO
RO
MoCl5, CH2Cl2
+
room temp.,
30 min, 47%
6
RO
OR
12 H. Bengs, O. Karthaus, H. Ringsdorf, C. Baehr, M. Ebert and
J. M. Wendorff, Liq. Cryst., 1991, 10, 161.
OR
3
OR
R = Bu
7
Scheme 3
Received in Cambridge, UK, 13th June 1997; 7/04130F
1616
Chem. Commun., 1997