Synthesis and polymerisation of α,ω-bis(3-pyrrolyl)alkanes

Article abstract of DOI:10.1016/j.tet.2007.03.010

The synthesis, characterisation and polymerisation studies of a homologous series of α,ω-bis(pyrrolyl)alkanes are described. These α,ω-bis(pyrrolyl)alkanes were produced using Friedel-Crafts acylation followed by reduction of the carbonyl group using Red-

Full text of DOI:10.1016/j.tet.2007.03.010

Tetrahedron 63 (2007) 4237–4242
Synthesis and polymerisation of a,u-bis(3-pyrrolyl)alkanes
Richard C. Foitzik,a Akif Kaynaka and Frederick M. Pfefferb,
*
aDeakin University, School of Engineering and Information Technology, Geelong, Victoria 3217, Australia
bDeakin University, School of Life and Environmental Science, Geelong, Victoria 3217, Australia
Received 29 November 2006; revised 14 February 2007; accepted 1 March 2007
Available online 6 March 2007
Abstract—The synthesis, characterisation and polymerisation studies of a homologous series of a,u-bis(pyrrolyl)alkanes are described.
These a,u-bis(pyrrolyl)alkanes were produced using Friedel–Crafts acylation followed by reduction of the carbonyl group using Red-
AlÒ. Chemical polymerisation of the resultant dimers using FeCl3 produced poly(a,u-bis(pyrrolyl)alkane) films, which were characterised
by SEM, FTIR and tested for conductivity.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
There has been a large number of publications on cross-
linked monomers of thiophene linked through the C-3
position such as a,u-bis(thienyl)alkanes (Fig. 1). These a,u-
bis(thienyl)alkane molecules when polymerised are reported
to have good conductivity and are highly flexible.24–32
In contrast, studies on the cross-coupling of pyrrole at the
C-3,30-positions are scarce and the published work has
been limited to theoretical studies, patent examples or syn-
thesis for specific purposes.33–35
The cross-linking of polymers has proven to increase the
strength, brittleness, rigidity or even the elasticity of the re-
sultant polymers. For example, one of the first processes to
form cross-linked polymers was the addition of sulfur during
the vulcanisation of rubber, which produces a harder, more
robust form of rubber.1
The cross-coupling of two pyrrole molecules for the purpose
of forming conductive polymers can occur at the nitrogen,
C-2 or C-3 positions. Previous work has shown that the intro-
duction of an alkyl chain to couple two heterocyclic mole-
cules at the C-2 position alters a variety of properties, such
as flexibility and elasticity.1–4 Aromatic spacers have also
been used for this purpose and upon polymerisation of such
dimers large changes in conductivity and solubility were
noted.5,6 Various pyrrole dimers have previously been syn-
thesised by coupling at the C-2 positions, these dimers have
been used for multiple applications, the major use being for
sensor technologies.7–11
Detailed herein are the synthesis, polymerisation and char-
acterisation of a dimer family of a,u-bis(pyrrolyl)alkanes.
The ideal structure of a,u-bis(pyrrolyl)ethane (Fig. 2) illus-
trates the high degree of cross-linking, the extent of which
(CnH2n
)
S
S
Figure 1. a,u-Bis(thienyl)alkanes.
Generally, polymers that possess a high degree of cross-
linking require a monomer that has three or more sites for
polymerisation and this is usually achieved by first synthe-
sising a dimer. For thiophene and pyrrole this is generally
achieved by the fixation of a linker from the C-3 position.
Pyrrole dimers can also be produced easily by cross-linking
through the nitrogen.12–22 However, for our future applica-
tions of these polymers, the nitrogen position must be
unfunctionalised as it is involved in fixation to specific
substrates.23
H
N
H
N
N
H
N
H
H
N
H
N
N
H
N
H
Keywords: Conducting polymers; Cross-linked polymers; Cross-coupled
pyrrole; Pyrrole dimers; Pyrrole.
* Corresponding author. Tel.: +61 3 52271439; fax: +61 3 52271040;
e-mail: thefef@deakin.edu.au
Figure 2. Ideal polymer structure for poly(1,2-bis(pyrrolyl)ethane).
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.03.010

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R. C. Foitzik et al. / Tetrahedron 63 (2007) 4237–4242
Table 1. Isolated yields for the a,u-bis(N-tosyl pyrrolyl)alkanoyl com-
pounds
can be reduced by the introduction of another monomer unit
during polymerisation, i.e., synthesis of copolymers.
Compound
n
Yield (%)
2a
0
22
2b
2
42
2c
3
52
2d
4
68
2e
5
74
2f
6
92
2g
7
91
2h
8
95
2i
10
93
2. Results and discussion
Compound
n
Yield (%)
3a
0
12
3b
2
14
3c
3
76
3d
4
37
3e
5
43
3f
6
83
3g
7
98
3h
8
96
3i
10
98
A four-step process was employed for the synthesis of the
pyrrole dimers (Scheme 1). Initially, pyrrole was protected
with a tosyl group, then Friedel–Crafts acylation of the
tosyl-protected pyrrole provided high yields of the 3-acyl-
ated product. Removal of the protecting group was achieved
under basic conditions and finally, Red-AlÒ reduction
yielded the desired a,u-bis(pyrrolyl)alkanes.
Compound
n
Yield (%)
4a
0
4b
2
4c
3
4d
4
80
4e
5
88
4f
6
90
4g
7
91
4h
8
96
4i
10
91
41a 55a 91
a
Yield of a black insoluble polymeric material.
yields were obtained. The reduction of the deactivating car-
bonyl group afforded unstable dimers that underwent instan-
taneous polymerisation. The rest of the dimers (4c–4i) were
more stable, resulting in high yields (Table 1) of the desired
compounds after purification using column chromatography.
Due to its ability to deactivate the C-2 and C-5 positions and
activate the C-3 and C-4 positions of pyrrole, the tosyl pro-
tecting group for the N-1 position on pyrrole was chosen. For
the acylation reaction the shorter carbon chain diacid di-
chlorides were more reactive and required inert conditions
(dry N2 gas) in order to obtain a reasonable yield. The longer
chain diacid dichlorides were more stable, allowing the reac-
tion to be completed without strict anhydrous conditions and
good yields were obtained. Purification by column chroma-
tography gave the desired products, with the yield generally
increasing as the chain length increased (Table 1, Fig. 3).
The polymerisation of the dimers 4a–4i was achieved by ox-
idation with FeCl3 (Scheme 2). The conditions for polymer-
isation were varied for different purposes. For formation of
polymers 5a–5i as powders stirring of a specific dimer 4a–
4i in solution (water or organic solvents) was followed by
the addition of FeCl3. If the polymerisation was performed
at low temperatures with rapid stirring very fine powders
were produced.
Removal of the protecting group involved refluxing a dimer
(2a–2i) in 1,4-dioxane with NaOH (5 M in water) overnight.
For the larger alkyl chain derivatives (3f–3i) purification by
recrystallisation in a petroleum spirits/methanol mixture
gave the product in high yields (Table 1). For the shorter al-
kyl chain derivatives (3a–3e) purification proved to be more
difficult, with column chromatography required. Unfortu-
nately the shortest chains, n¼0 (3a) and n¼2 (3b), polymer-
ised readily in the presence of air and light, with little
un-polymerised product left after the overnight reflux.
Polymer films were produced by polymerisation of the di-
mers using the minimum amount of solvent in a wide recrys-
tallisation dish. The dimer and oxidising agent were mixed
quickly in solvent and the solvent was allowed to evaporate
providing thin films that were simply peeled off the glass-
ware. The resultant thin films were very flexible (Fig. 4),
however, as the thickness of the film increased the flexibility
decreased.
The final step in the synthesis of the a,u-bis(pyrrolyl)-
alkanes was the reduction of the carbonyl group. This was
achieved using Red-AlÒ solution under inert conditions.
Once again for the two shortest linkers (4a and 4b) poor
The films that were produced were investigated by SEM for
surface characteristics. The films that were cast using water
produced a homogeneous surface as shown in Figure 5. The
films that were produced from the evaporation of chloroform
O
O
(2a) n=0
(2b) n=2
(2c) n=3
(2d) n=4
(2e) n=5
(2f) n=6
(2g) n=7
(2h) n=8
(2i) n=10
(CnH2n
)
O
O
N
S
N
S
N
S
Cl
(CnH2n
i
)
Cl
O
O
O
O
O
O
(1)
ii
(4a) n=0
(4b) n=2
(4c) n=3
(4d) n=4
(4e) n=5
(4f) n=6
(4g) n=7
(4h) n=8
(4i) n=10
(3a) n=0
(3b) n=2
(3c) n=3
(3d) n=4
(3e) n=5
(3f) n=6
(3g) n=7
(3h) n=8
(3i) n=10
O
O
(CnH2n
)
(CnH2n
)
iii
N
H
N
N
H
N
H
H
Scheme 1. Synthesis of a,u-bis(pyrrolyl)alkanes (4a–4i). (i) AlCl3, 0 ꢀC, CH2Cl2, 4 h; (ii) NaOH, reflux, 1,4-dioxane, 14 h; (iii) Red-AlÒ, 0 ꢀC, THF, 3 h.

R. C. Foitzik et al. / Tetrahedron 63 (2007) 4237–4242
4239
Figure 3. 13C NMR of 1,10-bis(N-tosyl-3-pyrrole)decane-1,10-dione (2h).
H
N
FeCl3
(CnH2n
)
(CnH2n
)
N
H
N
H
N
H
(4a) n=0
(4b) n=2
(4c) n=3
(4d) n=4
(4e) n=5
(4f) n=6
(4g) n=7
(4h) n=8
(4i) n=10
(5a) n=0
(5b) n=2
(5c) n=3
(5d) n=4
(5e) n=5
(5f) n=6
(5g) n=7
(5h) n=8
(5i) n=10
Scheme 2. Polymerisation of a,u-bis(pyrrolyl)alkanes (4a–4i).
exhibited a ‘bubbled’ effect (Fig. 6), which has been seen
before with poly(3-alkylpyrroles).23,36
The poly(1,10-bis(pyrrolyl)decane) films formed by the
evaporation of water were more brittle than those formed
by evaporation of chloroform, this was possible due to
encapsulated chloroform in the polymer matrix (Fig. 5).
The crack on the right side of the image is characteristic of
these films, whereas the films formed by the evaporation
of chloroform did not exhibit this brittleness.
Figure 4. Flexibility of poly(1,10-bis(pyrrolyl)decane) films (5h).
exhibited higher conductivity due to their continuous
surfaces; therefore, these films were used throughout the
conductivity tests.
The poly(a,u-bis(pyrrolyl)alkane) samples were prepared
for conductivity measurements by two different methods.
The growth of thin conductive films by solution polymerisa-
tion and subsequent evaporation of solvent was the first
method tried, the other was the grinding and compression
of the poly(a,u-bis(pyrrolyl)alkane) powders formed from
solution polymerisation into disks. The thin polymer films
Unlike the poly(3-alkylpyrroles), for which a steady de-
crease in conductivity was observed as the alkyl chain length
increased,36 the poly(a,u-bis(pyrrolyl)alkane) films showed
no obvious trends. Two conductivity maxima were observed
when 5-carbon (5c) and 8-carbon (5f) alkyl chain couplers
are used (Fig. 7) with the latter possessing the highest
conductivity.

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R. C. Foitzik et al. / Tetrahedron 63 (2007) 4237–4242
6
Dopant = Cl-
Dopant = AQSA
Dopant = tosyl
5
4
3
2
1
Figure 5. Scanning electron micrograph of 1,10-bis(pyrrolyl)decane poly-
merised with FeCl3 in a minimum amount of water, which was subsequently
evaporated. Magnification 3000ꢂ.
0
5
7
9
11
Alkyl cross-coupler carbon chain length
Figure 8. Conductivity testing of films produced by solution polymerisation
in the presence of Clꢁ, AQSA and tosyl dopants.
gap and promoting the movement of electrons along the
polymer chains.37 The use of other dopants was also investi-
gated, the results of which can be seen in Figure 8. Due to the
light and temperature sensitivity of the dimers 4a and 4b
they polymerised uncontrollably to produce polymers 5a
and 5b, and as these polymers were not produced under con-
trolled conditions they were not tested for dopant effects.
Three different dopants were used, namely Clꢁ (from
FeCl3), anthraquinone-2-sulfonic acid sodium salt (AQSA)
and tosyl sodium salt. The results obtained from the
AQSA and FeCl3 were approximately the same, with the
only difference being that the AQSA dopant produced poly-
mers that had a more uniform decrease in conductivity with
increase in alkyl chain length. A marked improvement in
conductivity resulted from the use of the tosyl dopant, which
showed approximately double the reported conductivity of
the polymers formed in the presence of the Clꢁ or AQSA
dopants. In general the trend for these polymers is a decrease
in conductivity as the cross-coupling carbon chain length is
increased. However, it is interesting to note that there is a di-
vergence in this trend for the poly(1,8-bis(pyrrolyl)octane)
(5f) films with respect to all three dopants tested.
Figure 6. Scanning electron micrograph of 1,10-bis(pyrrolyl)decane poly-
merised with FeCl3 in a minimum amount of chloroform, which was subse-
quently evaporated. Magnification 1000ꢂ.
3
2.5
2
1.5
1
Assessing all experimental data and the results from conduc-
tivity tests it was determined that eight carbons was the op-
timum chain length. Three factors led to this conclusion:
a high percentage yield, greater stability of the longer alkyl
chain dimers and the conductivity could be greatly increased
with a judicious choice of dopant.
0.5
0
2
4
6
8
10
12
Alkyl cross-coupler carbon length
In conclusion the synthesis of the a,u-bis(pyrrolyl)alkanes
was achieved in relatively high yields, however, the synthe-
sis of shorter cross-coupled carbon chain lengths (n¼0–2) of
a,u-bis(pyrrolyl)alkanes was difficult. Due to air and light
sensitivity, these shorter chain dimers polymerised uncon-
trollably. Stability of the larger chain a,u-bis(pyrrolyl)-
alkanes (n>7) allowed isolated yields in excess of 95% to
be accomplished.
Figure 7. Conductivity of films produced from the solution polymerisation
of a,u-bis(pyrrolyl)alkanes.
Polymers formed from controlled polymerisations using
FeCl3 have a higher conductivity due to the FeCl3 acting
as a source of Clꢁ as a dopant, thus decreasing the band

R. C. Foitzik et al. / Tetrahedron 63 (2007) 4237–4242
4241
The solution polymerisation method produced highly flexi-
ble conductive films when solvent evaporation from CHCl3
was employed. Modifying polymerisation parameters
allowed the resultant polymer to be forged in different solid
states such as powders. The optimum monomer for yield
and conductivity was poly(1,8-bis(pyrrolyl)octane). The
conductivity of the poly(a,u-bis(pyrrolyl)alkanes) can be
improved by the appropriate choice of dopants. The AQSA
dopant had little effect, however, the tosyl sodium salt caused
a substantial increase in the observed conductivity.
3.1.2. Synthesis of the a,u-bis(N-tosyl-3-pyrrolyl)alka-
noyl compounds (2a–2i). The appropriate alkyl diacid chlo-
ride (0.9 mol equiv) in dry CH2Cl2 (20 mL/g) was added to
a solution of AlCl3 (2.0 mol equiv) in dry CH2Cl2 (20 mL/
g of AlCl3) in one portion at rt under a nitrogen atmosphere.
After 10 min the reaction was cooled to 0 ꢀC and a solution of
N-tosyl-pyrrole (1) (2.0 mol equiv) in CH2Cl2 (10 mL/g) was
added dropwise over 5 min. This was stirred at 0 ꢀC for 2 h
before ice water was added cautiously and the product ex-
tracted using diethyl ether (3ꢂ200 mL). The organicportions
were combined, dried (Na2SO4), filtered and the solvent re-
moved under reduced pressure. The resultant products were
purified using column chromatography with petroleum
spirits/ethyl acetate (3:2) as the eluent. Products isolated
were white/grey solids or clear oils (22–93%). All yields,
1H and 13C NMR and elemental data are located in the
Supplementary data.
3. Experimental section
3.1. Materials and methods
All chemicals were purchased from Sigma–Aldrich (>98%
purity) and used as supplied. THF was distilled from
sodium/benzophenone ketyl under dry nitrogen. CH2Cl2
3.1.3. Synthesis of the a,u-bis(3-pyrrolyl)alkanoyl com-
pounds (3a–3i). An NaOH solution in water (5 M,
100 mL/g of the dimer 2a–2i) was added to a cross-coupled
tosyl-protected pyrrole (one of 2a–2i) dissolved in 1,4-diox-
ane (100 mL/g of 2a–2i) and the mixture refluxed overnight.
The mixture was cooled and extracted with diethyl ether
(3ꢂ100 mL). The organics were combined, dried (Na2SO4)
and the solvent removed under reduced pressure. The re-
sultant products 3a–3c were purified using column chroma-
tography with petroleum spirits/ethyl acetate (3:2) as the
eluent, yielding oils (12–76%), or recrystallised using petro-
leum spirits for the larger products (3d–3i) yielding white
powders (37–98%). All yields, 1H and 13C NMR and elemen-
tal data are located in the Supplementary data.
˚
was distilled from CaH2 and stored over 4 A molecular
sieves.38 Thin layer chromatography (TLC) was performed
on Merck Kieselgel 60 F256 aluminium backed plates, which
were developed in a chromatotank. Visualisation was
achieved using a UV lamp or by an oxidising dip comprised
of K2CO3 (1.0 g), KMnO4 (1.0 g) and H2O (100 mL). TLC
plates were dipped, then dried using a heat gun to ensure
that all compounds became visible. Column chromatography
was performed using silica gel (60–270 mesh). Solution
state NMR spectra were recorded using a Jeol JNM-GX
270 MHz or a JNM-ECP 400 MHz spectrometer as indicated
and referenced against tetramethylsilane (TMS).39 Two-
dimensional (2D) NMR spectra (COSY, HMQC and
HMBC) were recorded on the JNM-ECP 400 MHz spec-
trometer. Chemical shifts (d) are reported in parts per million
and coupling constants are reported in hertz. Melting points
were measured on a Reichart hot-stage microscope and are
uncorrected. Scanning electron microscope (SEM) images
were recorded on a Leica S440 Scanning Electron Micro-
scope. The samples were prepared by sputter coating a thin
layer of gold (w7–10 nm) onto the textile surface. New
compounds were characterised using elemental analysis
(Chemical and Micro Analytical Services Pty. Ltd. Belmont,
Victoria, Australia). Solid pellets of polymer were formed by
grinding the polymer and pressing into disks. Four probe
conductivity measurements were performed on the solid
disks or films using a Fluke multimeter.
3.1.4. Synthesis of a,u-bis(pyrrolyl)alkanes (4a–4i).
Under an atmosphere of nitrogen 3a–3i (31.00 mmol) was
dissolved in THF (125 mL) and cooled to 0 ꢀC while stir-
ring. A solution of Red-AlÒ (65% w/w, in toluene,
19.95 g, 64.00 mmol) was added slowly. The solution was
allowed to warm to rt and stirring continued for 3 h. After
heating the reaction mixture at 50 ꢀC for 1 h, the solution
was cooled to 0 ꢀC and cautiously hydrolysed by the slow
addition of water (100 mL). The crude product was extracted
with diethyl ether (3ꢂ50 mL) and dried (Na2SO4). The sol-
vent was removed and the product distilled at 0.01 mmHg to
yield a dark oil, or recrystallised from hexane/dichloro-
1
methane (1:1) to yield a white powder (41–96%). All H
and 13C NMR for 4c–4i are located in the Supplementary
data, 4a and 4b polymerised immediately and could not be
measured.
3.1.1. N-p-Toluenesulfonylpyrrole (1). A solution of pyr-
role (25.00 g, 0.37 mol), KOH (41.85 g, 0.75 mmol) and
tosyl chloride (66.73 g, 0.35 mol) in THF (1.10 L) was pre-
pared. The solution was refluxed for 4 h, cooled to rt, where-
upon diethyl ether (500 mL) was added and the reaction
mixture transferred to a separatory funnel. The reaction mix-
ture was washed with copious amounts of water (5ꢂ250 mL)
followed by a saturated NaCl solution (3ꢂ250 mL). The
organic layer was separated, dried (Na2SO4), filtered and the
solvent removed under reduced pressure. The resulting grey
solid was recrystallised from methanol (200 mL) to yield the
desired product as a fine white powder (73.70 g, 90%); mp
100–102 ꢀC; dH (270 MHz, CDCl3) 2.38 (3H, s, CH3), 6.26
(2H, t, J 2.2 Hz, Py), 7.14 (2H, t, J 2.0 Hz, Py), 7.26 (2H, d,
J 9.4 Hz, tosyl), 7.74 (2H, d, J 8.4 Hz, tosyl); dC (67.9 MHz,
CDCl3) 21.7, 113.6, 120.8, 126.9, 130.1, 136.1, 145.1.40
3.1.5. Synthesis of poly(a,u-bis(3-pyrrolyl)alkanes)
(5c–5i).
3.1.5.1. Method A. A dimer (one of 4c–4i) (5.0 mmol)
was stirred in CHCl3 (100 mL) while FeCl3 (5.0 mmol)
was added in one portion and the reaction left stirring for
3 h. The reaction mixture was filtered to separate the black
polymer, which was ground into a powder and washed with
copious amount of water to remove iron impurities. The
polymer powder was placed under reduced pressure
(0.1 mmHg) for 4 h before being processed into thin disks
for conductivity measurements. The disks produced were
extremely brittle making the conductivity measurements
difficult.

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R. C. Foitzik et al. / Tetrahedron 63 (2007) 4237–4242
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3.1.5.2. Method B. A flat dish with a large surface area
(200ꢂ200 mm) was filled with solvent (water or chloro-
form) to a depth of 2 mm. A dimer (one of 4c–4i)
(5.0 mmol) and FeCl3 (5.0 mmol) were added and the reac-
tion mixture was left, allowing the solvent to evaporate. This
occurred over varying lengths of time depending on the sol-
vent used. The resultant polymer film was soaked in water
followed by an aqueous HCl solution at pH 2. The film
was dried under reduced pressure (0.1 mmHg) for 4 h before
being tested for conductivity. The films that were produced
were flexible making conductivity measurements possible.
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The authors wish to acknowledge Australian Wool Innova-
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authors also thank Dr. Alan De Boos and Dr. Nathan Ly
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Supplementary data
Supplementary data associated with this article can be found
in the online version, at doi:10.1016/j.tet.2007.03.010.
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