An improved procedure for the synthesis of 3,4-ethylenedioxythiophene

Article abstract of DOI:10.3184/174751911X13073872921614

An improved procedure for the synthesis of 3,4-ethylenedioxythiophene is reported starting from ethyl chloroacetate. Reaction with sodium sulfide gave diethyl thiodiglycolate which was then reacted with diethyl oxalate, and then 1,2-dibromoethane to give 2,5-dicarbethoxy-3,4-ethylenedioxythiophene. Hydrolysis, and decarboxylation gave 3,4-ethylenedioxythiophene in 16% overall yield. The structures of the key intermediates in synthetic routes were confirmed, and every step was optimised, to give a procedure suitable for large-scale industrial production.

Full text of DOI:10.3184/174751911X13073872921614

JOURNAL OF CHEMICAL RESEARCH 2011
JUNE, 339–340
RESEARCH PAPER 339
An improved procedure for the synthesis of 3,4-ethylenedioxythiophene
Hua Zhang, Chao Qian and Xin-Zhi Chen*
Department of Chemical and Biochemical Engineering, Zhejiang University. NO.38, Zheda Road, Hangzhou 310027, P. R. China
An improved procedure for the synthesis of 3,4-ethylenedioxythiophene is reported starting from ethyl chloroace-
tate. Reaction with sodium sulfide gave diethyl thiodiglycolate which was then reacted with diethyl oxalate, and then
1
3
,2-dibromoethane to give 2,5-dicarbethoxy-3,4-ethylenedioxythiophene. Hydrolysis, and decarboxylation gave
,4-ethylenedioxythiophene in 16% overall yield. The structures of the key intermediates in synthetic routes were
confirmed, and every step was optimised, to give a procedure suitable for large-scale industrial production.
Keywords: 3,4-ethylenedioxythiophene, ethyl chloroacetate, diethyl thiodiglycolate, diethyl oxalate, decarboxylation
Poly (3,4-ethylenedioxythiophene) (PEDOT) is one of the
most promising polymers because of its high electrical
conductivity, high thermal stability and transparency in the
discussion. The conventional five-step reaction scheme suffers
from several major drawbacks from an industrial point of view,
including the hydrolysis of ethyl chloroacetate (1) in aqueous
sodium sulfide, the dependence on a high temperature and the
use of high boiling organic solvents, leading to low yield of
EDOT. The synthetic routes to several important intermediates
has been improved and optimised based on the classical five-
step method. Every step has been modified to be simpler and
the procedure is more suitable for large-scale production.
Previously, diethyl thiodiglycolate (2) was synthesised by
1–3
oxidized state. PEDOT is widely used although its annual
production is limited to the several-ton scale because of the
high production cost and the complicated synthetic technology
to prepare 3,4-ethylenedioxythiophene (EDOT) (6), the mono-
4,5
mer of PEDOT. Thus, much attention has been paid to
improving the synthesis of EDOT.
Conventional five-step method of synthesising EDOT was
6
8
reported by Stenger in 1998. In this technology, EDOT was
esterification of 2,2′-thiodiacetic acid with ethanol. However,
synthesised from 2,2′-thiodiacetic acid via esterification with
ethanol, reaction with diethyl oxalate using sodium ethanolate/
ethanol, followed by reaction with 1,2-dibromoethane
to obtain 2,5-dicarbethoxy-3,4-ethylenedioxythiophene (4).
Hydrolysis by aqueous sodium hydroxide, acidified by aque-
ous hydrochloric acid, and decarboxylation with Cu as a cata-
lyst in a high boiling organic solvents gave EDOT. Although
this process is established, the route is long and gives a low
yield of the product. In addition, the separations and purifica-
tion of the intermediates is also difficult. Another two-step
reaction method for the synthesis of EDOT was reported by
the raw material (2,2′-thiodiacetic acid) has a relatively high
price, and the esterification is also a reversible reaction leading
to a low yield. Another method of synthesising 2 is from a
derivative of 2,2′-thiodiacetic acid, such as 2,2′-thiodiacetyl
chloride. Obviously, this route needs additional steps to
prepare the derivative of 2,2′-thiodiacetic acid. In our study,
2 was synthesised by reaction of 1 and sodium sulfide. The raw
materials are cheap, cyclohexane was chosen as the better
solvent to avoid the hydrolysis of 1. After the reaction was
completed, the cyclohexane could be recovered and reused by
distillation with a high recovery rate.
7
Fredrik and his colleagues in 2004. Firstly, 2,3-dimethoxy-
Compound 4 was synthesised by the Williamson condensa-
tion of disodium 2,5-dicarbethoxy-3,4-dioxythiophene (3)
with 1,2-dibromoethane. Previously, this reaction was carried
out in DMF using potassium carbonate or potassium
buta-1,3-diene reacted with SCl to give 3,4-dimethoxythio-
2
phene, which was reacted with ethane-1,2-diol to give EDOT.
However, this route for synthesising EDOT was far from
industrialisation. Firstly, the starting material was very expen-
9,10
carbonate/tetrabutyl ammonium bromide as a catalyst. In
our research, triethylamine was selected to replace the tradi-
tional catalyst. The experimental result indicated that triethyl-
amine increased the reaction rate and selectivity of the desired
product.
sive and SCl used in this method must be purified before the
2
experiment. The intermediate 3,4-dimethoxythiophene was
very unstable, and must be preserved under an inert gas and
at low temperatures. Finally, column chromatography was
needed in the purification of the product, which was difficult
for large-scale production in industry.
Therefore, the five-step reaction scheme (Scheme 1) was
developed to synthesise EDOT in the light of the above
Decarboxylation was a key step in synthesising EDOT.
Generally, Cu was chosen as catalyst. More attention was paid
to the solvent used in this reaction. High boiling organic
solvents were used because decarboxylation demanded high
Scheme 1
*
Correspondent. E-mail: xzchen@zju.edu.cn

3
40 JOURNAL OF CHEMICAL RESEARCH 2011
1
1
temperature. Quinoline was selected as solvent by Coffey
filtrate was cooled to room temperature. The precipitated product was
filtered off, and the filter cake was dried in vacuum. The product
12
et al. This caused a difficulty in separating the products and
8
was 23.1g with yield of 53.8%. Melting point:146.0–146.9°C (lit.
recovering the solvent. Quinoline is a serious environmental
1
1
3,14
146.0–147.0°C). H NMR (400MHz, DMSO-d ) δ (ppm): 4.40
pollutant.
DMSO and DMF was selected as solvent by
6
H
1
5
(s, 4H, –O–CH
t, 6H, J=7.2Hz, –CH3).
,5-Dicarboxy-3,4-ethylenedioxythiophene (5): To a 500 mL four-
2
CH –O), 4.26 (q, 4H, J=6.8Hz, –OCH CH ), 1.27
2
2
3
Woon et al. The mixture demanded additional organic
solvent for extraction and distillation to obtain the desired
product. Thus, N, N, N′, N′-tetraethylethane-1,2-diamine was
selected as the solvent in our research. It could be recovered
and recycled for the next batch.
In summary, from the starting material of 1, EDOT was
obtained in five steps. The synthetic routes of key intermedi-
ates were improved and optimized to make the procedure sim-
pler with high yields. The present procedure is more effective,
competitive, and more suitable for large-scale production
(
2
necked flask were added 4 (21.5 g, 0.75 mol). 200 ml 10% aqueous
sodium hydroxide was added dropwise. The reaction was carried out
for 2 h with stirring under refluxing. Followed, the solution was
cooled to below 10°C. The aqueous hydrochloric acid was added until
the pH<3. The precipitated product was filtered off. The filter cake
was washed with H O (2×20ml) and dried in vacuum. The obtained
2
product was 14.0g with yield of 81.0%. Melting point: 319.5–321.2°C
1
2
1
(lit. 323°C). H NMR (400MHz, DMSO-d
) δ
6
(ppm): 13.10 (s, 2H,
H
1
–
COOH), 4.36 (s, 4H, –O–CH –CH –O). H NMR. The spectral data
2
2
were obtained from Wiley Subscription Services, Inc. (US). (500MHz,
Experimental
DMSO–d ) δ (ppm): 13.05 (s, 2H, –COOH), 4.35 (s, 4H, –O–CH –
1
13
6
H
2
H NMR (400MHz) and C NMR (100MHz) spectra were recorded
CH –O).
2
for DMSO-d solutions on a Bruker AC-400 instrument with TMS as
6
3
,4-Ethylenedioxythiophene (6): To a 250 mL four-necked flask
internal standard. The purities were determined by GC analysis
were added with 5 (11.5 g, 0.05 mol), Cu (1.3 g, 0.02 mol) and N, N,
N′, N′-tetraethylethane-1,2-diamine (100 mL). The reaction was car-
ried out for 4h with stirring at 180°C. The product was distilled under
reduced pressure and the fraction boiling at 92–100°C (0.004MPa)
was collected, to give 6 (3.4g). GC analysis indicated that the purity
(
Agilent 1790F using a 30 m long SE54 capillary column). Melting
points were determined on WRR melting point apparatus and uncor-
rected. Mass spectra were obtained on a Trace G2000 GC-MS spec-
trometer. All solvents, reagents and ligands used in this experiment
were AR.
was more than 98.5% and the yield was 47.9%. The overall yield
Diethyl thiodiglycolate (2): To a mixture of Na S (46.8 g, 0.6 mol)
2
1
was 16.0% from 1. H NMR (400MHz, DMSO-d ) δ (ppm): 6.53
and cyclohexane (200 mL) was added dropwise 1 (122.5 g, 1.0 mol)
with stirring at 15–20°C over 1h. The reaction was carried out for 3 h
with stirring under reflux. The mixture was cooled to room tempera-
ture, and the mixture was then filtered. The filtrate was distilled and
the fraction (cyclohexane) boiling at 80–81°C was collected. The
volume of obtained cyclohexane was 174.0 mL, and the recovery rate
was 87.0%. The residual oil was distilled under reduced pressure and
the fraction boiling at 160–170°C (0.01MPa) was collected, and gave
6
1
H
16
(
s, 2H, –C=CH), 4.17 (s, 4H, -O–CH CH –O). H NMR (lit. )
2 2
(
400MHz, CD SOCD ) δ (ppm):6.50 (s, 2H, –C=CH), 4.20 (s, 4H,
3 3 H
–
O–CH CH –O).
2 2
We greatly acknowledge the generous financial support by
grants from the Natural Science Foundation of the Zhejiang
Province (No. Y4090045, Y4100147) and the Fundamental
Research Funds for the Central Universities (No.
2011QNA4018).
2
(85.6 g). GC analysis indicated that the purity was more than 98.0%
and the obtained yield was 83.1%. MS (m/z, %):206 (44), 160 (100),
1
1
33 (56), 105 (50), 88 (12), 77 (40), 60 (14), 29 (16). H NMR
(
(
400MHz, DMSO-d ) δ (ppm): 4.16 (q, 4H, J=7.4Hz, –O–CH ), 3.58
6
H
2
s, 4H, –CH –S), 1.28 (t, 6H, J=7.4Hz, –CH CH ).
Received 26 April 2011; accepted 10 May 2011
Paper 1100675 doi: 10.3184/174751911X13073872921614
Published online: 11 July 2011
2
2
3
Disodium 2,5-dicarbethoxy-3,4-dioxythiophene (3): To a 500 mL
four-necked flask were added ethanol (200 ml) and sodium ethanolate
(
(
30.1 g, 0.44 mol). A mixture of 2 (41.2 g, 0.2 mol) and diethyl oxalate
32.2 g, 0.22 mol) was added with stirring at 20°C over 0.5h. The reac-
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8
9
0
1
2
1
1
1
cooled to 80°C. H O (200 mL) was added and the mixture was stirred
2
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3
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1
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