Synthesis and properties of butyl-capped dehydrothieno[14]annulenes and their conversion into terthiophenes

Article abstract of DOI:10.1021/ol801451x

(Chemical Equation Presented) A series of butyl end-capped dehydrothieno[14]annulenes (DTAs) has been prepared. These compounds were further transformed into electronrich terthiophenes (3TPs) by reaction with sodium sulfide. The DTA and 3TP macrocycles we

Full text of DOI:10.1021/ol801451x

ORGANIC
LETTERS
2008
Vol. 10, No. 18
3973-3976
Synthesis and Properties of
Butyl-Capped
Dehydrothieno[14]annulenes and Their
Conversion into Terthiophenes
Matthew J. O’Connor and Michael M. Haley*
Department of Chemistry and Materials Science Institute, UniVersity of Oregon,
Eugene, Oregon 97403-1253
haley@uoregon.edu
Received June 26, 2008
ABSTRACT
A series of butyl end-capped dehydrothieno[14]annulenes (DTAs) has been prepared. These compounds were further transformed into electron-
rich terthiophenes (3TPs) by reaction with sodium sulfide. The DTA and 3TP macrocycles were compared qualitatively using UV-vis spectroscopy,
and the latter were found to have lower energy absorptions. The conformations of these molecules were also examined by DFT-YFLP
computations.
Electron-rich unsaturated compounds have been of con-
siderable focus in recent years.1 Conjugated acetylenic
systems, in particular,2 have garnered the bulk of this
interest, which can be attributed to the ease of preparation3
and interesting materials properties such as nonlinear
optical (NLO) activity,4 liquid crystalline behavior,5 and
molecular switching.6 Our group has had a long-standing
interest in the preparation of these highly unsaturated
systems, particularly arene/acetylene motifs such as de-
hydrobenzoannulenes (DBAs).7-9
Concurrently, conjugated thiophene systems have also
attracted much interest due to their highly conductive
properties, optical properties, and relative environmental
stabilities over many of their benzene-based analogues.10,11
Incorporating thiophenes into arene/acetylene motifs was
originally explored by the groups of Youngs (e.g., 1, Figure
1)12 and Marsella (e.g., 2),13 which produced macrocycles
of higher order symmetries through intermolecular cycliza-
tion reactions of simple synthons.12,13 Our desire for
improved syntheses via intramolecular ring closure led to
our initial communication that focused on the [18]annulene
skeleton (e.g., 3);14 however, stability problems with the
intermediate thienyldiynes forced us to examine 14-
membered dehydrobenzothienoannulene (DBTA, e.g., 4, 5)
(1) Inter alia: (a) Topics in Current Chemistry (Carbon Rich Compounds
I); de Meijere, A., Ed.; Springer-Verlag: Berlin, 1998; Vol. 196. (b) Topics
in Current Chemistry (Carbon Rich Compounds II); de Meijere, A., Ed.;
Springer-Verlag: Berlin, 1999; Vol. 201. (c) Carbon-Rich Compounds: From
Molecules to Materials; Haley, M. M., Tykwinski, R. R., Eds.; Wiley-VCH:
Weinheim, 2006. (d) Functional Organic Materials; Mu¨ller, T. J. J., Bunz,
U. H. F., Eds.; Wiley-VCH: Weinheim, 2007.
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Chem. 2001, 11, 2943–2945. (b) Slepkov, A.; Hegmann, F. A.; Tykwinski,
R. R.; Kamada, K.; Ohta, K.; Marsden, J. A.; Spitler, E. L.; Miller, J. J.;
Haley, M. M. Opt. Lett. 2006, 31, 3315–3317. (c) Bhaskar, A.; Guda, R.;
Haley, M. M.; Goodson, T., III. J. Am. Chem. Soc. 2006, 128, 13972–
13973.
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Nielsen, M. B.; Diederich, F. In Modern Arene Chemistry; Astruc, D., Ed.;
Wiley-VCH: Weinheim, 2002; pp 196-216. (c) Acetylene Chemistry:
Chemistry, Biology, and Material Science; Diederich, F., Stang, P. J.,
Tykwinski, R. R., Eds.; Wiley-VCH: Weinheim, 2005.
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A., Diederich, F., Eds.; Wiley-VCH: Weinheim, 2004. (b) Transition Metal
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Davies, S. G., Murahashi, S., Eds.; Blackwell Science: Oxford, 1998.
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10.1021/ol801451x CCC: $40.75
Published on Web 08/20/2008
2008 American Chemical Society

thiophene ring by reaction with Na2S.17 The resultant
terthiophenes (3TPs, e.g., 13-18) contain more π-electron
density from the thiophene sulfur atom and better electron
delocalization than the diyne linker. Furthermore, the bridg-
ing 1,2-diethynylarene unit limits the rotation of the 3TPs,
ideally coplanarizing the 3TP moiety and thus enhancing
π-orbital overlap further. We report herein the synthesis and
optical properties of DBTAs/DTAs 7-12 and their conver-
sion into 3TPs 13-18 (Figure 2).
Figure 1. Previously prepared DBTAs and DTAs.
and dehydrothienoannulene (DTA, e.g., 6) topologies, with
greater success.15
To further our understanding and scalability of thiophene-
containing macrocycles, we elected to incorporate R-buty-
lated thiophenes as part of the macrocycle (e.g., 7-12) to
improve solubility and, more importantly, to protect the
compounds from unwanted oxidation.11,16 The added stability
conferred by the alkyl chain should also permit us to
investigate the conversion of the diacetylene linker into a
(6) (a) Gobbi, L.; Seiler, P.; Diederich, F. Angew. Chem., Int. Ed. 1999,
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Shirtcliff, L. D.; Haley, M. M. J. Am. Chem. Soc. 2005, 127, 2464–2476.
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Figure 2. Target thieno-fused dehydro[14]annulenes 7-12 and
3TPs 13-18.
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111, 946–954.
As in previous work, the macrocycles were assembled
from basic building blocks, namely core dihaloarenes 19 and
20 and differentially silylated diynes 24 and 26, through a
convergent design. Bromoiodothiophenes 19 and 20 were
prepared according to Scheme 1. Iodination of commercially
available 2-butylthiophene furnished 21. Use of the lithium-
halogen “dance” reaction18 afforded 22 in 42% yield for the
two steps. NBS bromination of 22 provided multigram
quantities of 19 in 36% overall yield. Regioisomer 20 was
easily prepared in an analogous manner: bromination of
2-butylthiophene and subsequent lithium-halogen dance
reaction on 2319 followed by iodine quench generated arene
(11) Handbook of Oligo- and Polythiophenes; Fichou, D., Ed.; Wiley-
VCH: Weinham, 1999. (b) Ba¨uerle, P. Sulfur-Containing Oligomers In
Electronic Materials: The Oligomer Approach; Mu¨llen K., Wegner, G.,
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University of Oregon, 2008.
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Org. Lett., Vol. 10, No. 18, 2008

annulene and are thus formed by sequential cross-couplings
to the dihaloarene “crown”. Scheme 3 shows a representative
Scheme 1
Scheme 3
20 in 90% overall yield. Halides 19 and 20 were subsequently
converted into protected diynes 24 and 26 by sequential
Sonogashira cross-coupling with (trimethylsilyl)acetylene
(TMSA) and (triisopropylsilyl)acetylene (TIPSA) in excellent
yields (vide infra).
[14]DBTAs 7 and 8 and [14]DTAs 9 and 12 possess two
fused thiophenes in a “symmetrical” orientation on the lower
portion of the annulene and thus were assembled via 2-fold
cross-coupling of the respective diyne to a central dihaloarene
core. Scheme 2 illustrates a representative synthesis starting
synthesis of “unsymmetrical” DTA 10. Dihalothiophene 20
was sequentially cross-coupled with TMSA and TIPSA under
Sonogashira conditions to afford 26 in 91% yield. Selective
desilylation of 26 with K2CO3, cross-coupling to 1 equiv of
arene 19, and then cross-coupling of the resultant bromodiyne
to monodesilylated 24 furnished 27 in 48% combined yield.
Macrocyclization was performed as before utilizing the
Pd(II)/Cu(I) catalyst system under oxidizing conditions,
affording 10 in 53% yield.
Scheme 2
Formation of the 3TPs was accomplished easily by treating
the corresponding [14]annulenes with Na2S in 2-methoxy-
ethanol (Scheme 2). A summary of the syntheses of all six
annulenes and the 3TPs is given in Table 1. Attempting 3TP
Table 1. Synthesis and Yields of DBTAs and DTAs 3-14
monodesilylated
diyne (equiv)
precursor, annulene, 3TP, yield
arene
24
26
yield (%)
yield (%)
(%)
from 19. Sequential Sonogashira reactions with TMSA and
TIPSA afforded diyne 24 in 94% yield for the two steps.
Selective removal of the TMS group with K2CO3 in MeOH
followed by cross-coupling to 1,2-diiodobenzene furnished
tetrayne 25 in 78% yield. Protiodesilylation with TBAF and
subsequent acetylenic homocoupling utilizing PdCl2(dppe)
catalyst, CuI cocatalyst, and iodine as an oxidant9a,20 gave
8 in 57% yield.
19
19
19
2
2
pre7, 80
pre8 (25), 78
pre9, 52
7, 64
8, 57
9, 52
13,46
14,38
15,43
16,58
17,16
18,42
2
20 1 (second) 1 (first)
20 1 (first)
19
pre10 (27), 48 10,53
1 (second) pre11, 56 11,66
2
pre12, 69
12,32
[14]DTAs 10 and 11 possess two fused thiophenes in
unsymmetrical orientations on the lower portion of the
formation on the previously reported nonalkylated
macrocycles15a afforded promising results; however, the
products were found to be extremely unstable to air and light,
decomposing rapidly after a few hours to give uncharacter-
(19) Mohanakrishnan, A. K.; Amaladass, P.; Arul Clement, J. Tetra-
hedron Lett. 2007, 48, 779–784
.
Org. Lett., Vol. 10, No. 18, 2008
3975

izable mixtures.15b In contrast, all of the butylated products
were much more stable to air, light, and silica gel, though
overall yields were somewhat lower than their nonbutylated
counterparts.15a
the lowest energy bands in 13 and 14 are significantly red-
shifted from 7 and 8 (381 f 415 and 422 f 442 nm,
respectively), thus reflecting enhanced electron delocalization
within the 3TP macrocycles over the diacetylenic DBTAs.
Additionally, thiophene connectivity via R-R linkages as in
14 affords a smaller HOMO-LUMO gap than the R-ꢀ
thiophene links in 13, which is a well-established phenom-
enon.11,21
The UV-vis spectra for DBTAs 7 and 8 and the
corresponding 3TPs 13 and 14 are shown in Figure 3a.
The UV-vis spectra for DTAs 9-12 are depicted in
Figure 3b. The most intense peak is approximately the same
for 9-12 (ca. 345 nm), with slight variations in vibronic
structure, and is red-shifted compared to 5 (335 nm), again
due to the electron donation of the butyl groups. In the low
energy region, the most red-shifted peak belongs to 9 (415
nm) followed by DTAs 11 and 12, with a common peak at
412 nm, and then 10 (409 nm). Analogous to what was
observed with the nonalkylated [14]DTAs,15a there are no
discernible trends of the low energy absorptions with respect
to thiophene orientation.
Sulfide-induced cyclization to generate 3TPs 15-18 results
in further red shifting (ca. 5 nm) of the main band to 350
nm, as well as a small bathochromic shift and broadening
of the low energy transitions (Figure 3c). In addition, there
is a new, very weak, low energy transition centered around
465-470 nm, a feature unique to the all-thiophene 3TPs
(Figure 3c inset).
DFT-YFLP calculations suppport the observed optical
behavior (see the Supporting Information). The optimized
structure for 14 shows a coplanar conformation across the
terthiophene having the lowest energy, whereas the structure
of 13 has an out of plane tilt of the central thiophene by ca.
20° due to H-atom interactions arising from the R-ꢀ
thiophene links. The coplanarity of 14 results in the lowest
energy/highest intensity absorption band, and the lack thereof
in 13 explains the reduced intensity and absence of fine
structure.
In conclusion, six R-butylated, thieno-fused dehydro[14]-
annulenes were prepared that exhibit enhanced stability and
synthetic scalability over their unsubstituted counterparts,
which allows for subsequent conversion to a class of linked
terthiophenes with enhanced planarity. Future reports will
discuss the electrochemical and emissive properties of the
butylated DTAs and 3TPs.
Figure 3. UV-vis absorption spectra of (a) 7-8, 13-14, (b) 9-12,
and (d) 15-18. All spectra were recorded in CH2Cl2 at analyte
concentrations of 15-25 µM.
Acknowledgment. We thank the National Science Foun-
dation (CHE-0718242) for financial support and Sean Mc-
Clintock for the DFT structure optimizations.
Compared to 4 and 5, electronic donation of the two butyl
groups in 7 and 8 red shifts the respective peaks by 6-12
nm. The previously noted trends with thiophene orientation
are also maintained: an increased extinction coefficient and
red shift are observed for both 5 (408 nm) and 8 (422 nm)
with “down” oriented thiophenes (sulfur atom adjacent to
the butadiyne linkage) compared to 4 (374 nm) and 7 (381
nm) with “up” thiophenes, whereas the highest intensity peak
is slightly blue-shifted for 5 (325 nm) and 8 (331 nm) over
4 (329 nm) and 7 (337 nm). Upon terthiophene formation,
Supporting Information Available: Experimental details
and spectroscopic data for all new compounds; DFT opti-
mized structures for 13 and 14. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL801451X
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Phys. 1985, 83, 1323–1329
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