Facile synthetic procedure for and electrochemical properties of hexa(2-thienyl)benzenes directed toward electroactive materials

Article abstract of DOI:10.1016/j.tetlet.2008.02.069

In the presence of RhCl₃·3H₂O and i-Pr₂NEt, the cyclotrimerization of di(2-thienyl)acetylenes proceeded smoothly to afford hexa(2-thienyl)benzenes. CV analysis of the hexa(2-thienyl)benzenes showed that they may be useful

Full text of DOI:10.1016/j.tetlet.2008.02.069

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 2363–2365
Facile synthetic procedure for and electrochemical properties of
hexa(2-thienyl)benzenes directed toward electroactive materials
Kenta Yoshida, Ichiro Morimoto, Koichi Mitsudo *, Hideo Tanaka *
Division of Chemistry and Biochemistry, Graduate School of Natural Science and Technology, Okayama University,
3-1-1 Tsushima-Naka, Okayama 700-8530, Japan
Received 16 January 2008; revised 12 February 2008; accepted 14 February 2008
Available online 19 February 2008
Abstract
In the presence of RhCl3Á3H2O and i-Pr2NEt, the cyclotrimerization of di(2-thienyl)acetylenes proceeded smoothly to afford hexa-
(2-thienyl)benzenes. CV analysis of the hexa(2-thienyl)benzenes showed that they may be useful as electroactive materials.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Rh/amine catalyst; Cyclotrimerization; Hexa(2-thienyl)benzene; Electroactive material
Over the past decade, extended p-conjugated com-
pounds1 have been studied for the use as materials in
organic electroluminescent devices2,3 and energy storage
devices.4,5 Recently, two-dimensional aromatic cores, such
as starburst hexaarylbenzene derivatives, have been synthe-
sized, and their electrochemical and photochemical proper-
ties have been intensively studied.6 However, there have
been only a few reports on the construction of hexahetero-
arylbenzenes, such as hexa(2-thienyl)benzene derivatives,
because they are difficult to synthesize. One way to con-
struct hexathienylbenzene derivatives is Stille-type coupling
of hexabromobenzene and thienylstannane, wherein the
reaction should use a large amount of toxic stannanes.7
Another way is transition metals-catalyzed cyclotrimeri-
zation of dithienylacetylenes.8 However, the trimerization
of internal alkynes bearing heteroaryl groups is inhibited
by the steric hindrance of products and the coordination
of hetero-atoms to the central metal of the catalyst. For
instance, Weber and co-workers reported that the reaction
of di-2-thienylacetylene catalyzed by RuH2(CO)(PPh3)3
gave a dimerized product, a benzothiophene derivative,
as the major product (63%) and hexa(2-thienyl)benzene
was obtained in only 5% yield.8a One approach to solving
this problem is to introduce large substituents at the 5-posi-
tion of thiophene moieties. Mullen and co-workers
¨
reported that Co2(CO)8 catalyzed the cyclization of di(5-
n-C12H25-thiophen-2-yl)acetylene to give hexathienyl-
benzene derivatives in 61% yield.8b To our knowledge,
there are no other reports on the efficient construction of
hexathienylbenzene derivatives, although they should be
novel, intriguing building blocks for not only electroactive
materials but also photo-materials.
Recently, we found that the cyclotrimerization of
internal alkynes proceeds efficiently in the presence of the
RhCl3/i-Pr2NEt catalyst.9 These successful results
prompted us to investigate the application of our methods
to the synthesis of hexathienylbenzene derivatives. We
report here the RhCl3/i-Pr2NEt-catalyzed cyclotrimeri-
zation of di(2-thienyl)acetylenes, and the electrochemical
properties of the resulting starburst-type benzene
derivatives.
First, we performed the trimerization of di(2-thi-
enyl)acetylene 1a (Scheme 1). In the presence of RhCl3Á
3H2O (8 mol %) and i-Pr2NEt (30 mol %), a solution of
di(2-thienyl)acetylene (1a) in toluene was heated to reflux
*
Corresponding authors. Tel.: +81 86 251 8072; fax: +81 86 251 8079
(H.T.).
E-mail addresses: mitsudo@cc.okayama-u.ac.jp (K. Mitsudo),
tanaka95@cc.okayama-u.ac.jp (H. Tanaka).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.02.069

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K. Yoshida et al. / Tetrahedron Letters 49 (2008) 2363–2365
Table 2
S
RhCl3/i-Pr2NEt-catalyzed cyclotrimerization of 1
S
RhCl3·3H2O (8 mol %)
Ar
i
-Pr2NEt (30 mol %)
S
RhCl3·3H2O (8 mol %)
i-Pr2NEt (30 mol %)
Ar
Ar
Ar
Ar
S
reflux, 24 h
S
S
Ar
Ar
14% (solvent = toluene)
49% (solvent = -PrOH)
1a
S
solvent, reflux, 24 h
S
i
1
Ar
2a
Scheme 1. RhCl3/i-Pr2NEt-catalyzed cyclotrimerization of 1a.
2
R
S
Ar
Yielda (%)
Recov. 1a (%)
for 24 h to afford hexa(2-thienyl)benzene (2a)10 in 14%
yield and 44% of 1a was recovered. Although the dimeriza-
tion did not take place, as we expected, the yield of 2a was
unsatisfactory. The low reactivity might be attributable to
the coordination of a thienyl group to the Rh center. To
suppress this coordination, we next used i-PrOH as a
solvent, and the yield of 2a increased to 49%.
Entry
1
R
Solvent
2
1
2
3
4
a
1b
1b
1c
1c
Me Toluene 2b 63
Me i-PrOH 2b 50
Ac Toluene 2c
—
18
20
—
4
Ac i-PrOH 2c 50
Isolated yield.
In a similar manner, we performed the cyclotrimeri-
zation of di(2-thienyl)acetylenes bearing substituents on
the 5-position of their thienyl groups (Table 2). In toluene
and i-PrOH, the cyclotrimerization of alkyne 1b, bearing a
5-methylthienyl group, gave the cycloadduct in respective
yields of 63% and 50% (entries 1 and 2).11 Notably, the
reactivity of 1b in toluene was similar to that in i-PrOH,
which is different from that of 1a, probably because the
methyl group on a-position of thienyl group might reduce
the coordination ability of the thienyl group. Indeed, in
toluene, the cyclotrimerization of alkyne 1c, bearing a 5-
acetylthienyl group which also can coordinate to the Rh
center, gave cycloadduct 2c12 in only 4% yield and 1c was
recovered in 20% yield (entry 3). With i-PrOH as a solvent,
the yield of 2c dramatically increased to 50% (entry 4).
These results suggest that RhCl3/i-Pr2NEt catalyst might
be more active in toluene than i-PrOH, but i-PrOH would
suppress the coordination of a thienyl group to the Rh
center.
To evaluate the catalytic activity of RhCl3/i-Pr2NEt, the
cyclotrimerization of 1a was carried out using several cata-
lysts (Table 1). Notably, the reaction using RhCl3Á3H2O in
i-PrOH showed higher reactivity than with other catalysts,
which are frequently used for the trimerization reaction of
acetylene derivatives (entry 1). With toluene or 1,4-dioxane
as a solvent, the yield of 2a decreased (entries 2 and 3).
When the reaction was carried out using RhCl(PPh3)3 (Wil-
kinson’s catalyst) in i-PrOH or toluene, 2a was obtained in
respective yields of only 5% and 26% (entries 4 and 5).
[Rh(cod)2][BF4] (cationic catalyst) was ineffective, and
starting material 1a was recovered (entries 6 and 7). When
Co2(CO)8 was used in i-PrOH, the corresponding product
was not obtained at all (entry 8). With 1,4-dioxane as a sol-
vent, the corresponding product was obtained in 37% yield
(entry 9). It is likely that RhCl3/i-Pr2NEt catalyst might be
electron-rich due to the coordination of i-Pr2NEt, and
could promote the efficient formation of metallacycle
intermediates.
Next, we subjected 2a to cyclic voltammetry (CV) mea-
surements (Fig. 1). The growth of redox waves was
observed in the potential range from 0.2 to 0.8 V during
the sequential potential cycling (Â100), which suggested
the formation of an electroactive film on the surface of
the working electrode. In fact, a film was observed on the
surface of the electrode. This suggests that the extension
Table 1
Cyclotrimerization of 1a using several catalysts
Ar
Ar
Ar
Ar
Ar
catalyst (8 mol %)
solvent, reflux
Ar
Ar
1a
Ar
Entry Catalyst
Ar
S
6
4
2
2a
Solvent
Time Yielda Recov. 1aa
(h)
(%)
(%)
1
2
3
4
5
6
7
8
9
RhCl3Á3H2O/i-Pr2NEt i-PrOH
24
49
14
11
5
26
—
—
—
37
49
44
87
73
54
68
71
—
—
RhCl3Á3H2O/i-Pr2NEt Toluene 24
RhCl3Á3H2O/i-Pr2NEt Dioxane 24
1cycle
10cycle
20cycle
50cycle
0
RhCl(PPh3)3
RhCl(PPh3)3
[Rh(cod)2][BF4]
[Rh(cod)2][BF4]
Co2(CO)8
i-PrOH
24
Toluene 48
i-PrOH 24
Toluene 48
i-PrOH 24
Dioxane 24
100cycle
-2
-0.5
0.0
0.5
1.0
Potential / V vs. Ag/Ag+
Co2(CO)8
Fig. 1. Cyclic voltammograms of 2a (10 mM) in TEABF4/PC (1 M)
solution. Scan rate: 100 mV sÀ1
a
Isolated yield.
.

K. Yoshida et al. / Tetrahedron Letters 49 (2008) 2363–2365
2365
4.0
2.0
References and notes
´
1. For a review, see: Baumgatner, T.; Reau, R. Chem. Rev. 2006, 106,
4681–4727.
0.0
2. For a review, see: Dini, D. Chem. Mater. 2005, 17, 1933–1945.
3. (a) Carpi, F.; Rossi, D. D. Opt. Laser Technol. 2006, 38, 292–305; (b)
Assaka, A. M.; Rodrigues, P. C.; Oliveira, A. R. M.; Ding, L.; Hu, B.;
Karasz, F. E.; Akcelrud, L. Polymer 2004, 45, 7071–7081.
4. For reviews, see: (a) Wohlgenanut, M.; Vardeny, Z. V. J. Phys.:
-2.0
-4.0
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
+
Potential / V vs. Ag/Ag
Condens. Matter 2003, 15, R83–R107; (b) Abruna, H. D.; Matsu-
˜
moto, F.; Cohen, J. L.; Jin, J.; Roychowdhury, C.; Prochaska, M.;
van Dover, R. B.; DiSalvo, F. J.; Kiya, Y.; Henderson, J. C.;
Hutchison, G. R. Bull. Chem. Soc. Jpn. 2007, 80, 1843–1855.
5. (a) Suematsu, S.; Mitsudo, K.; Katagiri, F.; Tanaka, H. Electro-
chemistry 2007, 75, 54–57; (b) Chen, T.; Wang, L.; Jiang, G.; Wang,
W.; Wang, X. j.; Zhou, J.; Wang, J.; Chen, C.; Wang, W.; Gao, H. J.
Electroanal. Chem. 2006, 586, 122–127; (c) Coppo, P.; Turner, M. L.
J. Mater. Chem. 2005, 15, 1123–1133.
6. (a) Chebny, V. J.; Shukla, R.; Rathore, R. J. Phys. Chem. A 2006,
110, 13003–13006; (b) Rosokha, S. V.; Neretin, I. S.; Kochi, J. K.
J. Am. Chem. Soc. 2006, 128, 9394–9407; (c) Mamane, V.; Gref, A.;
Lefloch, F.; Riant, O. J. Organomet. Chem. 2001, 637, 84–88.
7. Wu, I.-Y.; Lin, J. T.; Tao, Y.-T.; Balasubramaniam, E. Adv. Mater.
2000, 12, 668–669.
Fig. 2. CVs of 2a-based film formed after CVs in Fig. 1. Electrolyte: 2a
(10 mM) in TEABF4/PC (1 M) solution. Scan rate: 100 mV sÀ1. Number
of cycling: 5th cycle.
of p-conjugation of 2a might occur during the electrooxida-
tion. Next, to investigate the main coupling position of a
2a-based film, we measured the CV of 2b bearing a methyl
group at the 5-position of the thienyl groups. In CV, no
significant increase in redox waves was observed, which
suggests that electrooligomerization might occur at the
5-position of the thienyl groups. No film was observed on
the working electrode during the electrooxidation of 2b.
The generated film was then subjected to CV analysis
(Fig. 2). CV of the film showed two distinct redox responses:
one in the potential range from 0.2 to 0.8 V (p-doping), and
the other from À1.5 V to À2.8 V (n-doping), which are sim-
ilar to those of frequently used polythiophene derivatives
reported by Ferraris and co-workers.13 The maximum
potential difference between redox waves of n- and p-doping
was 3.5 V, which indicated a 3.5 eV band gap.
In summary, a simple method for constructing hexathie-
nylbenzenes has been developed, and their fundamental
electrochemical properties have been clarified. We found
that hexa(2-thienyl)benzene (2a) generated films by sequen-
tial potential cycling (CV). Though the exact structure of
the film has not been clear yet, it should be a novel type
polymer or oligomer containing thiophene linked at 2
and 5 positions. In addition, these 2a-based films may be
a candidate for the electroactive materials in energy storage
devices. Further studies on hexaheteroarylbenzenes are
underway in our laboratory.
8. (a) Lu, P.; Cai, G.; Li, J.; Weber, W. P. J. Heterocycl. Chem. 2002, 39,
91–92; (b) Geng, Y.; Fechtenko¨tter, A.; Mullen, K. J. Mater. Chem.
¨
2001, 11, 1634–1641.
9. Yoshida, K.; Morimoto, I.; Mitsudo, K.; Tanaka, H. Chem. Lett.
2007, 36, 998–999.
10. General procedure for Rh/amine-catalyzed cyclotrimerization of alkyne
1: To a solution of RhCl3Á3H2O (11 mg, 0.04 mmol) in i-PrOH
(3.0 mL) were added i-Pr2NEt (26 lL, 0.15 mmol) and di(2-thi-
enyl)acetylene 1a (96 mg, 0.50 mmol). The mixture was stirred at
reflux for 24 h. After being cooled to room temperature, the reaction
mixture was filtered and concentrated under reduced pressure. The
residue was purified by column chromatography on silica gel (hexane/
toluene 5:1) to afford hexa(2-thienyl)benzene 2a (47 mg, 49%) as
yellow solids: Rf = 0.27 (hexane/toluene 5:1); 1H NMR (600 MHz,
CDCl3) d 6.59 (dd, J = 3.6, 1.2 Hz, 6H), 6.68 (dd, J = 5.4, 3.6 Hz,
6H), 7.08 (dd, J = 5.4, 1.2 Hz, 6H); 13C NMR (150 MHz, CDCl3) d
125.8, 126.2, 129.1, 137.0, 140.7; IR (KBr) 3068, 2923, 2360, 1647,
1381, 694 cmÀ1; Anal. Calcd for C30H18S6: C, 63.12; H, 3.18. Found:
C, 63.08; H, 3.36.
11. Hexakis(5-methyl-2-thienyl)benzene (2b): Yellow solids; Rf = 0.23
(hexane/toluene 5:1); 1H NMR (600 MHz, CDCl3) d 6.33 (s, 12H),
2.30 (s, 18H); 13C NMR (150 MHz, CDCl3) d 15.2, 123.9, 128.7,
137.0, 138.8, 140.2; IR (KBr) 3068, 2912, 2855, 2357, 1747, 1442,
1219, 800 cmÀ1; Anal. Calcd for C36H30S6: C, 66.01; H, 4.62. Found:
C, 66.09; H, 4.53.
Acknowledgment
12. Hexakis(5-acetyl-2-thienyl)benzene (2c): Colorless solids; Rf = 0.07
(hexane/EtOAc 3:1), 1H NMR (600 MHz, CDCl3)
d 7.27 (d,
We thank the SC-NMR Laboratory of Okayama
1
J = 3.6 Hz, 6H), 6.67 (d, J = 3.6 Hz, 6H), 2.43 (s, 18H); 13C NMR
(150 MHz, CDCl3) d 26.7, 130.9, 131.8, 136.5, 145.8, 146.7, 190.7; IR
University for H and 13C NMR analyses.
(KBr) 3080, 1658, 1471, 1381, 1274 cmÀ1
.
Supplementary data
13. (a) Neef, C. J.; Brotherston, I. D.; Ferraris, J. P. Chem. Mater. 1999,
11, 1957–1958; (b) Loveday, D. C.; Hmyene, M.; Ferraris, J. P. Synth.
Met. 1997, 84, 245–246; (c) Guerrero, D. J.; Ren, X.; Ferraris, J. P.
Chem. Mater. 1994, 6, 1437–1443.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2008.02.069.