Improved synthesis of monohydroxytriphenylenes (MHTs) - Important precursors to discotic liquid crystal families

Article abstract of DOI:10.1016/j.tetlet.2010.10.152

Monohydroxypentaalkoxytriphenylenes are important intermediates for elaboration into complex covalently linked discotic liquid crystal assemblies. An improved, simple synthetic protocol is described that gives rapid access to these precursors in a single step.

Full text of DOI:10.1016/j.tetlet.2010.10.152

Tetrahedron Letters 52 (2011) 77–79
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Tetrahedron Letters
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Improved synthesis of monohydroxytriphenylenes (MHTs)—important
precursors to discotic liquid crystal families
a
a,
⇑
b
c
b,
⇑
Xiangfei Kong , Zhiqun He , Hemant Gopee , Xiping Jing , Andrew N. Cammidge
a
Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, China
School of Chemistry, University of East Anglia, Norwich NR4 7TJ, UK
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Monohydroxypentaalkoxytriphenylenes are important intermediates for elaboration into complex cova-
lently linked discotic liquid crystal assemblies. An improved, simple synthetic protocol is described that
gives rapid access to these precursors in a single step.
Received 20 September 2010
Revised 12 October 2010
Accepted 29 October 2010
Available online 4 November 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Triphenylene
Discotic liquid crystals
Monohydroxytriphenylenes
Statistical cyclisation
Ferric chloride
1
. Introduction
Certain disk-like molecules (‘discotics’) can form liquid crystal
most conveniently employing cross-coupling strategies to synthe-
sise bi- or terphenyl intermediates (Scheme 1).
These studies have highlighted triphenylene discotics as robust
materials with a strong tendency towards columnar behaviour.
Significant perturbation around the periphery can be achieved
with retention of the columnar mesophase behaviour so long as
phases. Most usually, the molecules assemble as columns to give
so-called columnar mesophases (which can be compared to the
smectic mesophases formed by rod-like/calamitic molecules)
8
(Scheme 1). In some cases a less ordered nematic phase is exhib-
conjugation to the aromatic core is preserved. However, the
ited in which the molecules retain only orientational order. Substi-
tuted triphenylene derivatives which present a flat, aromatic core
surrounded by flexible chains, are the most widely studied class
of discotic liquid crystals.1 Triphenylene discotics have received
particular interest because of their ability to act as one-dimen-
sional charge transport materials, acting as photoconductors or
semiconductors on doping2 and, alongside other discotic frame-
works, lend themselves to diverse electronic and optical
columnar mesophases of triphenylene, and other, discotic liquid
crystals tend to resist miscibility with other organics including
mesogens and other discotic systems. While such a phase separa-
tion holds advantages in certain applications in organic electronics,
incorporation of additional functionality within a columnar matrix
is a desirable feature, not least because the triphenylene core is it-
self an electron-rich component that can be further exploited in
device design.
3
applications.
Monohydroxypentaalkoxytriphenylenes can be considered as
1
,4–7
9
Synthetic advances
have permitted triphenylene discotics
important and versatile intermediates for the synthesis of
to be designed and synthesized so that an extensive range of struc-
tural features can be probed. Typically, symmetrically substituted
hexalkoxytriphenylenes (HATs) are most conveniently prepared
by oxidative cyclisation of 1,2-dialkoxybenzenes. Unsymmetrically
substituted derivatives have proved more challenging, but efficient
stepwise procedures have been developed to meet this demand,
twinned and linked structures. Retention of at least five alkoxy
chains promotes liquid crystal phase formation while the single
hydroxy group can be used as a conventional link point (ether or
ester formation) or converted into triflate for participation in
cross-coupling chemistry to introduce conjugated links (see
Scheme 2). MHTs are of course available through refined multistep
procedures as outlined above. However, these syntheses are an
over-elaboration and there is demand for simplified access to
material in reasonable quantities. It is known that monohydroxy-
triphenylenes MHTn can be prepared in a single step as byproducts
⇑
Corresponding authors. Tel.: +86 10 51688018, +86 10 51683933 (Z.H); +44
603 592011; fax: +44 1603 592011 (A.N.C).
1
E-mail addresses: zhqhe@bjtu.edu.cn (Z. He), a.cammidge@uea.ac.uk (A.N.
Cammidge).
9
from the cyclisation of dialkoxybenzenes. In this Letter we report
0
040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.10.152

7
8
X. Kong et al. / Tetrahedron Letters 52 (2011) 77–79
OR
OR
OR
OR
[O]
RO
RO
HATn
OR
OR
R = CnH2n+1
Columnar hexagonal
mesophase (Col )
A
A
h
A
B
B
E
B
E
B
B
C
A
A
F
C
C
C
C
[O]
C
[O]
D
D
B
A
F
Scheme 1. Conventional synthesis of symmetrical (HATn) and unsymmetrical hexaalkoxytriphenylenes that form columnar hexagonal mesophases.
are summarized in Scheme 3. At room temperature cyclisation
using ferric chloride in CH Cl /nitromethane was accompanied
2 2
OR
OR
OR
OR
OR
OH
OR
or
OR
by hydrolysis to the desired monohydroxytriphenylene MHT6.
TLC analysis of the crude product clearly showed the formation
of further polar compounds that presumably result from further
hydrolysis. MHT6 can be seen early in the reaction and it appears
that the rate of formation of HAT6 is comparable to the rate of
its hydrolysis to MHT6. However, further hydrolysis is even more
rapid so simply allowing the reaction to proceed for prolonged
periods at room temperature only increases the proportion of poly-
hydroxylated triphenylenes at the expense of MHT6. Performing
the reaction at lower temperatures was found to suppress hydroly-
sis. The cyclisation proceeds at a reasonable rate at 0 °C but the rate
of hydrolysis is more significantly slowed so that MHT6 only ap-
pears towards the end of the reaction and is formed in trace quan-
tities (<5%) along with products of further hydrolysis. The
dominant species responsible for hydrolysis in these reactions is
likely to be the HCl formed during cyclisation. This assumption
was verified by performing the cyclisation reaction over solid so-
dium carbonate and we found that under these conditions hydro-
lysis was suppressed almost completely.
RO
RO
RO
RO
RO
RO
O
A
O
O
B
OR
OR
Monohydroxypentaalkoxy-
triphenylene MHTn (R = C H2n+1)
n
OR
OR
OR
OR
C
RO
RO
cross-coupling RO
RO
or
OTf
OR
OR
OR
OR
OR
OR RO
RO
RO
OR
OR
D
OR
From the above experiments we can conclude that hydrolysis of
the formed hexaalkoxytriphenylenes is slowed significantly at 0 °C
and is suppressed completely by the addition of a base to neutral-
ize the HCl formed during the reaction. These conditions were
exploited in order to perform mixed cyclisations between 1,2-
dialkoxybenxene and alkoxyphenol, reasoning that a clean reaction
Scheme 2. Monohydroxytriphenylenes (MHTn) are important and versatile pre-
cursors to linked and twinned structures.
an optimized protocol for their synthesis via a mixed cyclisa-
5
a,10
tion
between 1,2-dialkoxybenzene and 2-alkoxyphenol.
In the development of an improved protocol for the synthesis of
monohydroxytriphenylenes a number of factors need to be consid-
ered. Although reaction yield is one important criterion, the ease of
isolation of pure material (in terms of time and cost) must also be
considered. Conceptually the most straightforward approach in-
volves the use of more vigorous conditions to those traditionally
employed to hydrolyse the symmetrical hexaalkoxytriphenylene
(
i.e., one that minimized significant further hydrolysis) would lead
to simple one-step access to MHTn. Consequently a solution of fer-
ric chloride in CH Cl /nitromethane was added to a 1:1 mixture of
,2-dihexyloxybenzene and hexyloxyphenol over solid sodium car-
2
2
1
bonate at 0 °C. Surprisingly, over the course of 3 h at 0 °C the for-
mation of HAT6 was observed along with some baseline material,
1
1
(
HATn) precursor. However, it is known that such conditions
FeCl3
OR
lead to the formation of additional hydrolysis products that are dif-
ficult to separate from the target material.11 Any useful strategy
R =n-C6H13
CH2Cl2/CH3NO2
OR
conditions (see text)
must, therefore, suppress excessive further hydrolysis to avoid
any complicated purification procedures. An ideal protocol would
involve the mixed cyclisation between a 1,2-dialkoxybenzene
and a 2-alkoxyphenol under controlled conditions. Clearly under
these conditions there are two possible origins of the target mono-
hydroxytriphenylene MHTn. It can be formed directly through a
mixed cyclisation or via hydrolysis of hexaalkoxytriphenylene
and so it was important to understand the factors that promote
or suppress the latter pathway under ferric chloride mediated cyc-
lisation conditions. Hexyl chains were chosen for the study and the
results from the initial experiments using 1,2-dihexyloxybenzene
OR
OR
OR
OR
OR
OR
RO
RO
RO
RO
further
hydrolysis
products
OH
OR
HAT6
MHT6
Scheme 3. HAT6 formation and hydrolysis using ferric chloride.

X. Kong et al. / Tetrahedron Letters 52 (2011) 77–79
79
FeCl3
oxy)benzene (1.11 g, 4 mmol) and 2-hexyloxyphenol (0.39 g,
2 mmol) was dissolved in CH Cl (15 ml), and was then added
dropwise into the FeCl solution over 30 min at 0 °C. The reaction
OR
OR
OR
OH
CH2Cl2/CH3NO2
conditions (see text)
2
2
3
was continued for 2.5 h at 0 °C. MeOH (15 ml) was added to stop
R =n-C6H13
the reaction and the mixture was washed twice with HCl (5%,
OR
OR
OR
OR
2
2 4
0 ml) and brine (20 ml). The organic layer was dried with Na SO ,
OR
OR
and the solvents removed under reduced pressure. The crude prod-
uct was purified by column chromatography (silica gel, EtOAc/light
petroleum 1:30–1:20) and afforded the two main products as col-
ourless solids; HAT6 (0.47 g, 42%), MHT6 (0.52 g, 35%).
RO
RO
RO
RO
OH
OR
Acknowledgements
H6TA (42%)
M6TH (35%)
Scheme 4. Controlled mixed cyclisation to give monohydroxytriphenylene MHT6.
We acknowledge support from the Chemical Analysis Centre at
Peking University China and the EPSRC Mass Spectrometry Service
but only trace quantities of MHT6. Variation of the reaction condi-
tions led to the further conclusion that addition of sodium carbon-
ate suppresses incorporation of the phenol and that, therefore, this
strategy to prevent hydrolysis of formed MHTn could not be em-
ployed. Fortunately, clean and efficient mixed cyclisations to give
the target MHT were achieved by reaction at 0 °C without sodium
carbonate. In these reactions there is a balance required so that a
reasonable reaction rate is achieved and the desired product (in
this case MHT6) can be isolated easily and purely. Separation of
MHTn from HATn can be achieved easily so long as MHTn is the
dominant component. However, separation of MHTn from prod-
ucts of further hydrolysis can be challenging if the latter are
formed in significant quantities. The most efficient protocol devel-
oped uses a statistical 2:1 mixture of 1,2-dihexyloxybenzene and
hexyloxyphenol at 0 °C and gives a yield of MHT6 of 35% (Scheme
(
(
(
Swansea). This work is supported by the funds from NSF China
20674004), ISTCP China (2008DFA61420), and the 111 project
B08002) of MOE, China.
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2
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