All-S,S-dioxygenated star oligothiophenes

Article abstract of DOI:10.1021/jo201012y

Star-shaped oligothiophenes are promising materials for applications in the organic electronics field. For the first time, a range of star-oligothiophenes was oxidized to the corresponding all-S,S-dioxides by using the HOF·CH₃CN complex. These

Full text of DOI:10.1021/jo201012y

NOTE
pubs.acs.org/joc
all-S,S-Dioxygenated Star Oligothiophenes
Shay Potash and Shlomo Rozen*
School of Chemistry, Tel-Aviv University, Tel-Aviv, Israel 69978
S Supporting Information
b
ABSTRACT: Star-shaped oligothiophenes are promising ma-
terials for applications in the organic electronics field. For the
first time, a range of star-oligothiophenes was oxidized to the
corresponding all-S,S-dioxides by using the HOF CH3CN
3
complex. These materials exhibit considerable thermal stability
and red-shift absorptions in the UV/vis relative to the parent
compounds.
ligothiophenes are widely utilized as building blocks in the
preparation of materials for use as organic semiconductors.1
oxygen atom. It was instrumental in quite a few successful
difficult transformations, including oxygenation of aromatic
rings,12 fast BayerꢀVilliger rearrangements validating Bayer’s
original proposed mechanism,13 converting azides, amines, and
vicinal diamines into the corresponding nitro14 and dinitro15
derivatives, epoxidation of electron-deficient olefins,16 and for-
mation of various N-oxides,17 including the synthesis of the then
elusive 1,10-phenanthroline N,N0-dioxide.18 This oxygen transfer
agent was also used for the oxidation of electron-deficient sulfides
to hard-to-get sulfones19 including glycosyl sulfones,20 episulfones21,
and many other first or difficult transformations.22
O
This is due to their excellent electronic properties and other
features such as chemical resistance and relative ease of synthesis.
As a result, there are many classes of oligothiophenes that serve in
applications such as organic field-effect transistors (OFET),2
organic light emitting diodes (OLEDs),3 and much more.4
A theoretical study predicted that by replacing all sulfur atoms
of polythiophenes with sulfone groups, the HOMOꢀLUMO gap
would be significantly reduced5—a very desirable feature. None of
the orthodox oxidizing agents, however, was able to convert all
sulfur atomsinvarioustypes of oligothiophenes to the correspond-
ing sulfones.6 We began to change the situation by employing the
The above, and the fact that SOTs could not be oxygenated
with traditional oxygen-transfer agents, prompted us to explore
the possibilities of reacting this family of oligothiophenes with
HOF CH3CN complex, and indeed, the HOMOꢀLUMO gap of
3
the conjugated all-S,S-dioxides narrowed considerably.7
the HOF CH3CN complex.
3
Recently, star-shaped oligothiophenes (SOTs) have received
considerable attention, not only because of their aesthetic appeal.
This family constitutes branched oligothiophenes, which are
promising materials for applications such as active layers in
multifunctional organic devices.8 These multi-dimensional
cross-conjugated materials have come to the forefront, both as
monomers in cross-linked semiconducting polymers9 and as
components of conjugated dendrimers.10
Both the synthetic challenge and HOMOꢀLUMO gap re-
duction met with success. We report here a general method for
the preparation of all-S,S-dioxide star-shaped oligothiophenes
using the HOF CH3CN complex.
3
Tris(5-bromo-2-thienyl)methane (1),23 with its three thio-
phene rings attached to a central methane unit, was reacted at
0 °C with 8 molar equiv of HOF CH3CN. Within 5 min, the
3
novel tris(5-bromo-2-thienyl 1,1-dioxide)methane (2) was
formed in 82% yield (Scheme 1). No partly oxidized or other
byproduct, usually associated with long reaction times and high
temperatures, condition characteristic to common oxidant,
was observed. It should be mentioned that reactions with
HOF CH3CN and sulfides always produce sulfones rather
3
Despite the understandable desire to check the properties of
the potential all-S,S-dioxide star-shaped oligothiophenes, it has
never been accomplished simply because none of the relevant
material was known. A few experiments we have conducted with
per-acids and dimethyl dioxirane revealed the reason. None of
the SOTs could be transformed into the all-S,S-dioxo derivative
by m-CPBA or by DMDO even after prolonged reaction times.
The acetonitrile complex of the hypofluorous acid, easily
prepared from diluted fluorine and aqueous acetonitrile,11 de-
rives its oxygen transfer ability from the highly electrophilic
than sulfoxides, unless special conditions are employed (see ref 19).
Similarly, the star-shaped oligothiophene 3a,24 in which three
thiophene units are placed around a central benzene ring, was
reacted with 8 molar equiv of HOF CH3CN for 2 min, affording
3
the previously unknown 1,3,5-tris(5-methyl-2-thienyl 1,1-dioxi-
de)benzene (4a) in 85% yield. In order to determine whether the
Received: May 19, 2011
Published: July 14, 2011
r
2011 American Chemical Society
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|

The Journal of Organic Chemistry
NOTE
Scheme 1. Star Oligothiophene with a Central Methane Unit
Scheme 3. Star Oligothiophene with a Central Thiophene
Unit
Scheme 2. Star Oligothiophene with a Central Benzene Unit
electron-donating methyl groups have something to do with the
ease of the electrophilic oxygenation through increasing basicity
at the sulfur atoms, we replaced all three methyls at the 5,50,500
positions with three electron-withdrawing chlorine atoms and
reacted 1,3,5-tris(5-chloro-2-thienyl)benzene24 (3b) with HOF
CH3CN. This change did not have any negative effect on the
reaction, and 1,3,5-tris(5-chloro-2-thienyl-1,1-dioxide)benzene
4b was obtained after 5 min in 96% yield (Scheme 2).
3
A more complicated star-thiophene system constituted of five
thiophene rings with four of them surrounding a central thiophene
unit. We synthesized the previously unknown 2,3,4,5-tetakis(2-
benzothienyl)thiophene (5) by reacting tetrabromothiophene
(6)25 with 2-tributylstannylbenzo[b]thiophene (7)26 under the
Stille cross-coupling reaction conditions. Reacting this crowded
molecule with 12 equiv of HOF CH3CN gave the symmetrical
3
octaoxide 9a as a sole product in practically quantitative yield. Any
attempt to fully oxidize 5 to its decaoxide derivative 9b by adding
an excess of HOF CH3CN was unsuccessful. Two factors con-
3
tributed to the inertness of 9a. The first is due to the formation of
four benzothiopheneꢀsulfone groups reducing the nucleophilic
character of the central sulfur atom to the extent of preventing an
additional electrophilic oxygen being transferred to it. The second
factor is a considerable sterical hindrance which the four peripheral
benzo-S,S-dioxothiophene units impose on the central ring. An
indirect approach had to be adopted starting with the central ring.
In the past, we have prepared tetrabromothiophene 1,1-dioxide
(10) from the tetrabromo parent compound 6.27 This derivative
which contains the sulfonyl moiety was then reacted with
2-tributylstannylbenzo[b]thiophene 7, under the Stille cross-
coupling conditions, to form in 72% yield the star-thiophene with
already oxidized central ring 9c. Next, the monodioxo 9c was
submitted to oxygenation with 12 equiv of HOF CH3CN, form-
3
ing at last the desired decaoxide 9b in 88% yield and which is the
single molecule (not part of a polymer) with the most thiophene S,
S-dioxide units (Scheme 3).
Two physical properties are characteristic to the oxygenated
star-thiophenes. The first is their thermal stability as evident from
their melting points which, in most cases, are above 300 °C.7b
The second is their electronic properties as deduced from their
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NOTE
Table 1. Absorption λmax (in nm) and HOMOꢀLUMO
Energy Gap (ΔEg,a in eV) in Solutionb
General Procedure for Working with Fluorine. Fluorine is a
strong oxidant and a corrosive material. It should be used only with an
appropriate vacuum line.17 For the occasional user, however, various
premixed mixtures of F2 in inert gases are commercially available, thereby
simplifyingtheprocess. Unreactedfluorineshouldbecapturedbya simple
trap containing a base such as soda lime located at the outlet of the glass
reactor. If elementary precautions are taken, the work with fluorine is
simple, and we have never experienced difficulties working with it.
c
compd
λmax (ΔEg)
ΔΔEg
1
215 (5.77)
325 (3.81)
302 (4.11)
337 (3.68)
303 (4.09)
347 (3.57)
374 (3.32)
369 (3.36)
415 (2.98)
415 (2.98)
467 (2.65)
2
1.95
0.43
0.52
3a
4a
3b
4b
5
General Procedure for Producing HOF CH3CN. A mixture of
3
20% F2 in nitrogen was used throughout this work. The gas mixture was
prepared in a secondary container prior to the reaction and passed at a
rate of about 400 mL per minute through a cold (ꢀ15 °C) mixture of
30 mL of CH3CN and 3 mL of H2O in a regular glass reactor. The
development of the oxidizing power was monitored by reacting aliquots
with an acidic aqueous solution of KI. The liberated iodine was then
titrated with thiosulfate. Typical concentrations of the oxidizing reagent
were around 0.5ꢀ0.6 mol/L.
9a
9b
9b
9c
ꢀ0.04d
0.33d
ꢀ0.33e
0.66d
a ΔEg = hc/λ. b CH2Cl2 solution. c ΔΔEg = ΔEg(nonoxygenated compd)
General Procedure A. Working with HOF CH3CN. An SOT
ꢀ ΔEg(oxygenated compd). d Compared to 5. e Compared to 9c.
3
derivative was dissolved in CH2Cl2, and the mixture was cooled to 0 °C.
The solution with the oxidizing agent was then added in one portion to the
reaction vessel. The reaction was stopped after a few minutes, and the
UVꢀvis spectra. Table 1 lists the maximum wavelength absorp-
tion (λmax), the HOMOꢀLUMO energy gap (ΔEg), and the
difference of the HOMOꢀLUMO energy gap between the S,S-
dioxo and the oxygen-free compounds (ΔΔEg). Table 1 reveals
that the per-oxygenation of these cross-conjugated compounds
resulted in considerable red-shift of the maximum wavelength
absorption (λmax). This indicates that very desirable lowering of
the HOMOꢀLUMO energy gap (ΔΔEg) has taken place.
The red-shifted absorbances, however, are different for each class
of materials. Oxygenation of the least conjugated 1 has the most
dramatic effect, a red shift of 110 nm (1.95 eV). Transferring the
cross-conjugated compounds 3 to 4 caused a red-shift of only
35ꢀ45 nm (0.43ꢀ0.52 eV). With compounds sprung from star-
thiophene 5 the trend is more complicated. The oxygenation of the
central ring to its corresponding SO2 derivative 9c causes the
strongest reduction of the HOMOꢀLUMO energy gap (93 nm,
0.66 eV), while oxygenation of the peripheral sulfur atoms alone (5
to 9a) resulted in a slight hypsochromic shift. Similarly, the last step
toward full oxygenation (9c to the all-SO2 9b) resulted in some-
what blue-shifted absorbances, probably due to the molecule’s
deviation from planarity, which decreases the π-orbital overlap.
In conclusion, it was shown that a variety of nonlinear, cross-
conjugated SOTs, could be effectively oxygenated using the
excessof HOF CH3CNwas quenchedwithsaturatedsodium bicarbonate.
3
The mixture was then poured into water and extracted with CH2Cl2, the
organic layer dried over MgSO4, and the solvent evaporated. The crude
product was usually purified by recrystallization.
General Procedure B. Stille-Type Cross-Coupling. Tetrabro-
mothiophene or tetrabromothiophene 1,1-dioxide, 2-tributylstannyl-
benzo[b]thiophene, and Pd(PPh3)4 were dissolved in dry toluene.
The mixture was degassed for 20 min and refluxed under nitrogen.
The solvent was washed with water, dried over MgSO4, and removed by
rotary evaporation. The crude product was purified by vacuum flash
chromatography followed by recrystallization.
Tris(5-bromo-2-thienyl 1,1-dioxide)methane (2). Com-
pound 2 was prepared from 123 (580 mg, 1.16 mmol) as described in
general procedure A, using 8 equiv of the oxidizing agent for 5 min. The
product was recrystallized from acetone/hexane. A greenish solid (570
mg, 82%) was obtained: mp > 300 °C; λmax 325 nm; 1H NMR (acetone-
d6) 5.22 (1H, d, J = 1.4 Hz), 7.30 (3H, dd, J1 = 5.1 Hz, J2 = 1.4 Hz), 7.39
(3H, d, J = 5.1 Hz); 13C NMR 31.0, 122.0, 128.9, 132.0, 136.3. Anal.
Calcd for C13H7O6S3Br3: C, 26.24; H, 1.19; S, 16.16; Br, 40.28. Found:
C, 26.19; H, 1.16; S, 15.70; Br, 40.67.
1,3,5-Tris(5-methyl-2-thienyl 1,1-dioxide)benzene (4a).
Compound 4a was prepared from 3a24 (190 mg, 0.52 mmol) as described
in general procedure A using 8 equiv of the oxidizing agent for 2 min. The
product was recrystallized from acetone/hexane. An off-white solid (205
mg, 85%) was obtained: mp > 300 °C; λmax 337 nm; 1H NMR (CDCl3)
2.23 (9H, s), 6.54 (3H, m), 7.08 (3H, d, J=4.4Hz), 8.03(3H, s);13CNMR
10.3, 123.4, 124.1, 125.1, 130.1, 139.9, 142.2. Anal. Calcd for C21H18O6S3:
C, 54.53; H, 3.92; S, 20.80. Found: C, 53.98; H, 3.80; S, 20.24.
1,3,5-Tris(5-chloro-2-thienyl 1,1-dioxide)benzene (4b).
Compound 4b was prepared from 3b24 (404 mg, 0.94 mmol) as
described in general procedure A using 8 equiv of the oxidizing agent
for 5 min. The product was recrystallized from acetone/hexane. An off-
white solid (472 mg, 96%) was obtained: mp > 300 °C; λmax 347 nm; 1H
NMR (acetone-d6) 7.31 (3H, d, J = 5.3 Hz), 7.66 (3H, d, J = 5.3 Hz) 8.22
(3H, s); 13C NMR (acetone-d6) 125.5, 125.7, 125.8, 130.4, 133.5, 138.8;
HRMS m/z calcd for C18H9O6S3Cl3Na 544.8525 (M + Na), found
544.8528. Anal. Calcd for C18H9O6S3Cl3: C, 41.27; H, 1.73; S, 18.36.
Found: C, 40.90; H, 1.65; S, 18.00.
HOF CH3CN complex with high yields and under mild condi-
3
tions. It seems that no other oxygen transfer agent can achieve
this goal. The oxygenation turns the compounds thermally more
stable and reduces the HOMOꢀLUMO energy gap. While the
reaction requires working with fluorine, it is much less compli-
cated than many may think. One can dilute the commercial
fluorine on the spot17 or purchase premixed fluorine/nitrogen
mixtures. The reactions were carried out in regular glass vessels.
We have never had any problems working with this element.
’ EXPERIMENTAL SECTION
General Experimental Procedures. MS spectra were measured
under MALDI conditions. UV spectra were recorded in CH2Cl2. 1H NMR
and 13C NMR were obtained at 400 and 100 MHz, respectively, with
CDCl3 (for compounds 4a, 5, 9a, and 9c), acetone-d6 (for compounds 2
and 4b), and DMSO-d6 (for compound 9b) as solvents and Me4Si as an
internal standard. In the 1HNMR spectra of 2, 4b, and 9b, signals at 2.96
and 3.33 ppm originate from water in acetone-d6 and DMSO-d6 solvents,
respectively. These peaks disappear upon addition of D2O.
2,3,4,5-Tetakis(2-benzothienyl)thiophene (5). Compound 5
was prepared from the reaction of 625 (1.175 gr, 2.94 mmol) and 726 (8.09
gr, 19.11 mmol) in 45 mL of dry toluene in the presence of Pd(PPh3)4
(220 mg, 6.5%) as described in general procedure B. The mixture was
refluxed over nitrogen overnight. The product was chromatographed and
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NOTE
recrystallized from benzene/methanol. A yellow solid (1.425 gr, 79%) was
Gorman, C. B.; Tiemann, B. G.; Cheng, L. T. J. Am. Chem. Soc. 1993,
115, 3006.
(5) Tanaka, K.; Wang, S.; Yamabe, T. Synth. Met. 1989, 30, 57.
(6) Barbarella, G.; Pudova, O.; Arbizzani, C.; Mastragostino, M.;
Bongini, A. J. Org. Chem. 1998, 63, 1742.
(7) (a) Amir, E.; Rozen, S. Angew. Chem., Int. Ed. 2005, 44, 7374. (b)
Oliva, M. M.; Casado, Juan; Navarrete, J. T. L.; Patchkovskii, S.;
Goodson, T., III; Harpham, M. R.; de Melo, J. S. S.; Amir, E.; Rozen,
S. J. Am. Chem. Soc. 2010, 132, 6231.
(8) Benincori, T.; Capaccio, M.; De Angelis, F.; Falciola, L.; Muccini,
M.; Mussini, P.; Ponti, A.; Toffanin, S.; Traldi, P.; Sannicolo, F. Chem.—
Eur. J. 2008, 14, 459.
(9) Belot, C.; Filiatre, C.; Guyard, L.; Foissy, A.; Knorr, M. Electro-
chem. Commun. 2005, 7, 1439.
(10) (a) K€ose, M. E.; Mitchell, W. J.; Kopidakis, N.; Chang, C. H.;
Shaheen, S. E.; Kim, K.; Rumbles, G. J. Am. Chem. Soc. 2007, 129, 14257.
(b) Mitchell, W. J.; Kopidakis, N.; Rumbles, G.; Ginley, D. S.; Shaheen,
S. E. J. Mater. Chem. 2005, 15, 4518.
1
obtained: mp = 286 °C; λmax 268, 302, 374 nm; H NMR (CDCl3)
7.66ꢀ7.55 (6H, m), 7.44 (2H, s), 7.25ꢀ7.35 (12H, m); 13C NMR 122.8,
123.0, 124.0, 124.5, 124.7, 124.8, 125.1, 125.3, 125.6, 127.5, 135.7, 136.3,
140.0, 142.0; HRMS m/z calcd for C36H20S5 612.0169 (M), found
612.0161. Anal. Calcd for C36H20S5: C, 70.55; H, 3.29; S, 26.16. Found:
C, 70.81; H, 3.30; S, 26.19.
2,3,4,5-Tetakis(2-benzothienyl 1,1-dioxide)thiophene (9a).
Compound 9awas preparedfrom 5(182 mg, 0.297 mmol) asdescribedin
general procedure A, using 12 equiv of the oxidizing agent for 10 min. The
product was recrystallized from chloroform/hexane. A yellow solid (220
mg, 100%) was obtained: mp blackens at 259 °C; λmax 321, 369 nm; 1H
NMR (CDCl3) 7.33ꢀ7.79 (20H, m); 13C NMR 122.3, 122.6, 126.9,
127.1, 131.1, 131.2, 131.4, 131.6, 132.0, 133.5, 135.5, 134.5, 134.6, 135.5,
135.6, 135.7, 137.1; HRMS m/z calcd for C36H20O8S5Na 762.9659 (M +
Na), found 762.9628. Anal. Calcd for C36H20O8S5: C, 58.36; H, 2.72.
Found: C, 57.81; H, 2.53.
(11) Rozen, S.; Brand, M. Angew. Chem., Int. Ed. Engl. 1986, 25, 554.
(12) Kol, M.; Rozen, S. J. Org. Chem. 1993, 58, 1593.
(13) Rozen, S.; Bareket, Y.; Kol, M. Tetrahedron 1993, 49, 8169.
(14) (a) Rozen, S.; Carmeli, M. J. Am. Chem. Soc. 2003, 125, 8118.
(b) Rozen, S.; Kol, M. J. Org. Chem. 1992, 57, 7342. (c) Kol, M.; Rozen,
S. J. Chem. Soc., Chem. Commun. 1991, 567.
(15) Golan, E.; Rozen, S. J. Org. Chem. 2003, 68, 9170.
(16) (a) Rozen, S.; Golan, E. Eur. J. Org. Chem. 2003, 10, 1915. (b)
Golan, E.; Hagooly, A.; Rozen, S. Tetrahedron Lett. 2004, 45, 3397.
(17) Dayan, S.; Kol, M.; Rozen, S. Synthesis 1999, 1427.
(18) (a) Rozen, S.; Dayan, S. Angew. Chem., Int. Ed. 1999, 38, 3471.
(b) Rozen, S.; Carmeli, M. J. Org. Chem. 2005, 70, 2131.
(19) Rozen, S.; Bareket, Y. J. Org. Chem. 1997, 62, 1457.
(20) Moralis, G. R.; Humphrey, A. J.; Falconer, R. A. Tetrahedron
2008, 64, 7426.
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(22) Rozen, S. Eur. J. Org. Chem. 2005, 12, 2433.
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Tetrahedron. Lett. 2005, 61, 9533.
(24) Kotha, S.; Kashinath, D.; Lahiri, K.; Sunoj, R. B. Eur. J. Org.
Chem. 2004, 4003.
(25) Punidha, S; Sinha, J.; Kumar, A.; Ravikanth, M. J. Org. Chem.
2008, 73, 323.
2,3,4,5-Tetakis(2-benzothienyl)thiophene 1,1-Dioxide (9c).
Compound 9c was preparedfrom the reaction of1027 (1.42 gr, 3.3 mmol)
and 727 (9.08 gr, 21.45 mmol) in 50 mL of dry toluene in the presence of
Pd(PPh3)4 (240 mg, 6.5%) as described in general procedure B. The
mixture was refluxed over nitrogen for 5 h. The product was chromato-
graphed and recrystallized from toluene/methanol, but we were not able
toget ananalyticalsample. A red solid(1.524 gr, 72%) was obtained:mp>
300 °C; λmax 467 nm; 1H NMR (CDCl3) 7.31ꢀ7.38 (8H, m), 7.47 (2H,
s), 7.58 (2H, d, J = 7.8 Hz), 7.75ꢀ7.80 (4H, m), 7.87 (2H, d, J = 7.8 Hz),
8.19 (2H, s). 13C NMR 122.7, 123.3, 125.4, 125.7, 125.8, 126.1, 127.4,
129.0, 129.2, 129.5, 131.0, 131.8, 134.9, 139.4, 139.7, 141.7, 142.7; HRMS
m/z calcd for C36H20O2S5 644.0067 (M), found 644.0046.
2,3,4,5-Tetakis(2-benzothienyl 1,1-dioxide)thiophene 1,1-
Dioxide (9b). Compound 9b was prepared from 9c (120 mg, 0.186
mmol) as described in general procedure A using 12 equiv of the oxidizing
agent for 10 min. The product wasrecrystallized fromchloroform/hexane.
An orange solid (127 mg, 88%) was obtained: mp > 300 °C; λmax 309,
1
415 nm; H NMR (DMSO-d6) 7.56ꢀ7.94 (18H, m), 8.395 (2H, s).
Solubility was too low to measure the 13C NMR spectrum. Anal. Calcd for
C36H20O10S5: C, 55.95; H, 2.61. Found: C, 56.32; H, 2.44.
’ ASSOCIATED CONTENT
(26) Fargeas, V.; Favresse, F.; Mathieu, D.; Beaudet, I.; Charrue, P.;
Lebret, B.; Piteau, M.; Quintard, J. Eur. J. Org. Chem. 2003, 1771.
(27) Rozen, S.; Bareket, Y. J. Org. Chem. 1997, 62, 1457.
Supporting Information. 1H NMR and 13C NMR spec-
S
b
tra for all new compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*Fax: +972 3 640 9293. Tel: +972 3 640 8378. E-mail: rozens@
post.tau.ac.il.
’ ACKNOWLEDGMENT
This work was supported by the Israel Science Foundation.
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