A versatile route to polythiophenes with functional pendant groups using alkyne chemistry

Article abstract of DOI:10.3762/bjoc.12.265

A new versatile polythiophene building block, 3-(3,4-ethylenedioxythiophene)prop-1-yne (pyEDOT) (3), is prepared from glycidol in four steps in 28% overall yield. pyEDOT features an ethynyl group on its ethylenedioxy bridge, allowing further functionalization by alkyne chemistry. Its usefulness is demonstrated by a series of functionalized polythiophene derivatives that were obtained by pre- and post-electropolymerization transformations, provided by the synthetic ease of the Sonogashira coupling and click chemistry.

Full text of DOI:10.3762/bjoc.12.265

A versatile route to polythiophenes with functional pendant
groups using alkyne chemistry
Xiao Huang*1, Li Yang2, Rikard Emanuelsson2, Jonas Bergquist1, Maria Strømme2,
Martin Sjödin2 and Adolf Gogoll*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2016, 12, 2682–2688.
1Department of Chemistry - BMC, Uppsala University, Box 576, 751
23 Uppsala, Sweden and 2Nanotechnology and Functional Materials,
Department of Engineering Sciences, Uppsala University, Box 534,
751 21 Uppsala, Sweden
doi:10.3762/bjoc.12.265
Received: 19 September 2016
Accepted: 22 November 2016
Published: 09 December 2016
Email:
Xiao Huang* - xiao.huang@kemi.uu.se; Adolf Gogoll* -
Adolf.gogoll@kemi.uu.se
Associate Editor: P. J. Skabara
© 2016 Huang et al.; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
electropolymerization; functional polymers; polythiophene;
Sonogashira coupling; thiophene
Abstract
A new versatile polythiophene building block, 3-(3,4-ethylenedioxythiophene)prop-1-yne (pyEDOT) (3), is prepared from glycidol
in four steps in 28% overall yield. pyEDOT features an ethynyl group on its ethylenedioxy bridge, allowing further functionaliza-
tion by alkyne chemistry. Its usefulness is demonstrated by a series of functionalized polythiophene derivatives that were obtained
by pre- and post-electropolymerization transformations, provided by the synthetic ease of the Sonogashira coupling and click chem-
istry.
Introduction
Currently organic conjugated polymers are attracting consider- fore, the chemistry of its building block 3,4-ethylenedioxythio-
able interest for various applications in plastic electronics. In phene (EDOT) and the functionalization of the basic structure
particular, poly(3,4-ethylenedioxythiophene) (PEDOT) [1] and have been attracting interest as well [13]. The vast amount of
its derivatives [2-7] play an increasingly important role in this research on functionalized polypyrroles [14] and polythio-
field. The attractiveness of PEDOT in organic electronics is due phenes [15] demonstrates that the attachment of functionalized
to its electrochemical stability in combination with conduc- pendant groups to the conjugated polymer backbone provides
tivity and solution processability. Recently demonstrated suc- access to novel properties and applications. An EDOT building
cessful applications include electrochromic materials [8], block to which highly functionalized pendants can easily be at-
energy storage materials [2-9], as well as ion sensing devices tached thus offers the possibility for rapid access to new func-
[10], biosensors [11], and thermoelectric polymers [12]. There- tionalized materials.
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Results and Discussion
and click chemistry is that it allows for reaction conditions
EDOT functionalization protocols typically involve manipula- tolerant for nearly all of the above mentioned functional groups.
tions of the ethylenedioxy bridge. Thus, the hydroxylmethyl de- Additionally, these functionalizations require only readily avail-
rivative EDOT-CH2OH [16], aminomethyl derivative EDOT- able starting materials. A related concept, i.e., attachment of a
CH2NH2 [17] and the methylenethiol derivative EDOT-CH2SH terminal alkyne moiety to the polymerizable thiophene deriva-
[18], as nucleophiles, as well as the halomethyl derivative tive ProDOT, an EDOT analogue, and its utilization for “click“
EDOT-CH2-Cl/Br [6,9] and the exomethylene-EDOT [19] as chemistry has been reported [28]. However, this involved an
electrophiles, can be used to form ether, thioether, ester, amine ether linkage. Furthermore, substantial differences between
and peptide linkages with the pendant groups. The polar reac- ProDOT and EDOT polymers regarding their electrochemical
tion conditions required for their synthesis exclude the use of properties have been described [29].
pendant groups featuring electrophilic or nucleophilic func-
tional moieties, such as esters and alkyl halide, or alcohols and Scheme 2 shows the synthesis of pyEDOT 3. It starts with
phenols. Furthermore, in the case of EDOT-CH2-Cl/Br, the glycidol that can be economically prepared by the epoxidation
α-hydrogen on the ethylene bridge is acidic and it thus favors of allyl alcohol [30]. The alkynediol 2 was prepared from
β-elimination under basic conditions leading to exomethylene- glycidol by using a four-step sequence reported by Pattenden
EDOT [19]. Therefore, the usage of basic nucleophiles is prob- and co-workers [31]. The presence of catalytic p-toluene-
lematic. Moreover, these heteroatom-based linkers between the sulfonic acid (p-TSA) in the toluene solution of 3,4-
polymer backbone and the pendant group usually have limited dimethoxythiophene and 2 resulted in a transetherification reac-
tolerance to acidic or basic conditions promoting hydrolysis, tion to form pyEDOT, which was isolated as light yellow oil by
and some of them are electrochemically redox active. The chromatography with a yield of 64%. The yield of the
stability of the linker would thus constitute an additional param- transetherification product was influenced by two factors. Sub-
eter to consider when designing functional polymers.
strate concentrations below 0.5 M resulted in much less poly-
merization, leaving more diols for transetherification. However,
The alkyne group is a versatile synthetic building block which still lower concentrations of less than 0.1 M resulted in lower
can be functionalized in a number of fashions [20-23]. These yields, probably due to decrease of the reaction rate. The yield
include, e.g., well-developed cross-coupling reactions, cycload- could also be improved by dividing the addition of the diol 2
dition reactions, radical reactions and reductive addition reac- into several portions, added over two days. Two or three
tions. A terminal alkyne can also react as nucleophile or serve portions were found to result in the highest yield, with more
as synthon for pyrrole rings [24,25]. Thus we here fuse the rich portions not changing the yield significantly.
alkyne chemistry to the EDOT backbone, resulting in a novel
EDOT derivative, the 3-(EDOT)prop-1-yne (pyEDOT, 3). Prior to the high yielding pyEDOT synthesis presented here, we
pyEDOT provides a useful synthon for the synthesis of a variety attempted to synthesize ethynyl-(EDOT) 8 (eEDOT) and
of EDOT-based polymerizable building blocks. This new ethynyltrimethylsilane-(EDOT) 8’ (etEDOT) with the alkyne
EDOT functionalization strategy (Scheme 1), including the po- moiety directly attached to the ethylenedioxyl bridge, albeit
lymerization of the resulting building blocks, we illustrate by with much lower yield (Scheme 3). The synthesis started from
introducing two examples of such functionalization using a economically available D-mannitol diketal, 1,2:5,6-bis-O-(1-
Sonogashira cross-coupling [26] and an azide–alkyne Huisgen methylethylidene)-D-mannitol which can be obtained via the
cycloaddition [27]. One of the advantages of the cross-coupling hydrogenation of common table sugar [32]. Oxidation of this
Scheme 1: Previous and present EDOT functionalization routes.
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large scale production, besides considering the environmental
impact of requiring a halogenated solvent. Comparing the struc-
tural difference between diols 2 and 7, these results emphasized
that the isolation of the alkyne from the ethylenediol by an sp3
carbon increased the yield of the transetherification reaction sig-
nificantly. Attempts were also made to attach an alkyne to
EDOT via the reaction between hydroxymethyl EDOT and
propargyl tosylate using DABCO as catalyst, but it led to a very
low yield and this EDOT-propargyl product was very sensitive
to acidic conditions. Considering the robust production of func-
tional alkyne-EDOT and its chemical stability, pyEDOT 3 was
thus selected as the precursor of choice for future synthetic and
electrochemical studies. pyEDOT can be stored at room temper-
ature over months in an acid-free environment.
Scheme 2: The synthetic route from glycidol to pyEDOT (3).
The synthetic utility of pyEDOT is demonstrated by the
diketal by NaIO4 led to glyceraldehyde [33], which was trans- following examples involving a range of pendant groups. The
formed into the dibromoolefin 5 by Corey–Fuchs reaction. electron acceptor units diethyl terephthalate (DET) and 9,10-
Dibromoolefin 5 was dehalogenated by adding 2 equivalents of anthraquinone (AQ) are of particular interest for their redox
n-butyllithium to produce 4-ethynyl-2,2-dimethyl-1,3-dioxo- chemistry in energy storage applications [34]. Their ester and
lane 6, which was deprotected in acid to give ethynyldiol 7. quinone moieties are vulnerable in nucleophilic and acidic envi-
However, the transetherification of 3,4-dimethoxythiophene to ronments. Furthermore, the viologen moiety, available from
produce 8 turned out to be difficult, with very low yield of only 4,4’-bipyridine (BP) has potential for energy storage and elec-
6%. Thus, diol 7 was modified by protecting the ethynyl func- trochromic applications [35,36], but it has previously been
tion with a TMS group, yielding 7’, expecting an improved anchored to a polymer backbone only by oxidative coupling via
yield from a better solubility of this diol in toluene. However, the nitrogen atom. As shown by our synthesis, all of these elec-
the transetherification reaction gave almost no conversion after tron acceptor units were tolerant to Sonogashira coupling condi-
2 days. By changing the solvent to dichloroethane a yield of tions, which produced new EDOT derivatives 9, 10, 11 and 12
12% could be obtained, but this would not be sufficient for (Scheme 4). The Sonogashira couplings were performed be-
Scheme 3: The synthetic route from D-mannitol diketal to eEDOT 8 and TMS-eEDOT 8’.
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tween the alkyne terminal of pyEDOT and a brominated ring
carbon of the pendant group precursors, with yields of 61% (9),
82% (10) and 65% (12), respectively. Methylation of bipyridyl
intermediate 10 by methyl iodide produced viologen derivative
11 after exchanging the iodide for the PF6− anion. For the meth-
ylation step drying of both the reactants 10 and methyl iodide
and the solvent was essential, since its omission resulted in po-
lymerization of the viologen-pyEDOT, as indicated by 1H NMR
(see Supporting Information File 1, Figure S11). We suspect
that the polymerization was triggered by the acid generated
from methyl iodide reacting with water. Interestingly, the colors
of viologen can be tuned by introducing various substituents at
the nitrogen sites [37]. The synthesis of 11 illustrates the ease of
functionalizing viologen on both nitrogens and it can poten-
tially be used for studying synergic electrochromism coupled
with PEDOT [35]. Diethyl 2-bromoterephthalate and 2-bromo-
4,4’-bipyridine were prepared according to reported procedures
[38,39]. The electron acceptor phthalimide (PT) was attached to
EDOT via cycloaddition between the alkyne and an alkylazide
to produce derivative 13 (Scheme 4). Regioselective formation
of 13 involved the Cu(I)-catalyzed Huisgen 1,3-dipolar cycload-
dition between N-(2-azidoethyl)phthalimide and the alkyne ter-
minal of pyEDOT under ultrasound conditions in 73% yield.
Detailed synthetic routes are presented in Supporting Informa-
tion File 1.
Scheme 4: New EDOT derivatives 9–13 accessible from pyEDOT with
bromo-pendant group precursors via Sonogashira cross coupling and
with an azide pendant group precursor via Huisgen cycloaddition.
Figure 1a shows the cyclic voltammograms (CVs) for electro-
chemical polymerization of pyEDOT on a glassy carbon
working electrode. For comparison, the CV for electrochemical (12) were obtained, similar to those of pyEDOT and EDOT as
polymerization of pristine EDOT under the same conditions is presented in Figure 1. For pyEDOT-MVPF6 (11), the polymeri-
shown in Figure 1b. Similar voltammetric responses were ob- zation was performed in a potential region covering both
tained for the polymerization of pyEDOT and EDOT, with in- EDOT-based oxidation and MV-centered reductions. The build-
creased current in the potential region from −1.2 to 0.6 V vs up of the MV-centered redox peaks is clearly evident at −0.8 V
ferrocene/ferrocenium (Fc+/Fc0) during polymerization cycling. vs Fc+/Fc0 and −1.2 V vs Fc+/Fc0 corresponding to the MV2+/+
Apparently, the presence of an ethynyl group on pyEDOT did redox reaction and the MV+/0 redox reaction, respectively. The
not disturb the polymerization of the EDOT backbone. The CVs CVs of these polymers can be found in Supporting Information
of pyEDOT indicated an irreversible reduction reaction at a File 1, Figure S39, and the polymer properties will be character-
potential lower than −2.4 V vs Fc+/Fc0 as shown in the inset of ized further in a separate study. The successful polymerization
Figure 1a. This is believed to indicate the reduction of the triple of these functionalized pyEDOT monomers exemplifies the
bond. The ease of pyEDOT polymerization should also allow capabilities that pyEDOT derivatives can bring for the synthe-
for further post-polymerization functionalization [40].
sis of new polymers and property studies.
Figure 2 presents the CVs of the pyEDOT derivative mono- The phthalimide-EDOT derivative 13 failed to polymerize. A
mers and their polymerization in MeCN solution. All com- plausible reason could be an interference of the electron rich ar-
pounds showed the electrochemical activities of both the omatic moieties with the thiophene radical cations formed
pendant group and the EDOT moiety individually, giving redox during polymerization, in analogy to previous suggestions from
reactions of the pendant groups in a negative potential region Bäuerle et al. for triazolomethyl-substituted EDOT. Post-func-
(the red curves in the insets of Figure 2) and oxidation of the tionalization of the azido-PEDOT polymer by click chemistry
EDOT unit in a more positive potential region (the black curves turned out to be a solution to this problem [41]. To confirm the
in the insets of Figure 2). Comparable voltammetric behaviors viability of this alternative, in a similar fashion, post-functional-
for the polymerization of pyEDOT-DeT (9) and pyEDOT-AQ ization of the poly(pyEDOT) was performed in an acetonitrile
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Figure 1: CVs of electrochemical polymerization of (a) pyEDOT 3 and
(b) EDOT in MeCN solution with 0.1 M TEAPF6, glassy carbon elec-
trode, 0.1 V s−1. Insets show the structure and voltammograms for the
monomers.
Figure 2: CVs of electrochemical polymerization of (a) pyEDOT-DeT
(9), (b) pyEDOT-AQ (12) and (c) pyEDOT-MVPF6 (11) in MeCN with
0.1 M TEAPF6, GC, 0.1 V s−1. Insets show the structure and voltam-
mograms for the monomers. Polymerization of pyEDOT-MVPF6 was
performed on a PEDOT-modified GC electrode.
solution of phthalimide-azide in the presence of catalytic
amounts of Cu+(CH3CN)4PF6− and elemental copper. The reac-
tion to correspondingly functionalized PEDOT was stopped
after three days at room temperature. After washing the
“click”–functionalized electrode it was characterized electro-
chemically. The redox peaks appearing at −2.0 to −1.5 V vs starting points for future new functionalized EDOT monomers
(Fc+/Fc0) indicated the attachment of phthalimide to the and following polymer or oligomer research.
PEDOT backbone (Supporting Information File 1, Figure S40).
Supporting Information
Conclusion
In conclusion, we have introduced a new functional pyEDOT
Supporting Information File 1
featuring a terminal alkyne which endows EDOT or PEDOT
functionalization with the rich chemistry of alkynes. We exem-
plify this application of a C–C bond forming synthesis for the
PEDOT functionalization by a Sonogashira coupling yielding
the first terephthalate functionalized EDOT monomer as well as
a viologen unit with symmetrical N-substitution attached to an
EDOT core. These new monomers have been successfully elec-
tropolymerized on a glassy carbon electrode, giving a robust
electroactive film. Additionally, we show that alkyne-PEDOT
can also be post-functionalized in a “click” fashion. All of these
proved the synthetic and electrochemical utilities of pyEDOT.
Thus, we believe that the pyEDOT synthon provides valuable
Experimental details, NMR spectra, IR spectra, and HRMS
for all products and electrochemistry data.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-12-265-S1.pdf]
Acknowledgments
The Swedish Research Council (VR), the Swedish Foundation
for Strategic Research (SSF), the Olle Byggmästare Foundation,
the Swedish Energy Agency, and the Swedish Centre for Smart
Grids and Energy Storage (SweGRIDS) are acknowledged for
their financial support. J. B. acknowledges the Swedish
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