Synthesis of functionalized triphenylenes by selective ether cleavage with B-bromocatecholborane

Article abstract of DOI:10.1055/s-1998-2133

This paper presents an efficient synthetic procedure for the preparation of mono-, di- and trifunctionalized triphenylene derivatives starting from the readily available hexakis(pentyloxy)triphenylene by selective ether cleavages with B-bromocatecholborane (2-bromo-1,3,2-benzodioxaboroles). Functionalized triphenylene derivatives are the precursor molecules for the preparation of processible triphenylene discotic dimers, oligomers, polymers and networks.

Full text of DOI:10.1055/s-1998-2133

SYNTHESIS
August 1998
1119
Synthesis of Functionalized Triphenylenes by Selective Ether Cleavage with
B-Bromocatecholborane
Sandeep Kumar, M. Manickam
Centre for Liquid Crystal Research, P.O. Box 1329, Jalahalli, Bangalore - 560 013, India
Fax +91(80)8382044; E-mail: uclcr@giasbg01.vsnl.net.in
Received 18 November 1997; revised 12 January 1998
Abstract: This paper presents an efficient synthetic procedure for
phenylene 2 is written as monohydroxy-H5TP or mono-
functionalized-H5TP and compound trihydroxy-tris(pen-
tyloxy)triphenylene 5 is written as trihydroxy-H5TP or
trifunctionalized-H5TP.
the preparation of mono-, di- and trifunctionalized triphenylene
derivatives starting from the readily available hexakis(pentyl-
oxy)triphenylene by selective ether cleavages with B-bromocate-
cholborane (2-bromo-1,3,2-benzodioxaboroles). Functionalized
triphenylene derivatives are the precursor molecules for the prepa-
ration of processible triphenylene discotic dimers, oligomers,
polymers and networks.
The monofunctionalized triphenylenes are very valuable
precursors for the synthesis of dimers, oligomers, side-
chain polymers and networks. The synthesis of mono-
functionalized triphenylenes 2 can be achieved by differ-
ent ways. Hexakis(pentyloxy)triphenylene 1 can be pre-
pared by oxidative trimerization of o-dialkoxybenzene
with chloranil,11 iron(III) chloride19 or molybdenum(V)
chloride.35 A nonselective cleavage of one of the alkoxy
chains of hexaalkoxytriphenylene can be done by the use
of a calculated amount of 9-Br-BBN30 which gives a mix-
ture of products containing unreacted hexaalkoxytriphe-
nylene (26%), monohydroxytriphenylene (39%) and mi-
nor amount of dihydroxytriphenylenes (10%). The de-
sired product can be purified by column chromatography.
Similarly, a nonselective synthesis can also be achieved13
by the partial alkylation of hexaacetoxytriphenylene to
monoacetyltriphenylene in low yield (26%) which can be
hydrolyzed to monohydroxytriphenylene. A selective
cleavage of the methyl ether of monomethoxypentaalkoxy-
triphenylene with lithium diphenylphosphide gives the
monohydroxytriphenylene in high yields.31 The mono-
methoxytriphenylene was prepared by the so-called bi-
phenyl route. The main drawback of this process is the
poor yield of the biphenyl in the classical Ullman coup-
ling reaction and the use of highly sensitive and hazard-
ous lithium diphenylphosphide. We have very recently re-
ported the synthesis of monohydroxytriphenylene by di-
rectly coupling tetraalkoxybiphenyl and alkoxyphenol us-
ing molybdenum(V) chloride as oxidant in dichlo-
romethane in moderate yield.35 Though this represents
one of the best method for the preparation of monohy-
droxytriphenylene, the moderate yield of the final oxida-
tive coupling reaction and the Ullmann reaction for the
preparation of the biphenyl results, overall, in a poor yield
of the final product. We have also reported that this com-
pound can also be obtained as a side product in the oxida-
tive trimerization of dialkoxybenzene with MoCl5 or
FeCl3 in about 25% yield.35, 38
Key words: functionalized triphenylene, hexakis(pentyloxy)tri-
phenylene, selective ether cleavage, discotic liquid crystals
Since the discovery of discotic liquid crystals,1 there has
been an ever increasing interest in the synthesis of new
monomeric and polymeric discotic liquid crystals.2
Though any device made of discotic liquid crystal has yet
to come, the potential applications of these material in op-
toelectronics have recently been the subject of many dis-
cussions.2–4 In the columnar mesophase, molecules stack
one on top of the other with a stacking distance of about
3–4 Å, making these molecules perfectly suitable for one-
dimensional charge migration. Conductivity, photocon-
ductivity and energy migration in the columnar phase of
various discotic liquid crystals (LCs) have been studied
extensively.5–10
Discotic LCs based on triphenylene core are the most
widely synthesized and well-studied materials in the fam-
ily of discotics. Due to their pronounced photoconductiv-
ity and high charge carrier mobility, triphenylene-based
columnar discotic LCs show great potential as molecular
organic materials for optoelectronic devices. Several re-
search groups are currently working on the synthesis of
symmetrical, unsymmetrical and functionalized triphe-
nylene derivatives.11–41 We have very recently reported a
highly improved synthesis of symmetrical, unsymmetri-
cal and monofunctionalized triphenylene derivatives us-
ing a novel reagent MoCl5.35 We have also reported earli-
er the synthesis of various functionalized triphenylene,4, 29
mixed tail triphenylene,33 low symmetry, fluorescent
triphenylene32 and core functionalized triphenylene dis-
cotic LCs.36–38 Because of synthetic problems in obtain-
ing the functionalized triphenylene derivatives in high
yield, the potential utility of discotic dimers, oligomers
and polymers has not yet been fully explored. In this com-
munication, we wish to report a simple, easy and high
overall yielding method for the preparation of mono-, di-
and trifunctionalized triphenylene derivatives. In order to
simplify the discussion hexakis(pentyloxy)triphenylene is
written as H5TP, and monofunctionalized means mono-
hydroxy-pentaalkoxytriphenylene. The pentyloxy groups
are not considered as functional groups. Thus, for exam-
ple, compound monohydroxy-pentakis(pentyloxy)tri-
The two hydroxy groups in a dihydroxytetraalkoxytri-
phenylene may be present in four different ways. These
are at the 2,3-, 2,6-, 2,7-, and 3,6-positions.28, 30 Synthesis
of dihydroxytriphenylene was achieved by statistical
methods. Alkylation of hexaacetoxytriphenylene with
alkyl halide gives a mixture of dihydroxytriphenylene in
29% yield.15 Difunctionalized triphenylene of well-de-

SYNTHESIS
Papers
1120
fined structure can be prepared by using the biphenyl
route and a selective ether cleavage.28, 30 The multiple step
synthesis involves the preparation of different dimethoxy-
tetraalkoxytriphenylene and finally the two methoxy
groups were selectively cleaved by lithium diphenylphos-
phide to dihydroxytriphenylene.28 This rational synthesis
is extremely important for the preparation of authentic
samples, but the generally poor yield obtained in the Ull-
man reaction, the use of highly sensitive, hazardous and
costly lithium diphenylphosphide, and overall poor yield
because of multiple steps (5–10% from catechol), make
this method uneconomical on a large scale.
Trifunctionalized triphenylenes are particularly important
for the preparation of “mixed tail” discotics, and as cross-
linkers for the preparation of discotic networks and elas-
tomers. Previously these compounds were prepared by the
selective ether cleavage of hexaalkoxytriphenylene with
bulky Lewis acid, 9-Br-BBN.4, 30 Very recently, the same
reagent has been used by Kilburn to prepare these trifunc-
tionalized derivatives in a modified way.41 Because of
handling problem of this pyrophoric moisture-sensitive
reagent, we started looking its alternative and have found
that B-bromocatecholborane is superior to 9-Br-BBN.
Scheme 1
atives, nothing goes to waste. In all the cases almost quan-
titative material balance was observed. Out of the four
possible isomers of dihydroxy-H5TP, we found that only
2,6- and 2,7-isomers, 3 and 4, respectively, are the major
products. These two 3 + 4 can be separated by a careful
and repeated chromatography over basic alumina in about
6:4 ratio. We actually separated only a small amount of
these isomers for physical and spectral characterization, in
all other cases global yield of the two isomers is given in
the Table. Trace (less than 1%) of the 3,6-isomer was also
present in some cases but the 2,3-isomer, as expected be-
cause of the bulkiness of the reagent, was completely ab-
sent.
The ability of B-bromocatecholborane (Cat-B-Br), also
named as 2-bromo-1,3,2-benzodioxaboroles or catechol
boron bromide, to cleave certain ether, ester and carbam-
ate groups under mild conditions is known from the liter-
ature.42, 43 We anticipated that this monofunctionalized,
bulky Lewis acid will have better regioselectivity than
commonly used boron tribromide and may cleave selec-
tively one, two and three alkoxy chains of hexaalkoxy-
triphenylene. The reagent can be readily prepared by re-
acting catechol with boron tribromide. The colorless solid
is stable for months, but can be stored more easily as
CH2Cl2 solutions.42, 43
The cleavage of hexaalkoxytriphenylene with different
concentration of Cat-B-Br is given in the Table. When
H5TP 1 is treated with 1–1.2 equivalents of the reagent in
dichloromethane at room temperature for 24–48 hours, it
gives mainly the monohydroxy-H5TP 2 with a minor
amount of unreacted H5TP 1 (Scheme 1). Increase in re-
action time and temperature does not change the yield of
monohydroxy-H5TP 2 significantly. An increase in the
concentration of the reagent produces cleaving of the oth-
er alkoxy chains and variable amounts of mono-, di-, and
trifunctionalized-H5TP are obtained (Table).
The real advantage of this monofunctionalized, bulky
Lewis acid was found in the preparation of trifunctional
triphenylene. Treatment of H5TP 1 with 3.6 equivalents
of Cat-B-Br in dichloromethane at room temperature for
36 hours gives exclusively two products, the symmetrical
2,6,10-trihydroxy-3,7,11-tris(pentyloxy)triphenylene (5)
(61%) and nonsymmetrical 2,7,10-trihydroxy-3,6,11-
tris(pentyloxy)triphenylene (6) (38%) (Scheme 2). Use of
less than 3.6 equivalents of the reagent ca. 3.0 equivalents
under similar conditions gives a small amount (6%) of di-
hydroxy-H5TP 3/4 in addition to the two trifunctional-
ized-H5TP 5 and 6. Reducing the time to 24 hours with
3.6 equivalents of reagent also gives about 5% of dihy-
droxy-H5TP. Excess reagent, ca. 4 equivalents, gives
more or less the same result obtained from the use of 3.6
equivalents of reagent (Table). The high selectivity of the
B-bromocatecholborane could be due to the very bulki-
ness of the reagent that allows the cleavage of only one
alkyl chain per ring. The two symmetrical and nonsym-
metrical trifunctionalized triphenylenes can be separated
very easily by simple column chromatography on silica
gel.
Reaction of H5TP with 2–2.4 equivalents of Cat-B-Br at
room temperature to prepare difunctionalized-H5TP al-
ways gives a mixture of mono- 2, di- 3 + 4, and trifunc-
tionalized-H5TP 5 and 6 (Table, entries 8 and 9). Howev-
er, the formation of trihydroxytriphenylenes 5 and 6 can
be checked by performing the reaction at 0 ˚C and the di-
hydroxy derivatives 3 + 4 can be isolated in reasonably
good quantity (Scheme 1). Nevertheless, all the products
can be easily separated by simple column chromatogra-
phy over silica gel and as all the functionalized triphe-
nylenes can be utilized to prepare a variety of other deriv-

SYNTHESIS
August 1998
1121
400 MHz Bruker NMR spectrometer. Mps were taken on a DSC 7
(Perkin–Elmer) with heating and cooling rate of 5°C per min and the
onset reading is given. Mps of the crystals obtained from solvent (1st
heating) differ from the mp of crystals obtained from melt (2nd heat-
ing) and, therefore, both are given. Compounds 2 and 3 show at least
one broad solid–solid transition prior to the melting. H5TP was pre-
pared as reported.35
B-Bromocatecholboronane:42, 43
To a cooled solution (–15°C) of BBr3 (18 g, 0.072 mol) in CH2Cl2
(10 mL) was added a suspension of catechol (7.1g, 0.065mol) in
CH2Cl2 (50 mL) in portions with stirring under argon. The mixture
was brought to r.t., the solvent removed and the product distilled un-
der vacuum. The white solid was used to make a 0.5 M solution by
mixing the product (11.0 g) with CH2Cl2 (100 mL) and this was used
for different ether cleavage reactions.
Cleavage of Alkoxy Chains of H5TP with B-Bromocatecholbo-
rane: General Procedure:
A solution of H5TP 1 (200 mg, 0.27 mmol) was dissolved in anhyd
CH2Cl2 (5 mL) and cooled to 0°C. To this was added the desired
amount of B-bromocatecholborane solution in CH2Cl2 under argon
and the mixture was stirred at r.t. for the specified time (Table). After
that it was poured over ice-water and extracted with CH2Cl2, the com-
bined extract was dried (anhyd Na2SO4), solvent was removed under
vacuum and the crude product was purified by column chromatogra-
phy (silica gel, hexane/CH2Cl2). The Rf value of the spots on TLC in
two different solvent systems, A (hexane/CH2Cl2 2:8); B (hexane/
EtOAc 8:2) is given.
Scheme 2
In summary, this work describes an efficient method for
the preparation of monofunctionalized, difunctionalized
and trifunctionalized triphenylenes starting from the easi-
ly available hexaalkoxytriphenylene. Functionalized
triphenylenes are extremely important for the preparation
of discotic dimers, oligomers, polymers, and networks.
The use of this reagent can be extended to other polyaro-
matic systems, e.g., truxene, dibenzopyrene, decacyclene,
etc., and this will open a route for the preparation of a va-
riety of new mixed tail discotics.
Monohydroxy-H5TP (2): mp 70.3°C (MeOH, 1st heating), 69.9°C
(melt, 2nd heating), (Lit.31 mp 70°C); Rf 0.73 (A); 0.63 (B).
2,6-Dihydroxy-H5TP (3): mp 85.8°C (MeOH, 1st heating), 85.9°C
(melt, 2nd heating), (Lit.30 mp 82.5°C); Rf 0.66 (A); 0.46 (B).
2,7-Dihydroxy-H5TP (4): mp 190.9°C (hexane/EtOAc, 1st heating),
186.2°C (melt, 2nd heating), (Lit.30 mp 180°C); Rf 0.61 (A); 0.48 (B).
3,6-Dihydroxy-H5TP: mp 143.3°C (hexane/EtOAc, 1st heating),
142.1°C (melt, 2nd heating), (Lit.30 mp 142.5°C); Rf 0.36 (A); 0.28 (B).
Table. Cleavage of H5TP with Cat-B-Br (CH2Cl2, r.t.)
Entry
Reagent
(Equiv.)
Time
(h)
Products Yield (%)
2,6,10-Trihydroxy-H5TP (5): mp 162.3°C (EtOH, 1st heating),
140.9°C (melt, 2nd heating), (Lit.30 mp 140°C, Lit.41 mp 157.8°C);
Rf 0.64 (A); 0.38 (B).
1
2
3+4
5
6
2,7,10-Trihydroxy-H5TP (6): mp 149.1°C (EtOH, 1st heating),
148.5°C (melt, 2nd heating), (Lit.30 mp 146°C, Lit.41 mp 119–
120°C); Rf 0.23 (A); 0.21 (B).
1
2
3
1.0
1.0
1.2
1.2
1.2
1.2
1.6
2.0
2.4
2.2
2.4
2.5
3.0
3.6
3.6
4.0
24
36
24
36
48
4
36
36
36
48
48
48
48
24
36
36
39
34
32
32
28
30
20
19
12
31
24
22
–
60
65
66
67
70
68
69
33
39
35
42
40
–
–
–
–
–
–
–
–
–
–
–
–
–
17
20
–
–
–
56
55
61
60
–
–
–
–
–
–
–
7
4
5
NMR data of all the derivatives were found to be in accordance with
literature data.30, 31
6a
7
–
6
8
9
14
15
10
18
20
6
5
–
–
10
–
–
We are very grateful to Professor S. Chandrasekhar for many helpful
discussions. The authors also gratefully acknowledge the technical
assistance of Mr. Sanjay K. Varshney.
10b
11b
12b
13
14
15
16
–
36
37
38
38
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