Diprotodecarboxylation reactions of 3,4-dialkoxythiophene-2,5-dicarboxylic acids mediated by AG2CO3 and microwaves

Article abstract of DOI:10.1080/00397911.2013.800212

An efficient and rapid method is reported to obtain 3,4-dialkoxythiophenes from 3,4-dialkoxythiophene-2,5-dicarboxylic acids through a diprotodecarboxylation reaction with Ag2CO3/AcOH as a catalytic system and microwave heating in dimethylsulfoxide (DMSO) as solvent. This methodology lets us obtain for the first time good performance with thiophenes bearing strong electron-donating groups such as alkoxides. This methodology eliminates the usage of harmful quinoline as solvent, as well as the long reaction times typically used (12-18 h) to obtain the 3,4-dialkoxythiophenes. The reaction of 7 diacids showed good yields (60-89%) following 20 min of microwave heating in a temperature range of 120-150 °C. [Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications for the following free supplemental resource(s): Full experimental and spectral details.]

Full text of DOI:10.1080/00397911.2013.800212

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Diprotodecarboxylation Reactions
of 3,4-Dialkoxythiophene-2,5-
dicarboxylic Acids Mediated by Ag2CO3
and Microwaves
Pablo A. Cisneros-Pérez a , Diego Martínez-Otero a , Erick Cuevas-
Yánez a & Bernardo A. Uribe-Frontana a
a Centro Conjunto de Investigación en Química Sustentable, UAEM-
UNAM , Toluca , Estado de México , Mexico
Accepted author version posted online: 27 Aug 2013.Published
online: 29 Oct 2013.
Click for updates
To cite this article: Pablo A. Cisneros-Pérez , Diego Martínez-Otero , Erick Cuevas-Yánez &
Bernardo A. Uribe-Frontana (2014) Diprotodecarboxylation Reactions of 3,4-Dialkoxythiophene-2,5-
dicarboxylic Acids Mediated by Ag2CO3 and Microwaves, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 44:2, 222-230, DOI:
10.1080/00397911.2013.800212
To link to this article: http://dx.doi.org/10.1080/00397911.2013.800212
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Synthetic Communications1, 44: 222–230, 2014
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2013.800212
DIPROTODECARBOXYLATION REACTIONS OF
3,4-DIALKOXYTHIOPHENE-2,5-DICARBOXYLIC ACIDS
MEDIATED BY Ag2CO3 AND MICROWAVES
´
Pablo A. Cisneros-Perez, Diego Martınez-Otero,
Erick Cuevas-Yanez, and Bernardo A. Uribe-Frontana
´
´
´
Centro Conjunto de Investigacion en Quımica Sustentable, UAEM-UNAM,
Toluca, Estado de Mexico, Mexico
´
´
GRAPHICAL ABSTRACT
Abstract An efficient and rapid method is reported to obtain 3,4-dialkoxythiophenes from
3,4-dialkoxythiophene-2,5-dicarboxylic acids through a diprotodecarboxylation reaction with
Ag2CO3/AcOH as a catalytic system and microwave heating in dimethylsulfoxide (DMSO)
as solvent. This methodology lets us obtain for the first time good performance with thiophenes
bearing strong electron-donating groups such as alkoxides. This methodology eliminates the
usage of harmful quinoline as solvent, as well as the long reaction times typically used (12–
18 h) to obtain the 3,4-dialkoxythiophenes. The reaction of 7 diacids showed good yields
(60–89%) following 20 min of microwave heating in a temperature range of 120–150 ꢀC.
[Supplementary materials are available for this article. Go to the publisher’s online
edition of Synthetic Communications1 for the following free supplemental resource(s):
Full experimental and spectral details.]
Keywords Ag2CO3; 3,4-dialkoxythiophene-2,5-dicarboxylic 3,4-dialkoxythiophenes; mic-
rowaves; protodecarboxylation
INTRODUCTION
3,4-Dialkoxythiophenes are very important substances in the preparation of
oligothiophenes and polythiophenes,[1] which have interesting electronic properties
and find applications in electronic devices fabrication, as organic light-emitting
diodes (OLEDs)[2a] and organic field-effect transistors (OFETs),[2b] electrochromic
materials,[2c] biosensors,[2d] chemosensors,[2e] organic solar cells,[2f] etc. One of the
Received April 4, 2013.
Address correspondence to Bernardo A. Uribe-Frontana, Centro Conjunto de Investigacion en
´
Quımica Sustentable, UAEM-UNAM, Carretera Toluca-Atlacomulco Km 14.5, Campus UAEMex
´
‘‘San Cayetano’’ Toluca, C.P. 50200, Estado de Mexico, Mexico. E-mail: bafrontu@unam.mx
´
´
222

PROTODECARBOXYLATION OF ALKOXYTHIOPHENE-DIACIDS
223
best known and useful polythiophenes is polyethylenedioxythiophene (PEDOT).
This polymer, when oxidized, has excellent conductivity (ca. 300 S=cm), is almost
transparent, and shows great chemical stability to the environmental conditions.[3]
In the classic synthesis of 3,4-dialkoxythiophenes such as 3,4-diethylendioxythio-
phene (EDOT, 2a), the monomeric precursor requires a double aromatic protodecar-
boxylation reaction from a thiophene bearing two carboxyl groups in positions 2 and
5 (Scheme 1).[4] The main disadvantages of this reaction are the long reaction time
(t > 12 h), high temperatures (T ꢁ 150 ꢀC) incompatible with some sensible functional
groups attached to the thiophene ring,[5] and poor yields with electron-rich aromatic
compounds as EDOT that do not react completely[6] (in the range of 45–64% from
diacid).[4b] Generally, this process is carried out using a copper catalyst and the toxic
and not environmentally friendly solvent quinoline.
In recent years more efficient protodecarboxylation methodologies for electron-
rich and electron-poor aromatic systems have been reported. Using the expensive
palladium(II) trifluoroacetate as catalyst, it is possible to decarboxylate electron-rich
aromatic systems. A method having broader scope was described by Gooben,[7] who
improved the protodecarboxylation of aromatic systems mediated by coopper cata-
lyst with microwaves; unfortunately the inconvenient of using high temperatures
(160–190 ꢀC) and the moderate yields (62%) reported with thiophene derivatives
makes its use unattractive with our starting materials. A third methodology,
developed independently by Gooßen et al.[8a,8b] and Larrosa et al.,[8c,8d] uses Ag (I)
as catalyst. In Larrosa’s methodology, the temperature does not reach more than
120 ꢀC and has been efficiently applied to some heteroatomic systems.[8d] Never-
theless, it has not been adapted to microwave heating and has been scarcely applied
to some thiophene derivatives. Larrosa only reported transformations with
Scheme 1. Classical synthetic route to obtain 3,4-dialkoxythiophenes and poly-3,4-dialkoxythiophenes.

´
P. A. CISNEROS-PEREZ ET AL.
224
halogen-substituted thiophenes, and there are no reports of protodecarboxylations of
thiophenes bearing strong electron-donating groups such as alkoxides. Thus, con-
sidering that as far as we know this procedure has never been used with the
3,4-dialkoxythiophene-2,5-dicarboxylic acids, in addition to the utility and value of
the 3,4-dialcoxythiophenes, it was decided to attempt Larrosa’s methodology with
microwave heating for this thiophene family. Our intent was to eliminate the use of
quinoline as the solvent and to reduce both the reaction time and temperature.
DISCUSSION
In our initial studies, 3,4-diethylendioxithiophene-2,5-dicarboxylic acid (1a)
was used as a model compound. The classical reaction with Cu2Cr2O7 and quinoline,
but using microwave heating instead of an oil bath, was carried out at 150 ꢀC only
for comparative purposes (entry 1, Table 1). This reaction occurred in a 78% yield
in only 15 min. This represents a substantial reduction in time and energy compared
with conventional heating where 20 h reactions are required (see Scheme 1).[4] The
Table 1. Optimization of diprotodecarboxylation conditionsa
Entry
Solvent
AcOH (mmol)
Catalyst (% mol)
T (ꢀC)
Time (h)
Yield (%)
1
2
Quinoline
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
—
0.05
0.05
0.5
1
A (34)
B (20)
B (40)
B (20)
B (20)
B (40)
B (40)
B (40)
B (40)
B (40)
B (40)
B (40)
B (40)
B (80)
B (60)
B (40)
B (40)
B(40)
150b
120
0.25
16
78
54
3
120
120
16
16
35
57
4
5
120
120
16
16
62
35
6
0.5
1
7
120
16
78
11
8
0.05
1
120b
120b
120b
120b
120b
120b
120b
85b
0.33
0.5
0.5
0.33
0.5
0.5
0.5
0.5
0.5
0.25
0.33
9
66
10
11
12
13
14
15
16
17
18
1
75c
61
2
2
48
2
60c
75
1
1
0
2
100b
150b
150b
17
1
10
89c
81c,d
aIn all experiments, 0.5 mmol of diacid was used.
bWith microwave heating (temperature of IR sensor).
cThe acid and catalyst were stirred for 5 min before adding AcOH and start microwave heating.
dUsing 5 mmol.

PROTODECARBOXYLATION OF ALKOXYTHIOPHENE-DIACIDS
225
reaction conditions used by Lu et al.[8d] for other compounds [10% mol Ag2CO3 for
carboxylic group, 0.05 mmol AcOH, dimethylsulfoxide (DMSO), 120 ꢀC, 16 h] in our
hands and using compound 1a and conventional oil bath heating, gave a moderate
yield of 54% (entry 2, Table 1), which is lower than using the Cu catalyst. An
increment in the amount of Ag2CO3 without adding more AcOH decreased the yield
to 35% (entry 3, Table 1). This can be attributed to the AcOH neutralization with
Ag2CO3 excess. Kozlowski determined that in aromatic protodecarboxylation
mediated by Pd(OCOCF3)2, the rate-limiting step is the protonation of Aryl-Pd
intermediate, and thus 10-fold excess of the compound source of proton
(CF3COOH) was required to complete the reaction.[6b,8d] After exploring different
conditions (entries 3–6, Table 1), the compound 1a was protodecarboxylated in a
yield similar to that with Cu catalyst and conventional heating, when 1 mmol of
AcOH and 40% mol of Ag2CO3 were used (entry 7, Table 1) after 16 h. These con-
ditions are more convenient than the classical synthesis depicted in Scheme 1,
because the reaction occurs at a temperature that is 30 ꢀC lower, uses DMSO rather
than quinoline as solvent, and uses a readily available and inexpensive proton
source. Unfortunately a long reaction time was necessary for complete disappear-
ance of the starting material using conventional heating. Therefore, to decrease
the reaction time, microwave heating was explored.[9,10] Heating for 0.5 h at
120 ꢀC, with the optimized conditions found with conventional heating for the dipro-
todecarboxylation reaction, afforded an acceptable yield of 66% (entry 9, Table 1).
However, a yield of 75% was reached when the experiment was repeated, after first
stirring the carboxylic diacid and Ag2CO3 in DMSO for 5 min, before the addition of
AcOH and microwave heating (entry 10, Table 1). This pretreatment substantially
favored the reactions (entries 12 and 13, Table 1). With a large excess of Ag catalyst,
the yield is also good (entry 14, Table 1) and equivalent to that found with prestirring
(entry 10, Table 1), demonstrating the advantage of this operation; nevertheless
further experiments demonstrated that 40% per mol of compound (20% per mole
of carboxylic group) was enough for carry out the reaction efficiently at low tem-
peratures. A larger excess of the proton source compound was not beneficial for
the reaction and the yield was lower (entries 11–13, Table 1). The use of temperatures
lower than 120 ꢀC provokes a drastic decrease of the yield (entries 15 and 16,
Table 1). This temperature is in accordance with the increment in pression inside
the reaction tube of reaction of the microwave reactor obseved when the temperature
reaches 110 ꢀC (Fig. SI-1), indicating that the diprotodecarboxylation reaction initi-
ates at this temperature. Using the best conditions found for dicarboxylic compound
1a at 120 ꢀC (1 mmol AcOH þ 40% mol Ag catalyst, corresponding to 20% per mole
of carboxylic group þ prestirring), the reaction was carried out at 150 ꢀC during
15 min of microwave heating, leading to a greater yield very close to 90% (entry
17, Table 1). Finally, when the reaction was scaled to 5 mmol using 150 ꢀC (entry
18, Table 1) the product 2a was obtained in a yield of 81%. In this reaction, the inter-
nal pressure in the vessel reached a constant maximum value of 12 bars after 20 min,
which is a convinient and safe value for the reaction vessels used with the microwave
system (max 30 bar). The utility and safety of the reaction conditions for larger-scale
transformations was therefore demonstrated. The protodecarboxylation optimized
conditions were tested with six additional analogs of 3,4-dialkoxythiophene-2,5-
dicarboxylic acids (1b–1g), and the results are shown in Table 2.

´
P. A. CISNEROS-PEREZ ET AL.
226
Table 2. Diprotodecarboxylation of 3,4-dialkoxythiophene-2,5-dicarboxylic acids, 1a–1g
Protodecarboxylated
Diacid Product
Yield (%)
Entry
Lit. Yield 120 ꢀC 0.5 h 150 ꢀC 0.25 h
1
54[12]
75
78
89
81
2
60[4b]
3
64[4b]
83
86
4
54[4b]
53
76
5
45[4b]
78
80
6
—
44
56
(Continued )

PROTODECARBOXYLATION OF ALKOXYTHIOPHENE-DIACIDS
227
Table 2. Continued
Protodecarboxylated
Diacid Product
Yield (%)
Entry
Lit. Yield 120 ꢀC 0.5 h 150 ꢀC 0.25 h
7
—
22b
37b
aUsing 0.5 mmol of compound 1, 1 mmol of AcOH, 40% mol Ag2CO3, prestirring of diacid and catalyst,
and microwave heating.
bMain product.
The 3,4-dialkoxythiophenes 2a–2e had been prepared by means of the classic
method with the Cu catalyst-quinoline system,[4b,11] and for comparison the reported
yields are depicted also in Table 2. As occurred with compound 1a, the corresponding
3,4-dialkoxythiophenes were obtained in greater yields and shorter heating times
(0.5 h) using lower temperature (120 ꢀC) than the previously reported with Cu catalyst
and without the harmful solvent quinoline (entries 2–5, Table 2). Increasing tempera-
ture to 150 ꢀC resulted in a slight increase in yield (3–23% depending upon the pro-
duct) with a time of reaction of only 15 min. Nevertheless, in the case of sensitive
compounds, limiting the microwave heating to 120 ꢀC is enough to obtain the target
compounds in good yields. O-Alkyl substituted derivatives such as 2f are prepared
throughout trans-etherification of 3,4-dimethoxy-thiophene,[2g,12a] a compound that
is relatively unstable. The present method overcomes this difficulty. The double pro-
todecarboxylation of 1g was not possible to achieve, because of a fast intramolecular
reaction between the acetal and one carboxylic group of the diacid that produces,
mainly lactone 2g and benzaldehyde. The identity of this compound was confirmed
by NMR (see Suporting Information) and x-ray crystallography (Fig. I). The two
enantiomers are present in the crystal of compound 2g as a racemate and it shows
planarity among the thiophene ring, the carboxylic group, the hydroxyl group, and
˚
the ethereal oxygen. Carbon 6 is 0.635 A out from this plane, so the conformation
Figure 1. Crystal structure of 2g. Ellipsoids are drawn at the 50% probability level for non-H atoms.

´
P. A. CISNEROS-PEREZ ET AL.
228
Scheme 2. Proposed mechanism reaction to achieve 2g.
of the six-membered ring is half-chair, the same conformation to reported in
compounds with similar six-membered ring fused to benzene.[13]
To determine if the intramolecular reaction of 1g occurs before or after the
protodecarboxylation reaction, it was followed by 1H NMR, heating the NMR tube
containing all the required reactants, but without the catalyst (Ag2CO3) (Fig. SI-2).
The experiment showed that spontaneous decarboxylation was possible when
temperature reaches 55 ꢀC, resulting in products 2g and benzaldehyde. The same
products were detected if the protodecarboxylation reaction is attempted without
AcOH (to minimize the acetal opening), showing that the reaction is not catalyzed
by this acid. A mechanism to explain the transformation of 1g to 2g is proposed in
Scheme 2, where an intramolecular hydrogen bonding between the acetal unit of 1g
and the acidic proton of one of the carboxylic acids promotes the 1,3-dioxalane
system opening. At the end of this rearrangement only one carboxylic acid is available
to react in the protodecarboxylation reaction.
CONCLUSION
In conclusion, Larrosa et al.’s protodecarboxylation procedure was modified
and adapted to carry out efficiently and rapidly diprotodecarboxylation of 3,4-
dialkoxythiophen-2,5-dicarboxylic acids using 40% mol Ag2CO3 (20% per mole of
carboxylic group), 2 equivalents of AcOH, and prestirring of the diacid and the
catalyst prior to microwave heating in the temperature range 120–150 ꢀC. This
methodology was applied to seven different substrates showing greater yields than
the classical Cu catalyst-quinoline protocol, eliminating the harmful quinoline and
saving energy because of the short time required in the reaction.
EXPERIMENTAL
All microwave reactions were carried out in closed vials using microwave
reactor Synthos 3000 from Anton Paar. Anhydrous DMSO, glacial AcOH, copper

PROTODECARBOXYLATION OF ALKOXYTHIOPHENE-DIACIDS
229
chromite, AgNO3, and anhydrous Na2CO3 were purchased from Sigma Aldrich, and
1
Merck Mallinckrodt, and used without further purification. H NMR spectra were
recorded at 300 MHz and are referenced to the residual solvent peak at 7.26 ppm
(CDCl3), 2.09 ppm (acetone-d6), and 2.50 ppm (DMSO-d6). 13C NMR spectra were
recorded at 75 MHz and are referenced to the solvent peak at 77.0 ppm (CDCl3),
29.9 and 206.7 ppm (acetone-d6), and 39.5 ppm (DMSO-d6).
General Procedure for Protodecarboxylation of 3,4-
Dialcoxythiophene-2,5-dicarboxylic Acids
The mixture of 3,4-dialcoxythiophene-2,5-dicarboxylic acid (0.5 mmol) and
Ag2CO3 (0.20 mmol, 55.15 mg) in DMSO (1.0 mL) was magnetically stirred. After
5 min, AcOH (1 mmol, 56 ml) was added, the microwave tube was closed, and the mix-
ture was heated using the microwave reactor. The reaction was carried out for 30 min
for the experiments conducted at 120 ꢀC and for 20 min when they were carried out at
150 ꢀC. After microwave heating, the reaction was cooled to room temperature and
quenched with 2 M HCl (2 mL), the mixture was filtered, and the aqueous phase
was extracted with EtOAc (5 ꢂ 5 mL). The combined organic layers were washed with
brine (2 ꢂ 25 mL) and dried (Na2SO4). Concentration under reduced pressure gave
a crude product, which was purified by flash chromatography in silica gel using
n-hexane–CH2Cl2 80:20 as eluent.
ACKNOWLEDGMENTS
P. A. C. P. thanks the CONACYT for doctoral fellowship. Funding through
PAPIIT Project IN202011 and CONACYT Project 179356 is also acknowleged.
´
Nieves Zavala Segovia, Alejandra Nun˜ez Pineda, Ruben Gavin˜o-Ramirez, Isabel
Chavez Uribe, Angeles Pen˜a Gonzalez, and Lizbeth Triana Cruz are recognized
by their technical support in this work.
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