Synthesis and characterisation of novel hexaalkoxytriphenylenes bearing an additional alkyl chain in the α-position

Article abstract of DOI:10.1016/j.tetlet.2009.03.089

Exhaustive alkylation of hexahydroxytriphenylene results in production of significant quantities of a side product bearing one additional alkyl chain originating from C-alkylation. A series of these novel materials have been isolated and characterised to

Full text of DOI:10.1016/j.tetlet.2009.03.089

Tetrahedron Letters 50 (2009) 3513–3515
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Tetrahedron Letters
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Synthesis and characterisation of novel hexaalkoxytriphenylenes bearing
an additional alkyl chain in the a-position
*
Andrew N. Cammidge , Hemant Gopee, Hitesh Patel
School of Chemical Sciences and Pharmacy, University of East Anglia, Norwich NR4 7T, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Exhaustive alkylation of hexahydroxytriphenylene results in production of significant quantities of a side
product bearing one additional alkyl chain originating from C-alkylation. A series of these novel materials
have been isolated and characterised to gain further insight on factors controlling mesophase formation
in triphenylene discotics.
Received 12 January 2009
Revised 20 February 2009
Accepted 5 March 2009
Available online 16 March 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Triphenylenes
Discotic liquid crystals
Alkylation
Certain disc-like molecules (‘discotics’) can form liquid crystal
phases. Most usually, the molecules assemble as columns to give
so-called columnar mesophases (which can be likened to the smec-
tic mesophases formed by rod-like/calamitic molecules) (Fig. 1).
Substituted triphenylene derivatives which present a flat, aromatic
core surrounded by flexible chains, are the most widely studied
class of discotic liquid crystals.1 In some cases a less ordered nema-
tic phase is exhibited in which the molecules retain only orienta-
tional order. Triphenylene discotics have received particular
interest because of their ability to act as one-dimensional charge
transport materials, acting as photoconductors or semiconductors
on doping2 and, alongside other discotic frameworks, lend them-
selves to diverse electronic and optical applications.3
Synthetic and applications science has advanced in parallel and
a significant challenge has revolved around unravelling and under-
standing the structural factors that control the formation of stable
mesophases. Triphenylene has proved to be a versatile scaffold for
such fundamental interrogation of structural factors controlling
mesophase behaviour (stability, type, etc.). New and improved
synthetic protocols have now led to characterisation of a wide
range of symmetrical (most common) and unsymmetrically substi-
tuted derivatives.4
tended core has a dramatic effect on the mesophase behaviour.
For example, hexaalkoxytriphenylenes (HATs) such as HAT6 are
the most studied triphenylene-based discotic liquid crystals and
are typically characterised as giving columnar hexagonal mesopha-
ses (Fig. 1).7 Replacement of one conjugating substituent with an-
other tends to maintain the mesophase behaviour.4d,7–9 However,
exchange with, or addition of, a non-conjugating group tends to
destroy the mesophase (Fig. 1).4g,8,10–12
We recently reported the unexpected first examples of meso-
phase-forming triphenylenes bearing a substituent linked to the
core by a saturated methylene group.13 These derivatives were
formed as side products when attempts were made to introduce
terminal alkene functionality on typical hexaalkoxytriphenyl-
enes. Observation of mesophase behaviour was surprising but
it was unclear if this series represented something of a special
case or whether this general structural motif supports meso-
phase stability. Consequently we decided to investigate the
same structural motif within the series of parent alkoxy/alkyl
triphenylenes.
Unsymmetrically substituted triphenylenes such as 10–13
could conceivably be prepared selectively from appropriate ben-
zene and biphenyl derivatives using precedented methodolgy.12
However, our previous work has demonstrated that alkylation
of hexahydroxytriphenylene produced the C-alkylated side prod-
uct in some cases and we reasoned that using this route would
be both convenient and allow us to investigate the reaction itself
to gauge its significance in such triphenylene syntheses (Scheme
1).
We have established a set of qualitative parameters governing
mesophase formation and stability in triphenylene discotics and
have argued that the central core of such discotic molecules ex-
tends beyond the central aromatic triphenylene unit to include
the attached conjugated substituents.5,6 Perturbation of this ex-
In a typical procedure therefore, hexahydroxytriphenylene 9
was reacted with an alkyl bromide (K2CO3/EtOH/reflux) for 24 h.
* Corresponding author. Tel./fax: +44 (0)1603 592011.
E-mail address: a.cammidge@uea.ac.uk (A.N. Cammidge).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.03.089

3514
A. N. Cammidge et al. / Tetrahedron Letters 50 (2009) 3513–3515
Br
C7H15
OHx
Hx = n-hexyl
OHx
Columnar hexagonal
mesophase (Colh)
HxO
HxO
HxO
replacement with
HxO
conjugating substituents
1
OHx
OHx
C7H15
replacement with
OHx
I 176 Colh
non-conjugating substituents
Br
OHx
2
HxO
HxO
Cr 70 I
addition of a
conjugating
substituent
addition of a
non-conjugating
substituent
OHx
OHx
OHx
OHx
OHx
OHx
OHx
OHx
Cr 70 Colh 100 I
HAT6
HxO
HxO
HxO
HxO
CH3
CN
(CH2)n
4
OHx
3
OHx
Cr 60 I
(CH2)n
O
Cr 93 Colh 214 I
O
(CH2)n
O
O
(CH2)n
(CH2)n
(CH2)n
O
O
(CH2)n
5-8
n = 3-6
Figure 1. Examples illustrating the importance of a central
p-extended core in triphenylene-based liquid crystals. Simple conjugating substituents, such as cyanide or
bromide, can lead to formation of columnar mesophases but non-conjugating ones (e.g., alkyl) suppress mesophase formation. The series 5–8 exhibit mesophase behaviour
and are the only known exceptions to this trend.
OR
OR
OR
OH
OH
OR
OR
OR
OR
OH
OH
RO
RO
RO
RO
Br
HO
HO
1-4
K2CO3/EtOH
R
OR
R = n-C5H11 - n-C8H17
9
HATn
70-76%
10-14
5-9%
Scheme 1. Alkylation of hexahydroxytriphenylene to give the C-alkylated product.
Symmetrical hexaalkoxytriphenylene (70–76%) was smoothly
formed. Once again, however, its formation was always accompa-
nied by the C-alkylated triphenylenes in a low but significant
and reproducible yield of 5–9%. Separation of the two compounds
was conveniently achieved by careful column chromatography and
the C-alkylated derivatives were easily identified by 1H NMR spec-
troscopy which revealed the unsymmetrical nature of the com-
pounds and showed triplets for the C-alkyl methylene group
around 3.2 ppm.14 Careful inspection of the mass spectra obtained
for the crude product mixtures revealed peaks corresponding to
formation of trace quantities of doubly C-alkylated materials but
no attempts were made to isolate these.
bers of the new series of C-alkylated materials melt directly
from crystalline solid to isotropic liquid, and therefore follow
the previously established trend whereby introduction of
a
non-conjugating substituent directly onto the core destroys
mesophase behaviour. This observation has more important
implications for the alkenyl series which further stands out as
unique in triphenylene discotic systems. Terminal alkenes are
known to have significant effects on calamitic systems,15 typi-
cally lowering transition temperatures compared to their
saturated counterparts. However, within the triphenylene discot-
ics we can now conclude that the subtle introduction of terminal
unsaturation can actually induce formation of a stable meso-
phase. This observation further underlines the fine structural bal-
ance that contributes to mesophase formation (and destruction)
in triphenylene discotics.
The thermal behaviour of the new materials is shown in Table
1 alongside data for the corresponding parent hexaalkoxytri-
phenylenes and the previously reported alkene series. All mem-

A. N. Cammidge et al. / Tetrahedron Letters 50 (2009) 3513–3515
3515
Table 1
Comparison of the transition temperatures of alkyl- and alkenyloxytriphenylenes
O(CH2)n
R
O(CH2)n R
O(CH2)n
R
O(CH2)n
R
R
O(CH2)n R
O(CH2)n
R
R
R
R
(CH2)nO
(CH2)nO
R
(CH2)nO
(CH2)nO
R
R
(CH2)nO
(CH2)nO
(CH2)n
R
(CH2)n
R
R
O(CH2)n
O(CH2)n R
O(CH2)n R
O(CH2)n
Series A
O(CH2)n
R
Series B
O(CH2)n
R
Chain length
Symmetrical
7-Chain
7-Chain
6-chain alkoxy
(HATn) R = CH2CH3
alkoxy/alkyl
(Series A)
alkenyloxy/
alkenyl
R = CH2CH3
(Series B)
R = CH@CH2
C5 (n = 3)
C6 (n = 4)
C7 (n = 5)
C8 (n = 6)
HAT5 Cr 69 Colh 122 I
HAT6 Cr 70 Colh 100 I
HAT7 Cr 69 Colh 93 I
HAT8 Cr 67 Colh 86 I
10 Cr 78 I
11 Cr 77 I
12 Cr 61 I
13 Cr 58 I
5 Cr 41 Colh 69 I
6 Cr 62 Colh 83 I
7 Cr 33 Colh 62 I
8 Cr 47 I
313; (c) Boden, N.; Bushby, R. J.; Cammidge, A. N. Tetrahedron Lett. 1995, 36,
8685–8686; (d) Boden, N.; Bushby, R. J.; Cammidge, A. N.; Duckworth, S.;
Headdock, G. J. Mater. Chem. 1997, 7, 601–605; (e) Boden, N.; Bushby, R. J.;
Cammidge, A. N.; Headdock, G. J. Mater. Chem. 1995, 5, 2275–2281.
Acknowledgement
The authors are grateful for the support received from the
EPSRC Mass Spectrometry Service (Swansea).
12. Boden, N.; Bushby, R. J.; Cammidge, A. N.; Headdock, G. Synthesis 1995, 31–32.
13. Cammidge, A. N.; Beddall, A. R.; Gopee, H. Tetrahedron Lett. 2007, 48, 6700–
6703.
References and notes
14. Triphenylene 10: A mixture of hexahydroxytriphenylene 9 (0.50 g, 1.54 mmol),
1-bromopentane (2.32 g, 15.4 mmol) and potassium carbonate (4 g) was
stirred in refluxing ethanol (30 mL) under nitrogen for 24 h. Aqueous work-
up followed by column chromatography (silica gel, CH2Cl2/petroleum ether)
and recrystallisation (ethanol) gave 10 as a colourless solid. Yield 65 mg, 5%;
Mp 78 °C; 1H NMR (400 MHz, CDCl3): 0.73–0.99 (m, 21H), 1.33–1.73 (m, 28H),
1.83–2.08 (m, 14H), 3.18 (t, J = 7.8, 2H), 4.03 (t, J = 6.5, 2H), 4.14 (t, J = 6.6, 2H),
4.19–4.24 (m, 8H), 7.73 (s, 1H), 7.77 (s, 1H), 7.79 (s, 1H), 7.80 (s, 1H), 7.88 (s,
1H); 13C NMR (75 MHz, CDCl3): 13.9, 14.0, 19.5, 22.4, 22.5, 22.7, 28.2, 28.3,
28.4, 28.9, 29.0, 29.1, 29.2, 29.6, 29.9, 30.1, 31.1, 31.8, 32.6, 32.9, 68.4, 69.1,
69.4, 69.6, 69.8, 72.8, 104.0, 106.8, 107.5, 108.2, 112.1, 123.9, 124.3, 124.5,
124.6, 126.9, 132.8, 147.4, 147.5, 147.9, 148.8, 149.3, 150.6; HR-MS (EI): calcd
for C53H82O6 [M+] 814.6106; found 814.6101.
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Chem. Commun. 2002, 966–967; (g) Cammidge, A. N.; Gopee, H. Mol. Cryst. Liq.
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Triphenylene 11: Prepared as above using hexahydroxytriphenylene 9 (0.50 g,
1.54 mmol) and 1-bromohexane (2.54 g, 15.4 mmol) to give 11 as a colourless
solid. Yield 127 mg, 9%; Mp 77 °C; 1H NMR (400 MHz, CDCl3): 0.83–0.95 (m,
21H), 1.25–1.57 (m, 42H), 1.82–2.08 (m, 14H), 3.17 (t, J = 7.7, 2H), 4.03 (t,
J = 6.5, 2H), 4.14 (t, J = 6.6, 2H), 4.18–4.24 (m, 8H), 7.73 (s, 1H), 7.77 (s, 1H), 7.79
(s, 1H), 7.80 (s, 1H), 7.88 (s, 1H); 13C NMR (75 MHz, CDCl3): 13.0, 13.1, 13.2,
21.6, 21.7, 21.8, 21.9, 24.8, 24.9, 25.0, 25.1, 28.4, 28.5, 28.7, 29.5, 29.6, 30.2,
30.7, 30.9, 31.1, 67.6, 68.3, 68.6, 68.8, 69.0, 103.2, 106.0, 106.7, 107.4, 111.2,
123.1, 123.5, 123.7, 123.8, 126.0, 131.9, 146.6, 146.7, 147.1, 148.0, 148.5,
149.8; HR-MS (EI): calcd for C60H96O6 [M+] 912.7201; found 912.7206.
Triphenylene 12: Prepared as above using hexahydroxytriphenylene 9 (0.50 g,
1.54 mmol) and 1-bromoheptane (2.76 g, 15.4 mmol) to give 12 as a colourless
solid. Yield 127 mg, 9%; Mp 61 °C; 1H NMR (400 MHz, CDCl3): 0.81–0.92 (m,
21H), 1.25–1.62 (m, 56H), 1.82–2.08 (m, 14H), 3.17 (t, J = 7.6, 2H), 4.03 (t,
J = 6.5, 2H), 4.13 (t, J = 6.7, 2H), 4.18–4.24 (m, 8H), 7.73 (s, 1H), 7.77 (s, 1H), 7.79
(s, 1H), 7.80 (s, 1H), 7.88 (s, 1H); 13C NMR (75 MHz, CDCl3):14.1, 22.7, 22.8,
22.9, 26.1, 26.2, 26.3, 26.4, 29.2, 29.4, 29.5, 29.6, 29.7, 29.8, 30.6, 30.7, 31.0,
31.3, 31.9, 32.0, 32.1, 68.6, 69.3, 69.6, 69.8, 70.0, 104.3, 107.1, 107.8, 108.5,
112.3, 124.1, 124.5, 124.7, 124.8, 127.1, 133.0, 147.7, 147.8, 148.2, 149.1, 149.6,
150.8; HR-MS (EI): calcd for C67H110O6 [M+] 1010.8297; found 1010.8285.
Triphenylene 13: Prepared as above using hexahydroxytriphenylene 9 (0.50 g,
1.54 mmol) and 1-bromooctane (2.97 g, 15.4 mmol) to give 13 as a colourless
solid. Yield 120 mg, 9%; Mp 58 °C; 1H NMR (400 MHz, CDCl3): 0.83–0.90 (m,
21H), 1.25–1.62 (m, 70H), 1.82–2.07 (m, 14H), 3.17 (t, J = 7.6, 2H), 4.03 (t,
J = 6.6, 2H), 4.13 (t, J = 6.5, 2H), 4.18–4.24 (m, 8H), 7.73 (s, 1H), 7.77 (s, 1H), 7.79
(s, 1H), 7.79 (s, 1H), 7.87 (s, 1H); 13C NMR (75 MHz, CDCl3):14.1, 22.7, 22.8,
26.2, 26.3, 26.4, 29.4, 29.5, 29.6, 29.7, 29.8, 29.9, 30.6, 30.7, 31.1, 31.3, 31.9,
32.1, 68.6, 69.3, 69.6, 69.8, 70.0, 73.1, 104.2, 107.0, 107.8, 108.5, 112.2, 124.2,
124.5, 124.7, 124.8, 127.1, 133.0, 147.7, 147.8, 148.1, 149.0, 149.5, 150.8; HR-
MS (EI): calcd for C74H124O6 [M+] 1108.9392; found 1108.9382.
5. Cammidge, A. N. Philos. Trans. R. Soc. A 2006, 364, 2697–2708.
6. Mesophase formation has recently been further correlated with the electron-
donating and -withdrawing nature of the attached substituents. However, in all
examples, the general correlation persists—groups which extend the p-core are
compatible with sustaining mesophase formation. Foster, E. J.; Jones, R. B.;
Lavigueur, C.; Williams, V. E. J. Am. Chem. Soc. 2006, 128, 8569–8574.
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1995, 5, 1857–1860.
9. Henderson, P.; Kumar, S.; Rego, J. A.; Ringsdorf, H.; Schuhmacher, P. J. Chem.
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953–963.