Oligothiophenes. Part 4: Formation and characterization of stable radicals from novel bis- and tris-bithiophene methanes

Article abstract of DOI:10.1016/j.tetlet.2006.11.028

Two star-shaped structures of the bis- and tris-thiophene methane types have been synthesized in a facile two-step synthesis. The resulting bis- and tris-thiophene methanes readily react with oxygen both as solids and in solution to yield stable radicals.

Full text of DOI:10.1016/j.tetlet.2006.11.028

Tetrahedron Letters 48 (2007) 281–283
Oligothiophenes. Part 4: Formation and characterization of
stable radicals from novel bis- and tris-bithiophene methanes
Harald Halvorsen,a Jan Skramstada and Morten Sørlieb,*
aDepartment of Chemistry, University of Oslo, PO Box 1033 Blindern, N-0315 Oslo, Norway
˚
Department of Chemistry, Biotechnology, and Food Science, Norwegian University of Life Sciences, As, Norway
b
Received 1 September 2006; revised 1 November 2006; accepted 5 November 2006
Available online 28 November 2006
Abstract—Two star-shaped structures of the bis- and tris-thiophene methane types have been synthesized in a facile two-step syn-
thesis. The resulting bis- and tris-thiophene methanes readily react with oxygen both as solids and in solution to yield stable radicals.
The formed radicals, which were investigated using electron paramagnetic resonance (EPR), were found to be residing on the central
carbons of the bis- and tris-thiophene methanes.
Ó 2006 Elsevier Ltd. All rights reserved.
Oligothiophenes continue to play an important role in
the development of new electronic and optical materials.
More than 200 papers were published last year in which
oligothiophenes were investigated for these purposes.1
The structures studied included both variously substi-
tuted linearly connected thiophenes2,3 as well as star-
shaped forms.4 The latter structures have shown a
great potential in the construction of nonlinear optical
applications.5
Stille reaction between I and tributylphenyl stannane
gave a 70% yield of bis(2,20-bithiophene-5-yl)-5-phen-
yl-2-thienyl methane III as a white solid melting at
91–92 °C (Scheme 1).7 Both reactions were carried out
under an atmosphere of argon to exclude oxygen from
the reaction mixtures. When the reactions were ended,
a saturated solution of potassium fluoride in methanol
was added to the residue, still under argon. The mixture
was stirred at an ambient temperature for 1 h and evapo-
rated with a small amount of silica gel. The residue was
eluted with degassed diethylether:hexane (8:1) on silica
gel using flash chromatography with argon as the pres-
surizing gas. As long as oxygen is excluded, compounds
II and III are stable as colourless solutions or as white
solids. When exposed to air, however, they turned dark
green. We believed this was due to oxidation to the cor-
responding radicals IIÅ and IIIÅ, which were confirmed
by EPR measurements (passage below). When com-
pounds II and III were incubated in water in the pres-
ence of oxygen, excess of potassium iodide, and starch,
the solution turned blue due to the formation of a
starch–iodine complex indicating that the production
of peroxide has oxidized iodide to iodine.
In this letter we report the synthesis of two star-shaped
structures of the bis- and tris-thiophene methane types
that show unexpected radical properties.
On reacting the readily available 2,20-bithiophene and 5-
bromo-thiophene-2-carbaldehyde with perchloric acid in
ethylene glycol at 50 °C, bis(2,20-bithiophene-5-yl)-5-
bromo-2-thienyl methane (I) was produced in a quanti-
tative yield.6
A Stille coupling of I with tributyl-2-thienyl stannane in
dry dimethyl formamide at 100 °C in the presence of
bis(triphenylphosphine)palladium chloride afforded
tris(2,20-bithiophene-5-yl) methane (II) as a white solid
melting at 150–151 °C in a 70% yield.7 Compound II
has previously been obtained by Krishnaswamy et al.
as a product of a photochemical reaction.8 Similarly, a
The EPR experiments were carried out by dissolving the
air-exposed compounds in toluene and keeping the solu-
tions anaerobic in gas-tight EPR tubes before spectra
were recorded at room temperature. The concentrations
of the observed radicals were kept at 50 lM to avoid
spin–spin interactions. The double integral of the signals
of IIÅ and IIIÅ were compared to that of a standard solu-
tion of DPPH (diphenylpicrylhydracyl).
Keywords: Thiophenes; Stille coupling; EPR.
*
Corresponding author. Tel.: +47 64965902; fax: +47 64965901;
e-mail: morten.sorlie@umb.no
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.11.028

282
H. Halvorsen et al. / Tetrahedron Letters 48 (2007) 281–283
2
+
Br
CHO
S
S
S
50 oC
HClO4/(CH2OH)2
S
S
S
S
S
Br
I
SnBu3
S
SnBu3
PdCl2(PPh3)2
DMF
PdCl2(PPh3)2
DMF
S
S
S
S
S
S
S
S
S
S
S
II
III
Scheme 1.
The experimental EPR spectrum of IIÅ (Fig. 1) shows an
isotropic signal with a g-factor of 2.0038. A satisfactory
simulated fit (WINEPR SimFonia) to the experimental
spectrum is obtained when the unpaired electron is cou-
pled with three protons with a hyperfine splitting con-
stant of 3.6 G, and with three protons with a hyperfine
splitting constant of 0.91 G, and furthermore with three
protons with a hyperfine splitting of 0.85 G, and a line
width of 0.77 G. Such a coupling pattern is consistent
with the C3-symmetry of compound IIÅ. Additional
hyperfine couplings cannot be resolved because their
magnitude will be smaller than the line width of 0.6 G
of the major peaks, whose presence are suggested due
to the broadness of the EPR signal.
These tris-thienylmethyl radicals are heterocyclic ana-
logs of the classical triarylmethyl radicals. The unsubsti-
tuted triphenylmethyl radical has a g-factor of 2.00266,9
The experimental EPR spectrum of IIIÅ (Fig. 2) shows
an isotropic signal with an increased g-factor of
2.0044. A satisfactory fit to the experimental spectrum
was simulated by coupling the unpaired electron with
three protons with a hyperfine splitting constant of
3.5 G. The presence of a phenyl group in III lowers
the symmetry compared to II. Other hyperfine inter-
actions are therefore not obtained. Their presence
are, however, suggested due to the broadness of the
signals.
3430
3440
3450
Magnetic Field (Gauss)
Figure 1. Depicted are simulated (top) and experimental (bottom)
EPR spectra of IIÅ. Instrumental settings: frequency, 9.65 GHz;
microwave power, 2.0 lW; sweep time, 82 s; modulation frequency,
100 kHz; modulation amplitude, 0.5 G.

H. Halvorsen et al. / Tetrahedron Letters 48 (2007) 281–283
283
Acknowledgments
We are grateful for the use of the Bruker ESP300E EPR
spectrometer at the Department of Physics, University
of Oslo, and to Dr. Matthias Kolberg and Professor
Yngve Stenstro¨m for helpful discussions.
References and notes
1. A SciFinder search with ‘oligothiophene’ as search crite-
rion resulted in 202 references for the period January–
December 2005.
2. Kanemoto, K.; Kato, T.; Aso, Y.; Otsubo, T. Phys. Rev. B
2003, 68, 092302.
3. Otsubo, T.; Aso, Y.; Takimiya, K.; Nakanishi, H.; Sumi,
N. Synthetic Metals 2003, 133–134, 325–328.
3430
3440
3450
Magnetic Field (Gauss)
4. Pei, J.; Wang, J.-L.; Cao, X.-Y.; Zhou, X.-H.; Zhang,
W.-B. J. Am. Chem. Soc. 2003, 125, 9944–9945.
5. Brasselet, S.; Cherioux, F.; Audebert, P.; Zyss, J. Chem.
Mater. 1999, 11, 1915–1920.
6. Halvorsen, H.; Hope, H.; Skramstad, J.; Oligothiophenes,
I. A. Syn. Commun. 2002, 32, 909–916.
Figure 2. Depicted are simulated (top) and experimental (bottom)
EPR spectra of IIIÅ. Instrumental settings: microwave power, 2.0 lW,
sweep time, 82 s, modulation frequency, 100 kHz, modulation ampli-
tude, 0.5 G.
7. Coupling of bis(2,20-bithienyl)-5-bromo-thienyl methane
with organostannanes: A mixture of bis(2,20-bithienyl)-5-
bromo-thienyl methane (506 mg, 1 mmol), bis(triphenyl-
phosphine)palladium(II) chloride (35 mg, 0.05 mmol) and
(1.5 mmol organostannanes) in dry DMF(5 ml) was
heated up to 100 °C under Ar for 14 h. All of the
organostannanes were commercially available.
close to that of the free electron (2.0023). Even the more
similar tris-biphenylmethyl radical has been investigated
using electron nuclear double resonance (ENDOR) with
respect to assigning hyperfine coupling constants.10 In
this work, the g-factor was not reported. Substitution
with heavy halogens results in an increase in the g-factor
to 2.0034 for tris(2,4,6-trichlorophenyl)methyl11 and to
2.0038 for 4-iodotetradecachlorotriphenylmethyl.12 In
the same manner, the presence of sulfur in IIÅ and IIIÅ
increase the g-factor from that of the free electron to
2.0038 and 2.0044, respectively. The same trend is ob-
served for the perhalogenated tris-thienylmethyl ana-
logues tris(3,4,5-trichloro-2-thienyl)methyl (2.0037) and
tris(3,4,5-trichloro-3-thienyl)methyl (2.0037).13 If the
unpaired electrons in IIÅ and IIIÅ were located directly
at one of the sulfur atoms of the thiophene rings, the
EPR signals would most likely have been either axial
or rhombic, rather than isotropic, as seen for other
organosulfur compounds.14–17
Tris(2,20-bithiophene-5-yl) methane (II) dH (300 MHz,
CD2Cl2) 6.02 (1H, s), 6.92–6.94 (3H, m), 7.00–7.02 (3H,
m), 7.06 (3H, d, J 3.68), 7.14–7.16 (3H, m), 7.22–7.24 (3H,
m). dC (75 MHz, CD2Cl2) 43.3, 123.5, 124.1, 124.9, 127.2,
128.2, 137.5 (2C), 145.8. m/z 508 (H+, 100), 475, 381. HR-
EIMS: Anal. calcd for C25H16S6, m/z 507.957631. Found
507.956104. Mp 150–151 °C.
Bis(2,20-bithiophene-5-yl)-5-phenyl-2-thienyl methane (III):
dH (300 MHz, acetone-d6) 6.05 (1H, s), 6.93–6.94 (2H, m),
6.99–7.02 (3H, m), 7.07 (2H, d, J 3.7), 7.15–7.16 (2H, m),
7.21–7.24 (3H, m), 7.26–7.39 (3H, m), 7.57–7.60 (2H, m).
dC (75 MHz, acetone-d6) 43.5, 123.0, 123.5, 124.1, 124.9,
125.9, 127.2, 127.5, 127.9, 128.2, 129.3, 134.5, 137.4, 137.5,
144.3, 146.0, 146.1. m/z 502 (100, M+), 336 (38.7). HR-
EIMS: Anal. calcd for C27H18S5 502.001210. Found
502.000199. Mp 91–92 °C.
There are several very interesting aspects of radicals IIÅ
and IIIÅ, in particular the ease of their formation and
their stability. In published syntheses, the reactions of
the parent carbinols with trifluoromethanesulfonic
acid/tetrabutylammonium iodide and perchloro acid/
zinc powder have been used to form halogenated radi-
cals tris(3,4,5-trichloro-2-thienyl)methyl and tris(2,4,5-
trichloro-3-thienyl)methyl and the parent tris(thien-
yl)methyl radicals, respectively.13 While the published
paper does not discuss the stability of the latter radicals,
the former are reported to be stable between 10 and a
few hours in solution. Compounds IIÅ and IIIÅ are
formed simply by an exposure to air either in solution
or as solids. When kept as solids, the radicals appear
to be stable even at room temperature. In toluene, the
radicals are stable over a period of weeks as evidenced
by EPR measurements.
8. Krishnaswamy, N. R.; Kumar, C. S. S. R.; Prasanna, S. R.
J. Chem. Res. Syn. 1991, 7, 166.
9. Sinclair, J.; Kivelson, D. J. Am. Chem. Soc. 1968, 90,
5074–5080.
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T. J. Am. Chem. Soc. 1968, 90, 4224–4231.
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˜
Molins, E.; Ruis, J.; Miravitlles, C.; Olivella, S.; Brichfeus,
J. J. Phys. Chem. 1987, 91, 5608–5616.
´
12. Ballester, M.; Castaner, J.; Riera, J.; Ibanez, A.; Pujadas,
˜
˜
J. J. Org. Chem. 1982, 47, 259–264.
13. Gronowitz, S.; Temciuc, M.; Eberson, L. J. Heterocyclic
Chem. 1995, 32, 65–67.
14. Box, H. C.; Freund, H. G.; Budzinski, E. E. J. Chem.
Phys. 1966, 88, 658–661.
15. Bonazzola, L.; Fackir, L.; Leray, N.; Roncin, J. Radiat.
Res. 1984, 97, 462–467.
16. Hadley, J. H., Jr.; Gordy, W. Proc. Natl. Acad. Sci. USA
1977, 74, 216–220.
17. Lassmann, G.; Kolberg, M.; Bleifuss, G.; Gra¨slund, A.;
Sjo¨berg, B.-M.; Lubitz, W. Phys. Chem. Chem. Phys. 2003,
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Further work is underway to establish the reactivity
pattern and other properties of these new radicals.