Helical polycyclic aromatics containing thiophenes: Synthesis and properties

Article abstract of DOI:10.1016/j.tetlet.2006.01.006

A series of planar and non-planar (helical) thiophene-based polycyclic aromatics has been synthesized through the Diels-Alder reaction followed by decarbonylation reactions and FeCl₃ oxidative cyclizations.

Full text of DOI:10.1016/j.tetlet.2006.01.006

Tetrahedron
Letters
Tetrahedron Letters 47 (2006) 1551–1554
Helical polycyclic aromatics containing thiophenes:
synthesis and properties
Jian Pei,* Wen-Yu Zhang, Jing Mao and Xing-Hua Zhou
Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry,
Peking University, Beijing 100871, China
Received 19 September 2005; revised 29 December 2005; accepted 4 January 2006
Available online 20 January 2006
Abstract—A series of planar and non-planar (helical) thiophene-based polycyclic aromatics has been synthesized through the Diels–
Alder reaction followed by decarbonylation reactions and FeCl3 oxidative cyclizations.
Ó 2006 Elsevier Ltd. All rights reserved.
During the past several decades, oligothiophenes and
polythiophenes have attracted significant interest due
to their applications in photonic and electronic dis-
S
plays.1 New synthetic methodologies and novel struc-
tures have been developed and significant effort has
S
S
S
S
been devoted to the investigation of the special intrinsic
properties by structural and chemical disorder,2 and the
easy modification of the thiophene structure of thio-
phene-based conjugated materials.3 The incorporation
of thiophene rings onto polycyclic aromatic frameworks
also exhibits considerable success in the design and syn-
thesis of unique organic semiconductors4 for organic
field effect transistors and high conductivity.5 [n]Heli-
cenes constructed through such modification have pre-
cisely ordered structures as a result of both the helical
conjugation and the stiff structure.6 Thus, they are espe-
cially suitable building blocks for organic materials with
extraordinary strong chiroptical properties.7
Br
Br
1
2
Br
Br
Chart 1.
effect of aromatization on the photophysical behaviors
of these thiophene-based polycyclic structure is further
investigated.
Scheme 1 outlines the synthetic approach to 1. The
Diels–Alder reactions between 2,20-bithienylethynylene
and 2,5-diethyl-3,4-dipheneylcyclopentadienone fol-
lowed by the decarbonylation afforded 3 in excellent
yield. Before the FeCl3 oxidation6d–f to form the
carbon–carbon bond at b-positions of thiophene rings,
we had to protect a-positions of thiophene rings in order
to prevent the polymerization of thiophene owing to the
high reactivity of a-positions. As shown in Scheme 1,
the bromination only occurred at a-positions of 3.
The oxidation of 4 using FeCl3 (6 equiv per C–C bond)
produced 18 in high yield. Neither dimerization nor
polymerization happened when excess FeCl3 was used,
which confirmed the specificity of such oxidative cycliza-
tions. This result also encouraged us to further prepare
other thiophene-based polycyclic aromatics.
Herein, we report a series of thiophene-based polycyclic
aromatics including planar ladder molecules (1 and 2) as
shown in Chart 1 and non-planar hetereohelicenes (9
and 10) with fused structures with the alternant benzene
and thiophene rings. These thiophene-based planar
polycyclic aromatics and helicenes allow for further
modification at a-positions of thiophene segments and
can ultimately be tuned into more extended molecules
and polymeric systems. Through such elaboration, the
Keywords: Polycyclic; Cyclization; Conjugation; Helicenes.
*
We also obtained 2 through similar procedures as
Corresponding author. Fax: +86 10 62758145; e-mail: jianei@pku.
edu.cn
shown in Scheme 2. The Diels–Alder reactions between
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.01.006

1552
J. Pei et al. / Tetrahedron Letters 47 (2006) 1551–1554
O
Et
Ph Ph
165 o
Et
FeCl3
NBS
CHCl3
CH3NO2
CHCl3/AcOH
S
S
C
0 o
C
S
S
S
S
S
S
Br
Br
Br
Br
3
4
1
Scheme 1.
O
Et
Et
Ph
NBS
Ph
165 o
CHCl3/AcOH
S
S
S
C
0
oC
Ph
Ph
Ph
Ph
Et
Et
Et
Et
Ph
Ph
Ph
Ph
S
S
FeCl3
CHCl3
Et
Et
Et
Et
CH3NO2
S
S
S
S
6
2
Br
Br
Br
Br
Scheme 2.
2,5-bis(2-thienylethynyl)thiophene and 2,5-diethyl-3,4-
dipheneylcyclopentadienone followed by decarbonyl-
ation as well as the bromination produced 6 with alter-
nant benzene and thiophene rings. In the presence of
more than two thiophene rings within one molecule,
FeCl3 oxidation of thiophenes usually polymerized
rather than underwent discrete monomer cyclization
due to the highly reactive nature of thiophene rings.
Fortunately, we obtained 2 without apparent polymeri-
zation by the formation of two C–C bonds in one step in
excellent yield.9 Therefore, such oxidative cyclizations
were efficient to develop well-defined, thiophene-cen-
tered polycyclic aromatics.
the proper analytical data to substantiate the accurate
structure of 9. We observed the exact loss of four hydro-
gen atoms from the MALDI-TOF MS spectrum of 9 in
1
comparison with that of 8. From the H NMR spectra
of 10, we observed the two doublet signals contributed
from the protons at the fluorene moieties moving to
the upfield at about 7.69 and 6.58 ppm (J = 7.8 Hz),
respectively, which were different with those of 2-thien-
ylfluorene.3d,6a This indicated the helical conformations
caused by the overlap of two fluorene rings.
The absorption spectra for 1, 2, 9, and 10 in dilute
CH2Cl2 solutions are shown in Figure 1. We observed
that the absorption of 2 peaked at 321 nm with a shoul-
der at 367 nm, which was red-shifted about 28 nm in
comparison with that of 1 (293 nm) due to the increase
of the effective conjugation length. Similarly, the absorp-
tion maximum peak (343 nm) of 10 was red-shifted
about 34 nm related to that of 9 (309 nm). However,
the absorption kmax of 9 was blue-shifted about 12 nm
in comparison with that of 2, which also suggested the
formation of helical structure. Therefore, compared to
those of all thiophene [7]helicene (256 nm)10d and all
benzene helicene,12 the absorption kmax of 9 obviously
red-shifted. The optical properties of all new compounds
in dilute CH2Cl2 solutions are summarized in Table 1.
Most recently, thiahetereohelicenes consisting of fused
thiahetereocycles have attracted extensive interest due
to their unique optical and physical properties and their
stable doped and neutral states owing to the p-conjuga-
tion on the thiophene rings.10 Both 1 and 2 were very
good building blocks for constructing the novel heli-
cenes. The synthetic route to helicenes 9 and 10 is illus-
trated in Scheme 3.11 Compound 7 was produced by the
Sonogashira reactions. After the Diels–Alder reactions
and decarbonylation as well as the oxidation, we
obtained helicene 9. Only two carbon–carbon bonds
were formed during the oxidation due to the helical
framework. To further confirm this structure, we
obtained 10 through Suzuki cross-coupling in excellent
yield of over 90%.
In conclusion, we have developed a reaction sequence to
synthesize a new series of planar and non-planar (heli-
cal) thiophene-based polycyclic aromatics. The Diels–
Alder reactions followed by decarbonylation produce
the precursors of the cyclizations. The regioselective
bromination of the unsubstituted a-positions of thio-
phene rings ensures the success of the oxidative cycliza-
1
In addition to H and 13C NMR spectra and analytical
data, MALDI-TOF MS we also employed to character-
ize the structure and molecular weight of all new com-
pounds (see the Supporting Information). We obtained

J. Pei et al. / Tetrahedron Letters 47 (2006) 1551–1554
Ph Ph
1553
Ph
Ph
Et
Et
S
O
Et
Et
S
Et
Et
S
Et
Et
Ph
Ph
Ph
Ph
FeCl3
Ph Ph
NBS
S
165 o
S
C
CHCl3/AcOH
CH2Cl2
0 o
C
S
S
Et
Et
S
Br
Br
7
8
Ph
Ph
Ph
Ph
B(OH)2
Et
Et
S
Et
Et
R: n-C6H13
R
R
S
Et
Et
S
S
Et
Et
Pd(PPh3)4, Na2CO3,
THF, reflux
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
S
S
Et
Et
S
S
Et
Et
Br
Br
( )
9
R
R
R
R
( )
10
Scheme 3.
that 9 exhibits the helical framework. The photophysical
properties of all compounds are investigated by UV–vis
measurement. The absorption spectra of the desired
materials are liable to be tuned by the introduction of
more conjugated moieties. These helicenes are good can-
didates for studying the one- or two-dimension mole-
cular wire in the field of organic semiconductors.
1
0.8
0.6
0.4
0.2
0
1
2
9
10
Acknowledgements
This research was financially supported by the Major
State Basic Research Development Program (No.
2002CB613401) from the Minister of Science and Tech-
nology, and National Natural Science Foundation of
China (NSFC. 20425207, 50473016, 50103001 and
90201021).
250
300
350
400
450
500
550
Wavelength (nm)
Figure 1. The absorption spectra of 1, 2, 9, and 10 in dilute CH2Cl2
solutions.
Supplementary data
Table 1. The optical properties of compounds 1, 2, 7, 8, 9, and 10 in
dilute CH2Cl2 solutions
Synthetic details for all new compounds 1–10 are avail-
able. Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/j.tetlet.
2006.01.006.
Compounds
kabs (nm)
k PL (nm)
1
293
321
337
298
309
343
305
338
395
312
342
424
2
7
References and notes
8
9
10
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1H NMR (300 MHz, CDCl3, ppm, d): 7.11 (m, 32H, Ph
and Th), 3.44 (m, 12H, CH2), 1.36 (m, 18H, CH3); 13C
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S 9.60%. Found: C 73.48%, H 4.33%, Br 12.05%, S 9.48%.
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J = 7.8 Hz, 2H, Ar), 3.55 (m, 12H, CH2), 1.86 (s, 8H,
CH2), 1.45 (m, 18H, CH3), 0.67 (m, 44H, CH2); 13C NMR
(75 MHz, CDCl3, ppm, d): 151.2, 151.0, 144.1, 141.9,
141.6, 141.3, 141.1, 140.8, 140.5, 136.5, 136.0, 135.7, 135.5,
134.9, 133.3, 132.9, 130.9, 130.5, 130.0, 129.8, 129.2, 128.5,
127.5, 127.2, 126.7, 126.0, 125.5, 122.9, 120.8, 120.6, 119.8,
119.5, 119.4, 55.0, 47.0, 40.1, 40.0, 31.3, 29.5, 27.1, 26.9,
26.3, 26.0, 23.5, 22.4, 16.1, 15.5, 15.4, 13.9; MALDI-TOF
MS m/z: 1841.8 (M+, 100%) (calcd for C132H128S4 1841.9).
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8. Compound 1 was obtained as white solids (yield: 85%). 1H
NMR (300 MHz, CDCl3, ppm, d): 7.74 (s, 2H, Th), 7.09
(m, 10H, Ph), 3.22 (q, J = 7.2 Hz, 4H, CH2), 1.19 (t,
J = 7.2 Hz, 6H, CH3); 13C NMR (75 MHz, CDCl3, ppm,
d): 141.3, 141.2, 135.7, 135.0, 133.3, 130.2, 127.2, 127.1,
126.1, 124.4, 115.3, 26.3, 15.5; MS m/z: 606.0 [(M+4)+,
100%]. Elemental analysis calcd for C30H22Br2S2: C