Facile synthesis and characterization of new polymerizable conjugated 2,5-di(selenophen-2-yl)pyrroles and 2,5-difuranylpyrroles

Article abstract of DOI:10.1016/j.tetlet.2010.12.019

A facile synthesis of novel π-conjugated 2,5-di(selenophen-2-yl)pyrroles (SeNSe) and 2,5-difuranylpyrroles (ONO) via Paal-Knorr reaction as the key step is presented. Photophysical and electrochemical studies of the various products have been explored. A

Full text of DOI:10.1016/j.tetlet.2010.12.019

Tetrahedron Letters 52 (2011) 711–714
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Facile synthesis and characterization of new polymerizable conjugated
2,5-di(selenophen-2-yl)pyrroles and 2,5-difuranylpyrroles
⇑
Pitchamuthu Amaladass, Kalyan Kumar Pasunooti, Zihuan Png, Xue-Wei Liu
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history:
A facile synthesis of novel
p-conjugated 2,5-di(selenophen-2-yl)pyrroles (SeNSe) and 2,5-difuranylpyr-
Received 6 October 2010
Revised 20 November 2010
Accepted 3 December 2010
Available online 9 December 2010
roles (ONO) via Paal–Knorr reaction as the key step is presented. Photophysical and electrochemical stud-
ies of the various products have been explored. A bathochromic shift of the emission maximum is
observed for all SeNSes over ONOs. Extended conjugation with a phenyl moiety further promotes the
bathochromic shift. These SeNSe and ONO derivatives exhibit lower oxidation potentials than their terse-
lenophene and terthiophene analogues.
Keywords:
Heterocycles
Ó 2010 Elsevier Ltd. All rights reserved.
Pyrrole derivatives
Polymers
Paal–Knorr reaction
Friedel–Crafts acylation
Heterocycles possessing oxygen, sulfur and selenium atoms
(group VI elements) play significant roles in the field of materials
chemistry.1 Over the past few years, significant attention has been
3,4-ethylenedioxy thiophene (EDOT) were reported by Yildiz
et al.10 The functionalization of 2,5-dithienylpyrroles with tetracy-
anoethylene (TCNE) was reported by two different groups for or-
ganic field-effect transistors (OFETs).11 Despite the many Letters
published on thienyl oligomers, very little is known on their close
analogues, di(selenophen-2-yl)pyrroles and difuranylpyrroles. The
exciting photophysical and spectroelectrochemical properties of
conducting oligomers based on 2,5-SNS derivatives, along
with our ongoing interest in the synthesis of functional mole-
cules,12 prompted us to research 2,5-di(selenophen-2-yl)pyrroles
2, (SeNSe) and 2,5-difuranylpyrrole derivatives 3, (ONO). Herein,
we report the first synthesis of these derivatives and present their
photophysical and electrochemical properties.
The pyrrole derivatives 2 and 3 were chosen as polymer precur-
sors for the following reasons: (i) synthesis using the Paal–Knorr
reaction represents an attractive one-step procedure, (ii) the oxida-
tion potential of SeNSe derivatives and ONO derivatives are lower
(about +0.8 V vs SCE and +0.68 V vs SCE) than those of their terse-
lenophene and terthiophene analogues (about +0.88 V vs SCE and
+0.95 V vs SCE) and (iii) good quality films of poly SeNSe as well
focused on
p-conjugated heterocyclic organic molecules (oligo-
mers/polymers) due to their wide range of applications such as, or-
ganic light-emitting diodes (OLEDs),2 nonlinear optics (NLOs),3
field-effect transistors (FETs),4 electrochromic devices5 and sensor
materials,6 which are useful in display technology and data stor-
age. In this connection, the design, synthesis and photophysical
studies of p-conjugated monomer 1, (SNS, Fig. 1) and analogues de-
rived from group VI elements represent a field of enormous growth
in materials chemistry. Conjugated oligomers/polymers, which are
obtained from these derivatives provide highly conducting proper-
ties, and particularly, a range of optical properties with electronic
band gaps, making them useful for numerous electrochemical
applications. However, the lack of research on these derivatives
is due to the poor availability of suitable synthetic methodologies
for the preparation of substituted monomeric precursors.
Ferraris and Skiles first proposed the use of poly-2,5-dithienyl-
pyrrole (poly-SNS) en route to well-defined polymers in 1987.7 In
addition, Johannsen and co-workers reported the synthesis of a ser-
ies of electropolymerizable 2,5-SNS derivatives towards conductive
polymers.8 The electropolymerizable thienyl pyrrole derivatives
have also been studied by different groups for electrochromic appli-
cations.9 Moreover, soluble conducting polymers based on 2,5-dit-
hienylpyrrole derivatives and a multi-chromic co-polymer with
N
R
N
R
N
R
Se
Se
O
O
S
S
2
3
1
⇑
Corresponding author. Tel.: +65 63168901; fax: +65 67911961.
E-mail address: xuewei@ntu.edu.sg (X.-W. Liu).
Figure 1. Conjugated pyrrole derivatives 1, (SNS), 2, (SeNSe), and 3, (ONO).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.12.019

712
P. Amaladass et al. / Tetrahedron Letters 52 (2011) 711–714
as poly ONO can be generated easily on a platinum disc working
electrode.
which is an important intermediate for the synthesis of 2,5-
difuranylpyrroles.13
We envisaged that the synthesis of the pyrrole derivatives 2 and
3 could be achieved via Paal–Knorr reaction with a key diketone
intermediate and a wide range of aliphatic and aromatic amines.
Therefore, our efforts to synthesize the pyrrole derivatives 2 and
3 started with judicious choice of the diketone 6. As shown in
Scheme 1, we initially studied the conventional double Friedel–
With the diketone 6 in hand, we next investigated its cycliza-
tion with different amines. Various acid catalysts, such as
propionic acid, p-toluenesulfonic acid (PTSA) and titanium tetra-
chloride were reported previously to promote cyclization to afford
dithienylpyrrole derivatives.14–16 However, it is highly important
to maintain the reaction medium (solvent) at an appropriate acid-
ity in order to avoid the formation of by-products. Hence, we car-
ried out carefully optimization reactions to fine-tune cyclization of
the diketone with primary amines by employing different solvents
and catalysts. Preliminary screening results are summarized in Ta-
ble S1(Supplementary data). Initially, we used the aliphatic amines
n-hexylamine, n-dodecylamine and n-hexadecylamine, and differ-
ent solvents and acid catalysts for SeNSe (2) and ONO (3) monomer
derivatives. From Table S1, it is evident that the yields of SeNSe (2)
Crafts acylation protocol where selenophene
4 and succinyl
chloride 5 were reacted in the presence of a Lewis acid (AlCl3).
However, this transformation was rather sluggish affording a very
poor yield (5%) of the desired diketone 6. Alternatively, the use of
the Weinreb amide precursor 8 (N1,N4-dimethoxy-N1,N4-dimethyl-
succinamide) led to the desired diketone 6, in a much better yield
(45%). Similarly, a previously reported synthetic protocol was
implemented to synthesize 1,4-di(furan-2-yl)butane-1,4-dione,
AlCl3, dry CH2Cl2
Cl
Cl
+
Se
º
Se
0 C, 5%
Se
O O
O
O
6
4
5
n-BuLi, THF
Se
4
º
0 C, 45%
O
OMe
N
O
H3CO
Et3N
Cl
OMe
N
NH2 Cl
Cl
+
Me
H3C
dry CH2Cl2
60%
O
O
Me
5
7
8
Scheme 1. Synthesis of 1,4-di(selenophen-2-yl)butane-1,4-dione (6) via double Friedel–Crafts acylation from succinyl chloride or the corresponding double Weinreb amide.
R-NH2, propionic acid
N
R
OO
X
X
X
X
X
toluene, reflux, 24 h
X = Se, O
N
C6H13
N
N
N
N
C16H33
X
X
X
X
X
X
X
X
X
C12H25
OMe
X: Se, (2a) 61%
O, (3a) 45%
X: Se, (2c) 63%
O, (3c) 51%
X: Se, (2d) 51%
O, (3d) 40%
X: Se, (2b) 65%
O, (3b) 55%
X: Se, (2e) 55%
O, (3e) 35%
N
N
N
X
N
N
X
X
X
X
X
X
X
X
X
O
Cl
F
OMe
X: Se, (2g) 53%
O, (3g) 45%
X: Se, (2h) 25%
O, (3h) 20%
X: Se, (2j) 15%
X: Se, (2f) 50%
O, (3f) 41%
X: Se, (2i) 27%
O, (3i) 21%
O, (3j) 10%
Scheme 2. Synthesis of 2,5-di(selenophen-2-yl)pyrroles 2, (SeNSe) and 2,5-difuranylpyrroles 3, (ONO). X = Se (2a–2j) 2,5-diselenylpyrroles, X = O (3a–3j); 2,5-
difuranylpyrroles, C6H13: n-hexylamine; C12H25: n-dodecylamine; C16H33: n-hexadecylamine.

P. Amaladass et al. / Tetrahedron Letters 52 (2011) 711–714
713
and ONO (3) products were not high enough when benzene was
Compound 2a
Compound 3a
Compound 2i
employed as the solvent. Similarly, the product yields were found
to be low when strong acid catalysts, such as PTSA were used,
especially in the case of aliphatic or less conjugated amines. How-
ever, when a weak acid such as propionic acid was utilized, the
yields increased compared to other acids. Among the other sol-
vents tested, toluene was the best in terms of reactivity and yields.
From the above investigations, toluene in the presence of propionic
acid was established as optimized reaction conditions for the Paal–
Knorr cyclization. Using the optimized conditions, a variety of
aliphatic and aromatic amines were subjected to acid-catalysed
cyclization to give 2,5-di(selenophen-2-yl)pyrroles and 2,5-difura-
nylpyrroles (Scheme 2).
In general, aliphatic amines furnished the desired di(seleno-
phen-2-yl)pyrroles and difuranyl pyrroles in good yields when
compared to aromatic and heterocyclic amines. It is worthy to note
that when long chain aliphatic amines, such as n-hexylamine,
n-dodecylamine and n-hexadecylamine were employed, di(seleno-
phen-2-yl)pyrroles were obtained in better yields than difuranyl-
pyrroles (2a–c and 3a–c, Scheme 2). However, there was no
impact from the aliphatic chain length on the yield of either the
furan- or selenophene-containing pyrroles. Substituted benzyl
amines gave moderate yields in the case of di(selenophen-2-
yl)pyrroles whereas difuranylpyrroles were obtained in lower
yields (2d–f and 3d–f). The reactions of different substituted ani-
lines possessing neutral (phenyl), electron-donating (4-methoxy)
and electron-withdrawing (4-fluoro) produced the heterocyclic
products 2h–j and 3h–j in low yields compared to other amines.
The 2,5-di(selenophen-2-yl)pyrroles and 2,5-difuranylpyrroles
synthesized were fully characterized by 1H, 13C NMR, HR-MS and
physical data (see Supplementary data).
1.0
0.8
0.6
0.4
0.2
0.0
300
400
500
600
Wavelength (nm)
Figure 2. Absorption spectra of 2a, 3a and 2i in dichloromethane.
0.00016
0.00014
0.00012
0.00010
0.00008
0.00006
0.00004
0.00002
0.00000
-0.00002
-0.00004
-0.00006
-0.00008
3c
2c
The physical properties were recorded for all the di(selenophen-
2-yl)pyrroles and difuranylpyrroles and the spectroscopic parame-
ters for compounds 2a–d, 3a–d, 2f–g, 3f–g, 2i and 3i are listed in
Table 1. Figure 2 shows the UV–vis spectra of 2a, 3a and 2i. A bath-
ochromic shift of the absorption maximum is observed for all 2,5-
di(selenophen-2-yl)pyrroles, compared to the corresponding 2,5-
difuranylpyrroles, which can be explained by the more aromatic
nature of the di(selenophen-2-yl)pyrrole systems over the furanyl
analogues. A significant bathochromic shift of the absorption max-
imum was observed for the monomers 2i/3i and 2j/3j over 2a–c/
3a–c, which can be explained by the extended conjugation due
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
E(V) vs NHE
Figure 3. Cyclic voltammograms of 3c and 2c in dichloromethane at a scan rate of
50 mV/s.
Table 1
Spectroscopic features for selected compounds (2a–d, 3a–d, 2f–g, 3f–g, 2i and 3i) in
dichloromethane (c 2.5 ꢀ 10ꢁ5 mol/L)
b
c
d
e
Compounda
kmax (nm)
klum (nm)
Eg (eV)
1Epa (V)
HOMOf
LUMOg
onset
ox
E
(eV)
2a
3a
2b
3b
2c
3c
2d
3d
2f
333
313
332
312
331
313
339
316
336
315
334
314
360
335
419
385
417
370
418
379
429
391
431
385
435
354
439
392
3.03
3.36
3.04
3.32
3.05
3.26
3.05
3.36
2.98
3.37
3.02
3.36
2.95
3.36
0.98
0.92
0.82
0.72
0.81
0.69
0.81
0.77
0.84
0.76
0.82
0.78
0.78
0.72
0.66
0.55
0.65
0.53
0.63
0.54
0.62
0.56
0.64
0.51
0.70
0.63
0.53
0.56
5.10
4.99
5.09
4.97
5.07
4.98
5.06
5.00
5.08
4.95
5.14
5.07
4.97
5.00
2.07
1.63
2.05
1.65
2.01
1.72
2.01
1.64
2.10
1.58
2.12
1.71
2.02
1.64
3f
2g
3g
2i
3i
a
b
c
d
e
f
Spectroscopic properties for selected compounds.
Measured in dilute dichloromethane solution.
Excited at the absorption maxima.
Estimated from the onset of absorption (Eg = 1240/konset).
1Epa denotes the first anodic peak potential.
Calculated using the empirical equation: HOMO ¼ ð4:44 þ E
Calculated from LUMO = HOMO ꢁ Eg.
onset
ox
Þ.
g

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P. Amaladass et al. / Tetrahedron Letters 52 (2011) 711–714
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The electrochemical parameters of selected 2,5-di(selenophen-
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to dithienylpyrroles. The oxidation potentials of monomers 3c and
2c occur at +0.67 and +0.81, respectively, on a platinum disc elec-
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experiment, it is obvious that the oxidation potentials of 2,5-dif-
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2,5-di(selenophen-2-yl)pyrroles are lower than the terselenophene
and the corresponding thienyl analogues.17
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In summary, we have presented a two-step synthesis of an ar-
ray of novel
p-conjugated 2,5-di(selenophen-2-yl)pyrroles 2,
(SeNSe) and 2,5-difuranylpyrroles 3, (ONO) via the Paal–Knorr
reaction as a key step. Photophysical and electrochemical studies
of the various products have been described. A bathochromic shift
of the emission maximum is observed for all SeNSes over ONOs.
The extended conjugation due to the presence of a phenyl moiety
further promotes the bathochromic shift. These SeNSe and ONO
derivatives exhibit lower oxidation potentials than those of their
terselenophene and terthiophene analogues. Electropolymeriza-
tion of the synthesized monomers is in progress in our laboratory.
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Acknowledgements
Financial support from Nanyang Technological University
(RG54/07) and the Ministry of Education Singapore (ARC24/07,
no. T206B1218RS) are greatly acknowledged.
Supplementary data
Supplementary data (experimental procedures, compound
characterization data and spectra) associated with this article can
be found, in the online version, at doi:10.1016/j.tetlet.2010.12.019.
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