Synthesis, structure, and properties of 2,5,8,11,14,17-hexamethyltriphenyleno[2,1-b:3,4-b′:6,5-b″:7, 8-b?:10,9-b″″:11,12-b′″″]hexathioph ene

Article abstract of DOI:10.3987/COM-08-S(N)103

2,5,8,11,14,17-Hexamethyltriphenyleno[2,1-b:3,4-b′:6,5-b″:7, 8-b?:10,9-b″″:11,12-b′″″]hexathioph ene has been synthesized by an iron(III-)mediated oxidative cyclization of hexakis(2-methyl-4-thienyl)benzene, and its structure has been determined by an X-r

Full text of DOI:10.3987/COM-08-S(N)103

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HETEROCYCLES, Vol. 76, No. 2, 2008, pp. 1043 - 1048. © The Japan Institute of Heterocyclic Chemistry
Received, 12th April, 2008, Accepted, 28th May, 2008, Published online, 5th June, 2008. COM-08-S(N)103
SYNTHESIS, STRUCTURE, AND PROPERTIES OF
2,5,8,11,14,17-HEXAMETHYLTRIPHENYLENO[2,1-b:3,4-b':6,5-b'':
7,8-b''':10,9-b'''':11,12-b''''']HEXATHIOPHENE
Masaru Endou, Yutaka Ie, and Yoshio Aso*
The Institute of Scientific and Industrial Research, Osaka University, 8-1
Mihogaoka, Ibaraki 567-0047, Japan. E-mail: aso@sanken.osaka-u.ac.jp
Dedicated to Prof. R. Noyori at the occasions of his 70th birthday
Abstract
–
2,5,8,11,14,17-Hexamethyltriphenyleno[2,1-b:3,4-b':6,5-b'':7,8-
b''':10,9-b'''':11,12-b''''']hexathiophene has been synthesized by an iron(III)-
mediated oxidative cyclization of hexakis(2-methyl-4-thienyl)benzene, and its
structure has been determined by an X-ray crystallographic analysis. Electronic
properties were investigated by spectroscopic and electrochemical measurements.
Polycyclic aromatic hydrocarbons (PAHs) represent an important class of fundamental π-systems.1
Triphenylene, one of the simplest framework of PAHs, and its derivatives have been extensively
investigated in terms of their unique structures and properties.2 To further modulate their inherent
structural and electronic features, many benzo derivatives have been synthesized.3 On the other hand,
the incorporation of thiophene rings onto PAHs exhibits considerable success in the design of unique
organic semiconductors.4 In this context, we have previously reported the synthesis, properties, and
S
S
S
S
S
S
S
S
S
1
2
3
Figure 1. Structures of 1, 2, and 3

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bowl-shaped structure of triphenyleno[1,12-bcd:4,5-b'c'd':8,9-b''c''d'']trithiophene (1)5 that is formally
isoelectronic with coronene. Here, we report on the synthesis, structure, and properties of novel
six-thiophene-annelated triphenylene 2, which is isoelectronic with hexabenzotriphenylene 3.6,7
S
S
S
Br
i )
ii )
S
S
S
S
S
S
4
5
S
S
S
S
iii )
5
+
S
S
S
S
S
S
S
S
2
6
Scheme 1. Synthesis of 2,5,8,11,14,17-hexamethyltriphenyleno[2,1-b:3,4-b':6,5-b'':7,8-
b''':10,9-b'''':11,12-b''''']hexathiophene (2)
Conditions
i ) PdCl2(PPh3)2, CuI, ethynyltrimethylsilane, DBU, H2O, toluene, 80 ºC, 95%;
ii) Co2(CO)8, 1,4-dioxane, 140 ºC, 87%;
iii) 1) Fe(ClO4)3·nH2O; 2) H2NNH2·H2O, CHCl3/MeCN, rt, 2: 13%, 6: 3%.
Although most of triphenylene derivatives have been synthesized by cyclotrimerizations of arynes,3 our
synthesis
of
2,5,8,11,14,17-hexamethyltriphenyleno[2,1-b:3,4-b':6,5-b'':7,8-b''':10,9-b'''':11,12-b''''']-
hexathiophene (2) is based on oxidative cyclization8 of hexathienylbenzene (5) as outlined in Scheme 1.
Firstly, bis(2-methyl-4-thienyl)ethyne (4) was prepared by the modified Sonogashira coupling reaction in
95% yield.9 Cobalt-catalyzed cyclotrimerization of 4 gave hexakis(2-methyl-4-thienyl)benzene (5) in
87% yield. Oxidative cyclization of 5 by Fe(ClO4)3·nH2O resulted in the formation of a mixture of 2
and 6. Finally, recrystallization of the mixture from a toluene solution gave single crystals of 2, whose
1
13
structure was unambiguously characterized by H and C NMR spectroscopy and HRMS measurements
as well as an X-ray crystallographic analysis. As shown in Figure 2,10 molecular structure of 2 proves to

HETEROCYCLES, Vol. 76, No. 2, 2008
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be a nonplanar conformation with C2 symmetry, and it exhibits bond alternation between the exo
(1.45–1.47 Å) and endo (1.40–1.41 Å) bonds in the central benzene ring. Similar nonplanar
conformation and bond alternation (exo: 1.44–1.45 Å; endo: 1.39–1.41 Å) have been observed on
hexabenzotriphenylene 3.7
Figure 3 shows UV–vis absorption and fluorescence spectra of 2 and 3 in CH2Cl2. They resemble each
other in shape, reflecting the isoelectronic structures, but the absorption and fluorescence maxima of 2 are
blue-shifted relative to those of 3. The electrochemical properties of 2 and 3 were also investigated by
means of cyclic voltammetry, and their voltammograms are shown in Figure 4. Hexathienotriphenylene
2 showed two reversible oxidation waves with half-wave potentials of +0.59 and +0.98 V vs. Fc/Fc+
couple. The first oxidation potential of 2 is approximately 0.23 V more negative compared with that of
hexabenzotriphenylene 3, indicating that the introduction of thiophene units to triphenylene framework
leads to increase electron-donating nature.11

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In conclusion, we report here
a
new class of heteroaromatic triphenylene derivative,
hexathienotriphenylene 2, prepared via oxidative cyclization of hexathienylbenzene. This study
provides a useful synthetic route for a series of heteroaromatic-extended triphenylenes. Further
investigations on the conformational study of 2 as well as the application to organic electronic devices
will be reported in due course.
ACKNOWLEDGEMENTS
This work was supported by Grants-in-Aids for Scientific Research from the Ministry of Education,
Culture, Sports, Science and Technology, Japan, and the Japan Society for the Promotion of Science. The
authors acknowledge Dr. N. Kanehisa at the Faculty of Engineering, Osaka University, for X-ray
measurement. Thanks are given to the Instrumental Analysis Center, Faculty of Engineering, Osaka
University, for obtaining high-resolution mass spectra.
REFERENCES AND NOTES
1. J. Wu, W. Pisula, and K. Müllen, Chem. Rev., 2007, 107, 718.
2. A. N. Cammidge, Phil. Trans. R. Soc. A, 2006, 364, 2697.
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Rec., 2007, 7, 326.
4. J. E. Anthony, Chem. Rev., 2006, 106, 5028.
5. K. Imamura, K. Takimiya, Y, Aso, and T. Otsubo, Chem. Commun., 1999, 1859.
6. N. P. Hacker, J. F. W. McOmie, J. Meunier-Piret, and M. V. Meerssche, J. Chem. Soc., Perkin Trans.
1, 1982, 19; D. Peña, D. Pérez, E. Guitián, and L. Castedo, Org. Lett., 1999, 1, 1555; D. Peña, A.
Cobas, D. Pérez, E. Guitián, and L. Castedo, Org. Lett., 2000, 2, 1629.
7. L, Barnett, D. M. Ho, K. K. Baldridge, and R. A. Pascal Jr., J. Am. Chem. Soc., 1999, 121, 727; M.
A. Barnett, M. R. Kopp, E. Wenger, and A. C. Willis, J. Organomet. Chem., 2003, 667, 8.
8. J. D. Tovar, A. Rose, and T. M. Swager, J. Am. Chem. Soc., 2002, 124, 7762; J. Pei, W.-Y. Zhang, J.
Mao, and X.-H. Zhou, Tetrahedron Lett., 2006, 47, 1551.
9. M. J. Milo, L. C. Kopel, J. B. Braun, T. L. Gadzikwa, K. L. Hull, R. B. Brisbois, C. J. Markworth,
and P. A. Grieco, Org. Lett., 2002, 4, 3199.
10. Crystal Data for 2: C36H24S6·0.5C7H8 M = 695.02, triclinic, Space Group P-1 (no. 2), a = 8.3928(17)
Å, b = 14.338(2) Å, c = 14.545(3) Å, V = 1663.0(5) Å3, Z = 2, Dcalc = 1.388 g·cm–3, F(000) = 722.00,
µ = 4.404 cm–1 (Mo Kα: 0.71069 Å ), 12226 reflections measured, 7011 unique, R = 0.0787 for I >
3σ (I), wR = 0.1230 for all data. The crystallographic data have been deposited with Cambridge
Crystallographic Data Centre as supplementary publication no. CCDC-687321.

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11. General Information. Column chromatography was performed on silica gel (40–50 µm). TLC
plates were visualized with UV. Preparative gel permeation chromatography (GPC) was performed
on a Japan Analytical Industry LC-918 equipped with JAI-GEL 1H/2H columns. Melting points are
13
uncorrected. 1H NMR and C NMR spectra were recorded on a JEOL JMN-400 spectrometer in
CDCl3 with tetramethylsilane as an internal standard. Data are reported as follows: chemical shift (δ)
in ppm, multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, quint = quintet), coupling constant
(Hz), integration. UV–vis spectra were recorded on a Shimadzu UV-3100PC. Fluorescence spectra
were recorded using a Fluoromax-2 spectrometer in the photo-counting mode equipped with a
Hamamatsu R928 photomultiplier. The bandpass for the emission spectra was 3.0 nm. All spectra
were obtained in spectrograde solvents. The concentrations of solutions were adjusted to yield
absorbance less than 0.1 in the absorption spectrum for any fluorescence experiments. Fluorescence
quantum yield was determined using 9,10-diphenylanthracene (Φf = 0.90, in cyclohexane) as
standard. Cyclic voltammetry was carried out on a BAS CV-50W voltammetric analyzer.
High-resolution mass spectrum (HRMS) was obtained on JEOL JMS-700.
Materials. All reactions were carried out under a nitrogen atmosphere. Solvents of the highest
purity grade were used as received. Unless stated otherwise, all reagents were purchased from
commercial sources and used without purification. 4-Bromo-2-methylthiophene and dibenzo[f,
j]phenanthro[9,10-s]picene (3) were prepared by reported procedure, and their 1H NMR data were in
agreement with those of the previously reported compounds.6,12
Bis(2-methyl-4-thienyl)ethyne (4).
4-Bromo-2-methylthiophene (1.77 g, 10.0 mmol),
ethynyltrimethylsilane (589 mg, 6 mmol), water (88 mg, 4.9 mmol), DBU (9.13 g, 60.0 mmol),
PdCl2(PPh3)2 (181 mg, 0.257 mmol), and CuI (89 mg, 0.47 mmol) were placed in a test tube and
dissolved with toluene (10 mL). The mixture was stirred at 80 ºC for overnight. Hexane was
added to the reaction mixture, and the organic layer was washed with NH4Cl aq and water and dried
over MgSO4. After removal of the solvent under reduced pressure, the residue was purified by
column chromatography on silica gel (hexane) to give 4 (1.03 g, 95%). Colorless plates; mp
98–99 °C; 1H NMR (CDCl3) δ 2.46 (d, 6H, J = 1.1 Hz), 6.81 (quint-like, J = 1.2 Hz, 2H), 7.22 (d,
13
2H, J = 1.4 Hz); C NMR (CDCl3) δ 15.1, 83.7, 121.7, 126.5, 127.6, 139.5; Exact mass Calcd for
C12H10S2: 218.0224, Found: 218.0226.
Hexakis(2-methyl-4-thienyl)benzene (5). A solution of 4 (1.03 g, 4.72 mmol) in dioxane (20 mL)
was degassed by Ar bubbling. To the solution was added Co2(CO)8 (270 mg, 0.79 mmol) in one
portion. The mixture was stirred at 140 ºC for overnight. After removal of the solvent under
reduced pressure, the residue was purified by column chromatography on silica gel (1:1
hexane/CHCl3) to give 5 (0.90 g, 87%). Pale brown powder; mp >300 °C; 1H NMR (CDCl3) δ 2.24

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(d, 18H, J = 1.0 Hz), 6.12 (quint-like, J = 1.2 Hz, 6H), 6.31 (d, 6H, J = 1.4 Hz); 13C NMR (CDCl3) δ
15.0, 121.8, 128.2, 136.2, 136.6, 140.3; Exact mass Calcd for C36H30S6: 654.0672, Found: 654.0678.
2,5,8,11,14,17-Hexamethyltriphenyleno[2,1-b:3,4-b':6,5-b'':7,8-b''':10,9-b'''':11,12-b''''']-
hexathiophene (2). Hexakis(2-methyl-4-thienyl)benzene (5) (331 mg, 0.506 mmol) were placed in
200 mL round-bottomed flask and dissolved with MeCN (20 mL) and CHCl3 (80 mL). To the
mixture was added Fe(ClO4)3·nH2O (2.16 g, 6.0 mmol) in one portion. The resulting dark blue
solution was stirred at rt for overnight. Hydrazine monohydrate (4 mL) was added, and the organic
layer was washed with NH4Cl aq and water and dried over Na2SO4. After removal of the solvent
under reduced pressure, the residue was passed through silica gel (2:1 hexane/CHCl3).
Recrystallization from toluene gave single crystals of 2 (44 mg, 13%). Purification of the filtrate
by preparative GPC gave 6 (11 mg, 3%). 2: Pale orange crystals; mp >300 °C; 1H NMR (CDCl3) δ
13
2.62 (d, 18H, J = 1.1 Hz), 7.47 (q, 6H, J = 1.1 Hz); C NMR (CDCl3) δ 16.2, 124.2, 126.1, 133.3,
136.7; Exact mass Calcd for C36H24S6: 648.0202, Found: 648.0208; UV/Vis (CH2Cl2) λmaxabs 338 nm
(log ε 5.08), λmaxems 455 nm (Φf 9.0 × 10–3).
6: Pale orange powder; mp >300 °C; 1H NMR (CDCl3) δ 2.42 (d, 12H, J = 1.1 Hz), 2.66 (d, 6H, J =
1.1 Hz), 6.02 (q, 4H, J = 1.1 Hz), 6.83 (m, 2H), 6.98 (d, 2H, J = 2.4 Hz); 13C NMR (CDCl3) δ 15.6,
16.1, 123.5, 125.9, 126.2, 129.2, 129.6, 132.9, 133.2, 134.2, 141.2, 144.3; Exact mass Calcd for
C36H26S6: 650.0359, Found: 650.0356.
12. J. Froehlich, C. Hametner, and W. Kalt, Monatsh. Chem., 1996, 127, 325.