General, efficient route to thiophene-1-oxides and well-defined, mixed thiophene-thiophene-1-oxide oligomers [12]

Full text of DOI:10.1021/ja992365t

9744
J. Am. Chem. Soc. 1999, 121, 9744-9745
thiophene-1-oxide oligomers. This methodology should prove
useful for incorporation of thiophene-1-oxide units into a variety
of oligomeric and polymeric structures.
General, Efficient Route to Thiophene-1-Oxides and
Well-Defined, Mixed Thiophene-Thiophene-1-Oxide
Oligomers
In general, the zirconocene coupling of alkynes tolerates the
presence of bromo-aryl functionalities and therefore provides
convenient routes to difunctional building blocks that can be
employed in carbon-carbon coupling routes to oligomers and
polymers.9 For example, the reaction of 4,4-bis(hexyloxymethyl)-
1,7-bis-p-bromophenyl-1,6-heptadiyne with zirconocene, as gen-
erated by the Negishi method,10 provides solutions of the
zirconacyclopentadiene 1 in high yield. However, an attempt to
prepare the thiophene monoxide 2c (see eq 1) by the reaction of
1 with thionyl chloride gave only a 10% yield of the desired
product, along with the corresponding thiophene (20%) and
several unidentified side products.
Considerably better results were obtained by the two-step
procedure outlined in eq 1. Generation of metallacycle 1 in THF
gave a brown solution, through which SO2 was bubbled to produce
a bright yellow solution of the desired product 2c within 10 min.
Compound 2c was isolated in 85% yield as yellow crystals after
standard workup. The IR spectrum contains a characteristic ν-
(SdO) stretching band at 1043 cm-1, and the pyramidal nature
of the sulfur atom manifests itself in inequivalent 1H NMR shifts
for the methylene protons of the fused, five-membered ring.
Interestingly, the λmax value for 2c (388 nm) is red-shifted with
respect to corresponding values for the thiophene (342 nm) and
thiophene dioxide (378 nm).
Biwang Jiang and T. Don Tilley*
Department of Chemistry, UniVersity of California at Berkeley,
Berkeley, California 94720-1460
ReceiVed July 8, 1999
Thiophene-1-oxides have attracted considerable attention due
to their highly reactive nature, their biochemical roles in
metabolism and toxicology,1 and because the development of
general methods for their preparation has proven difficult.2
Chemically they behave as reactive dienes, which tend to
decompose via Diels-Alder dimerization.2,3 In a few cases,
sterically protected thiophene-1-oxides have been prepared via
oxidation of the corresponding thiophenes, but this route appears
to be complicated by rapid, further oxidation to the 1,1-dioxides.2
The reaction of tetraphenylzirconacyclopentadiene, Cp2ZrC4Ph4,
with thionyl chloride has been shown to give tetraphenyl-
thiophene-1-oxide (2a) in moderate yield,4 but we have found
that this method is not generally reliable for the synthesis of
thiophene-1-oxides (vide infra).
We have been exploring the use of zirconocene couplings in
the synthesis of π-conjugated polymers and oligomers with novel
electronic properties5 and have recently targeted materials with
high electron affinities that might display n-type semiconductor
properties.6 In this context, thiophene-1-oxides and thiophene-
1,1-dioxides represent intriguing monomer units, given their
electron-deficient nature. A recent study by Barbarella et al.
demonstrated that introduction of thiophene-1,1-dioxide units into
thiophene oligomers results in decreased electronic band gaps and
higher electron affinities.7 Theoretical studies on the electronic
structures of poly(thiophene-1-oxide) and poly(thiophene-1,1-
dioxide) predict that these materials should have lower band gaps
than polythiophene but do not agree on which polymer would
have the lowest band gap.8 Here we report a versatile, high-yield
synthetic procedure for thiophene-1-oxides and mixed thiophene-
Table 1 provides examples of other one-pot reactions that
convert an alkyne or diyne to the corresponding thiophene-1-
oxide by the method of eq 1. The yields for these reactions are
(1) (a) Treiber, A.; Dansette, P. M.; El Amri, H.; Girault, J. P.; Ginderow,
D.; Mornon, J. P.; Nasuy, D. J. Am. Chem. Soc. 1997, 119, 1565. (b) Lopez-
Garcia, M. P.; Dansette, P. M.; Mansuy, D. Biochemsitry 1994, 33, 166.
(2) (a) Mock, W. L. J. Am. Chem. Soc. 1970, 92, 7610. (b) Li, Y.; Matsuda,
M.; Thiemann, T.; Sawada, T.; Mataka, S.; Tashiro, M. Synlett 1996, 461. (c)
Furukawa, N.; Zhang, S.; Sato, S.; Higaki, M. Heterocycles 1997, 44, 61. (d)
Pouzet, P.; Erdelmeier, I.; Ginderow, D.; Mornon, J.-P.; Dansette, P., Mansuy,
D. J. Chem. Soc., Chem. Commun. 1995, 473. (e) Nakayama, J.; Yu, T.;
Sugihara, Y.; Ishii, A. Chem. Lett. 1997, 499. (f) Hashmall, J. A.; Horak, V.;
Khoo, L. E.; Quicksall, C. O.; Sun, M. K. J. Am. Chem. Soc. 1981, 103, 289.
(g) Skaugset, A. E.; Rauchfuss, T. B.; Stern, C. L. J. Am. Chem. Soc. 1990,
112, 2432.
(3) Reviews: (a) Nakayama, J.; Sugihara Y. Sulfur Rep. 1997, 19, 349.
(b) Kellog, R. M. In ComprehensiVe Heterocyclic Chemistry; Katritzky, A.
R., Rees, C. W., Eds.; Pergamon: Oxford, U.K., 1984; Vol 4, p 713. (c)
Raasch, M. S. In Chemistry of Heterocyclic Compounds, Thiophene and its
DeriVatiVes; Gronowitz, S., Ed.; Wiley: New York, 1985; Vol 44, p 971.
(4) (a) Fagan, P. J.; Nugent, W. A. J. Am. Chem. Soc. 1988, 110, 2310.
(b) Fagan, P. J.; Nugent, W. A.; Calabrese, J. C. J. Am. Chem. Soc. 1994,
116, 1880. (c) Meier-Brocks, F.; Weiss, E. J. Organomet. Chem. 1993, 33,
453.
(5) (a) Mao, S. S. H.; Tilley, T. D. J. Organomet. Chem. 1996, 521, 425.
(b) Mao, S. S. H.; Tilley, T. D. Macromolecules 1997, 30, 5566. (c) Lucht,
B. L.; Tilley, T. D. Chem. Commun. 1998, 1645. (d) Lucht, B. L.; Mao, S. S.
H.; Tilley, T. D. J. Am. Chem. Soc. 1998, 120, 4354.
(6) (a) Lucht, B. L.; Buretea, M. A.; Tilley, T. D., manuscript in preparation.
(b) Jiang, B.; Tilley, T. D., unpublished results.
(7) (a) Barbarella, G.; Favaretto, L.; Zambianchi, M.; Pudova, O.; Arbizzani,
C.; Bongini, A.; Mastragostino, M. AdV. Mater. 1998, 10, 551. (b) Barbarella,
G.; Pudova, O.; Arbizzani, C.; Mastragostino, M.; Bongini, A. J. Org. Chem.
1998, 63, 1742. (c) Barbarella, G.; Favaretto, L.; Zambianchi, M.; Antolini,
L.; Pudova, O.; Bongini, A. J. Org. Chem. 1998, 63, 5497.
(8) (a) Tanaka, K.; Wang, S.; Yamabe, T. Synth. Met. 1989, 30, 57. (b)
Bakhshi, A. K. Solid State Commun. 1995, 94, 943.
generally quite high, indicating that oxo-transfer from the SO2
reagent to zirconocene is quite efficient as compared to the
analogous process with thionyl chloride, which gives Cp2ZrCl2
as the final zirconium-containing product. Note that tetraphe-
nylthiophene-1-oxide (2a, entry 1) is formed in very high yield,
whereas the synthesis from thionyl chloride is reported to give
yields of ∼50-60%.4 Zirconocene coupling of 3-hexyne followed
by reaction with thionyl chloride gave only a trace of tetraeth-
ylthiophene-1-oxide (2b), while the reaction with SO2 gave 2b
in excellent yield. Note that the few thiophene-1-oxides that have
been successfully isolated feature sterically bulky substituents
(e.g., Ph,2d,4 SiR3,2c,11 tBu2a,e) that protect against decomposition
reactions. Compound 2b is reasonably stable at room temperature
over several days as a pure oil, and only slight decomposition
was noted for a chloroform-d solution after 4 days.
(9) (a) Schlu¨ter, A.-D. In Handbook of Conducting Polymers, 2nd ed.;
Skotheim, T. A., Elsenbaumer, R. L., Reynolds, J. R., Eds.; Marcel Dekker:
New York, 1997; p 209. (b) Yamamoto, T. Bull. Chem. Soc. Jpn. 1999, 72,
621.
(10) (a) Negishi, E.; Cederbaum; F. E.; Takahashi; T. Tetrahedron Lett.
1986, 27, 2829. (b) Negishi, E.; Takahashi; T. Acc. Chem. Res. 1994, 27,
124.
(11) Furukawa, N.; Zhang, S.-Z.; Horn, E.; Takahashi, O.; Sato, S.;
Yokoyama, M.; Yamaguchi, K. Heterocycles 1998, 47, 793.
10.1021/ja992365t CCC: $18.00 © 1999 American Chemical Society
Published on Web 09/30/1999

Communications to the Editor
J. Am. Chem. Soc., Vol. 121, No. 41, 1999 9745
Table 1. Data for Thiophene-1-oxides Prepared from Various
Alkyne Starting Materials
Figure 1. ORTEP diagram for 2g. S(2)-O(1) bond distance, 1.495(3)
Å; bond angles (deg), C(5)-S(2)-C(12) 91.0(2), O(1)-S(2)-C(5) 112.3-
(2), O(1)-S(2)-C(12) 112.1(2).
The synthesis of the first mixed thiophene-thiophene-1-oxide
structures presented an opportunity to examine competing in-
tramolecular oxidation reactions. This is of interest given previ-
ously reported attempts to prepare thiophene-1-oxides via the
direct oxidation of thiophenes. It has been suggested that the
difficulty with this approach lies in the greater sensitivity of the
monooxide (vs the starting thiophene compound) toward oxida-
tion, such that the main product is typically the thiophene
dioxide.2,3 In fact, we have found that this reactivity difference
allows for selective oxidations of thiophene-1-oxide groups in
the presence of thiophenes. Thus, the oxidation of 2h by 1 equiv
of m-chloroperbenzoic acid (m-CPBA) at room temperature gave
the corresponding 1,1-dioxide in 78% isolated yield (eq 2).
b
a λmax value for the corresponding thiophene. λmax value for the
corresponding thiophene-1,1-dioxide.
In addition to 2c, the arylbromides 2d and 2e were obtained
in high yield. Furthermore, this method appears to be quite useful
in the efficient syntheses of mixed thiophene/thiophene-1-oxide
oligomers (2f-2i).12 All of the new thiophene-1-oxides in Table
1, except for 2b, are stable at room temperature in air over periods
of at least several weeks. As for 2c, the λmax values for these
oligomers are red-shifted with respect to those for the corre-
sponding dioxides and (in the case of 2h) the corresponding
thiophene. The molecular structure of 2g (Figure 1) consists of a
central diene fragment, with C(5)-C(6), C(6)-C(11), and C(11)-
C(12) distances of 1.357(6), 1.460(6), and 1.358(6) Å, respec-
tively. As expected, the oxidized sulfur atom exhibits a pyramidal
geometry and lies somewhat out of the C(5)-C(6)-C(11)-C(12)
least-squares plane (by 0.23 Å). The latter plane forms dihedral
angles with the substituent thiophene ring planes of 11.24° and
26.31°. It is interesting to compare the structure for an analogous
di(thienyl)silole derivative, 1,1-dimethyl-3,4-trimethylene-2,5-di-
(2-thienyl)silole, which possesses a five-membered ring fused to
the silole ring. Although the latter compound has significantly
lower dihedral angles between the thiophene and silole rings
(6.14° and 10.55°), it has a higher energy absorption (409 nm,
vs 428 nm for 2g).13
Similarly, 2i was converted to its corresponding thiophene-1,1-
dioxide in 75% isolated yield. Thus, mixed thiophene-thiophene-
1-oxide oligomers and polymers should serve as useful precursors
to the corresponding thiophene-thiophene-1,1-dioxide structures.
In conclusion, we report here a general and efficient method
for the synthesis of various functionalized thiophene-1-oxides
derivatives, which have not been previously available. This
method, based on oxo-transfer from sulfur dioxide to zirconocene,
should allow us to investigate electronic properties for a variety
of new oligomeric and polymeric structures incorporating thiophene-
1-oxide units.
Acknowledgment is made to the National Science Foundation for their
generous support of this work and to Dr. Fred Hollander for determination
of the crystal structure.
Supporting Information Available: Detailed experimental proce-
dures for the preparation and spectroscopic characterization of all
compounds. This material is available free of charge via the Internet at
http://pubs.acs.org.
JA992365T
(12) The lower yield for 2f results from some debromination during its
synthesis.
(13) Yamaguchi, S.; Itami, Y.; Tamao, K. Organometallics 1998, 17, 4910.